|
Lamego - Carruagem, Arco da Velha, Queimada, Cabeca de Pedra |
|
|
Minas Gerais, Brazil |
| Main commodities:
Au
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All papers now Open Access.
Available as Full Text for direct download or on request. |
|
|
The Lamego gold mine exploits the Carruagem, Arco da Velha, Queimada and Cabeça de Pedra orebodies that are distributed over a 5.7 km strike length of iron formation on the margins of a tight, SW-NE elongated, oval shaped reclined antiform (#Location: 19° 54' 37"S, 43° 46' 9"W).
Modern exploration in the Lamego district commenced between 1985 and 1990, resulting in the early recognition of several mineralised zones hosted in metachert and banded iron formation (BIF). These were the Arco da Velha, Queimadav and Cabeça de Pedra orebodies, while further drilling encountered the Carruagem orebody. After a program of testing and a positive feasibility study, the Lamego mine complex was established in 2010 with the first gold poured soon after. Lamego operates as a satellite to the Cuiabá Mine. Ore is transported to surface via ramps to be crushed, stockpiled and transported daily to the Cuiabá plant, where it is blended with Cuiabá ore on the ROM pad.
For details of the regional setting, see the Quadrilátero Ferrífero District Gold - Geological Setting record. See the detailed description of the Rio das Velhas Supergroup within the Nova Lima-Caeté Domain in that record, concentrating on the Nova Lima Goup, in particular the Mestre Caetano Formation in the middle of that group. The geological map on the same link also shows the location of the deposit.
Geology
The Lamego deposit is hosted within the Morro Vermelho Formation of the Nova Lima Group, as detailed in the link above. The stratigraphy in the deposit area is as follows (after Martins et al., 2016), from the base:
• Mafic volcanic rock unit, which is up to 200 m of meta-basalt, the protolith of which is a mainly green, fine-grained and massive mafic volcanic rock composed of plagioclase, amphibole, epidote and titanite, locally including chert layers up to 1 m thick. Where altered and foliated, it occurs as a chlorite-carbonate-sericite-quartz schist that encloses sigmoidal aggregates mainly composed of pyrite, and has an anastomosed and crenulated fabric. In the deposit area it has a preserved lateral extent of ~1.7 km in the core of the host antiform.
• Lamego chert and banded iron formation (the Lamego BIF), which ranges from 1 to 8 m in thickness and comprises intercalated chert and BIF (as defined by >15% Fe), with a locally interbedded 15 to 20 cm thick, foliated and sulphidised mafic volcanic horizon. The individual chert bands can be up to 2 m thick and vary from light cream to dark coloured, the latter due to impregnated carbonaceous matter with up to cm-thick carbon bands. The chert grades to ferruginous chert containing very fine siderite and rarely magnetite, which, in turn, grades into BIF as the Fe content increases. The BIF bands are characterised by alternating, dark carbonate-quartz ±magnetite and light quartz-carbonate bands. In the deposit area, in addition to quartz and carbonate, the BIF also contains chlorite, sericite, pyrite, arsenopyrite, pyrrhotite, with minor chalcopyrite, galena and sphalerite, and accessory minerals such as sericite, rutile and carbonaceous matter. The unit is laterally discontinuous, persisting over intervals of up to 500 m.
• Carbonaceous pelites, which are 1 to 5 m thick, and are dark, finely laminated and thinly bedded. They are composed of carbonaceous matter, quartz, chlorite, carbonate and minor sericite. Locally, these rocks also occur as 0.50 to 1 m thick lenses within the underlying mafic volcanic rocks and Lamego BIF.
• Micaceous pelites, which entirely envelope the Lamego antiform and are up to 200 m thick. They are a greenish to light grey, micaceous pelites that are laminated with gradational and compositional stratification. They are composed of sericite, muscovite, quartz, carbonate and subordinate carbonaceous matter.
• Dolerite intrusions, that occur in sections of the deposit area as both dykes and sills, with lengths of up to 200 m and average width of up to 5 m. They crosscut both carbonaceous and micaceous pelites, and the Lamego BIF, and are fine grained, textured, green rocks that are foliated near their contacts, but grade to medium grained over short distances inwards towards their core. They are composed of green hornblende, actinolite-tremolite, epidote, chlorite, carbonate, plagioclase, sericite and quartz, with accessory titanite and ilmenite.
Structure
The main structure at Lamego is a rootless, overturned/reclined, isoclinal, cylindrical fold with a NE-SW striking axial plane that dips at 30°SE and limbs that both dip at 20 to 30° to the SE. The hinge zone to the SW is thickened, whilst the limbs and core of the structure appear to be attenuated to the NE, until they pinch out, separating a tear shaped nose zone from the main fold. At surface, the structure occurs over a length parallel to the main fold axis of ~1.7 km and width of up to 450 m, with a rounded nose to the SW, and 'tapering-out' to the NE. Following the Lamego BIF at surface, the structure has a circumference of 4.8 km, although it expands with depth. The core of the structure is occupied by the mafic volcanic rock, rimmed by discontinuous lenses of Lamego BIF and an outer band of carbonaceous pelites, which, in turn, are overlain by the micaceous pelites. A few NE striking lenses of carbonaceous pelite and BIF are found to the NE of the 'pinch-out' of the main structure, one of which hosts the Carruagem orebody. The Cabeça de Pedra orebody occupies the fold closure to the SW, whilst the Queimada and Arco da Velha orebodies are on the normal northwestern and overturned southeastern limbs of the antiform respectively.
Two structural generations, G1 and G2, have been distinguished in the immediate Lamego deposit area. G1 is represented by: i). planar structures, such as the S1-2 foliation, V1 and V2 veins, and shear zones, which are generally oriented NE-SW and dip to the SE. The exception are the V1 veins that dip to the NE, and possibly represent the oldest structures at the deposit; and ii). linear structures which include the L1-2 lineation and F2 fold axis that plunge to the SE. G2 structures are represented by: i). the north-south striking, east dipping S3 crenulation cleavage; and ii). the F3 fold axis and L3 crenulation lineation that are north-south trending with a southerly plunge. The S3 crenulation cleavage is related to the regional Cambrian Brasiliano event D4, east-vergent, north-south trending regional folding and thrusting. These regional D4 folds are interpreted to have rotated the G1 structures from an original NW-SE trend, which may have represented the original orientation of the Rio das Velhas Archaean Belt (Martins et al., 2016).
The Queimada orebody plunges at 102°/29°, whereas Arco da Velha, Cabeça de Pedra and Carruagem plunge at 105°/25°, 120°/25° and 95°/22°, respectively (Martins et al., 2011).
Alteration
Three main hydrothermal alteration types dominate and affect all rock types. They are represented by quartz, carbonate and sulphides, which are developed parallel to the S1-2 foliation, and are best exposed in BIF and carbonaceous pelites, but are less well developed in the mafic volcanic footwall rock and micaceous pelites.
Silicification is the dominant alteration type at Lamego, occurring as smoky and milky quartz veins, wiith minor carbonates, sericite, pyrite and carbonaceous matter. These silicified zones are characterised by high-modal percentage, >85% quartz, which locally forms 1 to 35 m thick hydraulic breccias that may or may not be boudinaged. Quartz grains can be >0.5 cm long. Three generations of quartz have been recognised: i). deformed, coarse- to medium-grained smoky quartz, occurring as irregular masses that are both concordant and discordant within the mineralised zones; ii). fine-grained, granoblastic, white and milky quartz, formed from the recrystallisation of smoky quartz; iii). milky quartz in faults cutting across the preceding two generations.
Sulphidation takes the form of 0.5 to 7 cm thick sulphide veins which locally replace entire Fe-carbonate-rich bands within the Lamego BIF. The sulphides are principally pyrite, arsenian pyrite and arsenopyrite, with lesser chalcopyrite and sphalerite, and minor pyrrhotite and galena. This alteration style is also accompanied by minor sulphosalt minerals such as tennantite-tetrahedrite and nickeline, as well as the phosphate monazite.
Carbonate alteration is mainly defined by ankerite, and is best developed as coarse-grained, up to 2 cm crystals in BIF.
Mineralisation
Of the four main gold orebodies at Lamego, Carruagem is the most significant, followed by Arco da Velha, Cabeça de Pedra and Queimada. Carruagem contains the highest gold grades and over 50% of the resource. It is located at the junction of, or in close proximity to, two fold limbs in the northeastern segment of the main antiform. It is boudinaged, with two large pinch and swell disruptions in the structure, followed by eastward displacement. The gold mineralisation is mainly associated with hydrothermal alteration within the Lamego BIF.
The Queimada orebody is on the overturned northwestern limb of the Lamego antiform and carries the lowest gold grades at Lamego, and as such was not originally mined. Mineralisation is predominantly associated with the Lamego BIF.
The Arco da Velha orebody is on the normal limb of the structure and extends for ~250 m along strike. In the northeastern section of the orebody, the mineralisation is concentrated in the metachert, whilst in the southwestern portion it is concentrated in BIF. Carbonaceous phillite and chlorite/sericite schists occur in the hangingwall contact, while alteration within the meta-andesite occurs in the footwall. It contains a locally continuous chert layer, and much smaller zone(s) of massive sulphide associated with silicification.
The Cabeça de Pedra orebody is located in the hinge zone of the antiform to the SW. It contains the largest continuous mappable development of the Lamego BIF, with a thickness ranging between 1 to 2 m, and restricted massive sulphide zones associated with zones of silicification and chert layers. The areas with the best grades and thickness coincide with a predominance of banded iron formation versus chert within the Lamego BIF. Locally the structure is complicated by faulting.
At least three main gold mineralisation styles are recognise at Lamego, namely vein, replacement and disseminated. In the mineralised Lamego BIF, V2 veins dominate, with associated sulphide replacement of the Fe-carbonate and rarely magnetite. In the mafic rocks and carbonaceous pelites, disseminated mineralisation is dominant.
The vein style mineralisation includes quartz-carbonate ±sulphides veins and quartz boudins that range from 0.2 to 5 cm in thickness, and crosscut all lithologies, although they are best developed in BIF where they locally resemble a stockwork. Irregular smoky/milky quartz zones may be thicker than 35 m. In both vein and silicification zones, grade shells can range from 1.6 to 15.8 g/t Au, and locally reach 300 g/t.
Replacement style mineralisation is typical of the Lamego BIF, and where it is characteristically confined to that lithology, replacing favorable Fe-rich carbonate bands with sulphides. Gold grades range from ~0.03 to 6.5 g/t, with values as high as 90 g/t Au.
Disseminated mineralisation within the mafic volcanic unit and carbonaceous pelites is best exposed at the Cabeça de Pedra orebody, where they are commonly developed in 5 to 10 cm thick shear zones parallel to the S1-2 foliation. Grade ranges from 0.03 to 3.8 g/t Au.
In sulphide-rich BIF, gold grains vary from 0.04 to 0.05 mm across, and are hosted as inclusions within, and in 'cavities' in porous grains of, pyrite and arsenian-pyrite. In silicified zones gold may be 'free', included in pyrite, arsenopyrite and sphalerite, along arsenopyrite edges, and as trails in pyrite.
The ages of hydrothermal monazite grains in a mineralised mafic volcanic rock are interpreted to suggest mineralisation occurred at 2730 ±42Ma (U-Pb SHRIMP) during the G1 structural event. Younger, 2387 ±46 Ma monazite, and 518.5 ±9 Ma xenotime suggest Siderian and Cambrian overprints of the Lamego deposit. Whilst the Cambrian age reflects the late stages of the Brasiliano orogenic cycle, which is expressed at Lamego by the S3 crenulation cleavage, the Cambrian structural event is not associated with any hydrothermal alteration, and has not apparently introduced any new gold.
G1 deformation was responsible for NE-SW trending and SE dipping faults and shear zones within the F2 fold axial plane. These faults and shear zones were occupied by V1 and V2 veins, and served as hydrothermal fluid pathways during the Archaean (Lobato et al., 2007, 2001). Interaction between hydrothermal fluids and host rocks resulted in the precipitation of hydrothermal minerals such as quartz, pyrite, carbonate, sericite and gold. The richest orebodies represent the dilational loci where the mineralising fluids were concentrated during folding. As a consequence, high-grade gold lenses/shoots were preferentially hosted in F2 hinge zones and boudins. As such, the F2 fold axes then also control the plunge of the orebodies (Martins et al., 2016). These high-grade gold lenses/shoots have a spheroidal pattern, focussed on the hinge zones of F2 reclined folds where they coincide with the S1-2 foliation. Lower-grade gold lenses are controlled by pinch and swell, and local quartz boudins. The latter have two orthogonal directions, one to the NW-SE and the other to the NE-SW, thereby defining 'chocolate-tablet' style boudinage.
Reserves and Resources
Total Mineral Resources, at 31 December, 2009, prior to mining (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2009) were:
- 6.63 Mt @ 5.45 g/t Au, for 36.14 tonnes of contained gold.
Total Mineral Resources, and Ore Reserves, at 31 December, 2011 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2011) were:
Proved + Probable Ore Reserves:
Arco da Velha - 0.16 Mt @ 3.60 g/t Au, for 0.58 tonnes of contained gold;
Cabeça de Pedra - 0.07 Mt @ 3.33 g/t Au, for 0.23 tonnes of contained gold;
Carruagem - 0.81 Mt @ 5.75 g/t Au, for 4.65 tonnes of contained gold;
TOTAL - 1.04 Mt @ 5.26 g/t Au, for 5.46 tonnes of contained gold.
Measured + Indicated + Inferred Mineral Resources:
Arco da Velha - 0.76 Mt @ 4.15 g/t Au, for 3.14 tonnes of contained gold;
Cabeça de Pedra - 1.73 Mt @ 4.54 g/t Au, for 7.84 tonnes of contained gold;
Carruagem - 2.58 Mt @ 6.76 g/t Au, for 17.45 tonnes of contained gold;
Secondary areas - 0.94 Mt @ 3.77 g/t Au, for 3.54 tonnes of contained gold;
TOTAL - 6.01 Mt @ 5.32 g/t Au, for 31.97 tonnes of contained gold.
Total Mineral Resources, and Ore Reserves, at 31 December, 2021 (AngloGold Ashanti Mineral Resource and Ore Reserve Report, 2021) were:
Proved + Probable Ore Reserves:
Arco da Velha - 0.10 Mt @ 1.86 g/t Au, for 0.19 tonnes of contained gold;
Carruagem - 0.77 Mt @ 2.80 g/t Au, for 2.15 tonnes of contained gold;
Secondary areas, Queimada - 0.49 Mt @ 2.99 g/t Au, for 1.45 tonnes of contained gold;
TOTAL - 1.36 Mt @ 2.80 g/t Au, for 3.79 tonnes of contained gold.
Measured + Indicated + Inferred Mineral Resources:
Arco da Velha - 1.11 Mt @ 2.20 g/t Au, for 2.43 tonnes of contained gold;
Cabeça de Pedra - 2.07 Mt @ 2.84 g/t Au, for 5.90 tonnes of contained gold;
Carruagem - 5.04 Mt @ 3.43 g/t Au, for 17.31 tonnes of contained gold;
Secondary areas, Queimada - 1.26 Mt @ 3.56 g/t Au, for 4.46 tonnes of contained gold;
Secondary areas, Arco NE - 0.85 Mt @ 2.72 g/t Au, for 2.32 tonnes of contained gold;
Open pit, Arco da Velha, - 0.93 Mt @ 1.02 g/t Au, for 0.95 tonnes of contained gold;
TOTAL - 11.26 Mt @ 2.96 g/t Au, for 33.36 tonnes of contained gold.
NOTE: Mineral Resources are inclusive of Ore Reserves.
This summary is largely drawn from Martins et al. (2016) and from the AngloGold Ashanti Mineral Resource and Ore Reserve Reports (2009, 2011 and 2021).
NOTE: The map below erroneously shows the location of the Espiritu Santo open pit. This mistake is to be remedied.
The most recent source geological information used to prepare this decription was dated: 2021.
Record last updated: 9/1/2023
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Lamego
|
|
|
|
Martins, B.S., Lobato, L.M., Rosiere, C.A., Hagemann, S.G., Santos, J.O.S., Villanova, F.L.S.P., Figueiredo e Silva, R.C., Lemos, L.H.A., 2016 - The Archean BIF-hosted Lamego gold deposit, Rio das Velhas greenstone belt, Quadrilatero Ferrifero: Evidence for Cambrian structural modification of an Archean orogenic gold deposit: in Ore Geology Reviews v.72, pp. 963-988.
|
Martins, B.S., Rosiere, C.A., Lobato, L.M., Figueiredo e Silva R.C., Baars, F.J., Tschiedel, M.W., Oliveira, H. and Penha, U.C., 2011 - Mineralization Control of the Lamego Gold Deposit, Sabara, Quadrilatero Ferrifero, Minas Gerais, Brazil: in Proceedings of the Eleventh Biennial SGA Meeting, 11th SGA Biennial Meeting Lets Talk Ore Deposits, Antofagasta, Chile, 26-29 September 2011, Proceedings, pp. 583-585.
|
Morales, M.J., Figueiredo e Silva, R.C., Lobato, L.M., Gomes, S.D., Gomes, C.C.C.O. and Banks, D.A., 2016 - Metal source and fluid-rock interaction in the Archean BIF-hosted Lamego gold mineralization: Microthermometric and LA-ICP-MS analyses of fluid inclusions in quartz veins, Rio das Velhas greenstone belt, Brazil: in Ore Geology Reviews v.72, pp. 510-531. doi.org/10.1016/j.oregeorev.2015.08.009.
|
|
Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
