The Neo-Archean Ramagiri-Penakacherla belt is located in the eastern Dharwar craton of South India. It hosts auriferous quartz bodies in altered mafic and felsic volcanic rocks, intimately interlayered and co-folded with carbon phyllites. The sequence is metamorphosed to low greenschist facies. Pervasive quartz-carbonate alteration is a conspicuous feature in the gold-sulfide mineralized zone of the Kottapalle block of this greenstone belt.
The carbonate compositions spread along the dolomite-ankerite or calcite-ankerite join. The host volcanics are of tholeiitic basalt, andesitic and dacitic compositions representing a probable island-arc setting. The majority of carbonaceous samples fall in the range of_−20 and_−28 per mil δ
(PDB), suggesting a biogenic derivation of the reduced carbon and a sedimentary origin of the strata. Carbonate δ
C (PDB) have median values of_−2.4 per mil and mean of_−4.0 per mil. δ
S (CDT) values of pyrite and pyrrhotite cluster around +2 per mil. Primary fluid inclusions, rich in CO
with subordinate CH
, as observed in preliminary microthermometric studies, demonstrate the pervasive presence of a CO
-rich hydrothermal fluid responsible for the extensive carbonation of the auriferous host rocks.
Absence of any sedimentary carbonate strata in the sequence, and the stable-isotope data support the role of mantle-derived fluids in producing the quartz-carbonate alteration. The δ
C values of carbonates indicate that the hydrothermal fluid did not have any significant interaction with the reduced carbon in the ore zone. The sulphur isotope data points to the mantle-source of sulphur in the ore-bearing hydrotherms.