Geology

Northern Zacatecas and eastern Durango are situated geographically in Mexico's high central plateau known as the Central Meseta, host to many of the silver districts of central Mexico including Fresnillo, Zacatecas, Guanajuato, Real de Angeles and others.  (View Map)

Geologically, the plateau is underlain mainly by folded limestone, sandstone and argilllite formations of Jurassic and Cretaceous age, capped by flat-lying beds of basalt and rhyolite of Early Tertiary age that form distinctive buttes and mesas. This sedimentary and volcanic sequence is intruded by stocks of diorite, monzonite and granite of late Cretaceous to early Tertiary age. Extensive conglomerate beds of late Tertiary age cap all pre-existing rock units.

In common with other deposits in the region, the Miguel Auza deposit consists of structurally-controlled epithermal silver veins in argillite and sandstone of the Cretaceous age Caracol Formation. A small (6 km²) porphyry stock which intrudes the Caracol Formation near the town of Miguel Auza is the inferred source of the veins.

At least 17 polymetallic epithermal Au-Ag-Pb-Zn veins are currently being explored by Silver Eagle over an area of about 1,600 by 700 m near the southwest margin of the stock. The veins are termed "epithermal" because of observed textures such as open space filling, intricate banding and brecciation. Epithermal veins are believed to have formed close to the Earth's surface and their unusual textures are interpreted to result from repeated boiling and precipitation of precious and base metals from hydrothermal fluids. To most geologists epithermal veins are important for one reason alone: they host most of the world's bonanza gold and silver deposits.

Previous mining at Miguel Auza over the past 450 years focused on extracting easily mineable, near-surface, oxidized portions of veins above the water table, which is located at about 60 m depth. Silver Eagle is focusing on drill-testing the same veins below the water table to depths of 300 –460 metres — a depth consistent with the vertical extent of epithermal veins in other silver and gold mining camps in the region, and exploring them to their full lateral extent, currently at least 1,500 metres. Veins typically consist of argentiferous galena, argentite, silver sulphosalts, sphalerite, and pyrite in a gangue of quartz and calcite, with silver accounting for at least 50% of the metal value in most veins.

Although the vein swarm is essentially continuous without gaps or breaks, veins have been grouped for descriptive purposes into four zones: Calvario, North Zone, Mill Zone and East Zone. The steeply inclined Calvario silver base metal vein, with a strike length of at least 1.5 km and widths of up to 18 m, is the most continuous vein found to date. Gently dipping North Zone veins are for the most part bonanza silver veins (containing greater than 1 kg/tonne of silver) with very little associated base metals. Mill Zone veins to the west are mainly base metal veins with low silver content. Recently discovered East Zone veins, including the eastern extension of the Calvario vein, display high silver values similar to North Zone veins, together with moderate base metal contents.

A second style of mineralization occurs three kilometres northeast of the Miguel Auza deposit, on the eastern margin of the Miguel Auza stock. It consists of copper mineralization, mainly malachite and azurite, in an epithermal quartz vein. It is exposed by a shaft and several deep cuts. Consistent with most previous exploitation throughout the camp, no production records are available. The vein has been traced to the northwest for a distance of about one kilometre. It is one of numerous prominent, banded, epithermal quartz veins that cut the stock. The veins trend north-westerly, almost perpendicular to the trend of the silver base metal veins which they appear to post-date.

Yet a third style of mineralization found on the property consists of disseminated cassiterite (tin) in a Tertiary rhyolite plug on the eastern portion of the property. Minor artesanal mining was carried out years ago, evidenced by numerous pits and small quarries. Again, no production records are available and no modern exploration has as yet been conducted.
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