Determining the optimal soil sample grid spacing to explore for epithermal gold deposits in a tropical wet and dry environment: an example from the La India mining district, Nicaragua

Soil sampling is a useful tool for the assessment of large areas of land where low sulphidation epithermal vein mineralisation is known or suspected. It provides a quantitative means of mapping out the geometry of an epithermal system and identifying potentially economic gold concentrations. In this paper gold in soil geochemistry data from the La India gold mining district in Nicaragua is used to model the soil gold signature at different sample grid spacing over known gold resources. The most cost-effective or optimal soil sample grid spacing to explore for epithermal gold deposits in this tropical wet and dry environment is identified. La India gold mining district covers a 50 km² area of fault-fill gold-silver mineralized quartz-adularia veins in western Nicaragua. The distribution of gold veins is well understood with current estimates defining a mineral endowment of over 2.3 Moz gold. The exploration data includes full coverage of the district by a 200 m by 50 m soil survey of fine-fraction B-horizon soil with ultra-trace multi-element analysis. Soil close to the gold veins is enriched in gold at trace level. Gold resources and prospects can be effectively ranked using the soil data with the larger resources correlating to broader, higher-grade soil gold anomalies. The soil gold content can be used to map-out the geometry and distribution of the epithermal vein system. In this study the soil survey data is filtered to create simulated 200 m by 200 m and 400 m by 400 m soil grids. The simulated grids are assessed against the mapped veins, surface rock chip and trench results, and known mineral resources. It is concluded that a 200 m by 200 m soil grid is adequate to discover economic vein gold deposits in the soil type found in the tropical wet and dry zone of western Nicaragua. This grid spacing provides enough resolution to map out vein trends and the overall geometry of an epithermal vein system that is exposed at surface and provides sufficient resolution to rank the gold anomalies by size and grade with confidence that the ranking will broadly correlate with associated gold occurrences. A 400 m by 400 m spaced soil sampling does appear to detect the larger deposits but will miss smaller or partially hidden deeper-seated deposits, and the spacing is too wide to provide any useful information on epithermal vein orientation or network geometry.