Summary:
The Lindero Property contains two known porphyry gold-copper deposits. The Lindero Deposit which is the focus of current mining activities (the Lindero Mine); and the Arizaro Deposit which is located 3.2 km southeast of the Lindero Mine.
In the Central Andes, the altiplano or puna is a high plateau of more subdued relief between the Eastern Cordillera, a rugged region usually rising to between 3 km and 4.5 km, and the Western Cordillera, which is a high spine of mountains that may reach as much as 5 km in height. The Arizaro Volcanic Complex consists of two superimposed concentric volcanic centers, the Arizaro and the Lindero cones, located in the Archibarca volcanic arc at the southern margin of the Salar de Arizaro basin. Basement rocks crop out to the north of the Lindero Deposit, and consist of coarse- grained Ordovician granites uncomformably overlain by Early Tertiary red bed sandstones. The Lindero– Arizaro complex, a series of diorite to monzonite porphyritic stocks, intrudes these units.
Lindero Deposit
Mineralized zones at the Lindero Deposit form a semi-circular shape about 600 m in diameter which extends to a depth of 600 m, consisting of four different zones at the surface. The distribution of gold–copper mineralization at Lindero shows a strong relationship to lithology, stockwork veinlets, and alteration assemblages. Higher grades of gold–copper (approximately 1 g/t Au and 0.1 % Cu) are commonly associated with sigmoidal quartz, quartz–magnetite– sulfide, biotite-magnetite–chalcopyrite, magnetite–chalcopyrite and quartz–limonite–hematite stockworks that are strongly associated with K-feldspar alteration. This association is very common in the east zone of the deposit, where the highest gold grades occur. At other locations where one or more stockwork types are missing or the intensity of fracturing is lower, mineralization tends to be weaker and the grades of gold tends to be lower (approximately 0.4 g/t Au).
At the Lindero Deposit, native gold and electrum are finely disseminated in subparallel to stockwork quartz + sulfide ± magnetite ± anhydrite veins and in some cases in matrices of hydrothermal breccias. Magnetite is common to abundant in mineralized zones. These mineralized stockworks and potassic alteration are interpreted to have formed as the result of degassing of the early intrusive bodies. Fluid pressures during degassing triggered fracturing of the intrusions and wall rock, allowing gold-rich fluids to circulate and precipitate, forming a gold–copper orebody. Later intrusions resulted in weak to moderate gold–copper mineralization forming mostly along and immediately fringing these intrusive contacts. Finally, post mineralized intrusives were overprinted onto the north and west of the deposit.
Gold mineralization at Lindero is characterized by native, free-milling gold associated with chalcopyrite and/or magnetite grains with rare interstitial quartz.
Gold mineralization in the Lindero Deposit is hosted mostly in four different bodies that form an annular shape at the surface. The annular shape is related to the distribution of the earlier, mineralized rock units comprising FPD, CPD1, and PBFD lithologies with the CPD2, DDP, and PMI porphyries in the center of the deposit forming the barren to lowgrade core of the system.
The east/northeast body is the largest mineralized zone, extending north-south for 450 m. It is at least 190 m wide and extends for at least 300 m vertically, dipping steeply towards the center of the deposit. This body has the highest gold grades, averaging approximately 1.0 g/t Au. The southwest body forms an elliptical shape that is 270 m long, up to 100 m wide and extends for at least 400 m vertically. Grades average approximately 0.8 g/t Au. Mining in the first two years of operation has focused on extracting mineralized material from these two bodies with mapping in pit confirming the interpretation.
The northwest and west bodies are narrower, but thicken with depth, reaching a maximum width of 100 m. Both lie mostly beneath the PMI intrusion. Several other narrow mineralized bodies occur at depth, where bodies of well mineralized CPD1 and PBFD became rafted within the central, late- to post-mineral rock units.
Arizaro Deposit
The Arizaro volcanic center is characterized by fine- to medium-grained hornblende diorite to monzonite porphyritic stocks. The Arizaro Deposit is dominated by a main, moderately to strongly mineralized intrusive unit that crops out in the central part of the prospect area. It consists of fine hornblende porphyritic diorite intruded by several stocks, dikes, igneouscemented breccias and hydrothermal breccias. Smaller stocks are exposed in a few areas. Dikes of andesitic and dacitic composition are generally distributed radially to the main intrusive unit.
Gold–copper mineralization is associated with two different mineralizing events. The strongest is a non-outcropping intrusive (CDP) which occurs in the north part of the porphyry with an elongated shape trending northeast to southwest for more than 400 m with an estimated average width of 60 m, producing an Au-Cu mineralized zone approximately 500m by 150m, with average grades of 0.5 g/t Au and 0.15 % Cu.
The other mineralizing event is in the center of the system and is related to breccias and microbreccias which have a semi-oval shape at surface. In the center, there is a higher-grade core with a semi-ellipsoidal form, extending north–south for 480 m with an estimated average width of 50 m, averaging approximately 0.6 g/t Au at surface in the central core, and reducing to approximately 0.2 g/t Au on average in the margins of the body. Copper grades are more consistent across the deposit averaging 0.15 % Cu. The high relative gold to copper ratios suggests higher gold mobilization in the hydrothermal fluids with respect to copper and may be interpreted as representing the higher levels of metal precipitation from gold-rich, copperpoor, hydrothermal solutions.
The Arizaro Deposit has styles of mineralization with copper–gold grades which are strongly correlated with different alteration assemblages. Mineralization is mainly associated with potassic alteration. This occurs generally in multi-directional veins, vein stockworks and disseminations. In some areas, the vein density is high, forming vein stockworks in the intrusive rocks. These vein stockworks are limited to magnetite–biotite veinlets, quartz– magnetite–chalcopyrite veinlets, late magnetite breccias, and in late-stage mineralization events, anhydrite–sulfide veinlets.
Chalcopyrite and bornite are the main copper minerals. Gold is mainly associated with chalcopyrite, quartz, and anhydrite veinlets. Magnetite is common as massive replacements of the matrix in breccias, in veinlets and as disseminations. Dow (2002) reported the presence of elevated light rare earth elements (LREE) concentrations (La–Ce–Nd) hosted in monazite and allanite, and the presence of sub-micrometer-sized palladium intergrown with free gold in biotite-rich alteration assemblages.
Molybdenite is sporadically present and is associated with anhydrite–chalcopyrite veinlets. The presence of pyrite is limited to the distal margins of the system.
The copper oxide minerals found in the deposit include chrysocolla and brochantite. These occur as fracture-fill, fine veinlets with quartz ± sulfides and replacing feldspar crystals. The iron oxide minerals present are limonite, hematite, and very sporadic jarosite along fractures. Coarse gold was observed and confirmed with X-ray diffraction analysis at the University of Neuquen, Argentina, laboratory, from drill hole ARD-14 (154.5 m) and was also identified macroscopically in an anhydrite–chalcopyrite–molybdenite vein.