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Abstract

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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