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Moeda Conglomerate |
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Minas Gerais, Brazil |
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Au U
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Moeda Formation comprises a late Archaean to early Palaeoproterozoic unit of gold and uranium bearing pyritic cobble conglomerates that overlie deformed Archaean rocks in the Quadrilatero Ferrifero district, near Belo Horizonte in Minas Gerais, Brazil.
For a detailed description of the geological setting of the Moeda Formation, see the Quadrilátero Ferrífero District Gold - Geological Setting record.
This sequence lies on the southern margin of the Archaean São Francisco Craton. The Moeda Formation is found at the base of the Minas Supergroup, below the major exploited iron formations of the district, but unconformably overlies the older Rio das Velhas Supergroup which embraces the major iron formation hosted lode gold deposits of the district (eg. Morro Velho). It rims the Moeda and Gandarela synclines and other areas of outcrop of the Minas Supergroup, as illustrated in the main image in the record lined above.
The palaeo-environment comprises an upland fluvial drainage system in the north, leading southward to an alluvial fan and braided plain depository. Heavy mineral concentrates composed predominantly of coarse rounded pyrite, are concentrated in the matrix of cobble conglomerate that occur in the form
of discrete bars and sheets.
The lowest units in the Moeda Formation stratigraphy unconformably overlie the Nova Lima Group, and are sheet like pebbly diamictites, composed of angular quartz, chert, phyllite and yellow felsic lava clasts supported in a sheared argillaceous matrix. These diamictites are interpreted to represent mass flow conglomerates in a subaerial environment, and carry patchy 1 to 2 g/t Au grades over thicknesses generally of <1 m. They are overlain by oligomictic, cobbly, large-pebble conglomerates that fine upwards into coarse grained, trough cross-bedded quartz arenites. These represent channelised conglomerate-arenite couplets, the result of waning flow after flood deposition. They comprise increments of deposition that have accumulated into sheets up to 12 m thick, but generally averaging ~2 m. The conglomerates occur in channels with curved bases carrying very well rounded quartz clasts up to 30 cm across, indicating considerable maturity. The sedimentary form of these deposits are interpreted to suggests a braided steam gravel regime in water depths of less than 1 to 2 m.
To the south and east, the basal conglomerates are laterally extensive, whilst up the palaeo-slope to the north and west they are confined to discrete channels or wide, shallow valleys, containing individual conglomerate sheets up to 200 m wide and 600 m long. Heavy mineral concentrations, largely composed of rounded pyrite up to 30 mm, but averaging 5 mm in diameter, are found as bed load lag deposits at the base of the conglomerate lenses and as winnowed concentrations at the top of some conglomerate beds. Where the intervening arenaceous stages have been eroded, these pyrite concentrations help define the individual conglomerate beds. Gold mineralisation is associated with the rounded pyrite concentrations, especially at the basal contact, but also in the winnowed upper accumulations. In addition, trace, thin kerogen seams and granulated kerogen are evident at the base of the first conglomerate band. This lower conglomerate band thickens considerably to the south and east, fining upwards into an arenite that is up to 100 m thick that contains other less extensive conglomerate bands that may be poorly mineralised. In the north also, the basal conglomerates are locally overlain by a thin black siltstone several tens of cm thick interbedded with dark quartzite.
All of these conglomerates and arenite comprise the basal unit I, which is overlain by Unit II, a white, sheet-like, fine grained quartzite that is up to 30 m thick that fills and levels off the underlying channel topography. It is massive, although it does also appear to contain in-distinct planar ripple marks and trough cross-bedded structures locally. It has been interpreted to represent a transgressive, shallow marine setting. Unit II is overlain every where by unit III, the lower part of which contains numerous beds of oligomictic small to large pebble conglomerate that are mineralised with fine grained pyrite, and contain 4 to 5 x the radioactive background of the remainder of the group, but is not economic. Unit III is ~140 m thick to the south, but thins to the north.
The mineralised conglomerates outlined above are predominantly mineralised with sulphides, 95% of which is pyrite, occurring as partially rounded cubes up to 8 mm across, to well rounded spheroids composed of amorphous, concentric layered and porous masses up to 30 mm in diameter, as well as euhedral authigenic crystals. At surface these all oxidise to limonite, or are removed to leave large pock-marks. Other sulphides include arsenopyrite, also occurring as rounded grains, chalcopyrite, pyrrhotite, pentlandite, gersdorffite and covellite, usually occurring as inclusions in pyrite. Gold grains are generally <20 µm across and are included in pyrite. High grade porous pyrite contains remnant covellite after chalcopyrite. Silver generally comprises 5 to 10% of the gold particles, whilst mercury is ~2%. The rounded pyrite is generally more abundant and very coarse grained where gold is present, usually at the unconformable basal contact or at well defined scour surfaces. Large, shiny, rounded pyrite dominates over the more porous to bronze coloured pyrite. Black, fine grained kerogen granules and possible uraninite are also associated with well mineralised intervals, whilst fine gold visible under a hand lens is rare. Well mineralised layers may typically be 25 cm thick carrying 10 to 20 g/t Au and sometimes higher. The U:Au ratio is irregular, but tends to average between 40 and 50 over the total mineralised interval.
Although the mineralised Moeda Formation is extensive, to date no zones of sufficient grade to support economic extraction of gold and/or uranium have been established. The unit however has been a source of significant quantities of eluvial and alluvial gold.
The information in this summary is largely drawn from Minter, Renger and Siegers (1990)>
The most recent source geological information used to prepare this decription was dated: 1990.
Record last updated: 3/3/2023
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.
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Guimaraes, F.S., Freitas, M.E., Rios, F.J. and Pedrosa, T.A., 2019 - Mineralogical characterization and origin of uranium mineralization in Witwatersrand-like metaconglomerate of the Moeda Formation, Quadrilatero Ferrifero, Brazil: in Ore Geology Reviews v.106, doi.org/10.1016/j.oregeorev.2019.01.016.
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Minter W E L, Renger F E, Siegers A 1990 - Early Proterozoic Gold placers of the Moeda Formation within the Gandarela Syncline, Minas Gerais, Brazil: in Econ. Geol. v85 pp 943-951
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