Sulphide geochemistry and wall rock alteration as a guide to mineralization, mammoth area, NW Queensland, Australia |
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| Authors: | K M Scott |
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| Abstract: | Stratabound mineralization in the Mammoth area of NW Queensland occurs in steeply dipping, faulted Proterozoic arenites and dolomitic rocks overlying basic volcanics. Both syngenetic/diagenetic and epigenetic sulphides are present, with the latter divided into Mammoth- and South Mammoth II-styles. Syngenetic/diagenetic pyrite is distinguished from epigenetic pyrite by higher Co and Ni, and lower As, Mo, Sb and Tl contents. Chalcopyrite is the major copper sulphide associated with syngenetic/diagenetic pyrite and is characterized by low Ag, Bi, Mo, Ni and Tl contents relative to epigenetic chalcopyrite. No substantial wall rock alteration is associated with such sulphides. Disseminated syngenetic/diagenetic sulphides in the mine sequence may have induced deposition of epigenetic Cu mineralization, but alone, even when remobilized, do not reach economic grades.The four epigenetic Mammoth orebodies are all richer in chalcocite and bornite than syngenetic/diagenetic mineralization and have wall rock alteration characterized by alkali depletion and Fe enrichment in the zone between the ore and the fault considered the conduit for the hydrothermal ore-forming fluids. Pyrite associated with Mammoth-style mineralization has high As, Mo, Sb and Tl contents and was formed subsequent to the copper sulphides.Thick, essentially barren, pyritic sequences occur at South Mammoth II where the moderate As, high Mo, Sb and Tl contents reflect their hydrothermal origin but distinguish them from Mammoth-style pyrite. Their low As/Sb ratio and lack of wall rock alteration imply a lower temperature of formation than the Mammoth-style pyrite and possibly such mineralization represents the pre-ore stage of the Mammoth mineralizing hydrothermal system.An essential feature of economic Cu mineralization in NW Queensland is the operation of a hydrothermal system. As all such systems may not necessarily give rise to extensive wall rock alteration, use of the high As, Mo, Sb and Tl contents of hydrothermal pyrite can aid evaluation of mineralization intersected during drilling. |
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