GEOLOGY OF THE HOPE BROOK MINE, NEWFOUNDLAND, CANADA - A PRESERVED LATE PROTEROZOIC HIGH-SULFIDATION EPITHERMAL GOLD DEPOSIT AND ITS IMPLICATIONS FOR EXPLORATION

The Hope Brook gold mine is a high-sulfidation epithermal gold deposit hosted by Late Proterozoic rocks of the northern Appalachian Avalon z one. It is located within the Whittle Hill Sandstone, which is intrude d by a Late Proterozoic quartz-feldspar porphyry sill-dike complex of the Roti Intrusive Suite. The deposit (45 t Au) is enclosed within a z one of hydrothermal alteration >3 km long and up to 400 m wide. This z one of alteration is characterized by extensive advanced argillic alte ration with pyropyllite, kaolinite, andalusite, and alunite, which is developed mostly in the structural hanging wall of the ore zone, and t wo main stages of massive silicic alteration. The first, a buff-colore d stage of massive silicic alteration, extends for greater than or equ al to 3 km laterally away from the deposit. It constitutes a barren to weakly auriferous unit and most likely results from the pervasive aci d leaching of the original host(s). The second stage of gray massive s ilicic alteration is spatially coincident with gold mineralization and is characterized by vuggy quartz. The mineralization consists of seve ral percent of pyrite and lesser amounts of chalcopyrite and bornite a nd some tennantite with local traces of enargite. In addition to Au an d Cu, there is a very limited amount of other metals in the deposit, a lthough it has anomalously high amounts of Sb, Bi, Pb, and As. The age s of altered (pre- and late ore) quartz-feldspar porphyry and of an un altered (postore) intermediate dike cutting altered rocks bracket the age of mineralization-alteration between 578 and 574 Ma. This temporal ly rally and genetically links mineralization-alteration and Roti Intr usive 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 defo rmation. The deposit was formed similar to 150 m.y, prior to the devel opment of this fault zone. The existence of Hope Brook, along with a f ew Mesozoic (Pueblo Viejo), Paleozoic (Temora), Neoproterozoic (Brewer ), and Paleoproterozoic deposits (Enasen), highlights the potential fo r 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 are. Although the nature and di stribution of the initial buff silicic stage of alteration are critica l indications of a large-scale hydrothermal system, the ore at Hope Br ook is only localized in rocks displaying the gray-colored second sili cic stage of alteration. Consequently, tile recognition of the two sta ges of massive silicic alteration was critical in understanding the ge nesis of the Hope Brook deposit and constitutes an important explorati on tool in discriminating between potentially mineralized and barren z ones.