HYDROCARBONS LINKED TO STRATIFORM SEDIMENT HOSTED LEAD, ZINC AND COPPER MINERALIZATION IN LAJAS FORMATION, NEUQUÉN BASIN ARGENTINA

PONS, MARIA JOSEFINA; ANA LAURA RAINOLDI; MARTA FRANCHINI; ADOLFO GIUSIANO

Salta

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

SEGEMAR, AGA

Rocks mineralized with lead, zinc and copper sulfides were discovered at ~1000 m depth below the BardaGonzález and La Cuprosa stratiform sandstone-hosted copper deposits (SSHCD) at Aguada Baguales oil field, Neuquén Basin. Mineralized rocks occur in fluvio- deltaic sandstones of Lajas Formation (Aalenian-Bajocian) which is acurrent oil and gas reservoir rock. Mineralized rocks have been intersected by drill core AB-1010 in Aguada Baguales oilfield (973-1088 mbsl).Zinc, lead and copper minerals fill the porosity of coarse-grained facies from bed-load deposits irregularly distributedin the upper, middle, and lower section of the Lajas Formation. They consist of lithic feldspathic conglomerate andsandstone with quartz (40-35%), feldspar (15-20%), lithics (45-39%) and zircon as accessory. The porosity varies from3 to 15%, depending on the amount of diagenetic minerals. These rocks show moderate and high proportion ofhydrocarbon impregnations with yellow fluorescence. Six stages of porosity infill were recognized: 1) early quartzovergrowths 10-100 μm thick, 2) interstitial gypsum, 3) marcasite±illite (illite-smectite), 4) siderite, 5) Fe, Zn, Cu andPb sulfides and, 6) late well-crystallized kaolinite.Quartz overgrowths and detrital quartz are partially dissolved and covered by bitumen impregnations, earlymarcasite fills the pores and illite + illite-smectite mixed layers rim detrital grains. Locally, marcasite replacesprevious gypsum cement. Mn-bearing siderite (MnO up to 10%) fills the remained porosity and is replaced bypatches of galena, iron-poor sphalerite (< 1% Fe) and traces of chalcopyrite. Sphalerite is the most abundant sulfide,occurs as yellow, anhedral grains or as fibro radial crystals sourrounding black spots of solid hydrocarbons or previousmarcasite. Probably, this fibro radial habit resulted as pseudomorphs of early fibro radial marcasite. Late kaoliniteand calcite seal the remaining porous spaces.Isotopic sulfur values from marcasite varies between -3.5 to -2.2? (n=2), sphalerite presents heavier andhomogenous values between +10.4 to +10.8? (n=3) and galena shows an intermediate value with +5? (n=1). The equilibrium isotopic temperature calculated for sphalerite and galena was 120-130 °C using the equation of Liu et al.(2015).At Aguada Baguales oil field, Lajas Formation shows textural evidence that marcasite ± illlite and illitesmectitemixed layers are epigenetic, precipitated after quartz overgrowths and formed in contact with liquidhydrocarbons. Marcasite shows a maximum sulfur isotope fractionation δ34S ~19.2 to 33.5?, considering, that themain sulfur source could be the dissolution of evaporate layers of the basin with a δ34S between 17 to 30? (Lo Forte et al., 2005, Cavallaro et al., 2007). This fractionation range is just between the values obtained from biogenicsulfate reduction -BSR- (>20?) and those from thermochemical sulfate reduction -TSR- (H2S), capable of transporting Pb, Zn, Cu, and Fe (Cooke et al., 2000). In consequence, siderite representsthe first product of the brines-hydrocarbons interactions at temperatures lower than 150 °C and pH between 5 to 7(Cooke et al., 2000). The continue interaction of these oxides fluids with interstitial hydrocarbons produce additionaldecrease in oxygen fugacity and the precipitation of Zn, Pb and Cu sulfides after siderite formation (Cooke et al.,2000). Possibly, sulfur source of these later sulfides could be previous marcasite and the sulfate-bearing brines. Inconsequence lower isotopic fractionation (6.7 to 24?) occurred during Zn, Pb, Cu mineralization (galena, sphaleriteand traces of chalcopyrite) via TSR at slightly higher temperature than the previous marcasite (Machel et al., 1987).It is consistent with isotopic equilibrium temperature of 120-130 °C calculated for galena-sphalerite. This temperatureis coherent with the temperature (140 °C) obtained by Gómez et al. (2008) from microthermometric analyses offluid inclusions hosted in saddle dolomite from Lajas Formation in Puesto Gregor area. In consequence, the mainfactor to produce an abrupt change in solubility of these base metals was a decrease in oxygen fugacity when metalbearingoxidizing brines arrived into the oil reservoir rocks.New mineralogical, textural and analytical evidences indicate that Cu, Zn and Pb mineralization was epigeneticand results from the interaction of hydrocarbons with host rocks and metal-charged basinal brines via TSR, and markthe potentiality of Neuquén basin to host SSH-metal deposits of economic interest.