Porphyry-Epithermal Transition, Cajamarca Region, Northern Peru

Abstract

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 SiO2 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.