Formation of tin ore deposits: A reassessment

About 85% of all historically mined tin of about 27 million tonnes Sn is from a few tin ore provinces within larger granite belts. These are, in decreasing importance, Southeast Asia (Indonesia, Malaysia, Thailand, Myanmar), South China, the Central Andes (Bolivia, southern Peru) and Cornwall, UK. Primary tin ore deposits are part of magmatic-hydrothermal systems invariably related to late granite phases (tin granites, pegmatites, tin porphyries), and may become dispersed by exogenic processes and then eventually form placer deposits within a few km from their primary source, due to the density of cassiterite, its hardness and chemical stability. Alluvial placer deposits were usually the starting point for tin mining, and have provided at least half of all tin mined. The small-volume and late granite phases in spatial, temporal and chemical relationship to tin ore deposits are highly fractionated. Systematic element distribution patterns in these granite phases and their associated much larger multiphase granite systems suggest fractional crystallization as the main petrogenetic process controlling magmatic evolution and magmatic tin enrichment. Oxidation state controls the bulk tin distribution coefficient, with low oxidation state favoring incompatible behavior of divalent tin. Low oxidation state is also mineralogically expressed by accessory ilmenite (FeO TiO2) as opposed to accessory magnetite (FeO Fe2O3) in more oxidized melt systems. This difference in the accessory mineralogy and hence metallogenic potential (tin-bearing ilmenite-series versus barren magnetite-series granites), can be easily detected in the field by a hand-held magnetic susceptibility meter. The hydrothermal system is a continuation of the magmatic evolution trend and necessary consequence of the crystallization of a hydrous melt. The exsolved highly saline aqueous fluid phase, enriched in boron and/or fluorine plus a wide metal spectrum, can be accomodated and stored by the intergranular space in crystallized melt portions, or accumulate in larger physical domains, accompanied by focused release of mechanical energy (brecciation, vein formation), dependent on emplacement depth (pressure). The hydrothermal mobility of tin is largely as Sn2+-chloride complexes; the precipitation of tin as cassiterite involves oxidation. Tin typically characterizes the inner high-temperature part of much larger km-sized zoned magmatic-hydrothermal systems with the chemical signature Sn-W-Cu-As-Bi in the inner part (greisen, vein/stockwork/breccia systems, skarn) and a broader halo with vein- or replacement-style Pb-Zn-Ag-Sb-Au-U mineralization of lower temperature. This zoning pattern may also occur telescoped on each other. Active continental margins are the favorable site for both copper (−gold) and tin (−tungsten) systems. However, the narrowly segmented metal endowment and the episodic nature of ore formation suggest additional controls. These are the build-up of a subduction-derived metal and fluid inventory in the lower continental crust by flat-slab subduction (very little magmatism) for copper‑gold in the main arc, followed by large-scale intracrustal melting during mantle upwelling in the back arc for tin (chemically reduced reservoir rocks) and/or tungsten mineralization (less sensitive to oxidation state).