Porphyry Copper Deposits
Knowledge about many aspects of porphyry copper deposits has broadened immensely over the past several decades and especially since the mid-1960s. This review focuses on the deposits in two ways, by discussing their broad geologic setting and tectogenesis and their alteration and mineralization; however, the review is largely limited to the Mesozoic and Cenozoic deposits of the Pacific basin rim.
Radiometric age data show that porphyry deposits formed in three, clearly separated, broad intervals of time—the pre-Laramide, the Laramide, and the interval since ~40 m.y. ago. The deposits, regardless of age, occur within terranes having comparable distinctive geologic characteristics. On the eastern side of the Pacific basin, they are generally situated between older batholithic complexes and uplifted or exposed older cratons and are in moderately to strongly deformed mobile belts containing large numbers of parallel faults and folds. This environment is present in both accreted continental margins, such as the Canadian Cordillera, and in rigid margins, such as coastal Peru and Chile. Although the environment of deposits in the island regions of the western Pacific, most of which are of post-Laramide age, are more varied, most deposits occur along island or archipelago axes and in regions characterized by known or inferred parallel fault systems.
Petrologic data on rocks of the basin rim indicate two basic kinds of intrusions; the first, in the island regions, is characterized by low K2O/CaO ratios, the other, in continental settings, has relatively higher ratios. Differences in Na2O and K2O content also exist between the two settings. A distinctive alkaline suite occurs with certain deposits of the Canadian Cordillera. Most suites, where studied, are grossly of calc-alkaline nature, but differentiation trends are different in different settings, and there does not seem to be any specific kind of rock composition that is uniquely related to porphyry copper plutons. Initial Sr ratios for rock types of New Guinea and the American southwest are closely similar for both barren and productive plutons and for coeval volcanic rocks within regions, but the ratios differ considerably from one region to another.
A review of plate tectonic phenomena as they may relate to porphyry copper evolution suggests that episodes of intrusion in the Pacific basin rim correspond to times of higher convergent rates, to times when the convergence style changed and, most fundamentally, to destructive plate margins. Different convergence styles, such as oblique convergence, may prepare the crust to permit rapid access of magmas to shallow levels. Copper metallogeny has been related by some to subducted ocean crust; others interpret some metals as inherited from crust or mantle. Nevertheless, consideration of events of plate history in the context of intrusion episodes reveals, almost unequivocally, that a close cause and effect exists between the geotectonics of plate interactions and porphyry copper formation.
Intrusions related to porphyry copper-type mineralization differ from nonproductive plutons by having persistent and pervasive hydrothermal effects that extend into the surrounding wall rocks. Characteristic assemblages of alteration minerals exhibit systematic spatial and temporal relationships with respect to one another, but the patterns and compositions differ in quartz monzonitic and dioritic intrusions. Quartz monzonitic rocks have a central potassic or K-silicate zone containing hydrothermal K-feldspar and biotite, an intermediate zone of quartz + sericite, and a peripheral propylitic halo containing epidote, chlorite, and albite similar to green-schist facies metamorphism. Hypogene mineralization consisting of low (~1 vol %) total sulfides but high chalcopyrite/pyrite ratios occurs in the outer portion of the potassic zone surrounding a barren core; the surrounding quartz + sericite zone has considerably higher sulfide content, consisting almost entirely of pyrite. In dioritic rocks, central potassic alteration is pre-dominanted by biotite, and quartz + sericite alteration is largely lacking. Consequently, these deposits characteristically have lower sulfide contents than the quartz monzonite analogue, although hypogene copper may be higher in the diorite type. Alteration in silicate wall rocks, commonly continues outward from that of the central intrusion. In carbonate wall rocks, on the other hand, calc-silicate phases are developed, consisting chiefly of garnet and diopsidic pyroxene, which are bordered outward from the intrusion by marble. This anhydrous skarn is commonly crosscut by chlorite, epidote, talc, calcite, quartz, and/or clay minerals. Hypogene mineralization consisting of chalcopyrite and sphalerite takes place at the same time as the formation of anhydrous skarn, or at least prior to formation of the later hydrous assemblage. The upper portions of intrusions or preore lithocap contain chiefly argillic or solfataric alteration comprising clay minerals, fine-grained silica, and alunite accompanied, apparently, by chalcopyrite, enargite, pyrite, and hematite. It has been tentatively suggested that such mineralization is in part derived by leaching and remobilization of intrusion-related mineralization at depth.
Mineral stability diagrams provide a useful link for interpreting diverse alteration mineral assemblages in a variety of geologic environments. Most important is the suggestion that propylitic (chlorite ± epidote) alteration is a high Mg-Ca analogue to the quartz + sericite assemblage and may occur in dioritic environments. Additionally, stability of chalcopyrite requires a relatively restricted range of interrelated environmental factors, including temperature, activities of sulfur and oxygen, and the iron/hydrogen ratio in the hydrothermal fluid. Examination of mineral stability diagrams in view of the spatial and temporal relationships among hydrothermal minerals in the porphyry copper deposits indicates that with time, fluids within these systems produce alteration compatible with decreasing temperatures or lower cation/hydrogen activity ratios. Both changes can be called upon as explanations for certain observed zoning and paragenetic features.
Oxygen isotope studies indicate that early potassic alteration in intrusions was generated by fluids derived by magmatic processes, and that later quartz + sericite was produced from fluids of meteoric origin. Fluid inclusion studies suggest that magmatic fluids had high salinities (on the order of 30 to 60 wt % NaCl equiv.), whereas the meteoric variety were far more dilute (<15 wt %). Evidence for boiling (or condensation) of both fluid types is seen at various deposits, but the occurrence of such is neither systematic nor universal. Computed fluid-flow models predict that at various positions in the porphyry copper system, these two fluids would appear at different times and in different relative and absolute abundances. Such fluid flow patterns around and through a cooling pluton are greatly influenced by the distribution of rock permeabilities, which are directly related to the abundance and nature of fractures. With time, abundance of fractures appears to be increasingly confined to the intrusion-wall rock contact, with late hydrothermal effects generally concentrated in this space. No clear-cut evidence exists to relate fluids of either predominantly magmatic or meteoric origin to hypogene mineralization.
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