ORE GENESIS OF PGE MINERALIZATION IN THE SUKINDA CHROMITE DEPOSIT, ORISSA, INDIA

Abstract

Platinum Group of Elements (PGE), Os, Ir, Ru, Pt, Pd and Rh, along with gold (Au) and silver (Ag) are collectively known as noble/precious metals sub-grouped into: i. IPGE (Os, Ir, Ru) and PPGE (Rh, Pt, Pd). PGE behaves as a coherent group of siderophiles but behaves as chalcophiles under upper mantle and crustal conditions of fS2. PGE occur in nature as native state, alloys (with other members of the group and/or with base metals) or as discrete mineral phases i.e., Platinum Group Minerals (PGM) of sulfides, sulfo-arsenides, arsenides, oxides and silicates. Economic concentrations of PGE mainly occur in association with two major kinds of deposits i.e. Layered Ultramafic Complexes and Base Metal Sulfide (BMS) deposits. Former is further subdivided as 'Reef type (e.g. Merensky reef, Bushveld complex) deposits and PGE mineralization associated with chromitites (e.g. UG-2 chromitite, Bushveld complex). Unlike PGE mineralizations associated with sulfide deposits, those associated with chromite deposits especially with the layered complex are confined to specific stratigraphic horizons with remarkable persistent in tenor and thickness and are economically attractive. Owing to the anomalous concentrations of PGE in chromites and associated mafic/ultramafics, different ophiolitic complexes are also worked out by researchers for their PGE potentialities. But ophiolitic complexes are found to be discouraging for economic exploitation of PGE because of smaller size and erratic orientation of the host chromite lenses and, the lower whole rock concentration of total PGE with a dominance of IPGE those are relatively more difficult to beneficiate. Prerequisites for the formation of PGE deposits are: i. a melt enriched in terms of PGE compared to the mantle source to give rise to PGE deposits and requires their partitioning to the melt by suitable carrier phases i.e., sulfides; ii. Ascend of the magma essentially in a S-undersaturated condition to avoid loss of PGE through precipitation before its subsequent emplacement at crustal level; Segregation of an immiscible liquid by S-saturation and subsequent partitioning of PGEs to this liquid, to give rise to mineralization of economic importance. Different modes of origin for PGE deposits like magmatic, high temperature late magmatic and low temperature hydrothermal deposits in association with varied lithofacies are evoked by various workers for different deposits from different parts of the Globe. The overwhelming importance of chromite associated PGE deposits as the major contributor to the world's total PGE production attracted the author's attention to work on PGE mineralization associated with the chromite deposits of Sukinda Ultramafic Complex (SUC), Orissa, India. Such an idea has real aided motivation from recent discoveries of few PGE deposits of India, especially that of Baula-Nausahi complex which is in close spatial-temporal relation with SUC. Although, the major contribution of chromite production is shared by SUC, the area had not been taken up in detail for the PGE mineralization and its pattern, if there is any. Moreover, the chromite associated PGE deposits are long believed to be formed by the similar process as that of the chromites. Chromites are one of the best petrogenetic indicators that keep record of the magmatic history. The genetic aspect of SUC has long been a controversial topic amongst the geoscientist and there are broadly two schools of thought: i. stratiform type deposit associated with layered ultramafic complexes and ii. Podiform type deposit associated with ophiolitic complexes. With these considerations and in the virtual absence of any other primary phases excepting few relict olivines, the objective ofthe work mainly concentrated around study the primary PGE mineralization pattern in the light of Cr-spinel composition and sulfide phase composition, and their relation to their geotectonic set up of SUC. The SUC extends from Tomka (85°55E: 21°7'N) in the east to Kathpal (85°4VE: 2VVN) on the west spreading over an area of 40 sq. km in NE-SW direction in Jajpur and Dhenkanal districts of Orissa. The regional structure is broadly a westerly plunging syncline with easterly closure. Field evidences support the idea of a pre-tectonic emplacement of SUC with respect to Iron ore orogeny, and is intrusive into and co-folded with the quartzites of Iron Ore Group (IOG). The two generations of ultramafics of SUC are: i. older chromitite hosting altered dunitic-peridotites, and ii. younger orthopyroxenites devoid of chromitites. On the basis of physical character, structure and proportion of gangue, the chromite ores of Sukinda Valley may be classified as massive, banded, laminated varieties. Based on the level of alteration two different varieties of chromitites are identified: i. grey ores (hosted in serpentinites) iii and, ii. brown ores (hosted in limonites). The grey ores forms the main attraction for the present work as it relies largely on the primary phase relations, and least affected by the alteration, the Cr-spinels from these ores can provide the best possible approximation to the primary magmatic conditions. Grey ores are found mainly in two areas: i. Kathpal chromitites (Zone-1) virtually detached from main SUC defines a highly brecciated zone similar in disposition and intensity of deformation to that of the Bangur breccias of Boula complex, and ii. Kalarangitta chromitite (Zone-2) of main SUC presenting the Band-I of six regional bands reported. Systematic sampling of different varieties of grey ores and associated serpentinized host rock along individual sections of the two zones for detail study. Host rocks show dominance of serpentine psuedomorphs after olivine and/or pyroxene with meshwork, ribbon type and bladed mat textures in the petrographic study. Fibrous serpentines i.e. crysotiles indicative of shearing are also found. Qualitative mineralogical study of the host rock using XRD (IIT Roorkee) confirms the major phases observed microscopically. Trace elements concentrations are determined using ICP-MS in combination with acid digest technique (NGRI, Hyderabad). The Primitive Mantle (PM) normalized trace elements spider plots shows almost near mantle values with slight enrichment of LILE which may be attributed to mobilisation of these elements during serpentinization. The PM normalized plot of transition elements reveals that the serpentinites are enriched in Cr indicating anomalously high Cr concentration of the source magma and/or possible role of chromite crystallization consequent upon increase in f02. Secondary phases like haematite and goethite are also observed. IV Chromite proportion varies from <5% in host rocks to as high as 90% in massive chromitite with a dominant cumulus texture. Cr-spinel composition has been determined by EPMA (IIT Roorkee). The chromites show a general high Cr# and Mg# with low Ti02-content. The variation in the range of these ratios are dependent on the subsolidus redistribution of Mg2+ and Fe2+ between olivine and Cr-spinel, and depends upon their relative proportion. Hencethe primitive composition of the parent magma can best be commented on the basis of Cr-spinel composition of the chromitite. Cr# and Mg# range of the Cr-spinels of chromitite varies between 0.71-0.79 and 0.62-0.81 respectively indicating a high-Mg parentage formed from high degree of partial melting of the mantle and support the trace element signatures. The the relict olivines are also highly Mg-rich (~ Fo95) is also indicate the same. Such a high degree partial melting may remove high amount of S from the mantle and one of the pre-requisite for the potential PGE mineralization. Sulfides occur only in trace amounts not exceeding 1% by volume and are relatively higher with higher proportion of the silicates indicating a probably a decrease in a f02. The sulfides are analyzed by EPMA (IIT Roorkee) and SEM-EDS (IGEM Muscow) and are found to bedominated by Ni-rich magmatic varieties e.g. heazlewoodite and Ni-pentlandite, and altered to millerite etc. and mostly crystallized in magmatic crystallization sequence without seggragation. The whole rock PGE analyses are done by NiS FA ICP-MS (NGRI, Hyderabad). The PGE and, Ni and Cu ratios indicate a mantle-like source similar to the trace element data. Although the total PGE concentration is subchondritic (28 to 311ppb) but shows enrichment of IPGE with respect to the mantle like that of Cr. This is further in confirmation that the total PGE and IPGE values are high in the chromitites of respective to the host rocks of individual section and spotted ores and a possible fractionantion of IPGE during chromite crystallization. The dominance of IPGE is also observed from the rare micro nuggets of IPGE rich mineralogy as determined by SEM EDS (IGEM Muscow). The low modal proportion of sulfides, high IPGE and IPGE-rich PGM mineralogy indicates that chromite crystallization of SUC has a pronounced bearing on the kind and pattern of PGE mineralization associated with it. As the answer to the S-rich fraction of original melt it is proposed that this is concentrated in residual melt which is probably responsible for the PGE mineralization of Bangur breccia zone of Boula-Nausahi complex on the basis of similar pattern but elevated REE-concentration compared to serpentized chromite bearing host ultramafics, and PPGE rich PGM mineralogy of Bangur gabbro and similarity in the breccia zone reported from SUC to that of Bangur. Accordingly a model based on plume concept is suggested for the chromite and PGE mineralization of SUC and Boula-Nausahi complex to best explain the ophiolitic geochemical affinity as well as layered nature of these intrusive of Precambrian age with bulk peridotitic composition.