The only major phosphate rock reserves found up to now in the Sudan are the deposits discovered in 1983 during exploration work by geologists of the German Geological Group in the neighborhood of Jebel Kurun (Figure 1) on the eastern border of the Nuba Mountains [1] , at a distance of 690 kilometer from Khartoum and 1510 kilometer from Port Sudan.

This phosphate occurrence with minor base metal and uranium content forms a low elongated hill rising to about 25 meters above the surrounding plain. The hill extends for about 500 meter in a WSW-ENE direction and its width increases from about 100 meters in the west to 200 meters in the east [1] .

A preliminary resource estimate at Kurun indicates 1.68 million tonnes of phosphate ore grading 20% P₂O₅, which amounts to about 336,000 tonnes of P₂O₅. The depth is calculated to the base of the hills (12 m in the west and 20 m in the east), but the true depth of the phosphate mineralization is not known [1] .

K. Brinkmann (1985) [2] indicates that during weathering, sulphides were dissolved, sulphate was removed away, and zinc and copper were held by limonite. The apatite was dissolved and since Al⁻³ ions were abundant, alumino-phosphates were formed. Woodhouseite was formed in the presence of sulphate released from the weathering of sulphide and in the absence of sulphate, crandallite was precipitated. Wavellite was formed by loss of calcium as an ultimate product and in the presence of copper turquoise was formed.

The arithmetic average of the chemical analysis of the 52 samples gave 20.1% P₂O₅, in tests with formic acids. Soluble phosphates were proved to be less than 20% of total phosphates, and the visible phosphate reserves of the sampled portion of Jebel Kurun down to the elevation of the plain were estimated at 336,000 ton of P₂O₅ [1] .

The sampled area covered about 32,000 m² i.e. 200 × 50 = 10,000 m² in the west plus 275 × 80 = 22,000 m² in the east. The average elevation above the plain was taken at 12 meters in the west and 20 meters in the east. The average phosphate content is 20% P₂O₅ and the specific gravity = 3 ton/m³. The limits of the phosphate mineralization were not determined laterally and in depth and therefore an increase in the amount of the reserves is possible [1] .

Geological Research Authorities of the Sudan expect the continuation of phosphate towards the depth; therefore they expect a total reserve of phosphate may reach 81 million tons with average content of 20% P₂O₅ [3] . This estimation cannot be reliable without detailed drilling exploration.

Brinkman in 1986 [4] discovered two types of phosphate deposits in Kurun and Uro areas in the center of eastern Nuba Mountains. However, using of these phosphates as fertilizers, it is necessary to check them chemically, especially for radioactive uranium presence [5] .

Twelve selected samples from Jebel Kurun phosphate―bearing rocks were analyzed by X-ray diffraction method and were found to contains the following minerals (Table 1) [1] .

Giad industrial group (Sudan) reported the analysis results in 2017 for two samples (Figure 2) and (Figure 3) for the characterization of Jebel Kurun phosphate rock (Table 2) for grain size distribution and (Table 3) for chemical composition.

The analytical results indicate the following observations:

• A very high content of alumina in the rock.

• Most of the phosphate present as fluorapatite, that has an excellent digestibility in hydrochloric acid.

• Sand is present as quartz.

• Alumina and phosphate are present as Crandallite: CaAl₃(OH)₆(PO₃(O_0.5(OH)_0.5))₂.

• Assess the technical viability for the utilization of Jebel Kurun phosphate rock to produce phosphoric acid for fertilizer usage.

• Classification of J. Kurun phosphate rock and identification of types suitable for the production of phosphoric acid.

• Identification of the chemical characteristics and behavior of Jebel Kurun phosphate rock.

The statistical analysis results (Table 4 & Figure 6) shows the estimated phosphorus pentoxide ratio P₂O₅% in Jebel Kurun area has an average value of 19%, and ranges from 0.13% to 32.94% with a standard deviation of 9.05. There is strong negative correlation (Tables 4-8, Figure 7 & Figure 11) with value (−0.82) between phosphorus pentoxide and silicon dioxide, moderate positive correlation between phosphorus pentoxide and Aluminium oxide Al₂O₃, Calcium oxide CaO, Loss on Ignition LOI at 1000˚C, Vanadium V with values 0.66, 0.54, 0.63 and 0.61 respectively.

It is important to remember that the correlation coefficient between two variables is a measure of their linear relationship and that a value of r = 0 implies a lack of linearity and not a lack of association. a value of r equal to +1 implies a perfect linear relationship with a positive slope, while a value of r equal to −1 results from a perfect linear relationship with a negative slope [10] [11] [12] .

Uranium in Jebel Kurun area has an average value of 81.47 ppm (Table 7 & Figure 10) and ranges from 1.2 ppm to 208.3 ppm and a standard deviation of 53.89, a very weak positive correlation coefficient (0.16) (Table 7, Table 8 & Figure 11) with phosphorus pentoxide. Phosphate samples with an average uranium content lower than 60 ppm showed a higher correlation of 0.35 between uranium and P₂O₅ and represent 41.07% of total samples, a very weak negative correlation (-0.05) when uranium content is higher than 60 ppm for 58.93% of samples.

A research published by the Journal of the Argentine Chemical Society [5] stated that a positive relation exists between uranium content and P₂O₅ percentage

in phosphate ore samples from Kurun area (r = +0.58), which contain 56.63 mg/kg uranium in average. The chemical analysis of phosphate ore samples indicated that in the case of phosphate samples that contain low uranium concentration, there is a strong positive correlation between uranium content and P₂O₅%, but if uranium concentration increases over 60 mg/kg, no correlation exits.

Samples with P₂O₅ content higher than 15% and an average uranium content 88.93 ppm, indicate a weak negative correlation (-0.37) between P₂O₅ and uranium, and represent 78.57% of phosphate samples. Accordingly uranium would not be expected to be of economic interest, considering that samples that have P2O5 content lower than 15% and average uranium content 54.13 ppm exhibited a moderate positive correlation 0.61 between uranium and P₂O₅ and represent 21.43% of total samples.

Statistical analysis for trace elements (Tables 5-7 & Figures 8-11) indicates the mean values and correlation coefficient with phosphorous pentoxide.

Giad Industrial Group in Sudan admitted that the effective reserves for the type 1 and type 2 (Table 9) of phosphate rock in Kurun mining area should be more than twenty million tons to achieve economic viability.