Base metal mobility linked to brine and hydrocarbon migration at the Huincul High in the Neuquén Basin, Argentina: Implications for the formation of sediment-hosted base metal deposits

Abstract

This contribution discusses the discovery of PbZn mineralization in hydrocarbon exploration drill holes hosted in siliciclastic Jurassic rocks below the Cretaceous sediment hosted Cu mineralization located north of the Huincul High. In the Early Cretaceous, hydrocarbon and basinal water expulsion took place from the oil source rocks (Los Molles Formation) into the reservoir rocks Lajas Formation (Middle Jurassic), after the formation of diagenetic quartz overgrowths. Marcasite, pyrite and illite + illite-smectite mixed layer minerals then formed together with hydrocarbons. Marcasite (δ34S −3.5 and −2.2‰) and pyrite (δ34S −11.6 to 18.1‰) precipitated via bacterial sulfate reduction from fluids showing temperatures <80 °C and fluctuating pH conditions. During the Paleogene, Andean tectonism triggered migration of oxidized basinal brine containing dissolved leached Zn and Pb from the Los Molles Formation source rocks and S from the evaporites of the basin. This brine entered in the hydrocarbon reservoir of the Lajas Formation, resulting in the precipitation of sphalerite (δ34S +10.4 to +10.8‰) and galena (δ34S +5.2‰) by TSR at temperatures of 119–123 °C together with siderite. The Andean orogeny may also have triggered the migration of hydrocarbons + brines from the shallowest source rocks (Vaca Muerta Formation) and reservoir rocks (Mulichinco, Lajas and Lotena formations) into the red beds of the upper Cretaceous Neuquén Group, which became bleached as a result. Marcasite (δ34S −36.4 to 18.2‰) and pyrite (δ34S −60.2 to −24‰) precipitated by BSR, while calcite (δ13C −12.3 to −5.6‰) formed due to redox reactions accompanying formation of quartz overgrowths, clay mineral coatings (δD −92.4‰ to −82.8‰ and δ18O 16.2‰ to 18.40‰), and barite (δ34S −5.9‰). Quartz overgrowths and barite precipitated from brines (1.5–10 wt% NaCl equiv.) at temperatures of 91–120 °C, with barite precipitating close to the feeder zones. Calcite cements are more widespread and generated from several pulses of brine and hydrocarbons up flow, evidenced by the wide range of homogenization temperatures (100–185 °C) and diversity of UV fluorescence colors of hydrocarbon-rich fluid inclusions. In subsequent Miocene tectonic events, basinal brines were able to leach Cu from the underlying thick red beds of the basin, entering the Neuquén Group oil reservoir traps or carrier beds where they became reduced. Chalcopyrite and bornite precipitated at temperatures >100 °C close to the feeder zones at the expense of barite as a local sulfur source. Chalcocite group minerals (δ34S −21.3‰ to −7.3‰) precipitated outwards in pore spaces of the sandstones at lower temperatures (<100 °C) through BSR of sulfate in the mineralizing brines. Final exhumation of the Neuquén Group (Upper Miocene-Pliocene) promoted the infiltration of the meteoric water, the oxidation of the sulfides and formation of supergene FeCu, V and U minerals. Isotope geochemistry and fluid inclusion results combined with alteration and ore mineralogy document the processes controlling the distribution of base metals in the Huincul High region that can be applied to evaluate the metal potential in other areas of the Neuquén Basin and further extrapolated to similar geological systems elsewhere.