Great Southern Copper plc announced that it has increased its concession footprint at the La Colorada Lithocap prospect, part of the Especularita Project, which is highly prospective for porphyry Cu-Au type deposits. La Colorada Porphyry Cu-Au Prospect: Great Southern Copper's Especularita Project is centred around the giant La Colorada lithocap located within the Cretaceous-age coastal metallogenic belt of northern Chile. The prospect is defined by its distinctive high-sulphidation style alteration blanket mapped over an area greater than 75 km2 within a northwest-southeast trending structural lineament that links the prospect with notable porphyry Cu-Au mines and projects (Table 1) including Los Pelambres mine, Altar and El Pachon (Figure 1).
To the north the prospect is on trend of the Carmen de Andacollo porphyry copper (Teck) and gold mines (Galantas) which, significantly, are developed beneath an advanced argillic lithocap of similar type and dimension to La Colorada and hosted in similar aged rocks at similar elevation. Regional mapping at Especularita suggests that the La Colorada alteration system is controlled locally by the dominant northwest-southeast trending Victoria Fault and is off-set by regular late northeast-trending structures. The Cerro Negro high-grade Cu-Ag deposit is located within a southeastern dislocated section of the lithocap (Figure 1).
The new concessions further strengthen the Company's control over this highly prospective alteration system and are considered critical as they potentially occupy the central core to the mineralising system where alteration may be the most intense (hottest and most acidic). Deposit Company Stage Category Tonnes (Mt) Cu (%) Au (g/t) Mo (%) Ag (g/t) Los Pelambres Antofagasta Operation All resources incl. reserves 6,047 0.47 0.04 0.017 Carmen de Andacollo Teck Operation Mill ore reserves 417 0.34 0.12 Operation Heap leach ore reserves 1,300 0.29 Altar Aldebaran PFS M&I Resource 2,400 0.42 0.07 1.22 PFS Inf Resource 1,215 0.37 0.04 1.25 El Pachón Glencore Pre-development M&I&I Resource 6,000 0.43 - 0.013 2.2 Los Azules McEwen Development Prov&Prob Reserves 1,023 0.45 Development M&I Resource 996 0.26 Development Inf Resource 4,239 0.21 Llauhin Southern Hemisphere Exploration M&I&I Resource 218 0.38* Lithocaps and Porphyry Cu-Au Mineralisation: Advanced argillic high-sulphidation lithocaps are well known to occur above porphyry copper and copper-gold deposits globally although are not widely observed above deposits in Chile due to the actions of rapid tectonic uplift and erosion.
Notable exceptions include Carmen de Andacollo and Valeriano-El Encierro (Figure 3). Generally, lithocaps comprise subsurface, broadly stratabound alteration domains that are laterally and vertically extensive with lateral dimensions greater than 10 km and thicknesses of more than 1 km, hence they are often referred to as "blankets". They typically form when acidic magmatic-hydrothermal fluids rising off a porphyry intrusive at depth react with wallrocks during ascent towards the paleosurface.
Although lithocaps typically have steeply-dipping structural roots, there is a significant component of lateral fluid flow involved in their formation, either through permeable aquifers and/or a well-developed lateral fracture system. Although lithocaps are commonly barren, high sulfidation Au-Ag-Cu mineralisation typically occurs in silicic altered rocks, either as stratabound replacements, veins and/or breccia cement. The silica-clay-rich alteration assemblages are invariably magnetite-destructive and can obscure the magnetic signature of an underlying porphyry deposit.
Pyrite alteration is ubiquitous in lithocaps and mapping pyrite concentration can be an important vector indicator. Supergene weathering of pyrite (interaction with surface water) produces H2SO4, creating highly acidic fluid conditions which acts to leach the lithocap of minerals and metals, creating the distinctive and characteristic vughy silica texture (and may also produce supergene enrichment blankets at depth). Where present, the broad, flat-lying and chemical inert nature of the lithocap acts to mask (or blanket) the location of porphyry copper deposits below.
Explorers need to rely on other tools, such as geophysics and clay mineralogy, to attempt to identify the higher temperature and more acidic parts of the lithocap that sit above the feeder zones leading to the potassic-altered core of the porphyry mineralising system at depth (Figure 4). The lithocap may mask multiple porphyry copper deposits and the buried porphyry copper deposit may not necessarily be centrally located beneath the lithocap, highlighting the importance of understanding the lithocap geology and geochemistry (vectoring). Next Steps: Ground exploration covering the five new concession blocks will be incorporated into the broader ongoing La Colorada exploration programme including mapping and sampling.
Spectral mapping of alteration minerals will be key to vectoring toward hot, acidic feeder zones. Geophysics surveys (AMT and magnetics) are also planned to commence this quarter.
