Summary:
Berta corresponds to a porphyry copper system. In detail, Berta Sur exhibits evidence of alteration and mineralization which is comparable with that observed in the deepest parts of such systems. In particular the textural variations from TON to a crystal rich PTC, the background potassic alteration, the development of muscovite and greisen", and the Ca-Na alteration, are typical characteristics of the roots of porphyry systems (Dilles, et. al., 2000; Seedorf et. al., 2008). These characteristics also explain the lack of development of significant mineralization at depth. Rather the mineralization decreases in grade with depth or has a root-like shape becoming narrower in depth.
The interpretation at a local level shows variations from north to south, with the development of propyllitic alteration and NW oriented D type vein system with sericitic halo in the north passing to zones of breccia and the development of porphyry with potassic alteration in the south. This suggests a relatively deeper level of erosion toward the south and east, produced by NW oriented block faulting, which have segmented the porphyry system.
The copper oxide mineralization at Berta extends to depths of 30 to 100 m with mineralization outcropping at surface and with effectively no overburden. It has a simple mineral material and gangue mineralogy, excellent response to leaching and fairly continuous Cu grades and sharp contacts with low-grade margin mineralization. These favorable conditions are due to oxidation of the hypogene mineralization with simple alteration and mineralogy: dominant chalcopyrite hosted in breccias, porphyry and tonalite affected by potassic alteration. The lack of pyrite and unreactive hostrock has allowed the generation of in-situ oxidation, with only minor Cu re-mobilization and migration, without the formation of significant supergene sulfides.
The primary mineralization consists of Cu sulfide, dominated by chalcopyrite that occurs as breccia cement and disseminations in the breccias and their contact zones with PTC and TON. Occasionally bornite has been observed in the hypogene zones of Berta Sur breccias. There is gradation in the chalcopyrite percentage from higher grade centers (> 0.5% Cu) controlled by zones of greater permeability and potassic alteration in the breccia bodies, toward areas of lower Cu grades on the margins. This variation is truncated in the N by faults cutting the body, while in the S and SW part, there is an abrupt decrease to grades approximating 0.2% Cu, due to reduced dissemination of chalcopyrite. The mineralization is controlled by the distribution of the breccias and by the NE oriented pre-mineral structures. There is a close relationship between potassic alteration, with K-feldspar and grey green sericite, and the presence of chalcopyrite. The form of the sulfide mineralization is irregular however, such that good grade Cu oxides overly equivalent grade sulfides in some areas, in others this relationship does not exist and in contrast, an abrupt decrease of good grade oxides to underlying low grade sulfides is suggestive of an inverted cone-shape or root to the mineralization. These relationships, coupled with variable shape of the breccia bodies and variations in alteration with depth permits an interpretation of the deeper parts of the system.