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Information on the host rocks


Rock Alteration


Sulfide Mineralization


Petrochemistry of the Askot Crystallines

  Investigations of a series of rocks of the Askote Crystallines ranging from low-, through intermediate-, to high-grade, show that a systematic chemical exchange has been active during progressive metamorphism (fig. 1).  The rocks Comprising group A in fig. 1 are low-grade biotite-muscovite schists, group B consists of medium to moderately high-grade muscovite-biotite-garnet schists, while group C consists of granitic gneisses and migmatites.  It can be seen that with increasing transformation there is a significant decrease of Fe, Mg, Ti and Mn, coupled with an increase in Na and K. Si increases by a few percent and Al shows no significant change.

Fig 1: Geochemical comparision between low-grade (A, mean of 8 analyses), medium-grade (mean of 5 analyses) and high-grade (mean of 8 analyses)metamorphic rocks of the Askot Crystallines.

Fig 2: Geochemical comparision between low-grade sericite-chlorite schists (mean of 8 analyses), dioritic sills and amphibolites (mean of 4 analyses) and high-grade metamorphic schists and gneisses (mean of 6 analyses) of the Askot Crystallines.

The trace element distribution in low-, medium-, and high-grade metamorphic rocks also demonstrates clear-cut variations.  Cu, Cr, Zn, Ni, Co, and V decrease with increasing transformation while Pb, Be, and Li depict an increasing trend (Table 1) of the order of 100 percent.

Table 1. Trace element distribution in low-, medium-, and high-grade rocks of the Askote Crystallines.
Elements (ppm) Low-grade rocks (average of 8 samples) Medium-grade rocks (average of 4 samples) High-grade rocks (average of 6 samples)
Cr 99 78 51
Cu 43 21 19
Zn 98 84 74
Pb 15 17 38
Ni 65 50 37
Co 28 21 10
Li 17 19 58
V 118 85 7
Be 2 2 3

Comparison of the major oxide percentages of the dioritic sills and amphibolites occurring along the Askote Thrust, with those of the Crystalline rocks depicts a very significant trend.  The values of SiO2, Fe2O3, MnO, MgO, and TiO2 in the low-grade sericite-chlorite schists are intermediate between the dioritic amphibolites on the one hand, and high-grade schists and gneisses of the Askote Crystallines on the other (fig. 2).  The values of Fe2O3, MnO, MgO and TiO2 decrease from the amphibolite schists through the low-grade sericite-chlorite schists to high-grade metamorphics.  SiO2 however, shows an increasing trend.

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S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 91-571-2721150