Resolving the Relative Timing of Au Enrichment in Volcanogenic Massive Sulfide Deposits Using Scanning Electron Microscopy-Mineral Liberation Analyzer: Empirical Evidence from the Ming Deposit, Newfoundland, Canada

Abstract

The metamorphosed Cambro-Ordovician Ming volcanogenic massive sulfide deposit in northern Newfoundland, Canada, is locally overlain by a unit consisting of mafic to intermediate medium- to coarse-grained volcaniclastic breccia with up to 10 vol % sulfide clasts. Analysis via mineral liberation analyzer of two sulfide clasts, completed using scanning electron microscope observations, allowed the identification of a number of microscopic and submicroscopic electrum grains. These electrum grains occur in three types of textural settings: (1) free electrum grains with tellurides within gangue minerals, (2) inclusions of electrum with tellurides in pyrrhotite grains, and (3) free electrum grains with base metals and tellurides interstitial between base metals and along cataclastic fractures in pyrite. These three textural settings are similar to those in the underlying massive sulfide orebodies that represent very likely sources to the sulfide clasts. The polylithic nature and angularity of the volcaniclastic fragments in the breccia suggest a postmineralization gravity-controlled debris flow proximal to its source. The mineral assemblage and textures of electrum in the sulfide clasts implies evidence in support of syngenetic and predeformation Au introduction in the volcanogenic massive sulfide deposit, and argues against an orogenic overprint as the cause for Au enrichment.