Contrasting alteration assemblages in metabasites from Mount Isa, Queensland; implications for copper ore genesis

Two contrasting styles of hydrothermal alteration are preserved in metamorphosed mafic igneous rocks (metabasites) of the Middle Proterozoic Eastern Creek Volcanics at Mount Isa, Queensland. Regional-style alteration assemblages occur throughout the extensive regional domain and are also observed in undeformed parts of the more restricted Paroo fault domain. The Paroo fault-style of alteration is confined to the vicinity 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 alteration styles are interpreted to reflect interaction with compositionally, and possibly temporally, distinct fluids.In the regional domain, represented by Eastern Creek Volcanics outcropping to the east, north, and south of Mount Isa, large volumes of greenschist facies metamorphosed, 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-style alteration assemblages, termed epidosite and chlorite schist, respectively, developed by the channeling of large volumes of hydrous fluid through the host lavas and dolerites during burial and regional metamorphism. The mineral assemblages of both minimally altered and metasomatized regional domain metabasites resemble those in other low-grade metamorphic volcano-sedimentary piles and those observed within present-day midocean ridges.Regional domain carbonate delta ¹⁸ O values (10.4 + or - 1.8ppm), vein quartz delta ¹⁸ O values (11.3 + or - 1.1ppm), and vein chlorite delta D values (-58 + or - 4ppm) indicate equilibration at 325 degrees + or - 50 degrees C with waters derived by dehydration of an Eastern Creek Volcanics-type reservoir at the greenschist-amphibolite facies transition (T > 450 degrees C, delta ¹⁸ O _fluid [asymp] 5.5ppm, and delta D [asymp] -20 to -45ppm). delta ¹³ C _carbonate and delta ³⁴ S _sulfide values indicate that carbon and sulfur components of the metamorphic fluid were buffered by local wall rocks.Sulfide-oxide relations and sulfur isotope ratios of sulfides (mean delta ³⁴ S = 4.1 + or - 5.9ppm) in regional domain metabasites are incompatible with either the extensive channeling, or pervasive infiltration, of an oxidizing hydrothermal fluid. Thus, large-scale oxidative leaching is eliminated as a mechanism of copper mobilization and removal from the Eastern Creek Volcanics. In addition, copper abundances in metabasites and metasedimentary rocks from the regional domain indicate that copper mobility was probably limited to strata-parallel redistribution within, and near to, zones of enhanced fluid flow. Thus, fluid-rock interactions that produced the widespread regional-style alteration assemblages were probably not responsible for large-scale leaching of copper from the Eastern Creek Volcanics.In the Paroo fault domain, represented by Eastern Creek Volcanics situated between the Mount Isa and Paroo faults, a large proportion of observed metabasite is intensely deformed and altered to chlorite schist. In contrast to the regional domain, chlorite schist from this setting is futile rather than titanite bearing and commonly displays a trace element pattern which differs from that of the protolith. Where less deformed and altered, metabasite from the Paroo fault domain has the distinctive trace element and rare earth element characteristics of minimally altered metabasalts belonging to the older Cromwell Member of the Eastern Creek Volcanics.The Paroo fault-style alteration assemblages are consistently and strongly depleted in copper compared with average regional domain metabasites. The combined geologic, textural, and geochemical evidence suggests that Paroo fault-style alteration formed by syntectonic overprinting and replacement of earlier regional-style metabasites. The stable isotope compositions of carbonates, sulfides, and silicates from Paroo fault-style assemblages indicate equilibration with a fluid phase which resembled the regional domain fluid (in terms of T, delta ¹⁸ O, and perhaps delta D) with added components of ¹³ C-depleted carbon and ³⁴ S-enriched sulfur. Similar stable isotope systematics for corresponding mineral species in both the mineralized (Cu and Pb-Zn-Ag) and unmineralized Urquhart Shale at the Mount Isa mine (this and other studies) strongly suggests that the added components were derived from Mount Isa Group rocks. Thus, Eastern Creek Volcanics and Mount Isa Group rocks, within the Paroo fault domain, were apparently infiltrated by the same hybrid metamorphic fluid phase.Oxide-sulfide relations and inferred equilibria in the system Fe-Ti-S-O indicate that the physico-chemical characteristics of the hybrid metamorphic fluid (T, pH, f (sub S ₂ ) , f (sub O ₂ ) , and salinity) were such that from 10 to 1,000 ppm copper could have been carried in solution during the formation of chlorite schist beneath the copper orebodies. Given that Paroo fault-style alteration assemblages are associated with sporadically mineralized Mount Isa Group rocks along more than 50 km of major faults in the Mount Isa district, it is difficult to escape the conclusion that the hybrid metamorphic fluid was, under favorable circumstances, a copper-mineralizing fluid. However, to provide a sufficiently large copper source for the orebodies, copper-depleted chlorite schist must be present to a considerable depth below the Mount Isa mine, and/or the copper-leaching plumbing system must have once extended into the amphibolite facies of the Eastern Creek Volcanics.