Behaviour of Thallium in Ore-Forming Processes on Gold-Mercury and Antimony-Mercury Deposits

Evgeny Naumov (naumov@uiggm.nsc.ru)

Institute of Geology, Koptiuga pr. 3, 630090, Novosibirsk, Russia

Thallium is the important geochemical element of gold-mercury mineralization. The analysis of geochemical behaviour thallium in epithermal processes (Vink, 1993) shows that this element mainly occur in low temperature formations: travertine deposits both modern and ancient hydrotherms, gold-antimony, arsenic-antimony-mercury deposits. It is important to determine physical-chemical parameters for deposition at least of two thallium minerals carlinite (Tl₂S) and avicennite (Tl₂O₃), established in gold-mercury deposits. It should give a genetic explanation to occurrence of high thallium concentration in ores of gold-mercury deposits. For this purpose Eh-pH diagram of stability of thallium species at temperature 25^oC and 1 atm pressure was constructed. The thermodynamic constants for thallium species listed by Naumov, et al., 1971; Radtke and Dickson, 1975; were used in the calculations. The main ionic specie of thallium in a wide range of Eh and pH is Tl⁺ and the main solid phases are Tl₂S and Tl₂O₃. Stability fields of carlinite and avicennite occur in alkaline area, whereas stability field of Tl⁺ specie is situated in neutral and acid conditions. These data suggests that thallium minerals (sulfides and oxides) can not be formed in neutral and acid conditions. The hydrothermal fluids become alkaline as a result of degassing and removing CO₂ (Karpov, 1988) in epithermal conditions. In such conditions the alkaline hydrotherms can deposit the thallium minerals in association with adular, zeolite, illite and other minerals - indicators of the high pH values. These reasons also explain forming of thallium minerals and isomorphic impurity of thallium in sulfides and oxides at the gold-mercury deposits which formed in low temperature epithermal conditions. Thallium minerals occur at the gold-mercury deposits, where fluids inclusions are characterized by low concentration of carbonic acid. There are no thallium minerals at the Au-Hg deposits with high concentrations of CO₂ in the gas phase of inclusions.

Karpov GA, Recent Hydrotherms and Mercury-Antimony-Arsenic Mineralization, Nauka (in russian), 183, (1988).

Naumov GB, Ryzhenko BN & Khodakovsky II, Handbook of Thermodynamic Data, Atomizdat (in russian), 240, (1971).

Radtke AS & Dickson FW, Amer. Mineral, 60, 559-565, (1975).

Vink BW, Chem. Geol, 109, 119-123, (1993).

Crystal Chemistry of Tetrahedral and Octahedral Titanium-bearing Synthetic Diopsides

Sabrina Nazzareni (sabrina@flintstones.sct.unipg.it), Gianmario Molin (gmario@dmp.unipd.it) & Alberto Dal Negro (alberto@dmp.unipd.it)

Dipartimento di Mineralogia e Petrologia, Corso Garibaldi, 37, Università di Padova, Italy

A series of Ti-bearing diopside crystals synthesized by flux growth in boron-rich fluids at low fO₂, chemically investigated by SIMS and EMPA microanalyses (Skogby et al., this conference), was submitted to X-ray single-crystal diffraction study for structural determination of site occupancies. The investigated crystals, labelled Di64, Di80a, Di72, Di81 and Di73, showed increasing amounts of titanium from 0.082 (Di64) to 0.291 (Di73) atoms per formula units (a.f.u.). SIMS investigation showed the presence of boron from 0.014 (Di64) to 0.043 (Di72) a.f.u. Skogby et al. (this conference) provides evidence of Ti³⁺ and Ti⁴⁺ and correlates the presence of Na and B the total charge balance. Single-crystal X-ray diffraction data were collected up to (theta) = 45° (about 1500 non-equivalent reflections) using Mok(alpha) radiation. Structural refinements were completed in C2/c space group through SHELXL-97 using individual weights, ionic and atomic scattering factors (Mg²⁺, Ti³⁺, Ti⁴⁺, O^1.5-, Si^2.5+, Ca²⁺, Na⁺, B) converging between R_obs 2.08% (Di81) and 2.64% (Di73). Chemical composition, obtained by EMPA analysis of the investigated single crystals, was used as restrain in the refinements. Structural occupancies were calculated as follows: tetrahedron was constrained for B content, as derived from SIMS, whereas Si was refined against Ti⁴⁺; M1 site was constrained for Ti⁴⁺ whereas Ti³⁺ was refined against Mg; in M2 site Ca was refined against Na. Structural refinements indicate the presence of Ti⁴⁺ in the tetrahedron, in agreement with the spectroscopic results of Skogby et al. The substitution of Si with coupled ^IVTi⁴⁺ and ^IVB induces an enlargement in volume and regularisation of the tetrahedron; the substitution of Mg with Ti³⁺ causes an increase in site distortion, whereas site volume decreases slightly.

Thermal State of the Lithospheric Mantle and low Crust due to the Garnet-Orthopyroxene Thermobarometry of Xenoliths in Kimberlites and Alkali Basalts Data

Larisa Nikitina (nikita@ad.iggp.ras.spb.ru), Valentina Khiltova & Anna Saltikova

Institute of Precambrian Geology and Geochronology, Russian Academy of Science, Makarova emb. 2, St-Petersburg, 199034, Russia

The mantle and low crust thermal state is characterised by the geotherm positions on the P-T diagrams and the thermal gradient (TG) magnitudes, which is equal to T/h, °C/km (h=3.7*P in km, P in kb), determined due to P,T-equilibrium conditions of the mantle (garnet peridotites) and low crustal (granulites, garnet pyroxenites with plagioclase) xenoliths from kimberlites (E. Siberia, N.E. Europe, N. America, S. Afrika, India, S.E. Australia) and alkali basalts (E. Siberia, Mongolia, S.E. Australia, S. and N. America) data. To avoid a divergence in TG values, which is often caused by various thermobarometer application, the T,P equilibrium conditions of rocks were determined by the garnet-orthopyroxene thermobarometer (Nikitina, 1993) only. This gave a possibility to compare the TG values of the mantle and low crust in various regions. However we realize that values in this way obtained may not be considered as absolute. The points of mantle peridotites form two groups of geotherms: "cold" and "hot". The first (Khiltova, Nikitina, 1997; 1999) includes the geotherms of peridotite xenoliths from kimberlites and characterizes the thermal state of the mantle beneath Archean cratons (the coldest geotherm), Proterozoic collisional (the middle) and accretionary (the hottest) belts. These geotherms are described by linear equations, which reflect the TG invariability from 100-120 to 230-250 km. TG in the mantle beneath cratons, collisional and accretionary belts are equal to 6.4-6.7, 6.8-7.4, 7.2-8.1°C/km respectively. All geotherms intersect the continental geotherms with surface heat flows of 40-50 mW/m² (Chapman, Pollack, 1977). The second group includes the geotherms of peridotite xenoliths from basalts. They are close to the continental geotherm with surface heat flow of 60 m W/m². TG in the mantle in the region of alkali basalt vulcanism are 8.2-8.3(S. America), 8.3-8.8 (Zabaikalie), 8.6-9.9 (Mongolia), 9.0-10.8 (S.E. Australie), 10.4-10.8°C/km. Low crustal xenoliths from kimberlites and alkali basalts form the different geotherms. The xenolite points from basalts continue the 'hot" mantle geotherms to the region of lower P and T. The low crustal xenoliths from kimberlites are not continue the "cold" mantle geotherms. They form geotherms, which remove to higher temperatures, however they are not coinside with low crustal geotherms from basalts.