Andy Wilde

New constraints on the polychronous nature of the giant Muruntau gold deposit from wall-rock alteration and ore paragenetic studies

Muruntau in north-central Uzbekistan is one of the biggest gold deposits in the world with >5200 t of contained Au. Detailed observations from thin-section petrography of ore samples from the super-giant deposit and quantitative XRD analyses of variably hydrothermally altered host-rocks are reported here. These new data are used to argue that the unique size and nature of the deposit is the result of a combination of distinct and potentially temporally separated ore-forming events that probably spanned at least 60 Ma. Consequently, describing Muruntau as an entirely orogenic, thermal-aureole, or intrusion-related type of deposit might not be valid, and caution should be used when using deposit-scale features of this enormous occurrence in the formulation of genetic or exploration models. †Present address: Paladin Energy Pty Ltd, 502 Hay Street, Perth WA 6008, Australia.

Mount Isa copper orebodies: improving predictive discovery

The most likely source for oxidised brines that produced the Mount Isa copper–cobalt deposits is the hematitic meta-sediments of the <1500 Ma South Nicholson Group or its lateral equivalents. Downwards movement of the brines into relatively impermeable basement rocks at about 1370 Ma is suggested to have been a product of reactivation of NNW–SSE faults (e.g. Transmitter fault) during post-Isan inversion. Evidence for the involvement of such structures includes spatial association with major orebodies and mica–dickite–hematite alteration indicative of the passage of hot, acidic and oxidised fluid. Copper depletion and K enrichment in metabasalt of the ca 1830 Ma Eastern Creek Volcanics along such structures has previously been documented. Anhydrite-rich rocks preserved at the northern end of the copper orebodies represent a zone of high fluid input, where the characteristics of the original fluid are best preserved. The most plausible chemical process generating economic Cu is reduction of oxidised brine during reaction with carbonaceous meta-sediments of the Urquhart Shale. Many lithologies in the region would have had the chemical capacity to extract Cu from hydrothermal solutions. The availability of suitable reductant rocks was therefore not a limiting factor on Cu deposition. Rather, the distribution of porosity and permeability would have controlled deposit location. The Urquhart Shale is a particularly favourable host unit on account of pervasive mechanical anisotropy produced by regular interlayering of ductile metapelite and brittle metasiltstone. Strain partitioning during post-Isan inversion resulted in pervasive permeability development in metasiltstone, particularly when steeply dipping. An implication of this research is that rocks of the South Nicholson Group and equivalents could be targetted for sediment-hosted stratiform copper (SSC) deposits.

Significance of monazite EPMA ages from the Quamby Conglomerate, Queensland

Th – U – Pb electron microprobe (EPMA) dating of mainly detrital monazite from the Quamby Conglomerate in the Eastern Succession of the Mt Isa inlier reveals three distinct monazite growth/recrystallisation events at around 1640, 1580 and 1490 Ma. These ages are particularly significant with respect to the timing of deposition, iron and gold mineralisation, and deformation in the Mt Isa inlier. The oldest age probably represents provenance from igneous rocks. In the sample, the majority of monazite growth occurred at 1580 Ma, coeval with peak metamorphism in the Eastern Succession. The low metamorphic grade of the conglomerate and wide compositional range of monazite bearing this age indicates that the monazite grew elsewhere and was later deposited in the conglomerate. Purple bands in the rock are composed mainly of coarse specular hematite with recrystallised margins that contribute to high (up to 20%) Fe2O3 contents in the conglomerate. Gold is also present in some of the samples. Some of the monazite grains contain small, younger (ca 1490 Ma) domains that may have grown/recrystallised in situ during a lower grade syn- or post-diagenetic metamorphic/hydrothermal event that may have been related to hematite (re)crystallisation. Together, these ages bracket deposition of the Quamby Conglomerate to between ca 1580 and 1490 Ma, the latter age most likely representing diagenesis. This depositional age also represents a maximum age for north – south-striking, upright folds of the Quamby Conglomerate and implies that significant ductile deformation has affected parts of the Mt Isa inlier after 1580 Ma and probably after 1490 Ma.

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.

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.

Syn-metamorphic dates for tourmaline formation around Mount Isa, north-west Queensland, Australia

Pb stepwise leaching of tourmaline from the Palaeoproterozoic Western Fold Belt of the Mount Isa Inlier yield dates of 1559±17 Ma, 1577±52 Ma and 1480±90 Ma. Structural, petrological and textural data suggest that 1559±17 Ma is the best estimate for peak metamorphism in the area. Estimates of Pb closure temperature and comparison with peak metamorphic temperatures indicate that this age represents tourmaline crystallisation. These dates represent syn-metamorphic fluid flow and indicate metal mobility during the Isan Orogeny. A result of this event is regional-scale silicification that has the same relative timing as extensive silica deposition in the rocks that now host the Cu orebodies at Mount Isa.

Preliminary study of the source of base metals in MVT deposits of the Canning Basin, Western Australia

New bulk chemical and mineralogical data for unmineralized sediments and a review of published lead isotope and permeability/porosity data indicate that Carboniferous and Devonian feldspar-rich sandstones and siltstones are the most likely reservoirs for one-forming brines and metals in the eastern Canning Basin, Australia. The Anderson Formation and Tandalgoo Sandstone are characterized by extreme depletion in Pb and Zn relative to Cu. Conversely, limestones and dolomites of the Devonian and Carboniferous rift-fill are enriched in Pb and Zn relative to Cu. This enrichment, is interpreted to be due to hydrothermal processes, suggesting that carbonate rocks of the Carboniferous-Devonian rift-fill sequence could be targeted for MVT deposits. Furthermore, anomalous Au, As and Sb in limestone of the Pillara Formation suggests that the possibility of Carlin-style gold deposits merits some consideration.

Microbial remains in middle proterozoic rocks of Northern Australia

Investigation of the samples of the McArthur River complex ore deposit, one of the most zinc-lead mineral province in the world, brings us to conclusion about possibility of biogenic origin of sulfides in McArthur River ore deposit and to make supposition about formation of studied rocks in the photic zone of the sea.