Summary:
Deposit Types
The porphyry style mineralization at Caserones is a typical Andean porphyry copper/molybdenum deposit. A generic description from Pantaleyev (1995) summarizes the common features of porphyries as large zones of hydrothermally altered rock containing quartz veins and stockworks, sulphide-bearing veinlets, fractures, and lesser disseminations in areas up to 10 km2 in size, commonly coincident wholly or in part with hydrothermal or intrusion breccias and dike swarms. Deposit boundaries are determined by economic factors that outline ore zones within larger areas of low-grade, concentrically zoned mineralization (Amec, 2005).
Mineralization
The Regalito Deposit strikes SE-NW with a length of approximately 2,000 m and a width of approximately 1,500 m. The oxide and secondary copper zones form a surface-parallel blanket over 1,200 m in diameter with a central “core” of at least 1,000 m in diameter where thicknesses average 300 m and exceed 400 m in the central part. Secondary copper grades within this central area are also slightly elevated above those in the surrounding parts of the deposit. Outboard of this “core” the zone thins, and grades are generally lower, although the enrichment blanket is still open to the southwest. The secondary copper zone has been modeled to approximately 4,000 m in elevation and deeper locally. It is generally coincident with the porphyry intrusive (Amec, 2005).
The oxide zone forms a cap that sits on top of the secondary copper zone in the upper part of the northeast trending ridge that constitutes the northwest margin of the deposit, mostly above 4,400 m and thinning to a skin of a few metres, moving down the ridge slopes. Grades within the oxide zone are higher than the average grade of the secondary copper zone and it is open to the northeast.
Flanking the oxide zone and overlying the supergene zone, a zone of “leached” material varies in thickness from 0 to 200 m, averaging ~40 m over the majority of the deposit. This zone is not leached sensu strictu, as it contains appreciable amounts of pyrite and pods of chalcocite and copper oxide mineralization, which is poorly defined at the current drill spacing and were not included in the resource estimate.
Primary copper mineralization was not targeted by Lumina’s drill program and this zone remains open in all directions. Locally high-grade copper-moly mineralization associated with hydrothermal breccia bodies was cut by drill holes, notably RG04-57 and RG04-84, which may be associated with structures. Samples from the primary copper and molybdenum mineralization range from being relatively high grade to being low grade and it remains to be determined whether the primary zone is of economic interest.
It should be noted that molybdenum was not modeled as part of the resource estimate, since Lumina envisions a heap leach SX-EW operation from the oxide and supergene enriched horizons, which would not recover any molybdenum. Future studies could contemplate the potential of recovering the copper and molybdenum sulphides from deeper portions of the deposit through a milling and flotation operation.
Mineralization Zonation
At Regalito, interpretation of the results of the various drill programs has established four main sub horizontal zones of copper mineralization. These zones (leached, oxide, supergene sulphide, and primary) are characterized by distinctive assemblages of copper and molybdenum minerals (Amec, 2005).
Molybdenite is present in all of the zones and generally occurs with quartz in veinlets and rarely as disseminated grains.
Leached Zone
Typical leached zones in porphyry environments are formed when copper and iron-bearing sulphide minerals are leached from the host rock by groundwater and removed from the immediate area where leaching occurred, typically downwards, or laterally. The leached zone at Regalito contains only patchy, discontinuous copper mineralization and is well developed along the north and northeast sides of the deposit and less well developed over the central part of the deposit. Non-silicate minerals are dominantly pyrite with minor chalcopyrite and iron oxide minerals with significant, but patchy pods of secondary copper sulphide and oxide minerals. Leaching of sulphide within this zone is not complete and relic boxwork textures occur occasionally, mainly in veins, after pyrite and chalcopyrite.
Oxide Zone
The majority of the oxide zone at Regalito is restricted to the topographically higher part of the deposit. Oxide copper mineralogy is dominantly chalcanthite with subordinate chrysocolla, brochantite and minor amounts of malachite, azurite and anterlite noted locally. Oxide minerals are predominantly fracturing filling, with a fraction of the total disseminated as replacements of disseminated primary sulphides. Fracture filling mineralization is likely transported from overlying or lateral sources, and this is particularly evident where pods of copper oxide mineralization occur in monzogranite wallrock that contained only trace amounts of pre-existing sulphides. Gypsum is also present within the oxide zone, mostly as fracture fills and iron oxide minerals are dominantly goethite, hematite with lesser jarosite. Black manganese oxides also occur, most often in the peripheral parts of the deposit within the monzogranite host rock.
Secondary Sulphide Zone (supergene enriched)
Typical secondary sulphide (supergene enriched) zones form as a result of deposition of Cu-bearing secondary sulphide minerals from groundwater leading to enrichment of the primary grades. The secondary sulphide zone is moderately developed at Regalito and covers an area of about 2.0 km northsouth and 1.5 km east-west and averages about 355 m in thickness. The zone is restricted to elevations between 3,877 masl and 4,552 masl.
Supergene copper mineralization has been observed to preferentially replace chalcopyrite throughout the deposit, replacing pyrite to a much lesser extent and often leaving it pristine or coating grains with a sooty film.
The majority of secondary copper sulphides consequently occur as disseminations or hairline veinlets, mirroring the style of the primary sulphide mineralization. Regularly spaced, millimetric veinlets of chalcocite also represent a widespread mineralization type and veinlets generally occur with widths of 2-10 mm containing massive, crystalline, or granular aggregates of chalcocite. It has been noted that higher grade 2 m core samples (> 0.8% CuT) tend to contain one, rarely two, chalcocite veins in excess of 1 cm thick.
Primary Zone
The exploration program by Lumina, up to 2005, did not target the primary zone and most of the Lumina drill holes were stopped when primary mineralization was encountered, and many did not reach primary mineralization. Primary sulphide mineralization generally comprises 2-5% sulphides with 1 to 3% associated with areas of potassic alteration, frequently with 1-2% magnetite. Pyrite is the principal sulphide with subordinate chalcopyrite generally occurring in a proportion of less than 2:1. Molybdenite is also a subordinate sulphide and minor amounts of bornite, and traces of sphalerite have also been observed. Sulphides occur as disseminations and veinlets with pyritic veinlets more common in strongly phyllic altered areas. Gypsum occurs sporadically in this zone and anhydrite has been observed locally in the deeper parts of the deposit.
Mineralization at Regalito is typical of many Andean porphyry copper molybdenum deposits. The deposit consists of well defined leached, oxide, supergene sulphide, and primary zones. The supergene sulphide zone is moderately developed and is somewhat anomalous in that it is quite thick with fairly consistent copper grades throughout. There are no very high-grade zones within the enrichment as is common in many similar deposits.