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

摘要

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 content and isotopic ratios. The present-day lead isotopic signature of some of the crush-leach fluids is similar to that of granitic alkali feldspars from Dartmoor and other localities in southwest England, although some fluid samples contain a more radiogenic Pb component. The more radiogenic fluids fell into, or near, the granitic K-feldspar field upon correction of the present-day fluid Pb ratios for in situ U decay over ca. 280 Ma, but the lead isotope ratios of the less-radiogenic fluids became significantly lower than that of the granitic K-feldspars. The apparent overcorrection may be real, or may reflect either fluid mixing due to leaching of secondary fluid inclusions, differential leaching of U and Pb, the presence of an undetermined U-rich included phase (or fracture-filling) within the quartz, or a combination of the above. The variation of fluid U/Pb ratios also suggests some complications regarding either the selective retention of U and Pb on quartz surfaces, or the presence of submicroscopic U-rich inclusions. Therefore, we suggest that in situ decay corrections to lead isotopic data obtained using crush-leach techniques be applied with great caution. The fluid 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 corrected for in situ Rb-87 decay over 280 Ma, the fluids contain significant excess radiogenic Sr(Sr-i = 0.7118-0.7141), relative to the Dartmoor granite Sr-i (=0.7101). If the Sr in the fluids is derived exclusively from the granite, or some granite-related source, and closed system evolution of the Rb-Sr isotopic system is assumed, the amount of excess 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 intersection of granite and fluid inclusion Sr evolution curves are in agreement with published Ar-40-Ar-39 ages of late dikes and sericitic alteration at Birch Tor-Vitifer. Our results further suggest that the fluids related to quartz-tourmaline-cassiterite mineralization within the Dartmoor granite are themselves related to granite-derived, late-stage aplitic melts. [References: 44]