Vanadium mineralization at Los Chihuidos sediment-hosted stratiform Cu deposit

RAINOLDI, ANA LAURA; FRANCHINI, MARTA; BEAUFORT DANIEL; PATRIER PATRICIA; GIUSIANO, ADOLFO; PONS, JOSEFINA; CESARETTI, NORA NOEMÍ

Salta

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

BackgroundSediment-hosted stratiform copper (SSC) deposits represent the main metalliferous resource of the Neuquén Basin (Argentina). These deposits are spatially linked to oil and gas reservoirs and contain minor amounts of V, U and Ag. Los Chihuidos deposit outstands because contains major amounts of vanadium compared with the other SSC deposits of the basin. The Los Chihuidos deposit (37°50´30"S; 69°27´W) is located in the central part of the Neuquén Basin, near the Sierra Chata gas reservoir, and it is hosted in the Huincul Formation (Late Cenomanian-Early Turonian) of the Neuquén Group. The Neuquén Group represents a red bed sequence, but at the site of mineralization, the rocks are bleached. Bleaching resulted in colorful outcrops variations with the original red sandstone interbedded with white sandstone, whereas gray and brown sandstones are subordinated to the contact between the formers. The highest anomalies of V are always located below zones with high Cu grades, but at regional-scale, V is always present at the contact between the red and the overlying bleached sandstone, where gray sandstones occur. Uranium anomalies are more erratic, always associated with vanadium anomalies and characterized by the presence of U-V supergene minerals (i.e. carnotite group). The main differences between the sandstones with different colors are due to variations in the porosity and in the mineralogy of the authigenic minerals. This contribution presents the first mineralogical study of the vanadium-rich zones at Los Chihuidos, and discusses plausible sources of vanadium and the mechanisms for the metal precipitation, which has important implications for understanding the formation of vanadium mineralization in the SSC deposits of the Neuquén Basin.MethodsRepresentative samples of diagenetic facies from selected profiles were collected for laboratory analysis. Samples were analyzed first with a binocular polarizing microscope under transmitted and reflected light. Clay minerals were identified using a Bruker D8 Advance diffractometer. Representative samples were selected for SEM observations; paragenesis and chemical composition of mineral assemblage were studied with a JEOL® 5600 electron microscope equipped with a Bruker energy dispersive X-ray spectroscopy detector (EDS).ResultsRed sandstone of the Huincul Formation consists of feldspathic litharenite cemented by hematite coatings, pore-filling kaolinite, quartz and albite overgrowths, calcite I and minor barite and anhydrite. The red color of the sandstone is due to hematite coatings. White sandstone is characterized by a notorious porosity enhancement and abundant bitumen impregnations. Hematite coatings are lacking, only present below quartz and albite overgrowths. Previous kaolinite, calcite I, barite and anhydrite are locally to pervasively dissolved and new authigenic minerals include pyrite, calcite II and minor V-Cu-rich montmorillonite. Copper mineralization occurs in white sandstone in contact with bitumen impregnations and consists of chalcocite-djurleite with traces of very fine-grained pyrite replaced by supergene chrysocolla > malachite > covellite > brochantite > cuprite.Gray sandstone contains abundant bitumen impregnations, authigenic V±Cu-bearing clay minerals (montmorillonite, interstratified illite-smectite and subordinated chlorite-corrensite mixed-layers) and V-bearing hematite. Hematite can be also found in detrital martitized titanomagnetite, where Ti concentrations occur in magnetite and V in hematite. When Cu±V-rich chlorite-corrensite mixed-layers predominate over montmorillonite, the sandstone turns to brown color, defining the brown sandstone. Rarely, a secondary mineral enriched in Mn has been identified replacing chlorite-corrensite mixed-layers. Supergene volborthite and Cu-V-U minerals with high amount of V-rich clays are located at the contact between the gray and white sandstones, where copper mineralization occurs. Vanadium-rich montmorillonite, interstratified illite-smectite and hematite in the gray sandstone represent more oxidizing conditions, whereas Cu-rich chlorite-corrensite mixed-layers indicate chloritization of montmorillonite under more reducing conditions. Vanadium could have been carried by hydrocarbons and incorporated in clay and oxide minerals. The occurrence of vanadium has been observed previously in the Neuquén Basin in whole rock analysis from the Vaca Muerta Formation (Spaletti et al., 2014) but also worldwide in oil fields and hydrocarbon source rocks (Breit and Wanti, 1991; Parnell and Carey, 1995). The analyses of bitumen infill of veins (asphaltites) from the La Escondida deposit located 60km east of Los Chihuidos, yielded up to 0.2 wt.% vanadium (Parnell and Carey, 1995).ConclusionReddening of the sandstone took place during early-diagenesis when infiltration of meteoric water promoted hematite coating precipitation. With burial increases, kaolinite, quartz and albite overgrowths and calcite I precipitated. During the Andean tectonism, upflow of hydrocarbon-bearing fluids up to the red beds of the Huincul Formation promoted the development of a redox system where most cements including hematite were dissolved, iron was reduced and precipitated as pyrite, promoting the discoloration of the previous red sandstone. Hydrocarbons were oxidized and calcite II precipitated as by-product of the redox reaction. Vanadium-rich montmorillonite precipitated with pyrite and calcite II. Hydrocarbon-bearing solutions reached intermediate redox conditions at the contact zone with the white (reduced) and red (oxidized) sandstones. In this zone, abundant V-Cu-rich clay minerals (montmorillonite, illite-smectite and chlorite-corrensite) with V-bearing hematite and a Mn-rich mineral precipitated, defining the redox front. This mechanism could favored the destabilization of the soluble complexes generated during bleaching of the sandstone, precipitating the highest concentrations of V-rich montmorillonite, interstratified illite-smectite and interstratified chlorite-corrensite along the redox front, where the oxidizing-reducing conditions promote precipitation of such metallic elements whose speciation is very sensible to Eh conditions. A new stage of fluids migration resulted in copper mineralization in the white sandstone in contact with hydrocarbons. Minor copper was also incorporated in the gray and brown sandstones minerals like montmorillonite, illite/smectite and chlorite-corrensite mixed-layers. During the Andean uplift, infiltration of meteoric water promoted supergene alteration with precipitation of volborthite as oxidizing product of the vanadium ore at the redox front and, covellite, cuprite, brochantite, malachite/azurite and chrysocolla in the reduced white sandstone.