REPLACEMENT OF DIAGENETIC FE SULFIDES BY HYDROTHERMAL CU IN A CRETACEOUS CU-AG DEPOSIT IN THE ANDES: TEXTURES, PARAGENESIS, GEOCHEMICAL AND ISOTOPIC VARIATIONS

El Soldado, Chile, is a giant strata-bound, epigenetic copper deposit hosted by Lower Cretaceous basalt and rhyodacite. Previous work suggests that copper was concentrated preferentially where hydrothermal copper-rich solutions replaced pre-existing, low-temperature, diagenetic and biogenic pyrite, which is generally associated with pyrobitumen (solidified petroleum, forming chalcopyryte, bornite and chalcocite. This study tests the above model by analyzing samples of massive pyrite-chalcopyrite veins, some in the deepest roots of the deposit. Reflected light microscopy reveals textural evidence of pre-existing diagenetic pyrite, as well as textures indicative of paragenetically later hydrothermal sulfide growth. Diagenetic pyrite is characterized by framboidal structures of ca. 16µm diameter or smaller. Massive crystalline aggregates have evolved from individual microcrysts, through framboids, framboid clusters, to recrystallized megacrystic overgrowths, and there are also banded, concentric, "colloform" zones. All of these textures are compatible with a low-temperature genesis. Replacement by copper started in the finest-grained framboidal pyrite aggregates, probably due to their relatively large surface area. Post diagenetic, hydrothermal (ca. 350^oC) pyrite includes idioblastic cubes or pyritohedra in (late) calcite gangue. Electron microprobe analyses indicate local concentrations of arsenic (0.1-2.7 wt %) in the cores of framboidal pyrite as well as fresh overgrowths on idiomorphic pyrite; X-ray mapping identifies growth banding in framboidal aggregates, and relict cores in idiomorphic pyrite, not discernable by microscopy. Pyrite enriched in cobalt (0.1 - 0.5 wt %) is common. ³⁴S_VCDT values of sulfide separates vary from -7.4‰ to + 17.1‰, a range of 24.5‰, coinciding with previous work. Several samples yielded d³⁴S values overlapping with that characteristic of magmatic sulphur (± 2 ‰), thus allowing for the possibility of some degree of input of extraneous homogenized sulphur, perhaps (but not necessarily) from a magmatic source. The study fully supports the previous model, which invoked hot basinal fluids replacing biogenic, low-temperature pyrite, to form copper sulfides.