Geology of the Hope Brook Mine, Newfoundland, Canada; a preserved late Proterozoic high-sulfidation epithermal gold deposit and its implications for exploration

Abstract

The Hope Brook gold mine is a high-sulfidation epithermal gold deposit hosted by Late Proterozoic rocks of the northern Appalachian Avalon zone. It is located within the Whittle Hill Sandstone, which is intruded by a Late Proterozoic quartz-feldspar porphyry sill-dike complex of the Roti Intrusive Suite. The deposit (45 t Au) is enclosed within a zone of hydrothermal alteration >3 km long and up to 400 m wide. This zone of alteration is characterized by extensive advanced argillic alteration with pyrophyllite, kaolinite, andalusite, and alunite, which is developed mostly in the structural hanging wall of the ore zone, and two main stages of massive silicic alteration. The first, a buff-colored stage of massive silicic alteration, extends for > or = 3 km laterally away from the deposit. It constitutes a barren to weakly auriferous unit and most likely results from the pervasive acid leaching of the original host(s). The second stage of gray massive silicic alteration is spatially coincident with gold mineralization and is characterized by vuggy quartz. The mineralization consists of several percent of pyrite and lesser amounts of chalcopyrite and bornite and some tennantite with local traces of enargite. In addition to Au and Cu, there is a very limited amount of other metals in the deposit, although it has anomalously high amounts of Sb, Bi, Pb, and As.The ages of altered (pre- and late ore) quartz-feldspar porphyry and of an unaltered (postore) intermediate dike cutting altered rocks bracket the age of mineralization-alteration between 578 and 574 Ma. This temporally and genetically links mineralization-alteration and Roti Intrusive Suite plutonism. Despite a strong spatial relationship between a major fault zone (Cinq Cerf fault zone) and gold mineralization, the Hope Brook deposit bears no genetic relationship to major ductile deformation. The deposit was formed approximately 150 m.y. prior to the development of this fault zone.The existence of Hope Brook, along with a few Mesozoic (Pueblo Viejo), Paleozoic (Temora), Neoproterozoic (Brewer), and Paleoproterozoic deposits (Enaasen), highlights the potential for discovery of such Au-Cu intrusion-related deposits in older terranes. Despite their emplacement at high levels in the crust, the potential for preservation of such high-sulfidation deposits in older terranes may be high if tilting and/or burial took place early in the geologic evolution of the host volcano-plutonic arc.Although the nature and distribution of the initial buff silicic stage of alteration are critical indications of a large-scale hydrothermal system, the ore at Hope Brook is only localized in rocks displaying the gray-colored second silicic stage of alteration. Consequently, the recognition of the two stages of massive silicic alteration was critical in understanding the genesis of the Hope Brook deposit and constitutes an important exploration tool in discriminating between potentially mineralized and barren zones.