Abstract In Ramchandrapur, iron ore bodies occur in the form of lenticular bands of titanomagnetite closely associated with the quartzites and khondalites. The area bears evidences of polyphase deformation and metamorphism. Petrographic studies have revealed presence of two generations of magnetite. The earlier generation is represented by rounded to sub-rounded clastic grains of magnetite within the silicate rich groundmass and the later generation of magnetite occurs in the form of coarse-grained distinct bands following the dominant foliation (S2) of the host rock. The chemical analysis of the magnetite ores gives around 40.84 wt% Fe 2 O 3 , 14.04 wt% FeO, 13.68 wt% TiO 2 , 0.173 wt% V 2 O 5 on an average. Basic magmatic activity in two different phases might have produced magnetite ores localized in the high grade meta-sedimentary host. Recycling of original magnetite is suggested. Abstract Petrology, mineral chemistry and whole-rock geochemistry of the intrusive, layered (>3.1Ga) as well as the extrusive volcanic suites of the Nuggihalli Schist Belt (NSB) are presented in this paper. The dunite-metaperiodotite, serpentinite and syngenetic-stratiform chromite association forms the lower part of the Tagadur ultramafic-mafic complex. The upper part of the intrusion is represented by meta-gabbro-meta-anorthosites with vanadiferous titano-magnetite bodies. The extrusive ultramafic rocks are represented by pillowed, spinifex textured peridotites and high-Mg basalts of komatiitic affinity. Petrological studies of the ultramafic rocks reveal primary cumulus textures exhibited by olivine and chromite and secondary alterations, low-grade metamorphism and deformation. The chromitites hosts a variety, of unusual accessory silicate phases such as corundum, diaspore, rutile, zircon, monazite and baryte in addition to olivine, serpentine and amphibole (anthophyllite), chlorite (clinochlore). Native phases such as gold, silver, copper, iron and tin and other Fe-Ni-Cr-Cu alloys are found associated with cumulus chromite grains in the massive chromitites. Trace and REE geochemistry of these ultramafic-mafic suite reveal significant LREE enrichment and depletion, negative Ce and Eu anomalies and flat to enriched HREE pattern suggestive of their derivation from parental komatiitic magmas by high degree of partial melting of a metasomatized depleted mantle source in a subduction zone environment. Trace element and PGE geochemistry of the chromitites indicate that they are PGE poor chromitites but enriched in Au and other base metal alloys, formed under changing oxidizing and reducing conditions resultant of alteration of the primary sulphides by desulphidation processes during the Mesoarchaean. The associated ultramafic (peridotitic) and mafic (basaltic) komatiites devoid of chromite and PGE represent primitive melts, most probably derived from high degrees of partial melting of a common depleted mantle source. Abstract The vanadium bearing titaniferous magnetite ore deposits occur within the gabbro - anorthosite suite of rocks (Mayurbhanj basic igneous complex) extending for about 15 km from Kumhardubi in northwest to Hatichhar in south east, Mayurbhanj district, Odisha. The ore bodies occur as lodes, pockets and float boulders. The ore microscopic studies reveal the presence of primary Ti-magnetite, ilmenite, ulvospinel, spinel, martite, goethite, lepidocrocite and minor and trace amounts of diaspore, apatite, sulphides and silicate gangues. The titanomagnetite and ilmenite grains reveal various micro-intergrowths: (a) discrete coarse granular ilmenite, (b) external and internal granules of ilmenite, (c) trellis intergrowth of coarse and fine ilmenite lamellae, (d) ilmenite showing sandwich intergrowth, (e) magnetite-ulvospinel (± ilmenite) cloth intergrowth, (f) spinel lamellae in magnetite, (g) spinel-ilmenite intergrowth and (h) hematite lamellae in ilmenite. The evolutions of different types of micro-intergrowths are interpreted in terms of exsolution and oxidation above and below the magnetite - ulvospinel solvus. Ti magnetite grains show incipient to extensive alteration to martite due to oxidation during weathering. Bulk analytical results of the ores reveal that V 2 O 3 and TiO 2 contents range between 0.28 to 1.38% and 11.50 to 18.50% respectively. The maximum SiO 2 value is 4.30% indicating that the ores are almost devoid of silicate minerals. Thermo-barometric studies indicated that magnetite and ilmenite re-equilibrated within a temperature range of 777° to <550°C and oxygen fugacity of 10-13.4 to <10-24. The titaniferous magnetite ore deposits are genetically related to the associated gabbro-anorthosite rocks in which the magnetite and ilmenite occur as interstitial grains. The profuse crystallisation of plagioclase and pyroxenes resulted a residual liquid/magma highly enriched in Fe and Ti contents. The present disposition of the ore bodies is due to residual liquid injection of the Fe-Ti concentrated liquid. Densification and conversion of the ore-rich bodies essentially into monomineralic rock was possibly resulted either by (a) external granular exsolution or (b) post-cumulus sintering or annealing processes. Abstract Chromite is commonly used for deciphering the petrogenesis and geodynamic setting of ultramafic rocks. Chrome-spinels from the Indo-Myanmar ophiolite belt occurring in the form of podiform chromitites associated with the mantle sequence and associated volcanic rocks are studied to characterise their geochemical composition. We have measured major and trace element concentrations of the chrome-spinels. Cr/(Cr+Al) (Cr #) and Mg/(Mg+Fe 2+) (Mg #) range from 66 to 70 and 66 to 69 respectively in the chromitite and 47 to 56 and 67 to 70 in volcanic spinel. The computed crystallization conditions show that the studied samples crystallized in an upper mantle environment. Using the chrome spinel compositions, parental magma for the chromitite rock is calculated as boninite, and a MORB for the volcanic sample. Minor and trace element (Ga, Ni, Zn, Co, Mn, V, Sc) concentrations in the analysed spinels are comparable to other podiform chromitites hosted in the mantle section of the ophiolite complexes. Chrome-spinel compositions of the two rocks are used to recognize the probable crystallization condition and nature of the parental magma. Abstract Based on the field, petrological and geochemical data, the ultramafic rocks near Ikauna in Madawara Igneous Complex are found to occur as E-W trending lensoidal intrusive bodies in a major shear zone predominantly consisting of granite-gneisses in Bundelkhand craton, central India. Cumulates of olivine and pyroxene are the major mineral constituents while the intergranular spaces are occupied by chromite, pyrrhotite, pentlandite and magnetite. These rocks show variation in their chemical composition with moderately low SiO 2 and high Mg # (0.67-0.88), with high Ni, Cr and Co. The peridotites are Al-undepleted, while the pyroxenites are Al-depleted-Ti enriched showing chemical affinity towards Komatiitic composition although their textural characters do not reflect the same. SEM-EDS studies indicate presence of fine grained platinum group minerals (PGM) including other precious metal minerals (PMM) as disseminated grains in the interstitial spaces of olivine cumulates and pyroxenes. Ni/Cu, Cu/Pd and Pd/Ir ratios suggests late stage sulfidation prevailed over S-saturation conditions during crystallization of PGE/PMM. The concentrations of platinum group elements (PGE) and Au are variable (Ir 2.4-18.8 ng/g, Ru 34.8-79.6 ng/g, Rh 0.8 - 20 ng/g, Pt 6.8-128.2 ng/g, Pd 6.8-86.8 ng/g and Au 22.8-127.2 ng/g) and their distribution patterns represents relative enrichment of Ru, Pt, Pd compared to Ir and Rh. The trends of REE fractionation and geochemical discrimination diagrams suggest derivation of Ikauna ultramafic rocks at moderate depths from high degree of partial melting from fertile mantle. Abstract The Neoarchaean Sigegudda greenstone terrane is a small (ca. 30 km 2) greenstone belt, situated to the north of the Holenarsipur belt in the western Dharwar craton. Two compositionally distinct units of ultramafic rocks are recognized in the Sigegudda greenstone belt and have been interpreted as metamorphosed komatiites and komatiitic basalts. These komatiites have been analyzed for Platinum Group Elements (PGE) concentrations which provide significant petrogentic implications on the mantle melting conditions and sulphide saturation history of the parent magma. The PGE abundances of Sigegudda komatiites are characterized by 0.2-3.6 ppb Os, 0.02-1.2 ppb Ir, 0.4-1 ppb Ru among IPGE; and 0.4-4.8 ppb Pd, 0.4-7.6 ppb Pt, 0.2-1.2 ppb Rh among PPGE. These komatiites have lower concentrations of ΣPGE in the range of 1.02-15ppb. The chondrite normalized PGE patterns show variable PGE fractionation. The overall PPGE and IPGE variations are marked by both lower ΣPPGE/ΣIPGE (<1) and higher ΣPPGE/ΣIPGE (>1); Pd/Pt ratios lower (0.3-0.5) and higher (0.6-2.0) than the primitive mantle average of 0.55; Pd/Ir > Pt/Pd and Pt/Pd > Pd/Ir. Pd/Ir vs. ΣPGE and Pd vs. Cu variations suggest a sulphur saturated to undersaturated character of the parent magma which indicates variable degrees of mantle melting. PGE distributions of Sigegudda komatiites are consistent with olivine and sulphide fractionation trends of parent magma. Relative enrichment of PPGE over IPGE and pronounced enhancements of Pd relative to Pt in some komatiite samples reflect a multi-stage petrogenetic process marked by melt percolation and contributions from sub-continental lithospheric mantle (SCLM). Abstract The mafic-ultramafic (MUMF) suite of rocks of Archaean age occur as discontinuous pods north of Arsikere up to south of Vedavathi river in western Dharwar Craton, Karnataka. The mafic-ultramafic enclaves consist of meta-peridotites, tremolite-actinolite schists, serpentinites, amphibolites and metabasalts with gabbro dykes. The rocks have undergone green schist to amphibolites facies metamorphism. Our studies have shown nodular and ocellar, pillows, jack straw textures in these ultramafic–mafic suite (the talc-serpentine rocks). Talc-serpentine and tremolite-actinolite schists contain chromite and titano-magnetite concentrated at the grain boundaries. The overall major element composition of these rocks shows a high MgO (22-40%) and total iron (8-10%) contents and some samples also exhibit komatiitic affinity. The distribution of chromite grains in amphibole and associated matrix is also noticed. High contents of Ni and Cr (~3000, 2800 ppm) were obtained in some of these rocks. The REE data show slightly fractionated patterns with enriched LREE (LREE/HREE ~5.0) and negative europium anomalies. The reported Pt and Pd values are 6500 and 190 ppb. The element relationships indicate that different types of hydrothermal fluids re-distributed Ni, Cu, and (Pt+Pd) in different ways, which may have led to local concentration of Pt and Pd. The overall geochemical data and (Pt+Pd) contents imply that, at Antarghatta ultramafic belt, different genetic types of (Pt+Pd) enrichment processes were active. (1) Small amounts of immiscible sulphide melt collected relatively high amounts of PGE (relatively high (Pt+Pd) at low Ni and Cu contents) and represent a low sulphur type silicate PGE mineralization, but no major sulphide liquation event may have happened that would lead to a potential economic mineralization. (2) A hydrothermal Ni-enrichment trend (at low Cu) is reflected by the presence of millerite, which enriches the rocks beyond the Ni in the silicate minerals and the slightly higher values of Ni where elevated chromium values occur. (3) Where Cu (at low Ni contents) gets enriched by fluids, often (Pt+Pd) is enriched as well. Thus, the Antarghatta ultramafic belt warrants a multi-disciplinary approach to understand the critical parameters and processes involved in the genesis of Ni-Cu-(PGE) associated with these komatiites. Abstract Boggulakonda gabbroic complex (BKGC) (E 80°00'30", N 15°44'04") in the Prakasam Igneous Province (PIP) (southern India) is studied in detail using major, trace, REE and PGE geochemistry to understand its geochemical evolution. It shows sharp contact relationships with the Precambrian quartzofeldspathic mica schist and granite gneisses. It is one among the several gabbroic complexes, occurring to the east of Cuddapah basin, along the eastern margin of Nellore schist belt. It is predominantly composed of gabbro and gabbronorite with melanocratic to leucocratic variations. Petrographic studies show cumulus, poikilitic and porphyritic textures, micro-deformation features and symplectites/replacement textures. Magnetite and ilmenites are dominant opaques with subordinate pyrite and chalcopyrite. Cumulus phases of plagioclase, Cpx, are essential minerals whereas olivine, opx, amphibole and opaques appear as intercumulus phases. These rocks are classified as sub-alkaline, low Ti-tholeiitic gabbros. The normative compositions indicate that these rocks are quartz, plagioclase and pyroxene normative. The differentiation index (D.I.) determined from the norm compositions are in the range of 11.4 to 20.9 indicating that they are early-middle stage basaltic differentiates. These studies also indicate fractional crystalization was a dominant process of their genesis. The major, incompatible trace and REE geochemistry of BKGC rocks show LILE-LREE enrichment and HREE-HFSE depletion, pronounced Eu-positive anomalies, negative Nb anomalies relative to Th and low Ti/V ratios. Based on the above and other binary plots, it can be inferred that the BKG rocks are the products of ~5-15% partial melting of depleted mantle source strongly modified by the melt enrichment and addition of slab derived fluids in a subduction zone environment and emplaced as tholeiitic island arc magmas. BK gabbroic rocks indicate low abundances of PGE. PPGE’s are greater than IPGE; Pd/Ir and Pd/Pt ratios are greater than primitive mantle and chondrites. PGE geochemistry correlate well with the petrogenesis and tectonic discrimination studies made by using major, trace and REE geochemistry. Abstract Platinum group elements (PGE: Ru, Rh, Pd, Os, Ir, Pt) are highly siderophile and chalcophile, and their distribution within the crustal rocks is very heterogeneous and are generally present at exceedingly low concentrations (ng/g and sub-ng/g levels). As a result, their accurate determinations in terrestrial rocks are very challenging and reliable analytical techniques are required to obtain high quality data that are needed for both basic and applied studies. The past decade has witnessed several developments in the analytical instrumentation and sample preparation techniques which helped in generating high quality analytical data for PGE especially in their exploration studies. Reliable techniques were developed for the analyses of NiS and Pb beads directly by LA-ICP-MS and emission spectrometry which resulted in increased throughputs that are required for exploration studies. Hydro-geochemical investigations are useful in areas covered with thick vegetation without any outcrops, and also have the added benefit of functioning as baseline environmental data. On the micro-beam analyses front, spectacular developments have taken place particularly using laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS).This technique provided multi-PGE analyses of PGE-bearing sulfide minerals at ng/g levels leading to better understanding of PGE geochemistry in magmatic systems. This technique can be used in combination with electron probe microanalyses (EPMA) for minor and major elements and with quantitative image analysis to provide the data necessary to understand PGE behavior. This paper evaluates the recent advances made in related areas such as bulk chemical analysis, pathfinder elements, in-situ studies; hydro-geochemical prospecting methods, selective extraction methods, PGE certified reference materials and their contributions in the exploration studies as well as in related basic studies of these valuable metals.

Abstract Magadiite and cherts from the Magadi basin in southern Kenya and four other localities in the East African Rift all share distinctive compositional systematics: (1) negatively fractionated REE patterns; (2) high absolute concentrations of U, Nb, and Zr (up to 1,500 ppm versus a crustal average of 190 ppm); (3) normalized enrichments of U, Nb, and Zr relative to REE; (4) extreme fractionations of U-Th, Nb-Ta, and Zr-Hf; (5) positive Ce but negative Eu anomalies; and (6) normalized peaks at Mo, Ag, and Sb. Alkaline lake cherts composed of secondary quartz retain the trace-element patterns of the precursor, albeit at lower absolute element contents. Sublacustrine sinters at Lake Bogoria share some of the compositional features of the cherts from Magadi but lack the Ce and Eu anomalies and U-Th, Nb-Ta, and Zr-Hf fractionations. Cherts from ocean-ridge, pelagic, and continental-shelf settings are characterized by progressively larger Zr contents (≤ 170 ppm), but all are distinctly lower than in the alkaline-lake cherts, where micron-scale Zr-rich phases were identified as possible authigenic zircons. Both marine and alkaline-lake cherts share positive Ce anomalies. These result from scavenging of Ce by Fe-Mn oxyhydroxides in seawater, but from the solubility of Ce (IV) in oxidized alkaline brines for the lacustrine cherts. Europium anomalies range from small negative to positive in marine cherts, whereas alkaline-lake cherts feature large negative anomalies. These compositional systematics, which reflect the aqueous environment in which the chert precursor formed, confer a tool for interpreting the paleoenvironment of cherts in the geological record.