The Ajjanahalli gold mine is spatially associated with a Late Archean
craton-scale shear zone in the eastern Chitradurga greenstone belt of
the Dharwar craton, India. Gold mineralization is hosted by an similar
to100-m-wide antiform in a banded iron formation. Original magnetite and
siderite are replaced by a peak metamorphic alteration assemblage of
chlorite, stilpnomelane, minnesotaite, sericite, ankerite, arsenopyrite,
pyrite, pyrrhotite, and gold at ca. 300degrees to 350degreesC. Elements
enriched in the banded iron formation include Ca, Mg, C, S, An, As, Bi.
Cu, Sb, Zn, Pb, Se, Ag, and Te, whereas in the wall rocks As, Cu, Zn,
Bi, Ag, and An are only slightly enriched. Strontium correlates with
CaO, MgO, CO2, and As, which indicates cogenetic formation of
arsenopyrite and Mg-Ca carbonates. The greater extent of alteration in
the Fe-rich banded iron formation layers than in the wall rock reflects
the greater reactivity of the banded iron formation layers. The ore
fluids, as interpreted from their isotopic composition (delta(18)O =
6.5-8.5parts per thousand; initial Sr-87/Sr-86 = 0.7068-0.7078), formed
by metamorphic devolatilization of deeper levels of the Chitradurga
greenstone belt. Arsenopyrite, chalcopyrite, and pyrrhotite have
delta(34)S values within a narrow range between 2.1 and 2.7 per mil,
consistent with a sulfur source in Chitradurga greenstone belt
lithologies. Based on spatial and temporal relationships between
mineralization, local structure development, and sinistral strike-slip
deformation in the shear zone at the eastern contact of the Chitradurga
greenstone belt, we suggest that the Ajjanahalli gold mineralization
formed by fluid infiltration into a low strain area within the
first-order structure. The ore fluids were transported along this shear
zone into relatively shallow crustal levels during lateral terrane
accretion and a change from thrust to transcurrent tectonics. Based on
this model of fluid flow, exploration should focus on similar low strain
areas or potentially connected higher order splays of the first-order
shear zone.