Summary:
Gold mineralization within the Rosebel deposit is structurally controlled and exhibits similar geological, structural, and metallogenic characteristics to orogenic greenstonehosted gold deposits. Mineralization over the Rosebel property consists of quartz-carbonate tension and shear vein association. Rosebel hosts seven main deposits and several smaller gold occurrences in three mineralized domains.
The North domain is formed by two sub-parallel mineralized trends striking WNW-ESE: the southern trend comprises the PC and EPC deposits (and ETR exploration area), while the northern trend includes the KHJZ deposits (and MK exploration area). The mineralized trends are found on both flanks of an anticline exposing the volcanic rock and plunging 35° to the WNW. The volcanic rocks are overlain by the Rosebel Formation to the south, and by the Armina Formation to the North. A regional dextral strike-slip fault exhibiting late normal movement marks the southern limit of the North domain and is closely associated with the mineralization.
The South domain includes the MA, RM, and RH deposits. The local geology is characterized by the presence of a volcanic basement overlain by detrital sedimentary rocks of the Rosebel Formation. The MA, RM, and RH deposits are all hosted in the footwall of a major reverse fault striking E-W and which is closely related to the onset of mineralization. The sequence is folded into relatively open and slightly dipping (0 to 15°) east or west folds. Mineralization is associated with the major and/or subsidiary shear zones and in the hinges of anticlines.
The RB deposit is hosted in a sedimentary sequence of siltstone and arenite of the Rosebel Formation. RB is the only deposit not located along a volcano-sedimentary contact. The southern portion of the RB pit exposes one interval of conglomerate interbedded within a coarse grained, quartz rich arenite. This sequence evolves to finer grained arenites and siltstones suggesting a general northward younging direction. The sequence strikes 100° and is sub-vertical to steeply dipping to the north. The sedimentary sequence and the mineralization are intruded by three post-mineralization, sub-vertical, north-south diabase dykes.
Mineralization
Three mineralized/structural domains are observed at Rosebel: the North, Central, and South domains. The North domain includes the KH-JZ and PC deposits located along two trends following a WNW-ESE orientation. The Central domain includes the RB deposit, which strikes EW. The South domain is also E-W striking and hosts the MA, Roma (RM), and RH deposits.
Mineralization within the Rosebel deposits is structurally controlled and gold is hosted in both shear and tension veins which are tightly associated in space and time. Relations between veining and folding demonstrate that veining occurred after folding and has commonly borrowed pre-existent structures, such as extensional fractures, or along rock heterogeneities. As a result, elements such as an anticline hinges, lithological contacts and conglomeritic beds have provided structural traps for mineralized fluids. As the veins exhibit no significant signs of deformation, the mineralization is interpreted as being emplaced during the latest stage of the Transamazonian orogeny event.
The general vein mineralogy consists of a quartz - carbonate – tourmaline – plagioclase – pyrite/pyrrhotite assemblage. However, the proportion of the main minerals and the nature of the secondary and trace minerals vary between the different domains. In the South domain, the characteristic vein mineralogical assemblage is quartz + carbonate (calcite) + tourmaline ± chlorite ± sericite ± pyrrhotite ± pyrite, where accessory minerals comprise sphalerite, plagioclase, and magnetite. Alteration aureoles are defined by the presence of chlorite, carbonate (mostly calcite), sericite, pyrrhotite and locally tourmaline and pyrite. In the North domain, the vein mineralogy consists of in an assemblage of quartz + carbonate (calcite – ankerite) + plagioclase + hematite + chlorite ± sericite ± pyrite ± tourmaline. A zonation of vein mineralogy occurs at the deposit scale, with plagioclase and hematite constrained to the main shear zone, while all other minerals are distributed throughout the Rosebel deposits. In the Central domain, the characteristic vein mineralogical assemblage is quartz + carbonate (calcite + ankerite) ± chlorite ± sericite ± pyrite. Ankerite alteration halo is typical of the Central domain and is associated to the highest grade zones.