Analysis of supergene ore-forming processes and ground-water solute transport using mass balance principles

Abstract

Analysis of metal distribution in weathering profiles developed above ore deposits provides a simple and direct method to determine the geologic controls on surficial leaching and enrichment of metals by supergene (descending) fluids. This approach offers an understanding of the interrelationships of metal concentration, zone thicknesses, and bulk-rock density of the various zones within a weathered-rock column. Analytical solutions to mass balance equations provide expressions describing the chemical evolution paths of enrichment blanket metal grade, b, in terms of original protore metal grade, p, leached-zone metal grade, 1, and the ratio of total leached-zone column height to blanket thickness, L O T /B. Neglecting density terms for the sake of simplicity, this function is expressed as:b = p + L O T /B (p - 1).Linear data arrays in the coordinate system b versus (p - 1) for groups of drill holes in individual geologic domains, e.g., wall-rock types, strongly suggest that enrichment blanket metal grades evolve from a protore metal grade, p, with a fairly constant slope of L O T /B to a final value, b, determined by the extent of metal leaching in the zone of oxidation. The constancy of the ratio L O T /B within geologic domains indicates a simultaneous deepening of the zone of oxidation and thickening of the enrichment blanket under conditions of a descending ground-water table.Analysis of metal mass balance in drilling profiles also provides a means of reconstructing paleosurficial topography at the time of active oxidation even in highly eroded terrains. Application of these methods using rock density and assay data from the Butte district of Montana shows the paleo-oxidation surface to have been eroded by as much as 450 ft down to the present unconformity between weathered bedrock and overlying Tertiary gravels. Geomorphic stability and slow erosion rates often thought necessary for effective chemical weathering are therefore questioned. We propose instead a general balance of rates of erosion and descent of the ground-water table for optimal enrichment. Preservation of the fossil weathering profiles occurs by submergence of the zone of oxidation below the paleoground-water table resulting in cessation of supergene oxidation and preservation of the blanket. The mass balance methods derived in this study also relate overall leaching efficiency to protore characteristics, showing a strong hypogene alteration control at Butte as described by McClave (1973). Lateral copper transport is evaluated by identifying local anomalies in the computed oxidation surface. Zones of copper introduction near faults and zones of high permeability appear as apparent topographic highs whereas zones of depletion appear as topographic depressions.Preliminary analysis of this type at the porphyry copper deposit at La Escondida, Chile, indicates deeper oxidation and leaching effects than at Butte. The assumption of vertically homogeneous protore copper grades leads to the conclusion that erosion of at least 200 m of leached capping has occurred throughout much of the La Escondida district. Geologic domains defined by computed values of L O T /B correlate well with hypogene alteration zoning. Computation of the magnitude and direction of overall subsurface lateral metal fluxes indicates transport distances of at least 1 km in directions consistent with present surface topography and fracture patterns. There is a strong correlation of the thickest and highest grade enrichment zones with inferred positive lateral fluxes, whereas the most thoroughly leached cap rocks make up zones of inferred negative lateral flux which are likely source regions. Alternative explanations to the lateral flux hypothesis are evaluated, including assessment of variations in protore metal-grade projections, both in vertical and horizontal directions. Despite the approximations involved in the present calculations, the lateral flux pattern appears to be the most likely interpretation of the present metal distribution in the supergene system at La Escondida.