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Para, Brazil
Main commodities: Ni

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The GT-34 nickel prospect, which shares many of the characteristics of the Carajás IOCG deposits, is located ~12 km to the SW of the Sossego copper-gold deposit, 3 km from the township of Cedere III, and ~55 km SW of Parauapebas, in the state of Para, Brazil.

The deposit was first indicated as a highly conductive GEOTEM anomaly in an airborne survey conducted for CVRD (now renamed Vale) in 1999. Follow-up soil geochemistry and ground geophysics led to the identification of sulphide-rich breccias. In 2003, detailed ground geophysics and systematic drilling delineated an irregular, NE trending metasomatic zone that was ~1.5 km long and up to 500 m deep (Siepierski, 2008). Resources and grades were not available in the public domain in 2021.

For an overview of the regional setting, the stratigraphic units mentioned below and a geological map (Fig. 3) showing the location of the deposit, see the Carajás IOCG Province record.

The host sequence to the deposit predominantly comprises granitic orthogneiss, with local granodioritic to tonalitic orthogneisses which belong to the migmatites and bimodal syn-tectonic orthogneisses of the 3.0 to 2.83 Ga Mesoarchaean Xingu Complex. Both the deposit and other Mesoarchaean biotite and hornblende greenstones occur along a NE-SW shear zone associated with the regional Canaã Fault System. About 5 to 10 km to the south, separated by undifferentiated granitic gneisses, including Xingu Complex and Neoarchaean granitic gneisses, the host sequence is intruded by a large mass of diopside-norite that belongs to the Mesoarchaean Pium Complex. Smaller lenses of Neoarchaean mafic to ultramafic intrusive are also mapped, as are finer grained, presumably younger, mafic dykes. Approximately 7 km to the north, these rocks are overlain above a major south vergent thrust by rocks of the Neoarchaean Itacaiúnas Supergroup. The eastern limit of the prospect is marked by an ~5 x 8 km exposure of the Palaeoproterozoic Rio Branco granite, which crosscuts the regional foliation, and is part of the ~1.88 Ga Serra dos Carajá Suite.

GT-34 is a pentlandite-pyrrhotite-apatite-LREE-rich nickel deposit with rare iron oxides. It occurs along the NE-SW trending sub-vertical shear zone mentioned above. The deposit and immediate host rocks have undergone progressive Mg-alkalic alteration, and is predominantly hosted by granitic and locally by granodioritic to tonalitic orthogneisses that exhibit an incipient foliation. High-resolution SIMS U-Pb geochronology returned an age of 2828 ±4 Ma for zircon grains inherited from the host orthogneisses. The early stage of alteration is sodic, occurring as a scapolite-orthopyroxene assemblage. Scapolite occurs as fine to medium granoblastic euhedral crystals, whilst orthopyroxene is present as fine to coarse euhedral crystals and forms lenses <1 m long, enveloped by scapolite. This early assemblage is only partially preserved within the succeeding extensive pervasive calcic hornblende-chlorapatite ±plagioclase alteration zones that form as crosscutting fine veins and irregular replacive alteration halos. Hornblende is widespread, representing the main phase, and is locally associated with oligoclase and clinopyroxene. Oligoclase occurs as an alteration product of the scapolite of the earlier stage.

The nickel mineralisation is typically associated with zones where the calcic hornblende-chlorapatite ±plagioclase alteration is dominant, although it is not always restricted to those zones, and may cut the early scapolite-orthopyroxene alteration and, less commonly, partially preserved granite to tonalite gneisses. It predominantly occurs as discordant centimetre to metre scale nickel sulphide matrix-supported breccias, where a pentlandite-pyrrhotite rich matrix envelopes rounded clasts that are predominantly composed of hornblende and chlorapatite, with minor scapolite and orthopyroxene. The breccias are also variably clast supported with no apparent spatial control between the two typed. Nickel mineralisation also occurs as discontinuous, millimetre to centimetre scale zones of irregular veins to net-textured stockworks containing chalcopyrite-pentlandite proximal to the breccias. Chalcopyrite is relatively more common within veined to net-textured mineralisation than it is in the breccias. Pentlandite forms as 0.2 to 3.0 mm euhedral crystals with a marked octahedral partition along with 0.1 to 0.5 mm pyrrhotite crystals. Pyrrhotite contains abundant inclusions. Melonite, thorite and monazite are found at the contacts between sulphides and chlorapatite. Minor chalcopyrite also commonly occurs as fine filaments along cleavages in hornblende and around the borders of clasts. Palladium, Te, Ag and Au are present in small amounts associated with pentlandite and pyrite. Magnetite occurs as 0.2 to 0.5 mm euhedral to subhedral crystals typically associated with pyrrhotite and chalcopyrite. Pyrite is not common, but when present occurs as euhedral crystals associated with pyrrhotite. Chlorapatite occurs associated with the sulphide matrix, forming fine to coarse rounded crystals and is locally found as coronas around the breccia clasts. Based on whole-rock geochemistry, Siepierski (2008) observed a P and LREE enrichment associated with the Ni mineralisation. The mineralisation is reworked successively by later alteration.

Phlogopite ±talc ±actinolite alteration with late-stage veinlets crosscut both the earlier stage alteration zones and the mineralisation. This constitutes the first phase of potassic alteration, and begins with a phlogopite, talc and magnetite assemblage with associated fine actinolite needles and occurs as irregular veins, with chalcopyrite-pyrrhotite-magnetite commonly observed where the alteration intersects Ni mineralisation. Two sets of late-stage veinlets are recognised by Garcia et al., 2017). The first of these is referred to as 'K-mag alteration'. It cross-cuts and reworks the earlier alteration and mineralisation and is accompanied by irregular veins with fuzzy boundaries that commonly include chalcopyrite-pyrrhotite-magnetite where intersecting the Ni mineralisation. These veins are characterised by K feldspar, epidote, chlorite and calcite, and generally only persist for no more than 50 cm. This 'K-mag' phase is followed by a 'K-hem' alteration that is even more restricted. Veins are all <10 cm long and have sharp, well-defined margins. They are characterised by red K feldspar and albite that are both rich in hematite inclusions, with quartz, chlorite, epidote and calcite being the dominant parageneses. When K-hem alteration cuts the main nickel mineralisation sulphides are reworked, as with the phlogopite and 'K-mag alteration'. During this phase, sulphides are remobilised forming dominant millerite, hydroxyapatite and pyrite with local occurrence of galena and sphalerite. Hydroxyapatite is euhedral, occurring as fine, clear to white crystals, in contrast to the coarse grey to translucent chlorapatite. Both sets of veins carry varying amounts of epidote, chlorite, calcite and fluorapatite.

The timing of the mineralisation is indicated by the age of altered zircon grains from the scapolite-orthopyroxene and phlogopite ±talc ±actinolite alteration zones, defining an interval between 2751 and 2720 Ma (High-resolution SIMS U-Pb zircon). These ages are similar to that of the IOCG deposits in the Carajás IOCG Province and coeval with bimodal magmatism in the area. The orthopyroxene-marialite mineral chemistry supports a metasomatic crystallisation at high temperatures of >700°C and pressures between 5 and 7 kbar. Mineralogical and geochemical considerations suggest a metasomatic origin for the Ni sulphide mineralisation, rather than magmatic, consistent with the predicted P-T fluid conditions of the hydrothermal alteration. The detail above suggest GT-34 represents a Ni-rich variety of the IOCG family of deposits. The temperature and pressure estimates would suggest GT-34 represents the hottest and probably deepest IOCG-related metasomatic alteration known to date from the Carajás Mineral Province (Garcia et al., 2020). It has many similarities with the Jaguar deposit, some 110 km to the west in a similar setting.

No resource estimates can be found in publicly available sources as of late 2021.

The information and interpretations in this summary are drawn from Garcia et al. (2017) and Garcia et al. (2020).

The most recent source geological information used to prepare this decription was dated: 2020.    
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.

  References & Additional Information
   Selected References:
Garcia, V.B., Della Giustina, M.E.S., Oliveira, C.G. and Huhn, S.R.B.,  2017 - The GT-34 Nickel prospect: Deep alteration zones and evidence for magmatic fluids contribution at Carajas IOCG system: in   15th Amazon Geology Symposium - A different view of the geosciences for the sustainability of the Amazon, 25 to 29 September, 2017, Belem, Para, Brazil, 15 Simposio de Geologia da Amazonia   Proceedings 4p.
Garcia, V.B., Schutesky, M.E., Oliveira, C.G., Whitehouse, M.J., Huhn, S.R.B. and Augustin, C.T.,  2020 - The Neoarchean GT-34 Ni deposit, Carajas mineral Province, Brazil: An atypical IOCG-related Ni sulfide mineralization: in    Ore Geology Reviews   v.127, 19p. doi.org/10.1016/j.oregeorev.2020.103773.

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.

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