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 Detailed logging of core from drill holes in the Sur-Sur and La Americana breccias in the Andina portion of the Rio Blanco-Los Bronces porphyry copper deposit, supported by petrography, geochemistry, and study of fluid inclusions, has documented zonal and temporal patterns in ore breccias over 1,600 vertical meters. Breccia textures progress from incipient crackle breccia with tourmaline veining to increased rounding of fragments and filling of open spaces by mineralization products and rock flour, reflecting many repeated pulses of brecciation. Both angular breccias and rounded clast breccias with rock-flour matrix have been mineralized primarily by the quiescent flow of hydrothermal solutions between pulses of brecciation. Sharp contacts mark boundaries between pulses separated by a period of thorough cementation, while diffuse, gradational contacts mark boundaries between more closely spaced pulses. Refracturing of the rock mass continued after consolidation of rock-flour matrix breccias, as documented by local angular rebrecciation and ubiquitous postbreccia veining. The angular, tourmaline breccia with chalcopyrite-pyrite at Sur-Sur grades downward, with decreasing tourmaline and increasing biotite in the matrix, into breccia with biotite-alkali feldspar alteration and chalcopyrite-bornite. Variably tourmalinized rock-flour breccias at La Americana extend about 400 m higher than Sur-Sur, with upward increasing ratios of specularite/tourmaline and pyrite/chalcopyrite, and decreasing grades of Cu and Mo. Minor dikes of porphyry have intruded the breccias but are themselves fragmented and appear to be contemporaneous with brecciation. Dikes that have intruded deep, high-grade Sur-Sur breccia display intense biotite alteration and disseminated chalcopyrite-bornite. A high-grade interval of La Americana breccia has been intruded by a dike with intense sericite-chalcopyrite alteration. Dikes intruding poorly mineralized La Americana breccias are barren and are not biotized. The Sur-Sur breccias were formed contemporaneously with early-stage porphyry copper mineralization at depth, and are cut by a sequence of quartz-molybdenite and sulfide veins with sericitic halos that are typical of the evolution of veining in porphyry copper systems. In both the Sur-Sur and La Americana breccia matrices, highly saline fluid inclusions, saturated with NaCl (over filling temperatures from 225° to 500°C), coexist with vapor-rich inclusions and with fluid-rich inclusions from 150° to 450°C and 2 to 30 wt percent NaCl equiv. High-salinity fluids are most abundant at depth and with higher copper grades, while liquid-rich and vapor-rich fluids are dominant near surface, particularly at La Americana. This, and prior stable isotope evidence, is compatible with the interpretation that magmatic fluids, derived from magma giving rise to porphyry dikes and the PΔV energy for brecciation, were primarily responsible for mineralization of the ore breccias. Intermixed meteoric water, however, may have been responsible for the huge volume and complex reworking of the breccias, and for apparently wide fluctuations in temperature, pressure, f o 2 , and salinity, which are suggested by fluctuations in magnetite-hematite and anhydrite saturation in the breccias. This district represents nearly an end member in the wide range of variations that are characteristic of porphyry copper mineralization. The breccias are copper ores because they were formed and mineralized by intrusions derived from a differentiated magma chamber which became saturated with typical porphyry copper-ore fluids. Location, complexity, and geochemistry offer the explorationist clues to the relatively rare tourmaline breccia, which may be ore bearing.

Abstract Sediment-hosted stratiform deposits of copper, lead, and zinc are distinguished from volcanogenic massive sulfide deposits by a lack of associated volcanism. They are distinguished from Mississippi Valley-type deposits by an early timing of mineralization relative to deposition of host sediments, by greater conformity with their hosts, and by higher iron sulfide and silver contents. These two types, however, are probably part of a continuum in terms of characteristics and mechanism of formation. Characteristics of the diverse sediment-hosted stratiform deposit type are illustrated by 28 selected examples, which include several of the world s giant deposits. Metal ratios of sediment-hosted stratiform copper and lead-zinc deposits are distinctly separate from one another and unlike the range of ratios in volcanogenic deposits. Most sediment-hosted stratiform lead-zinc deposits contain abundant iron sulfide, in contrast to the copper deposits which are characterized by low sulfide iron contents. Barite is abundant and massive in several of the lead-zinc deposits but is sparse in stratiform copper deposits. Sediment-hosted stratiform deposits of both copper and lead-zinc range in age from about 2,000 m.y. to recent and occur in tectonically active intracratonic settings, commonly in fault-controlled sedimentary basins. They do not appear to be related to orogenic events or to events at margins of tectonic plates. Local tectonic settings and relations to basin margins and faulting are diverse. We interpret their unifying features to be that each provides a situation in which basinal ground water could be moved to a shallow site of sulfide deposition. Regional stratigraphie settings include (1) first marine transgressions over red terrestrial sections (e.g., Zambian deposits, Kupferschiefer, White Pine, Boleo), (2) within marine (and lacustrine) sections containing red beds (e.g., Dzhezkazgan, Udokan, McArthur River, Redstone River, Spar Lake, Irish deposits), (3) terrestrial sections containing red beds (e.g., Corocoro, Nacimiento, Largentière), and (4) marine sections without red beds (e.g., Sullivan, Meggen, Rammelsberg). Evaporites (bedded or interstitial gypsum, anhydrite, or carbonate after sulfate) or other evidence of aridity (salt casts, desiccation features) are closely associated with deposits in all settings but the last. Although they lack any close association with volcanism, most deposits occur in basins with some regionally contemporaneous volcanic activity or with a significant quantity of volcanic rocks in underlying stratigraphie sections. Sedimentary facies are important local controls on sulfide distribution, but host lithology varies widely from district to district and commonly within districts and is among the least definitive features of these deposits. Most sediment-hosted copper deposits (except in Shaba) are associated with bedded host or footwall units that retained high permeability after early diagenesis. The copper mineralization is invariably “disseminated” (including stockwork and breccia matrix). Most of the ore is in reduced sediments that contain (or can be inferred to have originally contained) abundant organic matter. Ore occurs at a redox interface with footwall or hanging-wall or laterally equivalent units that are hematitic (red now or before metamorphism) and barren of sulfide and organic matter. In contrast, most lead-zinc deposits are in sediments that became rather impermeable during early diagenesis. The ores are mostly either massive or banded (i.e., interbedded with sedimentary rock), although some are disseminated. Many deposits are also associated with evaporites and red beds, though less directly than the copper deposits, and the ore is not obviously controlled by the interface between oxidized and reduced conditions. Zonation in copper deposits typically follows the sequence: barren (hematite but no sulfide) chalcocite → bornite → chalcopyrite → pyrite, expressed both outward and upward. In many deposits this zonation does not reflect the paleogeography directly. Zonation is commonly continuous across alternation of depositional environments and probably reflects post-depositional hydrologie regimes and chemical gradients. In deposits with lead-zinc as well as copper, the general zonal sequence Cu-Pb-Zn-Fe is observed. In several deposits this sequence evidently marks increasing distance along an ore solution pathway. Sedimentary iron-formation, both sulfide and oxide facies, appears to have formed in distal portions of some lead-zinc mineralizing systems. Zoning patterns are varied. The timing of metal fixation relative to deposition of host sediments and their diagenetic modification is crucial but rarely constrained by unambiguous evidence. Concretions and authigenic minerals provide some of the best evidence, indicating fixation of base metal sulfides after early diagenesis at McArthur River and Shaba, but much earlier at Rammelsberg. The lack of definitive detailed documentation of evidence for timing and of subtle nonsulfide zoning patterns in most deposits seriously limits our ability to interpret their origins. The lack of information on temperatures of mineralization in most deposits is also critical. Unlike volcanogenic massive sulfide deposits, sulfur isotopes in sediment-hosted stratiform deposits do not reflect secular variations in the isotopic composition of seawater and appear to be dominated by processes at the site of ore deposition rather than the source of the sulfur. Narrow clustering of δ 34 S values in lead-zinc deposits appears to reflect systems dominated by mineralizing fluids, in contrast to scattered δ 34 S values in copper deposits which reflect sulfate reduction reactions at lower temperature and in systems dominated by the depositional environment. The data are ambiguous as to mechanisms and to definition of the ore-forming systems. At McArthur River, carbon and oxygen isotopes support an interpretation of mineralization dominated by a hydrothermal ore-forming fluid. We speculate that most sediment-hosted stratiform deposits of both copper and lead-zinc formed early in the diagenetic history of their enclosing sediments from brines derived from the sedimentary basin itself. They are thus diagenetic in the broad sense, even though the metals may have been introduced epigenetically into the system defined by the present physical limits of mineralization. The wide and gradational range of deposit characteristics derives from more or less independent variation of factors linked in a source-transport-trap chain of coincidence. The stratiform copper deposits are probably formed from cool sulfate-rich brines, derived at an early stage of basin evolution and migrating updip to reducing sites of deposition. The lead-zinc deposits are probably formed from similar but more evolved brines that have been heated and probably chemically reduced deeper within the basin. Some of the latter may have been formed by exhalation of dense basinal brines on the sea floor.