French Guiana lies at the east end of the Guiana Shield which extends 500,000 km², from Colombia eastwards to north-eastern Brazil. In addition to French Guiana, it also includes Suriname, Guyana, and part of Venezuela. It is the oldest and most stable geological structure on the South American continent, dating back some two billion years.
At that time, Guiana was still attached to the West African coast, part of the original supercontinent known as Gondwana, and its formation is the outcome of the various stages of continental drift which opened up the Atlantic Ocean. Guiana's subsoil therefore consists essentially of terrain comparable to that of West Africa (Ghana in particular). The potential for gold mineralization in French Guiana is therefore similar to that of the West-African craton to which it was attached in pre-Jurassic times.
The green rocks of Paramaca
The make-up of the Guiana Shield is similar to that of the other pre-Cambrian shields of a regional set of geological formations that include granites and ancient sedimentary and volcanic rocks, dubbed the green rocks of Paramaca. It is a set of rocks whose composition and texture closely parallel the formation of volcanic and sedimentary rocks thought to be favorable to the presence of gold deposits.
Recognition of 18 mining subjects in French Guiana
In the second half of the 20th century, French public authorities launched a major program to improve geological knowledge of the Guiana territory, which led to a project between 1975 and 1995 by the Geological and Mining Research Bureau (BRGM / Bureau de Recherches Géologiques et Minières) to carry out a mining inventory. This project, in which the French State had invested €42.7 million (280 million francs), first undertook aeromagnetic prospecting on more than half of the country (48 000 km²) which allowed it to locate favourable rocky geological structures ("green" rocks) that could reveal the presence of mineralized zones. It was followed by geochemical prospecting (with 120,000 samples taken) to delimit anomalous zones that suggest a high probability of discovering a deposit (excerpts from "L'Or en Guyane: éclats et artifices". Report submitted to the Prime Minister by Christiane Taubira-Delannon). The BRGM inventory led to the identification of 18 mining subjects in French Guiana, including the Dorlin sites, where anomalies were revealed in 1975-1976, and Yaou sites between 1981 and 1983.
Substantial potential for primary deposits
There are two types of gold deposits in French Guiana: primary deposits and secondary or alluvial deposits. Primary deposits are the result of geological anomalies: these are substances found in their original geological environment, corresponding to their initial location in the earth's crust, the ore having exceptionally high gold content. Secondary gold is the result of the break-up of primary gold deposits due to erosion. Gold mineralizations altered in this way are carried in the sedimentary deposits of riverbeds (alluvium). The presence of secondary gold close to the surface generally makes it easier to identify primary deposits. Notably, since gold was first discovered in 1855, most of French Guianese gold production has come from alluvial deposits. Virtually still intact, the existing potential for primary gold in Guiana is substantial.