Mineralogy and geochemistry of carbonaceous mudstone as a vector to ore: a case study at the Lagunas Norte high-sulfidation gold deposit, Peru

Abstract

The Lagunas Norte Au deposit in the Alto Chicama district of Peru is a ~14 Moz high-sulfidation epithermal deposit that is hosted by an atypical host-rock succession for this deposit type. Approximately 80% of the ore body is contained in the Lower Cretaceous Chimu Formation, which is composed of quartz arenite with interbedded carbonaceous mudstone, siltstone, and coal seams. The remainder of the ore is hosted by the Miocene volcanic rocks of the Calipuy Group, forming an irregular and thin veneer on the deformed sedimentary rocks of the basement. The host rock succession of the Lagunas Norte deposit has been affected by widespread hydrothermal alteration. The alteration is cryptic within most of sedimentary rocks as the quartz arenite was largely inert to alteration by the strongly acidic fluids. Vuggy textures associated with residual quartz alteration can only be recognized in the overlying Miocene volcanic rocks. However, the present study shows that mudstone of the Chimu Formation records acid-type alteration due to its originally high clay mineral content. The mineralogy of the mudstone broadly changes from residual quartz to quartz-alunite-pyrophyllite-kaolinite/dickite to illite-smectite away from the deposit. Mineralogical changes are paralleled by notable variations in the whole-rock major and trace element composition of the mudstone, allowing the definition of a set of alteration vectors to ore. Textural evidence suggests that alteration of the host rocks at Lagunas Norte predated precious metal mineralization. Residual quartz formed during the early acidic alteration is overgrown by later euhedral quartz that is texturally associated with the ore minerals. Fluid inclusion data suggest that the euhedral quartz and associated ore minerals formed from a low-temperature, low-salinity liquid. This liquid infiltrated the previously altered host rock succession, forming the sulfide ore zones of the epithermal deposit. Subsequent supergene weathering resulted in deep oxidation of the ore body.