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Bacuri
Para, Brazil
Main commodities: Cu


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The Bacuri IOCG copper deposit is located ~16 km east of the Sossego IOCG deposit, and ~40 km to the south of the Carajás townsite in the state of Para, Brazil.

For details of the regional and local setting see the main Carajás IOCG Province record. See also the record for the nearby, similar Jatoba, Castanha, Bacaba and Visconde deposits.

Geology

Like Sossego, Bacuri lies within the broad WNW-ESE trending regional Canaã dos Carajás shear zone, and in a NE-SW subsidiary transcurrent fault zone in the Mesoarchean basement, previously attributed to the Xingu Complex (DOCEGEO 1988; Machado et al., 1991). The host rocks include the:
The Serra Dourada Granite is greyish to pinkish, medium to coarse grained and predominantly isotropic, although it is also locally foliated. It includes subordinate pegmatitic and micrographic facies, and is cut by aplite dykes. It has been dated at ~2.84 Ga (U-Pb zircon; Moreto et al. 2011; Feio et al. 2013), and is classified as a syenogranite with an average composition of 35% quartz, 45% K feldspar and 20% plagioclase, with minor biotite (Moreto et al., 2011; Feio et al., 2013).
The Bacuri Porphyry, which is intrusive in the Serra Dourada granite and is the principal host rock in the Bacuri mineralisation. Its occurrence is restricted to a NE-SW fault, and it is commonly strongly mylonitised, although local relicts of igneous textures are evident. Ribbon structures and eye-shaped feldspar porphyroclasts are common. Where preserved, it is grey and porphyritic with a fine-grained phaneritic matrix composed of >50% quartz, ~15% K feldspar and ~20% plagioclase with a dacitic composition. The plagioclase includes ~15% idioblastic to subdioblastic phenocrysts of albite that are up to 3 mm long, show corrosion texture and are mostly fractured.
Gabbro dykes, which are dark green and have relicts of subophitic texture, defined by plagioclase laths and interstitial poikilitic augite.
Late quartz-feldspar porphyry dykes, which are subvolcanic rocks with quartz and K feldspar phenocrysts cross-cut hydrothermal alteration zones and are only incipiently altered. They are black or dark grey and isotropic to foliated, with a fine- to very fine-grained (>0.1 mm) matrix composed of quartz, alkali feldspar and plagioclase with rhyodacite to dacite modal composition. Blue quartz, 0.2 to 4 mm megacrysts with bi-pyramidal terminations, embayment, and undulose extinction are also evident as are local 1 to 3 mm subhedral plagioclase crystals.
Centimetre- to metre-thick quartz porphyry dykes cut all of the country rocks. They are isotropic, pink, and have a porphyritic texture within a fine-grained equigranular matrix. Irregular to vermicular granophyric intergrowths of quartz and alkali feldspar are common. The rock is composed of 1 to 4 mm phenocrysts and a fine-grained matrix that contains quartz, potassium feldspar and plagioclase, with minor biotite, magnetite and chalcopyrite. The modal composition varies from rhyodacite to rhyolite. These quartz porphyry dykes are unaffected by hydrothermal alteration and cut hydrothermally altered Serra Dourada Granite.

The Serra Dourada granite, the Bacuri Porphyry and gabbro dykes are all strongly foliated where they lie within the NE-SW subsidiary transcurrent fault. All three lithotypes are strongly foliated within the NE-SW subsidiary transcurrent fault, and exhibit a NE-SW subvertical mylonitic foliation and evidence significant grain-size reduction compared to the least deformed protoliths. In addition, the fine-grained matrix in these mylonites is predominantly composed of oriented hydrothermal minerals, re-crystallised quartz and intensely strained flattened grains of the pre-existing feldspar.

Geology of the central southern Carajas Copper Belt

Alteration

The Bacuri hydrothermal alteration is mainly controlled by the NE-SW subsidiary transcurrent fault zone, which is the locus of the copper mineralisation. Distal to the fault zone, the protoliths are still be recognisable. The hydrothermal alteration is influenced by the nature of host rock, for example, where hydrothermal biotite predominantly occurs in mafic rocks and K feldspar is in felsic host rocks. In proximal zones, pre-existing features of the host rocks are largely obliterated and different protoliths are developed, controlled by the hydrothermal paragenesis.

Early stage alteration is mainly identified in the distal portions of the deposit. whilst in proximal zones, overprinting of late pervasive alteration stages and telescoping prevail. Distal to the main mineralisation, the Serra Dourada granite predominates, affected by early pervasive sodic alteration with albite and minor scapolite, which replaces the igneous K feldspar and plagioclase respectively. Proximal to the main mineralisation, the main lithotype is the Bacuri Porphyry, although the Serra Dourada granite and gabbro are also recognised. A similar paragenetic sequence is recognised to that in the distal Serra Dourada granite, although hydrothermal alteration is more intense and pervasive, resulting in more strongly modified lithotypes. Zones of hydrothermal biotite-scapolite and chlorite predominate. Late quartz-feldspar porphyry dykes also cross-cut intensely altered rocks in proximal zones, but only have epidote veins and narrow haloes of K feldspar.

The distal early pervasive albite alteration results in the formation of albite-I to produce a white to pinkish colour in the altered Serra Dourada granite and Bacuri Porphyry. Hydrothermal albite-I with chessboard texture, either selectively replacing the igneous K feldspar, or infills veinlets. Scapolite-(magnetite) alteration and veining is intense and widespread, occurring in both distal and proximal alteration zones. In distal zones, early scapolite-I replaces igneous plagioclase along crystals edges and cleavage planes in the Serra Dourada granite and gabbro, and is also found in the quartz–feldspathic matrix of the Bacuri Porphyry. Scapolite-I infill veinlets are also common. Towards the centre of the deposit, veins of scapolite ±magnetite-I ±quartz that are up to 10 cm thick are also evident. The scapolite-1 crystals are fibrous, coarse-grained, up to 2 mm long, with undulose extinction. These are cross-cut by fine-grained scapolite-II crystals in deformed sections of the veins. All scapolite generations are indicated as having a marialite composition with low meionite content. Magnetite-I is closely related to scapolite alteration and occurs as tiny crystals on the edges of, and as inclusions within scapolite crystals in veins. Scapolite replacement by later hydrothermal mineral phases is also common. Networks of biotite veinlets cross-cuts scapolite crystals, which are also replaced by K feldspar, and thereafter, by fine-grained muscovite.

Potassic alteration with K feldspar and biotite overprints the sodic alteration. Incipient potassic alteration with biotite is recognised in least-altered Serra Dourada granite and Bacuri Porphyry, and is characterised by biotite-I crystals primarily in veinlets and in the interstices of the igneous quartz-fedspathic matrix of both host rocks. Biotite is also common in altered gabbro dykes, mainly replacing augite. Biotite-rich alteration is ubiquitous and occurs in >300 m wide zones that are crosscut by late alteration stages. Biotite alteration with accompanying scapolite is more intense toward shear zones, in which the rocks show S-C structures accompanied by crystal stretching and comminution of quartz, scapolite, and biotite. In zones proximal to mineralisation, biotite-scapolite alteration grades from selective and in fissures, to being pervasive, obliterating the protolith textures. It occurs as a mineral assemblage of biotite-I + scapolite-III ±magnetite-II ±quartz ±allanite ±monazite ±fluorite ±rutile, and commonly has a mylonitic foliation. The latter assemblage is grey to brown, due to the predominance of biotite-I, with very common white spots that are scapolite crystals. The mylonitic foliation is defined by the orientation of fine-grained hydrothermal biotite crystals and stretched quartz crystals. Scapolite-III crystals are deformed and commonly have pressure shadows. Networks of scapolite veinlets are also evident in the biotite-scapolite alteration zones. The second generation magnetite-II is broadly overlaps the scapolite-biotite rich rocks, occurring as subdioblastic to idioblastic crystals disseminated in the rock and concordant to the mylonitic foliation. A moderate increase in the size of magnetite crystals is accompanied by an increase in the biotite amount. In strongly mylonitised zones, deformed magnetite crystals are variably martitised and are associated with tabular hematite crystals.

Chlorite-(epidote) alteration and fissure veining is incipient in the distal alteration zones, but overall is the most common and well developed alteration in the Bacuri deposit, and is strongly controlled by the NE-SW fault zone. In the distal zones, chlorite-I replaces the biotite-I crystals, or occurs in veinlets and interstices of igneous crystals of the Serra Dourada granite but mainly in the Bacuri Porphyry. Inwards, towards the main mineralisation, the amount and size of chlorite veinlets increases and the alteration becomes pervasive. Where intensely altered, there is complete replacement of all igneous minerals by aggregates of fine-grained chlorite crystals. This alteration accompanies an increased intensity of the mylonitisation of the host rocks, resulting in strongly foliated rocks composed mainly of chlorite. The common mineral assemblage is chlorite-I + K feldspar-II + magnetite-III ± chalcopyrite-I ±quartz ±epidote-group minerals ±apatite ±tourmaline ±monazite ±titanite. K feldspar-II strictly surrounds or accompanies chalcopyrite, magnetite-III and epidote crystals in chloritic mylonites. Limited tourmaline, most likely schorl, is found in clusters of idioblastic crystals within the fronts of chlorite alteration and copper mineralisation. Epidote-group minerals, such as epidote-I, clinozoisite and allanite are specially associated with chlorite, K feldspar-II and chalcopyrite, and are observed disseminated parallel to the foliation and in veinlets.

Pervasive silicification and hydrothermal quartz veins cross-cut and are overprinted by the same hydrothermal alteration assemblages, implying recurrent silicification and veining. Distal to the deposit, silicification is less intense and largely associated with biotite-scapolite alteration. Whilst quartz veins are evident distal to mineralisation, silicification fronts predominate. However, in proximal locations, the host rocks are cut by quartz-I veins and subdioblastic quartz-I crystals up to 2 cm long grow in irregular alteration fronts and pockets. Quartz-II crystals in silicified zones and in early veins are strongly deformed, show subgrain formation and ribbons, and the veins are brecciated. Coarse-grained flaky muscovite crystals with kink bands, fluorite and chalcopyrite-II are common in brecciated quartz veins. Late milky quartz-II veins with chalcopyrite-III and minor chlorite commonly cross-cut the chlorite mylonites and brecciated quartz veins of previous generations. Coarse, up to 0.5 cm long biotite-II crystals fringe late quartz veinlets.

Late Hydrolytic alteration is represented by large amounts of greenish to whitish flakes of fine-grained muscovite with associated hematite replacing previous K feldspar, albite and biotite, whilst selvages of coarse-grained muscovite occur close to deformed quartz veins. This muscovite formation is late, and occurs as external haloes in relation to chalcopyrite-bearing quartz-(feldspar-muscovite) veins.

Late epidote veining occurs as zoned veins and veinlets with open-space filling textures that cross-cut biotite-scapolite, chlorite and even hydrolytic alteration. These late veins epidote-II, K feldspar-III, chalcopyrite-IV, calcite, chlorite-II and albite-II form crystals up to 3 mm long. K feldspar-III crystals are cloudy and euhedral, differing from the clear potassium feldspar-II and albite.

Carbonate veining, that occurs as ~2 mm thick veinlets of late calcite-(hematite) crystals cross-cut the previous stages of hydrothermal alteration listed above. Thin calcite veinlets cut quartz-muscovite and milky quartz veins.

Mineralisation

Copper mineralisation occurs as small lenses, veins and breccia bodies, which are spatially related to a NE-SW-striking transcurrent fault cutting the WNW-ESE trending regional Canaã shear zone. Copper minerals are found in i), chlorite-(epidote) alteration zones, where the copper mineralisation is represented by disseminated chalcopyrite crystals that are stretched and oriented parallel to the mylonitic foliation, as well as infilling fractures in hydrothermal apatite; ii), silicified zones and deformed/brecciated quartz-(muscovite-fluorite) veins, where chalcopyrite formed under brittle conditions; iii), undeformed milky quartz veins; and iv), late zoned K feldspar-epidote veinlets with well-developed open-space filling textures.

Chalcopyrite is associated with magnetite, monazite, epidote, allanite, clinozoisite, albite, chlorite, pyrite, and minor melonite, altaite, galena, and cheralite [(Ce, Ca, Th)(PO4)2].

Chalcopyrite-I is surrounded by clear K feldspar-II, and locally, infills fractures in hydrothermal apatite. Magnetite-III and pyrite are associated with chalcopyrite in centimetre-wide mineralised pockets. Andradite occurs as idioblastic inclusions in chalcopyrite crystals, whilst Melonite (NiTe
2, altaite (PbTe), galena and cheralite [CaTh(PO4)2] occur as small inclusions in chalcopyrite crystals that can only be identified with an SEM.

The main mineralisation are related to brecciated silicified zones and deformed quartz-(muscovite-fluorite) veins within intervals of intense chlorite alteration. Chalcopyrite-II and pyrite not only commonly infill network of fractures in deformed quartz and cleavage planes of muscovite, but are also concentrated in massive sulphide zones that replace the brecciated silicified rock. Chalcopyrite-III is also seen in late milky quartz veins and veinlets with cloudy K feldspar-III and epidote-II. The latter has comb and open-space filling textures, characterised by crustiform or symmetrical banding. Malachite [Cu
2CO3(OH)] represents the supergene copper-rich mineral phase, which appears at surface in the chlorite mylonites and in quartz veins.

The earliest hydrothermal mineral associations from the Bacuri deposit is dated at ~2.70 Ga, similar to the mineralization age of 2.71 to 2.68 Ga of the Sequerinho orebody at the Sossego deposit (da Costa Silva et al., 2015; Moreto et al., 2015).

Reserves and Resources

No reserve or resource estimations have been found during research for this record. However, it appears to be a small deposit, possibly representing a satellite to Sossego.

The information in this record is predominantly drawn from Melo et al., 2014, and to a lesser degree from Moreto et al., 2015, both of which are cited below.

The most recent source geological information used to prepare this decription was dated: 2015.    
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:
Melo G.H.C., Monteiro L.V.S., Moreto C.P.N., Xavier, R.P. and Silva, M.A.D.,  2014 - Paragenesis and evolution of the hydrothermal Bacuri iron oxide-copper-gold deposit, Carajas Province (PA): in    Brazilian Journal of Geology,   v.44, pp. 73-90. doi: 10.5327/Z2317-4889201400010007.
Moreto, C.P.N., Monteiro, L.V.S., Xavier, R.P., Creaser, R.A., DuFrane, S.A., Melo, G.H.C., Delinardo da Silva, M.A., Tassinari, C.G. and Sato, K.,  2015 - Timing of multiple hydrothermal events in the iron oxide-copper-gold deposits of the Southern Copper Belt, Carajas Province, Brazil: in    Mineralium Deposita   v.50, pp. 517-546. doi:10.1007/s00126-014-0549-9.


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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