Summary:
La Caridad is emplaced in a metallogenic province that is notable for Cu, Mo, gold (Au), silver (Ag), and platinum (Pt) Resources (Titley, 1995). It contains more than 50 deposits, some of which are considered giant ore deposits. Porphyries are Upper Paleocene to Lower Eocene in age and often exhibit supergene enrichment, which forms during exhumation of the hydrothermalized rocks towards the surface.
The largest mineralized districts in northwest Mexico occur in two main intervals, one at 59–63 Ma (Cananea), and the other at 53–55 Ma (La Caridad District), where associated magmatism overlaps in space and time. La Caridad porphyry intruded the thick-skinned Laramide orogen and is the southernmost giant porphyry Cu deposit belonging to the cluster of Arizona-Sonora Cu porphyries.
The deposit occurs exclusively in felsic to intermediate intrusive igneous rocks and associated breccias. The host rocks include diorite and granodiorite intruded by a quartz-monzonite porphyry stock and by numerous breccia masses that contain fragments of all the older rock types.
The main mineralization at La Caridad occurs in the Quartz-monzonite porphyry and hydrothermal breccias. The host rocks at La Caridad are andesites, with the oldest rocks corresponding to the Laramide volcanic rocks, which are regionally correlated with the Tarahumara Formation. These consist of lavas and tuffs of intermediate composition with aphanitic to porphyritic texture, including agglomeratic horizons and brecciated pyroclastic flows and fine-grained tuff. Locally, this andesitic volcanic sequence was intruded by a granodiorite which is well exposed to the east southeast of the La Caridad mine, which are in turn intruded by diorite dikes that range from fine to coarse grain. Discordantly overlying this igneous complex is a sequence of rhyolitic flows, dated by K-Ar at 51.3 ± 1.0 Ma.
Intense fracturing, with multiple fracture directions are observed at La Caridad. More than 560 local faults and fractures were measured in the La Caridad open pit at benches 1380 and 1305. This data exhibits two dominant trends to include the northeast (NE) and northwest (NW). Fracture density increases northward from the upper to the lower benches, consistent with the location of the mineralized center at the northern end of the open pit. Fractures appear to have provided important controls to the hypogene mineralization, particularly toward the margins of the deposit. Both pre mineral and post-mineral fractures influenced the supergene mineralization.
Primary mineralization occurs in the deposit in disseminated form, in fractures and in-filling breccia cavities. In the central part of the deposit, the presence of pyrite and chalcopyrite occurs mainly in disseminated form. There is a direct relationship between the amounts of pyrite and chalcopyrite mineralization and the quartz-sericite hydrothermal alteration. The primary disseminated mineralization occupies approximately 70% of abundance in the central part of the deposit, which as it moves away from the center toward the outside, decreases as dissemination and increases in the fractures and cavities of the breccias.
Molybdenite occurs gradually in fine aggregate crystals accompanied by variable amounts of quartz, filling thin fractures in the quartz monzonite porphyry. Generally, pyrite, chalcopyrite and molybdenite occur as a mixture filling fractures.
The Cu has been completely leached from the oxide zone of the deposit, commonly also referred to as leached exhaust. The oxide zone is thickest in the central part of the deposit and gradually decreases in thickness as it approaches the marginal areas of the deposit. The thickness of the leached zone varies from 10 m to 230 m with an overall mean of 50 m. This zone is mainly represented by Cu oxides and carbonates such as cuprite, tenorite, azurite, plus native Cu. The limits are approximately between elevations 1755 and 1665.