Metallogenesis and depositional environment of the Archean-Proterozoic carbonaceous phyllites from the Dharwar Craton, India

Highlights

• First report of PGE geochemistry in carbonaceous phyllites from Dharwar Craton.

• Pyrite chemistry indicates the role of hydrothermal fluid in gold genesis.

• Sulphur isotopic values suggest hydrothermal and sea water sulphate sources.

• Organic matter present in ocean anoxia aided to deposition of carbonaceous phyllites.

Abstract

Organic carbon rich meta-sedimentary rocks are important repositories of gold, PGE and vestiges of the biogeochemical cycles that prevailed during the Precambrian time. In this study, we report gold, PGE and δ³⁴S concentrations of carbonaceous phyllites from Neoarchean Chitradurga, Gadag greenstone belts of western Dharwar Craton and Mangampeta of Proterozoic Cuddapah basin, eastern Dharwar Craton to understand their metallogenesis and depositional environments. Chitradurga phyllites (avg. Au = 180 ppb) show elevated concentrations of refractory gold compared to Gadag (avg. Au = 38 ppb) and Mangampeta shales (avg. Au = 23 ppb). The Archean carbonaceous phyllites display three generations of pyrites and their mineral chemistry suggests hydrothermal imprints while Proterozoic shales have two generations of diagenetic pyrites. Gadag has high PGE concentration (ΣPGE = 55 ppb) compared to Chitradurga (ΣPGE = 37 ppb) and Mangampeta (ΣPGE = 27 ppb). The PGE geochemistry is characterized by enrichment of PPGE over IPGE with Au > Pt > Pd and the Au/Pd, Pd/Pt, Ru/Ir, Ir/Pd ratios coupled with negative Pt (Pt/Pt*) anomalies indicate hydrothermal source. The Chitradurga and Gadag samples yielded low sulphur isotopic signatures (δ³⁴S = 1.8–2 and 1.3–2‰ respectively) whereas the Proterozoic Mangampeta samples have comparatively higher values (δ³⁴S = 8.4–34.3‰) suggesting hydrothermal and sea water sulphate sources respectively. Sulphate present in Archean and Proterozoic sea water was converted to sulphur by bacterial sulphate reduction which led to the formation of pyrites. In the Archean phyllites, the metal bearing bisulphide complexes were adsorbed into the pyrites while the Proterozoic shales are characterized by the partitioning of organo-metallic complexes into the pyrite during the diagenesis. Strong positive correlations of transition metals, redox sensitive elements with PGE indicate hydrothermal source and V/(V + Ni), U/Th, Th/U, authigenic U attest euxinic paleo-redox conditions. Thus, biogenically mediated redox depositional environments which prevailed during Archean and Proterozoic times have enhanced the metal sequestration processes that subsequently resulted in the ore formation in these carbonaceous phyllites.

Introduction

Carbonaceous phyllites are distributed all around the world with wide range of depositional regimes in the stratigraphic record (Tourtelot, 1979, Eric et al., 2019). These organic carbon rich meta sedimentary rocks serve as tools to understand the global biogeochemical cycles and are also used to interlink geological events such as hydrothermal activity and marine anoxia (Schieber, 2011, Fakhraee and Katsev, 2019). Some beds of black shale contains minor elements in concentrations more than a hundred times their average crustal abundance (Krauskopf, 1955). During the cycles of weathering and erosion of the earth's surface some minor elements as well as sulfide tend to remain in resistant minerals that are transported to sedimentary basin. They are generally characterized by enrichment of redox sensitive trace elements and have potential metal resources such as PGE and gold (Pages et al., 2018, Khanchuk et al., 2018). Kotov et al. (1993) have suggested a two-phase development with syn-tectonic hydrothermal remobilization of Au from the black shales. Carbonaceous sequences often accommodate large gold deposits, such as Muruntau (central Uzbekistan), Bakyrchik (eastern Kazakhstan), Kumtor (Kyrgyzstan), and Sukhoi Log (Russia; Yermolaev et al., 1995). These stratiform deposits occur in carbonaceous shales containing fine-grain carbon of mainly biogenic origin as well as chemogenic origin (Laverov et al., 2000, Vinokurov et al., 1997). The presence of carbonaceous shales all along the geological time scale indicates that the geological processes played a prominent role in their deposition rather than the geological setting (Tourtelot, 1979). As these rocks show conspicuous confinement to ocean anoxia, they provide reliable insights into the microbial life and paleo-redox conditions that prevailed during the Precambrian (Marin-Carbonne et al., 2020, Jørgensen et al., 2019). Precambrian carbonaceous shales are characterized by enrichment of precious metals like gold and PGE while those of Phanerozoic have high hydrocarbon potential (Ohkouchi et al., 2015, Baioumy et al., 2018). The occurrence of carbonaceous shales in the sedimentary basins of India have been reported by many workers viz. Sandur greenstone belt (Manikyamba and Kerrich, 2006), Singhbhum Craton (Majumdar et al., 2019), Cuddapah Basin (Manikyamba et al., 2008, Basu et al., 2017), Vindhyan Basin (Paikaray et al. 2008), Krishna–Godavari Basin (Rao, 2001); Damodar Valley (Mani et al., 2015), Cambay Basin (Bhaskar, 2013), Kachchh Basin (Srivastava et al., 2018) and Mizoram Basin (Sawant et al., 2017). Though many workers have conducted extensive studies on these shales, reports on the Archean and Proterozoic shales of India are very meagre. In the present paper, we report the gold, PGE and δ³⁴S concentrations of carbonaceous phyllites from Archean Chitradurga, Gadag greenstone belts and Proterozoic Cuddapah basin to document the role of organic matter during the metallogenesis and deposition. As Precambrian era is intrinsically characterized by the rise of atmospheric oxygen i.e., Great Oxidation Event (GOE), the study of Archean and Proterozoic carbonaceous phyllites is vital to understand and decipher the Precambrian biogeochemical conditions.