The Origin Of The Hope Brook Mine, Newfoundland: A Shear Zone-hosted Acid Sulphate Gold Deposit

• Main Author: Stewart, Peter W
• Format: Text
• Language: unknown
• Published: Scholarship@Western 1992
• Subjects: Geology; > Mineralogy; Newfoundland
• Online Access: https://ir.lib.uwo.ca/digitizedtheses/2153; https://ir.lib.uwo.ca/context/digitizedtheses/article/3152/viewcontent/NN75383.PDF

The Hope Brook deposit is hosted by the Hope Brook Alteration Complex (HBAC) in the Hermitage Flexure of southwestern Newfoundland. The HBAC records a complex tectonic, magmatic, metamorphic and hydrothermal history. It is deformed within the Cinq Cerf Fault Zone (CCFZ), a Late Precambrian structure reactivated during Silurian tectonism when a composite pre-Silurian basement was faulted against the Silurian felsic volcanic La Poile Group. The principal protoliths to the HBAC are trondhjemitic Late Precambrian porphyry dikes of the Roti Intrusive Suite (RIS). Sedimentary rocks of the basement terrane are less common protoliths. Mafic dikes comprise three distinct geochemical populations in the HBAC. Late Precambrian group 1 and Silurian group 2 are low TiO{dollar}\sb2{dollar} continental tholeiites. Silurian high TiO{dollar}\sb2{dollar} mafic dikes are minor in abundance.;The HBAC is comprised of aluminous and silicified alteration zones consisting of quartz + rutile, with and without andalusite, muscovite, paragonite, pyrophyllite, and(or) kaolinite, respectively. Sericitic alteration of RIS injections in the subparallel dike swarm represents a third alteration domain. The orebody, comprised of disseminated gold {dollar}\pm{dollar} pyrite {dollar}\pm{dollar} chalcopyrite in silicified rocks, averages 4.5 g Au/tonne. Although minor gold ({dollar}\le{dollar}1.5 g/tonne) occurs over the 4 km strike of the HBAC, the orebody is confined to the innermost contact aureole of the posttectonic Devonian Chetwynd granite. Retrogression of contact metamorphic minerals and undeformed hydrothermal breccias attest to late hydrothermal activity.;Acid leaching, exemplified by the silicified and aluminous zones, and locally abundant alunite indicate the HBAC is an acid sulphate gold deposit. Early acidic alteration occurred during syntectonic RIS and group 1 magmatism in the CCFZ. Paragenetic relationships and the change from phengitic muscovites in CCFZ mylonites to weakly phengitic, sodium-enriched muscovites {dollar}\pm{dollar} ...