The
Oblaga Cu-Au skarn deposit and porphyry prospect is located in the south-eastern
edge of the Gobi Desert
in western Inner Mongolia. The deposit occurs
within metamorphosed middle Paleozoic volcanic, plutonic and sedimentary
rocks.

Ivanhoe geologists
recognise eight main rock types in the area. Calcareous sediments include
conglomerate, interlayered siltstone-sandstone, and limestone. Covering the
calcareous sediments is a slab of thrusted biotite granite, and above it is
rhyodacitic lithic tuff which, in places, is overlain by alluvial sediments.
Various plutonic rocks and related intrusion breccias have intruded these units.
Monzodiorite, quartz-monzonite and monzonite tend to form thin, high-angle
intrusions and the youngest andesitic intrusions are generally wider and
subhorizontal.

The
Oblaga Cu-Au skarn, which occurs in the calcareous sediments, shows typical
prograde and retrograde mineralogy. Petrographic studies show that prograde
skarn minerals have been overprinted by retrograde skarn minerals, and original
host rock minerals have been texturally obliterated and completely replaced. The
mineralogy of the Oblaga skarn deposit is very similar to other Cu-Au skarn
deposits worldwide.

Sericitic and minor
chloritic alteration occurs throughout the study area and may relate to
porphyry-style mineralisation. Generally, this alteration is not intense and
most rocks have retained their original texture. Geochemical analyses suggest
that K-metasomatism is not widespread at Oblaga. XRD analyses suggest an upward
and outward transition from sericitic to chloritic alteration.

Comparisons between
Oblaga and other porphyry Cu-Au deposits worldwide show that Oblaga has very
similar characteristics to deposits that have undergone sericitic alteration.
However, Oblaga is special because it also has relatively intense chloritic
alteration, which is uncommon in other porphyry Cu deposits described in the
literature.