Melissa J. Gregory

Geochemical process model for the Mt Isa Cu-Co-Ag deposits

We present the results of numerical simulation of chemical processes that formed the Mount Isa copper orebodies. Reduction through wall-rock reaction, is the most likely chemical process, although mixing of two fluids of contrasting oxidation state may also have contributed. Most rocks in the Mount Isa region have the ability to reduce oxidized fluids. Porosity (instantaneous fluid to rock ratio) is highlighted as a critical control on ore grade and ratio of Cu to Pb and Zn. Mechanical properties of the host rock may there-fore be more important than chemical ones.

Magmatic history of the Eastern Creek Volcanics, Mt Isa, Australia: insights from trace-element and platinum-group-element geochemistry

The Paleoproterozoic basalts of the Eastern Creek Volcanics are a series of continental flood basalts that form a significant part of the Western Fold Belt of the Mt Isa Inlier, Queensland. New trace-element geochemical data, including the platinum-group elements (PGE), have allowed the delineation of the magmatic history of these volcanic rocks. The two members of the Eastern Creek Volcanics, the Cromwell and Pickwick Metabasalt Members, are formed from the same parental magma. The initial magma was contaminated by continental crust and erupted to form the lower Cromwell Metabasalt Member. The staging chamber was continuously replenished by parental material resulting in the gradual return of the magma composition to more primitive trends in the upper Cromwell Metabasalt Member, and finally the Pickwick Metabasalt Member formed from magma dominated by the parental melt. The Pickwick Metabasalt Member of the Eastern Creek Volcanics has elevated PGE concentrations (including up to 18 ppb Pd and 12 ppb Pt) with palladium behaving incompatibly during magmatic fractionation. This trend is the result of fractionation under sulfide-undersaturated conditions. Conversely, in the basal Cromwell Metabasalt Member the PGE display compatible behaviour during magmatic fractionation, which is interpreted to be the result of fractionation of a sulfide-saturated magma. However, Cu remains incompatible during fractionation, building up to high concentrations in the magma, which is found to be the result of the very small volume of magmatic sulfide formation (0.025%). Geochemical trends in the upper Cromwell Metabasalt Member represent mixing between the contaminated Cromwell Metabasalt magmas and the PGE-undepleted parental melt. Trace-element geochemical trends in both members of the Eastern Creek Volcanics can be explained by the partial melting of a subduction-modified mantle source. The generation of PGE- and copper-rich magmas is attributed to melting of a source in the subcontinental lithospheric mantle below the Mt Isa Inlier which had undergone previous melt extraction during an older subduction event. The previous melt extraction resulted in a sulfur-poor, metal-rich metasomatised mantle source which was subsequently remelted in the Eastern Creek Volcanic continental rift event. The proposed model accounts for the extreme copper enrichment in the Eastern Creek Volcanics, from which the copper has been mobilised by hydrothermal fluids to form the Mt Isa copper deposit. There is also the potential for a small volume of PGE-enriched magmatic sulfide in the plumbing system to the volcanic sequence.

Potassic alteration and veining and the age of copper emplacement at Mount Isa, Australia

Preliminary 187Re/188Os dating of whole rocks and sulphide separates from the Mount Isa copper orebody has generated an isochron age of 1367 ± 80 Ma (MSWD=49; n=6). This age is approximately 150 myr younger than published biotite 40Ar/39Ar ages previously assumed to date the copper-forming event at ca. 1523 Ma. These older ages are from rocks in which biotite is likely to be metamorphic rather than hydrothermal in origin. Unambiguous potassic alteration related to copper formation is characterised by biotite replacement of metabasalt (brownstones) and potassium feldspar replacement of meta-tuffs. Previous 40Ar/39Ar dating of biotite from brownstone yields 1352 Ma to 1385 Ma ages, while 87Rb/86Sr dating of potassium feldspar altered tuffs gives 1323 Ma. Muscovite from the Buck Quartz Fault, considered a conduit for copper mineralising fluids, yields an 40Ar/39Ar age of 1324 Ma. We suggest that these ages more accurately reflect the age of copper emplacement, whereas the older 40Ar/39Ar ages more likely relate to cooling from peak metamorphism.

Understanding Copper Isotope Behavior in the High Temperature Magmatic‐Hydrothermal Porphyry Environment

Copper stable isotope geochemistry has the potential to constrain aspects of ore deposit formation once variations in the isotopic data can be related to the physiochemical conditions during metal deposition. This study presents Cu isotope ratios for samples from the Pebble porphyry Cu-Au-Mo deposit in Alaska. The δ65Cu values for hypogene copper sulfides range from −2.09‰ to 1.11‰ and show linear correlations with the δ18O isotope ratios calculated for the fluid in equilibrium with the hydrothermal alteration minerals in each sample. Samples with sodic-potassic, potassic, and illite alteration display a negative linear correlation between the Cu and O isotope results. This suggests that fractionation of Cu isotopes between the fluid and precipitating chalcopyrite is positive as the hydrothermal fluid is evolving from magmatic to mixed magmatic-meteoric compositions. Samples with advanced argillic alteration display a weak positive linear correlation between Cu and O isotope results consistent with small negative fluid-chalcopyrite Cu isotope fractionation during fluid evolution. The hydrothermal fluids that formed sodic-potassic, potassic, and illite alteration likely transported Cu as CuHS0. Hydrothermal fluids that resulted in advanced argillic alteration likely transport Cu as CuCl2−. The pH conditions also control Cu isotope fractionation, consistent with previous experimental work. Larger fractionation factors were found between fluids and chalcopyrite precipitating under neutral conditions contrasting with small fractionation factors calculated between fluids and chalcopyrite precipitating under acidic conditions. Therefore, this study proposes that hydrothermal fluid compositions and pH conditions are related to Cu isotope variations in high temperature magmatic-hydrothermal deposits.

Platinum-group element geochemistry of the Eastern Creek Volcanics, Mount Isa, Australia

The Eastern Creek Volcanics (ECV) are a series of continental flood basalts that occur adjacent to the giant Mount Isa copper deposit. Platinum-group element (PGE) geochemistry from the Cromwell and Pickwick Metabasalt Members of the ECV display trends that allow the evolution of the sequence to be defined. The Cromwell Metabasalt was produced from a contaminated, mildly S-saturated magma that was copper and sulphur-rich and PGE-poor. Conversely, the Pickwick Metabasalt formed from a S-undersaturated magma that was S- and Cu-poor and PGE-rich. The Cromwell Metabasalt was then further enriched in Cu during magmatic fractionation because the mild degree of S-saturation did not allow Cu to be stripped from the magma by precipitating sulphides. This Cu enrichment makes the Cromwell Metabasalt an excellent source rock for the copper that formed the Mount Isa deposit.