Petrography, geochemistry and stratigraphic relationships of outcropping units in the Cerro Las Minas sector, Andacollo Mining District, Neuquén, Argentina

Abstract

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The Andacollo Mining district is located on the western flank of Cordillera del Viento, Neuquén province, Argentina, and acquires relevance for its Au/Ag vein deposits. The present work provides the petrographic and geochemical analysis of rocks outcropping in the Las Minas hill that host vein deposits. In the area, the pre-Permian rocks mainly correspond to rhyolitic vitreous to lithic tuffs of the Arroyo del Torréon Formation which are intruded by the Dacita Sofía, a Carboniferous subvolcanic body. They are covered by sedimentary rocks of the Huaraco Formation. The Permian is represented by I and S types granites and granodiorites of the Huingancó Granite within the Huingancó volcanic-plutonic Complex. The post-Permian rocks are silicic rhyodacitic ignimbrites and andesites of the Cordillera del Viento Formation. All analyzed rocks have SiO₂ values between 61 and 78 %, and show arc-like signatures in active continental margins. The rocks present a general enrichment in LILE (large-ion lithophile elements), Nb, Sr, P and Ti anomalies, enrichment in REE relative to chondrite and enrichment in LREE (light-rare earth element) with respect to HREE (high-rare earth elements).  The highest enrichment patterns are presented by post-Permian rocks. Unlike the pre-Permian and Permian rocks, post-Permian rocks show an absence of negative Eu anomaly.