Mineralogy of the Rincón Blanco selenide occurrence, La Rioja, Argentina

Abstract

Rincón Blanco is a small selenide occurrence situated to the WNW of Jagüé, a small town of the General Sarmiento department, La Rioja province, Argentina. This occurrence, also called El Chire in recent years, is located (28°35'46.10"S, 68°44'35.78"W, 3120 m. a. s. l.) in the Precordillera environment, to the furthermost southwestern corner of the selenium ore district of Los Llantenes, defined as one of the representative metallogenic belts of the Gondwanic episodes. The selenide minerals occur as veinlets up to few centimeters or as patches or fine inclusions, widespread in calcite. We have identified eleven hypogene Se-bearing minerals (naumannite, tiemannite, chrisstanleyite, jagüéite, eucairite, clausthalite, fischesserite, umangite, athabascaite, berzelianite and bellidoite) mainly associated with gold, silver, hematite and various supergene minerals, including malachite, chalcomenite, and molybdomenite. The paragenesis should have formed from low temperature (< 133 °C), neutral to mildly alkaline pH and highly oxidizing fluids with increasing sulfur fugacity. The country-rock, rich in graptolite-fauna, could be considered, at least in part, the source for the Se and associated metals forming the minerals of this paragenesis, although this hypothesis has yet to be tested.