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Foskor |
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Limpopo (Northern) Province, South Africa |
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P
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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.
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The Foskor Phalaborwa operation comprises two phosphate-vermiculite deposits, the Northern and Southern Pyroxenite pits that are ~4 km apart. The operation is located immediately adjacent to Kruger Park, near the town of Phalaborwa in Limpopo Province, South Africa, 220 km ENE of Mokopane and 400 km NE of Johannesburg, #Location: 23° 57'S, 31° 8'E.
The deposits occur as two pipe-like apatite-bearing pyroxenite bodies within the composite, north-south elongated, 20 sq. km (~8 x 3 km) Phalaborwa Igneous Complex (2060 Ma), some 150 km to the east of the eastern lobe of the related ultramafic suite of the Bushveld Complex (2050 Ma).
The Phalaborwa Igneous Complex is predominantly composed of dunite, pyroxenite and apatite-rich pegmatoidal pyroxenite, with varying proportions of diopside, phlogopite and apatite, ranging in texture from fine and medium grained to pegmatoidal zones. A feldspathic pyroxenite zone occupies much of the outer rim with the intruded Archaean granite gneiss country rock.
The complex encloses three coalescing, pipe-like cores of pegmatoidal pyroxenite, with pyroxene and mica crystals up to 10 to 20 cm long, which are distributed along the long axis complex. These are the Northern, Southern and central Loolekop pyroxenites, which together account for around 53% of the area of the complex. The Northern and Southern pyroxenites host the Foskor deposits, while the Loolekop Pyroxenite encloses a central, vertically plunging ~2 x 1 km carbonatite zone that hosts the Palabora Cu-Fe-phosphate deposit (see the separate Palabora record). Foskor owns the rights to phosphate-bearing tailings from the Palabora Cu-Fe operation exploiting the Loolekop Pyroxenite. However, in 2003 a feasibility study of a tailings reclamation project revealed that the planned capital and operating costs would exceed actual benefits. The project was aborted and Palabora Mining Company continued to add to the tailings dam.
Each of these three pyroxenite cores has a concentric zoning structure. The Northern Pyroxenite encloses irregular zones of serpentine pegmatoid, as detailed below, but no major carbonatite mass, while the pipe-like carbonatite zone within the Loolekop Pyroxenite is progressively inwardly zoned from foskorite (olivine/serpentine-magnetite-apatite-calcite rock), to banded carbonatite and finally transgressive carbonatite.
The Phalaborwa Igneous Complex and Archaean country rock granite gneisses are overprinted by a destructive metasomatic halo, reflected by satellite intrusions, alteration zones, and aeromagnetic anomalies that extend for tens of kilometres from Palabora itself. This halo is largely the result of metasomatism (fenitisation) by K-Na-Ca-Fe-Mg and CO2 - rich magmatic chloride brines (with abundant volatiles, particularly fluorine) emplaced at a depth of ~12 km.
The Northern Pyroxenite is the largest of the three cores, and has been sub-divided into five roughly concentric zones, based on characteristic textures and mineral composition, from margin to centre of:
(i) Feldspathic pyroxenite, composed of diopside, microcline and apatite, which forms a virtually continuous marginal zone around the whole complex. In the Northern Pyroxenite it forms a transitional zone between syenite and massive pyroxenite. It has a chilled margin and sharp contact with country rock granites and the fenite alteration halo to the complex. There is a gradual decrease in feldspar content from its outer to inner margins;
(ii) Apatite-rich phlogopite-pyroxenite, containing diopside, phlogopite and apatite, is the most widespread phase of pyroxenite in the Phalaborwa Igneous Complex and the chief source of phosphate ore. This zone has been subdivided into three sub-zones, as follows from outer to inner: (a) Massive pyroxenite, which is the most homogeneous of the pyroxenites, although there is still a variation in the distribution of phlogopite and apatite. It occupies ~20% of the mined area and has gradation boundaries with enclosing phases, although sharp margins are also mapped; (b) Phlogopite-apatite pyroxenite and pyroxenitic glimmerite, which have no clear contact with each other, or with the enclosing massive pyroxenite or glimmerite, and may be regarded as transition stages between the massive pyroxenite an glimmerite. The former is the most abundant, with the pyroxenitic glimmerite only being lenticular and less extensive. Both show evidence of multiple intrusion and replacement. Both also have a weak vertical banding based on the presence of phlogopite; and (c) Glimmerite, occurring as apatite-rich glimmerite in this zone, as distinct from the very similar apatite-poor varieties in the apatite-poor pyroxenite zone. It occurs as irregular lenticular bodies up to 75 m across and accounts for ~7% of the mined area. It is usually schistose, but conformable with the structure of the complex, although discordant tongues are also evident. It has sharp margins with the massive pyroxene, but gradational contacts with phlogopite bearing rocks;
(iii) Apatite-poor phlogopite-pyroxenite, made up of diopside and phlogopite. This, and the following inner zones, is restricted to the Northern Pyroxenite, where it forms a broad irregular belt varying from massive pyroxenite to glimmerite and is virtually devoid of apatite. It is cut by numerous dykes of pegmatoid, carbonatite and syenite and has a sharp outer margin with the surrounding, visually identical apatite-rich phlogopite-pyroxenite;
(iv) Phlogopite-pyroxenite pegmatoid, predominantly composed of diopside and phlogopite, occurring mainly as dykes from a few cm to 5 m in thickness, with sharp contacts cutting the other pyroxenite rock types. The dykes have chilled margins, with the coarsest crystals in their centres. This phase is also found as irregular segregations with gradational boundaries with the surrounding pyroxenites. Although these segregations are concentrated in the apatite poor section of the pyroxenite adjoining the central barren core, the associated apatite-poor dykes do penetrate the surrounding apatite-rich section zones. However, pegmatoid dykes composed essentially of phlogopite and apatite with subordinate diopside are also mapped, and are younger than all of the other rock-types of the Northern Pyroxenite;
(v) Serpentine-phlogopite pegmatoid, predominantly composed of serpentine, phlogopite and minor diopside with little apatite, this is a heterogeneous and highly irregular central zone resulting from the disruption and metasomatic alteration of a dunite plug, occurring as small blocks enclosed within pyroxenite rocks. The dunite has been both serpentinised and phlogopitised. The phlogopite has been extensively altered to a depth of 50 m to vermiculite, forming the main exploited source of that mineral.
The Southern Pyroxenite hosts the largest phosphate resource in the complex, in part because it does not encompass any apatite-poor phase, with all of the pyroxenitic rocks being phosphate-bearing. The zonation is not as well developed as in the Northern Pyroxenite. Phlogopite-pyroxene-apatite pegmatite occurs as isolated dyke-like and segregation masses near the centre of the lobe. About 70% of the pyroxenite is occupied by equivalents of the apatite-rich phlogopite-pyroxenite and its sub-types described from the Northern Pyroxenite above. The pyroxenites are intruded by dykes of syenite and carbonatite, the latter becoming more abundant towards the Loolekop Pyroxenite
The Loolekop Pyroxenite is described in the Palabora record, although it should be noted that the core of Foskorite and carbonatite is surrounded progressively by phlogopite-pyroxene-apatite pegmatoid (not well developed, occurring within a 150 to 200 m wide belt outboard of the foskorite as small bodies with sharp contacts and as diffuse segregations), apatite-rich phlogopite pyroxenite (similar to the other two lobes, but with a lower apatite content) and feldspathic pyroxenite.
The Phalaborwa Igneous Complex is interpreted to have been emplaced in a repetitive cyclic manner. During the first cycle, a pre-existing dunite plug (now in the core of the Northern Pyroxenite) was broken up and phlogopitised by the emplacement of apatite-poor pyroxenite magma, followed in turn by extensive pipe-like bodies of apatite-rich pyroxenite, separate plugs of syenite, foskorite and carbonatite. Each pyroxenite stage comprised consecutive sub-stages characterised by increasing volatiles and hence phlogopite content, possibly overlapping each other, with accompanying metasomatic replacement producing heterogeneity. During the second cycle, the four main stages of the first cycle were repeated, but in a more restricted and transgressive manner within a more restricted area (Fourie and Jager, 1986).
Resource and reserve figures at 31 March, 2010, using a 5.5% P2O5 cut-off (Foskor website, 2012), were as follows:
North Pyroxenite Pit
Proved reserve - 433.0 Mt @ 7.04% P2O5
Proved reserve - 98.0 Mt @ 6.56% P2O5
South Pyroxenite Pit
Proved reserve - 997.0 Mt @ 6.91% P2O5
Proved reserve - 64.3 Mt @ 6.56% P2O5
Loolekop Pyroxenite - Palabora Tailings in 2003
Resource - 297.0 Mt @ 6.6% P2O5
Material with a grade of between 4% and 5.5% P2O5 is regarded as marginal ore and not included in these figures.
Most of this summary is derived from Fourie and Jager, 1986.
The most recent source geological information used to prepare this decription was dated: 2012.
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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Fourie P J, de Jager D H 1986 - Phosphate in the Phalaborwa complex: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 2239-2253
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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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