The geology of the area surrounding the old mining district is dominated by a large rhyolite dome complex that rises some 600 meters above the platform of limestones and shales which host the known Au-Ag veins. The dome consists of multiple flows of feldspar-quartz porphyry lavas and a basal sequence of quartz-feldspar ash tuffs and derived epiclastics which directly overlie the limestone basement.
Exposures west of Pinos, which includes the old mining district, are dominated by limestone whereas east of Pinos towards the El Africano vein the epiclastics are dominant. Further to the northeast, and at higher elevations, a partially welded tuff forms a capping unit which has produced the mesa-like table lands observed regionally.
This welded unit may represent the ultimate ignimbrite eruption associated with the collapse of the large caldera inferred to exist south of the Pinos District. According to this model, the limestones and epiclastics could represent calderal fill, whereas the rhyolite dome at Pinos is the product of resurgent volcanism along the rim of the larger caldera.
The mineralisation is underlain by a gently dipping sequence of Middle to Upper Cretaceous limestones and fine clastic sediments overlain unconformably by a succession of rhyolite flows, tuffs and associated epiclastic sediments. A complex set of high angle faults of several ages displaces the various rock units. One or more periods of hydrothermal activity has produced widespread epithermal gold-silver mineralization both as classic narrow vein occurrence as well as in potentially bulk tonnage settings.
The main surface host rock for the old district mineralization is the Cuesta Del Cura limestone, a widespread distinctive unit in central and northern Mexico. It is typically a thin bedded (~15 cm) sequence of light grey-weathering limestone interbedded with calcareous sandstones and thin black shales. Bands and nodules of black chert are a striking characteristic. This unit is at least 100 meters thick at Pinos and rests upon the so-called Black Lutites. This unit is only known from underground workings and drill core. It consists of black shales and calcareous mudstones interbedded with calcareous sandstones, limestones and intermittent bands of black chert.
Overlying the Cuesta del Cura limestone is a unit referred to as the Upper Clastic Formation. These rocks are not as brittle as the underlying limestones and lower clastics and usually the veins tend to narrow or to split into stockworks within them.
The rhyolite dome complex consists predominantly of multiple flows of porphyritic rhyolite and rests on a basal sequence of ash tuffs.
Numerous high-angle northeast striking faults are recognized in the district and were produced by the doming and/or caldera formation. Frequent northwest-trending normal faults with variable offsets appear to be younger than the northeast faults and are mainly responsible for the irregular topography observed today.
Examination of the regional maps strongly supports a caldera setting for the Real de Angeles Mine, 35 km to the west and suggests that a similar setting may exist at Pinos.
The initial discoveries of gold-silver at Pinos were made in the “old district” where a number of veins outcropped because the resistant, host limestone made ridges which protruded through the widespread alluvium cover.
Mineralization here is found in a large number of typical, epithermal quarts and quartz-calcite veins. These veins are grouped into four major veins zones. Such “zones” usually consist of two or more major veins and 15 or more smaller ones. The four vein zones are, from west to east:
- Cinco Estrellas Zone
- La Paz Zone
- San Ramon - Purisima Zone
- Candelaria Zone
Veins strike northerly, north-easterly and north-north-westerly and usually dip moderately to steeply west. Veins vary from a few centimetres to more than four meters in width and average about 1.5 meters.
This is a low sulphur system with minor pyrite and base metal sulphides only rarely seen. Mineralogical studies indicate that the gold and silver are fine grained and present mostly in native form, or as electrum with minor silver sulphides or sulphosalts.
Gold and silver tenors can vary significantly over short distances due to a pronounced nugget effect. An extensive sampling program carried out by Minera Apolo (~7,000 samples) throughout the district gave an average grade of 3.86 g/t gold and 36.6 g/t silver. Veins tend to be narrower but higher grade in the eastern part of the district with a higher ratio of silver to gold.
Almost all the veins have been worked to some extent and there are estimated to be 15 to 18 bonanza ore shoots (now completely mined out) where grades were in excess of 60 g/t gold. Such shoots occur at prominent deflections in veins or where two faults or fracture sets intersect. The sizes of such shoots are known to be 200 – 300 meters long. The vein systems themselves are at least 2.5km long and still open in both directions along strike.
Although irregular, the vein systems are very persistent along strike. Most of the shafts and important workings went to depths of between 90-120 meters, coinciding with the depth of the water table in various parts of the camp. Along the Candelaria vein system some workings went to at least 290 meters and possibly as much as 330 meters because of the installation of Cornish steam pumps in the later stages of the 19th century mining boom. There is no record of the exhausting the high-grade ore shoots at depth.