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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

Naturalis Scientias, 2025

Soil sampling is a useful tool for the assessment of large areas of land where low sulphidation e... more 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.

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Geological setting of gold-silver mineralisation in the La India mining district, Nicaragua

Naturalis Scientias, 2024

La India gold mining district covers a 50 km² area of fault-fill gold-silver mineralized quartz-a... more La India gold mining district covers a 50 km² area of fault-fill gold-silver mineralized quartz-adularia veins on the western margin of a Tertiary volcanic arc in western Nicaragua. Historic mining records and modern mineral exploration data, which to-date has defined a mineral endowment of over 2.3 Moz gold, provide a wealth of geological information. This paper draws on these observations and data to describe and classify the gold-silver mineralisation at La India, identify the geological controls, and interpret the timing of mineralisation within the regional tectonic setting. The district-scale gold-silver mineralisation at La India occurs in two adjacent geological settings with distinct mineralization characteristics and exploration potential: (1) an upland area of strongly faulted felsic to andesitic volcanics where the historic mine workings are located, and (2) an adjacent downthrown graben, the Sebaco Graben, where a thick sequence of andesite is preserved overlying the felsic volcanic sequence. Gold mineralisation is classified as rift margin-type low-sulphidation epithermal gold-silver fault and fracture-fill vein mineralization. In the historic mining area erosion has exposed the top of the high-grade epithermal zone. Minimal erosion in the Sebaco Graben means that the epithermal system is fully preserved at depth, with localised hydrothermal sinter outcrops, sporadic low-grade mineralised veins and a phreatic breccia pipe exposed at surface. Apart from the one phreatic breccia, the gold-silver mineralisation occurs in quartz veins and breccias that filled brittle faults and associated fractures and fissures which developed in an extensional tectonic setting. The structures containing the gold-silver mineralisation were formed as normal and trans-tensional faults with orientations consistent with southwest-directed extension: (1) a predominant northwest to north-northwest set parallel to the subducting plate; (2) secondary but locally extensive east-west, and (3) tertiary shorter and narrower northeast and north-striking veins. A district-scale north-northwest orientated through-going structure linking the major gold-silver deposits in the historic mining area is interpreted as a deep crustal conduit for the gold-silver bearing hydrothermal fluids. Other, as yet unidentified basement feeder structures may have fed mineralised corridors in the east and west of the district. The gold-silver mineralisation is best developed where structures pass through competent felsic volcanics and welded tuffs in the historic mine area, and also in the overlying andesite flows in the Sebaco Graben. Gold-silver mineralisation is less well developed where the structure passes through less competent unwelded tuffs and volcanic agglomerates. Gold-silver mineralisation is interpreted as occurring shortly before or at 8-10 Ma at the end of a long period of slab-rollback induced extension and arc volcanism. Post-mineralisation block faulting split the La India district into the upthrown, and subsequently eroded historic mine area where epithermal mineralisation is largely exposed at surface, and the well-preserved downthrown blocks such as the Sebaco graben where much of the gold-silver mineralisation is still hidden several hundred metres below surface.

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The ~1864 Ma Stubbins Formation and the prospectivity of orogenic gold in the early Tanami basin, Western Australia

The Tanami deep seismic reflection experiment: An insight into gold mineralization and Paleoproterozoic collision in the North Australian Craton

Tectonophysics, 2009

Imaging of a major collision zone between the Tanami region and Aileron Province of the Arunta Or... more

An example of a Palaeoproterozoic back-arc basin: Petrology and geochemistry of the ca. 1864Ma Stubbins Formation as an aid towards an improved understanding of the Granites–Tanami Orogen, Western Australia

Precambrian Research, 2008

... (1996) contains significant orogenic and intrusion-related ore systems. Examples in the weste... more

The use of magnetic susceptibility and trace element geochemistry for the correlation of fine-grained siliciclastic sequences: a Late Llandovery example from northwest England

Geological Magazine, 1999

High-resolution lithological and magnetic susceptibility logs were made of two sections of the Up... more High-resolution lithological and magnetic susceptibility logs were made of two sections of the Upper Llandovery Browgill Formation in northwest England: Stockdale Beck, the type section of the Browgill Formation, and Spengill. The Browgill Formation is composed of fine-grained deep marine siliciclastics, which can be divided broadly into two facies: a homogeneous grey mudstone, deposited under oxygenated bottom-water conditions, and subordinate beds of laminated, graptolite-bearing black mudstone deposited under low bottom-water oxygen levels. The latter facies is often partially or fully diagenetically altered to chlorite nodules, occasionally with manganese carbonate nucleii. Magnetic susceptibility logs are shown to reflect variations in the illite–chlorite ratio of the clay mineralogy. Chlorite is a paramagnetic mineral, so the bands of chlorite nodules produce magnetic susceptibility highs. Correlation demonstrates that diagenetically altered beds of laminated black mudstone ar...

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