Regional Copper Isotope Variations in the Giant Mount Isa Copper Mineral System—Toward a Basinal Origin

Analysis of regionally altered mafic rocks for δ56Fe and δ65Cu in the Mount Isa mineral province has identified a 25- × 50-km 65Cu-enriched zone to the east of the Mount Isa copper deposit. This δ65Cu anomaly coincides with the northern part of a zone enriched in 56Fe and depleted in copper. The timing of basalt alteration and the zones of 65Cu enrichment and copper enrichment overlap, such that the high δ65Cu zone is interpreted to mark the minimum extent of the mineral system that formed the Mount Isa copper deposit. The observed 65Cu enrichment in the altered mafic rocks can be modeled as residual copper left from copper depletion of the basalt, assuming that Rayleigh isotopic fractionation enriched the residual basalt in 65Cu, closely matching the observed spatial distribution. Copper isotope, light stable isotope, and geochemical data from the ores and altered mafic rocks suggest that the ore fluid was an evolved brine sourced either from the Isa and/or Roper superbasins after the main contractional phase of the Isan orogeny, and not a metamorphic or magmatic-hydrothermal fluid. Modeling of chalcopyrite deposition via Rayleigh fractionation using the δ65Cu value of the modeled ore fluid results in an increase in δ65Cu away from the point of ore fluid entry of about 0.8 to 1.0‰, consistent with an increase in δ65Cu of ~1.0‰ away from major structures, as noted in a previous study. In addition to contribut- ing to genetic understandings of mineral system processes, δ65Cu data may enhance mineral exploration by identifying metal sources and deposit-scale zonation related to ore deposition.

Economic Geology

Economic Geology

120

1554-0774

N/A

7

16