Patterns of mineralization and alteration below the porphyry copper orebody at El Salvador, Chile

DOI: https://doi.org/10.2113/gsecongeo.90.1.2

Three diamond drill holes, angled below the lowest haulage level at El Salvador, have doubled the vertical exposure of the deposit and revealed very different features of alteration and mineralization below this major porphyry copper orebody. Sulfide assemblages persist with depth, but the total sulfide content diminishes. Magnetite becomes a part of all sulfide assemblages, except very late pyritic D veins. Residual traces of pyrrhotite-chalcopyrite found locked in quartz and as abundant and wide- spread inclusions in pyrite apparently represent the remains of an early prograde mineralization obliterated by intense sulfidation of subsequent events. Relicts of specularite veinlets may be a similar phenomenon. Vein types change. Newly recognized, early biotitic (EB) veinlets, with and without sulfides, quartz, albite, anhydrite, and actinolite, have varied alteration halos containing albite, K feldspar, biotite, green sericite, anhydrite, and andalusite. They appear to be deeper equivalents of A quartz veins. Veinlets descriptively similar to both EB and A quartz veins formed as a second generation within the young intramineral L porphyry complex which truncates similar veinlets in older and better mineralized rocks. Granular A quartz-K feldspar-sulfide-anhydrite veins diminish in abundance and in content of sulfide and K feldspar with depth, and are hard to distinguish from B quartz-anhydrite veins with characteristic molybdenite. The latter have much better developed K feldspar alteration halos than seen above. Younger G sulfide veins with green sericite, biotite, and anhydrite, and halos with green sericite, alkali feldspar, and andalusite, cut B veins. They are older than relatively sparse D pyrite- quartz veins with sericite-pyrite-calcite-anhydrite halos and occasional tourmaline. Pervasive sericite- chlorite in the pyritic fringe terminates downward and biotitic alteration of andesite diminishes, revealing more restricted and residual actinolite hornfels. Ilmenite and then sphene appear as residual accessory minerals and minor vein constituents. Minor andalusite with alkali feldspar extends to deepest exposures, mostly within halos of B and C veins. Traces of corundum and cordierite occur with andalusite...