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Jatoba, Castanha, Bacaba, Visconde |
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Para, Brazil |
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Cu Au Ni
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Jatobá, Castanha, Bacaba, Visconde and Bacuri copper-gold deposits/prospects are distributed within the central section of the southern Carajás Copper Belt in Para State, and are located within 4 to 16 km to the north, NE and east of the major Sossego - Sequeirinho IOCG deposit cluster.
This group of deposits/prospects is part of the larger cluster that includes Sossego - Sequeirinho, Alvo 118, Bacuri, Cristalino, Jatobá, Borrachudo, Pedra Branca and Visconde that define the Carajás Copper Belt. They are spatially related to regional fault zones in the southern sector of the Carajás Mineral Province (Xavier et al., 2012; 2010). These deposits, although hosted in a variety of igneous and metamorphic rocks of differing ages, show similar hydrothermal alteration features, such as early sodic alteration, extensive pervasive potassic alteration haloes, high magnetite content, structural control of orebodies, and rare-earth element (REE) enrichment in the ore (Augusto et al., 2008, Monteiro et al., 2008, Xavier et al., 2010, Pestilho, 2011). Never-the-less, individual deposits also have differences, which are discussed below.
This record concentrates on the
Jatobá,
Castanha,
Bacaba and
Visconde
deposits which are a group of relatively minor deposits distally peripheral to the major Sossego - Sequeirinho cluster.
The Bacuri
deposit is a little further to the east, and is the subject of a separate record.
While some of these prospects may not contain economic mineralisation, they are described here as the information available on their geology, alteration and mineralisation contributes to the understanding of IOCG deposits in the Carajás Mineral Province and beyond.
For details of the regional and local setting see the main Carajás IOCG Province record, particularly Fig. 3 in that record, and the image below in this record.
Jatobá
The Jatobá deposit is located ~2.5 km to the NNE of the Sossego mine. Like a number of other IOCG style deposits in this part of the Carajás District, it is notable for its hydrothermal nickel-rich mineralisation. It is hosted by the Neoarchaean mafic and acid volcanic rocks of the Itacaiúnas Supergroup and is associated with pervasive and intense pre-, syn- and late-shearing hydrothermal alteration associated with subsidiary structures related to the WNW-ESE trending Canaã dos Carajás shear zone. This shear zone marks the contact between the Mesoarchean TTG orthogneisses, granitoids and greenstone belts to the south, and the Neoarchaean volcano-sedimentary regime of the the Itacaiúnas Supergroup to the north (see image below).
Mineralisation at Jatobá is hosted by Itacaiúnas Supergroup rhyodacites and felsic volcaniclastic breccias in the footwall zone, and amygdaloidal
basalts with associated mafic tuffs in the hanging wall. These rocks are overprinted by a high angle, WNW trending foliation, although heterogeneous deformation, results in isotropic structure and a low strain regime. All of the volcanic rocks are crosscut by sets of dolerite and gabbro dykes. Meta-ultramafic rocks are found close to the southern contact between felsic volcanic rocks and the Mesoarchean orthogneisses. All of the host rocks are hydrothermally altered, decreasing in intensity outwards from the high strain zones.
The least-altered of the host rhyodacite is light grey, fine-grained, isotropic to massive, and porphyritic. Phenocrysts of blue, locally rounded, bipyramidal quartz and plagioclase are set in a matrix of fine-grained, equigranular to in-equigranular hydrothermal quartz, biotite-I and apatite-I. However, the plagioclase phenocrysts are partially to totally replaced e.g., by biotite-I. In the Canaã dos Carajás shear zone the rhyodacite has a mylonitic foliation and S-C fabric, with almond-shaped albite and scapolite porphyroclasts surrounded by pressure shadows, quartz-ribbon, undulose extinction, subgrain formation, and grain boundary migration into the quartz phenocrysts.
The hanging wall amygdaloidal basalt is dark grey, isotropic and aphanitic, with an intergranular and intersertal texture composed of plagioclase laths, as well as relict tabular plagioclase crystals with well-developed polysynthetic twinning. Amygdales are mainly filled with hydrothermal biotite, quartz and euhedral magnetite, ias well as minor apatite . Where the basalts are deformed, magnetite crystals in the amygdales are surrounded by pressure shadows, with asymmetric deformation tails composed of quartz and scapolite. Strongly deformed and hydrothermally altered basalts form scapolite-biotite-hastingsite mylonites.
The Lapilli and crystal tuffs have been intensely hydrothermally altered. The lapilli tuffs are matrix-supported, with angular clasts that are <6 cm across, whilst the crystal tuffs contain >75%, often deformed, plagioclase crystals. The latter are dark grey or brown due, replaced by fine-grained biotite-I crystals, even in undeformed rocks. Dolerite dykes crosscut the volcanic and volcaniclastic rocks, and are dark green, phaneritic, fine- to medium-grained, and usually isotropic. These dykes may locally have a porphyritic texture, with plagioclase megacrysts replaced by clinozoisite and sericite. They also have a subophitic texture, characterised by cloudy, euhedral laths of sodic plagioclase, partially included in actinolite-I. Fine magmatic magnetite-I crystals with trellis texture are partially preserved in the dolerite. The actinolite-I is rimmed by ferro-pargasite. Plagioclase, in turn, is partially to entirely replaced by scapolite. Interstitial ferro-pargasite and biotite crystals make up the intergranular texture.
Hydrothermal alteration - is controlled by the Canaã dos Carajás shear zone, although an intense, pervasive alteration was also developed in weakly deformed to undeformed rocks. This alteration is zoned through distal to more proximal hydrothermal halos toward nickel and copper-(gold) mineralised zones, with recurrence of alteration stages, and telescoping. It has been divided into (Veloso, et al., 2020)
• Distal and Pre-Shearing hydrothermal alteration, that involved potassic-iron-silica; sodic; and sodic-calcic alteration:
- Potassic-iron-silica alteration, mainly comprising quartz, biotite-I, magnetite-II and apatite. Quartz, with associated biotite and Cl-apatite infills amygdales in undeformed basalts, whereas in isotropic mafic rocks, incipient silicification is accompanied by alteration of the magmatic magnetite-I, forming a trellis texture with ilmenite-I lamellae replaced by leucoxene. A similar association is seen in the rhyodacite with quartz, biotite, magnetite and apatite forming pervasive alteration fronts and vein infill. Extensive alteration zones are also developed where fine brown biotite-I is the main hydrothermal mineral, accompanied by weak silicification, with subordinated magnetite-II, quartz, Ce-allanite-I, Cl-apatite-I, K feldspar and tourmaline.
- Albite-I formation, part of a sodic phase, recognised in all host rocks, imparting a whitish-pink colouration in metamafic rocks and intense pink in rhyodacites, possibly due to very fine hematite inclusions. The albite is selective, occurring as cloudy replacement of pre-existing twinned plagioclase.
- Scapolite-I formation, representing the second expression of the sodic phase. It is the most common hydrothermal alteration in the Jatobá deposit, imparting a whitish to light cream colouration. Scapolite-I initially forms on the edges of cloudy albite-I crystals, locally, only leaving remnant albite cores. In pervasive alteration zones, scapolite is found as rounded, up to 6 cm diameter, millimetric to sub-centimetric porphyroblasts with irregular or reentrant rims, or as amoeboid-shaped crystals dispersed in the matrix of rocks. In mylonitic rocks, scapolite-I commonly is commonly found as porphyroclasts with pressure shadows.
- Ferro-pargasite formation, which represents the transition to sodic-calcic alteration, and occurs in the least altered dolerites. It has an intense bluish pleochroism and occurs as thin pseudo-prismatic crystals, replacing the borders of the actinolite. It also occurs on the borders of scapolite-I crystals.
- K feldspar formation, at the expenses of the nuclei of preexisting scapolite crystals, although in the rhyodacite, pseudomorphic albite megacrysts are commonly replaced, with associated allanite-II and clinozoisite. Potassic alteration fronts comprise fine cloudy aggregates of K feldspar
crystals, giving the rock an intense reddish-pink colouration due to fine hematite inclusions.
• Main Syn-Tectonic/Shearing hydrothermal alteration, which produced scapolite-hastingsite-biotite assemblages, controlled by the mylonitic foliation, with syn-deformation massive magnetite bodies representing proximal envelopes to the mineralised zones, overprinted by actinolite-apatite-magnetite and potassic-iron i.e., biotite-magnetite, alteration fronts.
- Scapolite-II formation rimming stretched porphyroclasts of scapolite-I which have cores that had been replaced by K feldspar and chlorite. Scapolite-II differs, in that it is limpid, compared with the cloudy appearance of scapolite-I.
- Hastingsite-I formation, mostly in mylonitic rocks of the hanging wall zone at Jatobá, predominantly within hydrothermally altered dolerite and basalts, giving the rocks a green-bluish colouration, but being only incipiently developed within the mylonitic foliation in rhyodacites. Crystals are prismatic and are aligned in the mylonitic foliation. Locally, hastingsite-I crystals pseudomorph scapolite-I and commonly occur in pressure shadows and deformational tails around porphyroclasts of the latter.
- Early coarsely crystalline magnetite-III and quartz veins and veinlets that are up to 3 cm thick. These veins crosscut foliated and intensely altered rocks, but are, in turn, commonly folded and transposed by the mylonitic foliation. The coarse magnetite-III crystals have distinct lamellae composed of ilmenite-II and hercynite. Fine cobaltiferous chalcopyrite crystals occur as inclusions within the magnetite. The recrystallised texture of quartz and the presence of micro-fractures in both quartz and magnetite are interpreted as evidence of superposed deformation. These veins also include titanite-I and ilmenite-II crystals, although the latter is commonly replaced by titanite-II and hematite, as well as subordinate brookite/rutile and pseudobrookite.
- Massive magnetite-IV, occurring as bodies related to replacement fronts that are controlled by the syn-shearing fabric. They occur as proximal envelopes to the the mineralisation along the contact zones between rhyodacites and basalts, and within the latter. These magnetite masses may be up to 60 m thick, and are composed of coarse euhedral to subhedral magnetite-IV) crystals that are commonly cut by fibrous actinolite-II and apatite-II.
- Actinolite-II formation in the footwall to mineralisation, where it selectively replaces rhyodacite along the mylonitic foliation, occurring as bundles of fibrous actinolite crystals with associated apatite, imparting a greenish colouration. Actinolite-II with associated Cl-apatite-II, Ce-allanite-III, and minor monazite-I, crosscut the massive magnetite-IV masses, closely related to the initial mineralisation stage at Jatobá. Rare fibrous actinolite-II has been found infilling older amygdales in the metamafic rocks, particularly in basalts.
- Hornblende formation in metamafic rocks, occurring as euhedral crystals elongated parallel to the mylonitic foliation, with subordinate cummingtonite and magnesio-ferri-hornblende.
- Biotite-II, Magnetite-V and K feldspar formation - biotite-II is extensively developed in all of the host rocks at Jatobá, including the felsic volcaniclastic lithotypes. It occurs as pervasive fronts of biotite-II and associated magnetite-V, overprinting the pre-shearing sodic, sodic-calcic and calcic alteration zones. It replaces the matrix in the rhyodacites that had been previously substituted by quartz, biotite-I, magnetite-II and apatite-I. The resultant alteration assemblage comprises dark brown to reddish Cl-biotite-II aggregates of fine anhedral crystals following the mylonitic fabric, accompanied by quartz, Co-magnetite-V (with minute ilmenite-III inclusions), Cl-apatite-III, Ce-allanite-IV and ilmenite-II.
• Late hydrothermal alteration, comprising fibrous scapolite veining, green biotite-scapolite-F-Cl-apatite veinlets, and chlorite-(epidote-quartz-adularia-calcite) veins, developed as follows:
- Scapolite-III formation, occurring as veins that are up to 1 m thick, composed of well developed, large, prismatic or fibrous, white to slightly creamy scapolite crystals, cutting foliated rocks, and earlier hydrothermal alteration zones. However, the scapolite-III has commonly undergone incipient retrograde alteration to sericite, pyrophyllite, chlorite, albite-II, and carbonate. These veins are proximal to mineralised intervals with associated chlorite alteration, that are characterised by chalcopyrite or Co-chalcopyrite with associated chlorite-epidote ±albite II ±calcite ±feldspar. These veins are often crosscut by late veinlets of coarsely crystalline biotite-III.
- Scapolite-IV, hastingsite-II and biotite-III formation, with quartz, and apatite-IV, which occur as replacement fronts, infilling late veins and veinlets, and overprinting fibrous scapolite-III veins. Biotite-III with quartz is also found in late barren or mineralised veins and differs from the previous biotite generations in its coarser crystals and paler brown or green colour.
- K feldspar-II formation, mainly as cloudy adularia infilling late veinlets and barren or mineralised veins, and as narrow replacement fronts, giving the rock an intense red colour due to fine hematite inclusions.
- Chlorite formation, first as chlorite-I, replacing amphibole, scapolite, K feldspar and biotite, including in poorly-deformed rocks. Chlorite-II occurs as pervasive alteration fronts and veinlet systems crosscutting previously altered rocks, and is associated with cobaltiferous-chalcopyrite that formed in the last and more extensive copper mineralising stage.
- Albite-II formation, occurring as veinlets of quartz and albite-II with minor fibrous-radiating stilpnomelane crystals.
- Calcite formation, as late veins overprinting rocks with potassic biotite-II and -III and K feldspar-II, and chlorite-I and -II alteration.
Mineralisation at the Jatobá deposit comprise swarms of vertical to sub-vertical orebodies with thicknesses ranging from 1.5 to 18.5 m, spatially related to dolerite dykes and contact zones between rhyodacites and basalts. It occurs as disseminations, massive replacement ore controlled by the mylonitic foliation, hydrothermal breccias, stockworks, veins and veinlet systems. Mineralisation was formed in four stages, as outlined below. The first two
were controlled by ductile structures related to the Canaã dos Carajás shear zone, evolving from replacement fronts controlled by mylonitic
foliation to ductile-brittle breccias related to overpressured fluids. The following two stages continued through the brittle-ductile to brittle deformational events, as follows:
• Early mineralisation stage I, which was spatially related to massive magnetite-(apatite-actinolite) masses, and has the highest nickel content. It is characterised by Ni-pyrrhotite-I, Ni-pyrite-I and subordinate Co-chalcopyrite, Ce-allanite-III, Co-pentlandite and Ce-monazite-I. This mineralisation evolved from replacement fronts controlled by the mylonitic foliation to hydraulic breccia zones. It is related to calcic alteration, especially to the development of actinolite-II in rhyodacites, controlled by the Canaã dos Carajás shear zone. Actinolite-II envelops the massive magnetite bodies, sometimes promoting their local brecciation.
• Mineralisation stage II, which was temporally and spatially related to the development of the pervasive iron-potassic alteration-II, which affects all of the host rocks, and is coeval with emplacement of Co-chalcopyrite (±Ni-pyrite ±Ni-pyrrhotite). This alteration involves biotite-II being formed at the expense of actinolite-II in mylonitic rocks formed in response to the Canaã dos Carajás shear zone. Associated mineralisation formed as a system of strongly oriented, interconnected bodies containing Co-magnetite-V, Cl-apatite-III, ilmenite-III, Ce-allanite-IV and quartz, with local discrete Ni-pyrite and Co-chalcopyrite.
• Mineralisation stage III, controlled by ductile-brittle and brittle structures, enabling the formation of veins with typical open-space textures filled by paler brown or green biotite, Co-chalcopyrite and siegenite [CoNi2S4] (±Co-pyrite, ±Co-magnetite, ±cassiterite). This stage is distinguished by being controlled by brittle structures and thus comprising stockworks and vein systems. It was coeval with the light brown or green biotite-III alteration, in veins of up to 4 cm thick containing coarse quartz crystals, as well as scapolite-IV, Cl-F-apatite-IV and hastingsite-II with typical open-space filling textures.
• Late Mineralisation stage IV, which was coeval with widespread chlorite alteration, and formed the bulk of the copper mineralisation at Jatobá. It occurs branching veinlets and breccias containing chalcopyrite, Co-chalcopyrite, Co-pyrite, sphalerite, molybdenite, uraninite, monazite, W-bearing hematite, and rare earth carbonates (bastnäsite, coskrenite and sahamalite). The high Cl contents of halogen-rich minerals (e.g., scapolite, biotite, apatite and amphibole) are interpreted to reflect the participation of highly saline, supersaturated fluids in the evolution of the mineralised system. Those metalliferous fluids were hot and highly focused to form hotter hydrothermal centres. High V and Ti contents in magnetite and its coexistence with hercynite and ilmenite are interpreted to indicate temperatures of >600°C. These conditions and element associations are interpreted as being consistent with magnetite formed in the magmatic to deep magmatic-hydrothermal transition, with nickel being leached from mafic and mafic-ultramafic rocks at depth, and transported in Cl-rich hydrothermal fluids, before being precipitation during episodic decompression. A temperature decrease to <380°C, as indicated by chlorite geothermometers, would have favoured the bulk of chalcopyrite precipitation in late mineralisation stages.
No resource or reserve figures have been encountered in publicly available literature to 2023 and this deposit remains a prospect.
The information in this summary is drawn from Veloso, et al., 2020)
Castanha
Like Jatobá, Castanha is hosted within volcanic and volcaniclastic rocks of the Itacaiúnas Supergroup, and is located ~7 km NE to ENE of the Sossego deposit. It is predominantly hosted by black to dark grey, hydrothermally altered, porphyritic subvolcanic and volcanic rocks of rhyodacitic composition, e.g., the 2745 ±4 Ma Castanha quartz-feldspar porphyry (U-Pb zircon SHRIMP; Moreto et al., 2015) and the ~2.74 Ga Itacaiúnas Supergroup sequence respectively. Where only weakly altered, they average ~45% quartz, ~5% plagioclase, ~35% K feldspar and ~10% fine-grained biotite. Where hydrothermally altered, these rocks can be distinguished by remnant plagioclase and quartz megacrysts. The quartz megacrysts are euhedral to subhedral, bipyramidal and 0.5 to 3 mm long, often with corrosion gulfs. They are also commonly blue, due to minute ilmenite inclusions. Inclusion-rich rims to these quartz megacrysts are often partially replaced by hydrothermal biotite or K feldspar. Plagioclase megacrysts are subhedral with albite or chessboard twining and 1 to 3.5 mm long. The surrounding matrix is fine-grained and phaneritic, with crystals that are 0.1 to 1 mm long of quartz, K feldspar and subordinate biotite. The biorite content is higher in mylonitised samples, where it exhibits S-C structures associated with mineral stretching of biotite, scapolite and quartz and comminution of quartz.
Hydrothermal alteration - albite-I represents the earliest hydrothermal alteration stage at Castanha, affecting all major host rocks. It has a pinkish colour, due to hematite mineral inclusions, and occurs as subhedral crystals, generally with chessboard twinning. The initial albite alteration was pervasive, and was subsequently replaced by scapolite, biotite, epidote, calcite and chlorite. Pervasive silicification and potassic alteration with K feldspar overprinted most of the early albite in the Castanha host rock.
Hydrothermal scapolite occurs in veins and as pervasive replacement. In veins, scapolite commonly occurs in textural equilibrium with quartz and fluorite, which are also associated with chalcopyrite and minor magnetite. Within pervasive alteration zones, scapolite crystals are found along the foliation in biotite-I, occurring as deformed crystals with pressure shadows. The scapolite crystals are commonly zoned, with a calcic-rich core and a sodic-rich outer-rim. Later alteration involving K feldspar, actinolite, biotite, sericite, calcite and chlorite also replaced most of the scapolite-rich zones.
Actinolite occurs in at least three distinct associations, namely:
• Actinolite-I, formed in an early calcic alteration stage, occurring as coarse-grained 1 to 3 mm long crystals associated with clinozoisite. This association replaces hydrothermal scapolite and albite, and envelops bi-pyramidal quartz megacrysts of the Castanha Porphyry.
• Actinolite-II, that is coeval with potassic alteration producing biotite, comprises fine-grained, up to 1 mm long crystals following the same foliation, accompanied by hastingsite, fine-grained biotite-I, and subhedral dark greenish-blue tourmaline. This mineral association represents a transition to the main potassic alteration event at Castanha, which is characterised by major developments of fine biotite-I and narrow potassic zones with K feldspar-I. Biotite-I and actinolite-II partially replace actinolite-I, scapolite and apatite.
• Actinolite-III as a later calcic-(iron) stage associated with the onset of iron metasomatism. These late, coarse-grained actinolite-III crystals are 1 to 3 mm long and occur in veinlets and comb veins that crosscut the actinolite-II association. It is well-developed and has a close spatial association with magnetite-rich zones and the Castanha porphyry host rock. Some scattered magnetite crystals have straight-line contacts with actinolite-III crystals, although actinolite is generally partially replaced by magnetite, chalcopyrite and associated minerals.
Magnetite-rich zones are closely-related to calcic-(iron) alteration, and are represented by magnetite in different associations:
• Magnetite-I occurs as disseminated, fine-grained, subhedral crystals, associated with the least-altered rhyodacite or with actinolitite, partially replacing actinolite-I, and has with straight-line boundaries with actinolite-III crystals.
• Magnetite-II, constituting up to 85% of the total rock where present, is associated with apatite + monazite ±actinolite ±pyrite ±chalcopyrite forming massive magnetitite bodies. Calcite and chlorite fill fractures in magnetite crystals and replace apatite. Apatite is also involved with stilpnomelane, which, in turn, occurs in straight contacts with monazite.
Subordinate quartz-biotite-II veins with minor chalcopyrite crosscut magnetite and actinolite bodies. Coarse-grained biotite-II and brown stilpnomelane replace scapolite-rich rocks.
Late K feldspar-II veins crosscut biotite-rich zones. The K feldspar has an association with randomly distributed minute hematite crystals that are <10 µm in diameter, and have a ruby-red internal reflection under reflected light.
Sericite alteration at Castanha deposit is not extensive. Proximal to magnetite bodies, it occurs in association with quartz ±K feldspar, chalcopyrite and biotite-II, or in veins with a quartz alteration aureole.
Chlorite selectively replaces scapolite, biotite and tourmaline, without twinning, and is associated with calcite,titanite and epidote, but is not a major component of the mineral assemblage.
Carbonate alteration occurs as a late pervasive event, spatially related to pyrrhotite-rich copper mineralisation, and is represented by calcite, REE-bearing carbonates and epidote, which occurs in the ore breccia matrix enclosing magnetite, chalcopyrite, and pyrite, and in altered host fragments. In veins and veinlets, calcite is associated with sulphides ± magnetite ±epidote ±chlorite and cuts across all the previous minerals. Post-mineralisation alteration styles include late pervasive and fissure-controlled barren carbonate alteration.
Mineralisation - the main mineralised zone at Castanha occurs as veins, veinlets, stockwork and particularly as breccias. The latter are composed of chalcopyrite, pyrrhotite and minor pyrite, virtually always enclosing rounded to sub-rounded fragments of altered host rocks forming a durchbewegung texture. Marshall and Gilligan, 1989). The enclosed lithic clasts are composed of host rocks replaced by biotite, chlorite, allanite, stilpnomelane, calcite and REE-bearing carbonate minerals. Allanite occurs as zoned euhedral crystals included in pyrrhotite and calcite and rarely, is replaced by chlorite.
In the most mineralised zones, the full mineralisation assemblage comprises chalcopyrite + pyrrhotite + pyrite ±sphalerite ±marcasite ±pentlandite. Cobalt-pentlandite [(Ni,Co,Fe)9.20S8.39] and pentlandite are commonly associated with pyrrhotite and occur as inclusions or in the pyrrhotite crystal edges, where they have granular or flames textures. Pd-melonite [(Fe,Ni,Pd)0.31Te0.61], are seen as thin crystals included in pyrrhotite.
Veins and veinlets are mainly found on the periphery of breccia bodies. Veins containing up to 75% chalcopyrite and up to 25% pyrite ±molybdenite + quartz + monazite + magnetite cross-cut magnetite and actinolite bodies. In more distal zones, pyrite veins are found, generally accompanied by minor chalcopyrite, quartz, calcite, albite, scapolite, Cl-apatite, fluorite and magnetite.
SEM analysis has identified a variety of minor minerals, including: uraninite, galena, molybdenite, monazite, sugakiite [Cu0.66(Fe,Ni)8.4S8.42], and Ni-pyrite [(Fe,Ni)0.79S1.72], (Mo0.34S2.06). Uraninite occurs in host rock clasts and as small (~1 µm) anhedral inclusion in sulphides. Similarly, galena occurs as small anhedral crystals (~1 µm) disseminated in lithic fragments within breccias in the mineralised zones, and occurs rarely associated with sphalerite. Monazite was observed in Cl-apatite rims, whereas euhedral molybdenite crystals are found as mineral inclusion in Cl-apatite and allanite.
No resource or reserve figures have been encountered in publicly available literature to 2023 and this deposit remains a prospect.
The information in this summary is drawn from Pestilho, et al., 2020)
Bacaba
The Bacaba deposit is located ~7.5 km east and ~3 km SSE of the Sossego and Castanha deposits respectively. Like the Bacuri deposit, ~8 km to the east, it lies within the confines of the host 2848 ±5.5 Ma Serra Dourada Granite (U-Pb zircon, Moreto et al., 2015), but also occurs within the 3004 ±7.8 Ma Bacaba Tonalite (U-Pb zircon; Moreto et al., 2011), and related gabbro bodies. In the Bacaba area and surrounds, mylonitisation and hydrothermal alteration of the host rocks result in the almost complete obliteration of the original rock textures.
At Bacaba, the Serra Dourada Granite is light greyish pink with a predominant isotropic structure and a 4 to 8 mm crystal-size coarse to medium equigranular texture. On average, it is contains ~40% quartz, ~30% white plagioclase, ~20% light red K eldspar, ~5% amphibole and ~5% biotite, with accessory pyrite and magnetite. Micrographic textures and pegmatitic facies have been recognised. Veins and veinlets crosscut this host rock, sometimes with open-space filling textures (e.g., comb texture), especially close to the strong copper mineralisation. Disequilibrium textures, interpreted to be related to pervasive potassic hydrothermal alteration fronts, are observed, even in apparently un-altered rocks. Mylonitic foliation is typically present in well-developed pervasive hydrothermal alteration zones.
The older Bacaba Tonalite is grey, isotropic to strongly foliated, medium-grained, and has an equigranular texture with 1 to 8 mm crystals. It is composed of ~40% quartz, ~35% white plagioclase, ~10% amphibole, ~10% biotite and ~5% light red K feldspar. It has similar hydrothermal textures to those of the Serra Dourada Granite, and the two are frequently difficult to differentiate in zones of intense alteration.
At Bacaba, the contact between gabbro and granitoids is characterised by intense hydrothermal alteration. The gabbros are distinguished by their sub-ophitic to ophitic textures, and are composed of plagioclase laths partially replaced by scapolite, and remnants of clinopyroxene overprinted by actinolite, hastingsite and biotite.
Hydrothermal alteration - the earliest hydrothermal alteration at Bacaba is albite, which affects all of the major host rocks. This early albite-I alteration was pervasive and imparted a pinkish colouration, due to fine hematite inclusions. It forms subhedral crystals, generally with chessboard twinning, and was subsequently replaced by scapolite, biotite, epidote, calcite and chlorite. Pervasive silicification and potassic alteration, characterised by K feldspar, overprinted most of the early albite in the Bacaba host rocks.
Hydrothermal scapolite occurs as veinlets, veins, stockwork veining and pervasive alteration. The latter envelopes thick scapolite veins and occur as orientated fibrous scapolite crystals, orientated along the mylonitic foliation, with grain boundary migration and sweeping undulose extinction. Two distinctive scapolite vein generations are recognised at Bacaba:
• Reddish-white scapolite-I veins that are up to 10 m in width, occurring as coarse-grained fibrous or prismatic crystals with low birefringence (marialite) associated with quartz, magnetite, fluorite and epidote.
• White scapolite-II veins up to 20 cm wide that crosscut the earlier veining. These are also composed of fine-grained marialite which is strongly oriented along foliation.
Both scapolite-I and -II are partially replaced by K feldspar, biotite and sericite. Hematite, and uranium- and thorium-bearing minerals, such as haiweeite [Ca((UO2)2Si5O12(OH)2)•3(H2O)] and calciothorite [(Th,Ca2)SiO4•3.5H2O), infill fracture between the fibrous scapolite crystals (Augusto et al., 2008).
Early magnetite occurs as elongated euhedral to subhedral crystals associated with scapolite-I in veins and along the mylonitic foliation. In intensely mylonitised scapolite zones, magnetite relicts occur as smaller xenomorphic crystals associated with Fe-Ti-(Mn) oxides and goethite. Substantial disseminated magnetite associated with K feldspar replaced the host rocks forming magnetitites, which are, in turn, crosscut by later K feldspar veinlets.
Hydrothermal K feldspar dominated potassic alteration, with minor rutile, forms pervasive, structurally-controlled potassic alteration zones that are associated with mylonitic foliation. Subhedral K feldspar crystals without any twinning are partially or wholly replaced by sericite, while late K feldspar veins commonly have a selvage of sericite or chlorite-calcite, and crosscut albite-, scapolite-, magnetite- and biotite-I rich zones.
Biotite-rich potassic alteration forms extensive and pervasive zones of fine-grained, from 1 µm, biotite-I, occurring as brown lamellar crystals that are mainly subhedral, with associated quartz, magnetite, K feldspar, scapolite, tourmaline, allanite, fluorite, zircon and rarely chalcopyrite and pyrite. A late biotite-II generation is found as pervasive alteration fronts and in quartz veins as coarse, up to 1 mm long crystals. It replaces and crosscuts scapolite-rich rocks, potassic altered zones with K feldspar, fine-grained biotite-I and K feldspar veins.
Chlorite replaces disseminated biotite-I in pervasive potassic alteration zones and is found in veinlets that crosscut K feldspar, albite, biotite, scapolite and amphibole.
Late hydrothermal Albite-II is associated with sulphide minerals such as chalcopyrite and pyrite which replaces K feldspar and scapolite, but is, in turn, substituted by sericite. It is found in pervasive alteration fronts and in veins.
Subhedral to anhedral sericite crystals replace K feldspar and albite, and is associated with quartz, and less commonly with chalcopyrite and biotite. Sericite, sericite-chlorite and sericite-quartz, and quartz ±pyrite veins and veinlets crosscut biotite ±quartz ±scapolite associations.
Late calcite and epidote associations are uncommon, although calcite veins/veinlets (calcite ±K feldspar ±musketovite ±chalcopyrite ±hematite or only calcite ±hematite associations are recognised mainly near copper mineralisation.
Mineralisation - Sulphides at Bacaba occur in veinlets, veins and in mineralised breccias, located in structurally-controlled replacement zones up to 30 cm thick following the mylonitic foliation. In these zones, chalcopyrite-I is the principal sulphide mineral, occurring in association with magnetite and minor bornite. Mineralised zones are spatially related to zones of potassic alteration with K feldspar and magnetite, in which the latter is commonly replaced by hematite. The potassic zone is crosscut by late mineralised veins containing chalcopyrite-II and pyrite, accompanied by albite-II, quartz, chlorite, calcite, epidote, clinozoisite, allanite, apatite, monazite, rutile and hematite. Musketovite and euhedral hematite lamellae have straight-line contacts with chalcopyrite-II. A thin, up to 1 mm thick albite-rich halo commonly separates the earlier K feldspar crystals from the crosscutting sulphides.
Melonite [NiTe2] and altaite [PbTe] are common accessory phases of the Bacaba mineralised zones, where they fill fractures or represent mineral inclusions into chalcopyrite or gangue minerals. Other minor minerals are galena, cassiterite, uraninite, hessite, cheralite [CaTh(PO4)2], olsacherite [Pb2SO4SeO4], and tsumoite [BiTe], which occur as thin inclusions in chalcopyrite. Hessite also is associated with melonite in chalcopyrite rims. Olsacherite is seen as needles in fractured walls within chalcopyrite crystals (Augusto et al., 2008).
No resource or reserve figures have been encountered in publicly available literature to 2023 and this deposit remains a prospect.
The information in this summary is drawn from Pestilho, et al., 2020)
Visconde
The Visconde Cu-Au deposit, like the others of this record, and the Sossego cluster, lies within the broad WNW-ESE trending regional Canaã dos Carajás shear zone that straddles the contact between the Mesoarchean TTG orthogneisses, granitoids and greenstone belts of the Xingu Complex to the south, and the Neoarchaean volcano-sedimentary regime of the the Itacaiúnas Supergroup to the north. It is located ~12 km east of Sossego, and 4.5 km ESE of Bacaba.
Mineralisation at Visconde is hosted by felsic metavolcanic rocks, the Serra Dourada granite, gabbros/diorites and, to a lesser extent, ultramafic rocks. These host-rocks are crosscut by zones of mylonitisation, and have been altered to an assemblage of abundant hydrothermal minerals, particularly biotite.
The felsic metavolcanic rocks, are grey, fine grained, isotropic to foliated metadacites (Craveiro et al., 2012). Where foliated, they comprise fine, alternating bands of quartz, plagioclase, muscovite or biotite, and/or hornblende.
The Serra Dourada Granite, which has been dated at 2848 ±5.5 Ma, (U-Pb zircon, Moreto et al., 2015), is greyish to pinkish at Visconde, and generally coarse-grained and isotropic, although incipiently foliated in places. Quartz, plagioclase and K feldspar are the principal constituent minerals, giving the rock a monzogranitic to granodioritic composition with a calc-alkaline affinity. This intrusion exhibits evidence of Na metasomatism evidenced by the common occurrence of chessboard albite (Craveiro et al., 2012; Feio et al., 2013).
Gabbro/diorites occur as stocks and dykes, and are apparently intrusive into both the metadacites and the Serra Dourada Granite. They are medium-grained, and where less altered, exhibit relict primary features such as sub-ophitic texture and straight-line contacts between plagioclase, Mg hornblende, or quartz crystals, in addition to primary magnetite crystals with exsolved ilmenite lamellae.
Lenticular Ultramafic bodies are found in some gabbros and within the Serra Dourada granite, occurring as magnesite- and talc-rich serpentinites, with less abundant pentlandite, Ni-bearing chalcopyrite and magnetite.
Most of these rocks have a strong east-west orientation and steep-dip imposed by the ductile-brittle to brittle shear deformation between 2.76 and 2.65 Ga in the Neoarchean (Pinheiro and Holdsworth 2000).
The Visconde Granite occurs immediately to the north of Visconde, and is a weakly altered, highly fractured, but un-mineralised granitic body. It is reddish-pink, isotropic, medium to coarse grained, with a heterogranular texture and local graphic intergrowths. The major constituent minerals include microcline, quartz, plagioclase and biotite, with a modal syeno-granite composition. Microcline crystals are coarse and commonly perthitic. Quartz crystals are anhedral to subhedral, some with weak undulose extinction. Plagioclase occurs as exsolution lamellae in microcline and as subhedral crystals, some showing kink bands and microfractures. Most biotite is interstitial occurring as isolated flakes. Zircon is the main primary accessory mineral, whilst minor sericite, chlorite, epidote and clay minerals are secondary phases that are generally concentrated near fractures. This granite has been described as a 1.88 Ga anorogenic granite (Vale 2003), although subsequently, petrographic data has led to it being tentatively correlated with the Planalto suite (Feio et al., 2012), with age of 2.74 Ga (Huhn et al., 1999; Sardinha et al., 2004; Feio et al., 2012). Despite being fractured, it shows no signs of alkali-metasomatism or mineralisation, suggesting emplacement post-dating the Visconde mineralising event.
Hydrothermal alteration - Staged hydrothermal alteration is indicated at Visconde, largely controlled by tectonic evolution from a ductile to a brittle regime. The initial stage involved early sodic-calcic alteration, in which the rock underwent pervasive replacement of primary mineral assemblages. Typical alteration minerals of this stage included chessboard albite that was developed in the Serra Dourada granite, whilst marialitic scapolite formed in the gabbros/diorites at the expense of plagioclase. Early associated veining during this stage included albite-epidote, scapolite-tourmaline, tourmaline-quartz, and actinolite, as well as aggregates of actinolite, locally with associated magnetite that had fine inclusions of ilmenite and rutile. Further magnetite was subsequently precipitated to form local massive magnetite bodies, or 'magnetitites'. Disseminated chalcopyrite and pyrite are also common during the early stage.
The next stage was characterised by potassic alteration, that produced biotite and K feldspar. Biotite is so abundant in highly deformed rocks, and the mylonitic foliation so pronounced that they resemble a schist. In addition to biotite, talc and magnesite were also formed in deformed ultramafic rock lenses. The overall assemblage in the potassic zone include quartz, allanite, magnetite, chalcopyrite and molybdenite.
This was followed by a new pulse of sodic-calcic and magnesian alteration in the distal zones of the deposit, accompanied by the precipitation of abundant sulphides in which chalcopyrite is >>bornite, accompanied by albite, epidote, chlorite, calcite and minor fluorite filling fracture-controlled veins and veinlets.
The final episode involved calcic-magnesian alteration, characterised by the precipitation of chlorite, calcite and local gypsum or hematite in veinlets and/or breccias.
Mineralisation occurs in tectonic breccias, veins, veinlets and disseminations. The breccias form bodies that overprint and cross-cut the potassic alteration zones and comprise clasts of hydrothermally altered rocks set in a matrix of locally >60 % sulphides, chiefly chalcopyrite, together with minor bornite, pyrite, chalcocite, covellite and digenite. Sulphides within veins and veinlets were co-precipitated with the sodic-calcic alteration assemblage.
The paragenetic sequence can be summarised as follow:
• Early sodic to sodic-calcic alteration episode involved the following, overlapping progression -
Albite-I → scapolite-I → epidote-I → minor tourmaline → actinolite-I → pentlandite and trace chalcopyrite begins → trace magnesite → talc → trace dolomite → magnetite → pyrite → quartz → ilmenite → trace rutile → calcite-I → apatite → molybdenite → allanite.
• Potassic alteration episode
Trace actinolite continues, while significant talc and magnetite also persist into the early potassic phase. Molybdenite and trace pyrite also continue into the the early potassic phase, as does apatite and allanite, while quartz grows in abundance. Biotite is developed throughout the potassic stage and K appears towards the end. Chalcopyrite becomes dominant at much the same time as K feldspar, while trace bornite makes a first appearance. Minor sericite and chlorite first appear.
• Late sodic to sodic-calcic alteration episode
is marked by the appearance and growing precipitation of albite-I, → minor scapolite-II → epidote-II and actinolite-II. Minor quartz-II appears, whilst sericite continues, calcite-II appears, and increases in intensity, as does chlorite and fluorite. Chalcopyrite continues to dominate the sulphide minerals, declining towards the end, as does bornite.
• Late sodic to calcic-magnesian alteration episode
Characterised by trace hematite → tapering sericite → calcite and chlorite continuing → gypsum.
Resources
According to Silva et al. (2015), quoting Benevides Aires, (pers. comm.), the Visconde deposit is estimated to contain between 20 and 50 Mt @ 1.0 % Cu, 0.28 g/t Au.
The information in this summary is drawn from Silva et al., 2015)
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.
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Selected References:
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Pestilho, A.L.S., 2011 - Sistematica de isotopos estaveis aplicada a caracterizacao da evolucao dos paleo-sistemas hidrotermais associados aos depositos cupriferos Alvo Bacaba e Alvo Castanha, Provincia Mineral de Carajas, PA.: in Masters dissertation presented to the Institute of Geosciences at Universidade Estadual de Campinas (UNICAMP) to obtain the title of Master in Geoscience, 77p.
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Pestilho, A.L.S., Monteiro, L.V.S., de Melo, G.H.C., Moreto, C.P.N., Juliani, C., Fallick, A.E. and Xavier, R.P., 2020 - Stable isotopes and fluid inclusion constraints on the fluid evolution in the Bacaba and Castanha iron oxide-copper-gold deposits, Carajas Mineral Province, Brazil: in Ore Geology Reviews v.126, doi.org/10.1016/j.oregeorev.2020.103738.
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Silva, A.R.C., Villas, R.N.N., Lafon, J.-M., Craveiro, G.S. and Ferreira, V.P., 2015 - Stable isotope systematics and fluid inclusion studies in the Cu-Au Visconde deposit, Carajas Mineral Province, Brazil: implications for fluid source generation: in Mineralium Deposita v.50, pp. 547-569.
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Veloso, A.S.R., Monteiro, L.V.S. and Juliani, C., 2020 - The link between hydrothermal nickel mineralization and an iron oxidecopper-gold (IOCG) system: Constraints based on mineral chemistry in the Jatoba deposit, Carajas Province: in Ore Geology Reviews v.121, 27p. doi.org/10.1016/j.oregeorev.2020.103555.
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