Geochemical prospecting investigations in the Copper Belt of Vermont

Abstract

Geochemical prospecting studies were made in 1954 and 1955 in the Copper Belt of Orange County, Vt. This district, which lies in the maturely dissected uplands of west-central New England, is underlain by highly deformed and metamorphosed rocks of early Paleozoic age. The ore in the various mines is principally pyrrhotite with subordinate amounts of chalcopyrite and sphalerite. The bedrock is covered nearly everywhere with a mantle of glacial deposits, which range from a thin veneer of basal till on the uplands to much thicker deposits of till and outwash in the valley bottoms. Podzolic soils are developed on this till. The field studies were designed principally to (a) determine whether anomalous chemical patterns of copper and zinc exist in the glacial soils in the vicinity of concealed pyrrhotitic copper deposits, (b) measure the physical and chemical characteristics of the dispersion patterns, and (c) establish optimum field and laboratory procedures for the detection of these patterns. Most of these field studies were made in the area of the Elizabeth mine, which lies in the extreme southern part of the Copper Belt. Anomalous patterns of copper are present in the soils over and adjacent to suboutcropping pyrrhotitic copper deposits concealed by a thin mantle of basal till. The total copper content of small soil samples collected in the top of the B soil horizon can be used as a guide in the search for anomalous areas. Copper values of 50 ppm (parts per million) and more are considered to be anomalous; background values range from 10 to 40 ppm. Anomalous values as high as 1,200 ppm were recorded from the B zone of soils located directly over suboutcropping ore containing about 1.1 percent copper. The highly skewed asymmetrical copper anomaly at the Elizabeth mine was formed by means of mechanical dispersion by glacial ice of a preglacial copperrich residual soil. Later, some copper was redistributed by ground water; thus, the original syngenetic pattern was modified and lateral seepage anomalies were produced on the west side of the ore zone. The presence of ore minerals and gossan in the till verifies the syngenetic origin of the pattern. At the Elizabeth mine, where the probable direction of ice movement makes a small angle with the strike of the ore zone, the anomalous patterns are detectable as much as 1,000 feet away from the zone. In general terms, the lateral extent, shape, and contrast of an anomaly associated with any given ore body are dependent on the tenor of the ore deposit; the geometrical rela-Bl B2 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS tionship between the shape of the deposit and the direction of glacial transport ; the thickness and type of the glacial mantle together with the intensity of glacial erosion; and the extent to which the original pattern has been modified by groundwater movement. Anomalies of cold-acid-extractable copper, according to limited data, have greater contrast than corresponding total-copper anomalies because most of the anomalous copper is in relativel...