Mining in French Guiana
Gold Exploration Update

Mineralization in the Paramaca Series is orogenic, quartz-carbonate veins along D2 and D3 shear corridors in mafic volcanics and volcaniclastic sediments, arsenopyrite-pyrite sulfide assemblages, sericite-chlorite-carbonate wallrock alteration. Core from the Ashanti Belt or the Houndé Belt looks the same.

The Inventaire Minier that BRGM ran from the late 1950s into the early 1990s covered large parts of the territory with 1:50,000 geological mapping, stream sediment geochemistry at about one sample per two square kilometers, soil traverses on priority anomalies, ground magnetics, limited drilling. Analytical methods were AAS on aqua regia digests, gold detection limits 5-10 ppb. Several permit areas currently held by juniors trace their anomaly pedigree to these surveys.

Most of this data has never been digitized. It sits in paper form in Orléans, field notebooks and draft maps and internal reports written by geologists who were simultaneously running Birimian programs in Côte d'Ivoire and Guinea. Whether anyone has re-assayed archived BRGM pulps with modern fire assay and ICP-MS multi-element suites cannot be determined from public filings. Surviving 1970s drill core could be re-logged against current alteration models for almost nothing compared to what a new drill mobilization costs.

Rosebel in Suriname has produced over 5 million ounces from the Marowijne Greenstone Belt, total camp endowment north of 12 million ounces. The Marowijne Belt is the Paramaca Series under a different name, continuous across the border. Soil anomalies and garimpeiro workings on the French Guiana side confirm gold along the same structural corridor. Drill density on the French Guiana side: negligible.

Aurora in Guyana carried underground reserves above 5 g/t in dilational shear zones within shield greenstone lithologies. French Guiana's shear corridors have not been tested at equivalent depth.

The regolith in French Guiana is thick. Charles Butt at CSIRO, who spent decades working on gold dispersion in lateritic regolith and published the foundational texts on the subject through the 1990s, would have recognized the Paramaca regolith profile immediately: ferruginous duricrust at surface, mottled zone beneath, pallid kaolinized saprolite preserving ghost textures of the parent rock, saprock, fresh bedrock. The profile develops to 50 meters in typical upland terrain. Over paleovalleys it can exceed 80. On ridges and along erosional scarps it thins to 15 or less. These thickness variations are not random. They map onto paleotopography, bedrock lithology, and fracture permeability in ways that a digital elevation model derived from TanDEM-X 12m data can partially resolve before anyone sets foot on the ground.

In the Yilgarn, Butt documented lateral gold dispersion distances of 50 to 200 meters from bedrock sources in mature laterite terrain. French Guiana's regolith is geochemically similar, the same ferruginous laterite over felsic-to-mafic saprolite with a seasonally fluctuating water table, but annual rainfall is four to five times higher than in the Yilgarn, and the organic acid load in groundwater is correspondingly heavier because of the equatorial forest biomass. The expectation is that hydromorphic gold dispersion distances in French Guiana are at least as large as in Australia, and possibly larger. Nobody has published a systematic study of gold dispersion halos in French Guiana laterite calibrated against known bedrock sources, which is a significant gap in the applied research.

What this means in practice is that a B-horizon soil sample at 30-50 cm depth returning anomalous gold does not tell you where to drill. It tells you that gold is present in the soil at that location. Whether the gold arrived there by residual concentration directly above a bedrock source, by lateral hydromorphic transport from a bedrock source offset by 100 or 200 meters, or by colluvial gravity transport from upslope, cannot be determined from the gold assay alone. All three mechanisms produce the same signature on a plan-view soil geochemistry contour map: a gold anomaly.

Exploration programs that collect soils on a grid, contour the gold values, and drill beneath the highest values treat soil geochemistry as if it were a direct projection of bedrock mineralization. In residual regolith terrain, this works. In transported or hydromorphically modified terrain, it fails. The hit rate on bedrock targets from programs in French Guiana that skipped regolith characterization is, based on drill outcomes from multiple companies over the past fifteen years, low. Anomalies are drilled. The holes find weakly anomalous saprolite and disappointing bedrock intercepts. The anomaly was hydromorphic, or colluvial, and the bedrock source is somewhere else.

The tools to avoid this are known and not expensive relative to drilling. Multi-element pathfinder geochemistry is one. Arsenic and antimony have lower aqueous mobility than gold in the pH-Eh conditions typical of laterite groundwater. A soil anomaly where Au, As, and Sb all peak at the same location is more probably residual than an anomaly where gold peaks alone, displaced from its pathfinder metals. This is Butt's framework. It requires no special technology, just requesting a broader element suite from the assay lab and interpreting the multi-element data rather than plotting gold in isolation.

Regolith landform mapping is the second tool. Mature undissected laterite plateaux preserve residual geochemical signatures and can be treated as windows into bedrock composition. Dissected slopes carry colluvial material. Alluvial flats carry transported sediment. Erosional scarps mark the boundaries between residual and transported domains. Mapping these domains from DEM data and aerial photography before interpreting soil geochemistry allows the geochemist to ask the right question of each sample: is this sample from a residual or transported landscape position? The answer changes what the gold value means.

Gold grain morphology from panned concentrates provides a third discriminator. Rough angular grains with crystal faces indicate a nearby primary source. Rounded polished grains indicate fluvial or colluvial transport. Grains coated in mercury amalgam indicate garimpeiro contamination, which needs separate discussion.

Then there is the nugget effect. Coarse supergene gold particles, sometimes visible, concentrated in ferruginous nodules or along the base of the duricrust, produce order-of-magnitude variance between replicate 50-gram fire assay charges from the same sample interval. Screen fire assay helps. BLEG on 2-5 kg charges smooths the reconnaissance data. Reliable grade estimation for the purpose of evaluating laterite-hosted oxide gold as a standalone heap leach target requires bulk samples of 50-100 kg per interval from large-diameter auger or excavated pits. Column leach metallurgy on French Guiana laterite, measuring cyanide consumption, permeability, recovery at various crush sizes, has not been published for most projects. The oxide opportunity sits in a data vacuum. Several West African mines launched as oxide heap leach operations before transitioning to sulfide milling, and the pathway is technically applicable to French Guiana, but the metallurgical evidence to evaluate it does not exist.

Most drilling stops at 80-150 meters. At those depths, most holes are still in saprolite or saprock. Sulfide mineralogy has been oxidized to goethite boxwork. Structural fabrics are partially destroyed. The geological information from a hole that bottoms in saprolite is categorically less useful than one reaching fresh rock where arsenopyrite-bearing veins sit in mappable sericite-ankerite alteration and oriented core allows three-dimensional structural modeling.

Orogenic deposits in equivalent terranes persist to 500 meters routinely, often much deeper. French Guiana's exploration has sampled the weathered top of these systems. Helicopter-supported diamond drilling in equatorial forest runs approximately USD 500/meter all-in. A 500-meter hole is a USD 250,000 commitment for a single data point. Junior companies cannot fund deep programs at resource-definition spacing at current capitalizations. At sustained gold prices above USD 2,000, deep drilling becomes fundable. The next generation of meaningful results from the territory will come from holes that reach fresh rock at 300-500 meters.

River sections along the Maroni and Oyapock are the only locations where bedrock structure can be measured at outcrop scale. Foliation attitudes, vein geometries, fault kinematics measured on river-cut exposures apply to forested upland targets along strike. The mapping requires boat access and extended field time.

Code Minier, EU directives, ministerial sign-off from Paris. Slow. Expensive. Filters out undercapitalized operators.

Montagne d'Or (Nordgold / Columbus Gold) defined a multi-million-ounce resource, proposed an open pit in primary forest, hit environmental and indigenous opposition amplified through national French politics, and stalled at the Conseil d'État. Open-pit mining in intact Amazonian forest is probably finished in French Guiana as a permittable concept. Underground mining is the remaining pathway. Projects need grades above about 3 g/t over mineable widths. All-in sustaining costs for an underground operation under French labor law: around USD 1,600/oz. Works at current gold prices. Does not work below USD 1,500.

Five to ten thousand illegal Brazilian miners across the interior, mercury amalgamation, Operation Harpie interdiction. Camp locations from military surveillance and Sentinel-2 change detection constitute a free placer prospectivity map. Mercury contaminates stream sediment samples downstream, requiring upstream sampling or gold grain screening to filter.

The Guiana Shield across its full extent has produced fewer large deposits relative to area than the West African Craton. Rosebel and Aurora are meaningful but neither approaches the 30+ million ounce scale of Obuasi or the production record of the Loulo-Gounkoto complex. The shield could be less well-endowed than the Birimian. No comparative endowment analysis exists, and the exploration density gap between the two cratons makes constructing one premature. The possibility is worth carrying alongside the optimistic terrane analogy rather than suppressing.

French Guiana also sits at the high end of the global cost curve. A deposit that works comfortably in Burkina Faso at USD 1,800 gold is marginal in French Guiana at the same price. Capital goes to the cheapest opportunities first. French Guiana gets funded when the cheap options thin out, or when the gold price is high enough. The current price environment is the closest the territory has come to sustained investment conditions. No major producer maintains a program here. The development model follows convention: juniors explore, majors acquire.