COMPARISON OF FLUID INCLUSIONS IN COEXISTING (COGENETIC) WOLFRAMITE, CASSITERITE, AND QUARTZ FROM ST-MICHAELS MOUNT AND CLIGGA HEAD, CORNWALL, ENGLAND

The St. Michael's Mount occurrence and Cligga Head deposit are typical examples of W-Sn mineralization in the Cornwall district in S.W. England. Mineralization consists of quartz, wolframite, cassiterite, and various amounts of base metal sulfides in sheeted veins. Associated alteration includes greisenization and argillization. In both areas textural evidence for contemporaneous deposition of the ore minerals, wolframite and cassiterite, with quartz is generally ambiguous. Previous studies state that the quartz and wolframite are intergrown, but do not address the genetic relation between them. In this study microthermometry was performed on fluid inclusions in wolframite, cassiterite, and quartz. The inclusions in wolframite were observed using an infrared microscope and those in cassiterite and quartz were observed with visible light. At St. Michael's Mount Th values for primary inclusions in quartz average 311°C and in wolframite average 369°C. Salinities average 7.3 eq. wt% NaCl in quartz and 4.2 eq. wt% NaCl in wolframite. At Cligga Head, the average Th values for primary inclusions in the various minerals are: cassiterite, 352°C; wolframite, 324°C; and quartz, 295°C. Fluid inclusion salinities average 5.3 eq. wt% NaCl in cassiterite, 3.9 eq. wt% in wolframite, and 6.0 eq. wt% NaCl in quartz. The magnitude of the changes in Th measurements between minerals cannot be accounted for by post trapping changes and therefore must be due to actual differences in the temperatures of deposition. These data suggest that the ore minerals, wolframite and cassiterite, were deposited earlier than the associated quartz, even though conclusive textural evidence is lacking. Wolframite and cassiterite were deposited at about the same temperature at St. Michael's Mount but at Cligga Head cassiterite was deposited at higher temperatures than wolframite. Certainly in these areas the gangue minerals do not provide reasonable estimates of the depositional conditions of the ore minerals even though they are intimately intergrown. © 1990.