TheSan Cristobal gold district is located 110 km east of the port of Antofagasta, Chile. It contains the San
Cristobal gold mine,
where Niugini Mining Ltd. commenced a heap leach operation, with initial
reserves of 10.1 million tonnes grading 1.34 g/t Au, in December, 1990.
The
San Cristobal
district occurs within an upper Cretaceous-lower Tertiary
magmatic arc. The main mineralisation occurs within a northwest trending
subvertical zone, 1km long and 70 - 100 m wide, which contains
rhyolite porphyry and minor breccia, and is locally termed the 'subvolcanic
complex'. At the
San Cristobal
mine, most gold mineralisation occurs in
subvertical quartz-pyrite veins which were emplaced in dilational sections of an
imbricate fault system. The bulk gold resource comprises four ore shoots which
formed where the vein system intersects the subvolcanic complex. Gold-bearing
quartz-pyrite veins also occur at separate prospects, the Three Sisters and Sol
prospects, west and south of the
San Cristobal
mine.

Hydrothermal alteration consists of potassic and
propylitic assemblages related to intrusion of the subvolcanic complex. This
alteration is overprinted by a phyllic stage which is associated with the
emplacement of the quartz-pyrite veins. The oxide zone is 180 m thick, and
contains native gold, electrum, limonite, and manganocarbonates. The underlying
hypogene zone contains pyrite with minor chalcopyrite, sphalerite, galena, covellite,
bornite, chalcocite, and arsenopyrite. Fluid inclusion studies indicate that the
deposit formed at approximately 270C, and the mean salinity of the fluid was 4%
NaCl equivalent. These observations enable the San Cristobal deposit to be
classed as a porphyry- related, mesothermal quartz-vein system of the low
sulphidation type.