Geological and Geochemical Characteristics of the IntrusionRelated Vein-Type Gold Deposits in the El Morado District, Coastal Cordillera, Northern Chile

摘要

El Morado is the only auriferous district in the north Chilean coastal Cordillera, and has breccia-associated Au-Fe-Cuvein-type gold deposits, such as Beatriz and Lilianita. In addition to geological and mineralogical descriptions, geochemical characterization of the two vein deposits, and fluid inclusion and oxygen isotope analyses of the Lilianita quartz vein were performed with the objective to elucidate their ore-forming characteristics. The two deposits are hosted in Late Jurassic to Early Cretaceous dioritic intrusions, and are characterized by the oxidized mineral association of electrum (Au75Ag25 to Au85Ag15), chalcopyrite, hematite and magnetite. Gold content of vein samples weakly correlates with Ag contents, but is not correlative with Cu contents. Fluid inclusion data of the vein quartz indicate relatively high-trapping temperature (290 to 340(circle)C) and low salinity (3.2 to 13.1 wt% NaCl) conditions with low-pressure boiling evidence. The d(O)(18) values of the corresponding quartz samples are in a narrow range of + 11.1 to + 12.5 %. These data combined with the fluid inclusion thermometric data suggest that the quartz-mineralizing fluid with d(O)(18) values between + 4.9 and + 6.2 % was derived from primary magmatic water with lesser amounts of low-temperature surface water, such as seawater or meteoric water. All these results show that the gold deposits in the El Morado district are intrusion- related oxidized veins, which formed from magmatic-hydrothermal fluids at relatively lowpressure shallow conditions, compared to the Cu-rich iron oxide-copper- gold (IOCG) vein deposits in the same province.