Role of thermochemical and biogenic sulfate reduction in the formation of the El Porvenir sediment-hosted stratiform copper deposit, Neuquén, Argentina

RAINOLDI, ANA LAURA; FRANCHINI, MARTA; BOYCE, ADRIAN; GIUSIANO, ADOLFO; CESARETTI, NORA NOEMÍ; PONS, JOSEFINA; RÍOS, FRANCISCO

Salta

Congreso; 15th Quadrennial International Association on the Genesis of Ore Deposits Symposium; 2018

BackgroundThe El Porvenir deposit is one of the numerous sediment-hosted stratiform copper (SSC) occurrences and deposits of the Neuquén Basin (Argentina). It is located in the west sector of the Huincul High structure, near hydrocarbon fields, and is hosted in the Huincul Formation (Late Cenomanian-Early Turonian) of the Neuquén Group. The mineralization at El Porvenir occurs in bleached sandstone, associated with bitumen, whereas fine-grained sandstone and siltstone remain red, reflecting a strong control of porosity and permeability in both alteration and ore formation. In the last decades, several hypotheses on the formation of these SSC deposits have been proposed, however, a comprehensive genetic model for the deposits of the Neuquén Basin is currently lacking. In this contribution, we present the summary of a petrographic and geochemical study of the El Porvenir deposit and discuss the processes responsible for the copper mineralization.MethodsRepresentative samples of fresh, altered and mineralized rocks from selected profiles and drillholes were collected for laboratory analysis. Polished thin sections were examined with a polarization-fluorescence microscope for fluid inclusion petrography and identification of trapped hydrocarbons. Microthermometric analysis were performed in fluid inclusions host in analcime and calcite cements using a Linkam MDS 600 (-180º/+600ºC) fluid inclusion cooling-heating stage at the Universidad Nacional del Sur and a Linkam FTIR600 stage at the Centro de Desenvolvimento da Tecnologia Nuclear. Sulfur, carbon and oxygen isotope analyses were carried out in sulfide and calcite at the Scottish Universities Environmental Research Centre, Scotland. For sulfide, purified gases were analyzed on a VG Isotech SIRA II mass spectrometer and for carbon isotope analyses, carbon dioxide was produced from calcite by reaction with 100% phosphoric acid and analyses were performed on an AP 2003 mass spectrometer. Oxygen, carbon and sulfur data are reported relative to the V-SMOW, V-PDB and V-CDT, respectively. ResultsThe Huincul Formation consists of well sorted and texturally immature feldspatic litharenite that was originally red. However, at the site of mineralization, this rock is now bleached. It contains lithic fragments (with predominance of basic and mesosilicic volcanic fragments), quartz, feldspar and accessory minerals. In the red sandstone, diagenetic minerals include hematite coatings, Na-rich corrensite, microquartz envelopes and quartz overgrowths, calcite I and analcime. Analcime hosts primary, two-phase fluid inclusions with Na-Mg-Cl-H2O fluids with salinities between 10.5 and 21.3 wt % NaCl equiv. that homogenize at an average temperature of 100ºC. In the white sandstone, hematite coatings are absent but remained preserved below quartz overgrowths. The interlayer charge of corrensite is compensated by Ca instead of Na, calcite I and analcime are dissolved, and pyrite (δ34S +10.2?) and calcite II (δ13C -12.9 to -8.3? and δ18O +18.6 to +22.2?) precipitated. Calcite II hosts aqueous fluid inclusions with Mg-Ca-Na-Cl-H2O fluids with salinities between 1.6 and 5.9 wt % NaCl equiv. that homogenize at an average Th of 133ºC. Aqueous fluid inclusions coexist with hydrocarbon-bearing fluid inclusions where the liquid shows blue fluorescence under UV light and formation of clathrates indicate the presence of carbonic species.Hypogene ore consists of chalcopyrite and bornite (δ34S +12.3?) after pyrite accompanied by coarse-grained calcite IIIa (δ13C -19 to -10.1? and δ18O +18.5 to +24.3?). Microthermometric studies in primary fluid inclusions from calcite IIIa indicate the presence of Na-Ca-Mg-Cl-H2O fluids with salinities between 10.1 and 21.3 wt % NaCl equiv. that homogenize at an average Th of 145ºC. Minerals from the chalcocite series (δ34S -11.7 to +4.1?) replace previous Fe-Cu sulfides and precipitated in the pore space with fine-grained calcite IIIb (δ13C -31.4 to -29.9? and δ18O of +17.7 to +20.6?). Fluid inclusions in calcite IIIb indicate the presence of Na-Ca-Mg-Cl-H2O fluids with salinities between 17.5 to 19 wt % NaCl equiv. that homogenize around 90ºC; aqueous inclusions coexist with one-phase fluid inclusions filled with oil, which shows a yellow fluorescence under UV light.ConclusionsThe sedimentary rocks from the Huincul Formation were subject to several stages of fluid-rock interaction. The spatial relationships between red, bleached and mineralized sandstones together with petrographic and geochemical evidence point to three distinctive episodes: diagenesis of red bed, bleaching of red bed and ore precipitation. The Huincul Formation was deposited in a fluvial system under oxidizing conditions, promoting the precipitation of hematite coating, which resulted in the reddening of sandstone. Arid climatic conditions likely favored the precipitation of Mg-rich smectite from alkaline water. With increasing temperature, Mg-rich pore water continued reacting with sediments transforming the smectite into corrensite with Na in interlayer position. Diagenetic quartz was deposited as microquartz envelopes or quartz overgrowths, covering the hematite coatings. Late cementing minerals like calcite I and analcime precipitated at higher temperature (~100ºC) from Mg and Na-rich saline connate fluids.During the Tertiary Andean tectonism, reactivation of faults promoted the migration of hydrocarbons, organic acids and formation water from the deeper source or reservoir rocks up to the Huincul Formation. These fluids reacted with the sandstone and oxidizing formation water causing the dissolution of early cements, including hematite. Ferric iron was reduced and precipitated as pyrite, bleaching the host rocks, whereas hydrocarbons were oxidized. Calcite II precipitated as by-product of the redox reaction from carbon sourced by oxidized hydrocarbons and extraformational saline (Na-Ca-Mg-Cl), warm and isotopically evolved oilfield fluids (δ18Ofluid of +3.5 to +8.2?), which also promoted the exchange of Na by Ca in corrensite. During subsequent uplift stages, incoming of metal-charged basinal brines precipitated sulfides characterized by an overall progressive and marked lowering of δ34S and δ13C from chalcopyrite-bornite (δ34S +12.3?) and calcite IIIa (δ13C -12.9 to -8.3?) to chalcocite-spionkopite (δ34S -11.7 to +4.1?) and calcite IIIb (δ13C-31.4 to -29.9?). The temperature also decreased from the formation of chalcopyrite-bornite-calcite IIIa (145ºC) to the formation of chalcocite-spionkopite-calcite IIIb (90ºC).Based on the isotopic and fluid inclusion data, ore precipitation was characterized by a Fe-Cu ore stage generated by thermochemical sulfate reduction and a final Cu stage dominated by biogenic reduction, using hydrocarbon as a reducing agent. The δ18O (+5.6 to +11.4?) of the fluids in equilibrium with calcite IIIa are similar to oilfield and basinal brines whereas fluids in equilibrium with calcite IIIb show a significantly lower δ18O (-0.6 to +2.3?). Lowering of δ18O in calcite IIIb, could be related to the incoming of cooler, metal-charged fluids and/or due to the infiltration of meteoric water during the Tertiary Andean uplift, facilitating bacterial activity. The results show that Cu ore was deposited - and refined - during a series of events that developed in an evolving basin, involving multistage reducing fluids, Cu-sulfate-bearing brine, and meteoric water penetrating through the Cretaceous red beds in response to Andean tectonism.