Kamal Lochan Pruseth

Neoarchean Greenstone Metamorphism in the Eastern Dharwar Craton, India: Constraints from Monazite U-Th-Pbtotal Ages and PT Pseudosection Calculations

The Archean Hutti-Maski greenstone belt (HMGB) and the south Kolar greenstone belt (SKGB) are the two richest gold provinces in India. Here, we present results of texturally constrained U-Th-Pbtotal/CHIME dating of monazites and PT pseudosection analyses to understand the geochronological and metamorphic evolution of the two greenstone terrains. The ages of felsic volcanism are constrained at ca. 2669 ± 22 Ma in the HMGB and 2661 ± 32 Ma in the SKGB. The HMGB rocks underwent midamphibolite facies metamorphism at ca. 2564 ± 12 Ma. The inferred PT path indicates a contemporaneous PT increase of up to ∼6 kbar and ∼620°C followed by postpeak near-isothermal decompression for the HMGB rocks. The SKGB rocks underwent a lower amphibolite facies metamorphism at ca. 2546 ± 12 Ma and are characterized by a synchronous PT increase of up to ∼4.6 kbar and ∼600°C, followed by decompressional cooling. These results indicate a mixed arc/plume setting for the evolution of both the greenstone belts, as suggested by several earlier works. The HMGB rocks record a pervasive postpeak metamorphic K-rich fluid alteration event at ca. 2414 ± 18 Ma that is discernible by monazite growth proximal to K-feldspar veins and garnet breakdown domains. Similar, although less prevalent, monazite age record of ca. 2414 ± 26 Ma from the SKGB rocks is manifested by postmetamorphic shear-induced hydrothermal activity. Considering the worldwide scenario, the HMGB and SKGB are ∼100 Ma younger and witnessed a higher grade of metamorphism than the greenstone belts in the Abitibi province and the Yilgarn Craton.

The minerals boulangerite, falkmanite and Cu-free meneghinite synthesis, new powder diffraction data and stability relations

Although falkmanite of the ideal composition proposed by Ramdohr & Odman (1940), Pb 3 Sb 2 S 6 , could not be synthesized even with the help of halide fluxes, a phase with composition similar to boulangerite, (Pb 5 Sb 4 S 11 ) but with dissimilar X-ray diffraction pattern was synthesized by sulfidation of bournonite at 400 and 500° C. This phase has been provisionally referred to as falkmanite in this study. Molar PbS:Sb 2 S 3 ratio in boulangerite varied from 2.41 to 2.47 and that in falkmanite varied from 2.51 to 2.57. Powder X-ray-diffraction pattern for boulangerite furnished an orthorhombic cell with a = 23.443 A, b = 21.183 A and c = 4.505 A, while that for falkmanite gave a monoclinic cell with a = 21.626 A, b = 11.409 A, c = 7.876 A and β = 96.19°. This raises doubts as to the identity of falkmanite as a member of the boulangerite homologous series, as proposed by Mozgova et al. , (1983). At 530° C, Cu-free meneghinite co-existed with falkmanite, signifying that the meneghinite solid solution field in the ternary Cu 2 S-PbS-Sb 2 S 3 touched the PbS-Sb 2 S 3 binary between 500 and 530°C. From 530 to 560° C, the proportion of Cu-free meneghinite gradually increased relative to that of falkmanite implying increased overlap of the meneghinite solid solution field with the PbS-Sb 2 S 3 binary. Falkmanite and boulangerite synthesized at 500° C and heated at 560° C for 2 hours provided similar X-ray-diffraction patterns, which corresponded with that of Cu-free meneghinite.

Compositions and petrogenetic significance of the eudialyte group minerals from Sushina, Purulia, West Bengal

The eudialyte-group of minerals (EGM) is one of the most important index minerals of the peralkaline (agpaitic) nepheline syenites. They crystallize in varied physico-chemical conditions ranging from the early-magmatic (orthomagmatic) to late-magmatic and even in the post-magmatic (hydrothermal) stage. In India, the only agpaitic nepheline syenite gneisses of the Sushina Hill region contain both late-magmatic as well as hydrothermal eudialytes. Compositionally these are Mn-Nb-Ca rich eudialytes and are comparable to the other EGM occurrences such as Ilímaussaq (Greenland), Tamazeght (Morocco), Mont-Saint Hilaire (Canada) and Pilansberg (South Africa). High Mn content (>6.5 wt.%) for both varieties of the Sushina EGM indicates that they are highly evolved in nature. In terms of the calculated site occupancy, particularly the [M(3)] and [M(2)], the Sushina eudialytes mimic some Pilansberg eudialytes. In addition to the eudialyte, the host nepheline syenite gneiss also contains an unknown Na-Zr-silicate (NZS) which is often found to be replacing both types of eudialytes. Compositionally these NZS can be tentatively represented as Na2Zr2S6O17. These NZS are characterized by much higher Zr, but lower Mn and Nb concentrations compared to the associated eudialytes. Two distinct varieties of eudialyte and NZS indicate subtle changes in the alkalinity during their formations. The formation of the late-magmatic as well as hydrothermal eudialyte essentially took place at somewhat elevated pH conditions. The replacement or alteration of eudialytes by NZS indicates a decreasing pH condition. In terms of the chemical composition the late-magmatic eudialytes can be represented as a solid-solution series between the kentbrooksite-taseqite-aqualite while the hydrothermal eudialyte represents solid-solution between kentbrooksitetaseqite -Ce-zirsilite.

Rinkite, cerianite-(Ce), and hingganite-(Ce) in syenite gneisses from the Sushina Hill Complex, India: occurrence, compositional data and petrogenetic significance

Abstract Accessory rare earth element (REE) minerals occur in small quantities in agpaitic and miaskitic nepheline syenite gneisses of the Sushina Hill Complex, India. The REE-rich minerals restricted mainly to the agpaitic rocks are rinkite, cerianite-(Ce), and cerian thorite. Rinkite, formed at the ortho-magmatic stage predates other REE-rich phases and is the most Nd-F-rich rinkite (6.62-7.45 wt.% Nd2O3; 8.75-9.74 wt.% F) with very high Nd/Ce (>2.46) ratios reported to date. Hydrothermal cerianite-(Ce), formed by the decomposition of eudialyte in the agpaitic rocks, occurs as small rounded crystals rich in Ce (~63-74 wt.% CeO2) and Y (6.03-11.69 wt.% Y2O3). The presence of cerianite-(Ce) indicates formation in an evolving hydrothermal fluid in an oxidizing milieu. Hingganite-(Ce) is present in the miaskitic unit and is considered to represent the superposition of an agpaitic mineral on an initial miaskitic assemblage. Hingganite-(Ce) is characterized by elevated contents of Ce (18.03-21.94 wt.% Ce2O3), and Nd (13.90-15.40 wt.% Nd2O3). Experimental data, coupled with the observed assemblage, suggest that the hingganite-(Ce) precipitated from the hydrothermal fluid between 400 and 300ºC followed by cerianite- (Ce) (<~300ºC). This conclusion implies that eudialyte decomposition was probably initiated above 400ºC.

Episodic tourmaline growth and re-equilibration in mica pegmatite from the Bihar Mica Belt, India: major- and trace-element variations under pegmatitic and hydrothermal conditions

Mica pegmatites from the Bihar Mica Belt contain three distinct generations of tourmaline. The major-element composition, substitution vectors and trajectories within each group are different, which indicates that the three types of tourmalines are not a part of one evolutionary series. Rather, the differences in their chemistries as well their mutual microtextural relations, can be best explained by growth of tourmaline from pegmatitic melts followed by episodic re-equilibration during discrete geological events. The euhedral, coarse-grained brown type I tourmaline cores have relatively high Ca, Mg (X Mg c. 0.37) and Al with correlated variation in Sr, Sc, Ti, Zr, Y, Cr, Pb and Rare Earth elements (REEs). They are inferred to have crystallized from pegmatitic melts. Monazites included within these tourmalines give chemical ages of 1290−1242 Ma interpreted to date the crystallization of the pegmatitic tourmaline. The bluish type II and greyish type III tourmalines with low Ca and Mg contents (X Mg = 0.16−0.27) and high Zn, Sn, Nb, Ta and Na, formed by pseudomorphic partial replacement of the pegmatitic tourmaline via fluid-mediated coupled dissolution–reprecipitation, are ascribed to a hydrothermal origin. The ages obtained from monazites included in these tourmalines indicate two alteration events at c. 1100 Ma and c. 950 Ma. The correlated variation of Ca, Mg and Fe and the trace elements Sr, Sn, Sc, Zn and REE within the tourmalines indicates that the trace-element concentrations of tourmaline are controlled not only by the fluid chemistry but also by coupled substitutions with major-element ions.

Magmatic, hydrothermal and subsolidus evolution of the agpaitic nepheline syenites of the Sushina Hill Complex, India: implications for the metamorphism of peralkaline syenites

Abstract The Proterozoic Sushina Hill Complex is the only agpaitic complex, reported from India and is characterized by a eudialyte-rinkite-bearing nepheline syenite. The complex is considered a ‘metamorphosed agpaitic complex’. This study describes the mineral assemblages formed during successive stages of evolution from magmatic to hydrothermal stages and low-temperature subsolidus reequilibration assemblage. The primary-late magmatic assemblage is characterized by albite, orthoclase, unaltered nepheline, zoned diopside-hedenbergite, rinkite, late magmatic eudialyte and magnesio-arfvedsonite formed at ~700°C with maximum aSiO₂ of 0.60. In contrast, a deuteric assemblage (400-348°C) is represented by aegirine-jadeite-rich clinopyroxene, post-magmatic eudialyte, sodalite, analcime and the decomposition assemblages formed after eudialyte with decreasing Asio (0.52-0.48). A further low-temperature subsolidus assemblage (≤250°C) represented by late-forming natrolite could be either related to regressive stages of metamorphism or a continuum of the subsolidus processes. Considering the P/T range of the greenschist - lower-amphibolite facies of metamorphism it is evident that the incorporation of a jadeite component within pyroxene is related to a subsolidus process between ~400°C and 348°C in a silica deficient environment. We emphasize that the deuteric fluid itself acted as an agent of metamorphism and the decomposition assemblage formed after eudialyte is retained even after metamorphism due to the convergence of subsolidus and metamorphic domains. The formation of jadeite-rich aegirine is not considered to result from metamorphism. Overall it is near-impossible to discern any bona fide metamorphic textures or mineral assemblages in these syenites which appear to preserve a relict mineralogy regardless of their occurrence in country rocks which have experienced greenschist - amphibolite facies metamorphism. The Sushina complex is very similar in this respect to the Norra Kärr complex (Sweden).

MATNORM: Calculating NORM using composition matrices

This paper discusses the implementation of an entirely new set of formulas to calculate the CIPW norm. MATNORM does not involve any sophisticated programming skill and has been developed using Microsoft Excel spreadsheet formulas. These formulas are easy to understand and a mere knowledge of the if-then-else construct in MS-Excel is sufficient to implement the whole calculation scheme outlined below. The sequence of calculation used here differs from that of the standard CIPW norm calculation, but the results are very similar. The use of MS-Excel macro programming and other high-level programming languages has been deliberately avoided for simplicity.

Trace-element geochemistry of pyrite and arsenopyrite: ore genetic implications for late Archean orogenic gold deposits in southern India

Abstract The distribution of Au and associated trace elements in pyrite and arsenopyrite from late Archean Hutti and Hira-Buddini orogenic gold deposits, eastern Dharwar Craton, southern India was investigated by laser ablation-inductively coupled plasma-mass spectrometry. X-ray element maps acquired by electron probe microanalyser reveal oscillatory zoning of Co and As indicating the crystallization of pyrite and arsenopyrite in an episodic fluid flow regime in which fluid salinity fluctuated due to fault-valve actions. The absence of any relationship between Au and As in pyrite obviate the role of As in the incorporation of Au into pyrite, particularly here and may be generally the case in orogenic gold deposits. On the other hand, positive correlations of Au with Cu, Ag and Te suggest possible influence of these chalcophile elements in the enhanced gold concentration in sulfides. Pb-Bi-Te-Au-Ag bearing micro-particles (<2 μm) are observed exclusively in micro-fractures and pores in arsenopyrite. The absence of replacement features and element gradient suggests direct precipitation of Pb, Bi, Te, Au and Ag from a fluid that was unreactive towards arsenopyrite. An intermittent fall in fluid pressure caused by the fault-valve action would have resulted in the sporadic precipitation of Au, Pb, Ag, Bi and Te.

Mantle-Derived Carbonate Fluid Alteration and Gold Mineralization in Southern Granulite Terrain, Wynad, India

The Wynad Gold Field, unlike Archaean greenstone-hosted gold mineralization that is commonly found in India, comprises a network (length >100 m, thickness ~ 1.5 m) of much younger (~450 Ma) auriferous hydrothermal quartz veins within migmatitic tonalitic gneisses of the Southern Granulite Terrain, north of the Moyar shear zone. The gold mineralization occurs closely associated with probable mantle-derived quartz-carbonate dykes. We have used field criteria, geochemistry of pyrite-hosted fluids and Rb-Sr geochronology of the quartz-carbonate dykes to constrain source(s) to these auriferous veins. Gold bearing quartz-pyrite veins, emplaced within faults and shears, represent late-stage hydrothermal veins originating from ~0.5 m thick, undeformed, pink quartz-carbonate dykes. Both hypidiomorphic and subvolcanic textures indicated by carbonate and muscovite megacrysts, set within a fine-grained groundmass, are observed. The groundmass assemblage is comprised of quartz + pyrite + phengite + chlorite + ankerite + ferroan calcite + albite + hematite + zircon. The auriferous veins contain additional pyrrhotite and chalcopyrite. The gold-bearing pyrites are essentially arsenian with relatively low Fe (Fe 0.82–0.90 S2 to Fe0.89–1.0 S2) as compared to ideal pyrite. Invisible gold varying in concentration from 0.11 to 0.17 wt% were analyzed within pyrite grains having abundant inclusions of calcite and KCl crystals, and CO2. Isotopic ratios from the carbonate dykes yielded an Rb-Sr age of 446 ± 26 (2σ), MSWD=2 with Sri=0.7029 ± 0.0004 (2σ). From field observations of direct continuity between the auriferous quartz veins with the quartz-carbonate dykes, low initial Sr isotopic composition (0.7029) and earlier reported mantle-derived carbon in fluid inclusions in auriferous vein quartz, we infer a distinct mantle origin for these dykes. This strongly suggests that there is at least one component that was directly extracted from the mantle which carried the gold. This mantle component probably was initiated due to structural and thermal events, synchronous with or post-dating the Pan-African orogeny, along the Moyar shear zone.

Phase equilibrium study in the system Cu2S-PbS-Sb2S3: non-stoichiometry in sulfosalts and isothermal variation in sulfur fugacity

Evacuated silica tube experiments (+halide flux) were conducted in portions of the system Cu2S-PbS-Sb2S3 at 440°C, using two-pyrrhotite indicator method to measure the sulfur fugacity. Product phases were identified by optical and X-ray powder diffraction methods supplemented with microprobe analyses. In addition to the previously reported mineral phases, famatinite (Cu3SbS4) appears to be a stable phase in the Sb2S3-rich portion of the system. Microprobe data indicate that almost all the sulfosalts depart from stoichiometry. Copper in Pb-Sb sulfosalts and Pb in chalcostibite and skinnerite are indicative of the coupled substitution 2Pb2+=Cu++Sb3+. Pb-solubility in skinnerite and Cu-solubility in zinkenite are dependent on the initial bulk composition of the charges. The compositions of meneghinite and boulangerite compare well with their natural analogues. The maximum isothermal variation of logfs2 falls in the range of-6.36 (1.06)≤logfs2≤11.12 (0.30). The experimentally derived logfs2 values for some two phase assemblages, compare reasonably well with the respective minimum logfs2 values calculated by the method of Craig and Barton (1973). The stable coexistence of famatinite with zinkenite plus stibnite instead of chalcostibite may be described by the sulfidation reaction: 3CuSbS2+1/2 S2=Cu3SbS4+Sb2S3.