The Renco mine is situated in the granulite facies Northern Marginal Zone (NMZ) of the Limpopo Belt in southern Zimbabwe. Gold mineralization is hosted by amphibolite facies shear zones in an enderbitic host rock. Shearing occurred during juxtapositon of the NMZ on the Zimbabwe Craton (Kisters et al., 1998). Amphibolite facies conditions during gold precipitation are constrained by garnet-biotite-thermometry and O-isotope thermometry yielding temperatures of 600°C and 680-710°C, respectively (Kolb et al., submitted). During deformation, strain has been partitioned in ductile mylonites and brittle-ductile lithons. Alteration of the host rocks was controlled by strain partitioning and caused hydration of the granulite facies mineral paragenesis in mylonites (i.e. formation of biotite, amphibole). In lithons, a typical high grade alteration assemblage of Qtz, Kfs, Bt, Gt with dominance of pyrrhotite in the ore mineral assemblage can be observed (Kisters et al., 1998). Structures of the lithons can be interpreted to have formed during hydraulic brecciation resulted in a dramatic decrease in pore fluid pressure and concommitent precipitation of sulfides and gold along fractures. Stable isotope data suggest an external crustal source for the mineralizing fluids.
The alteration characteristics and structural control of the gold mineralization give way to the possible interpretation of the Renco mineralization as a high grade member of a crustal continuum as is proposed for the lode gold deposits in the Yilgarn Block in Western Australia (Colvine, 1989; Groves et al., 1992; Groves, 1993). This model implicates giant plumbing systems and a deep, possibly mantle source of the ore forming fluids. However, in the present case the dry, granulite facies, felsic rocks underlying the Renco mineralization are unlikely to have produced the ore fluid.
The tectonic position of the Renco Mine favours a model involving a more lateral fluid plumbing system. During thrusting of the NMZ onto the Zimbabwe Craton prograde metamorphic reactions in the underlying granite greenstone assemblages may have initiated the formation of a fluid phase. This fluid phase is considered to have moved to the higher metamorphic rocks (Furlong et al., 1991) of the NMZ and further been focussed in the extensive shear zones of the thrust zone. The observation of enrichment of elements such as Cu, Co and Ni in the Renco ores, which could easily be derived from ultramafics of greenstone assemblages, may confirm this model for the fluid source. Precipitation of gold and sulfides occured in shear zones in the more competent Renco enderbite relative to the surrounding granulite geisses in a large scale low stress regime.
The crustal continuum model postulates crustal-scale hydrothermal systems, which can well explain the characteristics of lode gold mineralizations of the Yilgarn Block, but this model is not enterily applicable to the fluid plumbing system of Renco.
Colvine AC., Economic Geology Monogr., 6, 37-53, (1989).
Furlong KP, Hanson RB, Bowers JR., Rev. in Mineral., 26,, 437-506, (1991).
Groves DI, Barley ME, Barnicoat AC, Cassidy KF, Fare RJ, Hagemann SG, Ho SE, Hronsky JM A, Mikucki EJ, Mueller AG, McNaughton NJ, Perring CS, Ridley JR, Vearncombe JR, Geol. Dep. and Univers. Ext., The Univ. of Western Australia Pub, 22, 325-337, (1992).
Groves DI, Mineralium Deposita, 28, 366-374, (1993).
Kisters AFM, Kolb J, Meyer FM, Economic Geology, 93, 587-601, (1998).
Kolb J, Kisters AFM, Hoernes S, Meyer FM, Mineralium Deposita, (submitted)
Gold deposits are widely distributed in many of Arches greenstone belts of the world. In Russia these potentially perspective areas are poorly known. Investigated the Aldan Shield is most. On the Aldan Shield perspective areas on detection of "ancient" gold are greenstone belts, including Olondo Greenstone Belt (OGB). OGB is located in the central part of Olekma Granite-greenstone province (the west part of the Aldan Shield, Yakutia). It includes Archean granite-greenstone and strongly metamorphosed gneissic complexes and also Proterozoic sedimentary rocks. In the plan the belt represents V-figurative structure, composed by metavolcanic and intrusive rocks of different composition (from ultramafic to acid). All rocks are liable to folded deformation, attendant on the formation of thrusts and shifts. In fault zones this rocks were intensively sheared down to blastomilonitization. These events were accompanied by introduction of gabbro-alkali-basaltoic dikes with age 2.0-2.2 Ga (Popov et al., 1995). Increased tenors of gold in OGB are connected with zones of ancient (about 2 Ga) shearing and metasomatic reprocessing of mafic and ultramafic rocks. Metasomatic reformations are developed as moderate chloritization, biotitization, sulfidization and carbonatization. Zonal structure is typical for metasomatites. These zones of subvertical location are traced from 15-20 m to 100 m in extend. The significant transformations are characterized by formation of carbonat-phlogopite schists with tourmaline and increased aluminiferious. The contents of gold range from 0.1 g/t up to 2.5 g/t in poorly changed komatiitic metabasalts, reaches 4.5 g/t in komatiits and blastomilonits after metabasalts, and does not exceed 2.3 g/t in carbonat-phlogopite shists (average contents - 0,7 g/t). Metasomatic processes, probably, is favorable to the redistribution and accumulation of gold in rocks. The correlation connections of Au with other elements - Fe, Ti, K, Ca, LOI, inherent in biotite, chlorite, carbonate, sulfides confirm to it. The important conditions of accumulation of gold are carbonatization and sulfidization, and also introducing of insignificant phosphorus. Study of native gold from alluvial and delluvial deposits within OGB, and gold from a beats of host samples (toleitic metabasalts), shoed presence of two gold types -from zones of oxidation and. This confirms representations about accumulation of gold in conditions of metasomatic transformation of rocks and also allows us to assume presence of gold-bearing zones of oxidation. Fineness of fissure-streaky gold is 720-780, and isometric lump is 970-990, that indicates two sources. Rare finding of native gold in beats of host samples testifies that the overwhelming part of gold, probably, is presented by fine-dispersive grains (up to 10 micron). Independent grains are founded only in specific cases (up to 0,1-0,3 mm). Thus, the instituted researches allow us to speak about new and rather perspective types of gold displays on the Aldan Shield.
Popov NV, Smelov AP et al., Olondo Greenstone Belt, Yakutsk, YSC SB AS USSR, 172, (1998).
The most important tectonic feature in the Urals is the Main Uralian Fault (MUF), which represents a suture between the western foreland of the orogen and the internidic parts in the east. Along this structure, a number of small- to medium-sized gold deposits are developed, of which the Mindyak gold deposit, as an example, will be discussed here.
The Mindyak gold deposit is located in an intensely folded allochthonous tectonic mélange, which contains blocks of basaltic and dioritic rocks. Gold mineralization is particularly concentrated in brecciated diabase blocks in the mélange.
The wallrocks, consisting of serpentinites and carbonaceous schists, are wide-spread alterated by silification, carbonatization, sericitization and greenish Cr-muskowite (fuchsite). In the gold-bearing diabase, mafic minerals are altered to spots of fine-crystalline carbonates with ankeritic composition, rimmed by dolomite. The observed compositive zoning is considered to represent a change in fluid composition. The alteration is controlled by small veins, containing dolomite and ankerite of the same composition as in the altered rocks. Besides carbonate, chlorite is present in these veins. Chlorite thermometry points at alteration temperatures between 250 and 350°C. This initial alteration is characterized by a significant enrichment of Mg, Co, Ni, Sb and Ba, in places by a slight enrichment of gold. Other fluid components were potassium, which precipitated during sericitization, and CO2, which was countinuously consumed in the process of carbonatization.
The following mineralization is strictly vein-bound and located in veins and the quartz-carbonatic matrix of the hydraulically brecciated diabase. There is also a change in chemistry of the gangue carbonates from dolomitic-ankeritic to calcitic/ferrocarbonatic. Associated with the latter carbonates is the ore mineral assemblage, comprising predominantly pyrite, chalcopyrite, sphalerite, arsenopyrite and gold. With AsPy-Sph-therometry, mineralizing temperatures can be bracketed between 270 and 300°C. Investigation of fluid inclusions, measured in gangue quartz, show clear indications for a boiling system at about 300°C. None of the inclusions contain CO2 as a distinct phase, although rarely clathrathes can be observed. This observation is interpreted to reflect a late phase of the fluid, when CO2 was consumed during carbonatization and only a residual amount remained in the fluid.
Alteration parageneses point to a mesothermal environment for gold mineralization. However, fluid inclusion data and brittle deformation point to ore formation at a relatively shallow crustal level. Therefore, the Mindyak deposit is interpreted to represent an intermediate level between epi- and mesothermal.
The Shila low-sulphidation epithermal vein system is hosted by the Early to Middle Miocene calc-alkaline volcanites of the Tacaza Group in the Western Cordillera northwest of Arequipa. Since the mine went into production, it has produced (up to June 1997) 436,065 metric tons of ore grading 10.18 g/t Au and 265.3 g/t Ag.
The mineralized veins (or 'veta') of the Shila district trend east-west (Pillune, Sando Alcalde, Ticcla, Puncuhuayco, Mina Paula), northwest-southeast (Apacheta, Colpa, Tocracancha), and exceptionally northeast-southwest (Apacheta, Puncuhuayca, Ampato). The veins are generally steeply dipping (>75°) to the north or to the south.
Observations of the vein system indicate a general pattern that is characterized by the systematic association between a main vein, up to 1-2 m thick, and thin open satellite fractures filled with euhedral quartz in places associated with sulphides. The main vein commonly strikes N080°-N110° E, whereas the second-order fractures strike N120°-N135° E. Some veins do not fall into this general pattern: for example, 'veta 22' (Apacheta) strikes N040° E and shows no secondary fractures, and 'veta 2', in the same area, strikes N150° E and shows scarce secondary fractures that are not filled.
The formation of the mineralized veins in the Shila-Paula district is interpreted as the result of renewed movement and reopening of shear zones originating from an earlier tectonic event of Middle to Late Miocene age. A NE-SW to ENE-WSW shortening accompanying this early tectonic event would have been responsible for the formation of E-W sinistral strike-slip shear zones whose movement would have generated a contemporaneous shistosity along a general N120° E trend. These structures were reopened at the end of the Miocene and acted as channelways for the mineralizing fluids. The main veins are thus localized along old shear zones, whereas the secondary fractures follow the direction of the earlier shistosity. The reopening of the system could have been due to (i) the effect of a strong fluid pressure (fluid-assisted opening), (ii) the effect of a tectonic stress, or (iii) a combined action of the two. If the reopening was due to a tectonic stress, the orientation of the maximum stress would have been close to N120° E; i.e. the main direction of the secondary fractures, which would have behaved as tension gashes. End-Miocene shortening along this direction has never been described in the area.
Southern Siberia is the Russia's third platinum province, besides the-well known Tajmyr-Noril'sk and Kola-Karelia. Here a basic-ultrabasic plutonic belt stretchs for about 5,000 km from Baikal Lake to Okhotskoe sea. The Chiney pluton (a thickness of 3 km) provides the most considerable interest in respect of economic copper (with Co,Ni,Au,Ag and PGE) and unic vanadium (with Ti, Fe) mineralizations. Sulphur ores are most enreached in PGE. Sulphide mineralization is found in both intrusive and country rocks and, with respect to location and texture, is subdivided into following types: (1) sulphide-enriched horizons within titanomagnetite ores, (2) dissemination within alkaline rocks, (3) dissemination within leucogabbro, (4) sulphide mineralization within faults, 5) dissemination within skarns, 6) veins and lens-shaped bodies, 7) impregnated mineralization within sediments beneath the intrusion. Generally, economic interest is connected with the disseminated mineralization, wich is located within taxitic gabbrodiorite, monzodiorite, and leucogabbro comprising the lower part of the intrusion (the 2nd and the 3rd types) and with the veinlet-disseminated mineralization and small bodies of massive ore wich are concentrated within underlying sandstones (the 5th, the 6th and 7th types), endomorphic and exocontact ores. A very cooper rich composition is typical for Ciney ores, Cu/Ni ratio varies from 3 to 100. Over 100 samples have been analysed for PGE (exepting Os) and Au. Pt, Pd and Au are common in ores, Rh is very often found while Ru and Ir were detected in only two concentrates. Endomorhpic ores enriched in Pt in termes Pd comparing with exocontact. Mineral varities are typically specialized in PGE. Pyrrhotite and chalcopyrite ones are high-graded for Pt and Rh and for Pd and Au, respectively. Exocontact ores are characterized by strongly variable Pd/Pt ratio. In the ores PGE occur as own mineral species and as isomorphic admixture in some minerals. About 20 PGM have been recognized, including sperrylite, paolovite, copper-rich platinum froodite, michenerite, merenskyite, sudburyite, Pd-Au-Te, Pd-Se, Pd-Ag-Sb, Pd-Ag-Te, Pt, paolovite, polarite, potarite, sobolevskite, stibiopalladinite, mertieite, isomertieite. Pd-Bi-Te intermetallic compounds, first of all michenerite and merenskyite, are the most common PGM. PGM associate with arsenides and sulphoarsenides, forming includings in and intergrowths with gersdorffite, niccolite, maucherite, safflorite, lollingite. Pt, Pd impurities were discovered in niccolite (Pt - 0.42; Pd - 0.10 wt%) and maucherite (Pt, Pd - 0.42 wt%). Rare platinum elements have only been found in cobaltine. About 30 idiomorphic granes (size up to 20 mk) hosted in pyrrhotite have been analysed. The highest concentrations are typical for Rh. In respect of Rh content, all crystals may be subdivided into three groups, with high (9.33 - 12.39%), low (up to 1.00%) and free. Others PGE have been discavered in pure cobaltites (without Rh), their values are up to Os -0.68; Pd - 2.11; Ir - 0.21; Ru - 0.14; Pt - 0.19 (wt%). Economic titanomagnitite mineralization also containes platinum metals. Maximum grade of Pt+Pd corresponds to 2.5 ppm; besides, Ir of 80 ppb and Rh of 20 ppb are estimated for the same sample. Pt of 50-500 ppb and Pd= 20-700 ppb. Analyses of the distribution PGE in the rocks and in the ores shows, that thier origin is not connected with cristallization differenciation of massif.
The Tiferouine magnetite deposit is located 45 Km south-west of Marrakech (Morocco), in the southern part of the moroccan hercynian belt. It was discovred, by geophysics Method, under more than 100 m of miocene and quaternary cover. The magnetite deposit occurs in visean volcanic to sedimentary rocks which was deformed and metamorphosed during the Hercynian orogeny. It forms a NW-SE horizon, which plunge to NE with a stratiform nature, and layered internal structure. The primary mineral assemblage of the magnetite deposit is relatively simple and includes magnetite, pyrrhotite and small amounts of chalcopyrite. The magnetite of Tiferouine contains more than 1 weight percent of silica like those ascribed as "silican magnetite" (Shiga, 1988, 1989; Shimazaki, 1998), and occurs in natural hydrothermal environments. Individual grains of magnetite show zonal structure, displaying various contents of silica, manganese and calcium, similar to that repported by Westendrop and al. (1991), indicating the change of conditions during the cristal growth. Trace elements analysed in the magnetite from Tiferouine deposit, particularly Ni, Cr, V and Ti are clearly much lower compared with those from deposits of magmatic-segregation origin (So, 1978). They are clearly similar to those from other magnetite deposits of sedimentary origin. This is cofirmed by the low value of the Co/Ni ratio (<1). On the basis of the morphologic characters, paragenetic sequence and geochemistry of magnetite grains, we attributed the magnetite horizon of Tiferouine as exhalaison of hydrothermal fluids on the seafloor, similarly to the exhalatifs sediments described in the Hajjar volcano-sedimentary deposit (located about 10 Km north of Tiferouine) (Hibti, 1993). Locally in the western part of the deposit, a second type of ore is recognised. It is located in late fractures affecting the sedimentary serie, and is enriched in Arsenic, Bismuth and Gold. This ore is interpreted as a precipitation from residual fluids related to the late stage of hercynian orogeny.
Shiga Y, Mining Geol, 38, 437-440, (1988).
Shiga Y, Mining Geol, 39, 305-309, (1989).
So CS, Mineral. Deposita, 13, 105-117, (1978).
Hibti M, Thèse 3ème cycle Uni. Cadi Ayyad Marrakech, (1993).
Westendrop RW, Watkinson DH, Jonasson IR, Econ. Geol, 86, 1110-1114, (1991).
Shimazaki H, Ressource Geol, 48 n°1, 23-29, (1998).
High grade uranium mineralizations associated with unconformities at the base of Proterozoic sedimentary basins account for the most important U deposits of the world. The largest Canadian deposits of the unconformity-type are located in the Athabasca basin (Saskatchewan, Canada). The U deposits of highest grade (up to about 20% U) are polymetallic mineralizations (U, Co, Ni, As, etc.). Three major genetic models have been proposed for the origin of ore forming fluids in these U-deposits: a) a near-surface supergene origin, b) a magmatic or metamorphic hydrothermal origin, and c) a diagenetic-hydrothermal origin. Most recent studies are in favour of the diagenetic-hydrothermal model. However there is no consensus about the U source for the mineralizing fluids. The possible source rocks may be separated into three major groups, the Athabasca sandstones above the unconformity, the regolite at the unconformity (altered basement surface), and the underlying basement rocks. In order to constrain the proper source rock lithologies a detailed study of the distribution and remobilization of uranium and other trace elements within unaltered and hydrothermally altered country rocks of the U deposits was performed. Preliminary results indicate that generally more than 60% of the uranium content of most of the unaltered basement country rocks (except metabasites) is presently bound to monazite. Hydrothermal alteration of the basement lithologies along fault zones led to mobilization and partial redistribution of uranium. Small amounts of uranium are dispersed along microfractures and are absorbed by or incorporated within rutile or anatase. Monazite is often altered to thorite ± galena ± phosphates (apatite, goyazite-crandallite solid solutions). Co-, Ni-, Se, As-bearing sulfides have also been observed within or close to altered monazite. This suggest a genetic link between basement alteration and the formation of polymetallic U mineralization at the unconformity. Newly formed thorite shows two to four times lower U/Th ratios than the monazite which has been replaced in the basement rocks. It follows that up to 75% of the uranium bound to monazite could have been mobilized by hydrothermal fluids during alteration of some basement lithologies.
Anticlines develop when a layered sequence with competence contrasts is shortened horizontally. Fold wavelength is a function of the competence contrast of the folded layers and the layer thickness. When thick basin sequences are folded, multi-layer anticlines with kilometre-scale amplitudes are produced. Hydrothermal gold deposits like those in Victoria (Australia) or the Meguma terrain (Canada) formed where hot aqueous fluids flowed through multi-layer anticlines. It has therefore been argued that anticlines play a decisive role in focussing fluid flow into sites where hydrothermal ore deposits develop.
Here we present fluid-flow and reactant-transport simulations for static multi-layer anticlines in which competence contrasts between adjacent units are also reflected in permeability contrasts. The case of permeable flexural-slip planes and fold-radius-dependent jointing in concentric folds is also considered. All simulations are based on 2-D cross sections from natural folds.
We find that anticlines focus fluid flow if the overall composite-layer effective permeability along the axial plane is enhanced. This is likely, because deformation of the low-permeability layers is more intense in the fold core than in the fold limbs. This is especially true for chevron or kink folds. Simulations with a starting permeability which is isotropic and uniform in each layer of the static fold, but incorporating hydrofracture, also demonstrate flow-focussing by the anticlines. This occurs because hydrofracture becomes localized in sites of low effective stress below the highest base levels of the low-permeability units. Such apical sites are most vulnerable, because fluid pressure below low-permeability units changes on a hydrostatic vertical gradient while confining stress changes on a lithostatic gradient. The difference between fluid pressure and confining stress therefore reaches a minimum directly below the low-permeability layer where it occupies its highest structural position in the hinge zone of the anticline. In this case, the largest-amplitude folds in an anticlinorium would focus most of the flow.
Reactant transport through anticlines leads to a variety of mixing and hinge-zone discharge patterns. Thus, fluid composition in folded terrains may vary strongly on the scale of individual layers.
Sleza Variscan Ophiolite (SW Poland) is considered as the best-formed ophiolite complex in Central Europe. Ultramafic-hosted scattered-grain-ophimagnesite (SGOM) have been studied in serpentinite massifs (SW Poland). The CO2 yielded by decomposition of SGOM varies from 0.02 to 2.57 wt.%. The values varies in the range -15.99 to -1.17‰, and the 8O (SMOW) values range from 6.24‰ to 25.81‰. The magnesite concentration and 13C values increase, and, 18O values decrease, towards the centre of the Gogolów-Jordanów massif. These relationships possibly result from progressive precipitation of 13C-poor magnesite and 18O-rich serpentine minerals from external serpentinizing solutions, and/or from supersaturation of that solution with CO2 released from fluid inclusion from primary minerals of peridotite. The 34S sulphides and 34S-total S values varied from -1.13 to 6.39‰ and from 3.33 to 7.69‰, respectively. The chalcopyrite-pyrrhotite pair from in volcanic member yielded the temperature about 450°C. Temperature obtained for chalcopyrite-pyrite pair from gabbros (plutonic member) was 1120°C. Calculations due to Rayleigh distillation model for SO2 and H2S degassing processes suggest at least 20% loss of SO2 and/or degassing of at least 56% of H2S, respectively. The isotope mass-balance, calculated for this model, showed that at least 4 to 23% (or more) in the all the bulk sulphur is due to oceanic sulphur assimilation. Two generations of sulphide deposits (oceanic-hydrothermal and continental-hydrothermal) and 3 generations of magnesite formation (continental metamorhism, contact-hydrothermal and weathering) have been evidenced due to the isotopic results presented.
Cathodoluminescence (CL) investigations were undertaken to study carbonatites and related alkaline rocks from the Paleozoic (~380 Ma) Kovdor and Khibina complexes, Kola Peninsula (Russia). In the Kovdor massif, carbonatites are associated with alkaline ultramafic rocks (dunite, peridotite, clinopyroxenite and melilitolite) and with rocks of the ijolite-melteigite series while in the Khibina intrusion, carbonatites are linked to syenites, urtite, ijolite and apatite-nepheline-titanite ores.
CL microscopy appears to be a powerful tool not only to identify the carbonate minerals (i.e. yellow-orange calcite and more reddish dolomite) but also to unravel the growth history of minerals (i.e. apatite and zircon), to recognize minerals which are difficult to identify by optical microscopy (small inclusions of götzenite and pectolite in dark green aegirine-augite, ...) or to enhance the observations of subsolidus changes or hydrothermal alteration of alkali feldspars. Besides götzenite and pectolite, this study has revealed new CL minerals like mosandrite, lorenzenite and burbankite.
CL spectra and images were collected using a SEM coupled with a CL spectrometer in a wide wavelength domain, from 200 (UV) to 900 (IR) nm. The apatites which concentrate the rare earths elements and therefore control the REEs behaviour of igneous processes have been studied in detail. CL images of apatites provide important textural informations (oscillatory zoning, complex patchy zoning, resorption and overgrowth) that are not always detected by BSE images, especially in carbonatites. The observed peaks of the apatite CL spectra were indexed by comparison with those of synthetic doped chlorapatites. All the studied apatites (hydroxy-fluor-apatite and fluor-apatite) show quite similar CL spectra: the most prominent feature is the broad Ce3+ emission peak centered at 363 nm. This peak displays the highest intensity for all spectra and is slightly assymetrical towards longer wavelengths. Although most of the Ce emission is located in the UV part of the spectrum, the tail of the emission band extends in the visible domain from 400 to 450 nm which could explain the blue colour often observed in apatites from carbonatites. By contrast with the broad Ce3+ emission assigned to f-d electronic transitions, other trivalent REEs give narrow emission peaks due to f-f transitions: Pr3+ (247 and 277 nm), Nd3+ (874 nm), Sm3+ (565, 600 and 647 nm) and Dy3+ (483 and 576 nm). Very low contents of Mn2+ (a few ppm) are sufficient to produce a broad emission band centered at 565 nm. The Dy3+ and the three Sm3+ emission peaks are often poorly resolved as they are superimposed to the broad Mn2+ emission band.
Finite element modelling of 3-D convection in freezing magma chambers and attending hydrothermal circulation in surrounding country rock provides useful insight into the formation of mineral deposits. Presented are modelling results which include the effects of temperature dependent viscosity, solidification at the walls (multi-phase processes), latent heat, suspended crystal load, and geothermal gradient. These calculations attain reasonable Rayleigh numbers for magma chambers (e.g. 1014), i.e., are in the truly turbulent flow regime. Spatial variation in the permeabilityof the country rock and the configuration or shape of the geology has also been treated. The calculations are performed on commercial codes with extremely robust benchmark histories running over several decades and possessing large, technical support staffs. Some general observation are that 3-D calculations depart significantly from 2-D and that non-symmetric, complex structure is common (e.g., in temperature distributions) although completely symmetric boundary and initial conditions have been applied. The results have allowed the advancement of explanations for observations in the field that had previously been enigmatic such as mineralized cupola formation,some Mo/Sn deposits, the Zaaiplaats tin deposit, and mesothermal gold deposits such as that associated with Pilgrim's Rest, etc. Further, support for convective scavenging of important minerals into convecting boundary layers has also been derived; the PGE distributions in the Great Dyke of Zimbabwe being an example. Time evolution of these complexities will be shown by video. It is evident from the hydrothermal flow studies that physics supports the geological dictum that structure controls everything.
The aim of the AURROS project consisted in demonstrating the use of satellite radar data on sites representative of the problems encountered in geological and mineral exploration. The objective is to improve characterization methods for target zones, particularly in the early stages of the process leading to mining permit application.
The database integrates ERS (1 and 2) data, RADARSAT (broad-swath Scansar mode and fine bi-angular viewing mode) data, DEMs acquired by various means, and geophysical data (airborne magnetics, spectral radiometry). Various methods (filtering, geocoding and stereoscopy) have been integrated or developed by BRGM to create different map types. These data are analysed at various scales and validated using two approaches, namely checking in the field and fusion with airborne geophysical data (spectral radiometry and aeromagnetics).
The first results showed the radar to be a powerful mapping and deposit-prediction tool, and notably revealed that 1) the raw data can be used directly by geologists either for standard geological studies or as an exploration aid, and 2) fusion with geophysical data enables the identification of deformation structures where geophysics alone only reveals large masses, and rapidly and efficiently increases the possibilities of lithological discrimination.
Correct use of the data nevertheless involves taking into consideration the scales used and current restrictions concerning the most suitable study environments. The development possibilities associated with radar stereoscopy constitute a major progress factor for the near future.
This project was supported by the Aval SAR programme of CNES and the development programme (PRD 402) of BRGM's Research Division.
Beaudoin A, Le Toan T & Gwyn QHJ, IEEE Trans. on Geos. & Remote Sensing, 28 -5, 886-895, (1990).
Evans D, ISPRS Journal of Photogrammetry and Remote Sensing, 47, 79-99, (1992).
Feybesse JL, Milési JP, Precambrian Research, 69, 199-227, (1994).
Milési JP, Egal E, Ledru P, Vernhet Y, Thiéblemont D, Cocherie A, Tegyey M, Martel-Jantin B, Lagny Ph, Chron. rech. min, 518, 5-58, (1995).
Schreier G, SAR Geocoding: data and systems. ed. Wichmann, (1993).
Calzirtite, zirkelite, zirconolite, baddeleyite and zircon are primary minerals of Ti and Zr in carbonatites. The trends of the alteration of titanium and zirconium minerals: perovskite - baddeleyite - zirconolite, calzirtite - zirkelite - pyrochlore, zirconolite - pyrochlore (Bulakh, 1996), baddeleyite (and uranpyrochlore) - zirconolite - zircon and zircon - baddeleyite (Kapustin, 1964) were described previously. According to our data, zircon forms partial or complete pseudomorphs on the calzirtite in zones of hydrothermal alteration at the contact between the carbonatites and pyroxenites of the Seblyavr massif (Kola Peninsula, Russia). During the transformation of the calzirtite, zirconium is liberated to produce zircon, and titanium and calcium - to produce ilmenite and calcite, respectively. In such conditions zircon forms aggregates of crystals with rounded inclusions of baddeleyite. The inclusions of baddeleyite have larger sizes in central parts of zircon crystals, than in outer zones, where baddeleyite occurs as accumulations of fine inclusions. This association is, apparently, a consequence of the formation of zircon after baddeleyite. Zirconolite (non-crystalline) was observed in contact zones between calcite carbonatites and phoscorites. The zirconolite is hydrated to a different degree in the crystal outer zones and along cracks and contains veinlets of carbonates, sulphides and numerous inclusions. The inclusions consist of baddeleyite, calzirtite and complex oxides of Ba-Nb and Ba-Ti. Calzirtite and baddeleyite form fine relict inclusions with broken-off or corroded borders. The Ba-Ti oxide phase forms both separate crystals of rhombic section, and chains of irregular grains along cracks. The Ba-Nb phase forms rare irregular grains. The relationships between Ba-Nb and Ba-Ti phases and zirconolite indicate a later origin of the phases. Thus, hydrothermal alteration of rocks in the Seblyavr massif causes regular evolutional changes of mineral associations. The trends of the transformation of primary Ti-Zr-minerals can be presented as: 1) calzirtite - zircon (and ilmenite and calcite), 2) zircon - baddeleyite and 3) baddeleyite (and/or calzirtite) - zirconolite - complex oxides Ba-Ti and Ba-Nb.The work was funded by the Russian Foundation for Fundamental Sciences (grant 98-05-64365).
Bulakh A.G., Nesterov A.R., Acta miner.-petrogr., Szeged., 37, 112, (1996).
Kapustin Yu.L., Mineralogy and genetic features of alkaline massifs, 135-194, (1964).
Todorokite clusters from the Jabuka Pit (Central Adriatic) were studied for chemical composition, mineralogy, internal microfeatures and the mechanism of their deposition on mollusk shells and on other biogenic detritus.
In the Jabuka Pit, which represents the deepest (up to 275 m deep) part of the Central Adriatic, todorokite was recognized as a major and/or the sole constituent of the coated structures found only on the surfaces of mollusk shells and shell fragments exposed to sea water. Todorokite also forms a collomorph-banded and botyroidal aggregates, as well as 1.5 mm high stalactite structures composed of up to 5 µ m long plate-like crystals. Despite the heterogenety, cryptocrystallinity and very fine particle size of the todorokite clusters, a variety of techniques (microscopic examination in reflected light, XRD analyses, SEM and EDS examinations, as well as ICP and ICP/MS analyses) was used to determine the exact mineralogy, microstructure and chemical composition.
Mineral identification is based on peak position and relative intensities as compared to those in the standard powder diffraction files (JCPDS). In the samples investigated todorokite was recognized as the only manganese mineral phase by two strong diagnostic peaks at 9.62 Å, 4.85 Å and a weak one at 3.24 Å, which correspond to the basic reflections (001), (002) and (003) of the 10 Å manganite phase (Buser and Grütter, 1956). Other authigenic and/or detrital minerals including dolomite, aragonite, calcite, clays, quartz and feldspars were also detected in trace amounts.
During the SEM study todorokite was identified by EDS analyses on the basis of its high Mn (48 - 62%) moderate Mg (4.9 - 8.2%) and low Ca (1.4 - 2.5%) as well as Fe (3.6%) content. It also has a higher Ni content (up to 0.67%) than Co content (up to 0.41%), and is poor in Cu (123 ppm), Zn (109 ppm) and Pb (74 ppm). Todorokite clusters were also found to have REE concentrations about 5 times of less ((sum)REE = 64.5 ppm) smaller than the sediment ((sum)REE = 306.5 ppm), and a pronounced positive Ce anomaly of 1.40.
Taking into consideration the properties of dissolved Mn in seawater and the occurrence of todorokite only on surfaces exposed to the seawater, we believe that this mineral in the Jabuka Pit is essentially hydrogenic and precipitates direcly from seawater at the sediment-water interface . It should be pointed out that oxic diagenesis involving reactions in the oxidized upper part of the sediment most probably provide considerable fluxes of Mn and trace elements to precipitating todorokite coatings and clusters. This is consistent with the Mn enrichment within the upper 2 cm of the sediment and its decrease in the deeper parts, as noted by Paul and Meishner (1972).
Buser W, Grütter A, Schweitz miner. petrog. Mitt., 36, 49-62, (1956).
Paul J, Meischner D, Senckenbergiana Maritima., 8, 91-102, (1976).
Six members of the annite-siderophyllite join were synthesized in a three step process - crystallization of biotite from gels, decomposition of the fine-grained biotite under oxidizing conditions and resynthesis of Fe-Al biotite with planned compositions from these products - producing biotite crystals with thicknesses of up to 10 µ m. The biotite was characterized by microprobe, electron microscopy and X-ray diffraction. Heat capacities of these biotites were measured with a DSC (differential scanning calorimeter) over the temperature range 143 to 623 K. Using a least-squares technique, the data were fitted to a cp-polynomial, cp = k0 + k1 T-0.5 + k2 T-2 + k3 T-3. In the temperature range 143 to 250 K, heat capacities of the different annite-siderophyllite members decrease linearly with increasing Al content. At higher temperatures, however, the cp-polynomial of biotites with intermediate composition (except Ann79Sid21) exhibit a steeper slope than those of other biotites. This produces positive excess heat capacities in the annite-siderophyllite join at higher temperatures. The activity-composition data of the same binary derived from phase equilibrium experiments (Benisek et al., 1996) and the data of this study suggest two compositional regions along this join, with different extent of deviation from ideality. One at XSid < 0.3, characterized by a small deviation, one at XSid > 0.3 showing a higher nonideality, resulting in a discontinuity visible at this composition. Powder IR-spectra of these solid solutions were measured with a FTIR-spectrometer and used to calculate heat capacities according to the vibrational model of Kieffer (1979). The comparison of the vibrational function with the cp-polynomials shows that the vibrational function reproduces well the DSC-data of the siderophyllite-poor and -rich members, but deviates for intermediate compositions, where the excess heats of mixing occur. With increasing Tschermak vector, the tetrahedral rotation angle <alpha> increases from 0° to 13° for annite to siderophyllite, respectively. At the composition of the discontinuity, this rotation angle <alpha> reaches a value of ~ 8°. The processing of ~ 300 chemical data of natural biotites indicates that over 90% of them have a tetrahedral rotation angle that lies between 7° and 9°. It would appear that biotites with these structural characteristics are most stable.
Benisek A, Dachs E, Redhammer G, Tippelt G, Amthauer G, Contrib Mineral Petrol, 125, 85-99, (1996).
The Post-Karroo Chilwa Alkaline Province (CAP) was formed at the Jurassic-Cretaceous boundary after an earlier tholeiitic magmatic activity. The alkaline magmatism resulted in the formation of granites, syenites and nepheline syenites, together with carbonatite centres, nephelinites and alkaline lava flows. CAP is mainly intrusive, and differ in that way from other alkaline provinces associated with the Post-Karroo- and the East-African Rift Systems.
Of primary interest here are the Be-REE-bearing agpaitic pegmatites of the Zomba District (located within the central part of the CAP) which are relatively complex in mineralogy and spectacular not least for their well-crystallised species. The mineralogy of the agpaitic pegmatites bears a distinct resemblance to the occurrences at Narssarsuk, S Greenland (Petersen & Grossman 1994). Sporadically they carry miarolitic pockets rich in aegirine, K-feldspar, quartz, arfvedsonite, minor albite and sideritic carbonates, and REE-minerals such as bastnäsite, parisite, pyrochlore, fergusonite and rare polycrase (Johnsen et al., in press). Be-minerals are often present in small amounts, mainly as the dimorphs eudidymite and epididymite. Furthermore, zircon is omnipresent in several habits; from early formed, dark reddish brown, translucent crystals with complex morphology to later-stage long- and extremely short prismatic chocolate-brown opaque crystals as well as late prismatic, lemon-yellow translucent crystals. Recent finds also include rare galena and coarsely crystallised astrophyllite, the latter mineral further increasing the similarity with the Greenland occurrence.
The pegmatites seemingly cross-cut the syenitic to granitic pluton comprising the NW part of the Zomba-Malosa Plateau at Malosa Mountains. K-Ar hornblende ages from the Zomba-Malosa complex are 111±3 Ma and 115±3 Ma (Eby et al., 1995), which are interpreted to reflect the intrusion age of the plutons but more likely represent cooling ages. Single-zircon Pb-Pb evaporation (Kober, 1986) dating of intermediary-stage, reddish-brown euhedral zircons, collected from a cross-cutting pegmatite, yields an age of 123±4 Ma, interpreted to be the emplacement age of the pegmatite. The pegmatites represent the waning stage of the magmatic activity of the Zomba-Malosa complex, and thus, the 123±4 Ma age of the pegmatites gives indirectly the minimum age of the host rock, which must be older than the known K-Ar ages.
Castaing C, Tectonophysics, 191, 55-73, (1991).
Eby GN, Roden-Tice M, Krueger HL, Ewing W, Faxon EH & Woolley AR, J. Afr. Earth Sci, 20, 275-288, (1995).
Johnsen O, Ståhl K, Petersen OV & Micheelsen HI, N Jb Miner. Mh, (in press).
Kober B, Cont. Min. Pet, 93, 482-490, (1986).
Petersen OV & Grossman M, Min. Rec, 25, 29-38, (1994).
In the SW zone of the Larderello geothermal field a two mica granite was crossed by the well Carboli C-bis from 4200 m (CA 4200) to 4300 m (CA 4300) below ground level. Present-day in-hole temperatures are around 400°C. Both rocks, CA 4200 and CA 4300, show a porphyritic heterogranular texture. Perthitic K-feldspar phenocrysts are surrounded by andesine to labradoritic plagioclase, rounded and resorbed quartz, primary biotite and muscovite. Relatively abundant tourmaline and minor fluorapatite and zircon, ilmenite and pyrite are accessory minerals. Late stage small (5-20 micron) fluorite crystals fill fractures and cavities of plagioclase. The very high F contents in muscovite (up to 2.93 wt%) and the occurrence of fluorapatite and tourmaline suggest that the Carboli granite was enriched in F and B. Electron microprobe analyses on muscovite crystals revealed extremely variable compositions as shown by the following contents (in atoms per formula unit) of CA 4200: Si = 6.23-6.40; Ti = 0.03-0.07; Al tot = 4.66-5.27; Fe tot = 0.29 - 0.89; Mg = 0.24-0.28; F = 0.51-1.29. The muscovite of CA 4300 has similar ranges, notably Fe tot = 0.35-1.04 and F = 0.64-1.17. Backscattered electron SEM images of the muscovite crystals evidence a patchwork distribution of different grey nuances. The quantitative analyses show that light zones have higher Fe, Mg and F, and dark zones have higher Al and Si. The textural relations are very ambiguous. The only criterion to assign relative ages to the patchwork is the argument [1] that Fe+Mg-rich muscovites should form later in the evolution than Fe+Mg-poor ones. Therefore, the Fe+Mg enrichment of muscovite may be related to interactions with late magmatic / hydrothermal fluids.
39Ar/40Ar stepwise heating analyses followed the same approach of our work on white micas from the Radicondoli well [2], NE of the Carboli C-bis well. CA 4200 and CA 4300 confirm the anticorrelation between radiogenic Ar* and Cl-derived 38Ar: magmatic muscovite gives = 1 Ma with low Cl/K; an opposite signature is revealed by high Cl/K in the younger (= 0.3 Ma) ones. The question is whether the younger muscovites were formed hydrothermally (like Radicondoli) or if they are of late magmatic origin. We prefer the latter; the apparent difficulty of keeping the Carboli magma above solidus for over 0.5 Ma may be solved by noting that the F and B contents progressively led the solidus of the residual magma to lower and lower temperatures. For comparison, experimental data on peraluminous magmas indicate a solidus T = 450°C for the Macusani Li-B-F-rich system [3]. Finally, the present-day in-hole T, = 400°C, might not be excessively far from the solidus T for the last residual melts.
Cavarretta G & Puxeddu M, Contrib Mineral Petrol, submitted
Villa IM, Ruggieri G & Puxeddu M, Eur J Mineral, 9, 563-568, (1997).
London D, Morgan GB VI, Herwig RH, Contrib Mineral Petrol, 102, 1-17, (1989).
A granite sample cored at 4.5 km depth below ground level in the geothermal well Radicondoli 29 shows the sporadic occurrence of dumortierite. The rock is a porphyritic strongly heterogranular hypidiomorphic two mica S granite consisting of quartz, perthitic orthoclase, oligoclase to andesine plagioclase, biotite, muscovite and cordierite. Quartz crystals are rounded and resorbed with embayments. Plagioclase is sharply zoned with Ca-rich cores and Na-rich rims. K-feldspar forms cm sized poikilitic phenocrysts sometimes with graphic quartz. Biotite is locally altered in chlorite and epidote. Muscovite exhibits features of primary growth. Scattered crystals of cordierite show sector twinning. Tourmaline, fluorapatite, zircon, ilmenite, dumortierite and fluorite are the accessory minerals. Tourmaline, locally abbundant as a late mineral filling pockets, shows pleochroism from green to brown and sporadically from blue to green. Microprobe analyses yielded elbaite-rich compositions plotting in the field of the Li-rich granitoid pegmatites and aplites (Henry and Guidotti, 1985). Fluorite, associated with late muscovite, occurs as colourless to pale violet anhedral blades filling cavities and/or aligned along fractures and cleavages of early plagioclase crystals. Dumortierite appears as late interstitial fibrous-prismatic crystals. Pleochroism is: X=indigo blue, Y,Z=pale yellow or X=red purple to reddish violet, Y,Z=pale green. Nuances of colour are visible even in a single crystal. Dumortiertite is associated with blue-green tourmaline and with late primary muscovite that forms a network of branches along the neighbouring grain boundaries. Dumortierite crystals are often altered in chlorite and sericite. Preliminary microprobe analyses seems to indicate strongly homogeneous compositions, whose average is: SiO2=30.90, Al2O3=58.61, Fe2O3=0.44, MgO=0.47, CaO=0.02, F=0.15. The Al2O3 content is relativelly low and comparable to that of dumortierites having TiO2=2.8-4.6 and Fe2O3=3.7-4.4 known in literature (Huijsmans et al., 1982; Beukes et al., 1987). But the Larderello dumortierite has TiO2=0.00 and very low Fe2O3. The only explanation for this anomalous behaviour could be a much higher influence of the temperature sensitive substitution: 3H+octahedral vacancy=Al (Werding and Schreyer, 1990). This hypothesis is suggested by the very late crystallization of dumortierite near solidus temperatures strongly lowered by the F, B, and possibly Li enrichments of residual melts and fluids.
Henry DJ & Guidotti CV, Am Mineral, 70, 1-15, (1985).
Huijsmans JPP, Barton M & van Bergen MJ, N Jb Mineral Abh, 143, 249-261, (1982).
Beukes GJ, Slabbert MJ, de Bruiyn H, Botha PJV, Schoch AE & van der Westhuizen WA, N Jb Mineral Abh, 157, 303-318, (1987).
Werding G & Schreyer W, Contrib Mineral Petrol, 105, 11-24, (1990).
A unique association of practically iron-free but anomalously cobalt-enriched manganese carbonates and silicates was discovered in the Polar Urals during a study of manganese-bearing paleolaterites. This discovery can be interpreted as indicating that a supergenesis zone is important region of new mineral species formation. The paragenesis of Co-enriched minerals was found in the lower part of the profile of a manganese-bearing weathering crust. Identified among the carbonates were Co- and Co-Mg-calciorhodochrosites, Co-magnesiumrhodochrosite and Co-Mg-mangankutnohorite with CoO and MgO contents ranging 0.5-5.0 and 0.3-3.0 mass%, respectively. Discovered for the first time was Co-manganrhodonite of an unusual lilac-violet colour, found as an impurity in Co-Mg-Ca-Mn-carbonates. The empirial formula of the nev pyroxenoid can be given in the following form: (Ca,Mn)(Mn,Co,Fe,Ni)(Mn,Mg)3[Si5O15]. There is reason to think that the Co-bearing rhodonite variety is transient member of the isomorphous triad: CaMnMn3Si5O15 - MnMnMn3Si5O15 - (Ca,Mn)CoMn3Si5O15. Microscopic study revealed in Co-manganrhodonite grains still more unusual minute Co-silicate inclusions - potential new mineral species.
Mineral "O" displays subisometric grains with an orthoamphibole system of clevage fractures, pinkish-grey in colour, with weak pleochroism with absorption scheme: ng > nm = np. The mineral was found to contain the following components (mass%): SiO2 55.06-59.45; MgO 15.83-18.42; CoO 12.17-16.05; NiO 2.79-3.72; MnO 2.69-4.04; FeO 1.21-1.37; CaO 0.06-0.27. The chemical composition of the new amphibole-like mineral is easily calculated in terms of stoichiometry to yield anthophyllite and is approximately expressed by the following empirical formula:
(Mg,Fe,Mn)4(Co,Ni)2-3[Si8O22] (OH)2-3.
Mineral "Ts" occurs as elongated platy grains, pink or greyish-violet in colour, optically biaxial, positive. The major components are (mass%): SiO2 48.24-53.26; MgO 12.94-15.41; CoO 12.25-15.61; NiO 0-5.27; MnO 2.60-3.98; FeO 0.87-1.45; CaO 0.20-0.43. The chemical composition is stoichiometrically calculated to sepiolite and is approximated by the empirical formula:
Co,Fe)2-x (Mg,Mn,Ni)2+x [Si5-6O15](OH)2nH2O.
The discovered paragenesis of anomalously Co-enriched minerals has resulted from mineral formation under conditions of Mn,Co,Mg and Ca separation from Fe with their subsequent acid-alkaline differentiation in the profile of the zone of secondary carbonate-silicate-manganese enrichment.
Alkalic igneous rocks are typically enriched in rare metals such as Zr, Y, REE, Th, etc. However, extreme enrichments in these elements are commonly found in rocks that have undergone hydrothermal alteration. Examples are the Ilímaussaq complex in Greenland, and the Thor Lake and Strange Lake peralkaline granites in Canada. For the latter case, it has been shown recently that hydrothermal processes were responsible for concentrating Zr, Y and the heavy REE to ore grades (Salvi & Williams-Jones, 1996). This concentration is explained by transportation of these elements as fluoride complexes in an orthomagmatic fluid, and their subsequent deposition as a result of mixing with a Ca-rich meteoric fluid, derived from the surrounding metasedimentary rocks.
The Cretaceous Tamazeght complex (High Atlas Mountains) contains a wide variety of alkaline rocks, ranging in composition from carbonatites to peralkaline pegmatites. Among the most evolved units are agpaitic nepheline syenites and nepheline pegmatites that are particularly enriched in rare metals, locally to potentially economic levels. Some of these rocks show evidences of hydrothermal alteration similar to those observed at Strange Lake. Ca and F contents are unusually high, and primary minerals are altered to secondary rare-metal bearing phases (Khadem Allah et al., 1998).
Nepheline crystals adjacent to these mineral replacements host fluid inclusions that, based on textural evidence, are interpreted as primary with respect to the alteration event. The inclusions contain one or more solid phases in addition to a liquid phase and a vapour bubble. Opened fluid inclusions were investigated by cryogenic scanning electron microscopy according to the method outlined by Kelly & Burgio (1983). Minerals that were found recurring in several inclusions and that displayed similar volume ratios were considered to have crystallised inside the inclusions. One of the most common daughter minerals gave energy dispersive spectra showing light REEs as well as Ca peaks. Other daughter or trapped minerals include Ti, Zr (zircon?) and Nb minerals, a Zn-S crystal (sphalerite) plus K and Mn-bearing phases.
The presence of daughter minerals containing the light REEs (and possibly other rare metals) is a strong proof that at least some of the REEs were carried in solution during the alteration, adding to the existing pool of evidence that the rare metals can be mobile in some naturally occurring aqueous fluids. The association of Ca and F observed in the inclusions and mineral replacements may indicate that, similarly to the model presented for the formation of the Strange Lake deposit, these elements play a fundamental role in the process.
Khadem Allah B, Fontan F, Kadar M, Monchoux P & Sørensen H, Geochimija, 1998, 643-655, (1998).
Kelly WC & Burgio PA, Econ. Geol, 78, 1262-1267, (1983).
Salvi S & Williams-Jones AE, Geochim. Cosmochim. Acta, 60, 1917-1932, (1996).
The PIXE method allows the deep analysis of samples because the range of 3 MeV protons is more important than that of 15 keV electrons. It is a precious help for the non-destructive chemical characterisation of subsurface visible inclusions. However, the quantitative analysis is tricky because it needs the rigourous geometrical description of the inclusion under the beam. The fluid inclusion has a composite morphology and generally is only 10 - 20 µ m large whereas the proton beam size remains larger than 2 µ m.
In X-ray spectrometry codes, calculations can be performed to characterize superposed layers parallel to the target surface, these being considered very much larger than the beam size. When the fluids contents are calculated with the aid of this multilayer model, the estimated concentrations are then marred by mistakes due to the situation, size and morphology of the natural inclusions. Two effects are here considered :- variation of the inclination of the fluid plan under the surface;- presence of the quartz "walls" limiting the fluid on the side ("shadow corner" effect).
For the first effect, we used the calculations performed by the "multilayer" procedure of the GUPIX code, but with simulating the inclination variation of the fluid layer by the variation of the incidence angle of the proton beam. The parameters which influence the results are the variations of the distances covered in the fluid and in the quartz roof. They product variations of the proton induced X-ray emission and variations of the X-ray attenuation. With an inclination from +30° to -30° for example, it was observed on K lines 20% of underestimation for the elements above Fe.
For the second effect, the entire calculation was done again without using the "multilayer" procedure of GUPIX code. A parallellipedic inclusion was considered, perpendicular to the incident beam and of zero inclination with regard to the surface. Compared to the inclusion width w, the beam size <phi> varies from a minute value to a maximum defined by <phi> / w = 1. The results show clearly that the quartz wall produces a shadow corner which decreases when the inclusion size increases. This shadow corner comes from the greater attenuation of the RX in the quartz than that in the fluid. Then, it gives an underestimation of the elements concentrations in the fluid. For example, when <phi>/w=1, the relative deficits reach 56, 45, and 8% for Cl, K and Ca.
In conclusion, corrective terms must be brought to the contents calculated by the multilayer procedure. Their importance depends on the atomic numbers and on the relative sizes beam-inclusion. However, the risks of accuracy lacks can be neglected by operating concentrations ratios between the elements contained in each fluid inclusion.
The Hartkoppe, a small (c. 400-m diameter) rhyolitic stock of early Permian age exposed in a quarry near Sailauf, Spessart, exhibits both hypogene and supergene alteration as well as a complex vein-type Mn-Fe-U-As mineralisation controlled by NW-striking faults and fractures. In an attempt to constrain ages and temporal relations of coexisting mineral assemblages, a combined K-Ar and (U+Th)-He isotopic study on Mn + Fe ore minerals (braunite, hausmannite, cryptomelane, hematite) and authigenic clays (illite, celadonite) has been undertaken. The new data indicate that individual hydrothermal processes operated in distinct episodes ranging in age from 180 to 100 Ma. The following sequence of alteration/precipitation stages could be established: 1) 180 Ma: Initial high temperature alteration, involving illite growth from fluids percolating through a highly permeable circumrhyolitic fracture zone. K-Ar ages of different illite grain size fractions are internally concordant. 2) 160 Ma: Formation of a second generation of illite, now confined to veins or forming blebs in altered rhyolite. This event is coeval with the precipitation of braunite, as evidenced by several (U+Th)-He dates obtained on this main ore constituent. Regarding methodology, this observation adds an important new perspective to future geochronological investigations. 3) Subsequent crystallization of specularitic hematite is documented by (U+Th)-He ages clustering at 145 Ma. 4) 100 - 120 Ma: Final stage of hydrothermal alteration/mineralisation, identified by K-Ar dating of celadonite separates.By contrast, K-Ar and Ar-Ar dating of cryptomelane, found as vein-filling material and pseudomorphic replacement mineral after pre-existing braunite and hausmannite, yields much younger ages of 12 to 1.5 Ma. These results are interpreted as to reflect supergene alteration processes (subaerial weathering) subsequent to Neogene exhumation. Casting doubt on previous beliefs and speculation, our findings challenge both traditional perceptions of the origin of epigenetic Fe-Mn vein-type mineralisations within acid volcanics and the view that late Variscan and/or Tertiary magmatism may have played a crucial role. Rather, they add weight of evidence in support of the idea of widespread hydrothermal activities and corresponding ore formation during the Mesozoic - most probably related to heat pulses and tectonic movements resulting from Central and North Atlantic ocean opening. Together, our results demonstrate the importance of isotopic dating in providing rigorous data allowing for a coherent interpretation for all the events involved in the making of a deposit.
The Lueshe Niobium deposit (Kivu, Congo ex-Zaire) was formed by supergene alteration of carbonatitic and associated syenitic rocks. The present study is an extension of an EC project conducted in 1991-94 on ore beneficiation processes (Albers et al, Applied mineralogy of pyrochlore and related minerals in the weathe-ring zones of the niobium deposits of the Lueshe and Bingo carbonatites, Zaire, CCE project MA2 M-CT90-0038). The geological features of the laterites are discussed here in terms of whole-rock chemistry. An extensive analytical work (XRF, ICP-AES) has been done on fresh rocks and along several profiles in the laterites. These profiles are used to reconstruct the 3D organization of the different alteration facies. The Lueshe alkaline complex (Maravic et al, 1989) consists of two main units: a complex association of silicate-rich calcite carbonatite and syenitic rocks, and a plug of pure dolomite carbonatite. Primary niobium segregations are found associated with pyroxene/apatite/felspar concentrations within the calcite-carbonatites, or at its margins. The Nb-bearing laterites have a consistent geochemical signature, well constrained by a number of conservative interelement ratios involving Ti, Nb, Zr, REE, Al, Fe. The laterites are formed by in situ alteration of soevites and associated syenitic inclusions, without significant mixing of laterites derived from other rocks. Due to the high chemical heterogeneity of the protoliths, classical mass balance approaches cannot be used directly. The chemical evolutions of the laterites are interpreted, in terms of phase assemblages, using a combination of graphic projections and normative calculations specifically designed for phosphate minerals. A sharp front separates the two main horizons (lower: apatite-bearing, upper: crandallite-bearing) encountered in the studied area. In contrast with other supergene deposits of Nb developed on carbonatites, there is no secondary apatite enrichment at the lower part. The formation of crandallite is, at whole rock scale, strongly coupled with the breakdown of the primary silicates to kaolinite. It is suggested that the peculiarity of the Lueshe profile (absence of phophate accumulation) reflects to the high drainage conditions induced by the relatively steep topography of the site. The study intends to show that the whole rock approach of weathering processes can be more than merely descriptive, and how it can be used to understand, in conjunction with XRD and microscopic data, the evolutions of phase relations in an alteration profile.
Maravic Hvon, Morteani G, Roethe G, Jour. African Earth Sci, 9, 341-355, (1989).
The Ukrainian Shield is underlain by 136 500 square kilometers of Precambrian rocks subdivided into three blocks: Volyno-Podolsky (Western block), Central Ukrainian (Central block) and Priazovsky (Eastern block) separated from each other by north-to-south trending faults of the Golovanevskaya and Orehovo-Pavlogradskaya suture zones. Within these suture zones several different mafic-ultramafic complexes are recognized.
The most significant mineral deposits associated with Early Proterozoic ultramafic complexes of the Ukrainian Shield are located within the Golovanevskaya suture zone, which represents the transition area between the Volyno-Podolsky and Central Ukrainian blocks. The ultramafic complexes of the Golovanevskaya suture zone are consist of more than 60 plutons traced by gravity and magnetic anomalies and, in some cases, by detailed drilling profiles. The two groups of ultramafic complexes in this zone are: (1) layered intrusions and (2) plutons of ophiolitic (?) nature.
The first group of intrusions (Tarnavatsky, Grushkovsky, Lashevsky, Derenyuhinsky, Krymkovsky, Kumarovsky and unnamed others) are rounded, elongated and located within synclinal structures. The size of the intrusions vary between 70 to 640 meters in width and from 210 to 3350 meters in length. These plutons are mostly composed of layered sequences of serpentinezed dunite, peridotite and gabbro (norite) with poorly developed chromite and sulphide mineralization. However, the intensive lateritization during the Mesozoic period caused the formation of industrial nickel lateritic deposits (with nontronite as the predominant mineral) along with precious metal mineralizations.
The other group of ultramafic plutons (Kapitanovsky, Lipovenkovsky, Zavodskoy, Pervomaysky, Lipnyagovsky, Pushkovsky, Burtyansky and unnamed others) are represented by ophiolitic (?) dunite-harzburgite (with minor amounts of lherzolite and pyroxenite) and gabbro-peridotite in cumulative rock sequences. These plutons occur as 2500 meter long fault-bounded slices with the thicknesses of c. 40-350 meters. Among the dunite-harzburgite plutons in the Golovanevskaya suture zone, at least 11 contain chromite mineralizations; the significant reserves being within the Kapitanovsky and Lipovenkovsky plutons. Ore mineralizations occur as boudinaged and dismembered lenses and veins of massive and disseminated chromitites with significantly variable Cr2O3 content varies (up to 65 wt.%) averaging c. 29 wt.%. The subvertical ore bodies are 0.5-25 meters thick and have been traced to 300 meters depth. The dunite-harzburgite plutons are also covered by nickel lateritic deposits. The lateritic deposits contain gold and platinum-group element mineralizations, numerous zones enriched with high-grade vermiculite, and several zones with manganese (pyrolusite) mineralizations. Also included within the ultramafic rocks of the Golovanevskaya suture zone are veins and cavities filled by opal and other gemstones of the silica group.
Special work on the study of the amagmatic PGE concentration processes have resulted in the discovery of numerous PGE mineralization occurences, with their connection to typical hydrothermal processes as a main peculiarity. We used the results of investigation on black shale-hosted PGE-bearing giant deposits - Sukhoi Log and Muruntau to estimate the conditions of hydrothermal systems for this type of the mineralization (Distler et al., 1996). Disseminated gold, connected with quartz-pyrite and pyrite-pyrhotite ores contains increased platinum and palladium concentration of and is enriched with REE. New date on the Waterberg mineralization in metasomatic-changed felsites (Transvaal) and the Chudnoe Au-Pd deposit in fuchsite-albite metacomatites (Polar Ural) are used. Among the various factors of ore formation at the Sukhoi Log deposit a redox potential plays dominant role, regulating noble metals' transportation in fluid and the subsequent of their precipitation in more reduced conditions. Organic matter could come the basic reducing agent in black shale.
The composition of fluid inclusions in quartz indicates to medium temperature conditions of ores formation with CO2 prevaling in fluid (CO2:CH4=7-8). The indicators of PGE distribution, may be, are zones with the increased high-density nitrogen contents in fluid inclusions which formation is connect with hydrothermal reworking of nitrogen-bearing organic matter. The participation of Cl-complexes in PGE transportation is proved by constant presence chlorine-bearing phases. Changes iron and chromium valences can be the dominant factor in conditions unrelated to carboniferous environments.
We believe, that PGE hydrothermal deposits are characterized by distinct features, reproduced in different situation: 1) Constant association of PGE with gold and the presence of their high-reduced modes (native metals and alloys). 2) Development of REE mineralization in the same paragenesis with PGE. 3) High concentration of Cr, Ni, W, Co. 4) Development of mineral paragenesis, typical of sharp change of redox potential.
VV Distler, GL Mitrofanov, VK Nemerov, VA Kovalenker, AV Mokhov, LK Semeikina and MA Yudovskaya, Geology of Ore Deposits, 38, 413-428, (1996).
The Noril'sk deposits that are unique from the point of view of mineral composition and PGE concentration, are an excellent object for the study of PGE behavior in sulphide melts at different stages of their crystallization form parental magmas. Internal structure of the intrusions and regular downward change of the silicate and sulphide composition suggest the massifs were formed by liquid immiscibility and crystallization differentiation of ore-magmatic melt.
PGE are being concentrated in a liquid sulphide within the sulphide-silicate melt. The saturation of the liquid sulphide by PGE is determined by geochemical features of the melt injected into the chamber. The PGE fractionation occurs during the drowning of sulphide drops. Liquid sulphide is enriched by Pd in the lower ore differentiates of the intrusions, and by Pt - in upper zones. Late magmatic fluid-related PGE redistribution results in the formation of low-sulphide horizons in the intrusion upper endocontacts. Sulphide melt, forming massive and veinlet-disseminated ores in endo- and exocontacts of the intrusions, is PGE-poor as compared to the liquid sulphide in the massif itself. Sulphide melt internal immiscibility resulting in formation of Cu-reach and Fe-reach liquids lead to the accumulation of Pd and Pt in the former and of Rh/Ru and Ir - in the latter.
A distribution and forms of existence in solidifying sulphide melt is determined by several factors - sulphur fugacity and activity of metals at the formation and their successive transformation in subsolidus field as well as by fluidal regime at lower temperatures. At high temperatures, the PGE form solid solutions in MSS and ISS and than in sulphides. PGM and associated minerals crystallize from the residual sulphide melts and PGE-enriched fluids.
A new heavy mineral (HM) separation method was used to investigate PGE bearing minerals in serpentinite and recently discovered podiform chromitite (Vuollo, et al., 1995) from the Outokumpu Ophiolite Complex, Fenoscandian Shield, Finland. We used special ppb-separator (Knauf, 1996), which enables to separate fine ore mineral particles in water media within the wide range of their specific gravity. The procedure is performed in the practically closed system. It makes possible to quantify HM content in the sample, to avoid contamination, and to reach very high sensitivity and low HM detection limit (down to ppb level).
The minerals heavier than chromite were extracted from chromitite and two serpentinite samples. The fine fraction (-50 µ m) of disintegrated samples was used. Approximately 5 mg HM concentrates were obtained from the each 50-150 g sample. Accompanied losses of PGM did not exceed 30 wt.%.
The chromite, magnetite, ilmenite, pentlandite, nickeline, pyrite, gersdorffite, Co-gersdorffite, galena, hedleyite, baddeleyte, native iron, zinc, tin, lead, copper, and new unnamed Ni-Bi phase have been revealed in the HM fractions. In addition 36 PGM grains have been identified: erlichmanite, laurite, irarsite, osarsite, sperrylite and andouite as well as numerous grains of Os-Ir-Ru-bearing gersdorffite. The latter together with Ni-irarsite forms unusual concretion-type segregations. Mineralogical data demonstrate that the most of PGE are related with the secondary PGM.
General scheme of PGE history revealed from the intergrowth textures and the modes of mineral occurrence is the following. Nickel-bearing sulphides (Ni-laurite) were crystallised after primary magmatic PGE sulphides (laurite-erlichmanite) and further were replaced by Ni-isarsite, most likely during metamorphogenetic-hydrothermal processes. Finally PGE were concentrated in PGE-bearing gersdorffite (Ru 1.1-1.7, Os 6.4-9.3, Ir 3.2, wt.%).
Chromitites from Phanerozoic ophiolites are usually enriched in Os, Ir and Ru as well as in corresponding PGM (Page et al., 1986). The same features of PGE mineralization we revealed in the ancient Outokumpu Ophiolite Complex. Rough estimates based on the PGM composition and their content in the rocks give approximately 500 ppb of PGE in chromitite and first tenths of ppb in serpentinites. These figures compare well with the results obtained by conventional analytical method (Vuollo, et al., 1995). The presence of the concretion-type segregations of Ni-irarsite and PGE-bearing gersdorffite is the peculiar feature of the investigated rocks.
The extent of PGE economic potential in the Outokumpu Cu-Co-Zn-(Au) Ore District remains open to question. However, the PGE redistribution during secondary processes and crystallisation of Os-Ir-Ru-bearing sulphoarsenide indicate that gersdorffite mineralization could be the main target for further PGE prospecting.
Knauf VV, Proc. Rus. Min. Soc, CXXV, 109-113, (1996).
Page NJ, et al, Econ. Geol, 81, 1262-1271, (1986).
Vuollo J, et al, Econ. Geol, 90, 445-452, (1995).
In the Red Sea Hills and the Nubian Desert of the Sudan exist about 300 gold occurrences of different economic value. Four more important deposits were part of a more intense investigation, presented in this paper. Mining activity started in the pharaonic times and shows archeological evidence since Middle Kingdom time (about 1900 b.C.) and continued with long time gaps until arabic and modern times. In the beginning of this century english prospectors reopened a few mines which were abandoned around World War II. The latest mining has been initiated by Minex Co. in the 80's. During fieldwork different archeological periods have been distinguished by mill type, construction of buildings and ceramics. Gold mineralisation is structually controlled by a shear zone system consisting of steep to vertical dipping thrusts and faults probably reactivated during a posttectonic extensional event. Mineralization postdates the latest compressional event arguing for an age younger than 650 m.y. Two types of mineralisation have been identified. A first type (1) includes disseminated sulphides (pyrite, arsenopyrite) in highly sheared and altered host rock (with different lithology). The second type (2) is characterized by white to grey, massiv to brecciated carbonate-quartz veins with sulphides (pyrite, arsenopyrite, chalcopyrite) and native gold. Both types are always associated with acid to intermediate intrusions obviously delivering the thermal potential of the convectional cells of hydrothermal fluids. A detailed examination of wallrock alteration revealed two zones. The outer zone consists of quartz, sericite, chlorite and epidote due to breakdown of feldspar and amphibole. The inner zone is characterized by quartz, sericite, carbonates, epidote and ferrous sulphides resulting from more intense alteration. Geochemically the inner zone shows an increase in gold, arsenic, iron, calcium and potash and a decrease in sodium and silica.Carbonatization and sulphidisation indicate H2O-CO2-H2S-fluids responsible for Au-mineralization. Arsenopyrite geothermometrie revealed about 300°C formation temperature. Alteration pattern indicates a pH of about 4. In an acid enviroment gold is soluble in a Au(HS)0-complex. Precipitation of gold can be related in type (1) deposits to break-down of the gold-sulphide-complex by wallrock interaction. Alteration of pyroxenes and biotites produces free iron reacting with sulphide to pyrite or arsenopyrite. Gold is trapped in ferrous sulphides along the cristall faces. Relations to temperature in type (2) deposits have been observed, but preliminary fluid studies argue for boiling of the fluid precipitating gold as native element. Ferrous sulphide mineralization in the vein indicates a reaction similar to type (1) deposit. Finally analysis for gold revealed promising results, single samples giving values of 20-60 g/t. Unfortunately, the actual political system and the remote area have prevented further mining.
In the central Anti-Atlas, silver mineralization genesis has been studied in two hydrothermal deposits: Zgounder silver deposit and Bou Azzer Co-Ni-As-Ag-Au deposit, which have Panafrican host rocks (680-580 Ma). Both Zgounder and Bou Azzer are characterized by two major hydrothermal stages of mineralization. First stage is characterized by Co-Ni- arsenide and sulfide deposition and the second and later stage by Ag-Hg deposition.
At Zgounder, first stage is characterized by the development of E-W quartz veinlets with As-Co sulfide mineralization associated with H2O-CO2-CH4 (±N2) fluids trapped under high temperature conditions (450-300°C) and a rather wide range of pressures, attesting to a pressure fluctuation from hydrostatic (50-80 MPa) to lithostatic conditions (Pmin : 65-190 MPa).
Arsenide and sulfide stage at Bou Azzer and Ag-Hg- stage at Zgounder and Bou Azzer are associated with the circulation of similar hypersaline complex brines from the H2O-NaCl-CaCl2 system (24-40 wt.% NaCl+CaCl2 at Zgounder and 19-45 wt.%. NaCl+CaCl2 at Bou Azzer). First, these brines have deposited two different mineralizations at Zgounder and Bou Azzer, associated with quartz-carbonate-chlorite veins: Zn-Cu mineralization in N-S veinlets at Zgounder and As-Co-Ni paragenesis at Bou Azzer. Lately, brines have deposited the silver parageneses at Zgounder and Bou Azzer. General conditions for silver deposition are similar in the two deposits, with moderate temperatures and pressures (T around 200°C and P< 100 MPa).
Deep basinal brines from sedimentary basins, corresponding probably to sedimentary fluids which interacted with evaporites (with Infracambrian age-Adoudounian), are responsible for silver deposition at Zgounder and Bou Azzer. This basinal brines have circulated at a large scale in the central Anti-Atlas, in post-adoudounian fractures, probably with hercynian age. Silver has migrated as chloride complexes and dilution and slight cooling are the driving mechanisms for silver deposition in the two deposits.
Sierra de Guanajuato range (SGR) is integred by three well defined lithostratigraphic units (1): a) a cretacic basement formed by two formations a volcano-plutonic (2) one and by a volcano-sedimentary one; b) tertiary intrusions and, c) continental volcano-detrital deposits. SGR structure is due to compressive (laramide?) and distensive (cenozoique) deformations.
Metallogeny of SGR has a three-fold origin (3): volcano-sedimentary, granitic and volcanic. At SGR three metallogenic epoches have been defined (3): a) a cretaceous one (lens-shape stratiform bodies of massive pyrite of sedimentary-foreign filiation); b) a paleocene one (quartz-cordierite-sanidine veins and remplacement bodies of hydrothermal-metamorphic filliation: W+Se-Bi, Pb, Zn, Cu and pyrometasomatic bodies Cu, Pb, Zn (Ag), W and c) an oligocene one (quartz-calcite-adulaire epithermal veins of geothermal-volcanic filliation (4): Ag-Au; Guanajuato mining district is a classic exemple). The actual gold bonanza at Guanajuato mining district, specially at El Cubo mine, is linked to rhyolithic domes.
Martinez-Reyes J, Simp Sierra de Guanajuato. Inst Geol. Unam, 1, 32-48, (1987).
Monod O, Lapierre H, Martinez-Reyes J, Calvet P, Ortiz E, Zimmermann J, C. R. Acad. Sci. Paris, 310, 45-51, (1990).
Zarate-del Valle PF, VIII Conv Nal Soc Geol Mx, 1, 97-98, (1985).
Buchanan LJ, Ph. D., thesis, 1, 210, (1979).
Au-Ag-Pb-Zn ore from Beregovo epithermal deposit are within the limits of explosive caldera of sarmat age, which is filling by rhyolitic tuffs and ignimbrites. The frame of caldera is composed by rocks preneogene (Triassic - Cretaceous) basement (cherts, limestones, diabases) and neogene volcanic-sediment cover (rhyolitic tuffs, sandstones, mudstones).
The formation of minerals ores occurs in the following sequence: siderite + calcite + ankerite ± sphalerite + galena + native Ag + pyrite ±fluorite ±quartz-I + chalcopyrite + Bi-sulfosalts + electrum ±barite ±quartz-II + hematite.
On deep horizons of deposit (lower -200 v), in quartz veins, which cross altereted (quartz-adularia-sericite-kaolinite) rhyolitic tuffs, association is observed typically calcareous skarns: ilvaite, andradite - grossular, minerals of tremolite - actinolite range. It is observed only in intervals of ore bodies enriched presulphide carbonates and containing aggregates of quartz-1. Ilvaite and garnets are replaced by chalcopyrite, hematite, quartz-II. Relicts of limestones in field of skarns extending are not found.
The skarn paragenesis is formed in front of development of massive aggregates fine-grained quartz-1 and is kept only on the bottom horizons of deposit, where quartz-I has the minimal extending. Near to surface where quartz-I is widely advanced, and also the influence of the subsequent processes is strong, skarn association almost completely disappears. Unique skarn mineral, which is marked practically on all area of deposit and is the indicator of former presence of skarn mineralization, there is ilvaite. The skarn association observable at the Beregovo deposit concerns to original infiltration formations, which have arisen at influence enriched SiO2 of alkaline solutions forming quartz veins with carbonate hydrothermal veins.
The Ospinsky massif, located in southeastern of the Eastern Sayan, due to modern views, is referred to metaperidotite complex and belongs to the dunite-harzburgite type. The geology of this massif has been studied well enough, but the data about the composition of minerals are practically absent.
The massif is characterized by the relatively wide spread of fresh and weakly serpentinous rocks. It allows to carry out the detailed study of main and accessory rock minerals with the purpose of petrogenetic and paleogeodynamic reconstructions.
Chrome-spinellide is one of the most informational minerals. In the boundaries of the Ospinsky massif the two main types of chromite mineralizations have been distinguished: the scattered (accessory) dissemination and the veined and schlieren accumulations of massive chromite.
The investigations showed the clear dependence of chrome-spinellides composition on the whole composition of rocks. Thus, it has been determined that MgO content decrease in composition of chrome-spinellides from dunites to peridotites is compensated by FeO content increase. Besides, Cr2O3 content increase in these minerals from dunites to peridotites at the expense of Al2O3 contents. Thus, the general scheme of isomorphism will be like Mg+Al<->FeO2++Cr. As a resalt of chrome-spinellides composition study in the section from roof to bottom, the tendency of Cr2O3 content increase in these minerals upwards along the section has been revealed. On this basis, the conclusion about increase of the depletion degree of mantle peridotites in this direction has been made. The comparison of chrome-spinellides compositions of the Ospinsky massif with the ones from different massives of the Eastern Sayan ultrabasic rocks showed, that ultrabasites of the Ospinsky massif are the most depleted variety of the dunite-harzburgite type ultrabasites.
Cordierite (Mg,Fe)2Al4Si5O18 is an important mineral in many metamorphic rocks and plays an important role in geothermometry and geobarometry calculations. Heat capacities are neces-sary for the calculation of thermodynamic properties, for example, entropy and enthalpy. The hitherto measured values for anhydrous Mg-cordierite are more than 30 years old (Pankratz and Kelley, 1963; Weller and Kelley, 1963). Carey (1993) measured the heat capacity of anhy-drous Mg-cordierite between 295 and 425 K, but there is still a lack of new, precise, and more extensive data. No measurements have been made on the heat capacity of Fe-cordierite. Therefore, we synthesized Mg- and Fe-cordierite in order to undertake new Cp measurements.
Anhydrous Mg-cordierite was synthesized from a glass at 1 bar and 1400°C for 1 week. Its state of order was determined by powder diffraction. The distortion index equals 0.22, hence the Mg-cordierite is completely ordered. The absence of channel water was confirmed by powder IR spectroscopy.
Hydrous Fe-cordierite was synthesized hydrothermally from an oxide mix containing 5 wt.% seed crystals at 2 kbars and 720°C for 4 days in cold-seal vessels. Fe2+ was stabilized by using the IW-buffer. The first synthesis yielded a phase mixture consisting of 90-95 vol.% Fe-cordierite and 5-10 vol.% hercynite and quartz. In order to transform the latter into Fe-cordierite, the reaction products were finely ground and then resynthesized at the same P,T conditions. The second synthesis yielded single-phase Fe-cordierite. Dehydration was done at 1 bar and 650°C under an Ar stream, in order to avoid oxidation. The absence of water was also confirmed by powder IR-spectroscopy.
For the heat-capacity measurements the DSC method (Bosenick et al., 1996) was used. About 50-60 mg of sample were measured between 150 and 950 K.
The measured values for Mg-cordierite differ little from those of previous measurements. The deviation is not more than 1% and internal precision is also about 1%.Cp Polynomials were fitted to the experimental data giving for anhydrous Mg-cordierite:
Valid from 298 - 950 K:
Cp[J/(mol*K)] = 825.984 - 3111.363xT-0.5
- 34.93816x106 T-2 + 5435.177x106 T-3
Valid from 150 - 298 K
Cp[J/(mol*K)] = 1424.047 -19519.704xT-0.5
+ 17.23609x106 T-2 - 1219.325x106 T-3
For anhydrous Fe-cordierite we obtain:
Valid from 298-950 K
Cp[J/(mol*K)] = 744.598 + 224.582xT-0.5
- 55.59369x106 T-2 + 9407.029x106 T-3
Valid from 150 - 298 K
Cp[J/(mol*K)] = 1478.009 - 20114.309xT-0.5
+ 17.37480x106 T-2 - 1131.623x106 T-3
The measured values allow the calculation of the third-law entropy of Fe-cordierite. With these new data it is possible to calculate the position of various mineral reaction curves in the system FeO - Al2O3 - SiO2, whose positions are not presently known.
Pankratz LB, Kelley KK, US Bureau Min. Rep. Inv, 6555, (1963).
Weller WW, Kelley KK, US Bureau Min. Rep. Inv, 6343, (1963).
Bosenick A, Geiger CA, Cemic L, Geochim. Cosmochim. Acta 60, (1996).
Carey JW, Phys. Chem. Min, 19, (1993).
The method based on the study of solid microinclusions in minerals is untraditional for finding out the origin of highly metamorphosed complexes. It is for the first time applied to the rocks of the Lapland granulite belt. There are three basic hypotheses of the Lapland granulite origin: intrusive, supracrustal, infracrustal. They all imply that strongly metamorphosed rocks were brought up from certain depths. Micromineral investigations have indicated a great variety of mineral species, exceptional compositions of some mineral groups and some unique finds. The presented results pertain to SiO2 minerals only. The conditions of metamorphic changes and the occurrence of lonsdaleite in the Lapland belt (Golovnya et al., 1977) allowed us to consider optimistic the search of high pressure quartz polymorphs. The finds of diamond and coesite as inclusions in minerals of metamorphic rocks in the Kokchetav massif were the indirect stimulus for our investigations (Sobolev et al., 1994). Different forms of quartz occurrence (monomineral inclusions and the phases of polyphase inclusions) in garnets and zircons were examined. A preliminary study of the inclusions with microprobe CAMECA-46 allowed us to determine their compositions as SiO2 =99-100% and ascertain the distinction in colors of luminescence. The SiO2 polymorphs were subdivided according to the luminescence color during the study of microinclusions in the Kokchetav massif. It was necessary to test the structural state of SiO2 for those to reveal the nature of different luminescence. In this connection the Raman investigations of microminerals were performed with the Renishaw equipment (the laser wave length 632.8 nm) at Lulea University (Sweden). The spectra at 100-1500 cm-1 range have indicated the presence of Raman modes frequencies, whose intensities are reproduced for all inclusions. The most intensive peak has a frequency of 464-467 cm-1, which permits to determine the inclusions as <alpha>-quartz. The less intensive peaks have the positions close to pure quartz. The observed shifts are probably due to the influence of the mineral-host. The absence of some peaks may be connected with accidental degeneracy and low intensities of modes. The obtained results suggest the following conclusions: 1) the different colours of luminescence are caused by different admixtures in quartz, but no different structural states; 2) SiO2 microinclusions of are the <alpha>quartz and do not contain relics of a high pressure polymorph - coesite. The latter does not rule out the possibility of full structural conversion of phases. Although in this case it is not clear why there are no cracks surrounding the microinclusions, which must have appeared during the transform of the coesite with a more compact structure to <alpha>-quartz with a less compact structure. Therefore, according to the data obtained, an infracrustal origin of the Lapland granulite belt is less probable than supracrustal or intrusive.
Golovnya S.V., Chvostova V.P., Makarov E.S., Geochimiya, (in Russian), 5, (1977).
Sobolev N.V., Shatsky V.S., Vavilov M.A., Goryainov S.V., Dokl. AN SSSR, 334, 4, (in Russian), (1994).
Iron-formation-hosted manganese deposits of the Kalahari Manganese Field, South Africa, account for approximately 50% of the world's total land-based reserves of Mn-ore. The deposits occur in the uppermost part of the Palaeoproterozoic Transvaal Supergroup, in the form of three distinct units interbedded with essentially unmetamorphosed iron-formation (Tsikos and Moore, 1997). The origin of these deposits remains contentious, with contrasting volcanic-exhalative and chemical-sedimentary models having been proposed. The entire Fe/Mn succession contains a significant amount of carbonate compounds in addition to the major mineral constituents chert and Fe(Mn)-oxide, and lends itself to detailed stable isotope studies. Such investigations were conducted on the carbonate and silicate-oxide fractions of primarily the host iron-formation, with the purpose of shedding further light into the conditions of formation and subsequent alteration of the unusual Kalahari Mn-ores.
Carbon and oxygen isotope analytical data from the carbonate components of the iron-formation, manganese ore and younger limestone deposits compare well with data from the literature, and suggest that early suboxic diagenetic processes typical of iron-formation depositional environments were dominant, with coupled organic C oxidation - Fe/Mn reduction being the chief mechanism in producing the observed banded/laminated chert-carbonate-Fe/Mn-oxide mineral assemblages. Carbon and oxygen isotopic trends across the stratigraphy suggest a gradual upward enrichment in the heavy isotopes which, in conjunction with a respective increase in the amount of carbonate minerals, are strong indications of a generally shallowing-upwards sedimentary succession, coupled with progressively warmer/oxygenated conditions. Furthermore, oxygen isotope analyses of quartz-magnetite-calcite triplets in unaltered iron-formation samples, exhibit important similarities with other major iron-formation occurrences of the world of similar age and mineralogical composition, and suggest that the iron-formation and - by extension - the entire Fe/Mn succession were formed in an essentially open system, under diagenetic to very-low grade (lower greenschist facies) metamorphic conditions.
Subsequent to deposition, parts of the iron-formation and Mn-ores were subjected to weathering processes, burial, faulting and hydrothermal alteration, the latter producing a variety of exotic mineral phases in specific stratigraphic intervals. Oxygen isotope data from binary quartz-hematite pairs in severely altered iron-formation point towards conditions characteristic of a low-temperature supergene environment. The above, in conjunction with other geological evidence such as lithostratigraphic relationships and relative ages of tectonic events (eg. faulting), suggest that unconformity-related, low-temperature processes operating as early as during Palaeoproterozoic times, were critical in the alteration and economic upgrading of the Kalahari iron-formation and Mn-ores, a concept which casts doubt on previously proposed, fault-related alteration models (Gutzmer and Beukes, 1995).
Gutzmer J, and Beukes NJ, Economic Geology, 90, 823-844, (1995).
Tsikos H., and Moore JM, Economic Geology, 92, 87-97, (1997).
An on-going study of O and H isotopes in the lateritic profile of Lero, Guinea, was undertaken to investigate the potential record of paleoclimatic changes in the savanna regions of West Africa as a result of Africa latitudinal drift since Cretaceous times. Weathering kaolinite and goethite were sampled throughout the 70 m deep lateritic profile of Lero Mine. Groundwater and rainfall were also sampled. Acquisition of 18O- D data is not complete at this time because the mine pit has not yet reached the bottom of the weathering profile. However, preliminary results on kaolinite 18O in the top 40 m of the profile and groundwater 18O- D values already provide valuable climatic information.
Groundwater 18O- D values are consistent throughout the sampled depth interval, i.e., -17 to -62 m below surface. 18O values range from -4.7 to -4.3‰ (average = -4.5 ±0.2‰, n = 19) and D values from -33 to -30‰ (average = -32 ±1‰, n = 19) relative to SMOW. The data suggest some compartmentalization of groundwater isotopic composition with a slight enrichment (0.3‰ for 18O and 2‰ for D) of samples above -35 m relative to deeper samples. In 18O- D coordinates, groundwater samples plot on the Bamako (Mali) meteoric water line, which constitutes the closest IAEA station.
Kaolinite separates microsampled from supergene kaolinite developped from albitite stockwerk in the saprolite yielded 18O ranging from 17.8 to 18.9‰, and averaging 18.3 ±0.3‰ (n = 19). There is some suggestion in the data that kaolinite 18O values tend to increase slightly with depth between 10 and 45 m depth but this remains to be confirmed. If it is assumed that kaolinite formed in isotopic equilibrium with ambient water and using an average annual climatic temperature of 25°C for the site of Lero, the isotopic composition of kaolinite-forming waters can be calculated to be between -7 and -6‰ SMOW. This indicates that kaolinite from the saprolite in the profile of Lero is not in isotopic equilibrium with present-day groundwater, but rather with a older water depleted by about 2‰ relative to today's groundwater and reflecting different climatic conditions in the past. Kaolinite D values are presently being measured, and will hopefully confirm this conclusion.
The Soultz-sous-Fôrets site is located on the western border of the Rhine graben. It has been chosen for the european Hot Dry Rock program due to the high thermal anomaly encountered in this area. The Soultz granite, which is overlain by a thick sedimentary cover is crosscut by numerous fractures. Paleo-fluids have circulated through them and have developped hydrothermal alterations. Some of them are particularly intense and have lead to the total replacement of the plagioclases by a rather rare interstratified chlorite-smectite dioactedral clay mineral: tosudite. Its basic characteristics have been given by Ledésert et al. (1996 and 1997) using X-ray diffraction, optical and scanning electron microscopy and microprobe analyses. The present study gives new data based on transmission electron microscopy (TEM). At low magnification tosudite-rich samples display two types of occurrences. On one hand it consists of aggregates of curled crystallites of approximately 20 to 30 nm in size. On the second hand they consist of aggregated crystallites with a fan-shaped cluster fabrics. At higher magnification, high resolution images display lattice fringes showing a regularly alternating sequence of smectite (10Å, dehydrated smectite) and chlorite (14Å) layers. Moreover, primary observations attest of an intimate relationship between tosudite and organic matter (OM). Indeed, most of the aggregated crystallites display amorphous terminations certainly due to the presence of OM.
Ledésert B, Joffre J, Amblès A, Sardini P, Genter Aand Meunier A, Journal of Volcanology and Geothermal Research, 70, 235-253, (1996).
Ledésert B, Meunier Aand Genter A, abstract supplement n°1 to Terra Nova, 9, 544, (1997, EUG IX, Strasbourg France).
ODP Leg 168 investigated the transition from sediment-free to sediment-covered igneous crust through ten sites drilled across the eastern flank of the JdF ridge (Davis et al., 1997).
Normal tholeiitic pillow basalts were recovered at nearly all sites. Their alteration intensities systematically increase from <1 to 50% with increasing distance from the recent ridge both as a consequence of aging and increasing temperatures related to lateral hydrothermal flow. In both younger and older pillow units the alteration processes are mainly controlled by the presence of concentric and radial fractures which crosscut the pillows from the outer glassy zone to the holocrystalline interior. Pervasive permeation through porosity becomes significant only in the older pillow units. The concentric and radial veins clearly represent the most important channelways for the fluid circulation and the preferential sites where fluid-rock interaction started and developed to the adjoining rock.
Three alteration zones, characterized by different alteration style and mineralogical assemblages, formed as a consequence of the progressive evolution of the fluid-rock interaction: 1) oxidative zone (OZ), 2) intermediate zone (IZ), and 3) non-oxidative zone (NZ).
1) The OZ, 1-15 mm wide, is located immediately around the fracture and is characterized by its reddish to dark-brown pigmentation. Hematite, goethite, amorphous Fe-oxyhydroxides, iddingsite, celadonite, and Mg-rich, Fe-poor saponite are the most common secondary minerals. They occur in veins, in the adjoining rock, as patches within mesostasis, linings/fillings of vesicles and interstitial voids, and as pseudomorphs replacing olivine phenocrysts.
2) The IZ, 1-3 mm wide, marks the transition from the oxidative to the non-oxidative alteration styles. The pigmentation of this zone is strongly variable, from orange-reddish to yellowish to greenish. The mineral assemblages are extremely variable and often represent a complex mixture of zone 1 and 3 secondary phases.
3) The NZ is extremely variable in dimension and is scarcely developed in the younger pillow units. The main mineral assemblages are represented by Fe-rich, Mg-poor saponite ± zeolites ± carbonates ± sulphides (mainly pyrite and pyrrhotite). Carbonate precipitation is always the last alteration episode and is pervasively developed only in the older pillow units.
The temporal and spatial evolution through the different alteration zones show that low temperature alteration (<100°C) evolves from being "water dominated", along fractures and the adjoining selvages, to being "rock dominated" in the pillow interiors, where fluid seepage is the dominant process. The reactions are triggered by a progressively evolving fluid characterized by a general decrease of fO2 from the fracture towards the pillow interior. In the last stage the formation of Fe-rich, Mg-poor saponites + carbonates + sulfide assemblages suggest a sensitive increase in S activity together with a significant decrease of Eh.
Davis E., Fisher A.T., Firth J.V.& ODP Leg 168 Scientific Party, Proc. ODP. Init. Repts., 168, 470, (1997).
The Chivor emerald deposit is located on the oriental flank of the Colombian Eastern Cordillera. Emeralds are formed within lower Cretaceous black shales series during a strong fluid-rock interaction involving brines of evaporitic origin leading to massive albitization. In order to test and to quantify the element transfers during hydrothermal interaction process, a statistical study using PCA method was conducted on a database of eighty four samples analysed for major and trace elements (ICP-AES and ICP-MS analysis).
Two main correlation factors are evidenced allowing the distinction between the elements characterizing the lithology (CaO, MgO, Si02, TiO2...) and the hydrothermal alteration (Na2O, K20, Be, Cr...). The mineralogy of the alteration products can be indirectly approach through this geochemical study yielding to propose an equilibrated chemical reaction involving the transformation of detrital illite from the original black shales into neoformed albite. Computer assisted mapping of the mobile elements distribution allows to define the 2D geometry of the hydrothermal alteration halo on both sides of the main brecciated draining level. The genesis of emerald is related to a Na/Be exchange process between the hydrothermal brines and the black shales series, confirming the metallogenic model proposed by Cheilletz and Giuliani (1996; 1997). The comparison between altered and unaltered black shales shows that this rock can lose up to 80% (3 ppm) of its Be content.
Cheilletz A., Giuliani G., The genesis of Colombian emeralds: a restatement. Mineral Deposita, 31, 361-380, (1996).
Cheilletz A, Giuliani G, Comment se forment les e'meraudes? La Recherche, Nov. 1997, 48-52, (1997).
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