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Atlantic Coast Phosphate Province - Four Corners, Aurora, CF Hardee, Fort Green, White Springs |
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Super Porphyry Cu and Au
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The Atlantic Coastal Plain of the US contains a large phosphatic province that extends from the southern tip of Florida, to southern Virginia. It includes deposits both on land, and in the adjacent part of the continental shelf of the Atlantic Ocean. Historically, Florida has been the largest producer of phosphates in the US and one of the largest provinces in the world, with global resources in excess of 17 Gt of economic or marginally economic material with primary, or concentrated grades of >30% P2O5. However, by 2005 much of the accessible high grade ore had been exhausted and many mines had closed or were only working lower grade beneficiable material (Zhang et al., 2006), although a number of significant operations continue, with ~25 Mt of phosphate rock (plant capacity of ~31 Mt) being produced within the state per annum (IMC, 2001). Another group of now exhausted deposits was located near Savannah in Georgia (Riggs, 1979), while a large mine currently (2012) extracts ~4 to 5 Mt of phosphate rock per annum in North Carolina (PCS, 2012).
The most significant mining operations from 2001 to 2012 are:
Four Corners, Fort Green, CF Hardee, Kingsford, Hopewell, South Fort Meade, Hookers Prairie, Lonesome, Ona and Pine Level, all in the Central Florida Phosphate District, ~55 km to the SE of Tampa in western central Florida, within a 40 km radius of #Location: 27° 40'N, 82° 05'W.
White Springs and Suwannee River in the Swift Creek district in Hamilton County, northern Florida, ~110 km west of Jacksonville, at #Location: 30° 26'N, 82° 48'W.
Aurora (formerly Lee Creek) mine, 8 km north of the township of Aurora in Beaufort County, North Carolina, #Location: 35° 23'N, 76° 48'W.
The North American Atlantic coast had been on the passive spreading margin of the Central Atlantic from the Middle- to Late-Triassic, prior to which it had been adjacent to Senegal and Mauritania in west Africa. The Florida peninsular is currently the ~250 km wide, 750 km long, NNW-SSE elongated low relief emergent portion of a broad 850 km wide platform, that was emergent during Late Cretaceous to Quaternary low sea levels, and is bounded to the east and west by steep continental margins. To the north, as part of the Atlantic spreading extensional regime, a rift related ENE-WSW trending seaway straddling the Georgia-Florida border connected the Atlantic Ocean and Gulf of Mexico during parts of the Mesozoic and Tertiary, and separated Florida from the Appalachian highlands of the main North American plate to the north. To the south, the Florida platform abuts Cuba and the Bahama Platform (underlain by exotic South American crust) across a complex transcurrent structure and sutured collision margin (Hine, 2009).
Following the initial Triassic break-up with west Africa, deposition on the passive margin was in a series of block-faulted marginal basins between the eastern continental margin and the current east coast of Florida (below the section of the continental shelf known as the Blake Plateau), where thick (8 to 12 km) sections of Middle Jurassic to Lower Cretaceous, mainly shallow-water carbonate-rich sediments, are known from petroleum wells. These basins are bounded to the east by fault zones that correspond to the continental margin, and to the west by an exotic basement high block of former west African crust (Precambrian crystalline and Palaeozoic sedimentary, volcanic and granitic rocks) that underlies the current Florida Peninsular, where only an onlapping thinner Lower Cretaceous sequence of shelf sediments is recorded. Uplift during the Mid- to Upper Cretaceous is reflected by an unconformity truncating the Lower Cretaceous succession across the platform, followed by submergence and a thinner sequence of Late Cretaceous shallow-water carbonate sediments that blanketed the entire platform (Hine, 2009; Altschuler et al., 1994; Riggs, 1979).
This tectonic pattern is complicated by a generally ESE-WNW trending hinge line, or monoclinal flexure (Parallel to the Florida-Georgia border seaway to the north) half way down the peninsular, passing through Tampa Bay and the Canaveral salient on the west and east coasts respectively. To the south of this line, the Late Cretaceous and Tertiary sequences thicken considerably, from a thin clastic platform to the north to a thick carbonate basin to the south, separated by an intermediate marly facies in central Florida. A number of large domal structures are superimposed on the northern platform, resulting in the NNW-SSE trending Ocala Upland occupying the western half of the northern platform between the northern border of Florida and the Tampa Bay-Canaveral line, with other local highs to its east. These uplands were intermittently either emergent or shallowly submerged during the Tertiary (Altschuler et al., 1994; Riggs, 1979).
The elongate dome of the 300 x 50 km Ocala uplift trends NNW-SSE and plunges to the SSE. The bulk of the exploited phosphate beds in Florida are found over and fringing the Ocala Uplift and other domal highs to its east, and in the transition between the southern margin of the uplift and the thick carbonate platform to the south. Other phosphorite accumulations are found over domes in Georgia-Florida seaway in Georgia, and adjacent to the palaeo-shoreline of the emergent Appalachian high in North Carolina with the Atlantic continental shelf (Altschuler et al., 1994; Riggs, 1979).
In Florida, the uppermost part of the Upper Cretaceous, as well as the Palaeocene sediments of the central coastal plain, consist predominantly of authigenic glauconite with some dolomitic and phosphatic sediments, while the Eocene and Oligocene throughout the southeastern coastal plain are characterised by limestone and mixed limestone-terrigenous sedimentation (Riggs, 1979).
The oldest rocks exposed in Florida are Eocene limestones (mainly the Ocala limestone in northern Florida) representing the core of the Ocala uplift where they overlay Upper Cretaceous carbonate rocks only seen in drilling. In northern Florida, the: Ocala Limestone comprises a very pure limestone, which, where weathered, becomes soft, white and permeable and is commonly dolomitised. These are unconformably overlain by the Oligocene Suwannee Limestone which is 0 to a few metres thick and comprises limestone with minor sand and clay layers, but no phosphate, and where weathered contains secondary chert replacing limestone near the surface. Both the Ocala and Suwannee limestones have karst and highly eroded surfaces indicating periods of erosion and weathering prior to the Oligocene and Miocene deposition, and as such the top of the Suwannee Limestone is marked by an unconformity. It is overlain by the Miocene Tampa Limestone, a sandy, clayey limestone which contains chert nodules and trace phosphates. Where weathered it becomes a calcareous and sandy clay containing chert and phosphate, which is aluminium-rich near the surface. To the south and north these are respectively conformably and disconformably overlain, above an irregular surface, by the Miocene Hawthorn Formation, which is 5 to 90 m thick and composed of a buff, white or cream, sandy, clayey, phosphate-bearing limestone, which is dolomitic near the surface and includes interbeds of sand, clay or sandy clay. Where weathered it becomes a residual, calcareous sandy clay containing abundant phosphatic particles (the hard rock ore), grading downward into carbonate rock. This is in turn unconformably followed, above a very irregular, eroded karst surface, by the Pliocene Bone Valley Formation that includes a 5 to 8 m thick lower unit that comprises a green, brown and black sequence of sand, clay and gravel containing very abundant phosphate particles and matrix phosphorite. The lower unit is bedded, with graded- and cross-bedding and where weathered occurs as a phosphorite as for the unweathered sections. The contact with the 0 to 3 m thick upper unit is gradational, passing up into a green, clayey sand with minor apatite particles that are more abundant at the base. The clay minerals are montmorillonite, weathered to kaolinite at the top. It is finely bedded, with graded- and cross-bedding. Where weathered it is a white clayey sand, which is leached and indurated, with secondary aluminium phosphate minerals replacing clay and apatite. Where partially eroded, the aluminium phosphate alteration extends into the lower unit. All of these units may be unconformably overlain by an overburden of Pleistocene to Recent unconsolidated quartz sand with some organic content and ground water podsols (Altschuler et al., 1994; Riggs, 1979).
The phosphatic Hawthorn Formation in Florida, and the equivalent Pungo Formation in North Carolina, is an extensive unit of open-shelf carbonate rocks that is widespread throughout the coastal plains of the south-eastern US, extending over a strike length in excess of 800 km, from southern Florida in the south to southern Virginia in the north. This unit is the immediate source of significant secondary phosphate deposits in Florida, Georgia and North Carolina (Altschuler et al., 1994).
On the margins of the Ocala and related uplifts the primary Hawthorn Formation contains 2 to 10% low-grade silt- and sand-sized phosphate pellets dispersed in a matrix of fine-grained and partly bioclastic limestone and dolomite. Kazakov (1937) postulated that the primary precipitation of marine phosphorite of the type in the Hawthorn Formation was caused by upwelling along steep continental margins of deep ocean phosphate-rich water onto relatively shallow continental shelves. Phosphate solubility in sea water varies inversely with pH, and consequently varies directly with dissolved CO2, which is high in the cold waters of the deep ocean. Upwelling of such water into a shallow, hot agitated shelf environment releases CO2, consequent increase in pH, supersaturation and precipitation of phosphate (Altschuler et al., 1994).
Following deposition, exposed parts of the Hawthorn Formation were leached of carbonates during an erosional interval prior to the deposition of the Bone Valley Formation. The insoluble residues, further enriched in phosphate cement, with quartz and clay, accumulated at the surface, particularly in solution/karst depressions. These deposits take the form of a recemented solution breccia, rich in residual sand, pebbles and internal moulds of fossils, and contain both primary apatite nodules and secondary phosphatised limestone pebbles. Leaching of a thin cover of Hawthorn Formation has led to the phosphatisation of subjacent limestone to form hard rock deposits. The leached rock is generally bleached and mottled, deflated and softer, containing relatively higher concentrations of sand-size primary apatite pellets. These deposits of karst-controlled residuum and replaced limestone are locally termed hardrock deposits, but are generally small (50 to 100 000 tonnes), but of high grade. They occur principally as an arcuate belt between 28° 34' and 30° 03'N, centred on longitude 82° 30'W, and lies along the core/crest of the Ocala Uplift. Two other outlying areas are also known. All three cover a cumulative area of 3000 sq. km, of which approximately 10% is underlain by phosphate deposits (Altschuler et al., 1994; Cathcart, 1989).
Most of the phosphatised and residually enriched weathering debris on the Hawthorn surface was reworked during marine transgression into the cross- and graded-bedded, pebbly and clayey sands of the Bone Valley Formation in broad depressions in the upper Hawthorn surface. The phosphate is differentiated into low-grade coarse material along basement ridges, and high-grade, fine-grained material in the lowlands adjacent to ridges representing embayments containing material winnowed from the shallower coarse ridge deposits which remain a lag deposits. In general the depositional environment of the Bone Valley Formation and its equivalents is that of a shallow water, wide marine embayment or estuary. The ridges influenced influenced deposition throughout emplacement of the Bone Valley Formation, and have been considerably uplifted since then. Phosphate in the Bone Valley Formation therefor represents two main stages, namely: (i) weathering of the sources limestone to remove carbonate, concentrate phosphate particles and phosphatise surfaces and fragments of limestone and dolomite; followed by (ii) reworking in a marine environment to winnow, concentrate and redeposit phosphate particles to enrich P2O5 and U3O8 (Altschuler et al., 1994).
Lateritic weathering altered the phosphorite to form aluminium phosphate zones. The upper Bone Valley Formation is commonly compact and indurated with a light grey to green-grey colour due to the development of secondary kaolinite, with overprinted red or orange colouration. In many of the mines, the upper part of the section is irregularly transgressed by a zone of white leaching that produces a vesicular appearance and is either friable or indurated, and very light in weight. It comprises quartz sand, cemented and indurated by the secondary minerals wavellite, crandallite and locally millisite, with no remaining carbonate, with clay being phosphatised and apatite altered to aluminium phosphate. Secondary precipitation of chert, apatite, limonite and goethite are found below the leached lateritic zone, occurring a discontinuous hardpan cementing porous sands and encrusting the upper surfaces of clay (Altschuler et al., 1994)
The principal area of mining to the SE of Tampa where the mines listed above (with the exception of the Swift Creek district) are located, exploits a blanket of pebbly and clayey sand composed of quartz and clastic phosphate sands, nodules and pebbles locally known as land-pebble ore, and defines the Central Florida Phosphate District. This ore comprises the weathered Bone Valley Formation and covers an area of 4500 sq. km, a north-south elongated oval-shaped district of ~100 x 50 km. Economic mineralisation also occurs in the upper part of the underlying Hawthorn Formation, but is restricted to around one third of the area of that in the Bone Valley Formation. The better ore is concentrated within a ~40 km diameter circular area centred on 27° 45'N, 81° 55'W, immediately south of the Ocala Uplift, in the marly transtion zone from the northern clastic shelf over the Ocala Uplift, to the southern carbonate basin of south Florida as described above. A second area of this ore type incorporates the Swift Creek district in Hamilton County and adjacent Columbia and Baker counties in northern Florida around 30° 28'N, 82° 48'W. Other areas in Marion, Putnam, Lake and Orange counties are also known (Cathcart, 1989). The economic zone of the land-pebble district comprises the lower, unweathered unit of the Bone Valley Formation and the underlying residual phosphorite of the Hawthorn Formation, which ranges from 0 to ~15 m in thickness, averaging ~3.6 m, being thickest in karst depressions in the upper Hawthorn Formation surface. The ore is an unconsolidated mixture of phosphate pellets, granules, cobbles and boulders of phosphatised limestone, quartz sand and silt, and clay. The material mined in 1996 averaged about equal amounts of recoverable phosphate particles (i.e., >1 mm pebble and <1 to >0.1 mm concentrate), quartz sand (tailings) and slimes (<0.1 mm, including clays). The pebbles are generally lower in P2O5 than the concentrate fractions, while the reverse is true for U3O8, with grades of generally 0.12 to 0.2 kg/t U3O8 (Altschuler et al., 1994).
Early mining also exploited river-pebble deposits, which occurred as bars and in the flood plains of streams that cut through pre-existing phosphate deposits. These deposits, which were generally small, reworked and transported phosphatic pebbles, and were largely along the Peace and Alafia rivers, with small individual deposits distributed over lengths of ~80 and ~40 km respectively (Cathcart, 1989).
In North Carolina, the Miocene Pungo River Formation, an equivalent of the Hawthorn Formation in Florida and Georgia, contains ~2 Gt @ 2 to 21% P2O5, below 13.5 to 75 m of overburden, and occurs over an area of 1800 sq. km. The lower part of the overlying Pliocene Yorktown Formation also contains phosphorite. In addition, deposits occur offshore in NE Onslow Bay and Frying Pan Shoals on the Blake Plateau continental shelf.
At the Aurora (Lee Creek) mine in North Carolina, the host sequence unconformably overlies the Eocene Castle Hayne limestone, and comprises ~12 m of the Miocene Pungo River Formation, which is unconformably overlain by the generally barren Pliocene Yorktown Formation.
The primary Pungo River formation consists of dolosilt, coquinoid limestone, phosphatic sandy clay, dolstone and phosphatic doloclaystone. It is predominatly composed of inter-bedded phosphatic sands, phosphatic/calcareous/diatomaceous clays, dolosilts and indurated limestones and dolostones. The phosphatic sands are made up of fine to medium-grained phosphate (francolite, a species of the apatite family) and quartz, with varying amounts of silt and clay sized material which also contain phosphatised fossil fragments. The surfaces of individual phosphatic sand grains are commonly characterised by concentric bands or rings. They are typically smooth, glossy, black to brown in colour, and spheroidal to ovate in shape. Pebble-sized phosphate grains (>10 mesh) are normally <5 vol.% of the phosphatic sand matrix. The quartz fraction is typically clear, flat sided, angular to subrounded grains. Accessories within the phosphatic sands include calcite, garnet and ilmenite. Weathered shell material is often contained within the more clayey phosphatic sands.
The clays (<200 mesh) within the Pungo Formation, comprise ~20 vol.% of the phosphatic sand matrix, and are principally phosphatic and diatomaceous, although some are calcareous. Phosphatic clays are an olive drab-green colour, and often have a high dolomite content, and commonly after drying, appear fissile. The diatomaceous clays are light-grey-green in colour, and are composed of up to 90% diatom shells and fragments in a silt to clay sized groundmass, while the calcareous varieties are usually light-green to light-grey to white.
An internal capping unit low in the formation locally overlies indurated limestone and comprises a discontinuous greenish-yellow unit of bryozoan dolosilt hash known as the Chartreuse Bed. Diatomaceous clays are occasionally inter-bedded with the coquinas and calcareous clays. The principle phosphatic ore matrix of sandy clay lies above dolomitic sandstone and phosphatic doloclaystone at the base of the formation.
Several rock types form indurated zones within the formation, including phosphatic limestones, silty claystones, coquinas and dolostone. The phosphatic limestones are usually white to dark-grey, and are typically inter-bedded dense vuggy limestones that contain varying amounts of marly phosphatic clay, quartz sand and pebble phosphate. They contain casts and moulds of shell material. Locally, this lithology is entirely composed of cast and mouldic limestone. The dolomitic sandstones at the base of the phosphatic ore unit that are well indurated are very competent rocks that usually have a limited lateral extent and grade both laterally and vertically into poorly indurated rocks.
The silty claystones, formed by the cementation of phosphatic clays, are usually light to dark-grey, and are commonly localised, grading both laterally and vertically in the phosphorite. They are typically friable, but locally may be cemented and competent. The occurrence of coquina limestones is of limited extent within the district. They are creamy-white to light grey, composed of shell fragments, whole shells, and re-crystallised calcite, very locally containing significant amounts of pebble and cobble sized phosphate. They vary in degree of induration from very poorly- to well-cemented, competent rocks, and are inter-bedded with calcareous clays that are white to light grey. A coquina bed caps the Pungo River Formation.
The Pungo River Formation was deposited during a transgressive - regressive cycle of the sea into a NE-SW trending basin. Southeast of Beaufort County, in the deeper part of the basin, the formation is comprised mainly of clay.
The Pungo River Formation is overlain by ~10 m of the Pliocene Yorktown Formation, which comprises a central fine-grained bluish-grey sand interval, sandwiched between two muddy sand units, with concentrated shelly lenses. This unit is in turn unconformably overlain by a shell bed of the James City Formation and by surficial sands that are together ~ 15 m thick. This Aurora description is paraphrased after an article written by T. Gilmore, of PCS Phosphate, Aurora, N.C..
Reserve and resource figures for states, districts and deposits within the Atlantic Coastal Plain phosphatic province are as follows:
Florida resources, 1989 (Cathcart, 1989), 5.6 Gt @ >30% P2O5 concentrate grade,
proven reserves, 2000 (IMC, 2001), 874 Mt @ >29.75% P2O5, and 65.0 % BPL (Bone Phosphate Lime) concentrate grade,
potential reserves, 2000 (IMC, 2001), 3 Gt @ >27.67% P2O5, and 60.5 % BPL concentrate grade,
Georgia resources, 1989 (Cathcart, 1989), ~1 Gt @ >30% P2O5 concentrate grade,
North Carolina onshore resources, 1989 (Cathcart, 1989), ~9 Gt @ 30% P2O5 concentrate grade,
offshore Charleston Bump resources, 1989 (Cathcart, 1989), ~2 Gt @ >30% P2O5 concentrate grade,
Four Corners, central Florida:
Total proven reserves at Dec. 31, 2000, 77.1 Mt @ 64.8% BPL, 0.67% MgO concentrate grade (IMC, 2001),
Fort Green, central Florida:
Total proven reserves at Dec. 31, 2000, 50.0 Mt @ 63.8% BPL, 0.64% MgO concentrate grade (IMC, 2001),
Kingsford, central Florida:
Total proven reserves at Dec. 31, 2000, 8.0 Mt @ 65.2% BPL, 0.71% MgO concentrate grade (IMC, 2001),
Hopewell, central Florida:
Total proven reserves at Dec. 31, 2000, 3.9 Mt @ 71.1% BPL, 0.34% MgO concentrate grade (IMC, 2001),
Lonesome, central Florida:
Total proven reserves at Dec. 31, 2000, 44 Mt @ 67.6% BPL concentrate grade (IMC, 2001),
Ona, central Florida:
Total proven reserves at Dec. 31, 2000, 97 Mt @ 64.7% BPL concentrate grade (IMC, 2001),
Pine Level, central Florida:
Total proven reserves at Dec. 31, 2000, 100 Mt @ 65.7% BPL concentrate grade (IMC, 2001),
Hookers Prairie and South Fort Meade, central Florida:
Total proven reserves at Dec. 31, 2000, 135.4 Mt @ 64.0% BPL, 0.61% MgO concentrate grade (IMC, 2001),
CF Hardee, central Florida:
Total proven reserves at Dec. 31, 2000, 193.8 Mt @ 65.0% BPL, 0.70% MgO concentrate grade (IMC, 2001),
White Springs and Suwannee River, northern Florida:
Total proven reserves at Dec. 31, 2000, 39.4 Mt @ 67.0% BPL, 0.50% MgO concentrate grade (IMC, 2001),
White Springs, northern Florida:
Total proven + probable reserves at Dec. 2011, 38.2 Mt @ 30.66% P2O5 concentrate grade (Potash Corp, 2012),
Total measured + indicated resources at Dec. 2011, 76.4 Mt @ 30.66% P2O5 concentrate grade (Potash Corp, 2012),
Aurora, North Carolina:
Total proven + probable reserves at Dec. 2011, 115.4 Mt @ 30.66% P2O5 concentrate grade (Potash Corp, 2012),
Total measured + indicated resources at Dec. 2011, 177.2 Mt @ 30.66% P2O5 concentrate grade (Potash Corp, 2012),
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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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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