Title	MLP 6513-28(AK)
Participants	Y.J. Bhaskar Rao, B. Vijaya Gopal, EVSSK Babu, B. Sreenivas, T. Vijaya Kumar
Sponsoring Agency	CSIR-NGRI

Work Description:

First Finding of Native Gold from the Chromitites of the Mesoarchaean Tagadur Mines, Nuggihalli Schist Belt, Dharwar Craton, South India

Occurrence of native gold in ultramafic rocks is rather rare. Such occurrences serve as snapshots of the underlying processes leading to their formation and modifications thereafter. Presence of native gold as flakes were earlier reported from the chromites of rock massifs of Kraka, southern Urals, Russia. Gold spherules also occur as tiny inclusions (5-30 mm) in olivine, pyrrohotite and magnetite grains from the ultrabasic rocks of Loveld deposit, South Africa. For the first time, occurrence of native gold grains and flakes from the chromitites of the Mesoarchaean Tagadur Mines of the Nuggihalli Schist Belt (NSB), Dharwar Craton, south India are being reported. Geological Survey of India has already reported significant gold content from the bulk ultramafic rocks of Kempinakote area far south of the current study area. Broadly, the gold grains (Figure 21.1) occur in three distinct forms: (1) anhedral and dendritic flakes in the matrix (~1 to 480 mm); spherical and subspherical grains (~1 to 25 mm) and as inclusions in chromite grains (~ 5 to 35 mm). Petrographically distinct native gold grains also show some distinct compositional characteristics. The gold grains included in the chromite grains are nearly pure with a very minor component of copper. The spherical and subspherical grains in the ultramafic matrix have significant contents of Ag and Co. The anhedral and dendritic gold flakes in the matrix have significant Co content.

The petrographic character and the compositions indicate that the gold grains are syngenetic in nature and indicate the processes involved in the progressive crystallisation of the magma from which the minerals are crystallising. It further indicates that the gold in the native form is due to highly reducing conditions in the initial stages of crystallisation of the komatiitic melts from which the chromites are alleged to have formed.

(EVSSK Babu and DV Subba Rao)

Fig.21.1:
Location map of the Nuggihalli Schist Belt, Karnataka and the BSE images of the Chromite grains with native gold inclusions.