Study on gold mineralization in the El Callao Mining District, Venezuela. Implication of "invisible" gold remobilization for the deposit formation.

The Colombia Mine is localized in the El Callao Mining District Venezuela. It has Au reserves of 23 million ounces (= 740 t Au), which make it the largest gold mine in Guayana Craton, and grant it the label of giant gold deposit (i.e. > 500 t Au). The mineralization is hosted by the Paleoproterozoic El Callao formation, which is constituted by Fe-rich tholeiitic basalts that were formed in a plateau oceanic setting. Exploitation in the mine is focused on three "vein systems", known as América, Colombia and Hansa quartz veins. These veins systems are actually constituted by a network of interconnected quartz, albite and ankerite veins, which form a large stock enclosed between a hanging wall and a footwall. These interconnected veins include a large number of centimeter- to meter-size altered metabasaltic fragments, rich in pyrite and containing most of the gold mineralization. In the fragments, gold is always closely related to pyrite, either as inclusions, or filling fractures within it. We interpret these vein systems as a shear-fracture hosted mesh, formed in a fragileductile transitional deformation regime by a succession of micro-seisms. Each of these micro-seisms was accompanied by crystallization of a new core/rim pyrite sequence. Furthermore, local change in confining pressure from supra-lithostatic to near-hydrostatic accompanied each pressure drop causing boiling of the hydrothermal fluid, which, in turn, promoted precipitation of "invisible" gold. Later, during pyrite deformation, fluids reacted with pyrite crystals causing coupled dissolution-reprecipitation reactions. In this case, Au and others metals (i.e., Cu, Te, Bi, Pb, Sb) were lixiviated from pyrite and reprecipitated as visible gold grains, found either in inclusions or in fractures within pyrite, and are almost always accompanied by inclusions of chalcopyrite, tellurobismuthite, sphalerite and ankerite. We conclude that all visible gold in the Colombia Mine is secondary, and that it originated from remobilization of primary, invisible gold. For this reason, we consider that economic gold mineralization in the Colombia mine was favored by the brittle deformation that that took place relatively late in the genesis of the vein network, after hydrothermal pyrite precipitation.