JHAMARKOTRA ROCK PHOSPHATE PROJECT
JHAMARKOTRA ROCK PHOSPHATE PROJECT of Rajasthan State Mines & Minerals Limited
Jhamarkotra Rockphosphate Mines, RSMML
Jhamarkotra, Udaipur – 313 015, India
In earlier years, RSMML has been somewhat sheltered from global competition by import duties and protective tariffs against imported phosphate. These have since been removed as the Government of India adopts more open trade policies with the result that RSMML must now be able to sell their products at world prices, taking transportation costs into account. This implies the optimisation of both capital and operating costs.
Rock phosphates denote such rocks which contain one or more phosphate minerals, with a chemical composition of calcium phosphate. In such rocks, the proportion of calcium phosphate should be around certain optimum level, so that these can be commercially used as phosphates. Although all rock phosphates do not have the same chemical composition, but the chief constituent of these rocks is apatite. The chemical analysis of rock phosphate is usually reported as a percentage of P2O5 or a tetracalciumphosphate Ca3(PO4)2 Phosphate minerals are mainly used for the manufacture of fertilizer. Besides, phosphates are also used for the extraction of phosphorus metal, manufacture of ferro-phosphorus mixed-alloy and phosphoric acid.
The Jhamarkotra rock phosphate mines of Rajasthan Mines & Mineral Ltd. (RSMML), the largest rock phosphate mines in India, was discovered in 1968.The deposit falls between latitudes 24°27’30″ to 24°29’30″ and longitudes 73°49′ to 73°52′ under the survey of India toposheet no. 45H/15.The deposit is located 25 km south-east of Udaipur city and connected by tar road. There are two nearest railway stations i.e Kharwa Chanda & Umara situated at about 10 km south-west and 12 km north-west of the deposit respectively. Umara is connected by tar road where as Kharwa chanda is connected by metalled road. Both the stations are situated on the Udaipur-Ahmedabad meter gauge section of the Western Railway. The area is also well connected with Debari hydro-power grid and gets a smooth electric power supply through 33/11 KV substation.
The deposit extents over a strike length of about 16 km in a semi circular shape. For the sake of convenience in prospecting and mining activities the deposit has been divided in to 12 blocks i.e., A- extension, B, C, D, E, F, G, H, I, J & K.
Jhamarkotra area forms a basin structure surrounded by high hills. The heighest elevation in this area is about 780 m from MSL where as the elevation of the trough containing the deposit is around 480 m to 600 m. Quartzite being the hardest rock occupies the highest position of hills followed by the dolomite & dolomitic limestone in the slopping terrain and phyllites & schists in the lower horizon of the valley area. Jhamarkotra area is devoid of any major river system and is drained by the seasonal river Jhamari and small seasonal nallas originating from hills. There are two prominent nallas seen in Jhamarkotra area flowing north-west to south-east direction. One is originated from K block and joins with Bagdara lake where as the other nalla which is well known as Jhameswar nalla finally discharges in to the Jaisamand lake.
The Jhamarkotra area is a semi arid region experiencing with a typical tropical climate. The temperature of the area goes as high as 49 0C in summer and as low as 5 0C in winter. The yearly average rainfall in the area varies from 650 mm to 750 mm. Dusty wind and cyclone is often experienced in the area during summer.
The rocks around Jhamarkotra are in general not hydrologically potential as that are hard, compact and massive and can only fulfill the local demand. The dolomitic limestone is the principal water bearing formation of this area. Here the ground water occurs under confined to semi-confined condition in secondary openings like joints, fractures and solution cavities. The depth to water varies from 5 m to 52m below ground level. The ground water flow direction is from north-west to south-east, which is all most parallel to natural drainage in the area.
The ground water in this area is fresh, mostly calcium-magnesium bicarbonate type. The electrical conductance of ground water is low with total dissolved solids ranges between 294 and 929 mg/l. The concentration of fluoride and nitrate is also very low.
Description of Project
The Jhamarkotra rock phosphate deposit is situated 26 km southeast of the city of Udaipur in western Rajasthan, India. It is owned and operated by Rajasthan State Mines & Minerals Limited (RSMML), whose headquarters are in Udaipur. Mine offices are located at the site.
The deposit was discovered in 1968 and production began in 1969. Systematic mapping and exploration, including pitting, trenching and diamond drilling, by the State Department of Mines and Geology (DMG) and RSMML permitted delineation of the deposit and estimation of the resource. Two principal types of phosphate rock were identified for commercial exploitation, rock with a P2O5 content above 30%, called high-grade ore (HGO) and that below 30% referred to as low-grade ore (LGO). HGO is crushed, screened and sold as direct-shipping ore, principally to fertilizer plants, while LGO, averaging 17% P2O5, must be upgraded to 34% by beneficiation prior to sale.
The Jhamarkotra region is characterised by a Proterozoic meta-sedimentary sequence lying unconformably over Archean basement rocks that include granites gneisses, amphibolites and micaschists. Evidence of the angular discontinuity between basement rocks and the overlying Aravalli formation has been noted in former studies in the southern and western parts of the region.
The stratigraphy of the Jhamarkotra region is summarised in Table-1. The sedimentary unconformity and the presence of several folds in the phosphate-bearing rock and other sedimentary rocks show that movements have affected the entire area during the sedimentation process and subsequent geological periods. The geological work carried out as part of this study in the open pits and surrounding areas has revealed the presence of faults and stratigraphic sequences, and evidence of a number of mass movements which have occurred up until recent geological periods.
Stratigraphy of Jhamarkotra Region
|Intrusion of granite gneisses and veins of quartz|
|(Proterozoic)||Dolomitic limestone and its variants|
|Stromatolitic rock (phosphate) close to the base|
|Archean||Granite gneisses, amphibolites, granites, aplites|
The basal quartzite is generally massive, but in places shows a conglomeratic sediment containing small pebbles. This rock varies laterally and upwards from quartzite to dolomite including dolomite lenses. Quartzite and dolomite sometimes have a very well-defined contact.
There are several carbonate facies within the Jhamarkotra area, including ferruginous dolomite and marble. Their thicknesses vary from hundreds of metres to just a few centimetres. This formation lies between the quartzite or the gneiss-amphibolite and the phosphate units.
Phosphate Unit (Orebody)
At Jhamarkotra, phosphate beds has been recognised over a distance of more than 16 km. Their thickness varies from 1 m to 100 m in exceptional cases. In some cases, this variation is due to several episodes of folding.
A sequence of phyllite and micaschists lies between the carbonates and the metalithic arenite. Generally, this rock is finely schistose and varies in composition from ferruginous marly phyllite to dolomitic limestone near the contact.
In most cases, this rock has a poorly bedded aspect; it sometimes includes pebbles with various shapes and mineralogical compositions. Among these are found phyllites, dolomite, metalithic arenite and phosphate. This is another element that demonstrates the extensive mobility of this basin.
This quartzite can be seen in the north-eastern sector of the basin. As has been stated in earlier studies, this rock could have the same stratigraphic position as the metalithic arenite.
The Jhamarkotra open pits lie entirely in the Aravalli rock formation which is characterised by a complex relationship between structure (fold and faults) and weathering. The Jhamarkotra basin has been subject to deformation since the early stages of sedimentation, with four recognised episodes of folding being followed by numerous fault episodes. Phyllite planes cutting across the orebody in D Pit are a typical result of this tectonic action. These phyllite planes encountered everywhere (in the orebody and in the hanging wall) are in fact cataclasite rocks forming part of the fault plane.
In general, the local intensity of weathering increases with increasing fault activity. The rock flanking the orebody has been a stress relief area. Hence, a wide portion of the sedimentary sequence along the footwall and hanging wall presents fault planes. Within this area, the rocks are intensively crushed and weathered; however the rock in some exceptional cases is not weathered. Closer examination of the phyllitic planes in D Pit showed the movements to be subparallel to the strike of the orebody in both directions on a NW-SE axis.
Similarly in Pits B, E and F, weathering is associated with intense folding and faulting. As the orebody in Pits G and H is not linear, similar structures are expected. The orebody is at the centre of maximum deformation of the Jhamarkotra deposit. It is important to point out that the most recent faulting has less influence on slope stability than the earlier faulting and the weathered nature of the rocks.
D Pit, where the geological features are better exposed due to the depth and extent of the present excavation, formed the starting point for geotechnical mapping. By this means, it followed that the updating of the existing geological/geomechanical data could be extended to the other pits. The nomenclature adopted for each pit distinguishes between footwall, pit bottom and hanging wall.
Sufficient field measurements of fault orientation were taken in D Pit to ascertain the general pattern for the entire area. Hence, the few fault planes shown on drawings are not representative of the intensity, but rather are indicative of the existing geological structure within the mine. They have been used for slope stability studies in specific areas. In considering slope stability, it is noted that the intensity of weathering of the host rock increases with increasing ore grades.
Geology of the Jhamarkotra Area
The phosphate-bearing rock at Jhamarkotra is developed within an extensive horizon of stromatolitic dolomite of Proterozoic age. For ease of reference during initial exploration by the Department of Mines and Geology (DMG), the deposits in the mine area were divided into blocks, from A and A Extension in the west to H Block in the east.
The Geometry of the Phosphorite
The phosphorite occurs close to the base of a succession of likely Lower Proterozoic sediments, referred to as the Aravalli Formation, that lie unconformably on a pre-Aravalli basement, largely of granites and gneisses. In the area of Jhamarkotra, the phosphorite is contained within a synclinal basin whose overall axis trends approximately northwesterly. However, cross folds have caused considerable deformation, resulting in sub-basins trending approximately northerly (A, B and C Blocks) or northeasterly (F Block).
The phosphorite horizon has an average thickness of 15 m where well developed between the A and F Blocks. It displays pinching and swelling, however, and may be absent as between the F and G Blocks, or concentrated towards the axial zone of folds where tens of metres thickness may be clearly observed in the H Block.
Generally, the contacts are fairly sharp, suggesting that the phosphate concentrating stromatolites flourished over a relatively short, well-defined period. The phosphorite thus provides an excellent marker horizon.
The phosphorite horizon generally dips inward towards the principal basin at angles varying from 20 o to commonly 45 o. In places, however, the horizon may be sub-vertical or even overturned. Locally, it may dip outwards away from the basin as observed in both the C and H Blocks.
Faulting has also complicated the continuity of mineralization as, for example, between the B and C Blocks. Displacements, however, do not significantly modify the overall geometry of the deposits, but rather cause slope stability and water in-flow problems on a local scale.
The phosphate-bearing rock at Jhamarkotra is developed as an horizon of stromatolitic dolomite. The stromatolites have grown as an extensive colony of vertically standing columns. The columns are whitish on fresh surfaces, cylindrical, and may attain heights of almost one metre. In vertical section, they show growth septa that are convex upwards. In cross-section, the columns show a rounder outline whose typical surface area is some 2 cm2.
The stromatolites are embedded in a matrix of bluish-coloured dolomite over most of the areas. Locally, the matrix may be more clastic resulting in higher SiO2, Al2O3 and Fe2O3 contents. For example, in the A – Extension Block, the more clastic matrix is reddish brown from weathering.
The phosphate is almost exclusively contained within the stromatolites in the form of micro-crystalline apatite, Ca5(PO4)3F.
Two distinct types of phosphate rock may be observed at Jhamarkotra. Firstly, an unaltered matrix-dominated phosphorite is most widespread and generally contains less than 20% P 2O 5. This ore type is referred to as Low Grade Ore (LGO). Secondly, an altered stromatolite-dominated phosphorite occurs in which the dolomite matrix has been removed by solutions permeating through the rock mass leaving an apatite-enriched residual phosphorite commonly containing more than 30% P 2O 5. This ore type is referred to as High Grade Ore (HGO).
The HGO is evidently the result of increased permeability developed on account of fracturing and the subsequent passage of abundant, slightly acidic hydrothermal solutions through the rock mass. Locally, the alteration may be so intense as to produce a soft whitish enriched phosphorite that may be cut with a knife.
The LGO is normally a massive rock that is bluish grey where broken in the pit in contrast to HGO which is a friable rock that appears dark grey to brownish black when broken.
Both ore types may occur in juxtaposition but the contact zone is usually readily defined over a metre or two. Commonly a zone of fissured and discoloured, calcite-enriched phosphorite may occur between the two ore types.
Two other distinct styles of mineralization were observed. On the hanging wall in F Pit, an extensive area of altered dolomite occurs that is in-filled by silica, apatite and calcite. The in-filling by secondary apatite results in grades between 5 and 15% P2O5 and thicknesses of several tens of metres. On the hanging wall in the C Block, a laminated reworked detrital phosphorite horizon, approximately one metre wide, was observed.
The A Extension phosporite horizon extends over a strike length of some 1500 m. It is largely continuous and has been systematically trenched at 50 m intervals for a total of 35 trenches and pits. The horizon strikes at some 20° Az and generally dips steeply eastward at about 80°.
Twenty-seven boreholes (totalling 2523 m) have tested the zone that has an average true width of some 5 m, although this is variable on account of local pinching and swelling.
Phosphate grades are generally in the 20 to 30% P2O5 range although DDH 145 intersected 17.35 m of 32.14%. Thus, the zone is LGO but P2O5 contents are generally above average for this ore type. The assays indicate a high R2O3 content due to aluminium and iron enrichment of the clayey matrix and ore from this area will have to be carefully blended in small quantities. Silica contents are also high and the ore is hard to drill and blast. The high clastic component is reflected in the rusty weathered nature of the phosphorite in this section.
The wallrocks are typically cherty dolomites and appear quite massive and competent.
The A Block phosphorite horizon extends over a distance of 1300 m as a crescent-shaped outcrop, generally open and dipping to the east from 50° to 85° Az. It is largely continuous and has been trenched at 50 m intervals for a total of 25 trenches and pits.
Thirty-one holes (totalling 3763 m) have tested the zone which has an average width of 18 m. The phosphate grade is highly variable, ranging from 10 to 35%, although for the most part the zone contains low grade ore (LGO).
RSMML has developed a mining plan that will allow for the development of the A and B Blocks as a single pit. To date development has only taken place in B Block. The topography here would appear suitable to recovering the reserves at a favourable stripping ratio.
Mining possibilities include conveying run-of-mine ore across the intervening valley to the LGO crushing plant, or crushing and screening to 12 mm at site for direct sales.
The B Block phosphorite unit is continuous with the A Block from where it trends eastward over a strike length of 850 m before turning northward for a further 600 m of strike length. Along the northerly trace there is a surface discontinuity of the phosphorite over a distance of 120 m.
The B Block is folded and faulted along axes trending northwestward.
Drilling is largely confined to the more southerly, eastward-striking horizon of the B Block. Sixty holes (totalling 7123 m) have tested the zone, in addition to twenty-four trenches and pits. It has an average width of 30 m but varies from 7 to 100 m width in outcrop. The southerly zone generally dips to the north at between 45° and 60°.
The phosphorite includes a zone of highly friable HGO, somewhat silica enriched, but the majority of the mineralization is generally poor LGO averaging less than 15% P2O5. The B Block horizon where currently being developed shows extensive cavities, in-filled by clays and laminated sandstones. The ore zone and wall rocks generally appear to be of poor rock quality on account of the structural deformation in this area.
The C Block phosphorite unit is contiguous with B Block to the west and D Block to the east. It outcrops over a strike distance of 1050 m, is arcuate to the south, and dips northward generally at 45° with the mineralisation developed on a anticlinal fold crest. Westward, towards the B Block, the zone becomes sub-vertical to overturned.
Forty-six holes totalling 4657 m have tested the zone and systematic sampling has been carried out in sixteen trenches and pits at 50 m intervals.
The mineralisation is of LGO in the range of 19 to 20% P2O5 (the stromatolites being very well developed), thus this is a good zone to feed the beneficiation plant. The average thickness is approximately 15 m.
A reworked, detrital unit overlies the LGO zone as a well-banded, 1 m wide zone on the hanging wall.
Mining has only recently started here as a western extension to D Pit.
The bulk of the reserves and most of current production is within D Pit.
The block is some 1.5 km long and has been the subject of most exploration by trenching, diamond drilling and grade control drilling. Eighty-nine holes totalling 9895 m have tested the zone in addition to 30 trenches and pits.
Mineralization is largely HGO with lesser amounts of LGO. The average combined width is approximately 20 m, although there is a widening eastward. The zone strikes 115° Az and dips on average 45° to the north. The dip however steepens gradually eastwards.
While the footwall is generally planar, the hanging wall may be quite irregular. The HGO/LGO contact may be sharp or transitional but calcite in-filling is usually associated with the contact zone.
Most of the ore is HGO that may be massive (providing boulders for + ½ ” direct shipping ore), friable or totally decomposed with virtually no matrix. This latter material may contain more than 36% P2O5 with less than 1% MgO.
Mining started at 600 mRL and the lowest level is now at 460 mRL.
The phosphorite unit on the E Block trends 115o Az from the D Block over a distance of 700 m and then curves southerly for 300 m as E Extension. Over the east-west section, the phosphorite dips 35° N.
The zone has been tested by fifty-four holes totalling 7168 m and sixteen trenches and pits. It is distinguished by sharp contacts and shows widths ranging from 17 m at the centre to 40 m at the widest part.
The phosphorite is largely HGO with grades as high as 39% P2O5, with an average grade of 36% P2O5. The ore is dark grey to black, very friable, and shows little variation.
The E Block is part of the main pit, but is separated from the D Block by an unexcavated zone or bridge that currently forms the haulage way to the waste rock dump.
The E Extension area, which trends beneath a large hill towards the F Block, is not in the present mining plan but is reported to offer good exploration potential.
The F Block is an east-west trending zone some 600 m long and dipping 60° – 70° N. Considerable structural complexity exists here with bifurcations of the mineralized zone.
The zone has been tested by forty-three holes totalling 4224 m and 25 trenches and pits. While the original drilling by DMG had suggested a 10 m width, recent drilling has expanded the zone and significantly augmented the reserves. The mineralization is largely HGO.
The hanging wall zone, outside the main ore zone, is an area of extensive brecciation and the dolomite is silicified (60 – 70% SiO2) and remobilised from phosphatic solutions to form secondary ore. This material, ranging from 5 to 15% P2O5, is currently being stockpiled.
The F Block was not in the original mining plan but recent drilling and development have indicated a good potential. Current pit bottom is at 500 mRL and plans are to mine to 440 mRL.
Between the F and G Blocks, the phosphorite horizon becomes discontinuous where exposed at surface. The principal horizon trends east-west for 300 m and then turns northward to north-westward for over 1500 m and appears to follow an anticlinal axis (also trending north-westward). A repetition of the horizon occurs over some 400 m strike length to the east, likely as a result of folding. Both limbs dip eastward. The area is one of considerable structural complexity.
The block has received only minimal drilling, with twenty-six holes totalling 1986 m. The horizon has, however, been systematically surface sampled at 50 m intervals by fifty-one trenches and pits.
The main band has an average width of 7 m, the more easterly band a width of 15 m. The phosphorite is of low grade, ranging between 15 and 22% P2O5, and pinches at depth. Better grades, reportedly 30 to 32%, have been encountered towards the F Block. Zones of HGO were observed locally in outcrop.
Given the limited amount of drilling, the reserve potential of this block is unknown. The potential for good phosphorite development on the southeasterly plunging axial crest could be significant. This area, in the southern half of the G Block, also suggests potential reserves in a favourable topographic situation where the waste stripping ratio could be low.
The H Block is under application by RSMML for a mining lease. Development of the principal area of known reserves could be seriously constrained by the presence of the Jhameshwarji temple in the potentially minable zone. This temple, within an underground cavern, is the site of pilgrimage for over a million visitors annually, largely in the month of May.
The phosphorite horizon is discontinuous and highly variable in strike and in thickness, being close to a north-westward trending corridor of deformation.
The area has been tested by thirty-two drill holes totalling 3693 m. Systematic trenching has also been carried out that shows highly variable P2O5 contents over the eighty-one trenches and pits.
The largest area of phosphate development occurs in the extreme south-east of the block. The zone of phosphate has a 150 m strike length yet is up to 120 m wide. The phosphate appears to be concentrated around the crest of a southeasterly plunging anticline. The area is one of considerable ductile deformation and it is suspected that the extreme width of phosphate here is due to squeezing of the mineralization into the crestal zone of the fold. Traced northwestward, the phosphate quickly diminishes and zones of highly deformed stretched stromatolites may be observed as the horizon thins out.
To meet market requirements (for HGO) and beneficiation plant feed specifications (for LGO), RSMML have established specifications as described below. These refer not only to the grade (P2O5 content), but also to secondary components such as silica (SiO2), magnesia (MgO), calcium carbonate (CaO content) and oxides of iron and aluminum (combined as R2O3).
Mine production must be planned so that average grades do not exceed the limits listed. This implies careful grade control, selective mining and blending.
- P2O5 32% ± 0.5%
- SiO2 < 12%
- MgO < 1.5% if possible, maximum 2%
- CaO < 46%
- R2O3 < 3%, preferably below 2.1%.
The lower limit for HGO is generally about 26% P2O5. SiO2 content is generally below 16%. A high silica content (above 12%) affects the manufacture of di-ammonium phosphate. A high MgO content consumes acid and reduces reactivity. It is therefore an important specification for sales to fertilizer manufacturers.
HGO is screened at 12 mm (½”). Oversize is sold for the production of elemental phosphorus while undersize is sold to fertilizer manufacturers.
- P 2O 5 17.25% ± 0.25%
- SiO2 Maximum 9%
- MgO 10% ± 2%
Should the P2O5 content in feed to the plant exceed 18%, concentrate grade will fall below specification (34%). Below-grade concentrate is also believed to result from excessive amounts of calcite affecting flotation performance. For this reason, mixed altered ore from the fissured HGO-LGO contact zone should be sent to the HGO mixed ore stockpile (see below) rather than to LGO.
The lower limit for LGO is generally not below 12% P2O5 in situ.
LGO feed to the plant should be composed of real LGO, not a blend between HGO and low grade material which results in an excess of fines reporting to tailings and hence a drop in recovery.
LGO with a high silica content (between 9 and 30% SiO2) is screened at 12 mm, sometimes blended with locally-imported phosphate, and sold under the name ‘Rajphos’.
Material Grading 5-12% P2O5
By government regulation for the maximum conservation of resources, this material must be stockpiled for possible future recovery.
Rock grading less than 5% P2O5 is considered waste and is hauled to dumps.
There is generally little difficulty in producing HGO meeting the specification for P2O5 content. However, MgO content of HGO in situ is often over 2% and particular care must be taken with mining and blending to meet the specification for the final product.
HGO from the mine is hauled to one of two stockpiles at the HGO primary crushing plant. The first pile is for ‘clean’ HGO, normally grading 33 to 35% P2O5. Mixed or diluted HGO (running 22 to 26% P2O5) is delivered to the second pile. Blending is then accomplished by bulldozer, feeding the dump hopper from each pile in the proportions required as determined by management.
Following primary and secondary crushing, the HGO is screened at 12 mm, sampled and stockpiled by conveyor for sale as described above.
Initially, difficulty was experienced in meeting LGO feed specifications, largely because predicted grades in the CDE Pit (the main source of ore to date in Blocks C, D and E) were over 20% P2O5 while actual grades fed to the plant were closer to 17%. Mining of Blocks B and A, where LGO grades are lower, will facilitate blending to meet requirements.
Depending on the results of blasthole samples, LGO is delivered to one of three stockpiles at the LGO crushing plant:
- Pile A +14% -17% P2O5;
- Pile B +17% -25% P2O5;
- Pile C +10% -14% P2O5.
The run-of-mine ore is then blended in the proportions required to meet the desired content of 17% P2O5 and fed to the primary crusher. Following secondary crushing to 50 mm, the LGO is delivered to one of two 45,000 tonne stockpiles in a stacker-reclaimer system where it is stacked longitudinally and horizontally. It is subsequently reclaimed vertically, against the grade of the original stacking, to ensure a uniform feed to the tertiary crushers at the beneficiation plant.