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Grota Funda |
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Para, Brazil |
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Cu Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Grota Funda iron oxide copper gold (IOCG) deposit is located ~50 km WNW of the town of Parauapebas and 30 km SE of the Salobo IOCG deposit in the state of Para, Brazil.
Regional Setting
See the Carajas IOCG Province record.
Geology
The Grota Funda deposit is located in the northwestern Archaean (3.0 to 2.55 Ga) section of the Carajás IOCG province, and lies within the regional WNW-ESE striking Pojuca shear zone/fault system. The same shear zone also encloses the Gameleira Cu-Au and Pojuca volcanic-hosted massive sulphide Cu-Zn deposits, both of which are within 10 km to the WNW. Other structures intersecting the deposit are NE-SW and east-west trending subsidiary faults associated with the NW-SE Carajás fault system that is 10 km to the south.
The principal hosts to the Cu-Au mineralisation at Grota Funda are mafic igneous rocks (basalt and dolerite) correlated with the Igarapé Pojuca Group, whilst gabbro, felsic subvolcanic rocks and banded iron formations are also recognised in the deposit area. Intense and extensive hydrothermal alteration and brittle-ductile deformation have partially to completely obliterated the original textural and mineralogical features of the host sequence at Grota Funda. This has made recognition of the protoliths difficult.
Basalts are inferred by the presence of amygdaloidal textures in extremely fine grained rocks that are essentially composed of chlorite and quartz. The amygdaloidal cavities are lined by a fine-grained rim of chlorite and filled by microcrystalline quartz. The least altered dolerite and gabbro have similar compositions and poorly preserved subophitic textures, differing only in their grain sizes. Both are dark grey to green in colour and predominantly composed of plagioclase and augite, the latter being intensely altered to hydrothermal hastingsite and biotite. The least altered plagioclase crystals are generally tabular and have poorly preserved twinning, although most are converted into scapolite and subsequently surrounded by biotite and chlorite. Augite, when preserved, commonly has cleaved and simple twinned basal sections.
Felsic subvolcanic rocks are only locally recognised, but where observed, have been intensely modified by potassic and chlorite alteration to a greenish to greyish colour and are composed of a fine-grained (<1 mm) matrix of quartz and plagioclase, with subordinated K feldspar (microcline) that locally includes euhedral (bipyramidal) phenocrysts of bluish quartz. Bulk mineralogy suggests a dacitic composition. Trace amounts of zircon and epidote are also recognized within this rock. Chalcopyrite occurs as disseminations and fracture fill.
Banded iron formation has a compositional layering that is typically marked by the alternation of greyish-white quartz + grunerite and dark magnetite rich microbands, both of which are up to 0.5 mm thick. The contact between microbands is usually diffuse. Magnetite is predominantly fine grained and has been variably martitised. Grunerite crystals are typically acicular and randomly oriented within quartz-rich microbands.
Alteration and Mineralisation
The Grota Funda deposit has been subjected to intense and extensive hydrothermal alteration, including sodic-calcic, Fe metasomatism, potassic, chlorite-quartz-tourmaline and carbonate-quartz veining. There have been at least three recognised copper (-gold) mineralising episodes within the deposit, associated with Fe metasomatism, potassic and chlorite alteration stages respectively. The characteristics of these stages are as follows:
Sodic-calcic alteration - which is represented by two different assemblages:
• Fracture-controlled fine to medium grained albite ±quartz veinlets that crosscut the dacitic rock subvolcanic rocks. In contrast, within the mafic igneous rocks, pervasive Na-rich alteration containing albite is only locally evident, as they have been obliterated by overprinting scapolite, biotite and chlorite. Albite crystals commonly exhibit evidence of stress-related deformation, including stretching and usually contain tiny inclusions of hematite, which impart a reddish colour to the altered rocks.
• A well defined assemblage of hastingsite and scapolite overprints zones of earlier formed albite, and is best recognized in mafic protoliths, where typically bluish green hastingsite and light yellow marialitic ([Na,Ca]4[Al3Si9O24]Cl) scapolite respectively replace igneous clinopyroxene and plagioclase. Hastingsite has a relationship of mutual replacement with grunerite, taken to suggest the latter could have been formed earlier in the hydrothermal system.
Fe metasomatism - which occurs in the intense metasomatic central core of the deposit where massive precipitation of magnetite(I)-grunerite-almandine ±chlorite(I)-quartz took place. Veins containing coarse grunerite, magnetite and minor 1 to 2 cm molybdenite flakes overprint previous zones of sodic-calcic alteration. Grunerite replaces hastingsite to form a fine-grained mass of crystals, often involving rounded, up to 5 mm diameter almandine porphyroblasts. Magnetite, which is commonly granular to massive, is one of the most important minerals associated with this stage, locally comprising up to 55% of the rock mass over thicknesses of up to 10 m.
Almandine becomes progressively more abundant towards iron-enriched zones, and is itself Fe rich, and is generally poikiloblastic and riddled with grunerite, magnetite and quartz inclusions. Individual crystals are strongly fractured and selectively replaced by biotite. Elongated almandine grains with pressure shadows are seen in intensely deformed parts of the deposit.
An early generation of chalcopyrite is recognised within the magnetite-rich zones (Mineralisation I). This chalcopyrite (I) is predominantly xenoblastic and is typically intergrown with magnetite, grunerite and molybdenite. Light rare earth element-bearing (La-Ce-Nd) monazite, allanite, uraninite (UO2) and Te bismuthite are common accessories associated with this stage of mineralisation, usually occurring as very small inclusions in magnetite and chalcopyrite. Gold is essentially an inclusions in magnetite.
Potassic alteration - which is extensively developed and has affected all rock types, overprinting both sodic-calcic and Fe-rich alteration assemblages. Strong potassic alteration is well developed in high-strain zones, represented essentially by biotite + quartz, accompanied by subordinate apatite, tourmaline (I), allanite, ilmenite and Ti hematite. In these zones, biotite is usually found as a fine to medium grained groundmass of brownish tabular crystals intergrown with stretched quartz grains and tourmaline aggregates, which define a mylonitic foliation. Brown biotite is commonly selectively replaces hastingsite and grunerite. Millimetre scale veinlets filled by fine grained greenish to brownish biotite flakes occur as fracture fill in the mafic igneous rocks, frequently replacing almandine. They also crosscut coarse grunerite-magnetite veins. The tourmaline (I) crystals are mainly dark greenish blue and have a clear internal zonation. Biotite generally mantles the igneous fieldspar in felsic rocks, with evidence of later replacement by chlorite.
Main sulphide ore or Mineralisation (II) predominantly occurs in breccia bodies that can be up to 15 m thick, but is also found as subordinate veinlets and disseminations. The breccia ore is spatially associated with potassic alteration and is mainly hosted by mafic igneous rocks. The ore primarily comprises up to 70% massive chalcopyrite (II), followed by magnetite, pyrrhotite, pentlandite and sphalerite that occurs as breccia matrix fill. Apatite, quartz, tourmaline, biotite, chlorite and allanite are also commonly associated with copper ore.
Unlike in the iron-enriched zones, where magnetite is the dominant mineral phase, magnetite (II) is a secondary mineral phase compared to sulphides in the main sulphide ore. Pyrrhotite crystals contain fine-grained pentlandite exsolutions along their rims and cores. Sphalerite is mainly granular, commonly containing chalcopyrite inclusions (chalcopyrite disease) and typically has a low iron content. Bornite is very much subordinate and generally replaces chalcopyrite along its rims. Ilmenite occurs as a common accessory phase, especially in biotite-rich zones, and frequently has Ti hematite exsolution. Minor pyrite, siegenite, cobaltite and melonite (NiTe2) occur as inclusions in chalcopyrite.
Chlorite alteration - is extensively developed, mainly distal to the central core of the deposit. In zones of strong chlorite alteration, biotite and amphibole are altered to Fe-rich chlorite (I) (chamosite), and are also accompanied by silicification and tourmaline(II)-magnetite(III)-carbonate-actinolite precipitation. Mafic igneous rocks are particularly susceptible to intense chloritisation, completely obliterating their original igneous texture. Veinlets of chlorite(I) ±quartz-carbonate-chalcopyrite(III)-epidote commonly occur as fracture fill within felsic rocks.
Significant magnetite, tourmaline and actinolite are co-products of chlorite alteration. Magnetite (III), which is usually massive, occurs as fine to medium grained crystals, intergrown with tourmaline, chlorite, quartz and actinolite. Black to dark brown tourmaline (II) typically occurs as coarse-grained prismatic crystals up to 4 mm long, but unlike tourmaline (I), shows no evidence of internal zonation and is generally poikiloblastic, and riddled with chlorite, quartz and carbonate inclusions. Tourmaline-rich (up to 70 vol %) zones are recurrent within strongly chloritised zones. The paragenetic association between chlorite (I), magnetite (III), tourmaline (II) and actinolite may suggest that the actinolite represents a product of the chloritisation process. Actinolite generally occurs as coarse-grained prismatic crystals up to 3 cm long, frequently replaced by carbonate.
A third stage of chalcopyrite crystallisation (Mineralisation III) is spatially related to the chlorite-altered zones, and comprises up to 50% massive chalcopyrite (III) that accompanies quartz, chlorite, epidote, carbonate and minor Ti-bearing minerals (e.g., titanite, ilmenite and rutile). Two generations of chlorite were recognized within these mineralised zones, including an Mg-rich phase with anomalous dark-brown birefringent colours; chlorite (II), which apparently replaces the early formed chamosite.
Late carbonate veins - was the latest recognised stage of hydrothermal activity, and is represented by millimetre-scale veinlets, generally <4 mm width, composed of calcite-quartz ±chalcopyrite(IV)-pyrite (II), overprinting earlier alteration zones and crosscutting all host rocks. Galena and Ag tellurides occur as inclusions in chalcopyrite (IV). These calcite-quartz veins are commonly brecciated and contain open space-filling textures. In zones of pervasive carbonate-alteration, calcite usually replaces both igneous and hydrothermal feldspar crystals, Ca amphiboles and tourmaline.
The main sulphide ore predominantly occurs as breccia bodies that range from <10 to >50 m in width, with subordinate veinlets and fracture infillings and as disseminations. The ore paragenesis includes primarily chalcopyrite, magnetite, pyrrhotite, pentlandite and sphalerite, with minor ilmenite, Ti-hematite, siegenite, cobaltite and melonite. The association of chalcopyrite-magnetite-pyrrhotite-sphalerite indicates precipitation under low ƒS4 and ƒO4 conditions, whereas the replacement of chalcopyrite by bornite and the late crystallization of pyrite are evidence of fluid evolution toward higher ƒS4 conditions (Hunger et al., 2018).
Hunger et al. (2018) conclude the paragenetic evolution of the Grota Funda deposit points to significant changes in the physical-chemical conditions of the hydrothermal fluids, such as a temperature decrease from 540°C (during Na-Ca alteration) to <200°C (during late carbonate veining), accompanied by a decrease in salinity and pH. The early, high-temperature alteration assemblages of albite, scapolite-hastingsite may be intrinsically related to regional circulation of deep-seated hypersaline and hot metalliferous fluids, which transported metals as chloride complexes, whereas the onset of ore precipitation resulted from significant influx of relatively dilute (moderate salinity 29 to 24 wt.% NaCl + CaCl2 equiv.) and cooler fluids (Hunger et al., 2018).
Overall, the fluid inclusion data for the Grota Funda deposit is interpreted by Hunger et al. (2018) to point to a trend in fluid evolution that apparently involved the interaction between highly saline hydrothermal fluids and cooler, relatively dilute fluids. This process may have induced temperature and salinity decrease and, consequently, the precipitation of the main sulphide ore. Sulphur isotope compositions of chalcopyrite (δ34S = 0.9 ±0.9‰) suggest to a magmatic sulphur source, which was most probably leached from the mafic igneous host rocks present in the deposit area.
Molybdenite from grunerite-magnetite veins yielded a Re-Os model age of 2530±60 Ma, which is interpreted as the Mineralization (I) age (Hunger et al., 2018).
Resources
The Grota Funda IOCG deposit apparently contains 15 to 40 Mt at 1.2 to 0.8% Cu (VALE unpub. report, 2016; quoted by Hunger et al., 2018).
The information summarised in this record was drawn from Hunger et al. (2018).
The most recent source geological information used to prepare this decription was dated: 2018.
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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Hunger, R.B., Xavier, R.P., Moreto, C.P.N. and Gao, J.-F., 2018 - Hydrothermal Alteration, Fluid Evolution, and Re-Os Geochronology of the Grota Funda Iron Oxide Copper-Gold Deposit, Carajas Province (Para State), Brazil: in Econ. Geol. v.113, pp. 1769-1794.
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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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