Zoned Pyroxenes as Prospectivity Indicators for Magmatic Ni-Cu Sulfide Mineralization
Louise SchoneveldStephen J. BarnesHannu V. MakkonenMargaux Le VaillantDavid PatersonValentina TaranovicKaiyuan WangYa–Jing Mao
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Abstract:
Small intrusions dominated by olivine- and pyroxene-rich cumulates are well known to be favourable hosts to magmatic Ni-Cu-(Platinum Group Element - PGE) sulfide mineralization. Such intrusions are common in a variety of settings around the world, but only a very small proportion contain economically exploitable sulfides; these tend to be of conduit or chonolith style. If prospectivity could be discriminated from sparse sampling at early exploration stages, then the discovery rate for deposits of this type could be improved. To this end, a number of pyroxene-bearing samples from small intrusions containing magmatic sulphide deposits have been investigated including the Noril'sk-Talnakh camp in Siberia, the Kotalahti nickel belt in Finland, Ntaka Hill in Tanzania, Nova-Bollinger in the Albany-Fraser Orogen of Australia, Savannah in the Halls Creek Orogen of Australia, Jinchuan in central China, Xiarihamu in Tibet and Huangshanxi in the east Tianshan Ni province of NW China. To compare, samples from unmineralised intrusions in four of these regions were also investigated along with four mafic intrusions from other localities that are not associated with any known economic sulfide mineralisation. Using fine-scale (<5 μm/pixel) chemical imaging on the Australian Synchrotron, complex zoning in chromium was found in cumulate and poikilitic pyroxenes within the strongly mineralised intrusions. The zoning patterns can be separated into three distinct types: 1) abrupt zoning: a single change in trace element concentration with a sharp boundary; 2) sector zoning: hourglass style zonation; and 3) oscillatory zoning: small scale oscillations that are usually cyclic. Zoning of all three types can be present in a single grain. The presence of cumulus orthopyroxene with a combination of abrupt zoning, sector zoning and resorbed olivine inclusions has so far only been detected in mineralised intrusions. This combination of zoning patterns is postulated to be an indication of high magma flux and fluctuating cooling rates that accompany wall rock assimilation in dynamic conduits where sulphide liquid forms and accumulates. The distinctive zoning patterns reported here can, in many cases, be easily imaged using desktop microbeam XRF mapping techniques and may provide a useful fertility indicator for the exploration of new magmatic Ni-Cu-(PGE) deposits.Keywords:
Pyroxene
Prospectivity mapping
Platinum group
Ultramafic rock
Chromite
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Ultramafic massif of Bulqiza belongs to Eastern Jurassic Albanian ophiolite belt of IAT-BSV- type. This massif is the most important chromite-bearing ore. The mantle ultramafics have extremely refractory nature. This is due to the high partial fusion of upper mantle which is depleted in CaO and Al2 O3 . The chromitite is situated to different parts of ultramafic pile, from bottom Cpx harzburgites up to massive dunites and cumulate ultramafic but the mainly chromite potential belongs to mantle harzburgite –dunite level and to transition dunites partly. The chromite is chiefly of Cr-rich metallurgical type. The atomic ratios of chromite , Fo of olivine and some physical properties of them vary according to the chromitite setting and reflects the evolution of Ol-Sp equilibrium process depended of the chromite concentration, from baren dunitic lenses towards dunite envelops of the ore bodies and the interstitial and inclusions of olivine within chromite grains. Two particular chromite deposits are the Bulqiza- Batra tabular folded ore body and Shkalla, pencil –like ore body.
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The most productive chromite ore deposits are formed through crystallization of chromite-ore magmas under definite physico-chemical conditions. The formation of chromite ore is controlled mainly by the degree of differentiation of ultrabasic magma. How to measure the degree of ultrabasic magmatic differentiation is key to understanding the mechanism of formation of chromite ore. Many geologists base their understanding of ultrabasic magmatic differentiation on whether there are obvious petrographical-facies zones in ultrabasic rocks. This viewpoint needs further discussing because most ultrabasic rock bodies showing petrographical zonation contain no mineral deposits. On the contrary mineral deposits may occur in the ultrabasic rocks without obvious petrographical zonation. Therefore, the author suggests to use the chemical characteristics of minerals to determine the degree of ultrabasic magmatic differentiation as a guide to search for chromite ore deposits.
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Ultramafic complexes containing chromite-bearing units occur scattered over a large area of the Western Dharwar Craton (WDC). They are broadly separated into two groups; the older complexes, forming a part of the oldest known Sargur Group and occurring as dismembered enclaves within the Peninsular Gneissic Complex, and the younger, forming part of greenstone belt sequence having emplaced in the early stages of basin formation. In this study, chromitebearing units of the ultramafic complexes of the older group forming the Nuggihalli belt and Rangapura-Shivani complex and the younger group forming the ultramafic complexes of Channagiri, Shankaraghatta and Usgao, have been selectively examined. While Shankaraghatta ultramafite contains sparse disseminations, the other complexes include in addition mm-cm scale bands and lenses\pods of chromite; commercially workable deposits of chromite are confined only to the Nuggihalli belt. Further, whereas PGE mineralization of possible commercial value is recognized in the Channagiri complex, the Shankaraghatta complex encloses Ni-Au-PGE mineralization. Almost 200 spot analyses of chromite grains occurring in the five different ultramafic complexes of WDC have been obtained. Although it is the most resistant mineral of the rocks, in the intensely altered Channagiri complex it shows deep alteration whereas in the other less altered complexes the alterations are localized to outer zones and margins of the mineral. The least altered Cr-spinel composition (Spl) with X Mg 0.01-0.656, Xcr 0.553-0.955 and X, 0.014-0.215 is on the whole in the range of aluminian chromite, but, it shows significant variations from one cornplex to the other and even within the limits of a single complex, related to primary magmatic processes. An interesting finding is the presence of occasional relics of titanium chromite in the PGE mineralized Channagiri complex. In addition to common alteration of Spl to ferrian chromite (SpII) (with A1 2 O 3 :0.47-6.46 %; MgO: 1.4-4.196; Cr 2 0,: 36.1-54.9 %; Fe20,: 11.6-23.1 %), alteration of the mineral to chromian magnetite and magnetite (with A1 2 0 3 ,: 0.01-0.84 %; MgO: 0.01-0.07 %; Cr 2 O 3 ,: 1.17-28.81 %; Fe 2 O 3 ,: 34.7-63.6 %) is also recorded. These alterations, as well as near total conversion of olivine and pyroxenes into antigorite and chlorite, were brought about by pervasive low-grade regional metamorphism which accompanied penetrative fluid action and deformation. The alteration comprised of two way diffusion of elements. While the formation of ferrian chrornite was associated with quantitative outward migration of Al, Mg and very moderate Cr into the surrounding and inward migration of iron from the surrounding and oxidation of iron, the conversion of chromite into Cr-magnetite/magnetite was brought about by quantitative outward migration of Cr, A1 and Mg into the surrounding and inward diffusion of iron. As has been the case with many of the examples studied world over, the Cr-spinels of WDC too have chemical characteristics which largely overlap with those of both layered and ophiolitic/podiform examples and do not permit their characterization exclusively as layered or podiform types.
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The paper presents the results of a study of ore and accessory chromite mineralization in the ultramafic rock complex of the Valizhgen Peninsula. The chromite mineralization belongs to the podiform type, is located in lens-shaped dunite bodies, and is represented by several structural-textural varieties. The ore and accessory chrome spinels have some specific compositional features that differ in the rocks of either series: chrome spinels of the cumulative dunite-orthopyroxenite-chromitite series have elevated Cr, Ti and Fe3+ contents, while spinels from residual harzburgites are typically low in Ti and Fe Fe3+ and variable in Cr and Mg contents. The specific location of chromitites in mafic-ultramafic massifs of the peninsula and the composition of ore and accessory chrome spinels from ultramafics can be accounted for by interaction of primitive mantle magmas with host harzburgites. The calculations using mineral thermobarometry indicate relatively low pressure and high temperature and oxygen fugacity during the formation of chromite mineralization. The chemical and Re-Os isotopic compositions are also studied for PGE minerals from placers of the Valizhgen Peninsula. We suggest that chromitites of the mafic-ultramafic massifs are sources of these placers.
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Mineral redox buffer
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