Early magnetite-amphibole-plagioclase alteration-mineralization in the Island copper porphyry copper-gold-molybdenum deposit, British Columbia

Abstract

The initial hydrothermal event in the evolution of the ca. 377 Mt, 0.41 percent Cu, Island Copper porphyry Cu-Au-Mo deposit, northern Vancouver Island, was the formation of an extensive alteration-mineralization facies dominated by magnetite, calcic amphibole, and intermediate to sodic plagioclase. Early-stage quasipervasive magnetite-rich alteration and associated magnetite-rich veinlets in this Middle Jurassic, island arc-hosted, hydrothermal system developed in both an axial dacitic porphyry dike and contiguous basaltic flows and pyroclastic strata. The magnetite-rich alteration zone, 500 to 700 m wide and originally extending over at least 450 m vertically, exhibits an outward zonation from quartz-magnetite-albite (An (sub < or =6) )-amphibole (trace) - apatite (trace), through quartz-magnetite-amphibole-albite (An (sub 6-10) ) or oligoclase-andesine (An (sub 15-36) )-apatite (trace) + or - scapolite (trace), to amphibole-magnetite-oligoclase or sodic andesinc (An (sub 15-39) ) + or - quartz + or - apatite (trace) assemblages. Definition of early-stage mineral assemblages, impeded by unusually intense later intermediate argillic alteration and more local phyllic and advanced argillic alteration, was facilitated by the use of novel petrographic techniques, including incident-light Nomarski differential interference contrast microscopy. Early-stage mineralization comprises several well-defined vein-veinlet types which were demonstrably emplaced prior to the main-stage chalcopyrite-pyrite stockwork and associated K silicate alteration. Minor chalco-pyrite and pyrite occur in some early-stage veins, and magnetite is a constituent of many later main-stage sulfide-rich veins, but the distinction between these two major vein systems is clear.Mass-exchange calculations demonstrate that early alteration involved intense iron metasomatism of both felsic and mafic country rocks, with an Fe enrichment approaching 450 percent (20 g/100 cc) in the dacite dike, and lesser and more variable Na enrichment, with both Ti and Al behaving as mobile constituents. Paragenetically early fluid inclusions in the quartz phenocrysts of the dike are tentatively correlated with the magnetite-rich alteration-mineralization and imply that this event was initiated above 650 degrees C and at pressures of at least 1 kbar and was generated by FeCl 2 -rich brines, at first single phase and moderately saline (avg 15 wt % NaCl equiv), but latterly boiling at ca. 560 degrees to 645 degrees C and < or = ca. 0.55 kbars. The main-stage potassic alteration zone has the configuration of an annulus, ca. 100 to 150 m in width, which was entirely superimposed on the more extensive early alteration-mineralization zone at ca. 430 degrees to 575 degrees C and below 450 bars. Gold, although correlated overall with Cu and potassic alteration, was probably extensively introduced in the early stage.It is inferred that the scarcity of sulfides in the early alteration assemblages reflects the highly oxidized nature of the initial fluids, in which SO (super -) 2 exceeded both H 2 S and SO (super 2-) 4 . The deposition of abundant magnetite and the widespread and intense Fe metasomatism which define the early-stage record the high solubility of Fe as FeCl 2 in high-temperature brines in equilibrium with magnetite-bearing quartzofeldspathic rocks, as well as the retrograde solubility of magnetite between ca. 550 degrees and 750 degrees C. Whereas the high SO 2 /H 2 O + SO 4 ratios of the initial hydrothermal fluids at Island Copper, and in other similar deposits, may have resulted from the high inherent oxygen fugacity of the parental dacitic magmas, they may have directly reflected the composition of supercritical fluids expelled on quenching of underplating, S-rich, mafic melts. The mineral assemblages of the early-stage veins and associated quasi-pervasive alteration cannot be assigned to the accepted alteration-mineralization facies of porphyry systems, such as A veins and potassic alteration, and record specific and distinct conditions of fluid-rock interaction.