The Mine’s surface and mineral rights are owned by Minerales de Occidente, S.A. de C.V. (“Minosa”), a wholly-owned indirect subsidiary of Aura existing under the laws of Honduras.
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Summary:
The gold deposits at the Mine are hosted within Tertiary-aged felsic volcanic flows, tuffs and agglomerates, thick inter-bedded silica breccias, primarily containing volcanic fragments and tuffaceous sandstones. These volcanic units occur on the south (hanging wall side) of the San Andres Fault. The fault strikes west-east and dips at 60° to 70° south and it marks the northern boundary of the Water Tank Hill and East Ledge pits. The fault forms the contact between the Permian phyllites (metasediments) to the north and the volcanic units on the south. Mineralisation within the phyllites is limited to the Buffa Zone where quartz carbonate veining proximal to the San Andres Fault. South of the Mine area, where there is no alteration, the volcanic and sedimentary rocks have a distinctive hematite brick red color but, in the Mine area, they have been bleached to light buff yellow and grey colors due to alteration. The younger volcanic and sedimentary units typically have a shallow to moderate southerly dip and thicken to the south of the Mine area.
Structurally, the Mine area is transected by a series of sub-parallel, west to northeast-striking faults that are typically steeply dipping to the south and by numerous north and northwest-striking normal faults and extension fractures. The most prominent fault of the 35 first set is the San Andres Fault. The San Andres Fault is parallel to, and coeval with, a major set of west to north-northeast trending strike-slip faults that form the Motagua Suture Zone, which is continuous with the Cayman Trough.
The normal faults and extension fractures occur within the volcanic and sedimentary units on the south side of the San Andres Fault. Average strike of these structures is N25°W; dip is 50° to 80° to the southwest and northeast, forming grabens where the strata are locally offset. These faults and fractures are generally filled with banded quartz and blade calcite and have formed focal points the alteration and mineralisation fluids within the Mine area. These extensional structures are distributed over a wide area, from the East Ledge open pit to Quebrada Del Agua Caliente, approximately 1,500 m to the east, and from the San Andres Fault, for at least 1,200 m south and are coeval with the strike-slip faults.
The San Andrés deposit is classified as an epithermal gold deposit associated with extension structures within tectonic rift settings. These deposits commonly contain gold and silver mineralization, which is associated with banded quartz veins. At the Mine, however, silver does not occur in significant economic quantities. Gold occurs in quartz veins predominantly comprised of colloform banded quartz (generally chalcedony with lesser amounts of fine comb quartz, adularia, dark carbonate, and sulphide material). The gold mineralization is deposited as a result of the cooling and interaction of hydrothermal fluids with groundwater and the host rocks. The hydrothermal fluids may have migrated some distance from the source; however, there is no clear evidence at the Mine that the fluids or portions of the fluids have been derived from magmatic intrusions.
The rocks hosting the San Andrés deposit have been oxidized near surface as a result of weathering. The zone of oxidation varies in depth from 10 m to more than 100 m. The zone of oxidation is generally thicker in the East Ledge deposit compared to the Twin Hills deposit.
In the oxide zone, the pyrite has been altered to an iron oxide such as hematite, goethite, or jarosite. The oxide zone generally overlies a zone of partial oxidation, called the mixed zone, which consists of both oxidized and sulphide material. The mixed zone may not occur continuously, but where it is present, it reaches thicknesses of over 50 m. below the zone of oxidation; the gold is commonly associated with sulphide minerals such as pyrite. The sulphide, or “fresh”, zone lies below the mixed zone.
The gold contained in the oxide zone is amenable to extraction by heap leaching using a weak cyanide solution. The gold recovery is reduced in the mixed zone as a result of the presence of sulphide minerals and the gold cannot currently be recovered economically from the sulphide zone by heap leaching.
High clay content in the ore, resulting from alteration, is detrimental to the heap leaching process because of reduced through-put rates in the crushing plant and reduced permeability in the heap leach operation. This poor leaching situation is resolved by agglomerating the crushed ore by adding cement to increase the permeability of the heap prior to leaching.
Based on metallurgical studies, the gold is primarily contained in electrum as fine-grained particles. The particle size of the electrum grains varied from 1 micron (“µ”) x 1 µ up to 10 µ x 133 µ. One native gold grain was noted. The silver generally occurs at about the same grade as gold and the correlation between silver and gold is low at 0.24. Silver is not considered important because of the lower price for silver compared to gold and the lower metal recovery of silver.