FLUID INCLUSION ANALYSIS OF FLUORITE IN POST-ALLEGHANIAN VEINS, VALLEY AND RIDGE PROVINCE, CENTRAL PENNSYLVANIA

Fluorite-calcite-quartz mineralization was collected from post-Alleghanian veins in limestones of the Silurian-Devonian boundary near Allenport, Pennsylvania. Microthermometric analysis focused on 2-phase (liquid + gas) fluid inclusions in fluorite, which are dominantly in primary fluid inclusion assemblages. Temperatures of homogenization (T_h) (liquid + gas → liquid) were tightly constrained between 180 and 195°C. Ice melting temperature (T_m) showed a narrow range of -20.7 and -20.2°C. Collecting T_m was complicated by the formation of clathrates under freezing conditions, requiring that samples needed to be melted completely between observations of each T_m to allow the initial nucleation of the gas bubbles to be repeatable. A secondary fluid inclusion assemblage confined to a damaged corner of a euhedral fluorite crystal exhibited a much wider T_h range (115.0˚C - 195.0˚C) than the inclusions in the primary FIA, but produced consistent T_m values at -21.0°C. The tight T_h range from the primary FIA fits into the broad T_h range reported by Howe (1988) of 160 ± 60°C for Mississippi Valley-type occurrences in central Pennsylvania. However, Evans and Battles (1999) reported lower and broader T_h values (58 to 160°C) for vein minerals from Alleghanian veins in the same formation as our samples. T_h data from vein quartz in other local formations are radically different as well: 100 to 106.1°C (Curry, 2013) and >130 to 190°C (Scudder, 2014).