Porphyry and Hydrothermal Cu-Au Deposits: A Global Perspective

The Collahuasi porphyry copper deposit commenced operation in 1998, with a projected annual production rate of 380 000t of Cu. It is located in Northern Chile and has a global resource of 3.1 Gt @ 0.82% Cu, including exotic, supergene and predominantly hypogene ore. The operation is a joint venture between Falconbridge and Minorco, with minority Japanese partners.
Several different types of deposits are found within the Collahuasi district, but two copper porphyries are notable for their magnitude: Ujina and Rosario. The differences in the level of erosion and degree of pre- and post-mineralization faulting between the two deposits has resulted in a wide variation in size and morphology of the respective secondary enrichment zones and the characteristics of the subjacent primary ore.
The Ujina porphyry is located 7 km to the east of Rosario and is contained within a distinct hydrothermal system. One important characteristic is the presence of a sub-horizontal body of secondary enrichment, a fact that could explain why the column of primary mineralization has undergone a favourable enrichment process due to the absence of major mineralization controlling faults and a deeper level of erosion (4200 m above sea level) with respect to that in the Rosario deposit. Other aspects, such as the virtual absence of copper veins, the lower copper ore grades and amounts of pyrite in the primary ore that underlie the enrichment body, and the distribution of the hypogene alteration facies could also be explained by erosion that has exposed the deeper levels of the Ujina porphyry system.
The Rosario porphyry encompasses a 1 km² area within a 5 km radius hydrothermal alteration zone. The Rosario system also includes bornite and chalcopyrite (Bo-Cp) veins which crosscut the porphyry mineralization. The deposit is structurally complex due to the intense post-mineralization faulting. The level of erosion (down to 4650 m above sea level) exposes low grade hypogene mineralisation with propylitic alteration. The copper ore grade in hypogene mineralised zones and the Bo/Cp ratio increase with depth, while the pyrite content decreases. The morphological complexity and the low volume of enrichment material is the result of structural control on the supergene processes.