Laterite-Hosted Gold
In humid tropical climates, and with soil preservation, gold dispersion above and around gold-bearing rocks can result in the formation of gold nuggets in the overlying soils.
Artisanal gold miners target supergene gold enrichment in the soils above gold-bearing rocks.
Commonly Asked Questions
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The best soil sample is B Soil, which is the red laterite soil layer beneath the top soil.
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Although 28 cm on average can reach the B laterite soil, it is better to follow the soil layers instead of consistent depth.
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In the tropics, artisanal gold miners often target supergene gold nuggets that are found in the laterite soil.
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Because Mynah Exploration is highly experienced in all aspects of gold exploration, from grassroots to brownfields, with special expertise in tropical laterite regolith and soil geochemistry.
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A technical due diligence of your gold project involves: A deep dive into the project exploration data, artisanal mining history, geological interpretations and field site visit.
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Deliverables include: A powerpoint outlining the assessment of the gold project, including data interpretations, gold potential, ranked drill targets, geological model and exploration recommendations.
Key Facts About Tropical Laterite Supergene Gold
Are gold nuggets really formed in laterite soils?
Yes! Warm and humid climates, over hundreds of millions of years, have deeply weathered the rocks in the tropics.
Gold vein systems exposed to tropical weathering develop a mushroom-or umbrella-shaped supergene gold enrichment blanket within the overlying laterite and saprolite soils..
Oxidation of sulphides in the gold system, like pyrite, release acid to the groundwater, resulting in deep regolith formation and facilitating gold particle mobility.
Over geologic time, the gold particles adhere together into high-purity gold nuggets.
Through gravity the nuggets concentrate along the base of the laterite, above the water table.
Gold nuggets are the results of supergene gold formation in tropical climates.
Were Yukon gold nuggets also formed in laterite soils?
Yes! The gold nuggets from the 1896 Yukon Klondike Gold Rush, found in placer deposits within rivers and creekbeds, originated from ancient tropical laterite soil.
Long before the Yukon moved to its present northern latitude, the region was situated near the equator. Prolonged, intense tropical weathering broke down the host bedrock into thick, iron-rich laterite soils.
The surface exposures of gold vein systems developed gold enrichment caps with gold particles ranging from tiny granules to large nuggets.
The Yukon-Tanana terrane slowly rode the Pacific and Kula tectonic plates on a long, northward journey away from the tropical latitudes.
As this terrane eventually collided with the North American plate, mountain-building uplift occurred. In the unglaciated pockets of the Klondike, millions of years of sub-arctic weathering and stream erosion broke down the ancient laterite layers. Because of this intense mechanical grinding, much of the gold was reduced to fine gold dust.
Consequently, massive nuggets are rare, though legendary specimens weighing up to several pounds and more have been recovered.
Are Australian gold nuggets also the result of supergene tropical laterite weathering?
Yes! Australia’s incredibly large gold nuggets are the ultimate global masterpiece of tropical laterite weathering, supergene enrichment, and regolith preservation.
Tropical laterite processes in Australia are much more intense and deeper than elsewhere in the world.
Due to immense tectonic plate stability over geologic time, uninterrupted, very deep weathering occurred on flat landscapes.
As Australia drifted north, the climate shifted from wet tropical to arid. This dried the laterite into a hardened iron duricrust, while turning the groundwater highly saline and acidic. The results was exceptionally thick, leached soil horizons that concentrated pure gold nuggets close to the surface..
Key Facts About Laterite, Duricrust and Pisoliths
Is it laterite, ferricrete or duricrust?
Laterite: This is the soft, red, clay-rich residual soil (soft enough to push a thumb through).that can be massive or gravelly in texture. Typically, the gravel layer accumulates at the base of the laterite.
Duricrust: This is the hardened, sun-baked, red-brown, iron-rich, rock-hard layer formed from laterite after intense dehydration (climate change from humid tropical to dry savannah). It marks ancient, unglaciated plateau tops.
Ferricrete (very high iron), silicrete (high silica), bauxite (high alumina) are all types of duricrust.
What are nodules and pisoliths?
Nodules and Pisoliths: These round, pea-and gravel-sized iron-rich accumulations are the physical evidence of a changing climate.
When conditions shift from dry back to wetter, the tough duricrust naturally dismantles and breaks down, creating these distinct hydrated grains.
Nodules are agglomerates of pisoliths. If the yellow cutain (goethite)on nodules is still intact, the nodule has not been transported far and could be sampled for lack of soil sample.
Can you sample duricrust for geochemistry programs?
Ideally, no. Duricrust is leached and can contain transported material. It is also tough to break.
Duricrust tends to cover plateau tops, remnants of an ancient landscape.
Laterite, the B soil sample, is the ideal soil sample.
Where duricrust is re-incorporated into laterite (climate changed back to humid tropical), the laterite can be sampled.
Supergene Gold Mushroom Dispersal
In tropical environments, intense chemical weathering of a gold system breaks down primary sulfides, allowing gold to dissolve and reprecipitate to form nuggets in the overlying laterite soil..
The Opportunity: Chemical enrichment can turn a low-grade 1 g/t Au primary deposit into a 3 g/t Au oxide blanket near surface.
The Risk: Without a geological model, "nugget effect" in the laterite and/or saprolite can result in misleading resource estimates and failed drill programs.
Artisanal and small-scale gold miners (ASGM) also target the high purity gold nuggets that are in the relatively soft laterite and saprolite soils of the tropical regolith. The nuggets can be reached with shovel, and laterite and saprolite is washed with water across sluices to recover the supergene gold.
Exploration Targeting in Savannah Terrain
In tropical savannah climate the water table sits much lower than in tropical rainforest climates. As such, supergene gold development occurs deeper within the regolith, often within the upper saprolite.
The gold-enriched lower laterite and upper saprolite are targeted by the miners through deep vertical pits. Some pits are up to 30 meters deep and have underground side chambers.
