Mapping and Exploration Applications of Gamma Ray Spectrometry in the Bathurst Mining Camp, Northeastern New Brunswick
Robert B. K. Shives, Ken L. Ford, Jan M. Peter, 2003. "Mapping and Exploration Applications of Gamma Ray Spectrometry in the Bathurst Mining Camp, Northeastern New Brunswick", 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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In 1995, a helicopter-borne geophysical survey combining magnetic, electromagnetic, and gamma ray spectrometric sensors was flown over the entire Bathurst Mining Camp, northeastern New Brunswick. In 1997, field studies under the EXTECH II program were conducted in selected areas throughout the Bathurst Mining Camp to evaluate the gamma ray spectrometric component of that survey.
Herein, we describe results from 776 in situ K, equivalent uranium (eU) and equivalent thorium (eTh) abundance spectrometric readings and relate these to the airborne data and the most recent geological and geochemical knowledge of the Bathurst Mining Camp. Although there is considerable overlap in regional radioactive element signatures, distinctive trends are apparent within and between the felsic volcanic and volcaniclastic rocks of the various formations within the three geological groups surveyed (Tetagouche, California Lake, and Sheephouse Brook). These trends are largely due to different responses reflecting primary litho-geochemical variations and subsequent seawater hydrothermal alteration and/or greenschist metamorphic overprints. These trends are well supported by lithogeochemical data and were used to revise the bedrock geologic maps in several areas. Alkali element mobility attendant with hydrothermal alteration and metamorphism complicates interpretation of the potassium data, but good correlations between immobile, high field strength elements and thorium abundances illustrate the relevance and application of the gamma ray spectrometric eTh data in chemostratigraphic interpretation. Where chloritic hydrothermal alteration associated with sulfide mineralization (e.g., Brunswick 6, Halfmile Lake, Heath Steele, Restigouche, West Branch Forty-four Mile Brook, Devils Elbow, and Chester deposits) has mobilized certain elements, potassium depletion or enrichment can be readily quantified using in situ spectrometry.
The airborne magnetic and electromagnetic patterns provide direct exploration vectors to volcanic-hosted massive sulfide mineralization, but the gamma ray spectrometric patterns generally do not. This is because the technique does not detect the massive sulfides directly but rather the effects of alteration associated with some of the deposits. Further, K commonly behaves conservatively under the mild to moderately intense hydrothermal alteration effects typically observed at most of the deposits. The presence of overburden and host-rock cover in areas of nonoutcropping mineralization also reduces the effectiveness of gamma ray spectrometry in exploration. However, the presence of hydrothermally altered rocks present in outcrop or overburden at a few deposits coincides with subtle variation in the local airborne patterns, and these may be useful in the exploration for concealed mineralization at the deposit or property scales.
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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.