Summary:
Gold mineralization at the Gualcamayo Property occurs in three main areas: Quebrada del Diablo (QDD), Las Vacas (LV), and Salamanca. The QDD area includes the QDD Main, Condor, Potenciales, Amelia Inés and Magdalena (together AIM), Target D, QDD Lower, and Rodado deposits. There are also a number of small satellite deposits not currently included in the mine plan.
Three distinct mineralization types of economic interest occur in the Gualcamayo Property:
• sediment-hosted distal-disseminated gold (QDD);
• sulphide-bearing skarn deposits containing copper zinc and molybdenum with late stage gold-arsenic mineralization (Amelia Inés and Magdalena);
• porphyry style molybdenum mineralization (Quebrada Perdida).
Gold mineralization at QDD occurs in carbonate sedimentary rocks within conformable and discordant carbonate breccias and fractured limestone. The gold mineralization is related to a hydrothermal event overprinting the proximal skarns and extending into the surrounding marbles and limestones. The QDD canyon itself lies along a fault/dyke system, which is believed to be a reactivated, Ordovician rift structure that acted as the primary conduit for hydrothermal fluids migrating away from the intrusive contacts.
The mineralizing fluids were dispersed into a semi conformable, receptive limestone aquifer traveling up dip following the hydraulic gradient, more than 600 m away from the QDD feeder structure. The permeability was provided by several deformation and alteration factors forming large conformable collapse breccias and includes:
• early meteoric karsting of the Upper San Juan Formation and in particular the cliffy, bioturbated limestone member;
• hydrothermal dolomitization of the pre-existing diagenetic dolomite member of the upper San Juan Formation that initiated collapse and breccia development of the over lying karsted limestone;
• East-west faulting, tectonic brecciation along fold hinges, stylolite formation during the ongoing contractional, and transpressive deformation during the Andean orogeny.
These three factors produced a very permeable stratigraphic window (conformable breccia) within the Upper San Juan Formation that later focussed mineralizing sulphurous fluids through the earlier hydrothermal collapse breccias.
Higher-grade zones in the QDD deposit (> 2 g/t Au) are common and related to fold hinges, sediment infilled karst cavities, and brecciated intrusive contacts with limestone and marble. Mineralized breccia thicknesses range from 30 m to 150 m thick and extend more than 500 m outboard to the east and northwest of the QDD feeder structure. Due to the complex folding along north-northwest and east-west axes and strong lithological control, the mineralized collapse breccias form an undulatory ore deposit (dome and basin geometry) underlain by the hydrothermally altered dolomite member of the upper San Juan Formation.
Although more confined, gold mineralization remains open down dip along the QDD fault zone cutting the white recessive limestone unit (white marble) of the lower San Juan Formation.
The QDD deposit is a silica poor, high-level gold-arsenic system with fine marcasite and trace amounts of realgar and orpiment forming the main sulphide minerals. Silver values are generally less than 0.1 ppm. Barium is also elevated (200 ppm to 400 ppm) with higher concentrations localized along brecciated east-west striking dike margins. Mercury and antimony are weakly anomalous. Strong clay alteration along brecciated intrusive margins consisting of supergene kaolinite and alunite is also common suggesting a fairly acidic hydrothermal system. Gold occurs mainly as one to five micron inclusions within marcasite that was deposited with calcite along fractures and breccia matrices.
At AIM, late stage gold-arsenic mineralization overprints skarn zones and extends into the surrounding marbles of the San Juan Formation. Skarn hosted mineralization comprised of chalcopyrite, sphalerite, galena, pyrrhotite, and pyrite was deposited as a retrograde event preceding the introduction of the gold-arsenic mineralization.
Mineralization averaging 3 g/t Au is intimately associated with fine grained marcasite that lines late fractures and forms the chief component to marble and skarn breccias matrices. Brecciation and higher-grade gold mineralization are localized along the west to northwest trending marble– skarn contact and cross cutting east-west tensional structures. The rheological contrast between the brittle skarn and ductile marble is believed to have accommodated much of the movement during later wrench fault tectonics, forming localized east-west trending, dilational zones (i.e., breccia zones) that extend tens of metres outboard into the marble and skarn from the contact.
In addition to tectonic brecciation, gold mineralization was also enhanced by the presence of pre-existing karst cavities within the Cliffy Bioturbated member of the San Juan Formation and collapse generated by early hydrothermal dolomitization of the underlying Triplets member. These collapsed karst voids that have undergone late tectonic brecciation were later filled with semi massive, fine grained marcasite with dolomitized marble clasts over thicknesses greater than 10 m. This style of mineralization forms the highest-grade mineralized zones returning up to 30 g/t Au over 12 m.
Silver is anomalous but very sporadic returning greater than 30 g/t within the higher grade (>5 g/t Au) gold zones. Silver is localized within retrograde tetrahedrite veinlets and possibly electrum during deposition of the later stage fine grained marcasite. Inductively coupled plasma (ICP) analysis of the marcasite (sulfuros negros) also shows elevated lead, zinc, and molybdenum but relatively low silver levels.
Negligible gold mineralization extends into the intrusive stock to the north. Narrow (one to three centimetres wide) sheeted quartz-molybdenite veinlets are generally confined within the skarn at both Magdalena and Amelia Ines and extend outboard into the intrusive stock, localized along vertical, east-west tension structures.
Areas of high molybdenum content commonly lie peripheral to the gold enriched zones and overlap the intrusive contacts. Molybdenum mineralization is often associated with high manganese levels and occurs mainly in skarn with lesser amounts hosted by marble and intrusive rocks.