A fluid inclusion and geochemical investigation of the fluorite deposits of the Southern Pennine Orefield
The fluorite deposits of the Southern Pennine Orefield occur in vein fillings and metasomatic replacements. Primary and secondary fluid inclusions define five discrete phases of fluorite mineralization or remobilization. Homogenization temperature ranges for the five events are; (1) 100-171°C; (2) 62-82°C; (3) 65-99°C; (4) 73-106°C; (5) 66-68°C. Little evidence was found of east-west thermal gradients having operated during any of the events. The inclusion types are further characterized by distinctive melting point ranges of ice, hydrohalite or antarcticite. Major element concentrations of the ore fluids have been determined from cation ratios in inclusion leachates modelled in the ternary system NaCl-CaCl2-H20. High Ca/Na ratios in the type 2, 4 and 5 fluids indicate compositions atypical of present-day high salinity formation waters resident in deep sedimentary basins. Mineral solubility data has been used to determine limits for trace anion and cation activities. Ranges for oxygen and sulphur fugacity and pH have also been estimated for mineralizing phases 3 and 4. REE analysis of fluorite and carbonates showed LREE-enriched chondrite normalized patterns with negative cerium and europium anomalies. Total lanthanide abundances are low. Strontium isotope ratios range from 0.7082 to 0.7101 indicating ore fluid interaction with wall rocks enriched in radiogenic strontium with respect to Visean marine carbonates. Muscovite, biotite and K-feldspar can be demonstrated to have been unstable in the ore fluids at 100°C and represent potential fluorine source minerals. Dissolution or alteration of aluminium silicates could have produced fluoride release into the brines. The homogeneity of the inclusion fluids, the uniform REE content of the fluorite and equilibrium mineral textures indicate a non-mixing mechanism of mineralization. An open system, "one pass" model of mineralization with fluorite precipitated as a result of metasomatism of carbonates and possibly limited cooling, and sulphide deposition through pH increase is proposed.