Rare metal mineralization related to granites and pegmatites, Phuket, Thailand

Abstract

Rare metal mineralization on Phuket Island, southern Thailand, comprises deposits of Sn, Nb-Ta, and W. Mineralization is related to Cretaceous granites of the Khao Tosae Suite and their pegmatite derivatives. The Khao Tosae Suite granites and the pegmatites intrude other granite suites and the Carboniferous-Permian metasedimentary rocks of the Phuket Group. The Khao Tosae Suite consists of fine- to medium-grained, porphyritic biotite-muscovite + or - tourmaline granites. The pegmatites are grouped into three types according to their main mineral assemblages: K feldspar-muscovite-tourmaline, K feldspar-albite-muscovite, and albite- K feldspar-lepidolite dikes. Petrogenetic studies indicate that Sn and Nb-Ta mineralization is extended from the late-magmatic stage to the pegmatite and postmagmatic, hydrothermal stages of the Khao Tosae Suite. Besides its occurrence in the pegmatites, cassiterite precipitated during at least three periods in the Khao Tosae Suite granites, each of which has distinct petrochemical characteristics. Nb-Ta minerals occur in both the Khao Tosae Suite granites and the pegmatites as wolframoixiolite, niobian-tantalian rutile, and columbite- tantalite exsolutions in cassiterite. In addition, discrete grains of columbite-tantalite and microlite locally occur in the pegmatites. Wolframite is represented by huebnerite and is associated with the last stage of cassiterite deposition, occurring in late vugs and/or quartz veins in the Khao Tosae Suite granites. The Phuket deposits represent a rather rare example of a granite-related (Sn,Nb-Ta,W)-bearing system with substantial evolution of pegmatites as well as postmagmatic greisen and vein mineralization. The pegmatites are notable for their largely greenschist facies environment, lack of Be and P minerals, tourmaline sharply decreasing in abundance from early to late generations of pegmatite dikes, and a distinct accumulation of Sr and Ba in the otherwise most fractionated lepidolite-rich dikes. The pegmatites also provide the first example of Fe-Mn and Nb-Ta fractionation in successive generations of primary granitic to pegmatitic cassiterite (as reflected in its exsolved columbite-tantalite), which perfectly mimics fractionation trends established for primary columbite-tantalite in corresponding categories of pegmatites.