I recently had a new paper published in the Canadian Journal of Earth Sciences (CJES) with my colleagues John Hinchey and Greg Sparkes from the Geological Survey of Newfoundland and Labrador entitled “Volcanogenic massive sulfide (VMS) deposits of the Dunnage Zone of the Newfoundland Appalachians: setting, styles, key advances, and future research“. This paper was an invited paper for the 60th anniversary of CJES and represents a synthesis of research that Greg, John, me, and numerous graduate students and post-docs at Memorial (see below) have undertaken, as well as advances driven by the exploration and development undertaken by numerous companies over the last two decades.
The paper provides an overview of VMS deposits, their classification, and regional geological setting of VMS deposits in the Dunnage Zone. It then reviews the grade-tonnage data and classification of various deposits within a global framework. It then subsequently provides reviews and geological documentation for various VMS belts on the island and their contained mineralization. The discussion focuses on key themes that have come from research in the last 20 years. While not a new concept, there has been ample geological and geochemical evidence to show that VMS deposits in Newfoundland formed during rift events and formed either in forearc, arc, or back-arc rifts – this really validates early work by Scott Swinden and colleagues in the late 80s and early 90s who illustrated its importance and was subsequently recognized globally. One new concept raised in this paper is the importance of slab melting and the generation of Au-rich VMS deposits. My former PhD student Jean-Luc Pilote using numerous lines of arguments, illustrated quite conclusively that the rhyolitic rocks that hosted Au-rich VMS in the Baie Verte Peninsula were derived from slab melting. He further mused that transfer of volatiles during this process, including Au, were responsible for forming “epithermal suite” element enrichments in the rhyolites and subsequent deposits. This paper takes this concept a little further and suggests that such slab melts would also be S-undersaturated and would have dissolved sulfides in the mantle wedge during its transit to the surface, further enhancing the metal content of the magmas, which if degassed could have provided the Au and other elements found in these deposits (see Jim Mungall’s work for the inspiration for this idea here).
The second part of the discussion focuses on emplacement mechanisms of VMS deposits. It is well known that from modern seafloor massive sulfides that they exhibit “exhalation” on the seafloor and form “exhalative” mineralization. This is also common in many deposits in the ancient record, including those in Newfoundland such as the Lemarchant deposit in the Tally Pond belt and some of the in-situ deposits from the Buchans district. Previous workers (e.g., Geoff Thurlow, Eric Swanson, and Paul Binney’s work in Buchans) have also noted the importance of transported, clastic deposits, such as the Maclean deposits in Buchans, as another type of VMS emplacement style. More recently, however, there is increasing recognition that some deposits in Newfoundland formed below the seafloor and via subseafloor replacement, including deposits like those at Duck Pond and Boundary (e.g., work of Gerry Squires and my group). There are some deposits, including the Boundary deposit, which are some of the best examples globally of subseafloor replacement and relationships from there have impacted global models for this sub-type of VMS deposit.
The last two parts of the discussion focus on the sources of metals and sulfur in VMS deposits. Numerous workers have demonstrated that the metals in most VMS deposits come from leaching of footwall strata, be they basement or the volcanic/sedimentary sequence underlying the deposit. New research, however, has illustrated that some deposits have exceptional grades and/or tonnes that cannot be explained solely by leaching. For example, the exceptionally high grades found in many deposits in the Buchans district, coupled with mineralogical assemblages, alteration types, and other features point to the potential for magmatic-hydrothermal contributions of metals to these deposits. Likewise, the Au-rich deposits in the Rambler-Ming district likely had significant contributions of magmatic-hydrothermal fluids given their epithermal- and precious metal-enrichment, unique hydrothermal alteration assemblages, and sulfosalt-rich ore assemblages, assemblages commonly found in magmatic-hydrothermal deposits (e.g., Stef Brueckner’s and Jean-Luc Pilote’s research). Sulfur in VMS deposits has been shown to be from either leached footwall rocks (i.e., igneous sulfur) or from the thermochemical sulfuate reduction (TSR) of seawater sulfate, and this is the case in many VMS deposits in the Newfoundland Appalachians. New work, however, has shown considerable complexity in near vent hydrothermal sedimentary rocks (i.e., the plume fall out from black smokers) with considerable input from bacterial sulfate reduction (BSR) of seawater sulfate. Further, some deposits have evidence for magmatic-hydrothermal SO2 disproportionation and magmatic S in some VMS-related sulfides. A significant reason for new insight is because of the usage of in situ S isotope determinations (done in collaboration with Graham Layne and Glenn Piercey at Memorial), which has allowed greater numbers of S isotope determinations determinations and S isotopic data that is much nmore texturally and paragenetically constrained, something that is a significant with conventional S isotope work.
Despite all these new ideas, much is left to be done in VMS research globally and in the Newfoundland Appalachians, and these potential future research avenues are also outlined in the article.
As with any field of research, the work and synthesis above involved a lot of contributions from graduate students and post-docs. The pace of advancement of our knowledge and understanding of VMS in central NL has been greatly accelerated by my research team’s immense and high quality contributions. Research group members who have undertaken work on VMS in central NL, include (in alphabetical order):
- Stefanie Bruckner, PhD – Ming ore mineralogy and genesis of sulfide mineralization.
- Mike Buschette, MSc – alteration/lithogeochemistry of the Boundary VMS deposit.
- Jonathan Cloutier, PDF – S isotope systematics in opholite-hosted VMS in Lushs Bight Group and the reconstruction, hyperspectral, and alteration attributes of the Lemarchant VMS deposit.
- Michelle English, MSc – Hydrothermal alteration of the Hurricaine zone of the Boomerang VMS deposit.
- Alicia Escribano, PhD – Victoria Lake Supergroup magmatism and relationships to VMS deposits (ongoing).
- Shannon Gill, MSc – Ore mineralogy, mineral chemistry, and S-Pb isotope systematics of mineralization from the Lemarchant VMS deposit.
- Regan Jacobson, MSc – Geology and lithogeochemistry of the Powderhorn Lake VMS deposit, Newfoundland (ongoing).
- Kyra Kennedy, MSc – Ore mineralogy and causes of critical metal enrichment in the Tilt Cove and Betts Cove VMS deposits (ongoing).
- Marie-Eve Lajoie, MSc – Genesis of barite and relationship to mineralization in the Lemarchant VMS deposit.
- Stefanie Lode, PhD – Role of metalliferous mudstones in the localization and genesis of VMS mineralization in the Tally Pond belt.
- Conor McKinley, MSc – Volcanic and hydrothermal reconstruction of the Pilleys Island VMS district.
- Taylor Mugford, BSc(Hons) – Ore mineralogy and mineral chemistry of the Boundary VMS deposit.
- Jean-Luc Pilote, PhD – Geology, structure, petrogenesis, and alteration of the Ming VMS deposit.
- Helena Toman, MSc – Geology, ore mineralogy, and S isotope systematics of the Little Deer VMS deposit.
In addition, much of the work here builds on an outstanding historical database of material generated by numerous geologists in government and industry, and whose contributions deserve recognition, including: Terry Brace, Peter Dimmell, Greg Dunning, David Evans, George Jenner, Baxter Kean, Paul Moore, Brian O’Brien, Larry Pilgrim, Frank Santaguida, Tom Skulski, Kerry Sparkes, Gerry Squires, Peter Tallman, Scott Swinden, Geoff Thurlow, and Lawrence Winter. They have also provided their time and insights to me and I am grateful for those discussions and chances to learn from them.
This paper was a lot of fun to write and I hope that it provides some of you some enjoyment to read.
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