- The Inmaculada Mine is located in the Miocene belt of epithermal deposits that extends from southern Peru to northern Chile and Bolivia. This belt is known for its silver-rich epithermal veins that have been worked since colonial times. The Inmaculada Mine belongs to a mining district that includes, from north to south, the Selene, Pallancata, and Inmaculada mines. This study presents a new geologic map of a 100 km² area, including the Inmaculada Mine at a scale 1:25,000 and the results of laboratory studies that include petrography, geochronology, whole-rock geochemistry, as well as electron microprobe mineral analysis. Also, it reports adularia ages for samples from the Pallancata Mine.
The oldest dated volcanic rocks in the area, Aniso Rhyolite Volcaniclastics, Domes and Tuffs, are found north of the Inmaculada Mine and consist of volcaniclastic rocks, minor lacustrine limestone, and interbedded rhyolite tuffs which are intruded by rhyolitic dikes and domes. They are interpreted to potentially belong to an intra-caldera environment. The ⁴⁰Ar/³⁹Ar ages of sanidine from tuffs and rhyolite intrusions yielded two population a) ~ 23.7 Ma and b) 22.6 Ma, which are similar to the ages of regional Nazca 1 and Nazca 2 Ignimbrites.
The next youngest unit in the area is the Huallhua Rhyolite Ignimbrite, which is as thick as ~850 m was erupted at ~13.2 Ma. It crops out to the south of the Inmaculada Mine and consists of poorly sorted, massive, lithic-rich, poorly welded, and crystal-poor rhyolite tuff containing previously altered accidental fragments. The rock shows little variation in welding and varies from 62-74 wt% SiO₂ in composition. It is proposed that the contact between the 23-24 Ma Aniso Rhyolite Volcaniclastics Domes and Tuffs and the ~ 13.2 Ma Huallhua Rhyolite Ignimbrite is the San Salvador fault. This fault is the border of a proposed caldera named the Alpabamba caldera located southwest of the Inmaculada Mine. Immediately after the caldera collapse, the ~13.2 Ma Inmaculada Volcanics erupted, which range in composition from andesite to dacite and probably to rhyolite. Two pyroxene barometry suggests that the depth of pyroxene crystallization of the Inmaculada Volcanics ranges from 10 to 22 km. Therefore, it is proposed that after the rhyolite pyroclastic eruption and the caldera collapse, intermediate composition magma rose from a source at 10 to 22 km and erupted as the Inmaculada Volcanics.
The Inmaculada Mine is hosted in veins cutting the Inmaculada Volcanics and has two epithermal alteration styles, quartz-alunite, and quartz-adularia-illite, which may be genetically related. Only the quartz-adularia-illite alteration is associated with economic Ag-Au mineralization. Two Ar-Ar ages of adularia from Inmaculada yielded 12.42 ± 0.03 Ma and 12.42 ± 0.05 Ma; consequently, the quartz-adularia veins formed ~ 0.8 Ma after the eruption of the dated Inmaculada Volcanic host rocks. In the context of Andean uplift, the ~0.8 Ma time-period may have allowed the formation of a hydraulic gradient, which drove a lateral flow of hydrothermal fluids along the km-long northeast-southwest oriented vein system. The sphalerite-galena-pyrite-chalcopyrite-acanthite mineral assemblage indicates that the Inmaculada Mine is an intermediate sulfidation state epithermal deposit.
After the mineralization event, a ~10 Ma hornblende andesite was emplaced and is exposed to the northeast of the Inmaculada Mine. It is likely a subvolcanic intrusion. Then, the ~ 9.2 Ma Huancarama Rhyolite Ignimbrite covered the area of study. Based on its mineral assemblage plagioclase-quartz-sanidine-biotite-hornblende, it is thought to have erupted from an upper crust magma chamber. The igneous activity finalized at ~ 6.9 Ma when both the Chibchi Rhyolite Tuff and the Coñaccahua Andesite Lavas erupted. They crop out to northwest of the Inmaculada Mine.
Previous studies from the Selene Mine (Explorador vein) reported 14.2 ± 0.2 Ma (⁴⁰Ar/³⁹Ar of whole rock) as the possible age of Ag-Au mineralization. Adularia samples from the Pallancata Mine yielded ages of ~13.4 Ma. Therefore, there was a north to south migration of hydrothermal, and presumably causative magmatic activity in the district, from Selene to Pallancata to the Inmaculada Mine (~12.4 Ma) over a period of about 2 Ma.