Mineralium Deposita (v.43, #5)

The Osborne iron oxide–copper–gold (IOCG) deposit is hosted by amphibolite facies metasedimentary rocks and associated with pegmatite sheets formed by anatexis during peak metamorphism. Eleven samples of ore-related hydrothermal quartz and two pegmatitic quartz–feldspar samples contain similarly complex fluid inclusion assemblages that include variably saline (<12–65 wt% salts) aqueous and liquid carbon dioxide varieties that are typical of IOCG mineralisation. The diverse fluid inclusion types present in each of these different samples have been investigated by neutron-activated noble gas analysis using a combination of semi-selective thermal and mechanical decrepitation techniques. Ore-related quartz contains aqueous and carbonic fluid inclusions that have similar 40Ar/36Ar values of between 300 and 2,200. The highest-salinity fluid inclusions (47–65 wt% salts) have calculated 36Ar concentrations of approximately 1–5 ppb, which are more variable than air-saturated water (ASW = 1.3–2.7 ppb). These fluid inclusions have extremely variable Br/Cl values of between 3.8 × 10−3 and 0.3 × 10−3, and I/Cl values of between 27 × 10−6 and 2.4 × 10−6 (all ratios are molar). Fluid inclusions in the two pegmatite samples have similar 40Ar/36Ar values of ≤1,700 and an overlapping range of Br/Cl and I/Cl values. High-salinity fluid inclusions in the pegmatite samples have 2.5–21 ppb 36Ar, that overlap the range determined for ore-related samples in only one case. The fluid inclusions in both sample groups have 84Kr/36Ar and 129Xe/36Ar ratios that are mainly in the range of air and air-saturated water and are similar to mid-crustal rocks and fluids from other settings. The uniformly low 40Ar/36Ar values (<2,200) and extremely variable Br/Cl and I/Cl values do not favour a singular or dominant fluid origin from basement- or mantle-derived magmatic fluids related to A-type magmatism. Instead, the data are compatible with the involvement of metamorphic fluids that have interacted with anatectic melts to variable extents. The ‘metamorphic’ fluids probably represent a mixture of (1) inherited sedimentary pore fluids and (2) locally derived metamorphic volatilisation products. The lowest Br/Cl and I/Cl values and the ultra-high salinities are most easily explained by the dissolution of evaporites. The data demonstrate that externally derived magmatic fluids are not a ubiquitous component of IOCG ore-forming systems, but are compatible with models in which IOCG mineralisation is localised at sites of mixing between fluids of different origin.
Keywords: Osborne; Mt Isa; IOCG; Australia; Argon isotopes; Halogens

Integrated geological and geophysical investigations were carried out in 26 active mines in Northwestern China during the period 2001–2006 to explore for hidden extensions of known ore bodies and to search for new mineralization. This paper presents four case studies from northwest China: the Kuoerzhenkuoa volcanogenic hydrothermal gold deposit, the Nanjinshan breccia-associated gold deposit, the Duolanasayi deposit, associated with a ductile–shear zone, and the Hulu magmatic Cu–Ni sulfide deposit. In these studies, detailed mine-scale geological studies were carried out to determine the location and controls on ore formation. Based on these investigations and a review of previous exploration data, genetic models for the deposits were evaluated, and specific new targets were generated. These target areas were tested with surface geophysical surveys using the Stratagem EH4 system, a hybrid-source magnetotellurics (MT) method. Analysis of the data obtained in the surveys identified geophysical target anomalies that were subsequently drilled. Many of these test holes demonstrate the presence of Au and Cu–Ni mineralization. Evaluation of the geological models was crucial in developing conceptual targets as a basis for surface geophysical surveys. These models established the most likely target areas where Au and Cu–Ni mineralization could occur, but they did not define the limits or the geometries of the mineralized zones. Hybrid MT surveys played an important role in defining the location of buried mineralized systems and in testing the validity of the conceptual targets. The resistivity cross-sections obtained by imaging the MT data established the boundaries and geometries of the host rocks, including the distribution of lithology, structures, alteration, and mineralization. The four case studies in this paper show how this integrated geological and geophysical approach was used successfully to discover hidden mineral deposits.
Keywords: Prediction of hidden ore bodies; Hybrid magnetotelluric; Kuoerzhenkuola gold deposit; Nanjinshan gold deposit; Duolanasayi gold deposit; Hulu Cu–Ni deposit

High resolution X-ray computed tomography studies of Grasberg porphyry Cu-Au ores, Papua, Indonesia by J. Richard Kyle; Alison S. Mote; Richard A. Ketcham (519-532).
High-resolution X-ray computed tomography (HRXCT) provides unique information of the geological and metallurgical significance for gold and related ore minerals in the supergiant Grasberg porphyry Cu–Au deposit. Digital radiographs have proved to be an effective means of screening samples for the presence of gold for HRXCT studies. Digital radiograph effectiveness is limited by the thickness of samples (typically to ≤2 cm), as well as the associated minerals. Thus, preselecting samples for gold studies using HRXCT is most effective using digital radiographs combined with assay information. Differentiating between metallic mineral grains with relatively small differences in density, e.g., bornite (5.1 g/cm3) from chalcopyrite (4.2 g/cm3), is relatively straightforward for isolated monominerallic grains or composites in a similar lower-density matrix, but difficulties are encountered with the interpretation of typical intergrown ore minerals. X-ray beam-hardening artifacts lead to inconsistency in attenuation determination, both within and among slice images, complicating quantitative processing. However, differentiation of chalcopyrite and bornite has been successful in smaller-diameter (≤22-mm) cores of Grasberg ores. Small-diameter (≤10 mm) cores of the Grasberg stockwork Cu–Au ore were analyzed using HRXCT methods scanned at the minimum spacing currently available (7.5 μm), and data reduction protocols using the Blob3D program were modified to improve the quantification of grain sizes and shapes. Grains as small as 6.5 μm have been identified. All of these grains are in direct contact with chalcopyrite, providing support for gold distribution in porphyry copper systems being a result of exsolution from copper sulfides. HRXCT scanning (±digital radiography) precisely defines the in situ location of mineral grains of interest within a sample, which then can be studied in conventional petrographic sections, and other types of analytical studies conducted, e.g., gold trace element geochemistry.
Keywords: X-ray computed tomography; Porphyry Cu–Au; Mineralogy; Exsolution; Ore processing; Grasberg

Fluid and source magma evolution of the Questa porphyry Mo deposit, New Mexico, USA by Leonhard M. Klemm; Thomas Pettke; Christoph A. Heinrich (533-552).
Combined fluid inclusion microthermometry and microanalysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) are used to constrain the hydrothermal processes forming a typical Climax-type porphyry Mo deposit. Molybdenum mineralisation at Questa occurred in two superimposed hydrothermal stages, a magmatic-hydrothermal breccia and later stockwork veining. In both stages, texturally earliest fluids were single-phase, of low salinity (~7 wt.% NaClequiv.) and intermediate-density. Upon decompression to ~300 bar, they boiled off a vapour phase, leaving behind a residual brine (up to 45 wt.% NaClequiv) at temperatures of ~420°C. The highest average Mo concentrations in this hot brine were ~500 μg/g, exceeding the Mo content of the intermediate-density input fluid by about an order of magnitude and reflecting pre-concentration of Mo by fluid phase separation prior to MoS2 deposition from the brine. Molybdenum concentrations in brine inclusions, then, decrease down to 5 μg/g, recording Mo precipitation in response to cooling of the saline liquid to ~360°C. Molybdenite precipitation from a dense, residual and probably sulphide-depleted brine is proposed to explain the tabular shape of the ore body and the absence of Cu-Fe sulphides in contrast to the more common Cu-Mo deposits related to porphyry stocks. Cesium and Rb concentrations in the single-phase fluids of the breccia range from 2 to 8 and from 40 to 65 μg/g, respectively. In the stockwork veins, Cs and Rb concentrations are significantly higher (45–90 and 110–230 μg/g, respectively). Because Cs and Rb are incompatible and hydrothermally non-reactive elements, the systematic increase in their concentration requires two distinct pulses of fluid exsolution from a progressively more fractionated magma. By contrast, major element and ore metal concentrations of these two fluid pulses remain essentially constant. Mass balance calculations using fluid chemical data from LA-ICPMS suggest that at least 25 km3 of melt and 7 Gt of deep input fluid were necessary to provide the amount of Mo contained in the stockwork vein stage alone. While the absolute amounts of fluid and melt are uncertain, the well-constrained element ratios in the fluids together with empirical fluid/melt partition coefficients derived from the inclusion analyses suggest a high water content of the source melt of ~10%. In line with other circumstantial evidence, these results suggest that initial fluid exsolution may have occurred at a confining pressure exceeding 5 kbar. The source of the molybdenum-mineralising fluids probably was a particularly large magma chamber that crystallised and fractionated in the lower crust or at mid-crustal level, well below the shallow intrusions immediately underlying Questa and other porphyry molybdenum deposits.
Keywords: Porphyry molybdenum; Fluid inclusion; Questa

The syn-tectonic breccia-hosted Mount Isa Cu deposit in northwest Queensland is the largest sediment-hosted Cu deposit in Australia. Whole-rock samples of chalcopyrite-rich Cu ore form an isochron with a Re–Os age of 1,372 ± 41 Ma. This age is more than 100 Ma younger than the previously accepted age of Cu ore formation, an Ar–Ar mineral age for biotite separated from the host rocks within the alteration envelope to the Cu orebody. This discrepancy cannot be unequivocally resolved due to a lack of other absolute geochronological constraints for Cu mineralisation or the deformation event associated with Cu emplacement. The 1,372 ± 41 Ma date may reflect (a) the time of Cu deposition, (b) the time of a hydrothermal event that reset the Re–Os signature of the Cu ore or (c) mixing of the Re–Os isotope systematics between the host rocks and Cu-bearing fluids. However, a range of published Ar–Ar and Rb–Sr dates for potassic alteration associated with Cu mineralisation also records an event between 1,350 and 1,400 Ma and these are consistent with the 1,372 Ma Re–Os age. The 1.8 Ga Eastern Creek Volcanics are a series of tholeiitic basalts with a primary magmatic Cu enrichment which occur adjacent to the Mount Isa Cu deposit. The whole-rock Os isotopic signature of the Eastern Creek Volcanics ranges from mantle-like values for the upper Pickwick Member, to more radiogenic/crustal values for the lower Cromwell Member. The Re–Os isotope signature of the Cu ores overlaps with those calculated for the two volcanic members at 1,372 Ma; hence, the Os isotope data are supportive of the concept that the Os in the Cu ores was sourced from the Eastern Creek Volcanics. By inference, it is therefore postulated that the Eastern Creek Volcanics are the source of Cu in the Mount Isa deposit, as both Os and Cu are readily transported by oxidised hydrothermal fluids, such as those that are thought to have formed the Cu orebody. The Pickwick Member yields a Re–Os isochron age of 1,833 ± 51 Ma, which is within error of previously reported age constraints. The initial 187Os/188Os isotopic ratio of 0.114 ± 0.067 (γOs = −0.7) is slightly subchondritic, and together with other trace element geochemical constraints, is consistent with a subcontinental lithospheric mantle source. The Pickwick Member records a minimum age of ca. 1.95 Ga for melt depletion in the subcontinental lithospheric mantle beneath the Mount Isa Inlier prior to the extraction of the magmas which formed the Eastern Creek Volcanics. This corresponds with the end of subduction-related magmatism along the eastern margin of the Northern Australian Craton, which included the Mount Isa Inlier.
Keywords: Re–Os isotopes; Mount Isa; Cu; Eastern Creek Volcanics; Australia

Anhydrite pseudomorphs and the origin of stratiform Cu–Co ores in the Katangan Copperbelt (Democratic Republic of Congo) by Ph. Muchez; P. Vanderhaeghen; H. El Desouky; J. Schneider; A. Boyce; S. Dewaele; J. Cailteux (575-589).
The stratiform Cu–Co ore mineralisation in the Katangan Copperbelt consists of dispersed sulphides and sulphides in nodules and lenses, which are often pseudomorphs after evaporites. Two types of pseudomorphs can be distinguished in the nodules and lenses. In type 1 examples, dolomite precipitated first and was subsequently replaced by Cu–Co sulphides and authigenic quartz, whereas in type 2 examples, authigenic quartz and Cu–Co sulphides precipitated prior to dolomite and are coarse-grained. The sulphur isotopic composition of the copper–cobalt sulphides in the type 1 pseudomorphs is between −10.3 and 3.1‰ relative to the Vienna Canyon Diablo Troilite, indicating that the sulphide component was derived from bacterial sulphate reduction (BSR). The generation of $${ ext{HCO}}_3^ - $$ during this process caused the precipitation and replacement of anhydrite by dolomite. A second product of BSR is the generation of H2S, resulting in the precipitation of Cu–Co sulphides from the mineralising fluids. Initial sulphide precipitation occurred along the rim of the pseudomorphs and continued towards the core. Precipitation of authigenic quartz was most likely induced by a pH decrease during sulphide precipitation. Fluid inclusion data from quartz indicate the presence of a high-salinity (8–18 eq. wt.% NaCl) fluid, possibly derived from evaporated seawater which migrated through the deep subsurface. 87Sr/86Sr ratios of dolomite in type 1 nodules range between 0.71012 and 0.73576, significantly more radiogenic than the strontium isotopic composition of Neoproterozoic marine carbonates (87Sr/86Sr = 0.7056–0.7087). This suggests intense interaction with siliciclastic sedimentary rocks and/or the granitic basement. The low carbon isotopic composition of the dolomite in the pseudomorphs (−7.02 and −9.93‰ relative to the Vienna Pee Dee Belemnite, V-PDB) compared to the host rock dolomite (−4.90 and +1.31‰ V-PDB) resulted from the oxidation of organic matter during BSR.
Keywords: Anhydrite; Copperbelt; Cu–Co stratiform ore deposits; Fluid chemistry; Bacterial sulphate reduction; Democratic Republic of Congo

Late Carboniferous porphyry copper mineralization at La Voluntad, Neuquén, Argentina: Constraints from Re–Os molybdenite dating by Mirta Garrido; Fernando Barra; Eduardo Domínguez; Joaquin Ruiz; Victor A. Valencia (591-597).
The La Voluntad porphyry Cu–Mo deposit in Neuquén, Argentina, is one of several poorly known porphyry-type deposits of Paleozoic to Early Jurassic age in the central and southern Andes. Mineralization at La Voluntad is related to a tonalite porphyry from the Chachil Plutonic Complex that intruded metasedimentary units of the Piedra Santa Complex. Five new Re–Os molybdenite ages from four samples representing three different vein types (i.e., quartz–molybdenite, quartz–sericite–molybdenite and quartz–sericite–molybdenite ± chalcopyrite–pyrite) are identical within error and were formed between ~312 to ~316 Ma. Rhenium and Os concentrations range between 34 to 183 ppm and 112 to 599 ppb, respectively. The new Re–Os ages indicate that the main mineralization event at La Voluntad, associated to sericitic alteration, was emplaced during a time span of 1.7 ± 3.2 Ma and that the deposit is Carboniferous in age, not Permian as previously thought. La Voluntad is the oldest porphyry copper deposit so far recognized in the Andes and indicates the presence of an active magmatic arc, with associated porphyry style mineralization, at the proto-Pacific margin of Gondwana during the Early Pennsylvanian.