Genesis of the St. Lawrence fluorspar deposits as indicated by fluid inclusion, rare earth element, and isotopic data

Abstract

More than 40 separate fluorspar veins are associated with the Lower Carboniferous St. Lawrence Granite of southeastern Newfoundland, with more than 3.4 million short tons mined since 1933, and a further 9 million short tons of reserves contained in the three main veins. It is estimated from these tonnages that fluorine concentrations in the magma of only 19 ppm would have been adequate to form the veins, much less than the average of 1,567 ppm F found in the granite. Fluid inclusions show that salinities remain within the range of 5 to 15 equivalent weight percent NaCl, although homogenization temperatures fluctuate between 100 degrees and 500 degrees C. Evidence for boiling between 450 degrees and 223 degrees C indicates that the homogenization temperatures are trapping temperatures, at low pressures between 65 and 650 bars. Some 18 O depletion in the aqueous fluids owing to unmixing and loss of CO 2 . In the fluorite veins, delta 18 O quartz is approximately 14 per mil, and calculated fluid delta values are 10 per mil (450 degrees C) to -6 per mil (120 degrees C); waters released from low-temperature magmatic fluids which underwent transient mixing with a cool low 18 O meteoric reservoir.--Modified journal abstract.