Oxygen
and carbon isotope mapping of hydrothermally altered wall rocks was applied to
the blind ore prospecting of skarn Pb-Zn deposits in the Kamioka mining
district, centralJapan, where the
wall rocks consist of heterogeneous rock units. The results
(18OSMOW of -1.1 to +17.3 and 13CPDB
of -5.0 to +4.8
for limestones, and 18OSMOW of -0.8 to +12.5 for silicate rocks) show
isotope zonations of the wall rocks with depletions in heavy isotopes
(18O and 13C) toward the centre of mineralization. The isotope
zonations likely formed by interaction of migrating thermal waters with the wall
rocks during the mineralisation. The heavy-isotope depleted zone, which
indicates higher temperature or higher water-to-rock ratio of the system, occurs
in the footwall of a fault, indicating that the fault played the role of main
conduit of the ore-forming fluids. A heavy-isotope depleted zone was also found
in an area, where significant mineralisation had yet to be identified. Recent
drilling in this area has intercepted a zone with a grade of 16.0% Zn, 0.8% Pb,
and 35g Ag per metric tonne at a depth of about 360 m.