The Serra Grande mine is owned and operated by Mineracao Serra Grande S.A., a company registered in Brazil. Mineracao Serra Grande through AngloGold Ashanti Holdings plc., is wholly owned by AngloGold Ashanti plc.
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Summary:
The Serra Grande gold deposits are hosted in a typical greenstone belt sequence. The mineralisation is hosted by metasedimentary and metavolcanoclastic rocks, with quartz veins and massive or disseminated sulphides that have economic potential when controlled by geological structures. Two main deformational events control the mineralisation and have been identified in the region:
- The first, a thrusting event (D1 from west to east), developed with irregular thrust ramp geometry. This event was responsible for stacking and inverting the stratigraphic sequences.
- The second event (D2), was the thrusting of the Santa Terezinha sequence over the Crixás Greenstone Belt, folding the rocks (F2) and generating the structural controls for gold mineralisation, generally parallel to the fold axis.
Deposit type
The Serra Grande gold deposit is an orogenic mesothermal deposit, associated with the development of shear zones that belong to the Upper Archaean Crixás Group.
Gold mineralisation is associated with metasediments and metavolcanics from the Ribeirão das Antas and Rio Vermelho Formations respectively. The Crixás Greenstone Belt is surrounded by granitic gneiss terrains from the Ribeirão das Antas and Caiamar complexes and metasedimentary rocks from the Santa Terezinha Group, which is part of the Goiás magmatic arc.
Mineralisation style
The mine is in the Crixás Greenstone Belt sequence, in the central portion of Brazil, and the main host rocks are metasedimentary sequences associated with metavolcanic basic rocks.
Mineralisation at Serra Grande is associated with quartz veins and massive to disseminated sulphides in metasedimentary, metavolcanoclastic and metabasalt rocks, with variable degrees of hydrothermal alteration developed over orogenic stacked thrust layers (duplexes).
Mineralisation characteristics
Geometry of the mineralised deposits is typically complex with pinch-and-swell, and folded and boudinage shapes, dipping between 10° and 35° on average, with the greatest continuity along northwest plunging structures (azimuth 290°). In some cases, or some orebodies, the dip can increase to 65° which is seen for the deeper portions of the Mina III main quartz vein.
The mineralisation is hosted in five main domains (structures): Structure II, III, IV, Palmeiras and V. The mineralisation occurs as stacked lenses, generally concentrated in the same high deformation positions (with folds and disruptions) in the structures.
Structure II is a zone of quartz veinlets within graphite-schists and metagreywacke sequences which are structurally below Structure III. Mineralisation is hosted by quartz veins with free-gold and disseminated sulphides, which is represented by the Cajueiro target.
The mineralisation at Structure III, is hosted by quartz veins that are hosted in carbonaceous schists, where gold grades of up to 8 g/t can be found. Mina III (lower zone) and Ingá are typical of this mineralisation. This structure is also associated with massive and disseminated sulphides (mainly arsenopyrite and pyrrhotite) that occur in a sequence of hydrothermally altered schists. Massive and disseminated sulphide mineralisation hosted by dolomite lenses which are structurally above the lower zone schists and quartz veins is named the upper zone.
In Structure IV, the mineralisation consists of quartz veinlets and disseminated sulphide (arsenopyrite and minor pyrrhotite) hosted in graphite schists, such as Corpo IV, and Pequizão. The mineralised zones are hosted by graphite schists with massive and disseminated sulphide concentrated in fold-hinge zones. The ore shoots plunge to the north-northwest and dip between 6° and 35°.
Structure Palmeiras is associated with hydrothermal alteration of metabasalts, with sericite, chlorite, carbonate and massive sulphides (pyrrhotite and arsenopyrite).
Structure V hosts mineralisation within altered talc-schists which are structurally above structure Palmeiras. Alteration includes sericite, chlorite and carbonate, and mineralisation is hosted by quartz veins and disseminated sulphides (arsenopyrite and pyrrhotite).