A geochemical and mineral chemical assessment of sediment provenance and post-depositional alteration of auriferous conglomerates in the Singhbhum Craton

Abstract

The Mesoarchean Dhosrapahar Group and the Phuljhari Formation in the Singhbhum Craton host so far subeconomic gold occurrences in metaconglomerate beds, similar to the world-famous conglomerate-hosted gold deposits of the Mesoarchean Witwatersrand Basin in South Africa. In the studied metaconglomerates from India, gold occurs as invisible inclusions in pyrite and as native gold grains associated with detrital chromite, quartz, zircon and metamorphic chlorite. Unlike most Archean paleoplacer deposits, the inferred detrital gold here appears unrelated to detrital pyrite, thereby highlighting the decoupling of the sources for the various placer components. To this end, we attempt to delineate the possible source rocks and assess the extent of post-depositional alteration to better understand the controls on gold accumulation in these conglomerates. Elevated Ni, Co and As, to a lesser extent V, Cu, and Se contents in Au-bearing inclusion-rich detrital pyrite are suggested to be derived from Paleoarchean banded cherts in the Gorumahisani Greenstone Belt. Relatively immobile trace element ratios, as recorded by the conglomerate matrix, reflect a predominantly felsic source with minor sediment input from mafic/ultramafic sources. Fractionated chondrite-normalized light rare earth and flat heavy rare earth element patterns, along with a distinct negative Eu-anomaly, obtained for the samples from the Phuljhari Formation point to a differentiated felsic source, most likely the c. 3.05 Ga K-rich Mayurbhanj Granite. In contrast, depleted heavy rare earth element patterns and a weak negative Eu-anomaly in the conglomerates from the Dhosrapahar Group indicate derivation from a felsic source that had experienced garnet fractionation, the obvious candidate being the underlying c. 3.37 Ga Rairangpur tonalite-trondhjemite-granodiorite (TTG) suite. Detrital rutile is rich in Cr and V, and Zr-in-rutile geothermometry yielded crystallization temperatures between ~800 and 1000 °C. The most likely source of this rutile are 3047±17 Ma gabbro-anorthosite bodies in the basement. Post-depositional modification of the conglomerates is dominantly manifested as greenschist-facies metamorphic overprint, and is reflected also by detrital chromite texture and chemistry, and the occurrence of secondary rutile, euhedral pyrite, and secondary gold with metamorphic chlorite. Our provenance interpretations do not indicate any likely point-source of detrital gold, suggestive of the entire Archean hinterland, irrespective of composition, as a potential source, as already indicated for the Mesoarchean auriferous conglomerates of the Witwatersrand Basin. This contention is further substantiated by the evidence presented here for moderate-to deep chemical weathering of the Archean hinterland of the Singhbhum Craton.