GeoScienceWorld
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Andean Copper Deposits:

New Discoveries, Mineralization, Styles and Metallogeny

By Francisco Camus, Richard M. Sillitoe and Richard Petersen

Abstract

The copper deposits of Perú consist of porphyry Cu±Mo, Au, Ag, breccia pipe Cu-Mo, enargite vein and replacement Cu±Au, Ag, Zn, Pb, calcic skarn Cu±Fe, Au, Zn, amphibolitic skarn Cu±Fe, volcanogenic massive sulfide Cu-Zn, vein and manto Cu±Ag, Pb, Zn, Sn, W, and sandstone (“red bed”) Cu types. The vast majority of these deposits formed during the Andean Orogeny and are geographically and chronologically distributed in well-defined metallogenic domains. These domains correlate with geochemically distinct magmatic episodes.

The magmatic and metallogenic domains appear to be controlled in part by transverse growth-faults in the Mesozoic and older basement rocks underlying the intensely folded and thrust-faulted Mesozoic and Tertiary rocks of the higher structural levels of the Cordillera. During the Andean Orogeny the extent of magmatism and the corresponding metallogenic provinces were influenced by subducted plate segmentation and by continental margin basement tectonics. In addition, the lithologic nature of the host rocks played an important role in determining the types of copper deposits formed.

Porphyry Cu, breccia pipe Cu-Mo and calcic skarn Cu deposits are related to the Pomahuaca, Coastal and Caldera batholiths, as well as to felsic Cordilleran volcanism between 8° and 12°S. However, the largest and richest porphyry Cu deposits are related to the Caldera batholith. The Cobriza Cu-bearing skarn is the only significant copper deposit of pre-Mesozoic age.

Perú has many ore deposits associated with the Miocene felsic extrusive and intrusive rocks along the Cordillera, forming veins and disseminations in igneous rocks and noncarbonate sedimentary rocks, and replacement mantos, pipes and veins in limestones. Several are large and high-grade enargite-type deposits containing mainly Cu, Ag, Au, Pb and Zn, accompanied by significant amounts of Cd, Te, Se, In, Bi and Tl. Others are veins and mantos containing Cu±Ag, Pb, Zn, Sn, W.

The Mesozoic volcanosedimentary sequences along the coast host volcanogenic massive sulfide Cu-Zn and vein/manto-type amphibolitic skarn Cu±Fe deposits.

Red bed Cu deposits are relatively unimportant in Perú.

The following information on the history of copper mining in Perú has been condensed largely from Samame (1979), Petersen et al.(1990) and Benavides (1990).

In Perú, gold and silver were apparently used before copper. The latter was first mined and processed by the pre-Inca Chimú culture along the northern coast and by the Tiahuanaco civilization in the Lake Titicaca region.

Copper became an important metal during the Inca period,

  1. Page 1
    Abstract

    The copper deposits of Perú consist of porphyry Cu±Mo, Au, Ag, breccia pipe Cu-Mo, enargite vein and replacement Cu±Au, Ag, Zn, Pb, calcic skarn Cu±Fe, Au, Zn, amphibolitic skarn Cu±Fe, volcanogenic massive sulfide Cu-Zn, vein and manto Cu±Ag, Pb, Zn, Sn, W, and sandstone (“red bed”) Cu types. The vast majority of these deposits formed during the Andean Orogeny and are geographically and chronologically distributed in well-defined metallogenic domains. These domains correlate with geochemically distinct magmatic episodes.

    The magmatic and metallogenic domains appear to be controlled in part by transverse growth-faults in the Mesozoic and older basement rocks underlying the intensely folded and thrust-faulted Mesozoic and Tertiary rocks of the higher structural levels of the Cordillera. During the Andean Orogeny the extent of magmatism and the corresponding metallogenic provinces were influenced by subducted plate segmentation and by continental margin basement tectonics. In addition, the lithologic nature of the host rocks played an important role in determining the types of copper deposits formed.

    Porphyry Cu, breccia pipe Cu-Mo and calcic skarn Cu deposits are related to the Pomahuaca, Coastal and Caldera batholiths, as well as to felsic Cordilleran volcanism between 8° and 12°S. However, the largest and richest porphyry Cu deposits are related to the Caldera batholith. The Cobriza Cu-bearing skarn is the only significant copper deposit of pre-Mesozoic age.

    Perú has many ore deposits associated with the Miocene felsic extrusive and intrusive rocks along the Cordillera, forming veins and disseminations in igneous rocks and noncarbonate sedimentary rocks, and replacement mantos, pipes and veins in limestones. Several are large and high-grade enargite-type deposits containing mainly Cu, Ag, Au, Pb and Zn, accompanied by significant amounts of Cd, Te, Se, In, Bi and Tl. Others are veins and mantos containing Cu±Ag, Pb, Zn, Sn, W.

    The Mesozoic volcanosedimentary sequences along the coast host volcanogenic massive sulfide Cu-Zn and vein/manto-type amphibolitic skarn Cu±Fe deposits.

    Red bed Cu deposits are relatively unimportant in Perú.

  2. Page 19
    Abstract

    The Coastal Range of northern Chile is formed mainly by andesitic-basaltic lavas intruded by granitoids of intermediate composition. These rocks originated as a result of the development of a Jurassic-lower Cretaceous magmatic arc, which marks the beginning of the evolution of the Andean Tectonic Cycle. Copper deposits emplaced in these rocks constitute a polytypical, homochronic, and practically monometallic metallogenetic province.

    The distinctive characteristics of these mineral deposits are strongly controlled by the nature of the rocks in which they are hosted. These deposits can be grouped in:

    Volcanic-hosted deposits

    Buena Esperanza Type

    View this table:

    Other deposits and prospects, still little studied, occur as breccia pipes and stockworks related to intrusive rocks.

    The Buena Esperanza Type is the most studied and important deposit from an economical point of view. Several mechanisms have been proposed for the Buena Esperanza genesis including syngenetic volcanic, epigenetic post-volcanic, epigenetic and hydrothermal metamorphic. An important characteristic of this type of deposits is their invariable association with hypabyssal intrusive bodies initially interpreted as volcanic feeder conduits, but radiometric dating indicates they are 20 to 30 Ma younger. If, however, an early mineralization related to the volcanism is not discarded, new evidence indicates that there could have been mineralization related to later hydrothermal phenomena during the Jurassic and part of the lower Cretaceous.

  3. Page 33
    Abstract

    Large mineralized breccias are prominent features in the three giant late Miocene Andean copper deposits at Los Pelambres, Rio Blanco-Los Bronces and El Teniente in central Chile. The breccias were emplaced into Miocene igneous host rocks by the expansion of high-temperature, metal-rich fluids exsolvedfrom magmas. Minerals (tourmaline, anhydrite, biotite) precipitatedfrom these magmatic fluids in the matrices of different breccias have variable initial 87 Sr/86 Sr ratios, ranging from 0.7040 to 0.7049, and ∈.Nd values, ranging between +0.8 to +3.6. Although the fluids that generated the breccias may have leached some Sr from contained clasts of host rock, the ∈nd values of these breccia-matrix minerals are interpreted as the Nd-isotopic compositions of the magmas from which the breccia-forming fluids exsolved.

    The isotopic compositions determined for the breccia-matrix minerals differ from the host plutons. This implies that the fluids that generated the breccias were not derived from these plutons, which were already crystallized at the time of breccia formation as indicated by the angular nature of their clasts in the breccias. The fluids which generated the breccias must have exsolved from magmas crystallizing to form plutons not yet exposed at the surface, consistent with the fact that the roots of the mineralized breccias have not been encountered.

    Significantly, the isotopic compositions of the breccia-matrix minerals from different breccias in each deposit are variable. This indicates that the breccia-forming fluids were not derivedfrom a single magma, but from isotopically variable magmas. We suggest that the mineralized fluids that formed the late Miocene Cu-rich breccias in central Chile exsolved from multiple, compositionally variable magma batches cooling during the last stages of long-lived Andean magmatic systems. Cooling of these systems was triggered tectonically in the late Miocene as subduction angle and, as a result, subarc magma supply decreased.

  4. Page 43
    Abstract

    Exotic deposits form in conjunction with the supergene oxidation and enrichment of porphyry copper deposits. Supergene alteration primarily involves vertical solution movement, but percolation often involves a horizontal component whereby copper-rich acidic solutions migrate laterally within the vadose zone. Depending on Eh and pH conditions, copper may be transported through paleodrainage systems for distances of up to 8 km from source to produce continuous copper mineralization. The copper is deposited primarily in oxide form, but sulfide copper also may be present close to source.

    Favorable conditions for formation of exotic deposits existed between latitudes 12° and 27° Sin northern Chile and southern Perú. In Chile, twelve deposits are recognized, ranging from large deposits like Exotica, El Tesoro, and the recently discovered Damiana (1.2 to 3.5 million metric tons of Cu), through medium-sized ones like Huinquintipa and Sagasca (160,000 to 400,000 metric tons of Cu), to small ones like Quebrada Blanca and La Planada (100 to 10,000 metric tons of Cu).

    Geomorphologic conditions of the paleochannels and the chemical reactivity of the rocks and sediments in them influence the shapes of the exotic deposits confined to paleochannels (e.g., Sagasca). Other deposits are fan-shaped and cover large areas on the slopes of topographic highs underlain by the source deposits (e.g., Damiana). A few small deposits are controlled by faulting, the orientation of which controls their form (e.g., Ichuno).

    As a result oflateral flow of copper-rich solutions, exotic deposits display zoned alteration and mineralization patterns controlled by the reactivity of the host rocks, the CulFelS ratios of the source deposits, and the evolution of pH and redox state of solutions during deposit formation. Mineralization characteristics and mineral associations enable subdivision of deposits into proximal (0 - 2 km), intermediate (2- 4 km), and distal (4-6 km) zones. The changes in solution pH over time, from acidic to relatively alkaline, may give rise to vertical zoning.

    The origin of exotic deposits is linked to the transpressive tectonism and rapid uplift that produced the Andean chain in the late Oligocene-early Miocene interval. The generation of paleorelief caused water tables to fall, favoring formation of supergene enrichment and exotic copper deposits. The process was terminated by the change from semiarid to extremely arid conditions and, locally, by the deposition of extensive ignimbrite flows that sealed off access of supergene solutions.

    An empirical model for exotic deposits emphasizes the lateral and vertical zonation of mineralization and alteration. This zoning is interpreted in terms of successive solution fronts acting repeatedly during the period of mineralization. Low-pH solutions percolated through fractured rocks and rendered them impermeable and unreactive. Subsequently, more alkaline groundwater advanced over the impermeable kaolinized horizons, reaching higher levels and greater distances from the source and precipitatingoxide copper minerals in either unaltered gravels or basement rocks.

  5. Page 59
    Abstract

    The MM porphyry copper deposit is located 5 km south of the giant Chuquicamata deposit, and is concealed totally beneath >50 m of Miocene alluvial gravels. Hence geologic understanding is based on extensive core drilling and underground openings.

    The MM deposit and related mineralization farther south comprise a semicontinuous tabular zone that extends for 8 km from north to south, is inclined steeply west, and attains a depth of at least 1000 m. The deposit parallels and is delimited along its eastern side by the West fault, a major sinistral strike-slip structure of postmineral age.

    The mineralized zone is made up of four steeply inclined panels juxtaposed by postmineral faults subsidiary to the West fault. Panel one, abutting the West fault, comprises weakly chloritized and pyritized andesitic flows. Panels two and three consist of pervasively sericitized granodiorite containing pyrite-poor porphyry copper-type mineralization. The sericitic alteration largely destroyed a pre-existing K-silicate assemblage. Panel four is characterized by structurally controlled sericitic and advanced argillic alteration accompanied by enargite- and bornite-rich, high-sulfidation sulfide assemblages in granodiorite. These alteration-mineralization types grade westward in panel four through weak sericitic to pyrite-poor chloritic alteration, which affects granodiorite, dacite porphyry dikes, and roof-zone andesitic flows.

    The high-sulfidation copper zone in panel four comprises irregular stringers, patches, short veins, stockworks, and hydrothermal breccias filled with massive sulfides ± chalcedonic quartz. The hypogene sulfides are dominated by pyrite and enargite, with subordinate amounts of tennantite, bornite, digenite, chalcocite, and covellite, which can give rise to average copper grades of >2 percent. The north-striking structural system that localized the high-sulfidation copper zone is considered as an ancestral West fault.

    The MM deposit was assembled structurally as a series of fault slices that are considered to have traveled southward along the West fault. It would seem logical to presume that the slices were detached from the truncated western side of the Chuquicamata deposit. However, although MM and Chuquicamata geology possess similarities, especially with regard to the presence of structurally controlled, high-sulfidation copper zones, there are also several unexplained lithologic and alteration differences between them. These differences may be used to suggest another shallowly eroded porphyry copper system as the source of the MM components. Furthermore, MM lacks the mature supergene profile so important at Chuquicamata, and is essentially a hypogene deposit.

  6. Page 71
    Abstract

    Since the early 1950s, several companies and individuals have devoted considerable effort to exploration for precious metal and/or copper mineralization in the Chimborazo area, which is located in the Andes of northern Chile, about 180 km southwest from the port of Antofagasta. Minera Orion Chile, the predecessor of Minera Cyprus Chile, conducted exploration at Chimborazo, initially for precious metals and later for copper. The exploration for gold and silver had limited success, but that for copper lead to the discovery and delineation of a supergene enrichment blanket containing about 180 million metric tons averaging 0.8 percent Cu.

    The deposit occurs within volcanic rocks of andesitic to dacitic composition, cut by an early Tertiary intrusive complex composed of diorite and granodiorite. The main stages of alteration and mineralization comprise an early, late-magmatic episode, overprinted by a high sulfidation epithermal event. The early episode is represented by K-silicate and propylitic alteration accompanied by weak chalcopyrite mineralization plus minor pyrite and bornite. This episode was followed by emplacement of a series of hydrothermal breccias along a northeasterly structural trend. Advanced argillic alteration, associated with silicification and sericitization, is part of this later event. The later mineralization is complex and includes open space fillings, stockworks, and cementation of breccia fragments by pyrite, enargite, and variable, but smaller, amounts of chalcocite, chalcopyrite, bornite, and tennantite. Lead-zinc-silver mineralization, with some gold, was emplaced as peripheral veins during a final stage. Supergene activity created a profile consisting of about 100 m of leached capping, which contains some copper oxide remnants, underlain by an enrichement blanket containing chalcocite and minor covellite. The blanket is up to 200 m thick, elongated to give the shape of a canoe, and with its axis roughly coincident with the northeasterly trend of the hydrothermal breccias. In the western part of the deposit, chalcocite replaced pyrite in a sericitized intrusion, whereas toward the southeast and northeast, the supergene enrichment is superimposed directly over on breccias. Later northwesterly faulting, and diorite porphyry intrusion along the northeasterly fractures distorted the primary morphology of the system.

    Chimborazo is believed to represent the high-level portion of a porphyry copper system, preserved as the result of the special morphologic evolution of the Atacama Desert. The deposit illustrates the link between a porphyry copper deposit and a high level acid-sulfate epithermal system.

  7. Page 81
    Abstract

    The La Fortuna porphyry cluster is centered on a series of porphyritic stocks of Tertiary age that intruded a sequence of Paleozoic rhyolites and Jurassic red beds. Much of the area is covered by ignimbrites and gravels of Miocene age. Structure is dominated by regional, north-trending reverse faults crosscut by local, northwest-trending faults which are intimately associated with the porphyry-style mineralization. Two main types of mineralization are present in the area: porphyry copper-gold at La Fortuna itself and El Negro, and high-sulfidation epithermal gold at Cantarito.

    At La Fortuna, copper-gold mineralization is associated with pyrophyllite-rich advanced argillic alteration hosted by dacitic porphyries and hydrothermal breccias. Intermediate argillic alteration (sericite-chlorite) is preserved in deeper portions of the system. The mineralization occurs as supergene sooty chalcocite and hypogene assemblages of chalcocite, chalcopyrite, and bornite. Pyrite, specularite, and tourmaline are also important constituents. A gold-rich leached capping comprises abundant hematite and minor copper oxides.

    The El Negro system consists of a quartz diorite porphyry that intruded a sequence of andesitic volcanic rocks. Hypogene copper-gold mineralization is contained mainly in quartz-chalcopyrite-(bornite) stockworks associated with K-silicate assemblages composed of biotite, K-feldspar, and magnetite (averaging 6 vol. %). Copper and gold values display a sympathetic relationship.

    The high-sulfidation epithermal gold mineralization at Cantarito, consists of a core of vuggy and massive silica with abundant alunite, which is bordered outward by zones dominated sequentially by kaolinite, sericite, smectite, and chlorite. The mineralization is contained in the siliceous core, where native gold grains are associated with quartz-alunite in hairline fractures, veinlets, and breccias. Pyrite, barite, and scorodite are present locally.

    Available radiometric ages suggest that the hypogene copper and gold mineralization in the cluster was generated over a period of 3 m.y., between 35 Ma (K-silicate alteration at El Negro) and 32 Ma (advanced argillic alteration at Cantarito). Synmineralization uplift and erosion are interpreted to have been responsible for the unroofing of the hydrothermal systems and the superposition of epithermal associations over higher-temperature, deeper-seated assemblages.

    The style of the mineralization in the La Fortuna cluster displays many of the features that characterize the gold-rich porphyry copper deposits of the Philippines and Indonesia, as well as the porphyry gold deposits of northern Chile.

  8. Page 91
    Abstract

    This paper discusses the possible genetic relationship between two ore deposits in the Nevados del Famatina mining district, Argentina: Nevados del Famatina Cu-Mo-Au porphyry deposit and La Mejicana Cu-Au high-sulfidation epithermal deposit. The porphyry system crops out in three open annular areas dominated by porphyritic rocks at elevations between 4000 and 4800 m; the epithermal vein system is peripheral to the porphyry and crops out on adjacent ridges at elevations above 4500 m. The spatial, geologic, petrographic and geochronologic parameters suggest that there is a genetic relationship between the two deposits, and detailed mineralogic, fluid inclusion, and stable isotope studies and preliminary thermodynamic modeling provide additional support. The hydrothermal fluids, from which the ore, gangue, and alteration minerals were deposited, were dominated by magmatic volatiles, with admixture of groundwaters during the later vein stages in both the porphyry and epithermal deposits. Representative pressures, temperatures, and salinities decreased over the paragenetic stages of the porphyry and with increasing elevation in the late vein stages of the porphyry and epithermal systems. The fluids also became progressively more oxidized, more acidic, and lower in sulfur gas activity over the same stages and elevations. The temperature, pressure, and fluid composition (salinity, redox state, sulfur gas, acidity) inferred for the epithermal system are similar to those calculated for the last stage of the porphyry deposit. There are striking similarities in detailed mineralogy between the last porphyry vein stage and the epithermal veins. All available data support the hypothesis that the porphyry and high-sulfidation deposits are transitional and genetically related.

  9. Page 119
    Abstract

    Rio Blanco-Los Bronces, one of three giant late Miocene to early Pliocene copper deposits in the Andes of central Chile, formed as a result of emplacement of both multiple mineralized breccias and porphyry intrusions into early and middle Miocene plutonic rocks and Cenozoic lavas. The deposit is distinctive in that a significant proportion of the >50x106 metric tons of Cu it contains occurs within the matrix of mineralized breccia pipes or disseminated in the host rocks around the breccias. Approximately 50 percent of the Cu ore in the deposit occurs as breccia-matrix, stockwork, and disseminated mineralization in a zone of potassic alteration which formed during the emplacement of biotite-rich breccias of the Rio Blanco breccia complex and quartz monzonite porphyry intrusions. Following a period of uplift and erosion, younger tourmaline-rich breccia pipes, containing the other 50 percent of the Cu in the deposit, and weakly mineralized early Pliocene porphyries were emplaced both within and peripherally to the earlier zone of biotite breccias and potassic alteration. Clasts within the tourmaline breccias are sericitized. This sericitic alteration developed during the emplacement of these breccias, later than and independent of the earlier potassic alteration.

    Fluid inclusion and 0-, S-, and H-isotopic data indicate that the metal-rich fluids that generated both the older biotite-rich and younger tourmaline-rich breccias, and caused the potassic and sericitic alteration associated with these two breccia generations, were magmatic in origin. Sr- and Nd-isotopic data imply that the magmas that exsolved the breccia-forming fluids, as well as those that formed the late porphyries, were distinct isotopically from the older host rocks of the deposit. The breccia-forming fluids are believed to have exsolved from magmas that crystallized to form plutons that are still not exposed at the surface, consistent with the deep, as yet unknown roots of these breccias.

    The emplacement of the mineralized breccias and porphyries at Rio Blanco-Los Bronces occurred in conjunction with late Miocene changes in Andean magma chemistry and crustal thickness, within a period of<3 m.y. during the final stages of>15 m.y. ofMiocene magmatic activity in central Chile. The temporal changes in magma chemistry, the crustal thickening, uplift, and erosion which caused the younger mineralized tourmaline breccias to be superimposed on the earlier and deeper potassic alteration zone, and the decline of igneous activity in the Miocene magmatic belt all resulted from decreasing subduction angle beneath central Chile beginning in the middle to late Miocene.

  10. Page 131
    Abstract

    Los Pelambres, located 200 km north of Santiago, Chile, is a major porphyry copper deposit within a small intrusive complex hosted by Early Cretaceous andesitic lavas. The complex consists of a preore tonalite stock which was intruded by a series of porphyries, ranging from quartz diorite to quartz monzonite. K-Ar dates cannot discriminate ages of the individual units, which yielded an average age of 9.9 ± 1.0 Ma. Hypogene mineralization was introduced with quartz stockwork veining, potassic alteration, and breccia pipes. The entire complex was subjected to pervasive biotitization of hornblende. The stockworks show a sequence of types, including Granular Quartz veins lacking obvious alteration halos, quartz veins with K-feldspar halos, and Green Mica veins consisting principally of biotite and phengite. The main stage of mineralization consists of quartz veins with complex alteration halos containing quartz, K-feldspar, biotite, andalusite, and, less commonly, corundum. All these types of veins and alteration contain anhydrite, chalcopyrite, and bornite. Magnetite is present locally in the mica veins and alteration. A later stage of Comb Quartz veins introduced most of the molybdenum, with only minor K-feldspar and sericitic alteration. Late veins contain pyrite and quartz and have sericitic envelopes. Breccia pipes, up to 600 m across, have matrices of igneous rocks, biotite, K-feldspar, quartz, tourmaline, magnetite, chalcopyrite, bornite, pyrite, and molybdenite. Supergene enrichment produced 560 million tonnes of enriched ore with a grade of 0.93 wt. percent Cu. Sulfide minerals occupy a bornite-chalcopyrite zone at the center of the deposit, surrounded by a pyrite-chalcopyrite zone, with increasing pyrite outward related to the sericitic alteration. Molybdenum occupies a truncated dome-shaped zone near the center of the bornite-chalcopyrite zone, which is also the location of the largest breccia pipe. Metal reserves are in excess of 3,300 million tonnes with a grade of 0.63 wt. percent Cu, 0.016 wt. percent Mo, using a 0.4 wt. percent Cu cutoff

  11. Page 157
    Abstract

    The Manto Verde copper deposit is associated with specularite-rich breccias emplaced during extensional tectonism along brittle faults of the Atacama fault zone (AFZ). The AFZ is a complex sinistral strike-slip/dip-slip system that originated during oblique subduction in a Jurassic to Early Cretaceous magmatic arc. Activity of the AFZ is documented from the Middle Jurassic, with intermittent movements occurring at least into the late Miocene. South of Manto Verde, the AFZ is a major control on the emplacement of apatite-bearing magnetite deposits in the Cretaceous Iron Belt (CIB).

    The Manto Verde deposit is hosted by cataclased andesitic volcanics and coeval (?) diorite porphyry stocks of Early Cretaceous age affected by low-grade regional metamorphism. The deposit consists of three breccia units paralleling the Manto Verde fault (MVF) for at least 1500 m. The MVF is a 12 km long, north-northwest — striking- 40°-50°E — dipping structure that joins two major branches of the AFZ, and shows evidence of premineralization strike-slip motion followed by postmineralization, east-side-down, dip-slip movement. The hanging - wall Manto Atacama breccia is a ±100 m wide, specularite-rich, matrix-supported hydrothermal breccia that follows the MVF along strike and down dip, although it becomes narrower with depth. A Transition Zone is defined between the Manto Atacama breccia and the andesitic country rocks as brecciation intensity and associated copper mineralization decrease progressively away from the MVF. In contrast, the footwall Manto Verde breccia is represented by a ±20m wide cataclasite displaying a relatively sharp mineralization boundary.

    These units are all deeply oxidized to 200 m, so that the present mineralogy consists of a suite of copper carbonates, sulfates, and silicates with minor copper chlorides, which occur as fracture fillings, patches, and disseminations. Only a thin, poorly developed supergene enrichment zone exists. Hypogene sulfide mineralization is poorly explored and corresponds to chalcopyrite ± pyrite disseminated in a specularite matrix.

    District-wide chloritization and K-silicate alteration, characterized by microcline replacement and veining of the andesitic country rock, is recognized. Superimposed sericitic alteration is localized by the MVF and neighboring fracture zones. This late alteration event accompanied the main deposition of specularite and associated copper(-gold) mineralization dated by the KlAr method at 121 ± 3 and 117± 3 Ma.

    Preliminary studies indicate a predominance of three-phase fluid inclusions as well as the coexistence of vapor-rich and liquid-rich, two-phase inclusions evidencing boiling. Homogenization temperatures for two and three-phase inclusions in translucent quartz veinlets and fragments accompanying early iron - copper - (gold) mineralization range between 180° and 320° C, although they are mainly between 180° and 250° C. Salinities for the three-phase inclusions vary from 30 to 47 wt percent NaCl equivalent, although a decrease in fluid salinity (14-21 wt % NaCl equiv.J is detected in two-phase inclusions from late-mineralization, chalcopyrite-bear-ing calcite veins.

    The geologic features of the Manto Verde deposit suggest a common magmatic-hydrothermal origin with the apatite-bearing magnetite deposits of the CIB. Manto Verde probably represents the copper-rich, specularite-dominated, and possibly younger end member of a continuum of deposits that extends to copper-poor, magnetite-dominated deposits at the other extreme.

  12. Page 171
    Abstract

    Two superimposed alteration events control the regional distribution of five main alteration types in the Punta del Cobre belt, south of Copiapó, Chile. An early event of alkali metasomatism caused extensive albitization and was overprinted locally by potassic alteration. Albite-quartz-chlorite or K-feldspar-quartz-chloritelbiotite assemblages characterize the metasomatized rocks. The alteration resulted in marked changes in chemical composition, with extremes of 10 wt percent Na20 or 11 wt percent K2O. The second alteration event was associated with emplacement of a middle Cretaceous batholith, which is exposed in the western part of the area. The intrusion produced contact metamorphism which overprinted, with variable intensity, the areas affected by the alkali metasomatism. The contact effects are expressed as north-northeast-trending, largely overlapping zones which, from west to east, are characterized by: Caamphibole± biotite ± sericite, biotite ± chlorite ±sericite ± epidote, and epidote-chlorite ± quartz ± calcite. The Ca-amphibole is mainly actinolite, actinolitic hornblende, and magnesio-hornblende. The three alteration zones show only minor chemical modifications compared to the inferred average compositions of unaltered precursors.

    Chalcopyrite, pyrite, magnetite, and hematite are the principal ore minerals. Alkali metasomatism, in particular potassic alteration, is related spatially to mineralization both east and west of the Copiapó River. In contrast, calcsilicate assemblages, unrelated genetically to the Cu mineralization, occur only west of the river, close to the batholith contact. In the Punta del Cobre and Ladrillos districts, the mineralization appears to be controlled by north-northwest- to northwest-trending faults, which were active since the middle Cretaceous. A middle Cretaceous age for the mineralization is supported by a 40Ar/39Ar inverse isochron age of 114.9 ±0.5 Ma for hydrothermal biotite. The alteration pattern and ore formation temperatures (up to 400°-500°C) support an association of the Cu mineralization with deep-seated magmatic intrusion(s). Fluid inclusion data for postore calcite indicate involvement of saline fluids. A magmatic source of the sulfur is indicated by isotope ratios determined for chalcopyrite and pyrite (834S between -0.7 and +1.1 %o).The deposits in the Punta del Cobre belt are somewhat similar and possibly transitional to both the Chilean magnetite (-apatite) deposits and porphyry copper deposits.

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