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).
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 hypogene mineralization comprises pyrite, chalcopyrite and molybdenite in order of decreasing abundance, together with minor amounts of sphalerite, galena and bornite. Pyrite is by far the most abundant hypogene mineral.
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.
Chalcopyrite is most abundant in the central part of the deposit, where it has Cu grades in the order of 0.75% to 1.0%. Chalcopyrite contents gradually decrease towards the exterior of the deposit. The chalcopyrite-pyrite ratio in the primary zone is 2:1 and gradually increases towards the exterior with a ratio of 1:10.
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.
Significant amounts of molybdenite occur in the mid-central part of the deposit and grades of around 0.04% molybdenum (Mo) are found towards the east; towards the mid-western part of the deposit Mo grades are around 0.01%. Currently, the highest concentration of molybdenite in the deposit occurs within the pegmatitic zone with Mo grades in the order of 0.07 to 0.10%, associated with biotite, quartz, apatite, pseudomorphized to turquoise and sporadically, sphalerite, galena, and tetrahedrite.
The supergene sulfide zone or chalcocite zone is located at the base of the iron cap and represents the reducing environment below the paleo-water table. In this environment, Cu loses its solubility and is deposited on the hypogene sulfides, enriching them by processes of replacement of the Fe contained in them. Contrary to Cu, Fe is soluble under these conditions of low oxidation/reduction potential (Eh) and low to neutral potential of hydrogen (pH). The main constituents in this zone are chalcocite, covellite, native Cu, as well as Cu carbonates.
In porphyry Cu deposits, there is typically an increase in the quality and quantity of sulfide and oxide precipitation in zones of acidic groundwater infiltration. Typical supergene processes are solution, hydration, oxidation, precipitation, and reactions of ions in solution with ions from minerals.
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.
Pilares
The Pilares deposit consists of a volcanic breccia of lattitic composition, whose fragments are cemented by specularite and the main Cu mineralization is concentrated in the ellipsoidal-cylindrical brecciated structure found in the F and Tbl units. The shape of the blocks is variable, ranging from equidimensional to tabular; however, the finest fragments present laminar forms and develop a flow structure bordering the blocks (fragments of the blocks (larger fragments). The fragments that make up the Pilares breccia are cemented by quartz, chalcopyrite, pyrite and locally by specularite.
Based on its characteristics, Sillitoe (1985) classified the Pilares breccia as a classic magmatic-hydrothermal breccia.
A characteristic of the breccia is the orderly spatial distribution of the blocks; this arrangement resembles a jigsaw puzzle or mosaic, which ensures that the stratigraphic coherence of the lithological units is not lost. Fragment size is variable within the Pilares breccia. In general, the largest fragments are found in the center of the elliptical structure, which are mostly blocks whose diameters vary from 1 to 4 m. Toward the periphery, the size of the fragments gradually decreases, becoming much smaller at the edge of the structure (0.5 cm to 1 m).
The shape of the blocks is variable, ranging from equidimensional to tabular; however, the finest fragments present laminar forms and develop a flow structure bordering the blocks (fragments of the blocks (larger fragments). The fragments that make up the Pilares breccia are cemented by quartz, chalcopyrite, pyrite, and locally, by specularite.
The Pilares Breccia is the most important mineralized structure in the area, outcropping in the south-central portion of the area. It corresponds to a Cu mineralized structure that developed in the Andesitic Flow unit and in the Lapilli Tuff unit belonging to the upper units of the PVS, and at depth to the other underlying units. The structure has an ellipsoidal shape in plan with the major axis in NW30°SE direction and a cylindrical shape in section, the latter interpreted from drilling and direct works. The structure has a major axis measuring 550 m, the minor axis measures 250 m. The breccia is approximately 600 m deep.
Three mineralization events are recognized in the deposit, including:
- Mineralization event hosted in the ellipsoidal monolithic breccia with a hypogene origin. The best sulfide concentrations are hosted in veins, veinlets and disseminated with an annular-elliptical distribution in plan and a NW30°SE orientation. The breccia geometry and veinlet distribution define a typical chimney structure. The veinlets are mainly composed of quartz + pyrite + chalcopyrite + chalcopyrite + bornite ± galena ±magnetite.
- Mineralization event hosted in a tabular shaped breccia that overlies the ellipsoidal shaped breccia, this event has a hypogene origin: The tabular breccia has a NE60°SW orientation and is bounded by conjugate normal faults of the same orientation and is characterized by veins of specularite + quartz + chalcopyrite + chalcopyrite + pyrite + calcite.
- Mineralization event hosted in fractures and of supergene origin. Fracturing hosts secondary mineralization defined by bornite + covellite + chalcocite.