Characteristics of gold mineralization in volcanogenic massive sulphide deposits of the Notre Dame Bay area, central Newfoundland

Abstract

A reconnaissance study of 19 volcanogenic massive sulphide deposits in the Notre Dame Bay area indicates Au concentrations of up to 30 pprn (Betts Cove), and elevated gold contents (> 1 pprn Au) have been found in samples from 10 additional past producers and developed prospects. Systematic trends in the occurrence of gold are observed in two principal sulphide assemblages: polymetallic, pyrite -sphalerite -chalcopyrite -galena f arsenopy rite assemblages (type I) and pyrite-chalcopyrite + sphalerite f pyrrhotite assemblages (type 11). Type I assemblages occur in deposits with dominantly felsic host rocks, whereas type I1 assemblages are restricted to deposits in mafic-dominated ophiolite sequences. Free gold grains were observed in samples from eight different deposits in both type I and type I1 assemblages. X-ray emission spectra and electron microprobe analyses of the gold indicate that most grains are electrum, although a Au-bearing telluride occurs at Point Leamington. Ion microprobe analyses indicate that as much as 50% of bulk gold may be present as "invisible gold" locked in pyrite or arsenopyrite (up to 140 pprn Au at Point Leamington). Well-preserved primary depositional features in gold-bearing sulphides from several deposits suggest that the gold in type I assemblages is syngenetic. A strong correlation between gold and a polymetallic suite of Zn, Ag, Pb, As, and Sb, similar to that observed in Kuroko-type massive sulphides and in modem seafloor sulphides, supports a primary origin for gold in type I assemblages. In type I1 assemblages primary depositional features have been largely destroyed by deformation and annealing of sulphide grains. Gold is locally enriched in Zn-rich sulphides, and sulphides containing abundant pyrrhotite have the lowest gold contents. However, consistent geochemical associations with other elements are not observed, and this may reflect the strong remobilization of gold during structural deformation. The abundant free gold in some type I1 assemblages is a product of recrystallization during deformation and was derived locally from primary gold originally present in the host sulphides.

Comparison of the TAG mound and stockwork complex with Cyprus-type, massive sulfide deposits

“…This may indicate that important concentrations of gold in the Cyprus deposits were overlooked in the Zn-rich ores that were left unmined. A number of Zn-rich massive sulfides with high gold contents are known in the ophiolites of Newfoundland (e.g., Betts Cove and Tilt Cove: Hurley and Crocket, 1985;Tuach et al, 1988;Santaguida and Hannington, 1995) as well as in the Josephine ophiolite (Turner Albright: Kuhns and Baitis, 1987). In general, the very low Ag/Au ratios at TAG and Cyprus are characteristic of cupriferous pyrite deposits and reflect a combination of the source rock composition (low Ag/Au ratios in MORB) and the effects of hydrothermal reworking (e.g., Hannington et al, 1995b).…”

Preliminary observations on styles of mineralization and sulphide-mineral zonation in the Cambrian Zn-Pb-Cu-Ag-Au Lemarchant volcanogenic massive-sulphide deposit, Newfoundland and Labrador

“…Although the massive-sulphide deposits in this area represent a range of volcanogenic massive-sulphide deposit types and have been explored since the 1850s (Martin, 1983;Piercey and Hinchey, 2012), there are few well studied volcanogenic massive-sulphide deposits with significant precious-metal enrichment (cf. Hurley and Crocket, 1985;Santagulda and Hannington, 1993;Santagulda and Hannington, 1996;Brueckner et al, in press). The Lemarchant deposit is hosted in the Tally Pond volcanic belt of the Victoria Lake Supergroup (Rogers et al, 2006;McNicoll et al, 2010) and is a type example of precious-metal enrichment in Appalachian polymetallic volcanogenic massive-sulphide deposits.…”