Hydrogen and oxygen isotope ratios in minerals from porphyry copper deposits

Abstract

The O 18 /O 16 , D/H and C 13 /C 12 values, where applicable, were measured on quartz (40 samples), K-feldspar (12), biotite (16), sericite (23), and calcite (7) from potassic, sericitic and argillic alteration assemblages and from fresh igneous rocks and veins in 9 North American porphyry copper and molybdenum deposits, and in 4 other hydrothermal mineral deposits. The porphyry copper deposit at Santa Rita, New Mexico was sampled in detail.Both sericites and biotites appear to preserve their O 18 /O 16 and D/H values, as these isotope ratios are not affected by later low-temperature exchange with local ground waters. A systematic correlation is, however, observed between the D/H ratios of sericites (and clays) from most Tertiary porphyry deposits and those of meteoric ground waters; this requires the presence of a significant meteoric water component in the hydrothermal fluids involved in sericitization (and argillization). In contrast to the sericites, the very restricted range of D/H ratios of biotites from Bingham, Ely and Santa Rita (--74 + or - 10ppm) indicates the probable dominance of magmatic waters in the fluids associated with biotite alteration. The oxygen isotope data for the quartz-K-feldspar-biotite-chalcopyrite assemblages also support a high temperature of formation in the presence of waters with delta O 18 -values in the magmatic water range. However, at the Butte deposit meteoric waters were apparently associated with the early biotitization.Many problems are associated with the application of isotopic geothermometers to the alteration assemblages because of the general presence of: (1) retrograde exchange, especially of the K-feldspars, (2) isotopically heterogeneous minerals, such as quartz of different origins, that are intimately intergrown and thus difficult to separate physically, and (3) the common lack of independent criteria for recognizing isotopic equilibrium. In the quartz-sericite assemblage the delta O 18 of the igneous quartz is preserved and is different from that of quartz precipitated during the sericitization processes.The Santa Rita isotopic data are discussed in terms of two simplified end-member models to evaluate the importance of temperature variations relative to variations in the O 18 /O 16 ratio of the hydrothermal fluids; both types of variations are probable. Calculated temperature ranges for the combined models are 580 degrees to 390 degrees C for potassic alteration (note that these orthoclases are very K-rich, Or (sub 90-96) ), and 390 degrees to 285 degrees C for sericite alteration, with delta O 18 -water ranging from 7.3 to 3.9 per mil. Similar temperatures for sericite formation were obtained independently by direct isotopic analysis of some quartz-sericite assemblages, utilizing the quartz-muscovite O 18 /O 16 geothermometer. C 13 /C 12 values of hydrothermal vein carbonates in the porphyry copper deposits (delta = -- 2.6 to --5.9) tend to be a little heavier than primary igneous calcites from carbonatites (which typically have delta = -- 5 to --8ppm). The similarities between the isotopic variations in the calculated hydrothermal waters associated with sericitization and argillization and those of saline formation waters of North America suggest that heated Na-Ca-Cl brines originally present in the neighboring sedimentary and volcanic sections may be an important component of the hydrothermal fluids. The quantities of meteoric ground water that have interacted with the hot igneous stocks of the porphyry ore deposits, though very significant, are much smaller than the amounts involved in the vein systems at Butte and in many epizonal Tertiary igneous intrusions emplaced into permeable volcanic country rocks.