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Copper isotope ratios in magmatic and hydrothermal ore-forming environments


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Larson, PB, Maher, K, Ramos, FC, Chang, Z, Gaspar, M and Meinert, LD 2003 , 'Copper isotope ratios in magmatic and hydrothermal ore-forming environments' , Chemical Geology, vol. 201, no. 3-4 , pp. 337-350 , doi: 10.1016/j.chemgeo.2003.08.006.

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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.

Item Type: Article
Authors/Creators:Larson, PB and Maher, K and Ramos, FC and Chang, Z and Gaspar, M and Meinert, LD
Keywords: Cu; Isotope geochemistry; Hydrothermal; Mass fractionation
Journal or Publication Title: Chemical Geology
ISSN: 0009-2541
DOI / ID Number: 10.1016/j.chemgeo.2003.08.006
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