Mineralogical, sulphur, and lead isotopic study of the Lemarchant Zn-Pb-Cu-Ag-Au-VMS deposit: implications for precious-metal enrichment processes in the VMS environment

Abstract

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.

Potential for Volcanogenic Massive Sulfide Mineralization at the A6 Anomaly, North-West British Columbia, Canada: Stratigraphy, Lithogeochemistry, and Alteration Mineralogy and Chemistry

“…The occurrence of barite, while not observed in the thin section due to the small grain size, is common in many low-temperature VMS deposits, and is commonly associated with distal alteration [115,116]. However, the limited number of drill holes and the absence of a (semi-) massive sulfide lens make it difficult to determine the spatial relation of the altered felsic units (i.e., alteration pipe proximal to sulfide lens vs. semi-conformable alteration zone distally around sulfide lens; e.g., [17,117,118]).…”

Targeted Geoscience Initiative 4: Contributions to the understanding of volcanogenic massive sulphide deposit genesis and exploration methods development

“…Specific element suites, such as In, Te, and Bi in high-temperature Cu-rich ores and As, Sb, Hg, and Ag (the "epithermal suite") and complex sulphosalt assemblages in low-temperature Zn-rich ore, also have been linked to a direct magmatic contribution of metals into the ore-forming hydrothermal system Huston et al, 2011); both associations have been documented at Ming (Brueckner et al, 2014;Pilote et al, 2015) and at Lemarchant in central Newfoundland (Gill et al, 2013(Gill et al, , 2015. Efficient transport and precipitation mechanisms: Whereas some districts contain VMS deposits that are uniformly enriched in Au and/or Ag, some districts/ camps contain VMS deposits that are much more enriched than others in the same district (e.g.…”

“…The role of boiling/phase separation, which is largely controlled by the water depth, but also by the initial composition of the fluids, and the role of different ligands has been discussed in detail by Hannington et al (1999), Huston (2000), and others, and readers are referred to these papers for in-depth discussions. Boiling is considered to have been a major factor for precious and other trace metal enrichment at Lemarchant in central Newfoundland (Gill et al, 2013(Gill et al, , 2015Gill and Piercey, 2014;Piercey et al, 2014;Gill, 2015). At the same time, evolving conditions in a VMS-forming hydrothermal system can have caused major redistribution of metals through zone refining (see Ohmoto, 1996), for example, concentrating precious metals in some specific areas, lenses, or zones in a VMS deposit (e.g.…”