Summary:
Geology
Lamego
Lamego mine is located in the Iron Quadrangle, which is a geotectonic unit on the southern edge of the São Francisco Craton, comprising Archaean and Proterozoic terrains, and bordered by Neoproterozoic mobile belts. From a regional viewpoint, Lamego mine is located in the eastern extension of the Serra do Curral inverted homocline, located on the northern edge of the Iron Quadrangle.
The mine lithostratigraphy consists of an intermediate metamafic sequence of the greenstone belt type and is hosted in the Nova Lima Group, which is part of the Rio das Velhas Supergroup. This sequence is characterised by lower metabasaltic rocks at the base, overlain by Algoma-type BIF metasediments, a quartz layer (known locally as metachert), carbonaceous schist, graphite schist and a further sequence of sediments consisting of alternating metapelites and metapsammitic rocks with a volcanoclastic contribution. The upper sequence of the Rio das Velhas Supergroup is the metasedimentary Maquiné Group.
Cuiabá
Cuiabá mine is located in the Iron Quadrangle, which is a geotectonic unit on the southern edge of the São Francisco Craton, comprising Archaean and Proterozoic terrains, and bordered by Neoproterozoic mobile belts. From a regional viewpoint, Cuiabá mine is located in the eastern extension of the Serra do Curral inverted homocline, located on the northeastern edge of the Iron Quadrangle.
The mine lithostratigraphy consists of an intermediate metavolcanic sedimentary sequence of the greenstone belt type and is hosted in the Nova Lima Group at the bottom of the Rio das Velhas Supergroup. This sequence is characterised by metabasaltic rocks at the base, overlain by Algoma-type Cuiabá BIF, and carbonaceous schist. Above the metasediments is a sequence of metabasalts overlain by an alternating sequence of metapelites and metapsamitic rocks with minor volcanoclastics.
The Corrego do Sitio
The Corrego do Sitio (CdS) gold deposit is located in the eastern part of the Lower to Middle Greenschist Facies of the Rio das Velhas Archaean, in the Iron Quadrangle region, on the southern margin of the São Francisco Craton in Brazil
Deposit type
Lamego
The gold mineralisation at Lamego has features and characteristics that match the epigenetic orogenic gold deposit model presented for Archaean gold-lode deposits.
Cuiabá
Cuiabá is an Archaean BIF-hosted gold deposit that produces sulphuric acid as a processing by-product. It is characterised by hydrothermal alteration of the rocks, with the mineralisation occurring mainly in BIF layers, subordinate quartz veins or in schists. The host to the gold mineralisation in the volcanosedimentary Nova Lima Group that occurs at the base of the Rio das Velhas Supergroup. The upper sequence of the Rio das Velhas Supergroup is the metasedimentary Maquiné Group. The gold mineralisation at Cuiabá has features and characteristics that match the epigenetic orogenic gold deposit model typical of Archaean lode gold deposits.
The Corrego do Sitio
CdS is an orogenic gold deposit hosted in intensely deformed clastic, volcanoclastic, carbonaceous schists and metagreywackes in an approximately 30km northeast-southwest striking shear zone. Hydrothermal alteration phases associated with the mineralisation are dominated by sericite and carbonate.
Mineralisation style
Lamego
The gold mineralisation at Lamego is characterised by orebodies associated with two horizons of chemical sedimentary rocks: BIF and metachert, with shear zones containing abundant quartz veinlets. The proportions of these lithotypes vary substantially from one deposit to another. In the BIF, sulphide mineralisation is associated with gold, while in the metachert it is associated with quartz veins. The gold occurs either as native gold or in sulphides. Lamego has a similar rock assemblage to Cuiabá, but with higher structural complexity. The mineralised BIF is more structurally deformed and contains more silica when compared to Cuiabá, which reacted less with the hydrothermal fluid.
Cuiabá
Cuiabá mine’s gold mineralisation is associated with sulphides and quartz veins in BIF and volcanic sequences. Structural control and fluid flow are the most important factors for gold mineralisation with a common association between largescale shear zones and their associated structures. Where BIF is mineralised, the ore appears strongly stratiform due to the selective sulphidation of the iron-rich layers. Steeply plunging shear zones tend to control the ore shoots, which commonly plunge parallel to intersections between the shears and other structures. Mineralisation is hosted in the limbs of a fold system.
The Corrego do Sitio
The CdS I, II and III, gold deposits and associated targets are located in a gold trend that extends for approximately 14km in a north-easterly direction, from Grota Funda (CdS I) in the south to Anomalia (CdS III) in the north, which developed in a compressional tectonic regime. Gold is associated with quartz and fine-grained acicular arsenopyrite. The main gold targets and deposits are distributed over three trends, namely the CdS Trend and Cristina Trend hosted in metasedimentary rocks, and the Donana Trend hosted in BIF.
At CdS I, the main orebodies are Rosalino, Cachorro Bravo, Laranjeiras and Carvoaria, which constitute the current production sources and most of the Mineral Resource. At CdS II, the main orebodies are São Bento, Pinta Bem (both BIF-hosted) and Sangue de Boi (metasedimentary-hosted). At CdS III, where limited exploration has taken place, the Anomalia I orebodies are the best understood and have the highest potential, hosted in the metasedimentary and BIF sequences. The Jambeiro and Mina de Pedra targets are less drilled and less understood.
Mineralisation characteristics
Lamego
The mineralisation is characterised by sulphidation in the form of disseminated sulphide bands or as fracture fill and, more rarely, as massive sulphide hosted in BIF or metachert although sulphide bands are rare in the latter. The metachert (or quartz vein) is concentrated in the hinges of the Lamego structure and has free gold as the main mineralisation with a lesser amount associated with sulphides. The plunge of the mineralised zones coincides with both fold axes of the first two structural events and the stretching fabric.
Cuiabá
The main orebodies at Cuiabá are as follows:
- Normal limb: Fonte Grande Sul and Serrotinho;
- Overturned limb: Balancão, Galinheiro and Canta Galo.
Secondary orebodies occur in hydrothermally altered schists in the footwall of Galinheiro (Galinheiro footwall (GFW) orebody), hydrothermally altered schists/quartz veins near the footwall of Fonte Grande Sul and Serrotinho (Quartz vein orebody), and close to the hangingwall of Serrotinho (Viana orebody).
The Corrego do Sitio
The CdS deposits consist of narrow northeast to southwest elongated and folded lenses of mineralisation, parallel to the main regional deformational structure (S2), dipping 60° to 70° to the southeast and plunging 20° to 30° to the northeast. The orebodies are consistently folded, boudinaged and locally disrupted by younger structures. CdS is an orogenic type deposit comprising many hydrothermal lodes with quartz veins and low-grade sulphides, disseminated in the wall rocks. In general, the mineralisation consists of sericitic zones and quartz veinlets hosted in metapelite and BIF. The sedimentary sequence, and consequently the mineralised deposits, are cross-cut by a swarm of basic dykes of uncertain age, with a general orientation north-northeast to south-southwest dipping to the southeast, with thicknesses varying from 20cm to 20m. The gold occurs as native gold in smoky quartz veins and as microscopic or sub-microscopic inclusions in arsenopyrite (the main mineralisation style). It may also occasionally be associated with berthierite (FeSb2S4).