Role of evaporites in sodium metasomatism and formation of albite-rich rocks in IOCG provinces

Abstract

Hydrothermal Iron-oxide-Cu-Au (IOCG) deposits are important exploration targets for a number of metals of economic importance. Voluminous sodium metasomatism is ubiquitous in most IOCG provinces. The origin of the metasomatic fluid responsible for the formation of albite-rich rocks in such terranes is contentious; both evaporite-derived fluid and magmatic fluid have been advocated. The Singhbhum IOCG province in India is known for hosting several polymetallic deposits from which U, Cu, and apatite-magnetite are mined as principal commodities together with Ni, Au, Ag, Se, Te, Mo, and low-Ti magnetite as by-products. Voluminous albite-rich rocks occur at several sites along the mineralized belt. Metasomatic albite schist comprising predominantly albite and quartz at Pathargora contains tourmaline intricately associated with albite. Based on textures and mineralogical associations, tourmaline is classified as: tur1: coarse nodular tourmaline ± albite ± magnetite aggregate; tur2: disseminated fine-grained tourmaline in a schistose matrix; and tur3: tourmaline + biotite + magnetite + apatite clots. All tourmaline belongs to the alkali group and most to the dravite sub-group. Very low concentrations of Li (avg 3.8 ppm), Zn (avg 6 ppm) and Mn (avg 21 ppm) but high V (avg 2130 ppm) and Ni (avg 306.5 ppm) in tur1 core (tur1c) co-genetic with albite contrast with what is reported in magmatic/magmatic-hydrothermal tourmaline of granitic affinity. The δ11B values of tourmaline are mostly positive, ranging between –4.0 and 9.2 ‰ (avg 4.0 ± 3.0; n = 79); values of tur1 cores are 4.9–9.2 ‰. Previous fluid inclusion studies described the presence of halite and barite daughter crystals in primary fluid inclusions and demonstrated the involvement of a high-temperature (≥450 °C) and high-salinity (≥50 wt% NaCl equivalent) brine in the formation of cores of tur1. Calculated δ11B values of the hydrothermal fluid in equilibrium with tur1c range from 7.3 to 11.5 ‰ (for a crystallization temperature of 450 °C). Based on the textural setting of the tur1c-albite assemblage, geochemical and boron isotope composition of tur1c, and evidence of a high saline brine, we propose that the albite schist is the product of Na-metasomatism by brines that derived major constituents from the dissolution of marine evaporite. The inferred paleo-latitude and geological environment of sedimentation related to the host lithologies are consistent with the evaporitic model. We further suggest that a similar mechanism might have played an important role in the formation of albite-rich rocks in other IOCG provinces where the involvement of evaporite-derived fluids in mineralization/alteration have been demonstrated. Similar major element, trace element and positive boron isotope (1.5–6.3 ‰) composition of tur3 suggest similar source of fluid as that of tur1c, and tur3 formed either during the same alteration event or during a separate fluid pulse. On the other hand, the mode of occurrence, very low trace element concentration and weakly negative to positive δ11B (−4 to 2.3 ‰) value of tur2 suggest their metamorphic origin.