Modelling spatial anisotropy of gold concentration data using GIS-based interpolated maps and variogram analysis: Implications for structural control of mineralization

Abstract

Linear trends of anomalously high gold values in the Florida Canyon gold deposit, Nevada have been identified using a combination of contour maps of gold (Au) concentration developed with a geographic information system (GIS) and variogram maps created using a geostatistical analysis package. These linear trends are interpreted to represent major fault zones that exerted a prinicipal control on gold mineralization and therefore imparted a spatial anisotropy to gold concentrations. Oxidation state information such as oxide, sulfide or mixed was used initially to map and contour the lower limit of the oxidation zone. Linear trends on this surface suggest the location and trend of major structural elements in the deposit that guided late oxidizing fluids. Subsequently, four contour maps of gold concentrations in oxidized rocks were produced, each map representing 500 ft vertical intervals starting at 3500 ft above mean sea level (msl). Relatively high concentrations of Au that form linear trends on these maps suggest the presence of structural features, such as shear zones that controlled mineralization. Finally, to validate the observed trends, variogram maps of gold concentrations were derived through geostatistical analysis and the major axes of anisotropy were determined for each map. The results that emerge suggest linear trends of northeast, northwest and, less prominently, north–south orientations. The north–south and northeast trends match those of known and mapped major structures associated with the Florida Canyon deposit. However, the results imply a stronger control on mineralization by northwest-trending structures than previously recognized and the location of possible structures of all trends not previously mapped. They also serve to identify faults that controlled both early hydrothermal fluids and late oxidizing fluids since the gold distribution represents the time integrated effects of both fluid events. The linear trends derived by spatial analysis (contour maps, variogram maps) of geochemical data (i.e., gold concentration), combined with the results of the field observations prove to be advantageous in understanding the structural control of gold mineralization. Such spatial analyses of geochemical concentration data are particularly useful in the field of mineral exploration.