Abstract

The Ming volcanogenic massive sulfide (VMS) deposit (28 Mt at 1.48 wt % Cu, 0.06 wt % Zn, 1.99 g/t Ag, and 0.26 g/t Au) is part of the Baie Verte oceanic tract located in the northeast Canadian Appalachian orogen. The deposit is hosted in a NE-dipping volcanic succession (~487 Ma Rambler Rhyolite formation) composed of rhyodacitic to rhyolitic volcanic and volcaniclastic rocks that form the uppermost part of the Pacquet ophiolite complex.

The deposit consists of four elongated, stratiform semimassive to massive sulfide lenses (1807, 1806, Ming North, and Ming South zones) averaging 3 m in thickness, 50 m in width, and 500 m to 1 km in length, all spaced ~50 m apart from each another. The four semimassive to massive sulfide lenses are underlain by discordant stringers transitioning from sphalerite- to chalcopyrite-rich assemblages with stratigraphic depth. The chalcopyrite-rich stringer zone (Lower Footwall zone) defines a 26 million metric tons (Mt) mineral resource, oriented subparallel to the Ming North and 1806 zones.

Three units host the Ming deposit with the lowermost (unit 1.1) consisting of quartz-phyric to aphyric coherent rhyodacitic flows with marginal in situ massive hyaloclastite and peperite of similar compositions. Unit 1.1 is overlain by <200-m-thick felsic quartz-bearing volcaniclastic sequences (unit 1.2) with localized quartz-phyric to megacrystic rhyodacite flows at the top, which forms a flow-dome complex, immediately underlying the semimassive to massive sulfide of the 1806 zone. A coeval volcanic sequence, consisting of quartz-phyric to -megacrystic rhyodacite (unit 1.3a) and quartz-bearing tuff (unit 1.3b), hosts superimposed stratiform massive sulfide lenses in the Ming South zone, representing the waning stages of the Rambler Rhyolite formation-related volcanism. The succession hosting the Ming deposit is overlain by sedimentary and mafic volcanic and volcaniclastic rocks of the ≤479 Ma Snooks Arm Group.

Synvolcanic faults are recognized at Ming by abrupt lateral changes in lithofacies. Subsidence of unit 1.1 and coeval deposition of unit 1.2, as a result of these synvolcanic faults, occurred prior to the onset of the hydrothermal convection system responsible for the formation of the Lower Footwall zone and associated semimassive to massive sulfide lenses. The faults, which are interpreted to have been conduits for the metal-rich hydrothermal and magmatic fluids that deposited the metal-rich sulfide deposits, continued to be active until the deposition and eruption of unit 1.3, which is restricted to the Ming South zone.

Four major deformation events (D1-D4) are recognized at the Ming deposit, with D2 the most intense. Penetrative fabrics related to D2 are commonly oriented north-northeast, dipping east, with Cu-Au-rich semimassive to massive sulfides locally transposed and/or remobilized into these structures, together with drag folds, sphalerite exsolution layers, and boudinaged mafic dikes. Piercement structures of Cu-Au-rich sulfides associated with D2, commonly perpendicular to S2, are accentuated by D3 and D4 structures, representing significant targets for exploration and production.

The results presented here suggest that the linear distribution of the semimassive to massive sulfide zones relates to the original volcanic architecture (synvolcanic fault control) despite the previous interpretation of them being entirely controlled by structural elongation parallel to a regional stretch lineation; hence, this illustrates the importance of detailed reconstructions of lithostratigraphy, volcanic facies, and structure to the understanding of primary depositional controls on VMS mineralization in deformed orogenic belts. Furthermore, this study illustrates the effects of post-VMS deformation and metamorphism on the geometry of the ore lenses and on metal distribution in response to remobilization at various scales (secondary controls). A better understanding of primary and secondary controls on the nature and style of VMS mineralization have implications for exploration and development of deposits in ancient orogenic belts.

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