IMPLICATIONS FROM INCLUSIONS IN TOPAZ FOR GREISENISATION AND MINERALIZATION IN THE HENSBARROW TOPAZ GRANITE, CORNWALL, ENGLAND

Textural and geochemical studies of inclusions in topaz from greisens in the Hensbarrow topaz granite stock (St. Austell, Cornwall) are used to constrain the composition of fluids responsible for late stage gre isening and mineralisation. The topaz contains an abundant and varied suite of inclusions including aqueous liquid + vapour (L + V), quartz, zinnwaldite, albite, K-feldspar, muscovite, ilmenorutile, apatite, co lumbite, zircon, varlamoffite [(Sn, Fe)(O, OH)(2)] and qitianlingite [ (Fe+2,Mn+2)(2)(Nb,Ta)(2)W+6O10]. Primary L + V inclusions in topaz sho w relatively high T-h (mainly 300 to >500 degrees C) and a narrow rang e of salinities (23-30 wt% NaCl equivalent) compared with those in gre isen quartz (150-450 degrees C, 0-50 wt% NaCl equivalent). Textures in dicate that topaz formed earlier than quartz and the fluid inclusion d ata are interpreted as indicating a cooling of the hydrothermal fluids during greisenisation, mixing with meteoric waters and a decrease in pressure causing intermittent boiling. The presence of early-formed al bite and K-feldspar as inclusions in the topaz is likely to indicate t hat the greisen-forming fluid became progressively more acid during gr eisenisation. The most distinctive inclusions in the topaz are wisp- a nd bleb-shaped quartz, < 50 mu m in size, which show textural characte ristics indicating former high degrees of plasticity. They often have multiple shrinkage bubbles at their margins rich in Sn, Fe, Mn, S and Cl and, more rarely, contain euhedral albite, K-feldspar, stannite or pyrrhotite crystals up to 40 mu m in size. The quartz inclusions show similar morphologies to inclusions in topaz from quartz-topaz rocks el sewhere which have been interpreted as trapped ''silicate melt''. Thei r compositions are, however, very different to those expected for late stage topaz-normative granitic melts. From their textural and chemica l characteristics they are interpreted as representing crystallised si lica colloid, probably trapped as a hydro gel during greisenisation. T here is also evidence for the colloidal origin of inclusions of varlam offite in the topaz. These occurrences offer the first reported eviden ce in natural systems for the formation of colloids in high temperatur e hydrothermal fluids. Their high ore carrying potential is suggested by the presence of varlamoffite and the occurrence of stannite, pyrrho tite and SnCl within the quartz inclusions.