The forms of mercury in some australian stratiform Pb-Zn-Ag deposits of different regional metamorphic grades |
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| Authors: | W R Ryall |
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| Institution: | (1) CSIRO Division of Mineralogy, P.O. Box 136, 2113 North Ryde, NSW, Australia |
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| Abstract: | Mercury concentrations of four Australian stratiform lead-zinc-silver deposits of Proterozoic age are not directly related to their metamoprhic grade and are probably determined by local factors. Average mercury contents of H. Y. C. mineralization (unmetamorphosed) is 1300 ppb, Mount Isa ore (greenschist facies) 8300 ppb, Squirrel Hills mineralization (amphibolite facies) 100 ppb, and Broken Hill ore (granulite facies) ranges from 1000 ppb for A lode to 37000 ppb for No. 3 lens. In present-day sulfidic muds from the Coorong region of South Australia, mercury is present as mercury sulfide either as inclusions or chemisorbed on mackinawite. This may be analogous to the manner of mercury occurrence in sediments which predated the ingress of lead-zinc-silver mineralization into ancient basins. In bedded H. Y. C. mineralization mercury is present principally in pyrite, but recrystallized mineralization contains mercury in both sphalerite and pyrite. In bedded Mount Isa ore mercury is present in sphalerite, but in remobilized ore which is enriched in galena, freibergite and pyrrhotite, it occurs in both sphalerite and freibergite. Mineralization from Squirrel Hills contains no tetrahedrite, and sphalerite is the host for most mercury. Normal Broken Hill ore contains mercury parti tioned approximately evenly between sphalerite and tetrahedrite, but mercury is enriched in remobilized ore. Veins generated during retrograde metamorphism of the Broken Hill lodes contain high levels of mercury, and multiple veins produced in this fashion around other deposits could result in mercury haloes that can be detected during exploration programs. The way mercury is held in stratiform leadzinc-silver deposits, and the lack of decrease of mercury contents with increasing metamorphic grade, rules out total loss of mercury during metamorphism. Primary mercury haloes around this type of mineralization are due to mercury in disseminated ore-forming minerals or late-stage vein systems contain secondary mercury-bearing minerals. |
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