EL DISTRITO DE ORO EL BRONCE Y SU RELACION CON LA CALDERA MORRO HEDIONDO, REGION DE VALPARAISO, CHILE
Skewes, Milka Alexandra
RESUMEN:The El Bronce gold district is located in Central Chile (32°11'S-70°56'W), and contains epithermal polimetallic (Au, Ag, Cu, Pb, Zn) vein deposits. The district has been mined since the end of the 18th century and its estimated production up to the present is more than 4 x 105 oz Au and 106 oz Ag, besides an important volume of copper. The geological framework of the district is represented by Cretaceous volcanic and intrusive rocks. The oldest stratigraphic units are the Cerro Morado Formation (late Lower Cretaceous) and Las Chilcas Formation (late Lower Cretaceous-Upper Cretaceous (?)), formed by andesitie breccias, agglomerates and lava flows, with volcaniclastic interbeddings. An Upper Cretaceous eruptive centre unconformably overlies these units cutting the volcanic sequence. The centre is defined by the presence of a caldera structure ("Morro Hediondo Caldera"), of a 7 km radius semicircular section. Dacitic lapilli tuffs (K-Ar: 86±3 Ma), andesitic lavas (K-Ar: 82-80 Ma), and breccias, assigned to the Lo Valle Formation, are the rocks associated with this centre. Two groups of intrusive rocks are recognized in the area. The western group is the oldest one, and is composed of quartz monzodiorites belonging to a batholith dated (K-Ar) as 134 - 86 Ma, which extends between 31° and 32°S. The eastern group is younger and is composed by dioritie (andesitic) to granodioritic (dacitic) stocks, dikes and sills, among which the outstanding components are the "Porfido Petorea" (K-Ar: 86±3 Ma) and a "Ring Dike" (K-Ar: 80-79 Ma), which define the outer border of the above mentioned caldera. Extensive hydrothermal alteration zones considered as genetically associated with the intrusives, according to their spatial distribution and K-Ar data (109±4; 82±9; 81±14 Ma) also crop out in the district. The rocks of the area are cut by a number of mainly NW striking faults and dikes, which played an important role in the control of mineralization. Some of these structures show a radial or concentric pattern to me "Morro Hediondo Caldera". About 90 ore bodies, mainly polimetallie veins, some copper veins, and one copper breccia pipe (Dulcinea Mine) have been identified. The most important and productive ore deposit is the El Bronce vein, emplaced in a 7 km long, NS-N20°E striking fault. Different lenticular, structurally controlled ore shoots are observed along the fault, ranging from 100-600 m in length, 200-400 m in depth, and 1-20 m in width. The mineralogy consists of pyrite, sphalerite, chalcopyrite, galena, tennantite-tetrahedrite, bornite, arsenopyrite, pyrrhotite, hematite, and magnetite in that order of abundance. Gangue minerals are quartz (massive, microcrystalline, amethyst, and crystalline-transparent), ankerite, siderite, and barite. These minerals occur as massive concentrations, disseminated grains and in stockworks. A barren andesitic dike is also emplaced along the mineralized structure. A few meters thick, hydrothermally altered (sericite, clays, chlorite and carbonates) halo surrounds the vein. According to paragenetic studies, mineralization would have been produced in three stages, gold and silver being deposited during the two earlier ones, associated with quartz, pyrite, sphalerite and chalcopyrite. Fluid inclusion studies show homogenization temperatures ranging between 150° and 349°C, and salinities (weight %NaCI) from 1 to 10, while sulfide isotope analyses yielded values of 34S between +0,5 and +2,3. The above data support the hypothesis that the ore was formed at a depth of less than 1,000 m below paleosurface, due to a mixing process between hydrothemal magmatic ore fluids and meteoric ground waters. Keywords: Paragenesis, Hidrothermal alteration, El Bronce district, Epithermal ores, Gold, Morro Hediondo caldera, K-Ar Dating, Fluid inclusions, Upper Cretaceous, Valparaiso Region, Chile.