ALUMBRERA

GEOLOGY

The primary mineralized rocks of Bajo de la Alumbrera consist of a series of porphyritic intrusions into the andesites of the Farallon Negro complex. These alkalic porphyries were intruded some 8 million years ago into the roots of the Farallon Negro volcano.

The intrusion generated large-scale hydrothermal circulation which resulted in alteration and mineralization of the porphyry itself and its volcanic host rocks.

Subsequent erosion has exposed the upper part of the volcano and its porphyry system at a level that is favorable to mining.

The Farallon Negro host rocks are about 90% autobrecciated flows in a thick bedded sequence of fragment-poor to fragment-crowded, weak to strong porphyritic potassic andesite. The remaining 10% is comprised of lithic and non porphyritic flow units.

Topographically, the Alumbrera deposit is a bowl-shaped, north-east to south-west oriented ellipsoidal depression surrounded by ridges formed mostly by andesitic breccia of the Farallon Negro volcano. The floor of the bowl covers an area of 2.5 square kilometres.

A total of seven distinctive porphyritic intrusions have been recognized, which form stocks (earliest units) and dyke-like bodies (youngest units) that extend to the outer edge of the deposit with some of the dykes forming a radial pattern around the central stocks.

The mineralogy of the primary (unweathered) ore consists of chalcopyrite (+/ bornite), native gold and pyrite. Gold occurs mainly in chalcopyrite. Gold values correlate closely with copper values in primary mineralization and ratios are very consistent through the deposit.

Ore grades correlate with lithology. The highest copper-gold grades are associated with intense potassic (quartz-magnetite) alteration of two of the earliest mineralized porphyritic intrusions and in adjacent biotized or potassium feldspar altered andesites. Younger porphyries are less mineralized or in some cases barren. Most of the copper is primary and occurs as chalcopyrite in disseminated grains and in veinlets. Copper and gold are positively correlated, with gold occurring in association with early pyrite-chalcopyrite-magnetite as free gold grains in the 10 to 50 micron range. The occurrence of gold within chalcopyrite, together with magnetite and pyrite, suggests that the gold was transported by the same fluids as the other metals and that this fluid was oxidized.

The economic grade sulphide mineralization extends upwards almost to the surface. The upper portion of the orebody has been subject to weathering and can be subdivided into two distinct zones, an upper, thin, leached zone, and a lower sulphide enriched zone. These two zones however make up less than 10% of the total mineralization. The leached zone contains oxide and carbonate copper minerals, including soluble species. The sulphide-enriched zone is complex and contains chalcocite, covellite, native copper and chalcopyrite in varying proportions. The intensity of chalcocite decreases with depth and is absent in fresh (primary) ore where chalcopyrite is the dominant copper mineral. Leaching and oxidation near the surface generally does not extend to deeper than 30 metres.