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

PONS M. JOSEFINA; FRANCHINI MARTA; RAINOLDI ANA L.; GIUSIANO ADOLFO; CESARETTI NORA N.; MONTAGNA ALDO O.; HERRINGTON RICHARD

JOURNAL OF GEOCHEMICAL EXPLORATION

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2021

0375-6742

This contribution discusses the discovery of Pb-Zn mineralizationin hydrocarbon exploration drill holes hosted in siliciclastic Jurassic rocksbelow the Cretaceous sediment hosted Cu mineralization located north of theHuincul 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 (MiddleJurassic), after the formation of diagenetic quartz overgrowths. Marcasite,pyrite and illite + illite-smectite mixed layer minerals then formed togetherwithhydrocarbons. Marcasite (δ34S ?3.5 and ?2.2?) andpyrite (δ34S ?11.6 to18.1?) precipitated via bacterial sulfate reduction from fluids showing temperatures<80°C and fluctuating pH conditions. During the Paleogene, Andean tectonismtriggered migration of oxidized basinal brine containing dissolved leached Znand Pb from the Los Molles Formation source rocks and S from the evaporites ofthe basin. This brine entered in the hydrocarbon reservoir of the LajasFormation, resulting in the precipitation of sphalerite (δ34S+10.4-+10.8?) and galena (δ34S+5.2?) by TSR at temperatures of 119-123°Ctogether with siderite. The Andean orogeny may also have triggered the migrationof hydrocarbons + brines from the shallowest source rocks (Vaca MuertaFormation) and reservoir rocks (Mulichinco, Lajas and Lotena formations) intothe red beds of the upper Cretaceous Neuquén Group, which became bleached as aresult. Marcasite (δ34S ?36.4to 18.2?) and pyrite (δ34S ?60.2to ?24?) precipitated by BSR, whilecalcite (δ13C ?12.3to ?5.6?) formed due to redoxreactions 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 feederzones. Calcite cements are more widespread and generated from several pulses ofbrine and hydrocarbons up flow, evidenced by the wide range of homogenization temperatures(100-185°C) and diversity of UV fluorescence colors of hydrocarbon-rich fluidinclusions. In subsequent Miocene tectonic events, basinal brines were able toleach Cu from the underlying thick red beds of the basin, entering the NeuquénGroup oil reservoir traps or carrier beds where they became reduced.Chalcopyrite and bornite precipitated at temperatures >100°C close to thefeeder zones at the expense of barite as a local sulfur source. Chalcocitegroup minerals (δ34S ?21.3? to ?7.3?) precipitated outwards in pore spaces of the sandstones atlower temperatures (<100°C) through BSR of sulfate in the mineralizingbrines. Final exhumation of the Neuquén Group (Upper Miocene-Pliocene) promotedthe infiltration of the meteoric water, the oxidation of the sulfides andformation of supergene Fe-Cu, V and U minerals. Isotope geochemistry and fluidinclusion results combined with alteration and ore mineralogy document theprocesses controlling the distribution of base metals in the Huincul Highregion that can be applied to evaluate the metal potential in other areas ofthe Neuquén Basin and further extrapolated to similar geological systemselsewhere.