Summary:
Ministro Hales is a mega porphyry Cu-(Mo) deposit with the superposition of hydrothermal breccias/stockwork bodies containing Cu-(Ag-As) in its upper part. The porphyry and mineralization make up an elongated body resembling a large dike. It is situated west of a major strand of the West Fault system, an arc parallel set of (early dextral, later sinistral) transcurrent fault active in Cenozoic times. East of the fault, Paleozoic metamorphic, intrusive and volcanic basement rocks dominate, covered by ca. 1000 m of Cenozoic gravels. West of the fault volcanic rocks are intruded by Eocene intrusive bodies. Hypogene mineralization is distributed within a fanning, subparallel array of subvertical hydrothermal breccia bodies with a NS to N15o W elongation, parallel to the West Fault.
Supergene enrichment and oxidation are minor west of the fault, and an exotic oxide blanket occurs in the gravels to the east. Within the deposit, the older rocks are undifferentiated green andesitic flows and breccias with chlorite, epidote and pyrite, assigned to the Triassic Collahuasi Fm., although zircon dates indicate some rocks may be Paleozoic in age. The bulk of the hypogene ore is hosted by the pervasively altered, equigranular MM Granodiorite. It has a Triassic zircon U/Pb LAM-ICPMS age, and is presently considered to be the equivalent to the Este Granodiorite of Chuquicamata.
The dike like MM Porphyry appears deep in the deposit and adjacent to the West Fault; its U/Pb LAMICPMS age in zircon is 38.9 ± 0.4 Ma. It has an aplitic groundmass and scarce quartz eyes; it displays potassic alteration with K-feldspar (as veinlets and rims on plagioclase), biotite and green-grey sericite, cut by A type quartz veinlets. MM Porphyry is cut by early B type veinlets (with axial molybdenite and chalcopyrite). It is mineralized with bornite-chalcopyrite-(digenite).
The Quartz Porphyry was intersected in one deep drillhole. It is characterized by abundant quartz eyes. It has been affected by potassic alteration with green-grey sericite, K-feldspar and anhydrite, and contains disseminated bornite and chalcopyrite in fine stockworks. This porphyry cuts mineralized and altered veins of the MM Porphyry. Its U/Pb LAM ICPMS age in zircon is 35.5 ± 0.6 Ma.
Dacite dikes belonging to more than one generation intrude all the above units along faults but are cut by hydrothermal breccias; some are Triassic. They are aphanitic with crystals of hornblende, plagioclase and scarce quartz.
Hydrothermal breccia bodies rooted in the MM Porphyry host the highest Cu grades, as well as elevated values of As and Ag. These consist of isolated angular (dacitic) rock fragments within a matrix of sulfides (chalcocite, enargite and pyrite), rock powder, silica and alunite; laterally these bodies grade into mineralized random stockworks. Where the breccias show evidence of rotation of clasts, and banding, Cu grades are highest. Advanced argillic alteration (silica, alunite, pyrophyllite, sericite and dickite) accompanies these breccias bodies. At depth they contain assemblages with bornite-chalcocite-pyrite, at medium depths chalcopyrite-tennantite-pyrite, and closer to the surface chalcociteenargite-pyrite-(bornite)-(covellite). High-grade Cu zones (>2 %) contain high As, Ag, and Zn values. Minor gold is present in some bodies. This assemblage represents a transition between a typical Cu-Mo porphyry and “epithermal” high-sulfidation mineralization.
Pebble dikes: post-ore dike-like bodies of breccia occur within the NS fault and vein system; some are up to 30 m wide. Clasts are angular to rounded in a clastic matrix, barren of mineralization; they are interpreted to be products of fluidized beds during venting of highly pressurized steam.
Sedimentary rocks and exotic ores. Eocene and younger gravels are present east of the West Fault. A 10 to 50 m thick blanket of exotic copper mineralization is found in the lower MM Gravel unit below the contact with an upper thin brown unit. The unit contains chrysocolla, Cu-bearing cryptomelane, malachite, azurite, conichalcite and smectite.