Summary:
Mineral deposits in the Las Chispas district are classified as gold and silver, low to intermediate sulphidation epithermal systems, typical of many deposits in Sonora, Mexico.
In northwestern Mexico, much of the exposed geology can be attributed to the subduction of the Farallon Plate beneath the North American Plate and related magmatic arc volcanism. The host rocks to mineralization in the Las Chispas district are generally pyroclastic, tuffs, and rhyolitic flows interpreted to be members of the Lower Volcanic Complex. Locally, volcanic pyroclastic units mapped within the underground workings include rhyolite, welded rhyodacite tuff, lapilli (lithic) tuff, and volcanic agglomerate.
All rock types in the Las Chispas Operation area show signs of extensive hydrothermal alteration. Thin section and TerraSpec™ hyperspectral studies identified alteration consistent with argillic and advanced argillic alteration. Alteration minerals identified include smectite, illite, kaolinite, chlorite, carbonate, iron oxy/hydroxides, probable ammonium, gypsum/anhydrite, silica, and patch trace alunite.
Generally, the host rocks are above the existing water table. Oxidation of sulphides is observed from near-surface to depths greater than 300 m and the presence of secondary minerals is recorded from the Las Chispas historic underground workings approximately 60 m to 275 m in depth from the surface. Strong and pervasive near-surface oxidation is noted to occur in the Babicanora Area, where host rocks experienced faulting and advanced weathering to limonite, hematite, and clays.
Regionally, the Las Chispas Operation is situated in an extension basin related to a Late Oligocene half-graben of the Sonora River Basin. Multiple stages of normal faulting affect the basin. The main structures are steep, west-dipping (80°) and sub-parallel to the Santa Elena-Las Chispas normal fault, which is located along the western margin of the Las Chispas Operation, striking approximately 210°. The basin is further cross-cut by younger northwest–southeast trending normal faults that dip to the southwest, creating both regional and local graben structures. Locally, the graben structures are complicated by the effects of probable caldera collapse. Three structural controls, excluding bedding contacts, are considered to influence alteration and mineralization:
• 150–170° striking and are inclined at approximately 65–75° to the southwest
• 340–360° striking and are inclined 75° west to 75° east; and
• 210–230° striking and are inclined 70–85° to the northwest.
Mineralization is hosted in hydrothermal veins, stockwork, and breccia. Emplacement of the mineralization is influenced by fractures and low-pressure conduits formed within the rocks during tectonic movements. Mineralization can be controlled lithologically along regional structures, local tension cracks, and faulted bedding planes. Brecciated mineralization formed in two ways: 1) in zones of low pressure as hydrothermal breccia; and 2) as mechanical breccias. These breccia types are interpreted to occur at the intersection of two or more regional structural trends. The mineralization is 0.10–10 m in true width, and typically encompasses a central quartz ± calcite mineralization corridor with narrow veinlets within the adjacent fault damage zone. Stockwork and breccia zones are centred on structurally controlled hydrothermal conduits.
Generally, it appears that epithermal mineralization is higher in the system (closer to the paleo-surface) on the west side (e.g., La Victoria Vein and historic mine) of the Las Chispas district compared to the east side (e.g., Granaditas Vein and historic mine), where there is an observed increase in base metal content.
Silver mineralization visually dominates over gold throughout the Las Chispas Operation. Acanthite is the principal silver mineral. Electrum and native silver can be present. Silver is associated with galena, pyrite ± marcasite and chalcopyrite. Gold occurs as electrum, native flakes and in association with pyrite and chalcopyrite. Locally, gold and silver values have a strong correlation with each other. Base metal contents are low in veins.
The Las Chispas Operation is divided into the Las Chispas Area and the Babicanora Area, and currently has 62 epithermal veins, not including seven bifurcations. Mineral Resources were estimated for 38 veins, including bifurcations and Mineral Resources were estimated for 38 veins, including bifurcations, and Mineral Reserves were estimated for 19 veins, including HW, FW and splays, of which six veins (Babicanora Main, Babicanora FW, Babicanora Norte, Babicanora Sur, Babi Vista and Las Chispas) contain the majority of the Mineral Reserves.