A REINTERPRETATION OF THE ROLE OF GRANITOIDS IN THE GENESIS OF NEOPROTEROZOIC GOLD MINERALIZATION IN THE TELFER DOME, WESTERN-AUSTRALIA

The Telfer gold mine, in the Paterson province of Western Australia, i s one of Australia's largest gold deposits with more than 4 million oz (Mot; or 125 t) of gold produced since mining commenced in 1977. Host ed by low-grade, dominantly marine, metasedimentary sequences of the u pper Yeneena Group (similar to 1,000-750 Ma), the Neoproterozoic miner alization (700-600 Ma) is localized within the northwest-trending Telf er dome, which consists of two en echelon, asymmetric, doubly plunging anticlines called the ''Main dome'' and the ''West dome.'' Within the se two domes, gold and minor copper occur as a series of vertically st acked, strata-bound to stratiform, gold-copper sulfide quartz reefs (u p to several meters in thickness) linked by zones of intense stockwork and sheeted veins. Carbon, oxygen, boron, lead, and sulfur isotope da ta (including SHRIMP microanalysis) from ore sulfides and alteration m inerals (carbonates and tourmalines), in conjunction with tourmaline a nd pyrite compositional data, indicate that the ore fluid solutes were derived chiefly from sedimentary host rocks and not Neoproterozoic gr anitoids. Support for a dominantly sedimentary source of sulfur and or e metals includes: (1) a relatively wide range of delta(34)S values fr om hypogene pyrite in discordant veins and stratiform reefs from the M ain dome (-2 to +19.3 parts per thousand) and the West dome(-7 to +10 parts per thousand), which overlaps that of diagenetic-syngenetic sulf ides in carbonaceous host rocks (-23.8 to +11.2 parts per thousand, mo stly between 3 and 11 parts per thousand); (2) S/Se ratios in hypogene pyrite > 100,000, implying a sedimentary source of sulfur and seleniu m in the ore fluids; (3) lead isotope compositions of ore-associated s ulfides from the Main dome and the West dome close to those of the upp er Yeneena Group host rocks but distinct from those of Neoproterozoic granitoids; (4) carbonate delta(13)C and delta(18)O values from minera lized veins and reefs, which typically range between -3 to +3 and 13 t o 18 per mil, respectively. The calculated delta(13)C and delta(18)O v alues of coexisting fluids (at 250 degrees-450 degrees C) are consiste nt with carbon derived aa dissolution of primary marine carbonate in t he host rocks (i.e., rock buffered) and with oxygen comprising a mixtu re of formational, contact metamorphic, and lesser magmatic fluids (i. e., fluid buffered); (5) mineralization-related tourmaline composition s which are dravitic and major element source-rock discrimination diag rams, which indicate that Ca, Mg, Al, and Fe are most likely scavenged from pelitic and psammitic rocks; (6) calculated delta(11)B values of fluids (-9.0 to -0.6 parts per thousand) in equilibrium with minerali zation-related tourmaline (-14.0 to -12.6 parts per thousand), which e xclude boron, and indirectly sulfur, of marine evaporitic origin in th e ore fluids, but imply that boron is liberated from the elastic sedim entary host rocks during the similar to 700 Ma Paterson orogeny. A mod el for Telfer mineralization is proposed, such that granitoids act pri marily as sources of heat that drive thermal convection cells in which heated saline formational-contact metamorphic fluids scavenge gold, c opper, and sulfur from the surrounding sedimentary wall rocks at depth . This style of ore fluid generation shares some similarities with a c onvective porphyry copper (gold) model, although the lack of hornfelse s, proximal granitoids, and types of alteration characteristic of porp hyry copper deposits implies that the Telfer dome deposits did not for m in the near-porphyry environment. These data do suggest, however, hi ghly 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 plu ton(s). A regional-scale deep structure capable of focusing these deep er fluids to higher stratigraphic levels is tile north-northwest-trend ing 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 essen tially lithostatic fluid pressures, followed by rapid ingress of fluid s into the structure, triggered by a decrease in fluid pressure during reactivation.