Crixas - Serra Grande Mine
The Crixas mining district is a historic producer of gold, having yielded over 175 t of Au from 4 major mines in the belt over the last more than 100 years. Mineralisation is hosted by the ~2.7 Ga Archaean Crixas greenstone belt on the western margin of the São Francisco Craton in the Cental Brazil Shield, some 250 km NNW and NW of the cities of Goiana and Brasilia. The greenstones are surrounded by Archaean tonalitic gneisses and granodiorite. The current mine has been in production since 1989. It is hosted by an overturned sequence of foliated mafic meta-volcanics and meta-sediments that dip north at 25 to 40 degrees. The Crixas Greenstone Belt has 3 units, namely a western Basal Unit which is 600 m thick and is composed predominantly of serpentinites and magnesian schists with spinifex textures, and lesser ferruginous chert, quartz-chlorite schist and tremolite-albite-quartz-carbonate schist. The 800 m thick Middle Unit is dominantly a tremolite-actinolite-albite-chlorite-carbonate schist with minor pillow basalts, and more significant graphitic schist, quartz-chlorite schist and talc schist. The Upper Unit is areally restricted but is predominantly graphitic biotite-chlorite schist and meta-greywacke some 1600 m thick, with bodies of dolomite, massive quartz and magnetite-garnet-chlorite-chloritoid schist at its base, commonly containing appreciable gold., including the Crixas gold deposit. At Crixas, two main broadly concordant auriferous zones have been defined, aligned in a WNW direction, coincidental with a prominent lineation. The ore shoots plunge to the NW at 6 degrees and 35 degrees respectively, and have long axes that parallel isoclinal axes observed in the ore zone. The Upper Zone comprises ferroan dolomite schist and sericite-chlorite schist in which high gold concentrations are associated with massive arsenopyrite, pyrrhotite and chalcopyrite. The Lower Zone is approximately 120 m below the Upper and comprises a quartz cemented, graphitic pelite breccia in which gold is associated with arsenopyrite, pyrrhotite and chalcopyrite within and on the margins of graphitic pelite fragments. The breccia has 20 to 40% quartz cement. Gold is also found in association with sericite schists containing minor disseminated arsenopyrite and chalcopyrite. Pre-mining drilling defined 5.2 mt @ 12.7 g/t Au. The production rate in 1999 was 620 000 tonnes of ore \ per annum at a grade of 8.2 g/t Au, with a recovery rate of 95% and a operating cost per ounce of $US 184. The reserve at the end of 1999 was 0.56 mt @ 9.5 g/t in the Mina III, and 0.50 mt @ 4.7 g/t in Mina Nova. The mine is operated by Mineracao Serra Grande, a 50:50 joint venture between Anglogold and TVX Normandy..
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Morro do Ouro
The Morro do Ouro gold deposit at the Brasilia Mine of Rio Paracatu Mineracao S.A. in Minas Gerais is located some 240 km SSE of Brasilia and 500 km NNW of Belo Horizonte. It is hosted by Neo-Proterozoic sediments of the Brasilia Fold Belt on the western margin of the SÑo Francisco Craton within the Central Brazil Shield. These 850-1000 Ma sediments were deformed between 680-650 Ma. The mineralisation is hosted by carbonaceous phyllites of the Paracatu Formations which comprises a thick succession of such graphitic phyllites with interbedded layers and lenses of impure quartz sandstones which are generally quartzitic. These have been thrust over carbonates of the Vazante Formation that belong to the same stratigraphic package. The Morro do Ouro gold deposit is localised within a structure that is a subsidiary of a larger thrust. The extensive flat lying orebody lies within the horizontal segment of a monoclinal thrust that is characterised by a strong mylonitic foliation within the host graphitic phyllites. In detail the better gold is found along the hinge zones of isoclinal folds and along S-C surfaces which were transformed into dilatant sites during progressive shearing. The same progressive shearing produced intense boudinage of quartz veining introduced in preceding episodes of the same deformational event. In the oxide zone gold occurs as free metal in the quartz boudins, accompanied by sulphides and minor carbonate and after decomposed sulphides in the enclosing phyllite, concentrated in pressure shadows produced by the presence of the boudins. The initial viability of the deposit was determined by the flat lying attitude of the ore zone which forms a low hill with no overburden, enhanced by the deep oxidation of the ore. The oxidation has released gold from the sulphides and carbonates to be free milling, while it has also decomposed the rock so no crushing or blasting is required in the oxide zone. In the primary zone gold is distributed in the lattice or in micro-fractures within iron and arsenic sulphides, and in association with carbonates. The operation is one of the lowest grade profitable mines in the world. In 1999 some 17.47 mt of ore were mined at an average grade of 0.45 g/t Au. The gold recovery was 73%, for 5.7 t (0.185 Moz) of gold at a cash cost of $US 176 per oz. Reserves and resources stood at 124 mt @ 0.41 g/t Au. The mine has been in production since 1988. Rio Paracatu Mineracao is controlled by the Brazilian subsidiary of the operator Rio Tinto (51%) with the balance held by TVX Normandy.
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Overview Briefing on Quadrilátero Ferrífero Gold
The Quadrilátero Ferrífero district of Brazil which surrounds the major city of Belo Horizonte in Minas Gerais is one of the most significant mining centres in Brazil. It represents the home of mining in Brazil with the countries first and largest gold mines, and the being the hub of much of the iron mining of the country outside of the Carajas. Gold mining commenced in the Nova Lima district some 200 years ago, with one of the earliest mines being Morro Velho. The geology of the Quadrilátero Ferrífero is dominated by Archaean and Proterozoic volcano-sedimentary sequences and Precambrian granitic complexes. The oldest rocks in the district are an Archaean gneiss-migmatite complex which is overlain by the late Archaean Rio das Velhas Supergroup. This supergroup is sub-divided into the basal Quebra Osso Group, succession of komatiitic ultramafic and mafic rocks with BIF intercalations, overlain by the Nova Lima Group, a middle sequence of schists and phyllites that contain lenses of dolomitic and calcic white marble and by thin beds of iron formation, and finally the upper Maquine Group of quartzites, conglomerates and phyllites. These are discordantly overlain by the Palaeo-Proterozoic Minas Supergroup quartzites, schists, phyllites, meta-conglomerates, carbonates and iron formations that produce the major iron deposits of the district. The Morro Velho mineralisation lies within the Nova Lima Group.
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Morro Velho Complex
The Morro Velho complex of mines have been worked for over 200 years, while Morro Velho itself was in continuous operation from 1834 to 1960. The mineralisation at the Morro Velho complex is hosted by the volcano-sedimentary Nova Lima Group. Gold mineralisation is associated with sulphides and quartz veins in banded iron formation (BIF), mafic and ultra-mafic sequences and as disseminations throughout the volcano-sedimentary sequence. The current genetic theory involves an epigenetic model with strong structural control and an association of deep granitic intrusions. The host rocks at Morro Velho are BIF, Lapa Seca (a quartz-dolomite-ankerite rock) and mafic (principally basaltic) volcanics. Mineralisation is commonly associated with large scale shear zones and their associated structures. Where BIF is mineralised (as at the old Morro Velho mine) the ore is stratabound due to the selective sulphidation of the iron rich layers. Steeply plunging shear zones tend to control the ore shoots and commonly plunge parallel to intersections of shears and other structures. At Morro Velho the controlling structures were apparently the intersection of a thrust fault and tight isoclinal folds in a ductile environment. Mineralisation is believed to result from the interaction of low salinity CO2 rich fluids with the high iron BIF and basalts and the carbonaceous graphitic schists. Sulphide mineralisation comprises pyrrhotite and arsenopyrite with subordinate pyrite and late stage post gold chalcopyrite. Wall rock alteration is typically carbonate, potassic and silicic. Mining in the complex has in recent years been restricted to the Espiritu Santo open pit and the Cuiba and Mina Velha underground mines. During 1999, some 6.1 t (197,500 oz)of gold were produced at a cash cost of $US 124 per oz and a total production cost of $US 176 per oz. The recovered grade was 7.01 g/t Au. The operation is controlled by AngloGold through its wholly owned subsidiary Morro Velho.
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The São Bento mine is located in the eastern portion of the Quadrilátero Ferrífero district. Mineralisation at São Bento is restricted to the fine grained sediments, tuffs and chemical sediments of the Nova Lima Group. Four formations have been differentiated on the property, namely the Carrapato, the Lower Iron, the Basal Iron and the São Bento Formations. The ore is localised within the Basal Iron Formation which has been continuously mined from the surface to a depth of 1100 m. Based on lateral lithologic variations, the basal Iron Formation has been divided along strike into three portions, namely Orebody No. 1, Orebody No. 2 and Orebody No. 3 (otherwise known as the SÑo Bento/Pinta Bem Orebody). The No. 1 Orebody represent the best developed mineralisation, reaching a thickness of 35 m and displaying al least 6 periods of gold/sulphide mineralisation. The ore zone is distinctly banded and comprises alternating layers of sulphide and iron carbonate. the gold is closely associated with the sulphides, being present either as free metal, on crystal boundaries or enclosed within sulphide grains. The gangue and sulphides are generally 0.5 to 5 millimetres across, while the gold varies from 1 to 250 microns, averaging 70 microns. The average gold content is very consistent as are the ratios between gold, arsenic and sulphur. The ore deposits dip at 55 to 60 degrees and have an average true thickness of 1.4 m. In 1999 540, 014 tonnes of ore were milled to produce 3.9 t (0.127 Moz) of gold at an average grade of 8.18 g/t Au. The total cash cost was $US 189 per oz. The mineable reserves were 3.238 mt @ 8.95 g/t Au for 29 t (0.932 Moz) of gold while the in-situ resource was 4.04 mt @ 10.6 g/t Au for 43 t (1.38 Moz) of Au. Total production to that stage was 35.1 t (1.13 Moz) of Au. The mine is operated by Eldorado Gold Corp., which is 36% owned by Gold Fields Ltd.
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For more information contact: T M (Mike) Porter, of Porter GeoConsultancy (firstname.lastname@example.org)
This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF). While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.
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