Sphalerite-galena geothermometers; distribution of cadmium, manganese, and the fractionation of sulfur isotopes

Distribution of cadmium and manganese, fractionation of sulfur isotopes between coexisting galena and sphalerite, and filling temperatures in sphalerite have been studied as a test of sphalerite-galena geothermometers in samples from more then 30 deposits. The distribution patterns of cadmium range from linear to scattered. Scattered distribution patterns of manganese are typical of most samples studied, although there is a small tendency for linear distribution in samples from a few deposits. The relationship delta ³⁴ S _ZnS < delta ³⁴ S _Plus is found for most coexisting galena and sphalerite pairs studied, but the relationships delta ³⁴ S _ZnS nearly equal and delta ³⁴ S _ZnS < delta ³⁴ S _PbS were also found. The formation temperatures of a large number of sphalerite-galena pairs, estimated on the basis of the Cd distribution, range from 320 degrees to 370 degrees C. These values tend to be about 40 degrees to 100 degrees C higher then the filling temperatures. The observed differences are considered to lie within the pressure correction values for fluid inclusions and a reasonable assumption is that there is good agreement between galena- sphalerite cadmium temperatures and filling temperatures of fluid inclusions for the studied samples. The cadmium galena-sphalerite geothermometer gives reliable temperatures for high-temperatures assemblages (>250 degrees C). Its applicability for low-temperature deposits is doubtful. The assumption that coexisting sphalerite and galena equilibrated with respect to cadmium distribution at the majority of deposits studied seems to be reasonable. Several samples studied gave a good agreement between temperatures based on manganese distribution and those estimated from filling and the cadmium galena-sphalerite geothermometer. However, most of the samples gave rather unreliable temperatures. From these results it is concluded that geothermometry based on manganese distribution between galena and sphalerite is not useful. The isotopic sphalerite-galena geothermometer yielded temperature ranges from 90 degrees to 690 degrees C. Approximately 30 percent of the samples gave unrealistic values. Most of samples studied display Delta (sub ZnS-PbS) values which seem to be much smaller then the expected equilibrium values and therefore give unrealistic temperatures. Part of the filling temperatures are within 100 degrees C of the sulfur isotope curves, but in some cases the discrepancies are larger. An important result is that no samples for which all three geothermometers were employed have given concordant temperatures. This is evidence of chemical and isotopic disequilibria at least on the scale of the volumes sampled.