SULFIDE CHEMISTRY IN PORPHYRY-STAGE AND MINOR LATE POLYMETALLIC MINERALIZATION IN THE GABY DISTRICT, NORTHERN CHILE

In porphyry systems, the precipitation of metal-rich sulfides involves multiple hydrothermal events forming a continuum of porphyry-style alteration and mineralization commonly overprinted by a late polymetallic stage. Despite its significant potential for elucidating ore-forming processes, the trace-element geochemistry of sulfides, such as chalcopyrite, pyrite, and bornite, remains largely unexplored in porphyry systems.

The textures and chemical compositions of chalcopyrite, pyrite and bornite from both porphyry mineralization and a minor late polymetallic stage, as well as supergene chalcocite, are evaluated in the Gaby district (northern Chile), using optical microscopy, automated mineralogy (QEMSCAN), electron microprobe, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Copper mineralization is developed in a suite of Eocene porphyries intruding a Carboniferous–Permian volcano-sedimentary sequence and a Permo-Triassic plutonic complex.

In the Gaby district, three stages of copper introduction are identified. An early stage, coeval with potassic alteration, occurs in veinlets and disseminations with chalcopyrite, bornite, molybdenite, and minor pyrite intergrown with K-feldspar and secondary biotite. A second major stage of copper precipitation overprints the potassic alteration and is dominantly found as disseminations of chalcopyrite and molybdenite, paragenetically associated with white mica and chlorite alteration. Traces of luzonite formed at the expenses of high-temperature Cu-Fe sulfides are also observed. A minor third late stage of copper precipitation is observed in polymetallic intermediate-sulfidation veinlets, sporadically identified at Gaby, and consisting of Fe-poor (< 0.4 wt% Fe) sphalerite, galena, chalcopyrite, tennantite-tetrahedrite, pyrite, locally wittichenite and native gold, intergrown with early Fe-rich dolomite or minor rhodochrosite and late siderite and Mn-poor calcite. Secondary chalcocite is formed during supergene processes and can be detected down to > 500 m from the present-day surface. New LA-ICP-MS analyses show higher In content (5 - 20 ppm) in porphyry-stage chalcopyrite compared to the polymetallic veins. High Co content (up to 2 wt. %) is detected in pyrite from polymetallic veins hosted in volcanic rocks.