Summary:
Suriname Rosebel Gold Mine is part of a world-renowned orogenic gold deposit cluster, with mineralization conditions similar to the West African Leo Shield.
Rosebel and Saramacca are orogenic greenstone-hosted gold deposits. Rosebel includes seven main deposits, with mineralization controlled by shear zones and folds: Royal Hill (RH), Mayo (MA), Rosebel (RB), Koolhoven-J Zone (KH-JZ), Pay Caro (PC), and East Pay Caro (EPC). Saramacca mineralization is concentrated along the Faya Bergi fault and subsidiary shear zones.
Rosebel
The Rosebel deposits are hosted by a volcano-sedimentary sequence of the Marowijne Supergroup and by the overlying detrital sedimentary sequence of the Rosebel Formation. Five types of lithologies are distinguished on the Rosebel property: felsic to mafic volcanic rocks, felsic intrusion, flysch sequence, arenitic sedimentary rocks, and late diabase dykes. Gold mineralization is predominately hosted in the sedimentary and volcanic rocks, while the intrusion rarely mineralized, and the late diabase dykes are barren.
The volcanic and the felsic intrusive lithologies are interpreted as being part of the regional greenstone — tonalitetrondhjemite-granodiorite
(TTG) suite. The volcanic assemblage, part of the Paramaka Formation, consists of andesite in the northern portion of Rosebel, and of felsic (rhyolite) to mafic (tholeiitic basalt) rock to the South. In the southern portion of Rosebel, the volcanic rocks surround a tonalite intrusion (Brinks intrusion), while in the northern portion, up to the Charmagne concession, they form bands a few kilometres thick alternating with the sedimentary rocks of the Armina Formation.
The flysch sequence pertaining to the Armina Formation is found in the northern portion of Rosebel. The sequence consists in an assemblage of greywacke alternating with finer mudstone beds interbedded with conglomerate lenses of few metres to several metres thick, continuous over a few kilometres. The sedimentary rocks of the Rosebel Formation form the central sedimentary basin unconformably overlaying the volcanic rocks. The Rosebel Formation consists of an arenitic sequence interlayered with finer sedimentary rocks and continuous conglomeratic beds.
On the eastern portion of Rosebel (near the RB deposit) the rocks are intruded by three post mineralization north-south trending diabase dykes of the Permo-Triassic Apatoe dyke swarm.
Saramacca
Saramacca is underlain by metabasalt of the Paramaka Formation. The main volcanic units are a massive basalt overlain by a thinner amygdular basalt unit and a thick unit of pillow basalts, with a southwest to northeast younging direction. Rocks have been metamorphosed to the greenschist facies and have developed an assemblage of actinolite-chlorite-epidote-plagioclase. Rare, barren, thin felsic dykes crosscut the volcanic pile.
The massive basalt is a homogeneous, green, medium grained unit in which leucoxene sporadically develops. The true thickness of the unit is unknown, exceeding 50 m. The basalt’s northeastern contact with the amygdular unit is commonly obliterated by hydrothermal alteration. The amygdular basalt unit is a greenish grey to buff colour where hydrothermally altered. Quartz amygdules are generally one to three millimetres in diameter and constitute up to 5% of the rock. The pillow basalt is over 75 m thick and exhibits typical periodic arcuate selvages in the core. It is of a medium to dark green colour and is commonly moderately magnetic. Some graphitic shears appear to be spatially associated to the main mineralized structure.
The Faya Bergi fault zone is localized along the contact between the massive and pillow basalts along the thinner amygdular unit. The Faya Bergi fault zone and the Brokolonko structures represent a major brittle-ductile vertical dip-slip fault zone to which gold mineralization is associated.
Mineralization
Rosebel
Three mineralized/structural domains are observed at Rosebel: the North, Central, and South domains. The North domain includes the Koolhoven-J Zone (KH-JZ), Pay Caro (PC), and East Tailing Road (ETR) deposits located along two trends following a WNW-ESE orientation. The Central domain includes the Rosebel (RB) deposit, which strikes EW. The South domain is also E-W striking and hosts the Mayo (MA), Roma (RM), and Royal Hill (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.
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.
Saramacca
The Saramcca (SM) deposit mineralization is principally hosted within a series of N-NW trending structures ranging from two metres to 40 m in width over a strike length of 2.2 km and is open along strike.
Several sub-parallel structures have been identified, however, the Faya Bergi and Brokolonko structures are the primary mineralized structures over a continuous distance. The other structures are variably mineralized, though more drilling is required to test their prospectivity.
Dolomite breccias observed in the main fault zone are characterized by repeated ‘‘crack/seal’’ and dilatational infilling textures. These veins are also boudinated and folded, having formed within an active dip-slip environment. Higher grade gold is typically associated with dolomite breccias and pyrite mineralization, with the best gold grades located along thick fault segments to the northwest and the southeast.
The alteration pattern enclosing the fault zone shows the destruction of magnetite and the formation of leucoxene at distal ranges. Carbonate-chlorite alteration becomes more dominant with increasing proximity to the Faya Bergi fault. Within the fault zone, the protolith is destroyed by quartz-dolomite-pyrite and minor mica. The alteration footprint is commonly wider in the northeast block (pillow basalt) and can extend up to 50 m from the fault zone, while in the southwest block (amygdaloidal and massive basalt) it is observed up to 15 m to 20 m from the fault zone. The larger northeast alteration footprint may be ascribed to the presence of smaller, variably mineralized, subsidiary fault and shear zones northeast of the Faya Bergi fault.