CONTRASTING ALTERATION ASSEMBLAGES IN METABASITES FROM MOUNT ISA, QUEENSLAND - IMPLICATIONS FOR COPPER ORE GENESIS

Two contrasting styles of hydrothermal alteration are preserved in met amorphosed mafic igneous rocks (metabasites) of the Middle Proterozoic Eastern Creek-Volcanics at Mount Isa, Queensland. Regional-style alte ration assemblages occur throughout the extensive regional domain and are also observed in undeformed parts of the more restricted Paroo fau lt domain. The Paroo fault-style of alteration is confined to the vici nity of major faults and fault zones such as the Mount Isa, Paroo, and Western faults. The contrasting mineralogy, trace element chemistry, stable isotope systematics, and geologic relations of the two alterati on styles are interpreted to reflect interaction with compositionally, and possibly temporally, distinct fluids. In the regional domain, rep resented by Eastern Creek Volcanics outcropping to the east, north, an d south of Mount Isa, large volumes of greenschist facies metamorphose d, minimally altered, and minimally deformed mafic lavas (metabasalt) and dolerites (metadolerite) are interspersed with smaller volumes of intensely epidote- and chlorite-altered equivalents. These regional-st yle alteration assemblages, termed epidosite and chlorite schist, resp ectively, developed by the channeling of large volumes of hydrous flui d through the host lavas and dolerites during burial and regional meta morphism. The mineral assemblages of both minimally altered and metaso matized regional domain metabasites resemble those in other low-grade metamorphic volcano-sedimentary piles and those observed within presen t-day midocean ridges. Regional domain carbonate deltaO-18 values (10. 4 +/- 1.8 parts per thousand), vein quartz deltaO-18 values (11.3 +/-1 .1 parts per thousand), and vein chlorite deltaD values (-58 +/- 4 par ts per thousand) indicate equilibration at 325-degrees +/- 50-degrees- C with waters derived by dehydration of an Eastern Creek Volcanics-typ e reservoir at the green-schist-amphibolite facies transition (T > 450 -degrees-C, deltaO-18fluid almost-equal-to 5.5 parts per thousand, and deltaD almost-equal-to -20 to -45 parts per thousand). deltaC-13(carb onate) and deltaS-34(sulfide) values indicate that carbon and sulfur c omponents of the metamorphic fluid were buffered by local wall rocks. Sulfide-oxide relations and sulfur isotope ratios of sulfides (mean de ltaS-34 = 4.1 +/- 5.9 parts per thousand) in regional domain metabasit es are incompatible with either the extensive channeling, or pervasive infiltration, of an oxidizing hydrothermal fluid. Thus, large-scale o xidative leaching is eliminated as a mechanism of copper mobilization and removal from the Eastern Creek Volcanics. In addition, copper abun dances in metabasites and metasedimentary rocks from the regional doma in indicate that copper mobility was probably limited to strata-parall el redistribution within, and near to, zones of enhanced fluid flow. T hus, fluid-rock interactions that produced the widespread regional-sty le alteration assemblages were probably not responsible for large-scal e leaching of copper from the Eastern Creek Volcanics. In the Paroo fa ult domain, represented by Eastern Creek Volcanics situated between th e Mount Isa and Paroo faults, a large proportion of observed metabasit e is intensely deformed and altered to chlorite schist. In contrast to the regional domain, chlorite schist from this setting is rutile rath er than titanite bearing and commonly displays a trace element pattern which differs from that of the protolith. Where less deformed and alt ered, metabasite from the Paroo fault domain has the distinctive trace element and rare earth element characteristics of minimally altered m etabasalts belonging to the older Cromwell Member of the Eastern Creek Volcanics. The Paroo fault-style alteration assemblages are consisten tly and strongly depleted in copper compared with average regional dom ain metabasites. The combined geologic, textural, and geochemical evid ence suggests that Paroo fault-style alteration formed by syntectonic overprinting and replacement of earlier regional-style metabasites. Th e stable isotope compositions of carbonates, sulfides, and silicates f rom Paroo fault-style assemblages indicate equilibration with a fluid phase which resembled the regional domain fluid (in terms of T, deltaO -18, and perhaps deltaD) with added components of C-13-depleted carbon and S-34-enriched sulfur. Similar stable isotope systematics for corr esponding mineral species in both the mineralized (Cu and Pb-Zn-Ag) an d unmineralized Urquhart Shale at the Mount Isa mine (this and other s tudies) strongly suggests that the added components were derived from Mount Isa Group rocks. Thus, Eastern Creek Volcanies and Mount Isa Gro up rocks, within the Paroo fault domain, were apparently infiltrated b y the same hybrid metamorphic fluid phase. Oxide-sulfide relations and inferred equilibria in the system Fe-Ti-S-0 indicate that the physico -chemical characteristics of the hybrid metamorphic fluid (T, pH, f(s2 ), f(O2), and salinity) were such that from 10 to 1,000 ppm copper cou ld have been carried in-solution during the formation of chlorite schi st beneath the copper orebodies. Given that Paroo fault-style alterati on assemblages are associated with sporadically mineralized Mount Isa Group rocks along more than 50 km of major faults in the Mount Isa dis trict, it is difficult to escape the conclusion that the hybrid metamo rphic fluid was, under favorable circumstances, a coppermineralizing f luid. However, to provide a sufficiently large copper source for the o rebodies, copper-depleted chlorite schist must be present to a conside rable depth below the Mount Isa mine, and/or the copper-leaching plumb ing system must have once extended into the amphibolite facies of the Eastern Creek Volcanics.