Summary:
The Chapada Mine area consists of multiple zones, including Chapada Cava Central and Suruca, with stratigraphy comprising metasedimentary and metavolcanic layers. The mineralization is primarily hosted in biotite gneiss and amphibole-biotite gneiss, with copper and gold associated with early- to inter-mineral porphyry stocks.
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.
Brasiliano Orogenic System
The epigenetic hydrothermal fluids are responsible for the remobilization of gold and base metals in both systems (Chapada and Suruca). Chapada is characterized by the transformation of magnetite-biotite gneiss to biotite schist (biotitization) and, in Suruca, the mineralization is disseminated in a propylitic halo that is not usually a skarn system. However, it is not clear if these epigenetic hydrothermal processes contributed new gold and copper metal to the Chapada and Suruca deposits.
Mineralization
The gold at the Suruca deposit is related to folded quartz veins/veinlets with sericitic and biotitic 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 southwest area of Suruca is similar to Chapada, with sulphide disseminations and sulphides associated with stockwork quartz veinlets. Generally, Suruca mineralization occurs primarily by chalcopyrite and pyrite, with subordinate sphalerite and molybdenite.
The Suruca deposit comprises three distinct zones, divided according to the contained metals and oxidation zones: Suruca Oxide (gold-only), Suruca Sulphide (gold-only), and Suruca SW (copper-gold). The gold-only portion of the Suruca deposit is approximately 4.3 km in length, 1.0 km in width, and up to 540 m in depth. The Suruca copper-gold deposit is approximately 4 km in length, 700 m in width, and up to 540 m in depth.
The Suruca Oxide zone is hosted in a thick weathering mantle with an average thickness of 35 m to 40 m, with a well-defined zoning from top to bottom, composed of soil, mottled rock, fine saprolite, coarse saprolite, and altered rock.
The remaining mineralization is hosted in the Suruca Sulphide zone and the lithologies are grouped into five domains, including:
- ANF: medium grained to finer grained amphibolite to quartz amphibolite, in which epidote and chlorite are common accessory minerals.
- MTS: the metasedimentary layer and upper metavolcano-sedimentary layer (A layer).
- MVI: metavolcano-sedimentary layer (B layer).
- QDP: intrusions of porphyritic metadiorite composed of quartz, biotite, and plagioclase.
- AQS: an interlayering between lithotypes with metasedimentary protoliths (e.g., garnet-biotitequartz schist and garnet-amphibole-quartz schist) and metavolcanic protolith (e.g., biotite-quartz schist).
The main mineralization pre-dates the documented deformation at Suruca. The gold and copper-gold zones are therefore believed to be associated with calcic skarns that were subjected to amphibolite and subsequent greenschist-facies regional metamorphism; however, some structurally controlled features are also observed.