Geology of the Frontino-Morrogacho Gold Mining District and Metallogeny of the El Cerro Igneous Complex

Abstract

The Frontino-Morrogacho gold district is located on the western flank of the Western Cordillera, NW of Antioquia Province. Gold mineralizations in the area are spatially and genetically associated with the cooling  of three mid- to late-Miocene age intrusive centers in the form of stocks and dikes (12-9 Ma): Cerro Frontino,  La Horqueta and Morrogacho (El Cerro Igneous Complex). These composite magmatic pulses, with  ultramafic to intermediate compositions, vary into diorite-, gabbro- and monzonitic-bearing phases.  Mineralization in the complex is present as several structurally controlled fault veins, shear-related veins,  sheeted quartz extension veins and quartz-carbonate tabular extension veins, with the development of  swarms and nests of veins-veinlets, breccias and stockworks. Structures range from centimeter-wide  individual veinlets to several meter-wide swarms of veins developed within broad mineralized structural  corridors, with a metallic signature that consists of Au + Ag + Cu + Zn + Pb + As (± Te ± Bi ± Sb ± Hg ± W)  assemblages. Veins are composed of multiple stages of mineralization, and the formation of these  structures is enhanced by the presence of a local regime of extension and E-trending structures, including  evidence of faults and shear zones with right-lateral displacement, which are likely involved in pluton  emplacement and cooling. The ore mineralogy is composed of pyrrhotite, pyrite, chalcopyrite, sphalerite,  galena and arsenopyrite assemblages formed in two or more mineralization stages, with complex Bi, Te, Sb  and Hg mineral specimens associated with Au and Ag. Mineralized structures of the district present a preferential E-strike with dominant vertical to subvertical and occasional subhorizontal S-dips and secondary N- and NW-strikes that are steep to vertically E-dipping. The Frontino-Morrogacho Gold district presents characteristics related to the architecture, mineralogy and  alteration of reduced (ilmenite-series) intrusion-related gold systems but is genetically associated with a  parental oxidized magma source. The gold content is associated with three different families involving electrum, tellurides and alloys: gold rich (66 to 78% Au, 22 to 34% Ag), average (50 to 60% Au, 40 to 50% Ag)  and silver rich (32 to 40% Au, 60 to 68% Ag). The formation of these bodies is associated with an N-S  magmatic-metallogenic trend of Au-Ag-Cu deposits, which extend for more than 300 km along the Western  Cordillera of Colombia. Similar plutonic suites span from the south of Chocó Province to the north of Antioquia Province, which indicates that the Frontino-Farallones-Botón arc can be proposed as an individual  metallogenic belt.