Dispersion of soil gas around micron gold deposits

Experimental weathering of sediment-hosted micron gold (SHMG) ore from the Alligator Ridge deposit indicates that ore and adjacent Pilot Shale wallrock release gas with larger concentrations of COS, CS2 and CH4, and lower CH4/(CH4 + C2H4) and CH4 / (CH4 + C2H 6) ratios than does barren Pilot Shale. The COS and CS2 are derived from decomposition ofdisseminated pyrite, which is prob-ably more abundant in mineralized zones, and variations in the CH-gas ratios are thought o reflect increased thermal maturity of organic material in the mineralized zones. Surveys designed to deter-mine whether these gases are present in detectable haloes around SHMG mineralization were carried out over the Yankee prospect, a relatively similar deposit several km south of Alligator Ridge. The surveys included analysis of interstitial soil gas, gases desorbed from soils, and gases desorbed from two sets of Carbotrap collectors, one left on the bedrock interface (at depths of about 30-70 cm below the surface) and another placed in holes drilled about 3 m into bedrock. All of the gases observed in the experiments were detected, as well as organo-sulfur and higher alkane and alkene gases. The sur-veys indicated systematic variations in gas dispersal related to faults and/or mineralization, with the most consistent patterns being elevated COS values and depressed CH4/(CH4 + C2H4) ratios over mineralization. Similar highs in COS, as well as other gases including SO2 and ethyl mercaptan, wer