Alan Yule

The Hope Brook Gold Inc. mine is located in southwestern Newfoundland (47o44'N lat., 58o06'W long) and exploits a large (>11.0 Mt) low grade (4.5 g/t Au, 0.3 wt.% Cu) disseminated gold-chalcopyrite-pyrite deposit. Discovered in 1983 by Selco Division of BP Resources Canada, present reserves of >50 t Au make Hope Brook the most significant gold discovery in the Appalachians in the last 50 years. It is situated at the southwestern end of the Hermitage Flexure, within the Gander tectonostratigraphaic Zone of the Canadian Appalachians. The La Poile and Bay du Nord Groups form the principal stratigraphic units in the area.

The deposit lies immediately to the south of a large NE-striking, steeply S-dipping belt of intense ductile strain (here informally named the Cinq Cerf Fault) which separates two distinct lithologic blocks that were juxtaposed during major Siluro-Devonian deformation. The Northern Block includes greenschist facies volcanic and subvolcanic (Hawks Nest Pond Porphyry) rhyodacites and rhyolites, terrigenous, conglomerates and composes the La Poile Group. The Southern Block, which hosts the deposit, is dominated by transitional greenschist-amphibolite facies greywackes, and andesites and basalts, with abundant hypabyssal intrusions of basalt and andesite, and quartz-feldspar porphyry derived from a high-level granodiorite pluton (Roti Granite). The Roti Granite bounds this block in the south. These rocks show more oceanic affinities, and their regional correlation is uncetrtain. To the northeast, a post-tectonic syenogranite (Chetwynd Granite) truncated the Cinq Cerf Fault and contact metamorphosed the deposit.

Submicroscopic free Au, narrow, irregular chalcopyrite veinlets, and disseminated pyrite occur within a vuggy, pervasively silicified rock. North of the deposit, an intensely silicified and deformed, pyritic rock forms the structural footwall. To the south, quartz-andalusite-sericite schists form the structural hanging wall and probably represent the regional metamorphic equivalents of advanced argillic and phyllic alteration. Acid hydrolysis post-dated silicification-mineralization, and also affected an old generation of mafic intrusions within the deposit. These intrusions may represent lower grade equivalents of cordierite-anthophyllite metabasite rocks.

The data gathered for this study are compatible with alteration-mineralization having occurred prior to peak regional metamorphism and deformation, and having mainly affecting volcanic and subvolcanic rocks cogenetic with the Roti Granite. The deposit probably formed at moderate depths in a hot hydrothermal system evolved from the cooling of the Roti Granite; the tectonic environment was possibly an oceanic island-arc. The preferred genetic hypothesis makes Hope Brook a sheared, regional and contact metamorphosed, massive replacement mesothermal gold deposit.

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Pages: 264
Supervisor: Marcos Zentilli