Diverse Tourmaline Compositions from Orogenic Gold Deposits in the Hutti-Maski Greenstone Belt, India: Implications for Sources of Ore-Forming Fluids

Abstract

The Late Archean Hutti-Maski greenstone belt hosts the largest operational gold mine in India at Hutti, as well as two satellite mines at Hira-Buddini and Uti. Tourmaline is a common accessory mineral in the auriferous altered wall rocks, proximal to the gold-bearing veins of these orogenic gold deposits. We report here major, rare earth element (REE), and selected trace element compositions of tourmalines from these alteration zones at Hutti and Hira-Buddini, and from spatially associated volcaniclastic rocks at Uti, in order to compare and evaluate possible fluid sources related to tourmaline precipitation and gold mineralization. Tourmalines at Hutti occur as tourmaline-calcite microveins within proximal alteration zones, and as disseminated grains along the biotite-chlorite–defined mylonitic foliation. These tourmalines belong to the dravite/oxydravite group, with an evolutionary trend towards magnesio-foitite, and commonly are Mg rich (up to 2.06 apfu) with low Na (<0.61 apfu) and up to 53% X-site vacancies. Characteristics are low abundances of total REE (0.25–0.67× chondrite) and a flat trend, with minor HREE enrichment accompanied by strong positive Eu anomalies. At Hira-Buddini, tourmalines of the oxydravite-povondraite series replace amphiboles and hydrothermal biotite. Based on textural and chemical characteristics, three generations of tourmalines are present at Hira-Buddini. Earliest type I tourmaline has moderate Fe content (up to 1.46 apfu), high Na (average 0.80 apfu), and moderate X-site vacancies up to 22%. Type II tends to be relatively enriched in Fe (up to 2.08 apfu), have similar Na contents (average 0.79 apfu), and low X-site vacancies (up to 8%). These tourmalines contain higher total REE (0.93–2.23× chondrite) and display a flat trend with slight middle rare earth element (MREE) depletion accompanied by a small positive Eu anomaly. The latest tourmalines (type III) are Fe-poor (up to 1.52 apfu) and Na-poor (average 0.67 apfu), with relatively high X-site vacancies (up to 21%). This tourmaline type has relatively low total REEs (0.21–1.05× chondrite) and similar abundance patterns to those of Hutti tourmalines, accompanied by large positive Eu anomalies. Similarity in tourmaline compositions with evolutionary trends in compositional space from different textural associations suggests that a single low-salinity and reduced metamorphogenic fluid, with inherently low ∑REE, was responsible for proximal alteration and gold mineralization at Hutti. The type I and type II tourmalines from Hira-Buddini display oxydravite-povondraite trends, with compositional overlap that suggests fluid mixing at a paragenetically early stage. The early hydrothermal fluid was more saline, oxidizing, and likely had a granite-derived component with high ∑REE contents. A later evolved, relatively low-salinity and reduced fluid is recorded by the last generation (type III) tourmalines at Hira-Buddini. The tourmalines from Uti metapelites are characterized by dravitic composition, low Na contents, high X-site vacancies (31–47%), and an evolutionary trend toward magnesio-foitite. These tourmalines have higher ∑REE contents (1.16–2.08× chondrite), LREE-enriched patterns together with large positive Eu anomalies, and low concentrations of most trace elements, all features that suggest a metamorphic origin.