Pranjit Hazarika

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.

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.

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.