Evidence for Syngenetic Precious Metal Enrichment in an Appalachian Volcanogenic Massive Sulfide System: The 1806 Zone, Ming Mine, Newfoundland, Canada

Abstract

The Ming deposit, Baie Verte, Newfoundland, Canada, is an Early Ordovician bimodal-mafic, Cu-Au-(ZnAg) volcanogenic massive sulfide (VMS) deposit. The deposit consists of a number of ore lenses that are stacked in the uppermost part of the Rambler rhyolite. One of the uppermost lenses, the 1806 zone, is enriched in Au and Ag. The deposit has been affected by Silurian-Devonian greenschist to amphibolite grade metamorphism and polyphase deformation and this has led to debates as to whether the Au-Ag enrichment in the deposit is syngenetic or a product of a later metamorphic and structural overprint.The 1806 zone consists predominantly of discordant sulfide stringer, stratabound semimassive to massive sulfides and a weakly mineralized silicified cap zone. The ore is largely hosted within the footwall Rambler rhyolite that is strongly altered to quartz-sericite ± green mica with sporadic chlorite-biotite. Base and precious metal zoning is developed from the down-plunge portion (Cu-Au) to the up-plunge portion (Cu-Zn-Au-AgPb) of the 1806 zone. The ore mineralogy and mineral chemistry are complex and are interpreted to be of an intermediate sulfidation type. Pyrite and chalcopyrite are the dominant sulfide species with minor to accessory sphalerite, galena, pyrrhotite, and arsenopyrite. Sulfosalts including Ag-bearing tennantite-tetrahedrite, stannite, boulangerite, and loellingite, and precious metal-rich phases are common throughout the deposit. Precious metals occur as (1) mercurian electrum (7.93-20.57 wt % Hg) and (2) various Ag phases (miargyrite, pyrargyrite, mercurian stephanite, unnamed AgCuFeS phase, Ag-Hg ± Au alloys) that are more abundant in the up-plunge portion of the 1806 zone. Tellurides and bismuthides are present in trace amounts. Oxides (cassiterite and magnetite) are present exclusively in the down-plunge portion of the 1806 zone.The deposit geometry, metals distribution, complex ore assemblages, abundant sulfosalts, and the significant concentration of the epithermal suite of elements (e.g., Au, Ag, As, Hg, Sb, Bi) in the ore strongly contrast with those of orogenic Au deposits and support a syngenetic or synvolcanic origin for precious metals in the Ming VMS deposit, including a possible magmatic input into the 1806 zone ore-forming system.Evidence for a late, syndeformation precious metal emplacement via orogenic overprinting is lacking at the 1806 zone and all data point to a syngenetic origin for precious metal enrichment, indicating that Ordovician Appalachian VMS deposits are favorable targets for Au. Nevertheless, Silurian-Devonian metamorphism and deformation have resulted in textural recrystallization and local remobilization of precious metals and the main fabrics largely control the current geometry of the host succession and of the Ming orebody.

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

“…data, 2013); however, the nature of Au in this unit is yet unclear. Although numerous lines of evidence support a syngenetic Au mineralization at the Ming deposit from recent work exclusively on the (semi-)massive sulfide orebodies (Brueckner et al, 2014, the results of this study and the nature of Au in this clast-rich unit provide an additional critical test of the syngenetic versus epigenetic origin for Au enrichment. Here we use for the first time on samples from a different unit at the Ming deposit the combination of scanning electron microscopy (SEM)-mineral liberation analysis (MLA) to identify the textural and mineralogical setting of Au-bearing minerals.…”

“…The silicified horizon capping and the stringer zone underlying the 1806 zone also show this type of precious metal texture (Fig. 8a, b; Brueckner et al, 2014. However, within the sulfide clasts, electrum is also in contact with epidote.…”

“…The orebodies are Cu rich, precious metal bearing, and locally enriched in Zn. The sulfide assemblage in the orebodies is complex and is dominated by a pyrite-chalcopyrite assemblage with minor pyrrhotite-sphalerite and traces of galena-arsenopyrite ± cubanite ± sulfosalts ± tellurides ± (sulfo-)antimonides ± precious metals ± oxides (Brueckner et al, 2014. Three generations of mafic to intermediate intrusive rocks are present in the deposit.…”

“…Brueckner et al (2014 showed that Au occurs predominantly as (mercurian) electrum at the Ming deposit orebodies. Although the relative abundances of Ag, Au, and Hg in electrum vary (Brueckner et al, 2014, distinctions between the different Au-bearing alloys in the clasts are difficult to make using MLA due to the semiquantitative detection and the small size of the analyzed grains. Hence, the term electrum is used in this paper for all Au-bearing phases that have >10 wt % Au.…”

“…The goals of this study are as follows: (1) the identification of Au-bearing minerals inside the sulfide clasts, (2) characterization of their textures and mineral associations, and (3) comparison with the previously studied underlying massive sulfide orebodies (Brueckner et al, 2014. These results are then utilized to provide insight into Au provenance within the clasts and to assess whether Au enrichment was syngenetic or epigenetic.…”