The area of the East South "C" (ESC) orebody of the Dickenson mine, Red Lake, consists of variably altered and mineralized basalt, basaltic volcaniclastic rocks, minor sulphidic iron formation, and a series of mainly postdeformation dykes. Except for the dykes, the rocks are in general well foliated. The macroscopic structural geometry of the stratiform rocks has been determined to a large extent by movement on schistosity-parallel faults.Three broad types of mineralization or alteration are recognized: an Na-Ca-Mg depletion with associated Fe-Mn enrichment controlled by primary permeable structures in basalt; a series of carbonate and quartz or "chert" veins emplaced into fissures; and auriferous silicified and sulphidized zones controlled by vein-filled fractures. The last is the main mineralization type in the ESC orebody on the 24th level of the mine, which was the focus of this study. Although all mineralization types occur within the mine, they are not directly associated either temporally or spatially on a mesoscopic scale. All, however, appear to have been overprinted by or formed synchronously with the amphibolite-facies metamorphism.A rich variety of metamorphic mineral assemblages occurs in the volcanic rocks because of the chemical effect of pre-or synmetamorphic hydrothermal alteration. These assemblages and the composition and mineral associations of arsenopyrite in the ESC orebody closely constrain the conditions of metamorphism to 520-540°C and 3.8-4.2 kbar (380-420 MPa) fluid pressure.La rkgion est-sud "C" (ESC) de la masse mintraliske de la mine Dickenson, Red Lake, est form6 d'un basalte minkralisk marquk par diverses altkrations, de roches volcanoclastiques, de formation ferifkre B faible teneur de sulfures et d'une famille de dykes dont la majoritk sont apparus suite a la dkformation. A part les dykes, les roches en general sont foliackes. La gkomttrie structurale mtgascopique des roches stratiformes fut dCterminCe en grande partie par l'ttude du mouvement des failles paralltles a la schistositk.Trois types principaux de mintralisation/altkration sont reconnus: un appauvrissement en Na-Ca-Mg associk avec un enrichissement en Fe-Mn sous le contr6le des structures primaires permtables dans la basalte; un ensemble de veines de carbonate et de quartz ou "chert" mises en place dans les fissures; et des zones auriferes et silicifites de sulfures relikes aux fractures remplies par des filons. Le dernier type correspond a la minkralisation principale de la rkgion ESC du gisement au niveau 24 de la mine et constitue l'objet central de notre ktude. Bien que tous les types de minkralisation dCcrits apparaissent dans la mine, l'ktude mksoscopique montrent qu'ils ne sont pas directement associCs entre eux, ni dans le temps et ni dans l'espace. Cependant tous semblent porter l'empreinte d'une activitt mktamorphique du faciks des amphibolites ou bien leur formation pourrait &tre synchrone B ce mttamorphisme.Une grande variCtt d'assemblages de mintraux mttamorphiques est obsemke dans les roches volcaniques ...

The Borrowdale Volcanic Group (BVG) in the SW of the English Lake District (SWLD) consists predominantly of subaerial pyroclastic rocks that host syn-volcanic intrusions ranging in composition from basalt to dacite. The petrology and geochemistry of the sills verify a broad compatibility with the BVG in the central Lake District. The geochemistry indicates a marginal continental arc setting, a petrogenesis involving c. 15% partial melting of a lherzolite mantle source and subsequent fractionation of olivine, ortho-and clinopyroxene, plagioclase and Fe-Ti oxide. Three groups of basaltic sills are recognized in the SWLD, one of which bears a close resemblance, geochemically, to the lower BVG (Birker Fell Formation) basaltic lavas and appears to have been emplaced early in the SWLD succession. The other two groups, which may be cogenetic, appear to be somewhat geochemically distinct from the first, with trace element and rare-earth characteristics suggesting a source slightly more enriched in incompatible elements. It is suggested that the fault-dominated tectonic setting in the Millom Park area of the SWLD may account for the least-evolved magmas being allowed to migrate directly to a high structural level, accounting for a higher proportion of basic lithologies in the upper BVG succession of the SWLD than is found in other parts of the Lake District.

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