Milton C. Graves

The study of fluid inclusions and the analyses of mineral phases in the Fe-As-S system allow temperatures and pressures to be estimated at 432^o + 60^oC and 2.3 + 1.0 kilobars for the crystallization of groups of quartz-carbonate-arsenopyrite veins which are concordant with the bedding of the flyschoid Meguma Group (Cambro-Ordovician) of Southern Nova Scotia. The bedding and the enclosed veins are deformed by upright folds with north-east-trending axial-plane slaty cleavage defined by greenschist-grade regional metamorphic minerals. The vein mineral assemblages were in equilibrium with the greenschist assemblages. The formation of veins by "open-space" filling followed by regional F1 folding ending with the fixing of slaty cleavage appear to have occurred during the regional metamorphism to greenschist grade and under a relatively homogeneous strain regime. Emplacement of Devonian plutons, small-scale kink folding, and faulting occurred later and under a different strain regime.

Microprobe analysis of arsenopyrites (32.1 + 0.9 mean atomic percent arsenic) from seventeen groups of veins indicate a temperature of crystallization of 432^o + 60^oC. No trends of composition can be ascertained within grains, across or along veins, or between groups of veins. Although the present resolution of the arsenopyrite geothermometer only allows broad temperature estimates, the data obtained in this work may indicate that arsenopyrite crystallization in the veins took place under conditions of relatively constant temperature.

Fluid inclusions in quartz of the veins are two-phase (water as a liquid and carbon dioxide as a vapour). No solid phases were observed. They fall into two populations: one with a very constant vapour-to-liquid ratio (pseudosecondary), which homogenize at 262 + 10^oC and a second population which mainly occur in groups and along planes with varying vapour-to-liquid ratios (secondary) and homogenization temperatures of 210 + 6^oC. If the arsenopyrite crystallized together with the vein quartz at 432 + 60^oC and the pseudosecondary population of inclusions represents the crystallizing fluid, a pressure of 2.3 + 1.0 kilobars can be estimated.

Hydraulic fracturing could account for the formation of sub-horizontal veins of the observed mineralogy and paragenesis if greenschist grade metamorphic fluid was sufficiently overpressured under conditions of tectonic stress. If the above hypothesis correctly predicts vein formation, the bulk transport of vein constituents including gold and tungsten by means of greenschist grade regional metamorphic dewatering may be an important feature of mineralization in similar terrains.

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Pages: 174
Supervisor: Marcos Zentilli