Abstract

The Lamaque and Sigma ore zones are composed of a multiplicity of gold-bearing quartz veins extending vertically to depths greater than 4,000 feet. The mines are about 2,500 feet apart horizontally and they are separated by a steeply dipping fault zone called the Highway Shear,the movement on which was hitherto unknown.The Au/Ag values from annual mint returns taken over the past 30 years from both the Lamaque and Sigma ore zones represent very large samples. These values when plotted against the annual mean depth of mining are found to increase with depth in both mines at about the same rate with Lamaque having the higher values. If it is assumed that both ore bodies formed under similar conditions, the higher Au/Ag values found at Lamaque suggest that this ore body has been raised approximately 1,300 feet with respect to Sigma by post ore movement on the Highway Shear. The direction of movement is consistent with geological evidence.Slope samples taken over a vertical range of 2,285 feet at Lamaque were analyzed for Au and Ag by two independent methods. The Au/Ag values determined by conventional analysis (i.e., Au by fire assay; Ag by atomic absorption) are quite variable but are in the same range as the ratios from mint returns. There is no evidence that intra-stope variations are caused by such geological features as the composition of the wallrock or the thickness or grade of the vein.The Au/Ag values obtained by electron probe microanalyses of gold grains taken from the stope samples were approximately three times higher (18:1) than conventional analyses (5.5:1). There appears to be no change in gold fineness with depth. These results indicate that both the intra-stope variations and the increase in the Au/Ag values with depth are caused by the presence of Ag-bearing minerals (or the absence of Au-bearing minerals) other than the native alloy. Silver was found to occur in petzite (25% Au, 42% Ag) and tetrahedrite (12% Ag), but these minerals are rare in the mines and could not adequately account for all the missing silver. Pyrite is the most common sulfide, making up about 2.5 percent of the ore; it contains about 2.3 oz of silver per ton of pyrite, which is sufficient to account for the difference in silver content obtained from native gold and silver recovered either from individual stope samples or from the mill as reported in mint returns.These results clearly indicate that Au/Ag values from mint returns may have some practical significance around a mine but that conclusions on ore genesis based on Au/Ag values are open to question.

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