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The bimodal felsic Zn-Pb-Cu-Ag-Au Lemarchant volcanogenic massive sulphide (VMS) deposit is located in central Newfoundland. Characteristic of the deposit is a mineralized barite lens that contains abundant sulphosalts and anomalous precious metals. While much of the barite is massive, bladed aggregates of barite are locally present. The mineralized barite lens consists of three mineral assemblages (or facies) that grade from the outer barite-rich mineralization toward the sulphide-rich base as follows: 1) facies A: white to honey-coloured sphalerite+pyrite+chalcopyrite+bornite>enargite±Au; 2) facies B: white sphalerite+galena+pyrite+tetrahedrite>tennantite+stromeyerite+Ag-tetrahedrite; and 3) facies C: honey-brown (and minor red) sphalerite+chalcopyrite+pyrite±galena. Iron content of sphalerite grades from higher values in the outer barite lens and proximal rhyolite, to lower values in the central barite lens. Minor visible free gold is present in barite-rich facies A mineralization. The interpreted primary mineralization sequence of these three facies begins with deposition of sphalerite and fine-grained pyrite, and penecontemporaneous crystallization of tennantite-tetrahedrite, galena, and enargite, followed by sulphide replacement by bornite and chalcopyrite, late-stage stromeyerite mineralization, and recrystallization of euhedral pyrite. The Lemarchant deposit is similar to other barite-rich, Kuroko-style VMS deposits, and is especially notable for its sulphosalt-rich mineral assemblage and precious-metal-bearing minerals. Further detailed mineral chemistry, sulphur and lead isotope analyses, and thermodynamic calculations will be undertaken to understand the siting of precious metals and the processes that resulted in precious metal enrichment at Lemarchant.

The Lemarchant deposit is a Cambrian volcanogenic massive sulphide (VMS) deposit located in the Central Mobile Belt of the Newfoundland Appalachians. Unlike other polymetallic VMS deposits in the bimodal felsic Tally Pond group, Lemarchant is enriched in precious metals. The deposit is composed of contrasting styles of sulphide mineralization, and formed in three discrete stages: Stage 1: barite-rich, low-temperature (<250oC) VMS mineralization; Stage 2: 150 to 250oC intermediate- to high-sulphidation epithermal-style mineralization; and Stage 3: polymetallic, high-temperature (>300oC) VMS mineralization. Sulphur isotopes suggest that S is derived from three sources: thermochemically reduced seawater sulphate, leached igneous basement rock, and magmatic SO2. Lead isotopes indicate that Pb is primarily derived from evolved crustal material, with some input from juvenile volcanic rock (i.e. arc-rift). Precious metals associated with epithermal-style mineralization are consistent with a magmatic contribution to the hydrothermal fluid. Precious metals were precipitated from intermittently boiled fluids, at relatively shallow (<1500 m) water depth.

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The precious-metal-bearing, polymetallic, bimodal felsic Lemarchant volcanogenic massive-sulphide deposit is located in the Tally Pond belt, Dunnage Zone, Newfoundland Appalachians and consists of a stratiform, massive to semimassive sulphide zone and an underlying stringer sulphide zone. Five principal types of mineral assemblage are present: 1) semimassive white (low-Fe) sphalerite-granular barite-recrystallized pyrite-galena-minor tetrahedrite; 2A) bornite-galena-stromeyerite±chalcopyrite; 2B) bladed barite-coarse-grained tetrahedrite-galena-electrum-colusite±bournonite-polybasite-miargyrite; 3) massive red (high-Fe) sphalerite-fine- to medium-grained pyrite-chalcopyrite-galena; and 4) chalcopyrite-pyrite±orange sphalerite stringers. The stratiform sulphide zone contains the type 1 assemblage, which is crosscut by the type 2A and type 2B assemblages. The type 3 assemblage overprints the type 1 assemblage at the top of the stratiform zone. The basal stringer zone is host to the type 4 assemblage. The type 3 and type 4 assemblages represent minor zone refinement of the stratiform and stringer zones, as the hydrothermal fluids from which they were deposited were relatively hotter (>300°C) than the lower temperature fluid (<250°C) from which the type 1 and type 2 assemblages were deposited. The Lemarchant deposit shows zone refinement typical of Kuroko-style volcanogenic massive-sulphide mineralization; however, the precious-metal-enriched low-Fe sphalerite, bornite, electrum, and sulphosalt-rich type 2 assemblages suggest processes analogous to high-sulphidation epithermal-style volcanogenic massive-sulphide mineralization early in the evolution of the deposit. A direct magmatic contribution to the hydrothermal fluid and intermittent boiling during deposition of the type 1 and type 2 assemblages may be partially responsible for precipitation of epithermal-suite minerals and precious-metal enrichment.