SPHALERITE-GALENA GEOTHERMOMETERS - DISTRIBUTION OF CADMIUM, MANGANESE, AND THE FRACTIONATION OF SULFUR ISOTOPES

Distribution of cadmium and manganese, fractionation of sulfur isotope s between coexisting galena and sphalerite, and filling temperatures i n sphalerite have been studied as a test of sphalerite-galena geotherm ometers in samples from more then 30 deposits. The distribution patter ns of cadmium range from linear to scattered. Scattered distribution p atterns of manganese are typical of most samples studied, although the re is a small tendency for linear distribution in samples from a few d eposits. The relationship delta(34)S(ZnS) > delta(34)S(PbS) is found f or most coexisting galena and sphalerite pairs studied, but the relati onships delta(34)S(Zns) approximate to and delta(34)S(ZnS) < delta(34) S(PbS) were also found. The formation temperatures of a large number o f sphalerite-galena pairs, estimated on the basis of the Cd distributi on, range from 320 degrees to 370 degrees C. These values tend to be a bout 40 degrees to 100 degrees C higher then the filling temperatures. The observed differences are considered to lie within the pressure co rrection values for fluid inclusions and a reasonable assumption is th at there is good agreement between galenasphalerite cadmium temperatur es and filling temperatures of fluid inclusions for the studied sample s. The cadmium galena-sphalerite geothermometer gives reliable tempera tures for high-temperatures assemblages (>250 degrees C). Its applicab ility for low-temperature deposits is doubtful. The assumption that co existing sphalerite and galena equilibrated with respect to cadmium di stribution at the majority of deposits studied seems to be reasonable. Several samples studied gave a good agreement between temperatures ba sed on manganese distribution and those estimated from filling and the cadmium galena-sphalerite geothermometer. However, most of the sample s gave rather unreliable temperatures. From these results it is conclu ded that geothermometry based on manganese distribution between galena and sphalerite is not useful. The isotopic sphalerite galena geotherm ometer yielded temperature ranges from 90 degrees to 690 degrees C. Ap proximately 30 percent of the samples gave unrealistic values. Most of samples studied display Delta(ZnS-Pbs) values which seem to be much s maller then the expected equilibrium values and therefore give unreali stic temperatures. Part of the filling temperatures are within 100 deg rees C of the sulfur isotope curves, but in some cases the discrepanci es are larger. An important result is that no samples for which all th ree geothermometers were employed have given concordant temperatures. This is evidence of chemical and isotopic disequilibria at least on th e scale of the volumes sampled.