Chemical weathering of garnet in Banded Iron Formation: Implications for the mechanism and sequence of secondary mineral formation and mobility of elements

Abstract The chemical weathering of rocks and minerals on the Earth’s surface leads to the mobilization and redistribution of elements and has (a) significant control on the chemistry of the soil-hydrosphere-atmosphere system, (b) a profound effect on the bioavailability and toxic effects of elements, and (c) applications in mineral exploration among others. In this study, we describe the sequential weathering of spessartine- and pyrope-bearing almandine garnet in metamorphosed banded magnetite quartzite from the Khetri Copper Belt, western India. Based on detailed microtextural relations the following sequence of garnet weathering is proposed: garnet → alumo-goethite1 → alumo-goethite2 → kaolinite ± (monazite + xenotime ± thorite) → haematite. Our study thus reports for the first time the presence of alumo-goethite, kaolinite, monazite, xenotime, and thorite in weathering assemblage of garnet. The sequential chemical weathering of garnet was both grain boundary-controlled and fracture-controlled. Tectonic fractures (developed during exhumation), reaction-induced fractures (developed due to volume expansion), and secondary porosity (developed due to volume reduction) collectively controlled the propagation of weathering. Integrating multiple lines of evidence, e.g., corrugated contact between the weathering phases, presence of pores, relative molar volume changes, and high mobility of most elements it is argued that the weathering progressed through an interface-coupled dissolution-precipitation process. In situ major and trace element measurements and mass balance calculations show that several major (Fe, Na, K, P) and trace elements (Cl, F, alkalis, alkaline earth elements, and first series transition and base metals) were added while Si, Al, Ca, Mg and Mn were lost during the garnet to alumo-goethite transformation. During kaolinitization, all the major elements except Si and Al were lost to the weathering fluid. The alkali and alkaline earth, first series transition elements, base metals excepting Cr were lost during this weathering stage. Phosphorous, REEs, and Th were locally immobilized as monazite, xenotime, and thorite. In the more advanced stages of weathering, kaolinite was replaced by haematite. This stage involved re-addition of several major (except Al and Si) and trace elements. It is seen that although alumo-goethite, kaolinite, and haematite can host significant quantities of several trace elements, alumo-goethite and haematite have a much better potential of immobilizing most trace elements including toxic ones compared to kaolinite in the weathering profile.