Sistemas hidrotermales asociados a calderas en el arco volcanico paleogeno de la region de Antofagasta, Chile: Distritos El Guanaco, Cachinal de La Sierra y El Soldado

Abstract

Resumen. Los yacimientos epitermales El Guanaco (Au, Cu), Cachinal de La Sierra (Ag) y El Soldado (Ag), ubicados entre 24°52'-25°06' y 69°32'W, estan geneticamente relacionados con un sistema de calderas anidadas del Paleoceno-Eoceno Inferior. Las rocas y estructuras volcanicas del area permiten identificar las etapas del desarrollo de una caldera, Segun los modelos evolutivos aceptados. Asi, las unidades Diorita Cachinal y Lavas Andesiticas de Piroxeno (62-60 Ma) caracterizan a las etapas iniciales; la Unidad de Tobas Cachinal-Guanaco representa el gran volumen de emision de Tobas a los 61 Ma. Luego, un complejo de lavas y domos es emplazado asincronicamente de norte a sur (60-55 Ma, respectivamente). El domo resurgente se caracteriza por las tobas del piso de la caldera flectadas anticlinalmente e intruidas Por diques lacoliticos. En las etapas finales, se originaron campos fumarolicos y actividad hidrotermal asociados Con el emplazamiento de los domos. Los yacimientos El Soldado, Cachinal de la Sierra y El Guanaco se relacionan con Etapas tardias del sistema de calderas y estan controlados por sus estructuras. El mineral El Soldado tiene vetas polimetalicas Dispuestas radialmente a la caldera del mismo nombre. Cachinal de la Sierra consiste en un sistema de vetas Paralelas a la fractura del borde de la Caldera Cachinal. La asociacion de minerales de alteracion, en estos dos distritos, Es semejante a la descrita para los yacimientos del tipo Adularia-Sericita. El Guanaco esta controlado por una estructura Aproximadamente este-oeste que corta por el sur a la misma caldera. La mineralogia de alteracion presente es equivalente A la descrita para los yacimientos del tipo Acido-Sulfato o Enargita-Oro. Abstract. The epithermal deposits of El Guanaco (Au, Cu), Cachinal de La Sierra (Ag) and El Soldado (Ag) are located 24°52'/ 25°06'S, and 69°32'W, and genetically related with a Paleocene-Eocene volcanic arc. A two resurgent cauldrons model is proposed for the evolution of the volcanic sequence, between El Soldado and El Guanaco districts. This hypothesis is supported by a detailed geological mapping, petrographic studies and 26 K-Ar datings carried out on hydrothermally altered to fresh mineral concentrates of the area. The western part of the 30 km diameter cauldron (Caldera Cachinal) IS the Only one preserved, and Cachinal de La Sierra as well as El Guanaco ores are emplaced at its margin. The El Soldado ore Is located at the intersection between this cauldron and another one (Caldera El Soldado), 8 km in diameter, which was Emplaced, 7 Ma later, north of the former one. Early stages are represented by a dioritic porphyric stock (Diorita Cachinal), spatially and genetically related with pyroxene andesitic lavas (Andesitas de Piroxeno Unit), with crystallization ages of 62, 61 and 60 Ma. The Tobas Cachinal-Guanaco Unit, as a major ash-flow eruption at 61 Ma. The collapse of the Cauldron is observed at the Penafiel fracture as a remarkable N-S structure. The uplift of the resurgent dome is extended eastward where the raised floor of the caldera is depicted by local structures parallel to dacitic dikes and laccolithic Stocks. The dacitic domes (Domos Daciticos Unit) were emplaced along the Penafiel fracture. These domes were emplaced with their related lava-dome (Dacitas y Andesitas de Plateau Unit) and asynchronously emplaced from north to south (59-55 Ma, respectively). The ore deposits are controlled by the cauldron structures, and mineralization is genetically related with the latest stages of the dacitic domes, since the age of sericite in two different veins is 59 Ma. Mineralization of El Soldado deposit is emplaced in faults striking N15°W, north 01 the area. Ag and minor polymetallic Ores (Zn, Pb, Au) have been mined. The Cachinal de La Sierra veins present alteration assemblage mineralogy similar to the Adularite-Sericite type deposits, with restricted pervasive silicification of the host tuffs. Mineralization is mainly formed by argentiferous galena, pyrargirite, together with quartz (amathiste)-adularite-fluorite gangue minerals. At El Guanaco the same is related with E-W fracture-controlled strong silicification zones, surrounded by an irregularly distributed advanced argillic alteration zone (quartz, alunite, kaolinite, and dickite). The enriched ore bodies are composed of native gold, barite, hematite and alunite, located in fractures within the silica zones. Mineralization consists mainly of Pyrite, enargite, luzonite and minor chalcopyrite. These mineral assemblages are similar to those described for the Acid-Sulphate or Enargite-Gold type deposits.