Uranium of Algeria
1- History :
Uranium is the chemical element with atomic number 92, symbol U. It is part of the actinide family, it is the 48th most abundant natural element in the earth's crust, it is found everywhere in trace amounts , including in sea water, it is widespread in the depths of the terrestrial globe and more abundant in nature than gold or silver , especially in granite and sedimentary soils. It is a radioactive metal (alpha emitter), its 235U isotope is the only natural fissile isotope which makes uranium the main raw material used by the nuclear industry thanks to its immense fission property.
The ore of this fuel of tomorrow is called uraninite, or pitchblende, its first discovery was highlighted in 1789 by the Prussian chemist Martin Heinrich Klaproth from the analysis of a piece of pitchblende rock that had been brought to him from the Saint Joachimsthal mine.
Before the nuclear power event, uranium ores were sometimes used as a ceramic dye, a source of radium, and in some countries, a source of vanadium, long before perhaps because of the yellow coloration. and bright orange of its secondary minerals, it could be used as a ritual painting in primitive man, after the discovery of nuclear fission, a new industry was born with all the problems posed by the market.
In Algeria and from 1954, the French CEA (Commissariat à l'Energie Atomique) with the collaboration of the BRMA (Bureau de Recherche Minière in Algeria) launched the first uranium exploration program at Hoggar; which made it possible to highlight timid clues (El Bema, Furon, Timgaouine…) of no economic interest. In 1969, SONAREM (Société Nationale de Recherche et d'Exploitation Minière) began for the first time general reconnaissance work on uranium which targeted the verification of already known indices but also the formations of the Precambrian base as well as the land. Paleozoic sedimentary that surrounds it. It was on this basis that from 1971 a vast exploration program was developed and carried out.
In 1973, following the spectacular rise in uranium prices and until 1980, all the indexes listed were explored and evaluated. Over 80,000 meters of drilling have been completed. The major results obtained are:
The delimitation of the main geological provinces favorable to the establishment of uranium mineralization;
The constituency of ten serious clues;
Identification of 78 points of mineralization, 319 geochemical anomalies (punctual and superficial) and 80 aerospectrometric anomalies;
The discovery of four uranium deposits (Timgaouine, Abankor, Tinef and Tahaggart) with measured resources estimated at over 12.7 million tonnes at 0.187% U, or 23,000 tonnes of uranium metal.
The types of uranium mineralization identified are as follows:
• Quartz veins and molybdenum-uranium grinding zones associated with deep schear zones;
• Paleozoic carbonate and detrital sediments containing phosphate - uranium;
• Silicified uranium sulphide grinding zones housed in neoproterozoic black shales.
2- The deposits discovered:
2-1 - The TIMGAOUINE deposit:
The deposit is located 220km southwest of Tamanrasset,
The uranium mineralization is controlled by a shear zone comprising a system of parallel submeridian fractures with a dip of the order of 65 ° to the west. The uranium is carried by quartz veins and red quartzo-feldspathic breccias rich in iron oxide, calcite and siderose. The host rocks are represented by biotite granites.
2-1-1 -Works carried out:
• 1958-1960: Reconnaissance of the deposit by short soundings (20 to 30m) and
trenches (CEA, France),
• 1969-1970: General exploration by soundings and radiometric prospecting (SONAREM-GEOMINES),
• 1970-1972: General exploration of the areas of Abankor and detailed exploration of the areas of Timgaouine and Tinef (GEOMINES),
The work carried out amounts to:
• Geological surveys at 1/25000 (345 Km2), 1/1000 (1.85 Km2), 1/5000
(0.12 Km2), 1/2000 (2 Km2),
• Topographic survey at 1/1000 (1.85 km2) covering the deposit
• Radiometric prospecting at 1/10 000 (120 Km2), 1/500 (3 Km2).
• Trenches (500 m3),
• 99 boreholes (23,500 m) at regular mesh size of 80x40 m and 80x80 m
• research well (24.5 m),
• exploration well (93 m),
• gallery (140.3m),
• cross-benches (99.5m).
2-1-2 Geological setting:
It is a Neoproterozoic tectonic block developed over an area of more than 50,000 km². It is bounded to the east and west by deep submeridian shear zones materialized by mylonites and cataclasites. It comprises volcano-sedimentary and sandstone-detrital formations, structured in parallel linear bands of north-south stretches. They are separated from each other by calcoalkaline granitoids batholiths which represent more than 70% of the outcrops. Mafic and ultramafic rocks outcropping in small isolated massifs are intimately linked to volcano-sedimentary formations. Finally, the Iskel terrane includes several sub-circular bodies of post-orogenic granites known for their richness in wolfram, tin, beryl and sometimes uranium mineralization.
2-1-2-1 Mineralization:
The mineralization is associated with the large granitic batholith of Timgaouine which outcrops in a wide flat valley about 140 km long and 40 km wide. It is intrusive and formed essentially of an alkaline two-mica granite composed of idiomorphic quartz, perthitic orthosis, albite-oligoclase and secondarily apatite.
Mineralization carried by quartz veins, breccias and metasomatites line the roof of a submeridian shear zone called the 4 ° 18 fault. It is underlined by crushed, weathered granites. The lengths of the mineralized zones vary from 300 to 1200m. The geological section (fig. 1.2) established from soundings shows from West to East the following succession: pink granites with two mica; a silicified granite breccia; a stockwork area and associated veins
• 1976: Additional geological work on the Timgaouine and Abankor zones (Montan Consulting)
to a diabase dyke; a metasomatic red granite with calcite, hornblende and epidote; veins of syenite (or episyenite); and finally pink granites with two mica. The mineralogical association is characterized by pitchblende, occurring in three generations (I, II, III), black uranium oxides containing molybdenum (uranium molybdates?), Secondary uranium minerals (gummite, autunite?), molybdenite, two-generation pyrite, the first of which is contemporary with pitchblende I and some sulphides (chalcopyrite, marcasite, galena, sphalerite. Primary pitchblende (I) shows collomorphic structures of spherolitic appearance and irregular shrinkage structures. identified thanks to its reduced reflectivity, pronounced hardness and above all by the absence of shrinkage cracks. This rests on the primary pitchblende by forming outer rings. The pitchblende III marks the passage towards the black oxides. The Pechblende I and II are the most widespread and appear even at the -90m level (see cross-bench). Black oxides containing molybdenum are localized in highly cataclated and metallized granites. asomatized. They fill the cracks in the quartz grains with carbonates, where they form small veins. The yellow to greenish yellow secondary minerals have been described as either autunites or a mixture of uranotile and gummite.
2-2 The ABANKOR deposit
The Abankor uranium deposit is located east of Timgaouine. The mineralization is of the vein and stockwork type. It fits into two fracture systems, oriented respectively N 10 ° E and N 40 ° -50 ° E. These fractures are materialized by mylonites and cataclasites in an environment dominated by hyperalcaline granites with egyrine riebeckite.
2-2-1 Work carried out:
The work carried out on all the study sectors (Abankor center,
Abankor Nord, Daïra Nord, Daïra Sud, Hichem,) are as follows:
- Geological survey at 1/10000 (35 km2), 1/1000 (3.2 km2), 1/500 (0.1 km2),
- Topographic survey at 1/10000 (17km2), 1/1000 (3.2 km2), 1/500 (0.3km2),
- Radiometric prospecting at 1: 10,000 (296 km2), 1/5000 (80 km2),
- Trenches (2400 m3),
- 55 Boreholes totaling 9,878 m according to a mesh of 80x40m,
- Research well (42 m),
- Galleries (323.5 m),
- Collection and analysis of 1668 samples,
2-2-2 Geological Setting:
As with the Timgaouine deposit, the Abankor deposit is located in the eastern part of the Iskel terrane (Figure 2.1). The geological environment is marked by the presence of two Neoproterozoic volcano-sedimentary formations separated by a discordance
The lower series is mainly composed of marbles, quartzites and
chlorite schists associated with meta-andesites and meta-basalts in pillows It is intersected by basic and ultrabasic intrusions as well as by syn to late-tectonic batholiths dated respectively at 870 Ma and 840 Ma The upper series begins with base conglomerates, topped by an alternation of aged and pellites. The latter are covered by significant calc-alkaline volcanism (andesite, rhyodacite) which may be linked to a subduction zone.
All these series are cut by large calc-alkaline pre-syntectonic batholiths. Finally, intermediate series of molassic type outcrop in residual basins and grabens, in disagreement with the two series described above.
2-2-2-1. Mineralization:
The Abankor deposit appears as a small hill (h = 15m) with a high radioactive background (> 50µRh) compared to other areas studied in the region. It is characterized by a vein network
associated with stockworks or clusters of various shapes. The central part of the deposit is represented by two ore bodies. It is a vein called "vein of the wall" and a large cluster called "inflection" representing between them more than half of the reserves of the deposit with high grades (0.3-0.35% U). The main mineral is pitchblende which appears in three generations:
- primary pitchblende I, recognizable thanks to its collomorphic structures
spherolitic or cellular in appearance.
- pitchblende II, resulting from the transformation and replacement of pitchblende I. It is characterized by reduced reflectivity and the absence of shrinkage cracks. It forms outer rings.
- pitchblende III, very widespread, it marks the passage towards black oxides.
Compared to the Timgaouine deposit, the Abankor deposit is
characterized by :
- The presence of uraninitis;
- the abundance of black products with absence of molybdenum or in very tiny quantities;
- the abundance of pechblendes II and III;
- the scarcity of copper and zinc minerals as well as the low Pb contents. It follows that unlike Timgaouine which is a molybdenum uranium deposit, that of Afankor is almost a monometallic deposit. The mineralogy suggests, as in Timgaouine, the possibility of significant secondary enrichment.
It should be noted that the initial quartz feldspathic rock has undergone some
transformation process namely: mylonitization, hematitization, albitization, silicification, desilicification, carbonatization, pyritization, sericitization, chloritization and argilization.
2-3 TINEF deposit
The Tinef deposit, developed over an area of 21 km², is located 30 km north of the Timgaouine deposit, near the western edge of the batholith.
2-3-1 Work carried out:
The research work was carried out over a total area of 21km 2 The cross-cutting holes, carried out during the preliminary stage, were implanted in the perspective radiometric perimeters following an interval of 2-3km, then 1 - 1.5 km.
Prospecting (percussion) soundings were carried out over the entire extent of the area (over approximately 10km from North to South) with a mesh size of 200 x 200m. In heavily mineralized areas, the size has been reduced to 50x60m.
Sometimes, on a line of holes, the mesh was reduced to 25x50m, in order to verify the variation of the mineralization according to its estimated parameters (grade and thickness).
2-3-2 Geological setting:
The Tinef deposit as well as those of Timgaouine and Abankor are localize in the eastern part of the Iskel terrane where two volcanosedimentary cycles, separated by a discordance, have been distinguished: The Pharusian I and The Pharusian II.
Pharusien I: It is mainly composed of trees, quartzites, and chlorite schists associated with meta-andesites and meta-basalts in pillow. It is intersected by basic and ultrabasic intrusions as well as syn to late tectonic batholiths of Tin Tekadiouit and Taklet, dated respectively 870 Ma and 840 Ma.
Pharusian II: It rests in discordance with Pharusian I and begins with
base conglomerates followed by sandstone-pelitic filling. These terms are overcome by significant calc-alkaline volcanism (andesite, rhyodacite). This set is cut by large calc-alkaline pre-syntectonic catholites including that of Aouilène dated at 629 Ma .. The latest events closing the Pan-African orogeny correspond to the establishment of circumscribed, post-tectonic granites called "Taourirts".
This building is surmounted, in discordance with the formations of Pharusian II, by series of molassic type (Bourzzekal series) which outcrop in residual basins and grabens.
2-3-2-1 Mineralization:
The uranium mineralization of the Tinef zone is similar to that of the other two deposits, Timgaouine and Abankor. It is inscribed in quartz veins, breccias and metasomatites along the roof of a submeridian shear zone called the 4 ° 18 fault.
Uranium minerals are represented by yellow products (uranolite, gummites and sporadically autunite) and black products (uranium oxide). These minerals come from the weathering of the primary minerals of the "pitchblende - uraninite" group and may be coffinite. The more abundant yellow products appear macroscopically in the form of millimetric crusts with a resinous or vitreous luster, filling in cracks and cracks. The mineralogical association mainly comprises: uranolite, gummite, carbonates (calcite, dolomite, siderose), quartz, oxides and hydroxides
iron and titanium minerals. Under the microscope, gummites appear in various forms: bushy cryptocrystalline masses, in small patches and sporadically in aggregates of crystals.
Black products appear in the form of scabs, plating
millimeter and as a film coating. They are localized in the cracks of the host or in the pockets of dissolution often associated with carbonates, quartz, and sometimes sulphides.
The distribution of mineralization (in thickness and grade) is irregular both in surface area and in depth due to the fact that it is variously distributed: first in cracks, then in hematitized and carbonate rocks, finally in nodes (small enriched zones) formed by the crossing of veins.
2-4 TAHAGGART deposit
The Tahaggart deposit is located on the Asséo sheet, about 300km SE of the town of Tamanrasset.
Besides the Tahaggart deposit, other indices (Timouzzeline and Tamart-NBlis) are highlighted, and make this border of the Paleozoic basin (several hundred km ²) an interesting mining district.
Uranium mineralization can be localized both in soils
alteration only in the marginal parts of the first basal levels of the Paleozoic sedimentary cover.
2-4-1 Work carried out:
- Geological surveys at 1/1000 (2.70km2) and at 1/2000 (1.71km2), survey
topographic at 1/1000 (2.7km2).
- Radiometric prospecting at 1/10 000 (291km2), radiometric details at 1/2000 (1.50km2), at 1/200 and 1/400 (1.22km2).
- Percussion and core drilling (9881m).
- Mining works: trenches (110m3) and wells (15m).
- Sampling (270 points).
The soundings are implanted according to a square mesh 20x20m and 40x40m. In the parts where the mineralization is discontinuous, they are carried out according to a loose mesh greater than 40m.
2-4-2 Geological setting:
The Tin Siririne basin (Figure 3.1) is represented by tabular deposits of Paleozoic age.
• Cambro-Ordovician
It rests in discordance on the crystalline base. The first layers are
made up of sandstones on the side of In-Azzaoua. On the NE flank of the Tin Siririne syncline and in other places more or less distant from the Tahaggart area appear large pebble conglomerates of quartz (Ø> 3cm) with a power of about 2-10m, forming the inferior transgressive and discordant term on the base. Above the conglomerates lie coarse sandstones alternating with
micro conglomerates and followed by fine sandstones. The latter are marked by a crisscross stratification comprising lenses of clays and widely dispersed quartz grains. The thickness of these sandstones is variable and can reach 50m, towards the axis of the syncline.
• Silurian (Gothlandian)
Identified thanks to the presence of graptolites, it forms a more or less narrow band between the Cambro-Ordovician and the Devonian. There are places where the Silurian disappears and the Devonian is directly transgressive on the crystalline base.
• Devonian
The Devonian formations are the most developed in this syncline. We distinguish the Lower Devonian and the Upper Devonian.
• Lower Devonian
The base of the Lower Devonian consists of coarse sandstones and
conglomerates sometimes up to 300m thick (In-Atei). This complex is surmounted by a clay - sandstone layer with carbonate levels. In the southern zone of the syncline, we also note the presence of an intersecting stratification of conglomerate, sandstone and clay levels.
• Upper Devonian - Carboniferous
In In-Atei, above the formations described, appear sandstone slabs of Ginetian age on which the Upper Devonian begins. It is represented by a distinctly sandstone and very ferruginous formation. In the Devonian area, there is also the Carboniferous, thus forming the series known as "Tiberia series" about 450m thick which extends southwards to the border with Niger.
2-4-1-1 Mineralization:
The mineralization of the Tahaggart zone is located at the contact between the Eburnean crystalline basement and the Cambro-Ordovician sediments. It is represented by the secondary products of uranium rich in phosphorus (autunite, thorbernite) in the form of fine lamellar aggregates, in the form of nests, beaches and as fine impregnations in cement and veins.
At the surface, the mineralized sectors are unevenly distributed. Several ore bodies stretched in a North West - South East direction, dipping to the South West, were discovered in the study area (6Km²). It was established that the frontal parts of these layers were the most mineralized (up to a few percent), with decreasing uranium contents in the direction of the dip.
The thicknesses of the horizontal ore bodies vary from 1 to 8 meters. The secondary minerals are: autunite, sabugolite, thorbernite, carnotite. These minerals are found in the form of nests and venules.
- The important macroscopic characteristics of the selected samples:
• The Tahaggart (Tah) - gneiss deposit is altered, mineralized with very high radioactivity,
radiometric background >>> 20000cps;
• The Tahaggart deposit (GBT) - basal, oxidized and mineralized conglomerate, with yellow uranium mineralization, with very high radioactivity. Radiometric background >> 20,000 cps;
• Occurrence of uranium ore (As) - weathered whitish pink granite, syntectonic. Radiometric background> 3000 cps;
• Occurrence of uranium (A1) ore - altered red granite, post-tectonic, presence of mineralization with a radiometric background> 2000cps;
• Occurrence of Tamart-N-Iblis (TMB) uranium ore - arkosic sandstone, weathered and mineralized with a yellow product. Radiometric background> 3500cps; Occurrence of Timouzaline uranium ore (TMZ) - arkosic sandstone, mineralized, presence of a yellow product. Radiometric background> 4000 cps;
• Occurrence of Tedjart uranium ore (Tdj) - arkosic sandstone, oxidized and mineralized, the presence of a yellowish product with a very strong radiometric background> 18000cps.
2-4-2 Resources:
The resources were calculated by the geological block method.
The rich zones have been prospected by square mesh soundings (20x20m and 40x40m). On the other hand, the zones where lamination is discontinuous, the prospecting by sounding grid is rather loose (> 40m).
The overall resources in the Tahaggart area are estimated at
800,000 tonnes of ore grading 0.215% U, or 1,700 tonnes of Uranium metal.
3- Types of mineralization:
a) Uranium mineralization linked to the inconformities:
The discordance which separates the Pan-African basement from the Paleozoic lands constitutes a geological environment favorable to the establishment of uranium mineralization (mineralization linked to surfaces of nonconformities). This inconsistency (discrepancy) is developing all around the Hoggar massif, representing an overall area of around 30,000km². It is often represented by silico-ferruginous cement conglomerates in which uranium weathering minerals are found. The thickness of the bottom layer is about 1-3m.
b) Uranium mineralization of stratiform type:
The uranium mineralization identified in the sedimentary formations is located in 3 Paleozoic basins. These are Tin Seririne, Tafassasset and Ugarta. Apart from the deposits linked to the irregularities (crystalline basement-Paleozoic sedimentary basin interface, there is also a uranium-bearing sandstone benchmark level in the Devonian formations which first recalls the Arlit deposit which is located in the Carboniferous. Tamart In Iblis, covering an area of about 300 km ² This type of formation is very developed especially on the northern edge of the Hoggar shield over an area of more than 1000 km.
c) Lode type uranium mineralization:
It is represented by multiple showings of uranium mineralization which are associated with the crushing and silicification zones affecting volcanic terrigenic soils. Thus, the grinding zones discovered near the Tin Ezzararine showing (Laouni leaf), in the Neoproterozoic black schists, are mineralized in Uranium and Thorium with sulphides (pyrite).
It spatially coincides with the area of extension of the platform cover from the South to the Hoggar. Taken together, this sub-formation seems to offer prospects for the eventual detection of large uranium deposits from phosphate-uranium and rare-earth uranium metalliferous formations. The following surfaces of the sub-formation appear to be the most favorable:
i. Tahaggart metallogenic zone
ii. The metallogenic zone of Tamart-In-Iblis
iii. The Tamega metallogenic zone
iv. The Eridjane metallogenic zone
v. The Intabarekkat metallogenic zone
4- The Metallogenic Belts of Hoggar Central:
There is no doubt that the uranium metallogenic zones of the Hoggar Central metallogenic belt are interesting targets that should be further investigated.
a) The Tazoud metallogenic zone (20) corresponds to a pluton of the granodiorite-granite formation of the Neoproterozoic. The pluton and its host comprises multiple Vendien-Cambrian dykes and bodies, and possibly even mesozoic activation (?). The Ait Oklan - El Bena mineralized zone is considered to be potentially the most favorable zone.
b) The metalogenic zone of Teg ’Orek (32) corresponds to an intensely active granitogneissic block, cut by two submeridian tectonic accidents. A geochemical anomaly and a radioactive anomaly have been identified in this area which also includes the Teg ’Orak showing.
c) The metallogenic region of Timgaouine which is subdivided into two metallogenic zones of the molyddeno-uranium formation.
- The area of Timgaouine - Tinef
- The Afankor area
It is obvious that the execution of mapping and prospecting work geological detail and the performance of specialized radiometric work would have the effect of enhancing the mining prospects of these two zones and would eventually allow the localization of new deposits. In addition to the aforementioned metallogenic zones, the uranium mineralization, in association with gold and molybdenum, could probably still be localized in the metallogenic zone of the 4'50 'accident as well as in other adjoining structures or was manifested the magmatic activity of the Vendian-Cambrian or even of Mesozoic age.
5- Exploration methods:
The most widely used methods for uranium are based on
radioactive characteristics of uranium and its progeny. These elements emit radioactive radiation which can be measured (radiometry). However, geochemistry and other geophysical methods are also very useful.
5-1. RADIOMETRY:
Radioactive radiation is of several types Alpha (α), Beta (β) and Gamma (γ). The type and intensity of the radiation is characteristic of the element or parentage. The intensity of the radiation is proportional to the amount of uranium and daughter products. Alpha and gamma radiation are used in radiometry.
Beta radiation being very easily stopped by a physical barrier is not used in prospecting.
5-1-1 Gamma radiation:
Gamma radiation, because of its penetrating properties, is the most widely used.
5-1-1-1 Scintillometer:
The most widely used radiometric method for prospecting for uranium is scintillometry. The scintillometer replaced the Geiger counter. A sodium iodide crystal in the instrument is excited by gamma radiation. The resulting light excitation is then transformed into electric current through a photocell. This makes it possible to measure the total count of the gamma radiation. The discrimination of gamma-specific radiations of the three main radioactive elements (Uranium, Thorium and Potassium) can be done using spectrometry.
5-1-1-1-1 Aero spectrometric prospecting
Aerial prospecting generally measures:
• the total count of gamma radiation
• gamma radiation from Uranium, Thorium and Potassium.
The principle is the same, but the sodium iodide crystal used in the aircraft is large.
5-1-1-1-2 Ground prospecting
A portable scintillometer measuring the total count of gamma radiation is an instrument small enough (less than 30cm in maximum dimension) to be used on the ground for prospecting outcrops and making aerial anomaly verification counter plans. This makes it possible to make a first approach.
The use of a portable spectrometer then makes it possible to distinguish between gamma radiation due to uranium and that due to Thorium.
5-1-1-1-3 Downhole radiometric prospecting
The "downhole" logging used for prospecting for uranium includes a scintillometric method that measures the total count due to gamma (natural gamma). By making a correlation curve between gamma radiation measurement and geochemical uranium measured in the core, the contents can be calculated from the measurements.
radiometric. In addition to measuring natural gamma, modern logs generally include the following measurements:
• Spectral Gamma
• Gamma - gamma (Density)
• Neutron (porosity)
• Electrical measurements (conductivity, resistivity)
• Sonic measurements (porosity)
5-1-2 Alpha radiation:
Alpha radiation is used in prospecting for uranium to measure a daughter product: radon gas, which is an alpha emitter. This method makes it possible to detect deep uranium deposits. The radon gas emitted by the deposit rises through the overlying geological layers and if these are sufficiently permeable, the radon rises to the surface. There are two types of methods for measuring alpha radiometry due to radon:
• Radon pump. This instrument has a metal tube which is
sunk into the ground. A pump brings the gas from the ground
in an ampoule, the wall of which is covered by a product which becomes
luminescent in the presence of alpha radiation. Luminescence is
transformed into electric current and the measurement can be made.
• Radon cups. This method is easier to use. An alpha-sensitive film is attached to the bottom of a plastic cup. The cup is buried, turned upside down, in a 10 to 30 cm deep hole. After days, the cup is picked up and the impacts of the alpha particles on the film are counted. Specialized laboratories supplied the blank cups and took care of the counting for a unit price. In both cases the measurements are made following a grid which makes it possible to obtain counter plans.
5-2 GEOCHEMISTRY:
Geochemistry is used proportionately less for prospecting for uranium than it is for prospecting for other metals. The main reason being that for many uranium deposits a correlation can be obtained between radiometric measurements and geochemical measurements, therefore it is more profitable in this case to use the "radiometric grade". However, a minimum of geochemical measurements must be made during ground prospecting to ensure that the gamma radiation is due to uranium.
5-3 OTHER GEOPHYSICAL METHODS:
All other geophysical methods can be used depending on the characteristics of the deposits sought. For example, electromagnetic methods will allow better localization of the Proterozoic “sub-discordance” deposits which are generally associated with graphite.
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