Ancient Metallurgic Processes: A Mineralogical Investigation of Slags

Andrea Manasse (manasse@unisi.it), Marcello Mellini (mellini@unisi.it) & Cecilia Viti (vitic@unisi.it)

Dipartimento di Scienze della Terra, Via Laterina 8, 53100 Siena, Italy

The Campiglia Marittima area (Southern Tuscany) has been long exploited for Cu-Pb-Ag mineralizations. An important mining activity took place in Etruscan time (VIII-III b.C). Segregation of metals was obtained by melting processes (smelting) in appropriate ovens and by subsequent gravimetric separation of metals (at the bottom of the oven) and silicatic melts (at the top of the oven). The crystalization of the silicatic melts gave rise to slag deposits, particularly abundant in the Capattoli valley. Slags are mainly formed by SiO₂(mean value of 28.0 wt%), FeO (47.9 wt%) and CaO (9.2 wt%), with minor Al₂O₃, MnO, Na₂O and K₂O contents. The Cu, Pb and Zn contents may give information about the efficacy in metal segregation. The Cu content is low (mean value of 2700 ppm), whereas Zn is systematically higher (mean value of 36700 ppm), thus suggesting that this element was neglected in the smelting process. Pb occurs in variable amount, (5300 - 33900 ppm). In particular, we identify two different compositional groups, high Pb in CaO-poor slags and low Pb in CaO-rich slags, the latter corresponding to higher segregation efficacy. X-ray powder diffraction shows that slags are mostly formed by olivine, with variable Fe/Ca ratio: in particular, CaO-poor slags consist of fayalite whereas CaO-rich slags are formed by two different olivines, a Ca-rich fayalite and a Fe-rich kirschteinite. Pb and Zn occur as sulphides, in galena and sphalerite, respectively. The mineralogical characterization of slags from the Capattoli valley has been useful to understand the ancient metallurgic technique used in this area, and the efficacy of the metal segregation process: Cu segregation was attained in all slag deposits. In contrast, the segregation of Pb was successfully attained only in the ovens that produced CaO-rich slags: this could reflect different mineralogical compositions of the raw material as well as different techniques.

Experimental Fragmentation of Bubble- and Crystal-Bearing Haplogranite Melts

Caroline Martel (caroline.martel@uni-bayreuth.de)¹, Donald B. Dingwell¹, Oliver Spieler¹, Michel Pichavant² & Max Wilke³

¹ Bayerisches Geoinstitut, Germany

² CNRS-CRSCM, France

³ Universität Hannover, Germany

Explosive volcanism is controlled by a complex interplay of physico-chemical properties, leading to a wide range of eruptive features. Dome-related eruptions range from low-energy coarse-grained pyroclastic flows to high-energy fine-grained surges. We are experimentally reproducing some of the conditions under which domes fragment using a fragmentation bomb apparatus (Alidibirov and Dingwell, 1996). This apparatus simulates material fragmentation under rapid decompression up to 30 MPa in 1 ms and at up to 950°C. We first hydrate a synthetic haplogranite powder containing Al₂O₃ crystals of ~350 or <90 µm in diameter (called phenocrysts and microlites, respectively) with 1 to 4.5 wt% H₂O in an internally heated pressure vessel. Some of the hydrated samples (highly crystallized) are characterized by fractured crystals. The hydrated, crystal-bearing glass cylinders (20 or 8 mm in diameter, 20 mm in length) are placed in the vertically-working autoclave of the fragmentation bomb for H₂O exsolution at 600°C or 800°C and pressures between 6 and 30 MPa for 30 minutes. The subsequent rapid decompression of the sample is induced by the disruption of the three diaphragms on the top of the autoclave. The fragments are collected for analyses.

The very sharp outlines and glass failure textures of the fragments suggest a brittle fragmentation. Fragment size distributions are characterized by 1 to 4 main modes attributed to 1) the influence of pressure and crystal content (this peak at ~1.5 mm in diameter is nearly almost present), 2) low fragmentation pressures (generating a ~6 mm in diameter peak), 3) crystal size, and 4) crystal strength to decompression (fractured crystals produce a ~0.07 mm mode). We found that an increase of the decompression magnitude generates finer fragments, in contrary to an increase of the large crystal content. Large crystals have more influence on the fragment size distribution than small ones.

Alidibirov M & Dingwell DB, Nature, 380, 146-148, (1996).

Gold Concentration by Felsic Magma Migration

Lucienne Martin (065edin@cosmos.wits.ac.za)¹, Gary Stevens (065gary@cosmos.wits.ac.za)¹ & Wojciech Przybylowicz (przybylowicz@nac.ac.za)²

¹ Economic Geology Research Institute, Dept. of Geology, P/Bag 3, University of the Witwatersrand, WITS 2050, South Africa

² Materials Research Group, National Accelerator Centre, PO Box 72, Faure, 7131, South Africa

Experiments have been conducted to investigate gold solubility in granitoid and syenitic magmas. Glasses of syenitic, and A-, I- and S-type granitoid compositions were encased in gold capsules with 4.4wt% H₂O, and with 1wt% of either Cl^-, F^- or S^-, as potential gold complexing agents. The experiments run at 1500bars, 750^oC and fO₂ ~NNO buffer, produced quenched glasses containing a high temperature silicate, oxide and sulphide assemblage, reflecting the bulk composition, as well as small gold crystals. As they were designed to be vapour absent, the presence of the gold crystals is of particular importance. These indicate that gold was mobilised by diffusion through the melt and indicates that, at least locally, gold saturation of the melt occurred.

The gold concentration of the glasses was analyses using a combination of LA-ICP-MS and PIXE techniques. Both magma type and the nature of the gold complexing agent exert a control on the gold concentration of the magmas. The minimum and maximum gold concentrations measured were, respectively 0.17 ppm in the A-type melt with Cl^- added and 2700 ppm in the syenitic melt with S^- added. These data have profound implications for models of gold deposit genesis as they demonstrate that under realistic geological conditions, gold is likely to behave as a strongly granitophile element. Current theories of granitoid pluton genesis envisage many high-level granitic intrusions as low-proportion partial melt extracts (~3-10%). Our data indicate that granitoid and silica-undersaturated alkaline magmas are potentially excellent scavengers of gold, and assuming the operation of viable concentration mechanisms, even the lowest gold concentrations measured in this study could produce sizeable gold deposits from a single pluton. Our data further explain: (1) low gold concentrations in post-anatectic, restitic metamorphic terranes; (2) why low-grade terranes with associated granitoid intrusions are commonly gold enriched; and (3) the association of gold deposits with strongly alkaline mantle-derived melts.