Exhalative Sn and W deposits associated with mafic volcanism as precursors to Sn and W deposits associated with granites

Precambrian and Early Palaeozoic submarine exhalative Sn and W deposits are most commonly present in thick deep water sequences comprising pelites, carbonate, calcsilicate and cherty rocks, mafic volcanics and quartz-tourmaline rocks. The exhalative deposits are commonly low grade and, in places, the sequence is transgressed by granites with economic Sn or W mineralisation. These thick deep water sequences as components of sinking trenches are more likely than most other rocks to undergo high grade metamorphism and anatexis. Thus elements such as Sn, W, B, F, Mo, Bi, Li, Be and base metals associated with the exhalites rapidly partition into the anatectic melt, lower the liquidus and solidus, and are concentrated in the late stage aqueous fluids and residual melt fractions to form Sn or W deposits associated with highly fractionated granitic rocks.