Gold mineralization at Maricunga has been interpreted to be porphyry style gold systems.

The Verde and Pancho gold deposits at Maricunga occur in the Maricunga Gold Belt of the high Andes in northern Chile.

The Maricunga volcanic-intrusive complex is exposed over an area of 12 km2 and consists of andesitic to dacitic domes, flows, and breccias that are intruded by subvolcanic porphyries and breccias (Muntean 1998).

At Verde West, gold mineralization is centered about an elliptical porphyry plug measuring 175 by 100 m and oriented at N30°W. At Verde East, the porphyry plug measures 130 by 80 m and is oriented at N35°E. The porphyries occur within a sequence of intermediate tuffs, porphyries and breccias that are the host rocks to the gold mineralization. Lithological interpretation at Verde has identified six major lithologic units. These are:

• Post mineral intrusives (barren)
• Mineralized post mineral intrusives
• Verde breccia
• Dacite porphyry
• Dacite tuffs
• Laguna tuff (barren)

The most favourable ore hosts are the Verde Breccia and Dacite Porphyry units. The dacite porphyry is a volcanic to hypabyssal intrusive rock with 20% to 40% plagioclase phenocrysts in a fine-grained matrix. It contains phenocrysts of biotite, hornblende, and sparse quartz. The unit is the best host for stockwork veining, with some portions containing up to 20% quartz-magnetite ± pyrite veinlets.

The Verde Breccia consists of intrusive breccia and/or volcanic tuff breccia. Its geometry suggests an intrusive origin. The unit consists of breccia with angular to rounded clasts, generally matrix supported. The breccia is generally green to greenish gray, which is one of the diagnostic features of the unit. The color of the rock is largely due to chlorite and occasionally epidote. The fragments range from 2 mm to greater than 2 meters in size. Locally the unit is mostly matrix-sized material with only sparse clasts. Clasts in the unit are generally monolithic volcanic fragments, which are often porphyritic with white plagioclase laths up to 5 mm in length. The unoxidized rock typically contains 0.5 to 1% pyrite. Quartz-magnetite veinlets are common in the mineralized portions of the unit. In places, the breccia is cut by fine-grained matrix material that forms clastic-like dikes. These are generally > 2 cm in width. They may have been formed as “fluidized” material injected into fractures at the time of the formation of the unit.

Gold mineralization at Verde is interpreted to be the result of the fracturing and concentration of fluids in the carapace of an intrusive plug or stock. Gold is closely associated with quartz, magnetite, calcite, and garnet stockworks. Approximately 80% of the stockwork veins are generally dark grey in color, finely banded, with magnetite. The remaining 20% are principally white quartz veins. Gold mineralization is postulated to have resulted from at least 2 phases of mineralization, the first is a lower grade phase associated with copper and probably the porphyry emplacement event. The second phase is a higher-grade gold only event possibly associated with the structurally emplaced veinlet swarms and northwest trending sheeted veinlet zones more evident in Verde East. Verde East and Verde West cover an area of 1.4 km in length by 700 km in width. Drilling has identified gold mineralization to a depth of a 600 m.

The porphyritic diorite intrusives and intrusive breccias are the main hosts for mineralized veins. Mineralization is also hosted by the volcanic rocks. The mineralization is commonly related to the various geologic features listed below:

• directly associated with the presence of B, A and T veinlets
• in silicified zones, normally micro-granular and a dark grey color, with or without the
presence of magnetite
• associated with intrusive breccias
• in contact zones such as the contact between the intrusive breccia with the volcanic breccia or the contact between the intrusive breccia and the diorite porphyry.

In the porphyry the veins are present in stockwork or sheeted veins. They are generally subvertical and have a strong, preferred north-westerly strike. The northwest structural control is evident not only at outcrop scale but is also reflected in the northwest alignment of intrusives and the three centers of mineralization in the district, Verde, Pancho and Guanaco. The mineralization outlined to date at Pancho comprises an area of 800 m by 700 m. Gold porphyry mineralization was identified in drilling at depths exceeding 600 m.

Maricunga is an open pit mine. The ore is transported by haul truck to the three-stage crusher and then to the heap leach pad.

The ore is transported by haul truck to the three-stage crusher and then to the heap leach pad. Cyanide solution is applied by drip emitters at a concentration on the order of 0.5 grams per liter (g/L) at a rate of approximately 10 liters per hour per square meter (L/hr/m2). The solution percolates and is collected at the base of the pad by drains and pipelines that convey the rich solution to the solution ponds at the Adsorption-Desorption-Refining (ADR) Plant. The ADR Plant uses activated carbon for gold recovery. Cyanide is added to limit the adsorption of copper onto carbon, and a cold cyanide strip of the carbon is performed to remove copper. The cold strip Cu-CN solution is fed back to the heaps. The ADR plant consists of five carbon adsorption tanks, with Zadra pressure elution of carbon coupled with gold electrowinning. Overall gold recovery from ore to gold doré at Maricunga is 65 to 70 percent.

A portion of the rich solution proceeds to the Sulfidization, Acidification, Recycling, and Thickening (SART) Plant for recovery of copper and regeneration of cyanide. SART involves adding chemical sulfide ions, such as sodium hydrosulfide (NaSH), to an acidified CN solution to precipitate copper, silver and zinc (if present) as metal sulfides, and convert cyanide to HCN, under weakly acidic conditions. HCN is a very soluble gas, and remains in solution until the Cu2S, Ag2S or ZnS are removed by thickening and filtration. Importantly, gold does not precipitate to any significant extent during the SART process, and remains in solution to be recovered on activated carbon. The HCN solution is then neutralized with lime or caustic soda, prior to recycling to the leach pad.