Guyana-Surinam Bauxite - Linden, Ituni, Aroaima, Kwakwani, Tarakulli, Canje, Bonasika, Pakaraima
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The Guyana-Surinam bauxite belt is marked by a crescent shaped zone containing numerous bauxite deposits within the Atlantic Ocean coastal plains of these two countries. This zone is near 800 km long and 20 to 40 km wide and is outboard of the Guyana Shield. The bulk of the deposits form a 170 km long, NNW-SSE trending chain in northeastern Guyana from near the capital Georgetown, extending into northwestern Surinam. After a gap with less frequent occurrences, a second group of deposits form another ~160 km long, east-west oriented string in northeastern Surinam, extending eastward from immediately to the south of the capital Paramaribo.
The southern boundary of this belt is defined by the line beyond which the laterite deposits are too ferruginous and too siliceous to be economic, or where un-lateritised basement rock is exposed at the surface. The northern boundary largely corresponds to the point where the overburden is too thick, even
if bauxite is present at depth (Moses and Michell, 1963).
In addition to this line of deposits on the coastal plains, a number of others are also developed south and west of the craton margin on remnant plateaux surfaces representing Tertiary and older peneplains.
From ~1945 to ~1955, Guyana (then British Guiana) and Surinam produced around 50% of the world's bauxite. Although the tonnage produced continued to rise to a peak of ~10.5 Mtpa in 1970, compared to the ~7 Mtpa in 1955, the share of world output declined markedly as both demand and supply from other sources increased substantially.
Deposits later found to be bauxite were first noted in British Guiana by government geologists in 1869 and 1873, although the bauxite was not identified as such until 1897 during research by the tropical weathering expert John Harrison (Harrison 1934). The first shipment of bauxite was in 1917. From that year to the end of 1960, some 39 millon tonnes of bauxite had been produced. The Demara Bauxite Company (a subsidiary of Aluminium of Canada) produced the bulk of this tonnage, until nationalised as the Guyana Bauxite Company (GUYBAU) in July 1871. Reynolds Guyana, the second largest producer commenced mining in 1952, after taking over the Berbice Bauxite Company (a subsidiary of American Cyanamid) which had been mining at Kwakwani since 1942. Reynolds was also nationalised in 1975 and the company renamed Berbice Mining Enterprise BERMINE). Since then new and existing operations have continued, but production declined. The debt-ridden Guyana Mining Enterprise (GUYMINE) that was established to manage the government ownership of GUYBAU and BERMINE was liquidated and split into Linden Mining Enterprise (LINMINE) and BERMINE. Protracted efforts were made to privatise both during the 1990's, culminating in the acquisition of a 90% interest both enterprises by the Russian aluminium company RUSAL in 2004, with the residual 10% owned by the Guyana Government. The operating company in 2016 was the Bauxite Company of Guyana. The company's annual bauxite production capacity in 2015 was 1.7 Mt of bauxite. Most is shipped to the Nikolaev and Aughinish refineries in Ukraine and Ireland respectively. The Chinese company Bosai Minerals began operations at Omai, in the south of the Linden group of deposits in 2007 (Sources - USGS Professional Paper 1076-B (1986), various blogs and RUSAL website).
Bauxite was known in Surinam from the beginning of the 20th century, with the first shipment by Alcoa (Aluminium Company of America) in 1922. Alcoa operates locally in Surinam as Suriname Aluminium Company LLC (Suralco LLC). A second company NV Billiton Maatschappij Suriname began mining in 1941. A state owned company NV Grassalco was established in 1971 to enter into joint ventures with foreign companies for the exploitation of bauxite in the West Suriname area, particularly at Bakhuis. When both Alcoa/Suralco and NV Billiton were active, Surinam was the worlds largest producer, with a total cumulative production to 1960 of >50 Mt of bauxite. Since then peak production reached 7.8 Mtpa in the late 1970's but has varied since then with a low of 3 Mtpa in 1982, and 5.3 Mtpa in 2008. Since 1987, no bauxite has been exported with all processed at the Paranam refinery which was originally inaugerated in 1965 with a smelter by the Suralco to provide a fully integrated industry to make both alumina and aluminium. These plants use hydroelectricity generated from the Suriname River at Afobaka, and in 2016 produces ~3150 tpd of aluminium. In 2000 the smelter was decommissioned and dismantled. From the early 2000's until recently, the mines and refinery south of Paramaribo were operated by a joint venture between Suralco and BHP Billiton.
Geology and Mineralisation
The main bauxite deposits of Guayana are scattered through the interior coastal plains, specifically Linden (Mackenzie) along the Demerara River, 105 km from its mouth, Ituni, 60 km to the west of the Berbice River and Kwakwani, 10 km to the east side of the same river, while Aroaima (and the new Kurubuka-22 mine) is ~18 km to the north of the latter on the west bank of the river. Other deposits, such as Bonasika are 50 km north of Linden and 12 km east of the Essequibo River, in the coastal lowlands to the SW of Georgetown.
Aluminous laterites are also found in uplands at intermediate altitudes and are surrounded by coastal plains sediments (e.g., Blue Mountains and Pomeroon, NW of the Essequibo River mouth). However, much larger deposits of this type are developed on the higher interior plateaus (e.g., those spread over a 200 km belt in the Pakaraima Mountains, on the western border with Venezuela, ~300 km SW of Georgetown, and southwest of the main Guyana-Surinam bauxite belt). Similar deposits are found in the higher plateaus of Venezuela to the west.
The Pakaraima Mountains are known in three areas. Those in the Kopinang River area are reported to cover an area of 1160 km2 and average 3.5 m in thickness, with average grades from 35 test pits of 40.4% Al2O3, 6.1% SiO2, 2.1% TiO2, 27.6% Fe2O3. The deposits of the Sukabi River cover an area of 965 km2 with very limited sampling of the upper 5 m in three pits suggesting grades of 34.4% Al2O3, 16.5% SiO2, 1.9% TiO2, 25.8% Fe2O3. In the third area, the Kamarang-Kukui basin of the Mazaruni River, the upper 4.3 m in 16 pits averaged 33.3% Al2O3, 20.3% SiO2, 19.7% Fe2O3 (Patterson et al., 1986).
These deposits are developed on old peneplains, locally resting on weathered crystalline bedrock, particularly over mafic igneous rocks (greenstones), and generally grade downwards progressively from bauxite into a saprolite, a zone of sparock to weathered and fresh bedrock. In some cases they are overlain by Al-rich ferruginous hard-caps approaching bauxite grade.
As many as five peneplains are recognised across the Guiana Shield from the Amazon Basin to the Atlantic Ocean, at i). >65 Ma in the Jurassic to Cretaceous; ii). 50 Ma, early Tertiary, Paleocene to Eocene; iii). 25 Ma, late Tertiary I, Oligocene to early Miocene; iv). 5 Ma, late Tertiary II, Pliocene; and v). 1 Ma, Pleistocene to Recent (Aleva, 1981).
Marked peneplains are developed at 700 to 800 m and 400 to 500 m in the Pakaraima Mountains, with the lower of these sloping seaward, marking the plateau top at the Blue Mountains remnant mesa to the east at an altitude of ~300 m. The laterites on these uplands have formed on old emergent peneplains, while the deposits in the coastal plains were developed during regression and non-deposition reflected by unconformities that sandwich the bauxite layer in the coastal plains sequence.
The coastal plains bauxites may locally rest on weathered crystalline basement, but are dominantly found within the sedimentary sequence that comprises the Corentyne Group. The main bauxite layers are hosted within the kaolinitic clays and kaolinitic sands of the Coesewijne Formation, which is separated from the underlying basement by as much as 35 m of gravels, sand, lignite and banded clay of the Onverdacht Formation. The bauxite is overlain by the Berbice or White Sand Formation of the upper Coesewijne Formation, comprising white sand, sandy clay, silty clay and silt that was most likely deposited in brackish water. This sequence is variable in thickness, being almost absent at Moengo in the east of Surinam, thickening to >40 m at Onverdacht near Paramaribo, 100 km to the west, and varies from <1 to >10 m in Guyana. These are overlain, in turn, by the Pleistocene Coropina Formation, composed of 2 to 20 m of laminated mottled and hard clays with fine sand lenses and zones rich in organic material, that pass upwards into sandy clay and cemented sand. The uppermost units are the Recent Demerara Formation, comprising a lower unit of dark organic rich swamp clays and a thin layer of modern swamp.
The footwall beds are of Late Cretaceous (Maestrichtian) to early Eocene age, while those immediately above are early Oligocene, limiting the bauxite formation to the span from the early Eocene to early Oligocene. The Cainozoic sequence is up to 250 m thick in the central Berbice Basin, although in the region in which the bauxite deposits are found it is rarely more than 50 to 100 m.
The bauxite in most of the deposits of the coastal plain are underlain by clay, consisting mainly of kaolinite and lesser halloysite. According to Patterson et al. (1986), several authors (e.g., Kersen, 1956) agree that the kaolin layers preserved beneath the bauxite are the immediate parental material of the bauxite. This is suggested by the observation of electron- and photo-micrographs of partially leached kaolinite crystals and kaolinite partially altered to gibbsite.
Two main styles of bauxite formation are suggested in the Guyana-Surinam bauxite belt, namely i). in situ lateritisation, evident in the inland plateau deposits and at at one deposit in the Mackenzie district, and another at Ituni; and ii). a two stage process involving in situ weathering, followed by transport of the weathering clay materials, deposition and bauxite formation.
According to Dixon (1989), in tropical conditions, such as those in Guyana and Surinam, where there are alternating wet and dry seasons, alumino-silicate minerals decompose into aluminium hydroxide minerals and silica. At the same time, iron-bearing minerals in the same rock are broken down into iron hydroxide and iron oxides. In the wet season, soluble material, such as alkalies and alkaline earth minerals, are readily removed in solution, while normal groundwater (which is slightly alkaline) will slowly remove silica. An organic-rich soil cover will impose reducing conditions that result in some iron being taken into solution in the wet season. During the ensuing dry season, near surface evaporation promotes upward migration of pore fluids by capillary action, resulting in iron being brought to, and precipitated near the surface. This results in an iron rich cap, overlying a bauxite layer, beneath which is a clay-rich saprolite interval and then sap-rock, grading into weathered and finally fresh bedrock. This is in situ lateritisation. The development of such a profile requires the continuation of tropical climate conditions over a long period in a tectonically stable crustal environment.
Most of the coastal plain deposits in Guyana and Surinam are not typically of this character. The bauxite textures, its heavy/resistate mineral content and distribution (which occur as discrete layers, e.g., at Onverdacht) suggest the bauxite was developed from arkosic sediments. However, there is no evidence it was directly derived from the sedimentary rocks below the kaolinitic clay layer that underlies the bauxite (Dixon, 1989). Instead a two stage process is envisaged, involving deep weathering of metamorphic and igneous rocks inland over the Guiana Shield, followed by erosion, transport and deposition of the kaolinitic weathering products as kaolinitic swamp clays in the coastal lowlands below the shield margin scarp (Moses and Michell, 1963; Vletter, 1963). It is suggested by these authors and Patterson et al. (1986), that the coastal plains bauxites were formed by desilicification of these transported kaolinitic clay beds during an extended period of tectonic stability, probably as long as 5 to 15 Ma, from the Eocene to Oligocene.
Additional bauxite was locally developed from parent Pleistocene illite clays of the Coropina Formation beneath decaying organic matter in swamps. Halloysite occurs as an intermediate product between the illite and gibbsite (Kersen, 1956).
Aleva (1981) describes the coastal plains deposits as forming bauxite cappings on Late Cretaceous to early Eocene poorly consolidated rocks that separate it from the basement, as decribed above. The bauxite cappings have the shape of broad domes or flat inverted saucers, and top steep-sided, escarpment-flanked plateau hills of irregular shape and highly variable size. The bauxite caps of these plateau hills range in maximum size from a few metres to as much as 20 m. Individual deposits contain from 0.1 to 1 Mt. They are incised to a depth of about 4 m by dendritic drainage systems which drain to both the west and east sides of the plateaux. Some of these bauxite-capped hills are visible in the terrain in the southeasterly part of the coastal plain, although they are mostly overlain by the Oligocene and younger sediments thickening toward the NW.
Aleva (1981) summarises the main bauxite horizon on the coastal plains as follows, largely based on sections in the Onverdacht-Lelydorp area south of Paramaribo. It is composed of three main layers:
• The bottom layer is predominantly laminated bauxites, enclosing pockets of massive bauxite, and has a characteristic box work and cavity texture with hard gibbsitic partition walls enclosing open voids and/or kaolinitic or gibbsitic clay. When gibbsite fills the space between the partitions, the material as a whole constitutes bauxite, containing <1% ferric iron and <2% silica. This bottom layer is generally best developed in the buried plateau hill deposits. The lower boundary of the bauxite horizon with the underlying pure kaolinitic clay, although sharp, is very irregular, with blocks of kaolinite up to 1 m across found above the base of the bauxite at Onverdacht. Also at Onverdacht, small but significant layers of heavy/resistate minerals are found in this lower, laminated layer. The bottom layer in places also exhibits large-scale (1 to 30 m in size) relict structures of sedimentary beds.
• The middle layer is light tan-coloured, and composed of a high grade nodular, concretionary, or cellular bauxite with both the Fe2O3 and SiO2 content usually <3%.
• The top layer, which is up to 3 m thick, originally consisted of dark brown iron-rich bauxite or bauxitic iron laterite, with pseudobreccia textures, and locally with a vertical columnar structure. In the buried deposits, this iron-rich top layer is only present as pebbles and remnants of a few tens of metres across, with the remainder of the layer being altered, probably under the influence of the covering layers and brackish ground water, into a pinkish-orange low iron bauxite with dykes and veins of younger in situ formed kaolinite and metahalloysite. Overall this upper layer commonly carries 20 to 25% reactive silica and is discarded with the overburden.
Brindley and Sutton (1957) investigated the composition of bauxite at a group of representative mines and found that gibbsite [Al(OH)3] is the principal mineral in all deposits investigated, with some samples containing up to 95% of the mineral. Boehmite [AlO(OH)] is also present at all deposits, but with a very irregular vertical distribution, generally varying from 1 to 3%, with 1 deposit having layers containing up to 5%, and individual samples of up to 15%. Anatase [TiO2] is uniformly distributed through most bauxites which contain 1 to 2% of the mineral. Either goethite or hematite are found at most deposits. Goethite rarely exceeds 5%, although in some, layers of 5 to 15% are recorded. Hematite sometimes forms layers with up to 10%.
Districts and Deposits
This following summarises the main deposit clusters, from NW to east, around the main crescent shaped Guyana-Surinam bauxite belt:
Pomeroon Group - ~70 km NW of Georgetown, in Guyana (#Location: 7° 13' 24"N, 58° 43' 34"W).
At least five occurrences of bauxite are indicated over an area of 3 x 2.5 km, centred ~4 km NW of the Essequibo river mouth, and within 2 to 8 km of the Atlantic coast. In this area, Tertiary marine and fluvial sands with clays and silts of the Berbice Formation overlie the basement rocks. Bauxite pebbles containing 57% Al2O3 were first reported in 1916, leading to prospecting along creeks and shallow pitting investigations in the 1920’s. Barima Minerals Limited drilled selected targets in the Pomeroon area during 1957. A number of showings consisting of re-cemented boulders of bauxite were located as well as several discrete tabular bodies of bauxite. To date (2016) no significant bauxite has been mined. Although the bauxite in the Pomeroon area is overlain by the Berbice Formation, it is uncertain if it was developed on transported weathered kaolin, or directly from the underlying crystalline basement.
Blue Mountains - 65 km WSW of Georgetown, in Guyana (#Location: 6° 33' 54"N, 58° 46' 29"W).
Some 65 holes were drilled during 1955-56 on a plateau in the eastern Blue Mountains, all of which reportedly penetrated commercial-grade bauxite layers. The clay-rich bauxite in this area is overlain by <1 to 3.5 m of high-iron laterite containing up to 70% Fe2O3 and the bauxite, of unspecified thickness, returned a grade of 50.52% Al2O3, 19.31% Fe2O3, and 2.43% TiO2.
The bauxite layer is developed on a gently SE dipping peneplain surface, underlain by crystalline basement of the Guiana Shield. To the south, on the banks of the Essequibo River, granite and gneiss are exposed, intruded into the Palaeoproterozoic Barama Group sequence of metamorphosed volcanic and sedimentary rocks. Granitic rocks are reflected in an undulating topography, whereas the Barama Group metamorphic rocks impart a more rugged surface. South from the ENE-WSW trending White Creek, vertically dipping amphibole and chlorite schists, quartzites, phyllites and gondites are tightly folded along east-west axes. The Blue Mountain bauxites trend parallel to and are 1 to 3 km to the south of White Creek. Immediately to the south of White Creek, an elongated flat laterite plateau with an easterly dip overlies chlorite and amphibole schists of the Barama Group. Further to the south again, available basement exposure is predominantly of biotite granite. Tertiary marine and fluvial sands with clays and silts of the Berbice Formation overlie the basement rocks, although the hills appear to have been capped with laterite or ferruginous laterite prior to the deposition of the marine and/or fluvial Berbice sediments. Erosion prior to deposition of the Berbice sediments removed portions of the upper part of the laterite caps and the colluvial and detrital debris were deposited around the flanks of the hills. The detrital deposits were covered by Berbice sediments and renewed erosion produced further debris from the remaining hills which were incorporated into those sediments as boulders and pebbles. The bauxitic boulders that were derived from the original laterite capping may not have initially been bauxitic, but may have been iron-rich initially and then leached to form a suitable combination of SiO2, Fe2O3 and Al2O3 to constitute ore grades of bauxite.
Bauxite float occurs along the southern flank and eastern end of the WNW-ESE trending Blue Mountains plateau. Holes drilled into this area did not encounter economic bauxite. However, pisolitic bauxite occurs as cappings on low hills that are ~20 metres above the surrounding low swampy areas between the Blue Mountains laterite (to the north) and adjacent hills of Berbice sediments (to the south). On the eastern part of this ridge, on a 130 metre asl plateau, 65 holes, drilled by Harvey Aluminum in 1957, penetrated red-brown vesicular bauxite with an average grade of 50.52% Al2O3, 5.54% SiO2, 2.43% TiO2, 19.31% Fe2O3. The bauxite on this plateau extends over a strike length of ~11 km, and width of up to 750 m in three aligned blocks separated by intervals of 750 to 1500 m. The bauxite is capped by 1 to 4 metres of laterite, with grades of 70% Fe2O3. Red residual clays with variably high alumina content and preserved schistose textures form the base of this occurrence, and the bauxite appears to have been derived directly from a band of aluminous rocks within the crystalline basement, but overlain by Berbice Formation sediments. (Summary drawn from "Meixner, H.M., 2008 - Technical report on the Bonasika Bauxite Project, Northeastern Guyana, Prepared for Academy Ventures Inc., 42p").
Essaquibo Group - 60 km west of Georgetown, in Guyana (#Location: 6° 29' 22"N, 58° 29' 25"W).
The Essaquibo Group of at least 5 economic deposits and >12 occurrences of bauxite define a NNW-SSE trending linear zone that is over a 50 km long and ~10 to 15 km wide. This trend is subparallel to, and within 20 km to the east of the Essaquibo River, in the northern part of the Guyana Coastal Plain to the west of the Demerara River.
The economic deposits of Bonasika 1, 2, 5, 6 and 7 are distributed over an interval of 11 km in the northern section of this trend. The ore reserve estimate, and a brief description of each deposit is as follows:
Bonasika 1, which is ~1000 x 400 m, elongated NE-SW. The average thickness of the bauxite horizon has been estimated to be 4.5 m and the average overburden depth to be 5.6 m although the bauxite was found to be locally exposed at surface.
Proved reserve - 1.398 Mt @ 54.6% Al2O3, 12.6% SiO2, 2.15% Fe2O3, 1.9% TiO2, 27.8% LOI,
Probable reserve - 0.063 Mt @ 52.7% Al2O3, 14.6% SiO2, 2.93% Fe2O3, 1.9% TiO2, 27.0% LOI,
Sub-Total - 1.461 Mt @ 54.5% Al2O3, 12.7% SiO2, 2.18% Fe2O3, 1.9% TiO2, 27.7% LOI,
Bonasika 2 - ~1.5 km SW of Bonasika 1, with dimensions of ~500 x 200 m, elongated NNW-SSE. The bauxite lens averages 2.4 m in thickness, with 6.0 metres overburden.
Proved reserve - 0.330 Mt @ 53.9% Al2O3, 14.8% SiO2, 1.77% Fe2O3, 1.9% TiO2, 27.0% LOI,
Probable reserve - 0.076 Mt @ 54.0% Al2O3, 14.6% SiO2, 1.93% Fe2O3, 1.8% TiO2, 27.0% LOI,
Sub-Total - 0.406 Mt @ 53.9% Al2O3, 14.8% SiO2, 1.80% Fe2O3, 1.9% TiO2, 27.0% LOI,
Bonasika 5 - ~3 km NW of the Bonasika 1, with dimensions of ~800 x 200 m, elongated NE-SW. The bauxite lens averages 7.9 m in thickness, with 4.8 metres overburden.
Probable reserve - 0.637 Mt @ 54.2% Al2O3, 13.9% SiO2, 2.08% Fe2O3, 1.7% TiO2, 27.1% LOI,
Note: The ore reserves quoted above for Bonasika 1, 2 and 5 are preliminary, Non NI 43-101 compliant reserve estimate as of November 2011, calculated by Met-Chem Canada Inc., for First Bauxite Corporation. The reserves for Bonasika 6 and 7 below are NI 43-101 compliant.
Bonasika 6 - ~6.75 km SE of Bonasika 1, with dimensions of ~1600 x 600 to 1000 m, elongated north-south. The deposit is essentially flat lying, although it is covered by variable thicknesses of overburden, ranging from 20 to 60 m, averaging 40 m. The average thickness of the bauxite lens is 4.1 m, with a maximum of 9.24 m. The iron content of both Bonasika 6 and 7 is significantly lower (~1%) than at Bonasika 1, 2 and 5 (~2%), and there is also much less laterite in the
Direct Feed Bauxite - Probable reserve - 2.037 Mt @ 61.21% Al2O3, 4.36% SiO2, 0.85% Fe2O3, 2.35% TiO2, 30.7% LOI,
Regular Grade Bauxite - Probable reserve - 2.912 Mt @ 57.13% Al2O3, 11.15% SiO2, 0.84% Fe2O3, 2.16% TiO2, 28.1% LOI,
Total probable reserve - 4.949 Mt @ 58.81% Al2O3, 8.35% SiO2, 0.84% Fe2O3, 2.24% TiO2, 29.2% LOI, 65.179 Mt of overburden, 13.17 strip ratio.
Bonasika 7 - ~4.5 km SSW of Bonasika 6, with dimensions of ~1600 x 500 to 1000 m, elongated NE-SW. The deposit is essentially flat and largely continuous, with the exception of local clay seams that likely result from sub-vertical fracturing and in-filling subsequent to bauxite development. The bauxite lens averages 4.2 m in thickness with an average overburden cover of 28.7 m, although it outcrops in the SW extremity, with the overburden thickening toward the east to a maximum of 42 m. It is similar in character to the bauxite developed in the Linden producing area immediately to the south, although not as thick, and is higher in silica.
Direct Feed Bauxite - Probable reserve - 1.874 Mt @ 60.20% Al2O3, 4.58% SiO2, 0.75% Fe2O3, 2.63% TiO2, 30.9% LOI,
Regular Grade Bauxite - Probable reserve - 2.890 Mt @ 55.75% Al2O3, 12.39% SiO2, 0.87% Fe2O3, 2.34% TiO2, 27.7% LOI,
Total probable reserve - 4.765 Mt @ 57.50% Al2O3, 9.32% SiO2, 0.82% Fe2O3, 2.46% TiO2, 29.0% LOI, 40.736 Mt of overburden, 8.55 strip ratio.
Each of these lenses of bauxite rests on white kaolinitic clay whose depth has not been determined (to 2015), other than in one drill hole at Bonasika 7 where decomposed gneisses was encountered at a depth of close to 40 m, beneath 27 m of increasingly saprolitic clays.
The bauxite lenses are low-lying, but outcrop locally where the overburden has been eroded, or the bauxite layer has been tilted. However, they are generally covered by poorly consolidated, shallow marine, deltaic and swamp sediments that were deposited during the late Tertiary transgression that submerged the outer margin of the Guiana Shield. The dominant of these overlying sequences is the continental to deltaic sequence of the Oligocene to Pleistocene Berbice White Sand Formation, that covers a wide extent of northern Guyana and largely conceals the principal areas of bauxite mineralisation. This sequence thickens to the north and NE and includes unconsolidated sands and poorly indurated sandstone, with lesser siltstone, mudstone, carbonaceous mudstone and clay.
No iron capping is seen on the bauxite profile associated with the Bonasika deposits, although local areas of more lateritic bauxite are evident. The transition from the bauxite lenses to the Berbice White Sand Formation is marked by a capping of mudstones, with increasing concentration of lignite, siderite and iron-sulphides towards the base. Below the lignites found towards the base of this layer, the mudstones commonly carry interspersed kaolinite and gibbsite.
Whilst the bauxite lenses are essentially flat lying, reported sections (Meixner, 2008; Ausenco, 2015) suggest they have measurable gradients of the order of 10 m vertical in 200 m laterally. These gradients may be relatively uniform in one direction, although in some cases (e.g., Bonasika 1 and 5) the lens has a downward gradient outward on either side of the core of the lens as the lens feathers on its outer margins. Tilting may in part by due to block faulting, or alternatively draping over basement plateaux. Bleackley (1964) notes post-Berbice Formation faulting, along the lower Essequibo River valley as well as east-west faulting to the north of the Bonasika area. The present day topography also exhibits evidence of block faulting with discrete more-or-less orthogonal escarpments (Meixner, 2008). This faulting, tilting or draping likely played a key role in providing the relief necessary to promote the appropriate geomorphic and hydraulic conditions to leach the weathered clays and and channel water (Meixner, 2008). Water flow within the bauxite appears more horizontal than vertical, largely through the granular horizons (Ausenco, 2015).
The bauxite lenses occur as a sedimentary pile with a granularity that ranges from clays to fine gravels. They have cores of higher grade mineralisation encompassing inter-layered units of hard massive bauxite, coarse, porous granular bauxite and very fine gibbsite units that have the appearance of clay. These high grade cores are bracketed above and below and laterally by an envelope of clayey bauxite, capped by a blue-grey bauxitic clay. The bauxite is largely composed of gibbsite and minor boehmite, with kaolinite, anatase, siderite and pyrite. Clasts of gibbsite and less frequent 'flint clay' (indurated kaolinite) are commonly found within the bauxite lenses.
The bauxite lenses are composed of the following facies (after Butty et al., 2011; Ausenco, 2015):
• Bauxitic clay, which is white to light grey, compact, with patches of pinkish to whitish bauxite (gibbsite) supported within the matrix. The bauxite patches gradually increase downwards in the profile and may reach one cm in diameter. Bauxite makes up less than 50% of the lithology. A transition may sometimes be observed from mudstone into a bauxitic clay with increasing white patches of kaolinite (and pink gibbsite) developing downwards into the bauxite horizon.
• Clayey Bauxite, which is cream to light grey, with 10 to 50% clay, comprising a high proportion of cream to pink grains or nodules of micro-crystalline gibbsite. Locally, siderite-enriched pockets may be associated with this unit, either, i). at the base of the bauxite profile, ii). near the contact with carbonaceous mudstone at the top of the profile, iii). at the base of the clayey bauxite unit, with minor pyrite occurring locally throughout. The clayey bauxite tends to form an envelope around a higher grade core zone of massive, granular and fine bauxite. Thus the bauxite horizon is usually capped by clayey bauxite and merges into clayey bauxite on the margins of the deposit.
• Fine bauxite, is a soft cream coloured variety, typically composed of very fine gibbsite. The gibbsite grains can become very coarse or nodular, but are always supported with a very fine cream colored gibbsitic matrix, making up >50% of the rock mass. It tends to be associated with the high grade core zones and typically occurs at the base of the bauxite profile, although it can also be seen at the top of the high grade core zone and sometimes intercalated within it.
• Granular Bauxite is a coarsely granular rock, largely composed of bauxite clasts or fragments and <10% contained clay. It is a vuggy, typically cream colored bauxite with interstices, veins and vugs that may be in-filled with clay. Amorphous gibbsite (and likely boehmite) may re-cement the mass and also partially infill the vugs. Washing reveals nodules of coarse to fine cream to pink gibbsite. Very locally, siderite may be associated with the lower part of the granular horizon.
• Massive Bauxite is composed of cream coloured, very compact micro-crystalline to amorphous gibbsite with minor boehmite and contains <10% clay material.
The granular, massive and fine bauxite lithologies constitute the higher grade core zones of the Bonasika deposits.
Linden - Mackenzie Group - 90 km south of Georgetown, in Guyana (#Location: 5° 59' 44"N, 58° 16' 30"W)
A cluster of significant mined and unmined deposits and occurrences of bauxite cover an area ~25 km in diameter, south of a gap of ~20 km from the southernmost occurrence of the Essaquibo Group. Within this area, a north-south elongated cluster of mined deposits occupies and area of ~20 x 8 km straddling the Demerara River.
At the Montgomery Mine, bauxite is seen to have formed in place from residual clay or granite. However, elsewhere in the cluster, the kaolin is separated from basement by Cretaceous to Tertiary sedimentary rocks. The individual bauxite deposits in the Linden Group may exceed 10 Mt, with remaining reserves of bauxite at East Montgomery in the order of 60 Mt (Butty et al., 2011). As at Bonasika, the bauxite lenses may outcrop locally, although the Berbice Formation overburden thickens to the NE to as much as 70 m at the active East Montgomery Mine. The district includes current (2016) significant producing mines, operared by RUSAL’s Bauxite Company of Guyana Inc.
Ituni Group - 140 km south of Georgetown, in Guyana (#Location: 5° 30' 46"N, 58° 15' 11"W)
The Linden and Ituni clusters appear to be separated by a further 20 km gap. The latter comprises a NW-SE elongated group of deposits and occurrences of bauxite covering an area of ~40 x 15 km. As in the Linden district, the bauxite and footwall clay is developed either directly on weathered crystalline basement as in the southern part of the district, or is separated from basement by Mesozoic to Tertiary sedimentary rocks.
Bauxite mined in the Ituni and surrounding district in the southern part of the belt in Guyana (Pollard and Barron, 1955) ranged in composition from 59.03 to 60.45% Al2O3, 4.78 to 7.07% SiO2, 1.17 to 2.85% Fe2O3, 1.90 to 2.53% TiO2 and 29.61 to 30.43% LOI.
The district includes current (2016) significant producing mines, operared by RUSAL’s Bauxite Company of Guyana Inc.
Aroaima - 165 km SSE of Georgetown, in Guyana (#Location: 5° 22' 3"N, 58° 1' 27"W)
The Aroaima deposits are situated on the west bank of the Berbice River, 14 km NNE of Kwakwani, and ~30 km SE of Ituni. The district includes the currently producing Kurubuka-22 mine, operared by RUSAL’s Bauxite Company of Guyana Inc.
Kwakwani Group - 180 km SSE of Georgetown, in Guyana (#Location: 5° 11' 49"N, 58° 1' 17"W)
The Kwakwani cluster of bauxite deposits and occurrences are mostly to the east of the Berbice River, distributed over a NW-SE elongated interval of ~20 km, separated from the Ituni group of deposits by a gap of ~20 km. The district includes current (2016) significant producing mines, operared by RUSAL’s Bauxite Company of Guyana Inc.
Tarakulli and Canje - Canje is ~180 km SSE of Georgetown (#Location: 5° 16' 59"N, 57° 38' 38"W; Tarakulli - 5° 8' 17"N, 57° 23' 53"W)
Tarakulli is an unmined (2016) bauxite deposit in southeastern Guayana, located ~90 km east of the Aroaima-Kwakwani bauxite mines, and ~15 km west of the Corentyne River, which defines the Guyana-Surinam border. The deposit was discovered by the Reynolds Metals Company in the 1960s and outlined by >700 drillholes on an ~500 x 500 m pattern within an area of 6.5 x 1.5 km. The bauxite lens was estimated to have an average thickness of 6.61 m, based on 40 contiguous intersections with a 1.82 m thickness cut-off, and is overlain by ~46 m of sand and clay overburden.
Tarakulli - Probable reserve of in situ wet bauxite - 62.8 Mt @ 58.6% Al2O3, 4.7% SiO2, 3.3% Fe2O3, 2.5% TiO2.
Half of the tonnage was metallurgical grade bauxite (3.0 to 5.5% SiO2 and 1.5 to 4.5% Fe2O3) and the other half of chemical grade bauxite (4.5 to 6.0% SiO2 and <1.5% Fe2O3). These chemical classifications were those accepted at the time of drilling.
The Canje deposit, is some 34 km SW of Tarakulli, and ~40 km east of Kwakwani, and was tested by Reynolds Metals in the 1960s, with 516 drill holes intersecting bauxite. Based on only 25 of these drill holes indicate bauxite over a thickness of 0.5 to 10 m, buried under 27 to 75 m of overburden.
For details of the deposits of this belt within Surinam, see the separate "Guyana-Surinam Bauxite - Onverdacht, Lelybad, Lelydorp, Kaaimangrasi, Klaverbad, Moengo, Bakhuis, Nassau" record.
The total combined reserve/resource estimates in Guyana is variable, as quoted by USGS Professional Paper 1076-B (1986) were as follows:
• Data supplied to the International Bauxite Association by the Guyana Government (1978),
Measured High Grade reserves - 520 Mt @ 50 to 63% Al2O3, 1 to 10% SiO2.
Measured Low Grade reserves - 170 Mt @ 30 to 55% Al2O3.
Possible High + Low Grade reserves - 1160 Mt including undeveloped resources.
• U.S. Bureau of Mines estimate (Baumgardner and McCawley, 1983),
Economic reserves - 700 Mt + Sub-economic resources - 300 Mt,
Historic production to 1960 - 39 Mt.
Much of this summary is drawn from Patterson, S.H., Kurtz H.F., Olson, J.C. and Neeley, DC.L., 1986 - World Bauxite Resources; U.S. Geological Survey Professional Paper 1076-8."
The most recent source geological information used to prepare this summary was dated: 2009.
Record last updated: 11/2/2016
This description is a summary from published sources, the chief of which are listed below.
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