Abstract The Hualgayoc district in the Cajamarca region of northern Peru experienced middle Miocene andesitic to rhyolitic magmatism and magmatic hydrothermal activity that produced Au and Cu mineralization, including the Cerro Corona porphyry Au-Cu, Tantahuatay high-sulfidation epithermal Au, and the AntaKori skarn Cu-Au-Ag deposits. We examined 32 samples from 22 units that encompass the entire igneous rock record in the district. Our new U-Pb dating of 454 zircon grains from these 22 igneous units yielded Concordia ages showing continuous magmatic activity in the district, from 14.8 to 9.7 Ma. Igneous activity in the eastern part of the district took place between 14.8 and 14.0 Ma, including the Cerro Corona intrusive complex that hosts a porphyry Au-Cu deposit. Magmatism in the western part of the district occurred between 13.7 and 11.5 Ma and included the Tantahuatay Volcanic Complex that hosts a high-sulfidation Au deposit. The different styles of mineralization and ages of igneous rocks in the eastern and western parts are accounted for by deeper erosion to the east. Bulk-rock compositions indicate that parental magmas originated from amphibole-rich juvenile lithospheric mantle or lower crust and evolved through amphibole fractional crystallization. Amphibole and zircon compositions indicate that parental magmas of all igneous units in the Hualgayoc district were water-rich, >3 wt % H 2 O, and oxidized above the fayalite-magnetite-quartz (FMQ) redox buffer. Our observations in the Hualgayoc district suggest that oxidized conditions and high-water contents are necessary to produce porphyry-type mineralization, but that other factors were also critical to form mineralization, such as a shallow depth of magma emplacement, with near-vertical shape of intrusions. These findings are likely applicable elsewhere.

Abstract At least 14 porphyry copper-gold deposits and 19 epithermal gold deposits are known within 60 km of Cajamarca. The partly explored porphyry deposits vary in grade, Cu-Au-Mo proportions, and depth of erosion. Associated epithermal mineralization occurs at Perol, Peña de las Águilas, Kupfertal, Yanacocha Norte, Maqui Maqui, and Pampa Verde but not at Michiquillay, El Galeno, Chailhuagón, Cerro Corona, La Sorpresa, Colpayoc, and Chamis. These deposits are associated with Miocene magmatic activity, northwest-trending folds and thrusts, and northeast-trending faults. In the porphyry deposits, granular A quartz veins, associated with K-feldspar-biotite alteration and disseminated chalcopyrite-magnetite with bornite or pyrite, are typically present within and about multiple coeval porphyry intrusions. Banded quartz veins occur near the tops of some shallowly eroded systems, and late sericite-pyrite ± chalcopyrite is superimposed on most. Epithermal mineralization is mostly of high-sulfidation character, with pyrite-enargite-covellite typically underlying oxide Au zones leached of Cu. Epithermal Au-Cu is associated with multiple stages of brecciation and intense silicification, zoned outward and downward with decreasing SiO 2 and Au through quartz-pyrophyllite-diaspore-alunite-dickite to quartz-alunite and kaolinite. Structurally controlled, high-grade Au is apparently late and associated locally with intermediate-sulfidation assemblages, barite, and chalcedony. The transition between porphyry and epithermal environments is exposed at Perol and Huaylamachay, La Zanja, and especially Tantahuatay and Yanacocha. At Perol and Huaylamachay, porphyry gold-copper deposits are adjacent to generally contemporaneous volcanic vents altered to quartz-alunite with minor structures containing quartz-pyrophyllite-alunite-Au. At Perol, the dacitic vent is intruded by a later mineralized porphyry, whereas at Huaylamachay the vent breccia contains clasts with quartz-molybdenite veins and is cut by banded quartz veins, which we interpret as indicating a second, deeper porphyry Au system. At Tantahuatay, an andesitic dome complex is pervasively brecciated and altered to quartz-alunite-pyrophyllite-diaspore ± dickite, with extensive pyrite-enargite-covellite-(bornite) veins and disseminations beneath Aurich oxide mineralization. A gusano texture of soft, round patches of pyrophyllite-diaspore and/or alunite in a silicified matrix is widespread and associated with anomalous concentrations of Mo. Only one of several drill holes to 600-m depth encountered A quartz veins and minor porphyry intrusions. This hole provides evidence for prograde advance of quartz veining associated with one or more porphyry intrusions into the epithermal environment and subsequent retrograde collapse. At Yanacocha, the most abundant evidence of direct, albeit complex, spatial and temporal relationships between multiple centers of epithermal mineralization and porphyry intrusion and mineralization has been partially deciphered. At Kupfertal, the matrix of gusano alteration above the top of the porphyry becomes increasingly silicified and patchy downward, developing very contorted wormy quartz veins that overlap the top of A quartz veins. Intense pyritic quartz-pyrophyllite-diaspore-alunite and underlying sericite alteration is superimposed on K-feldspar-biotite alteration of the early stage. Fluid inclusions in quartz are vapor dominant, with downward-increasing proportions of high-salinity inclusions and amounts of minute relict chalcopyrite ± bornite grains “locked” in A vein quartz. A-veined and advanced argillic-altered xenoliths in pyroclastic rocks intruded by porphyries and hosting gold mineralization demonstrate multiple generations of porphyry and epithermal mineralization. Early Cu and Au of the porphyry event appear to have been remobilized and incorporated into the overlying epithermal system.

Abstract With premining reserves plus resources of 10.2 million oz (Moz) of gold, 424 million metric tons (Mt) at 0.75 g/t, La Quinua is one of several world-class gold deposits that comprise the Yanacocha mining district in the Cajamarca province, northern Peru. Unlike volcanic-hosted high-sulfidation gold deposits that characterize much of the Yanacocha district, La Quinua is hosted by unconsolidated gravel. In 1996, Newmont and Minera Yanacocha geologists discovered gold-bearing gravel while drilling an alluvial basin to test “blind” basement targets. Exploration and development drilling proceeded rapidly; 1 yr after initial discovery the reserve plus resource stood at 7 Moz. Production began in mid-2001. At La Quinua, there is infilling of coarse clastic sediments in two structural basins situated along the western flank of the Yanacocha Sur and Yanacocha Oeste gold deposits. Gravel fans reach a maximum thickness of 350 m on the downthrown side of the basin-bounding La Quinua fault. Sediments fine down gradient from chaotic boulder gravel in proximal facies to gravelly silt and sand in distal facies. Bedding becomes more pronounced down gradient with a decrease in bed thickness. Five deposit-scale stratigraphic units are recognized. These include, from bottom to top, (1) regolith directly overlying basement rocks, (2) high-to low-energy deposits of clay-and alunite-bearing sand and gravel, (3) low-energy deposits of organic-rich mud, peat, and bog iron, (4) ferruginous gravel, and (5) high-energy deposits of pebble-cobble-boulder gravel grading distally to fine-grained sandey silt. La Quinua gold was derived from erosion, transportation, and deposition of gold particles and mineralized clasts from the Yanacocha Sur and Yanacocha Oeste deposits. However, a portion of the gold may have resulted from chemical mobilization and reprecipitation. Gold particles are mostly micron sized, liberated within mud matrix, and disseminated within mineralized clasts, although liberated particles to 0.2 mm have been observed. Gold is disseminated throughout the deposit with only gradual lateral and vertical grade transitions. Placer “paystreaks” have not been encountered. The La Quinua gold-trapping system was efficient; grade dilution from source deposits is <25 percent. The Ag/Au ratio is 6/1, compared to >10/1 for the Yanacocha Sur and Yanacocha Oeste deposits. Copper and iron are locally enriched in specific stratigraphic horizons. Copper values up to a few percent are associated with detrital and authigenic minerals. Copper mineralization is erratic and not considered economic and may present future ore-processing challenges. Authigenic iron is present as bog-iron lenses, ferricrete cement, and gravel matrix impregnation. La Quinua formed in response to dynamic interaction of climate and tectonics. A subsiding tectonic basin preserved gold-bearing sediments during periods of intense mechanical weathering in the adjacent highlands. Cold alpine climatic conditions resulted in two pulses of rapid sedimentation and basin infill. Temperate conditions resulted in diastems marked by organic accumulation and surficial iron deposits. La Quinua is characterized by a paucity of channel deposits, lack of coarse-grained placer gold, and preservation of fine-grained gold. Structural offset and warping of sedimentary units indicates that basinal tectonism continued after deposition of the gravel sequence. Gold production from La Quinua will exceed 1 Moz/yr during 5 yrs of an 8-yr mine life, with a peak of nearly 2.5 Moz predicted in 2006. Light blasting improves mining efficiency. Oxide ore is belt agglomerated prior to placement on the leach pad. The majority of gold is recovered in carbon columns with a smaller portion recovered in a Merrill-Crowe facility. Mine equipment includes Hitachi 5500EX shovels, Cat 992 loaders, and Cat 785 and 793 haul trucks.

Abstract A 40 Ar/ 39 Ar age of 16.94 ± 0.34 Ma on hypogene alunite from the La Virgen sedimentary and volcanic rockhosted Au deposit and new ages of 16.06 ± 0.11 to 15.58 ± 0.12 Ma on porphyry Cu-Au deposits at Minas Conga confirm the existence of early Miocene and earliest middle Miocene mineralization in the eastern part of the Miocene metallogenic belt of northern Peru. A new K-Ar age of 15.3 ± 0.3 Ma for the Magistral porphyry-skarn Cu-Mo deposit extends the belt of known early and middle Miocene deposits southeast of La Virgen. A 40 Ar/ 39 Ar age of 20.02 ± 0.15 Ma is reported for a late intrusive phase at the Michiquillay porphyry Cu deposit. A 40 Ar/ 39 Ar plateau age of 15.61 ± 0.12 Ma on alunite from the San Pedro Sur zone at the La Zanja epithermal Au district is consistent with the location of the district within the middle Miocene Quiruvilca-Pierina subbelt of the Miocene metallogenic belt. 40 Ar/ 39 Ar mineralization ages of 20.79 ± 0.10 and 21.78 ± 0.11 Ma have been obtained on mineral deposits of the Malvas and Huinac districts in the Cordillera Negra to the south. Together with a published age of 18.7 ± 0.6 Ma on the Churropampa Au prospect in the eastern part of the Cordillera Negra and a new Re-Os age of 18.15 ± 0.06 Ma on the Pachagón porphyry Cu-Ag prospect northeast of Trujillo, an early Miocene mineral activity in the western part of the metallogenic belt is indicated. The new ages show that clearly magmatically related mineralization was formed at the same latitudes in both the eastern and western parts of the Miocene metallogenic belt during early Miocene and earliest middle Miocene times. The deposits of the giant Yanacocha Au district, which were formed at about 11 Ma, are younger than deposits to the west and to the east. Deposits of the Hualgayoc district to the north, which have yielded mineralization ages from 12.4 ± 0.4 to 14.4 ± 0.2 Ma, may overlap in time with those of Yanacocha. These districts, which are located within large coeval volcanic fields, could reflect a younger episode of magmatic activity of late middle Miocene age.

Abstract The Miocene metallogenic belt of central and northern Perú, extending for at least 900 km along the Western Cordillera and the adjacent high plateaus province, is defined by a large number of hydrothermal mineral deposits of different types that formed between about 6 and 20 Ma. The belt, centered east of the Mesozoic and early Paleogene Coastal batholith, is on mature continental crust that has undergone multiple episodes of compressive deformation from at least middle Paleozoic to latest Neogene time. Mineralization began before the early Miocene Quechua I compressive event and spanned later Quechua II tectonism. Mineral deposits are mostly hosted by shelf carbonates and other sedimentary rocks of Late Triassic,Jurassic, and Cretaceous age and by volcanic and intrusive rocks mainly of Neogene age. Base metal and precious metal mineralization was intimately associated in time and space with the eruption of calc-alkalic volcanic rocks of intermediate composition and the emplacement of mineralogically and chemically similar dikes and stocks. These igneous rocks are moderately potassic and the few available data suggest relatively nonradiogenic Sr, Nd, and Pb isotope compositions. Mineral deposits range from porphyry and associated proximal skarn deposits to polymetallic, precious metal, and mercury deposits formed at relatively lower temperatures. Porphyry deposits include the La Granja Cu porphyry, the Au-bearing Michiquillay Cu porphyry, the Mo-bearing Cu porphyritic rocks of Toro Mocho, Pashpap, and Páraq, the Mundo Nuevo-Tamboras-Compaccha Mo-W porphyry system, and the Cerro Corona, Minas Conga, Collpayoc, Laguna Chamis, Carhuacayán, and Puy-Puy Au-Cu porphyry deposits. Many of the classic base and precious metal deposits of central and northern Perú are within zoned polymetallic districts, some with one or more porphyry centers. Many districts have veins or replacement bodies containing enargite in their central parts, and a number are characterized by deposits of both vein and limestone replacement type. At a number of polymetallic districts, for example, Julcani, Yauricocha, Morococha, Casapalca, Huarón, Raura, Antamina, Pasto Bueno, Quiruvilca, Algamarca, and Hualgayoc, stocks containing high-salinity fluid inclusions are exposed, known from drill-hole data, or can be confidently inferred from fluid-inclusion or isotope data. Vein and limestone-replacement Pb-Zn ± Ag ± Cu deposits are common, and range from vertically persistent, high-temperature deposits, such as the veins of Casapalca, to largely stratabound deposits such as Cercapuquio and Azulcocha, that were formed at temperatures below 200°C. Although certain writers have interpreted some manto deposits to be diagenetic or syndiagenetic, field relations and lead isotope compositions argue strongly for an epigenetic origin. Vein systems or epithermal paragenetic stages in which silver is the economically most important metal, such as those of Milluachaqui, Millotingo, and Colqui, typically contain appreciable amounts of base metals and can best be considered a variant of the polymetallic vein group. The Huancavelica mercury deposit represents an extremely large geochemical anomaly, perhaps developed at the top of a polymetallic system. High-sulfidation-type Au-Ag deposits, such as Pierina and those of the Yanacocha district, are economically important. At Tantahuatay and Colquijirca, oxidized Au-bearing, vuggy silica rock occurs at higher elevations than surrounding, zoned, enargite-cored Cu-Pb-Zn-Ag veins and strata-bound replacement deposits. In contrast to the association of precious metals with enargite, tetrahedrite, and barite at Julcani and other reduced-type deposits, in moderate- to high-grade ores at Pierina and probably certain deposits in the Tantahuatay and Yanacocha districts, most of the gold is very late, following initial quartz-alunite-pyrite alteration, the destruction of alunite to form vuggy silica rock, and the subsequent deposition of pyrite and enargite accompanied by small amounts of gold. Gold and silver in economic quantities were then introduced by compositionally distinct, late fluids that oxidized pyrite and enargite, leached Cu, Zn, Se, Te, Tl, and other elements, and introduced Hg, Pb, Bi, Sb, and large amounts of barite. An analogous case for a distinct, compositionally different Au-Ag mineralizing pulse perhaps can be made for the sedimentary rock-hosted gold deposits of Purísima Concepción in Yauricocha. The ubiquitous presence of enargite, and the spatial and temporal association in several districts of pyrite + enargite, with modest gold content, and oxidized Au-rich ores, support the interpretation that bulk-mineable, volcanic-hosted gold deposits of a high-sulfidation type represent one of the many types of deposits related to the general class of porphyry-related, zoned polymetallic systems. The sandstone-hosted gold deposits of northern Perú also appear to be related to subjacent magmatic systems, although there are certain geological, mineralogical, and chemical differences from both volcanic-hosted, high-sulfidation and Purísima-type gold deposits. High W and Sn content of many of the sandstone-hosted ores of the Angasmarca district suggest that they are high-level manifestations of subjacent W-Mo ±Au systems such as are exposed at the nearby, more deeply seated Mundo Nuevo-Tamboras-Compaccha and Pasto Bueno districts. Several subsidiary belts are recognized within the Miocene metallogenic belt. A group of deposits in northern Perú, including the polymetallic deposits of the Quiruvilca district, the several Cu-Mo porphyry systems at Pashpap, and the Pierina high-sulfidation Au deposit, defines the 13 to 15 Ma or older Quiruvilca-Pierina subbelt in the western part of the metallogenic belt. The provisional Michiquillay-El Toro subbelt, including the Michiquillay Cu porphyry, the El Toro Au prospect, and probably the Au-Cu porphyry systems of the Minas Conga district, appears to have formed in northern Perú along the eastern margin of the metallogenic belt between about 18 and 20 Ma. A narrow, late Miocene subbelt that comprises a number of deposits dated at less than about 10 Ma, including Huachocolpa, Yauricocha, San Cristóbal, Morococha, Puy-Puy, Carhuacayán, Huarón, Raura, Huanzalá, Antamina, Pasto Bueno, and Angasmarca, extends from the Huachocolpa district at the southern end of the belt to the latitude of Santiago de Chuco in northern Perú. Deposits of the late Miocene subbelt postdate the 9 to 10 Ma Quechua II compressive pulse, and the initiation, location, and narrowness of the subbelt may have been related in some manner to this tectonic event. Intersections of successive, magmatic mineral axes with northeast-trending and other fault systems of probable crustal scale may have combined to influence the location of individual mineral deposits or clusters of deposits. Mineralization had ceased, and possibly was terminated, by the 5 to 7 Ma Quechua III compressive event. The emplacement of the 5.2 Ma late phase of the Cordillera Blanca batholith and the eruption of approximately coeval units of silicic ash-flow tuff and lava in northern and central Perú may reflect the subsequent relaxation of compressive stress, leading to the switching of axes of least and greatest principal stress indicated by 4 Ma north-south-trending dike systems in central Perú. Four important older districts within the Miocene metallogenic belt (Quicay, ca. 37.5 Ma; Uchucchacua, ca. 24.5 Ma), or bordering it on the east (Atacocha and Milpo, ca. 29-30 Ma), are related to older, and perhaps in part less intense, periods of magmatic activity. Although gold deposits may prove to be more important in northern than in central Perú, there is little indication that the concentrations of other metals vary markedly along or normal to the Miocene metallogenic belt. For example, porphyry molybdenum deposits are found in both the eastern and western parts of the belt. Moreover, particular types of deposits do not appear to be preferentially restricted to a given time period: several sandstone-hosted gold deposits in northern Perú have yielded ages ranging from less than 9 to greater than 18 Ma, and Au-bearing porphyry systems include examples of early, middle, and late Miocene age. Local geology and depth of erosion may be more important controls of deposit type. If future work shows that individual subbelts are as narrow and continuous as the present data suggest, areas within the narrow subbelts may prove to be the most prospective for mineral exploration.