U-PB AND RH-SR ISOTOPIC SYSTEMATICS OF FLUIDS ASSOCIATED WITH MINERALIZATION OF THE DARTMOOR GRANITE, SOUTHWEST ENGLAND

Quartz veins from early hydrothermal mineralization within the 280 Ma Dartmoor granite contain abundant fluid inclusions, which were sampled using crush-leach techniques and analyzed for Rb, Sr, U, and Pb conte nt and isotopic ratios. The present-day lead isotopic signature of som e of the crush-leach fluids is similar to that of granitic alkali feld spars from Dartmoor and other localities in southwest England, althoug h some fluid samples contain a more radiogenic Pb component. The more radiogenic fluids fell into, or near, the granitic K-feldspar field up on correction of the present-day fluid Pb ratios for in situ U decay o ver ca. 280 Ma, but the lead isotope ratios of the less-radiogenic flu ids became significantly lower than that of the granitic K-feldspars. The apparent overcorrection may be real, or may reflect either fluid m ixing due to leaching of secondary fluid inclusions, differential leac hing of U and Pb, the presence of an undetermined U-rich included phas e (or fracture-filling) within the quartz, or a combination of the abo ve. The variation of fluid U/Pb ratios also suggests some complication s regarding either the selective retention of U and Pb on quartz surfa ces, or the presence of submicroscopic U-rich inclusions. Therefore, w e suggest that in situ decay corrections to lead isotopic data obtaine d using crush-leach techniques be applied with great caution. The flui d Rb-87/Sr-86 values vary little (1.40-1.74), as do their present-day strontium isotope signatures (Sr-87/Sr-86 = 0.71814-0.71968). When cor rected for in situ Rb-87 decay over 280 Ma, the fluids contain signifi cant excess radiogenic Sr(Sr-i = 0.7118-0.7141), relative to the Dartm oor granite Sr-i (=0.7101). If the Sr in the fluids is derived exclusi vely from the granite, or some granite-related source, and closed syst em evolution of the Rb-Sr isotopic system is assumed, the amount of ex cess Sr-87 is a function of the time difference between Sr closure in the granite and Sr closure in the vein. Age estimates from the interse ction of granite and fluid inclusion Sr evolution curves are in agreem ent with published Ar-40-Ar-39 ages of late dikes and sericitic altera tion at Birch Tor-Vitifer. Our results further suggest that the fluids related to quartz-tourmaline-cassiterite mineralization within the Da rtmoor granite are themselves related to granite-derived, late-stage a plitic melts.