CHEPTER v
Nature of phosphate Mineralisation and Behaviour of Phosphorite ore
Bodies at Jhamarkota, Udaipur District, Rajasthan
By
-S.B.L. Srivastava, Director, Mines and Geology Deptt., Udaipur
Jamrkotara rock phosphate deposit is unique in its mode of occurrence of
phosphorite which is confined to the orange-sedimentary called
stromatolites. The nature of mineralization is therefore controlled by the
density of stromatolites, deformation and leaching resulting in grade
variations. The algal phosphotic stromatolites are confined to the
calcareous deformation and farcies (diplomatic limestone) or rocks
overlying basal quantize and occurs in three distinct forms i.e. columnar
and brecciated and stratiform. of these columnar and braccaited forms are
most common in
this area while startiform. of these columnar and
brecciated forms are most common in this area while start form type is
confined to H and 1 block only. The dolomitic limestone enclosing the
phosphorite bed is generally fine grained hard compact grey, massive or,
cherty ferruginous and solidified Towards the footwall side the dolomitic
limestone is siliceous and ferruginous while on siliceous or fine grained
greyish .
The phosphatic stomatolites occur as layered branched or underbranced
columns in which laminae are well developed. The ore is in general
bluish grey to dark grey in colour which on account of its characteristic
lithology, appearance and continuity constitutes stratigraphic and
structural market bed. The ore body shows considerable variation in
shape, size width and depth persistence in different parts of the area. it is
roughly horse-shoe- shaped and extends for a strike length of about 16
km with an average with of 15 mt. In the western sector comprising A,A
(extension) I and J blocks the general trend of ore body is N-S to NNESSW dipping due east with moderate to dipp. The body show stratirfoms
or laminated from in I block, brecciated s. The ore body show stratirfom
or laminated from in A block, brecciated form in J, northern part of A,A
(extn.) block and columnar in A block. There is gradual increase in width
from 4 mts. in J block to 15 Mts. (average width) in the A block. But the
grade on the whole falls as one proceeds from north to south. In the
central sector Comprising B,C,D,E and F
block
the stormatolitic
phosphate is columnar in most part of the central part of block B,D,E
and F mostly brecciated, deformed, fragmental and friable ore has been
found to occur. The grade of phosphorite is usually high (+30%P 2O5) in
D,E, and F block and central part of B block, whereas it is of medium
grade (12-25% P2O5) in the west of B and C block. The ore body shows
decrease in width from west to east, the verge width of 30 meters in B
block decreases to 7-10 meters in E and F block. The trend of phosphorite
in this sector os E-W to NNW- SSE with low to moderate to dips (25-60)
due north.
In the eastern sector where G and H blocks are located the general nature
of phosphate ate body is patchy & discontinuous and exhibit number of
bands with considerable varrition in shape, size width and grade It is
stratiform in H block while columnar and brecciated in G blocks. The
southern part of G block contains high grade (+30%P 2O5) from 15 to
22% P2O5 In block K (near base camp) where is Considerable Varrition
in attitude and width of the ore body (Wirth Varying from 1 to 50 mt.)
with an average grade of 23% P2O5 the stromatolites are columnar thus it
will be seen that phosphatic stromatolites occur in various physical forms
like massive, compact with well developed stromatolites (as in block
A,B,C,G & K) and brecciated fragments, deformed, crushed and
pulverized as in parts of A extn. B,D,E,F&G block (Where stromatolitic
columns are beyond the limit of recognition ) and stratifoms in H and 1
block. The grade of the ore density of their compaction by deformation
usually massive, compaction by deformation and leaching of their of nonphosphatic
constituents
usually
massive,
compact,
columnar,
stromatolites, show low values of P2O5 while brecciated loosed and
powdery from contains high P2O5 content. The microstructure of
stromtotic columns show laminae of phoshatic and carbonate material
varying considerably in their relative properties ultimately determining
the grade. The intercolumnar space is practically free of phosphate. A
general P2O5 os associated with high silica and low carbonates while low
P2O5 is associated with low silica and high carbonates.