Local Geology
Epithermal systems form near the surface, usually in association with hot springs, and to depths on the order of a few hundred meters. Hydrothermal processes are driven by remnant heat from volcanic activity. Circulating thermal waters rising through fissures eventually reach a level where the hydrostatic pressure is low enough to allow boiling to occur. This can limit the vertical extent of the mineralization, as the boiling and deposition of minerals is confined to a relatively narrow range of thermal and hydrostatic conditions. In many cases, however, repeated healing and reopening of host structures can occur, imparting cyclical vertical movement of the boiling zone and resulting in mineralization that spans a much broader range of elevation.

Local Geology Of the geological formations associated with the Guanajuato district only the Esperanza and La Luz Formations occur in the Bolañitos mine area with mineralization residing primarily within the La Luz Formation. Mineralization is known to dissipate at the contact with the Esperanza Formation.

The vein dips from 35° to 55º to the southwest with measured displacement of around 1,200m near the Las Torres mine and 1,700 m near La Valenciana mine. The most productive veins at Bolañitos strike parallel to the Veta Madre system.

Bolañitos mineralization is directly related to faulting. Mineralization occurs as open-space fillings in fracture zones or impregnations in locally p ........

Mining methods used at Bolañitos include long-hole stoping and conventional cut and fill mining.

The mine operates on two 10- hour shifts, 7 days per week, whereas the mill operates on a 24/7 schedule, two 10-hour shifts per day.

Cut and fill stopes are generally mined 15m along strike and in 1.5 - 1.8m high cuts, and long hole stopes are 15m long and 20m high (20m between levels floor to floor). Access to the stoping areas is provided by a series of primary and secondary ramps located in the footwalls of the target structures. In Bolañitos numerous veins are mined. The ramps have grades from minus 15% to plus 12%, with plus or minus 12% as standard. The ramps and crosscuts are generally 4 m by 4 m.

The ground conditions at the San Miguel, La Luz and Lucero mines are considered to be good. The rocks are competent and require no special measures for support other than occasional rock bolting and regular scaling. Cable bolting is sometimes required during the preparation of stopes for longhole blasting. The cable bolts are installed by drilling holes in the hanging wall and fixing the bolts in place with cement pumped into the hole.

Conventional drill and blast methods are used to extract the ore at Bolañitos, and access to the mining areas is provided by ramps and audits. Mine development headings are drilled by jumbo and by jackleg. Traditionally a conventional bottom-up cut and fill mining method was employed with waste rock brough ........

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