Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and Copper-Gold Deposits at Los Pelambres, Central Chile
José Perelló, Richard H. Sillitoe, Constantino Mpodozis, Humberto Brockway, Héctor Posso, 2012. "Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and Copper-Gold Deposits at Los Pelambres, Central Chile", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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The porphyry copper mineralization at Los Pelambres is contained in two contiguous deposits, Los Pelambres (Cu-Mo) and Frontera (Cu-Au), which together constitute the third largest copper concentration (∼36 million metric tons (Mt) Cu) in the Miocene to early Pliocene belt of central Chile. Los Pelambres is centered on a composite, N-oriented, ∼ 4.5- × 2.5-km precursor quartz diorite stock emplaced within the regional, NNW-striking, E-vergent Los Pelambres reverse fault. The fault places intensely deformed Late Cretaceous volcanic and late Oligocene to early Miocene volcanic and volcanosedimentary rocks of the Los Pelambres Formation over gently folded early Miocene volcanic rocks of the Pachón Formation. Copper-gold mineralization at Frontera is hosted mainly by andesite of the Pachón Formation.
Hydrothermal alteration at Los Pelambres-Frontera conforms to the classic zonal pattern in which a potassic center grades laterally to an annular sericitic zone surrounded by a propylitic halo. The bulk of the hypo-gene metal resource is hosted by multiple veinlet generations within potassic alteration, of which type 4 (quartz ± K-feldspar ± biotite ± sericite ± phengite ± andalusite ± corundum), A, and B types are volumetrically and economically the most important. The type 4 veinlets are regularly distributed throughout Los Pelambres and Frontera, whereas highest intensities of A and B veinlets display a spatial correlation with at least 20 small (∼ 200-m diam), SE-plunging magmatic-hydrothermal centers. These centers comprise one or more intermineral porphyry intrusions of dacitic (porphyry B) and andesitic (porphyry A) compositions along with igneous and hydrothermal breccias, the apical parts of which contain aplite and pegmatite pods. These centers acted as a series of miniature porphyry copper deposits whose coalescence generated the Los Pelambres-Frontera ore-body. This coalescence also led to deposit-scale sulfide zoning, from internal chalcopyrite-bornite through chal-copyrite-pyrite to external pyrite. Abundant hydrothermal magnetite accompanies the gold-bearing copper mineralization in biotitized andesite at Frontera. The sericitic alteration is largely pyritic, but a NE-striking, SE-dipping corridor of D-type veinlets that overprints the potassic alteration in the northwestern quadrant of Los Pelambres contains copper sulfosalts. The internal portions of this corridor are characterized by advanced argillic assemblages, defining the roots of a once more extensive lithocap.
On the basis of detailed U-Pb zircon dating, the intrusive magmatism at Los Pelambres-Frontera lasted ∼ 3.8 m.y., from emplacement of the precursor Los Pelambres stock between ∼ 14 and 12.5 Ma, through generation of numerous porphyry B and A phases and associated magmatic-hydrothermal centers between ∼ 12.3 and 10.5 Ma, to intrusion of late mineral porphyry at Frontera at ∼ 10.2 Ma. Similarly, the copper, molybdenum, and gold mineralization was introduced during a protracted interval of ∼ 1.7 m.y., between 11.8 and 10.1 Ma, as constrained by Re-Os molybdenite geochronology. The entire system cooled to nearly ambient temperatures by ∼ 8 Ma, as supported by temporally overlapping K-Ar, Ar/Ar, and (U-Th)/He ages, and was exposed to the effects of supergene oxidation and immature enrichment by ∼ 5 Ma. Plio-Pleistocene glaciation partially eroded a former, more widespread supergene chalcocite blanket, the remnants of which accounted for the bulk of the ore mined during the first 10 years of the Los Pelambres open-pit operation.
The southeast-inclined geometry of the entire Los Pelambres-Frontera system, including the porphyry centers and northeast structural corridor defined by sericitic and advanced argillic alteration, are ascribed to synmineral tilting. The tilting accompanied regional tectonic uplift during crustal shortening and thickening, which were controlled by thick-skinned reverse faults active ∼ 60 km farther east in Argentina.
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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.