Geologic and Structural Setting of the Bald Mountain Volcanogenic Massive Sulfide Deposit, Northern Maine: Cu-Zn-Au-Ag Mineralization in a Synvolcanic Sea-Floor Graben
Published:January 01, 2003
Michael P. Foose, John F. Slack, Cathy J. Busby, Klaus J. Schulz, Michael V. Scully, 2003. "Geologic and Structural Setting of the Bald Mountain Volcanogenic Massive Sulfide Deposit, Northern Maine: Cu-Zn-Au-Ag Mineralization in a Synvolcanic Sea-Floor Graben", Massive Sulfide Deposits of the Bathurst Mining Camp, New Brunswick, and Northern Maine, Wayne D. Goodfellow, Steven R. McCutcheon, Jan M. Peter
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The Bald Mountain volcanogenic massive sulfide (VMS) deposit of Early Ordovician age in northern Maine contains 30 million metric tons (Mt) of Cu-Zn-Au-Ag sulfides. It is exceptionally well preserved, lacking penetrative deformation, and having experienced only prehnite-pumpellyite–grade regional metamorphism. The deposit occurs within a homoclinal west-dipping volcanic sequence that consists of, from bottom to top, basalt and basaltic andesite, crystal-poor rhyolite ignimbrite, massive sulfide and related units, crystal-rich rhyolite ignimbrite and intercalated andesite, carbonaceous argillite, and rhyolitic volcaniclastic rocks. Basalts stratigraphically below the massive sulfide are intruded by an elongate body of tonalite-plagiogranite; gabbros intrude rocks both above and below the massive sulfides. The basal contact of the host volcanic sequence is believed to be a thrust with underlying Middle Ordovician clastic sedimentary rocks; the upper contact is depositional with the Middle to Upper Ordovician Winterville Formation and, in places, with Silurian conglomerates.
Ordovician synvolcanic faults that predominantly strike 025°, 050° to 060°, 325° to 335°, and 350° formed a small (320 × 275 m) synvolcanic graben in which as much as 215 m of massive sulfide accumulated. Hydrothermal solutions utilized these faults as fluid conduits, causing structurally controlled epidote and silica alteration in the deep footwall. Structurally controlled alteration is also indicated by the presence of magnetic low areas in mafic rocks up to 1 km below the deposit. Movement of zinc- and copper-rich fluids was controlled by the location of the Ordovician faults. Zinc-rich fluids were concentrated along faults that bound the northern, western, and southern sides of the small graben; copper-rich fluids moved along faults that define the eastern side of the graben. Rocks overlying the massive sulfide body show little evidence of the growth faulting that occurred within and below the deposit, indicating that most extensional deformation ceased shortly after exhalative sulfide deposition. Synvolcanic Ordovician faulting and graben formation are the principal causes for the small lateral dimensions of the Bald Mountain deposit relative to those of most VMS deposits of comparable tonnage.
Postsulfide deformational events occurred in the Late Ordovician to Early Silurian when rocks hosting the Bald Mountain deposit were thrust over Ordovician clastic sedimentary rocks and in the Early Devonian when Acadian faulting and folding segmented the deposit and tilted it to the west.
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Massive Sulfide Deposits of the Bathurst Mining Camp, New Brunswick, and Northern Maine
The Mining and mineral processing industry is important to the Canadian economy and in 2001 contributed $35.1 billion, or 3.7 percent, to the Gross Domestic Product and employed approximately 376,000 Canadians (Minerals and Metals Sector, Natural Resources Canada). However, over the past decade, Canada’s base metal reserves have declined by more than 25 percent, and significant new discoveries will be required if Canada’s role as a major base metal producer is to be maintained into the twenty-first century. The Bathurst Mining Camp is one of Canada’s most important base metal mining districts, accounting in 2001 for 30 percent of Canada’s production of Zn, 53 percent of Pb, and 17 percent of Ag. In 1999, the Bathurst Mining Camp accounted for 32 percent of the Zn, 80 percent of the Pb, and 25 percent of the Ag reserves (Minerals and Metals Sector, Natural Resources Canada). The value of production from the Bathurst Mining Camp in 2001 exceeded $500 million and accounted for 70 percent of total mineral production in New Brunswick. Approximately 2,000 people are directly employed by the mining industry in the Bathurst Mining Camp. Without the discovery of new ore reserves, however, production will decline and will cease within about 10 yr at current production rates, and with it the principal source of economic activity in northeastern New Brunswick will also disappear.
To address the major decline of mineral resources in Canada’s economically important mining districts, EXTECH (Exploration and Technology) projects were established by the Geological Survey of Canada. EXTECH-II is a multidisciplinary, integrated and collaborative project that has focused on the Bathurst Mining Camp with four principal objectives: (1) update and expand the geoscience knowledge base, (2) develop and test new and improved methods of exploring for massive sulfide deposits, (3) conduct ground and airborne, geophysical and geochemical surveys to identify new exploration targets, and (4) build a multiparameter, comprehensive, coregistered, and internally consistent digital geoscience database of the entire Camp. Although EXTECH-II was initiated by the Geological Survey of Canada in 1994, it was a collaborative project involving earth scientists from the Geological Survey of Canada, the Department of Natural Resources and Energy of New Brunswick, universities, and mining and exploration companies.
A similar multidisciplinary project was established at about the same time by the U.S. Geological Survey to study the well-preserved Bald Mountain Cu-Zn-Ag-Au massive sulfide deposit in northern Maine. This project, which began in 1995 and ended in 1999, also included selected research on the Mount Chase Zn-Pb-Cu-Ag-Au deposit 70 km to the south of Bald Mountain.