Copper isotope ratios in magmatic and hydrothermal ore-forming environments

Abstract

Multi-collector inductively coupled plasma mass spectrometry now provides for precise and accurate measurements of Cu isotope ratios. Copper minerals prepared by direct dissolution with and without chromatographic purification yield identical Cu isotope ratios within analytic precision of about 0.04x(1r). Cu isotope ratios have been measured for copper minerals from worldwide magmatic and hydrothermal copper deposits, and for several weathered deposits. Natural variations in d65Cu values, relative to NBS976, range over 9x. Chalcopyrite samples from mafic intrusions lie within a narrow range of about 1.5x, and most cluster tightly between 0.10xand 0.20x. This range lies within the broader black smoker chalcopyrite and iron meteorite ranges, and possibly represents a bulk mantle Cu isotope ratio. Most values for hydrothermal native copper from the Michigan native copper district also show a narrow range just larger than 0.1xand suggest a common homogeneous source for Cu in this large hydrothermal system. Later copper sulfide and arsenide minerals from this district range to values more than 2x lower than native copper. Ratios for chalcopyrite and bornite from moderate to high-temperature porphyry, skarn, and replacement deposits as a group and within individual deposits exhibit a broad range of values. Variations of nearly 1xare observed over distances on the order of 1 m. In some cases, these variations may result from multiple mineralization events or copper remobilization during retrograde or later hydrothermal activity. Fractionations between chalcopyrite and bornite, where they occur in the same sample or in related samples, cluster near 0.4x, suggesting equilibrium Cu isotope fractionation between them at moderate temperatures. In addition, weathering of hydrothermal copper minerals produces a wide range of values in secondary copper phases. In the supergene environment, cuprite typically has higher values than associated native copper. Therefore, redox states appear to exert a significant control over fractionation at low temperatures. Significant questions remain to be answered. Before the source of copper in hydrothermal environments can be fully addressed, source reservoir Cu ratios need to be determined, and chemical and physical factors that control Cu isotope fractionation must be quantitatively defined. D 2003 Elsevier B.V. All rights reserved.