Resumen de El Distrito de oro El Bronce y su relación con la Caldera Morro Hediondo, región de Valparaiso, Chile
Francisco Camus Infanta, Ricardo Boric Perellano, Milka Alexandra Skewes
The El Bronce gold district is located in Central Chile (32° 11'S-700 56'W), and containsepithermal polimetallic (Au, Ag, Cu, Pb, Zn) vein deposits. The district has been mined since the end ofthe 18th century and its estimated production up to the present is more than 4 x lO' oz Au and 106 oz Ag, besidesan importantvolume 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 f1ows, with volcaniclastic interbeddings. An Upper Cretaceous eruptive center unconformably overlies these units cutting the volcanic sequence. The center is defined by the presence of a caldera structure ("Morro Hediondo Caldera"), of a 7 km radious semicircular section. Daeitic lapilli cuffs (K-Ar: 86 ± 3 Ma), andesitic lavas (K-Ar: 82 - 80 Ma), and breceias, assigned to the Lo Valle Formation, are the rocks associated with this center.
Two groups of intrusive rocks are reeognized in the area. The westem group is the oldest one, and is eomposed 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 (daeitic) stocks, dikes and sills, among which the outstanding components are the "Pórfido Petorea" (K-Ar: 86 ± 3 Ma) and a "Ring Dike" (K-Ar: 80- 79 Ma), whieh define the outer border of the abo ve mentioned caldera.
Extensive hydrothermal alteration zones considered as genetically associated with the intrusives, according to their spatial distribution and K-Ar data (109 :t 4; 82 ± 9; 81 ± 14 Ma) also crop out in the distriet.
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. Sorne of these struetures show a radial or concen tric pattern to me "Morro Hediondo Caldera".
About 90 ore bodies, mainly polimetallie veins, sorne eopper veins, and one eopper 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-N200E striking fault. Different lenticular, structurally controlled ore shoots are observed a10ng 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 oecur as massive concentrations, disseminated grains and in stockworks. A barren andesitic dike is also emplaced along the mineralized structure. A few meters thick, hydrothermally a1tered (sericite, c1ays, chlorite and carbonates) halo surrounds the vein
