Lithostratigraphic and structural reconstruction of the Zn-Pb-Cu-Ag-Au Lemarchant volcanogenic massive sulphide (VMS) deposit, Tally Pond group, central Newfoundland, Canada

Abstract

The Lemarchant volcanogenic massive sulphide (VMS) deposit (1.24Mt grading at 0.58% Cu, 5.38% Zn, 1.19% Pb, 1.01g/t Au, and 59.17g/t Ag) is a bimodal-felsic VMS deposit hosted within the Late Cambrian (∼513–509Ma) Tally Pond group of the Exploit Subzone in central Newfoundland, Canada. The deposit is hosted by andesitic volcaniclastic and volcanic rocks with subordinate dacite flows. The mineralisation is hosted by the dacites and is overlain by pillowed and massive basalts.Four structural breaks offset the local stratigraphic sequences including: 1) the LJ syn-volcanic shear zone; 2) the KJ syn-volcanic shear zone; 3) the Lemarchant thrust; and 4) the Bam normal fault. Deformation of the Lemarchant likely occurred during the Penobscot orogeny (486–478Ma). Early deformation is marked with the local deformation of the LJ and KJ syn-volcanic shear zones during NW-SE compression which coincided with the development of the Lemarchant thrust. A late (<465Ma) east trending normal fault, the Bam fault, affected the central portion of the Lemarchant area and down-faulted the southern portion of the study area relative to the northern portion.Immobile element systematics of all the sequences from the Lemarchant deposit are tholeiitic with transitional Zr/Y ratios (1.9–6.6), Lan/Smn ratios <1 (normalised to upper crust), and have primitive mantle extended rare earth elements profiles with slight light rare earth element (LREE)-enriched patterns with flat heavy REE (HREE), and weak to strong negative Nb, Zr, and Ti anomalies. Together, these geochemical features, coupled with an FIIIa signature, and existing mineralogical and Nd-Pb isotope data, are consistent with the rocks at the Lemarchant deposit having formed within a shallow (<1500m) arc or migrating cross-arc seamount chain located within a young peri-continental rifted arc along the margin of Ganderia, within the Iapetus Ocean. The estimated shallow water emplacement of the deposit likely allowed boiling near or at the rock-sea water interface, ultimately resulting in precious metal enrichment of the Lemarchant deposit. It is suggested that cross-arcs within rifted arc environments may represent favourable exploration targets for precious metal-enriched VMS deposits.