26° 
  
  
  
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IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002 
  
  
92° 30 b 2" in 
PT . 0-47" 
KEY MAP 250  500mfs. 
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INDEX 
(GES) svente 
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7 7| SHLLONG GROUP 
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MEDIUM GRAINED GRANITE 
PROPHYRTIC GRANITE 
BIOTITE GRANITE 
BASIC DIKES 
TRACHYTE 
MAGNETITE PROVSKITE ROCK 
OLIVINE 
GABBRO à 3 | 
[Ap- ] ALKALI PEGMATITE ; 
(e CARBOMATITE /UOGLITE T 
[SS] VERTKAL JOWT Vax 
[CK] DIPPING JONT INGTIGAON 
E SOR COVER ees x x 
— À XXX 
26* 
6° dr. 
V lu. 9291-407] 
Fig.4. Geological map of Jasara ultra mafic- alkaline complex, 
Assam. 
   
   
  
  
  
The Jasara alkaline complex presents a very interesting and tell 
tale magnetic signature of its lithic components. The Shillong 
plateau- Mikir Hills massif was covered by airborne magnetic 
surveys by NGRI in the year 1980. The Jasara alkaline complex 
is manifested as a reversely polarized uni polar anomaly 
indicating its emplacement during a period of magnetic reversal 
in Cretaceous. Unlike the bipolar nature of other coeval alkaline 
complexes in this terrain, the unipolar nature of the complex is 
probably due to its emplacement in a higher latitude during 
Cretaceous period. 
This complex preserves a wide petrologic spectrum represented 
by (i) ultramafites: pyroxenite and its variants with layers and 
lenses of titano-magnetic; (ii) mafites: olivine gabbro and basic 
dykes; (iii) alkaline rocks : syenite, trachyte, alkali pegmatite 
and ijolite and (iv) carbonatite and fenite. 
The characteristic features of the Jasra complex are : (i) crudely 
defined concentric compositional zoning, i.e. basic center to sub 
silicic / silicic margins. (ii) The occurrence of prominent alkali 
gabbro bodies running parallel to the boundary of the complex. 
(iii) The presence of gabbroic layers in pyroxenites. (iv) The 
presence of layers of lenses of titano-magnetite in pyroxenites 
as manifestation of magmatic sedimentation in a magma 
chamber and (v) The presence of macroscopic and microscopic 
layering in pyroxenites. 
Rare metal (niobium and thorium) and rare earth element 
potentialities related to the complex include magnetite, ilmenite, 
perovskite and thorium-bearing minerals. The titano-magnetite 
on an average contains 26.17% FeO,, 43.42% Ti O2, 0.07% 
ThO, and 2468 ppm of niobium. Soil cover over this lithounit 
approximately contains 8053 ppm of total rare earth elements. 
Ubiquitous sulphide mineralization occurring as dissemination, 
veins and fracture fillings in pyroxenites was also noticed. The 
sulphide minerals identified include pyrite, chalcopyrite, bornite 
and covellite. 
ACKNOWLEDGEMENTS 
The authors gratefully acknowledge the technical assistance 
rendered by Shri C. Shanti Kumar, Dr.B.V.S.N.Raju, S/Shri. 
Abhinav Kumar, B.Saraavnan and T.C.Sunil. Scientists and 
staff members of the Department who have proved the WahKyn 
uranium deposit will ever be remembered. 
References: 
1. R. Mamallan, A.B. Awati, S.N. Kak and R.K. Gupta 
(1995), "Effective utilization of geomorphology in 
uranium exploration; A success story from 
Meghalaya, northeast India," Current Science. Vol.68, 
No.11, p 1137-1140. 
2. P.K. Sharma, and R. Mamallan, S.N.Kak (1992), 
Exploration target identification by Remote sensing 
techniques — A case study from Meghalaya, India 
Proc.Nat.Symp. on Remote sensing for Sustainable 
Development, Lucknow, India. 
3. R. Mamallan (1995), A report on the photogeological 
studies carried out in parts of West Siang district, 
Arunachal Pradesh and Khasi Hills district, 
Meghalaya, unpublished report of Atomic Minerals 
Director for Exploration & Research. 
4. D’ Cruz Eric, Mathur, Padhi, A.K., S.K., Sachaan, 
A.S., Sen, D.B.(1998), On the discovery of uranium 
mineralisation in WahKyn area, W.Khasi Hills 
district, Meghalaya, India., Current Science, V 75, 
No.11, P1117-1119 
5. JDhirendra Kumar, R. Mamallan, B. Saravanan, S.K. 
Jain and P. Krishnamurthy (1989), Geology and 
geochemistry of Samchampi alkaline carbonate 
complex, Karbi Anglong district, Assam, India, , 
Exploration & Research for Atomic Minerals, Vol.2, 
p 183-199. 
6. R. Mamallan, Dhirendra Kumar and R.K. Bajpai 
(1994), “Jasara ultramafic-mafic-alkaline complex: A 
new find in Shillong plateau, northeast India” Current 
Science, Vol.66, No.1, p 64-65. 
537