A reinterpretation of the role of granitoids in the genesis of Neoproterozoic gold mineralization in the Telfer Dome, Western Australia

University of Western Australia, Department of Geology and Geophysics, Nedlands, West. Aust., Australia

The Telfer gold mine, in the Paterson province of Western Australia, is one of Australia's largest gold deposits with more than 4 million oz (M oz; or 125 t) of gold produced since mining commenced in 1977. Hosted by low-grade, dominantly marine, metasedimentary sequences of the upper Yeneena Group ( approximately 1,000-750 Ma), the Neoproterozoic mineralization (700-600 Ma) is localized within the northwest-trending Telfer dome, which consists of two en echelon, asymmetric, doubly plunging anticlines called the "Main dome" and the "West dome." Within these two domes, gold and minor copper occur as a series of vertically stacked, strata-bound to stratiform, gold-copper sulfide quartz reefs (up to several meters in thickness) linked by zones of intense stockwork and sheeted veins. Carbon, oxygen, boron, lead, and sulfur isotope data (including SHRIMP microanalysis) from ore sulfides and alteration minerals (carbonates and tourmalines), in conjunction with tourmaline and pyrite compositional data, indicate that the ore fluid solutes were derived chiefly from sedimentary host rocks and not Neoproterozoic granitoids. A model for Telfer mineralization is proposed, such that granitoids act primarily as sources of heat that drive thermal convection cells in which heated saline formational-contact metamorphic fluids scavenge gold, copper, and sulfur from the surrounding sedimentary wall rocks at depth. This style of ore fluid generation shares some similarities with a convective porphyry copper (gold) model, although the lack of hornfelses, proximal granitoids, and types of alteration characteristic of porphyry copper deposits implies that the Telfer dome deposits did not form in the near-porphyry environment. These data do suggest, however, highly channelized fluid flow to the domal sites of mineralization after an initial period of extensive deep fluid circulation and leaching of sulfur and metals from the sedimentary wall rocks adjacent to the pluton(s). A regional-scale deep structure capable of focusing these deeper fluids to higher stratigraphic levels is the north-northwest-trending Telfer lineament upon which the Telfer dome is situated. Periodic reactivation of this structure, during the the Paterson orogeny, would allow for episodes of widespread fluid advection at depth under essentially lithostatic fluid pressures, followed by rapid ingress of fluids into the structure, triggered by a decrease in fluid pressure during reactivation.

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