The San Dimas deposit occurs in the central part of the Sierra Madre Occidental volcanic pile which contains rocks reflecting two separate periods of magmatism which were active between 100-45 million years ago and 32-23 million years ago. The veins are contained within rhyolitic, andesitic and granodioritic rocks of the older magmatic episode.
Regionally, the geological setting at San Dimas shows two major volcanic successions and a number of intrusive events. The Eocene aged Lower Volcanic Group (LVG) is predominantly composed of andesitic and rhyolitic flows and tuffs, and is intruded by a number of bodies including the dominant Piaxtla Granite (part of the Sinaloa composite batholith).
The Tertiary aged Upper Volcanic Group (UVG) is composed of a lower andesitic horizon capped by rhyolitic ash flows and tuffs, and is separated from the LVG by an erosional and depositional unconformity. Subsequent rhyolitic and basic dykes intrude both the LVG and UVG.
Structurally, the San Dimas district lies within an area of complex normal faulting along the western edge of the Sierra Madre Occidental. Compressive forces first formed predominantly east-west and north-northeast tension gashes that were later cut by transgressive north-northwest striking slip faults. The strike-slip movements caused the development of secondary north-northeast faults, with right lateral displacement.
Locally, ore is contained in over 120 gold/silver epithermal veins characterized by low sulphidation and adularia-sericitic alteration, and formed prior to deposition of the UVG. Veins pinch and swell and commonly exhibit bifurcation, horse-tailing and cymoidal structures. The veins vary from a fraction of a centimetre in width to over 15 metres, but average 2 metres. They have been followed underground from a few metres in strike-length to more than 2 kilometres.
Mineralization is typical of epithermal vein structures with banded and drussy textures. Three major stages of veining have been recognized in the district, each containing variable amounts of mineralization. The second stage produced the bulk of the ore deposits, itself having three sub-stages characterized by distinct mineral assemblages, quartz-chlorite-adularia, quartz-rhodonite, and quartz-calcite. Sulphide minerals include pyrite, sphalerite, chalcopyrite and galena as well as lesser amounts of argentite, polybasite, stromeyerite, native silver and lectrum.
The ore deposits are typically found in what is referred to as the ‘Favourable Zone’, analogous with the boiling zones. Strike length of these Favourable Zones can exceed 2,000 metres and dip length can exceed 300 metres. Exploration generally relies on a combination of structural geology, geochemistry, and fluid inclusion studies to identify the Favourable Zone. Historical reconciliation has shown that, once the Favourable Zone is defined, over 30% of the vein material contained within the zone will be classified as ore.