Summary:
The Chapada and Suruca deposits are located in the Eastern Belt of the Mara Rosa Volcano-sedimentary sequence. The Eastern Belt in the vicinity of the mine comprises a thick package of amphibolites succeeded by volcanic and volcanoclastic rocks overlying metasedimentary rocks.
The Suruca deposit comprises three distinct zones, divided according to the contained metals and oxidation zones: Suruca Oxide (Au-only), Suruca Sulphide (Au-only), and Suruca SW (Cu-Au).
Currently, the most accepted metallogenetic model for Chapada is a metamorphosed porphyry model associated with skarn system. The porphyry, skarn, and epithermal system can be separated into three distinct mineralization styles, based on hydrothermal alteration and metal association:
• Copper-Gold Porphyry System (Chapada Corpo Principal, Corpo Sul, and Sucupira);
• Gold (Silver-Lead-Zinc) Distal Skarn (Suruca); and
• Copper-Gold Proximal Skarn (Suruca SW).
Mineralization
The copper-gold deposit at Chapada comprises products of hydrothermal alteration of the copper-gold porphyry system. Alteration styles include biotitization, sericitization, argillization, and propylitization. The primary copper-gold mineralization at Chapada is epigenetic. Copper is principally present as chalcopyrite with minor amounts of bornite. Fine grained gold is closely associated with sulfide mineralization and was likely to be contemporaneous with copper mineralization. Other district targets include mineralization styles associated with skarn alteration.
The gold at Suruca is related to folded quartz vein/veinlets with sericitic and biotitic alteration, rather than high sulfide concentrations. The second generation of quartz veins/veinlets with sulfides (sphalerite + galena + pyrite), carbonates, and epidote also host gold, which is related to zinc. Mineralization predominately pre-dates deformation, so the gold (Suruca) and copper-gold (Suruca SW) are associated with skarn features, however, some structurally controlled features are also observed.
Suruca
The Suruca SW mineralization was discovered in 2017 exhibiting similar geological features to the Chapada deposit. The mineralization was delineated along a 2.1 km strike, 650 m width, and average depth of 50 m, and was partially exposed on surface.
The gold at Suruca is related to folded quartz veins/veinlets with sericitic and biotite alteration, rather than high sulphide concentrations. The second generation quartz veins/veinlets with sulphides (sphalerite + galena + pyrite), carbonates, and epidote also host gold which is related to zinc. The copper mineralization in the Suruca SW displays same features as Chapada, with sulphide disseminations and sulphides associated with stockwork quartz veinlets. In general, Suruca SW mineralization is formed predominantly by chalcopyrite and pyrite, with subordinate sphalerite and molybdenite.
Mineralization predominately pre-dates deformation so the gold (Suruca) and copper- gold (Suruca SW) zones are associated with skarn features, however, some structurally controlled features are also observed.
Skarn system – Suruca
The Suruca deposit is host to gold-zinc mineralization associated with sericite- chlorite-epidote carbonate (- biotite) and the argillic to propylitic alteration which corresponds to a skarn system.
According to Sillitoe (2014), the Suruca metal concentrations marked by zinc (sphalerite), lead (galena), and gold associated to epidote/calcite and/or garnet/amphibole rich schists, point to a distal gold (Ag-Zn-Pb) skarn system. These epidote-calcite rich schists are interpreted as skarns, which were subjected to amphibolite and subsequent greenschist facies regional metamorphism. Also, the possible presence of deformed and metamorphosed diorite porphyry in the copper- gold zone suggests that Suruca may be related to a discrete porphyry centre represented by Suruca SW.