The Agnew-Wiluna greenstone belt of Western Australia is the largest komatiite-hosted nickel sulfide belt in the world and contains two world-class Ni-Cu-(PGE) deposits and a host of smaller deposits. This study focuses on the broader scale geology of this greenstone belt in order to understand the key controls on the genesis of the komatiite-hosted Ni-Cu-(PGE) deposits, with specific focus on camp to district controls. We apply multiple sulfur isotopes to this geologic framework and conclude not only that the addition of crustal sulfur is a prerequisite for ore genesis in komatiite systems, but above all that the sulfur required to generate world-class deposits is most likely derived from barren volcanic massive sulfide lenses, which are spatially and genetically associated with felsic volcanic and volcaniclastic sequences that were emplaced coevally with large komatiitic sills and channelized lava flows. Multiple sulfur isotope data can be utilized in exploration at the deposit to district scales. At the deposit scale, the spatial pattern of mass-independent S isotope values (Δ 33 S) provides crucial insight into the identification of proximal high-grade and high-tenor ores in mineralized systems. In fact, sulfur data reflect the assimilation process that occurred upon komatiite emplacement, whereby hot turbulent magma thermomechanically eroded and assimilated exhalative sulfides spatially located close to vent with negative to near zero Δ 33 S values, whereas less turbulent flows interacted with distal sulfidic shales having Δ 33 S values above 0 per mil. Accordingly, the spatial variation of multiple sulfur isotope values in magmatic sulfides and associated host rocks may be utilized as a vector towards high-grade ores of poorly known systems. At the district scale, rather than ascertaining what controls the distribution of komatiite-hosted Ni-Cu-(PGE) deposits, the appropriate question to ask is what controls the distribution of country rock sulfides, considering that exhalative sulfides may be crucial to ore genesis in komatiite systems. We propose that felsic lava domes unambiguously mark their vents and can be directly mapped or inferred from gravity data. This work provides the first step in identifying district-scale control on komatiite-hosted Ni-Cu-(PGE) deposits. This is the scale that has high impact on exploration for new komatiite-hosted nickel sulfide belts globally.
The spatial distribution of mineral deposits is a critical component of predictive estimation of undiscovered mineral resources. Nickel sulfide deposits in the Kalgoorlie terrane of Western Australia, the world’s premier province for komatiite-hosted nickel sulfide deposits, are generally perceived to be clustered. We apply distance-based spatial analysis methods (nearest neighbor and K function) to determine the spatial distribution pattern of nickel sulfide deposits in the Kalgoorlie terrane. Results of these spatial analyses indicate the komatiite bodies that contain the nickel sulfide deposits in the terrane are clustered. In contrast, nickel sulfide deposits within komatiite bodies are either randomly distributed or dispersed and not clustered. Therefore, the apparent clustering of nickel sulfide deposits within the Kalgoorlie terrane may be a mere expression of the underlying clustering of the host komatiite bodies. These findings have two main implications: (1) nickel exploration models implemented through area selection based on localization controls of favorable komatiite bodies, followed by direct detection of deposits within komatiite bodies have spatio-statistical validity, and (2) a Poisson distribution could be a plausible initial model for predicting the number of nickel sulfide deposits within a komatiite body.
Une analyse isotopique in situ de l'amphibole ignee provenant de la ferropicrite de Boston Creek, ceinture de l'Abitibi, en Ontario, une unite mafique-ultramafique de 100 m d'epaisseur, fournit une nouvelle information a propos de l'evolution de magmas archeens enrichis en H 2 O, derives du manteau, et mis en place dans la croute. Les analyses faites avec une microsonde ionique de la magnesiohastingsite interstitielle d'une roche ultramafique indique un grand intervalle de valeurs de δD, allant de -47 a +54‰ et une teneur de 2-3% H 2 O (poids) a une echelle intragranulaire de 100 a 1000 μm, mais une composition relativement uniforme en termes des elements majeurs et des elements traces incompatibles. Ce grand intervalle depasse largement les valeurs de δD obtenues pour les roches globales, -50‰, les valeurs inferieures a +10‰ pour l'amphibole ignee des filons-couches ultramafiques, les valeurs entre -80 et -30%c pour les fluides hydrothermaux-metamorphiques de la region, et les valeurs entre -90 et -60‰ pour les materiaux du manteau. Les valeurs de δD anomalement elevees ne sauraient etre attribuees a la seule perte de H de l'amphibole au cours de sa recristallisation secondaire. Le contraste avec la composition et les teneurs en H 2 O et en elements traces relativement uniformes excluent la possibilite que les valeurs de δD temoignent d'un fractionnement variable entre phase fluide et mineral. C'est plutot a la composition du magma ferropicritique a partir duquel l'amphibole s'est formee que nous attribuons les valeurs anomalement elevees en 8D. Compte tenu de la distribution repandue et de l'abondance des bulles dans la ferropicrite de Boston Creek, ces values semblent indiquer un degazage de l'hydrogene par rapport au deuterium, possiblement par dissociation de H 2 O pendant la mise en place du magma a faible profondeur dans la croute. L'augmentation en degre d'oxydation du magma residuel qui en resulta a cause la cristallisation d'un oxyde Fe-Ti, ce qui diminua la teneur en FeO du magma et declencha une mineralisation en sulfures de Cu-PGE-Ag associee a la formation du gabbro a magnetite, la roche hote.
We present a new lithogeochemical method to target prospective komatiites that may host Ni-Cu-(PGE) deposits. The new methodology is based on the geochemical properties of ruthenium (Ru) and chromium (Cr), elements that are immobile under most conditions; it relies on a restricted number of carefully selected representative samples and is applicable in highly altered terrains. Ruthenium is a platinum-group element (PGE) that exhibits contrasting geochemical behavior in sulfide-saturated and sulfide-undersaturated komatiites. Similarly to other PGEs, Ru shows highly chalcophile behavior in magmas that equilibrate with an immiscible sulfide phase. However, Ru is also compatible in chromite in sulfide-undersaturated systems. If we consider Cr concentration as an index of chromite abundance in chromite-saturated komatiites, we observe that Ru increases or decreases systematically with increasing Cr according to the sulfide saturation state of the magmatic system. In rocks that crystallized from sulfide-saturated melts, Ru contents decrease with increasing Cr. Conversely, in rocks that crystallized from sulfide-undersaturated melts, Ru contents increase with increasing Cr. As a result, on the basis of the Ru-Cr variation it is possible to discriminate whether a komatiite melt equilibrated with a sulfide liquid during crystallization. The strength of this method compared to previous PGE-based lithogeochemical techniques derives from combining the traditional use of the geochemical properties of a highly immobile and chalcophile element that records the ore-forming process (ruthenium) with the occurrence of a mechanically and chemically resistant mineral phase (chromite), which is generally preserved in highly altered komatiites.
A two-pronged approach involving GIS-assisted manual prospectivity analysis and GIS-based fuzzy prospectivity analysis was used for identifying the most prospective ground for hydrothermal Ni deposits in Tasmania. The manual analysis involved (1) defining the process model and identifying key mineralization processes responsible for formation of hydrothermal Ni deposits; (2) manually estimating the probability of occurrence of each mineralization process based on the available information; (3) combining the above probabilities to derive overall probability of occurrence of hydrothermal Ni deposits in various geological regions of Tasmania. The Geological Information System (GIS)-based analysis involved representing the mineralization processes in the form of spatial predictor maps and using a fuzzy logic model to integrate the predictor maps and to derive a hydrothermal Ni prospectivity map of Tasmania. The results indicate that the most prospective areas for hydrothermal Ni in Tasmania are located along the boundary between the Dundas and Rocky Cape regions. This study further indicates that parts of the Ferrar Large Igneous Province (LIP) in central and Eastern Tasmania, which are presently considered non-prospective, could be potentially prospective for hydrothermal Ni deposits. A major advantage of the approach is that it can be easily adapted and applied to hydrothermal Ni prospectivity analysis worldwide.
Abstract In situ trace element and isotopic data of hydromagmatic phases from the Valmaggia peridotite pipe provide insights into the origin of the metasomatic fluids that affected the Ivrea‐Verbano Zone (NW Italy) during the late Carboniferous and shed new light on genetic models for the formation of Ni–Cu–PGE deposits. Volatiles implicated in the formation of hydromagmatic phases are not hydrothermal and did not derive from crustal assimilation. Low boron concentrations exclude the implication of fluids derived from dehydration of a subducted slab and indicate an origin from mantle‐derived juvenile water. Metasomatism introduced elevated contents of alkalis, Cu, PGEs and S into the depleted mantle of the Ivrea‐Verbano Zone. Increased water activity caused the harzburgite to undergo partial melting, thus producing pockets of volatile‐rich sulphide‐bearing ultramafic magma that evolved to form independent intrusions that host Ni–Cu–PGE mineralization.
A new style of komatiite-associated sulfide-poor platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) mineralisation has been identified at Wiluna in the strongly nickel sulfide (NiS) mineralised Agnew – Wiluna Greenstone Belt, Western Australia. The komatiite sequence at Wiluna is ∼200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate-textured peridotite core, which is overlain by rhythmically layered wehrlite, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic (harrisitic) domains are common features, whereas spinifex-textured horizons and flow-top breccias are absent. The presence of anomalous PGE-enriched horizons (ΣPt – Pd = 200 – 500 ppb) in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained (1 – 10 μm) platinum-group minerals (PGMs). More than 70 PGM grains were identified, and a considerable mineralogical variability was constrained. However, only Pd – Pt-bearing phases were identified, whereas no Ir – Ru-bearing PGMs were found in any of the sections examined. Interestingly, all PGMs are not in paragenetic association with sulfides, and only sulfide-poor/free intervals contain significant PGM concentrations. The whole-rock PGE sequence largely reflects the PGM distribution. It is hypothesised that the Pd – Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir – Ru depletion in the upper mafic section of the sequence are the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulfide-undersaturated environment. The crystallisation of clinopyroxene in the oikocrystic pyroxenite horizon may have triggered the formation of Pt – Pd-bearing alloys and arsenides, which were the first PGMs to form. Stratiform sulfide-poor PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich sulfide-poor mineralisation zones in thick differentiated intrusions, rather than to other PGE-enriched zones in komatiite-hosted systems, where PGE enrichment is directly associated with accumulations of magmatic sulfides.
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