Compositions of Sphalerites from the Zoned Hydrothermal Lead-Zinc Deposits at Zeehan, Tasmania

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Hydrothermal lead-zinc veins in the Zeehan district of western Tasmania show clear mineralogical zoning around the southeast margins of the intrusive Heemskirk granite, manifested principally in a transition from pyritic to sideritic gangue. The zoning is thought to have resulted from both progressive spatial changes in the environment of ore deposition and changes with time in the source composition of the ore fluids.Sphalerite was deposited in fissure veins over most of the field. FeS and MnS contents of the sphalerites, determined by an extensive program of microprobe analysis, both show systematic declines paralleling the mineralogical zoning. The decline in MnS contents is abrupt and coincides with the first zonal appearance of substantial gangue carbonate; it is thought to indicate preferential partitioning of manganese into the carbonate minerals. Variation in FeS contents is more gradual and is consistent with zonal declines in temperature and/or sulfur fugacity. Much of the sphalerite may have been deposited in equilibrium with pyrite, but pyrrhotite is rare. The compositional data therefore are insufficient to determine either temperature or sulfur fugacities prevailing during crystallization of the sphalerite. They are consistent with zonal ranges of as much as 500 degrees to 200 degrees C in temperature and 10 (super -5) to 10 (super -12) bars f (sub S 2 ) , but they do not preclude narrower ranges in either parameter.Some of the sphalerites show textural evidence of partial reequilibration during the deposition of paragenetically younger minerals, notably galena and quartz. The altered sphalerites are chemically more homogeneous than their unaltered equivalents and they usually contain more FeS. Many also contain small inclusions of pyrite and/or pyrrhotite, thought to have formed during the reequilibration. Compositions of sphalerites from sphalerite + pyrite + pyrrhotite assemblages are of interest, since many contain less FeS than would be predicted from the results of recent experimental studies in the region above 300 degrees C. Microprobe analyses suggest, however, that most of the pyrrhotite is probably monoclinic, which may account for the low FeS contents of sphalerites from these and perhaps other similar occurrences.