Summary:
The vein-hosted mineral deposits within the San Dimas Mine district are considered to be examples of silver- and gold-bearing epithermal quartz veins that formed in a low-sulphidation setting.
Mineralization
Within the San Dimas district, the mineralization is typical of epithermal vein structures with banded and drusy textures. Epithermal-style veins occupy east–west-trending fractures, except in the southern part of the Tayoltita Block where they strike mainly northeast, and in the Santa Rita area where they strike north–northwest.
The Favourable Zone concept for San Dimas was developed in the mid seventies in the Tayoltita Block, based on the San Luis vein, which was mined out in the late 1990s. The mine geologists observed that bonanza grades along the San Luis vein were spatially related to the Productive andesite unit and/or to the interphase between the Productive andesite and the Portal rhyolite and/or the Buelna andesite. This spatial association of vein-hosted mineralization to a favorable zone within the volcanic sequence is now recognized in other fault blocks and constitutes a major exploration criterion for the district.
The veins were formed in two different phases. The east–west striking veins developed first, followed by a second system of north–northeast-striking veins. Veins pinch and swell and commonly exhibit bifurcation, horse-tailing, and sigmoidal structures. They vary in width from a fraction of a centimeter to as much as 8 m wide, but average 1.5–2.0 m. The veins have been followed underground from a few meters in strike-length to more than 1,500 m. An example of these veins, the Jessica Vein, which extends for more than 1,000 m in the Central Block.
Three major stages of mineralization have been recognized in the district:
• Early stage;
• Ore-forming stage;
• Late-stage quartz.
These three distinct sub-stages of the ore-forming stage can be discriminated by distinctive mineral assemblages with ore-grade mineralization occurring in all three sub-stages:
• Quartz–chlorite–adularia;
• Quartz–rhodonite;
• Quartz–calcite.
The ore-forming vein stage mineralogy consists primarily of white to light grey, medium-to-coarsegrained crystalline quartz. The quartz contains intergrowths of base metal sulphides (sphalerite, chalcopyrite, and galena) as well as pyrite, argentite, polybasite [(Ag,Cu)6(Sb,As)2S7], stromeyerite (AgCuS), native silver, and electrum. The veins are formed by filling previous fractures and typical textures observed include crustification, comb structure, colloform banding and brecciation.
Mineralized shoots within the veins have variable strike lengths (5–600 m); however, most average 150 m in strike-length. Down-dip extensions of mineralized shoots are up to 200 m in length and are generally less than the strike length.
West Block
The West Block is limited to the west by the Don Porfirio Fault and to the east by the Sinaloa Fault. It covers an area of 2,700 m in the northeast–southwest direction and 7,700 m in the southeast– northwest direction. In this approximately 21 km2 of surface, a total of 17 veins have been identified. The veins are hosted by the Portal rhyolite and Productive andesite stratigraphic units and andesitic intrusions.
The strike direction of the veins in this block is east–northeast–west–southwest, dipping at 30–60° to the northwest. The highest vein in elevation is San Rafael, located at 1,100 masl; and the lowest is Santa Rosa vein at 340 masl. The strike length varies from 100–500 m. The average thickness is 1.5 m, the distance between veins varies between.
Graben Block
The Graben Block is limited to the west by the Sinaloa Fault and to the east by the Limoncito Fault. It covers an area of 1,200 m in the northeast–southwest direction and 6,000 m in the southeast– northwest direction. Within this approximately 7 km2 area, a total of eight veins have been identified. The veins are hosted by the Portal rhyolite and Productive andesite stratigraphic units and dioritic rocks.
The strike for most of the veins is northeast–southwest dipping from 30–90° to the northwest. The highest vein in elevation is Santa Regina, located at 1,060 masl and the lowest is Victoria at 250 masl. The strike length varies from 100–1000 m. The average thickness is 2 m, and the distance between veins varies from
Central Block
The Central Block is limited to the west by the Limoncito Fault and to the east by the Guamuchil Fault. It covers an area of 3,200 m in the northeast–southwest direction by 8,500 m in the southeast–northwest direction. Within the block, a total of 21 veins have been identified in the last two decades.
Two significant veins in the Central Block are the Roberta and Robertita veins, which are 1,500 long by 500 m high by 2.5 m average thickness. These two veins are almost mined-out. The Santa Jessica vein, which is one of the main veins in the district, is also hosted in the Central Block. The veins in Central Block show northeast–southwest strike direction, dipping to the northwest. In terms of elevation the highest vein is Santa Jessica at 1,231 masl and the lowest is Robertita at 110 masl. Within the veins, the high-grade mineralized shoots generally plunge to the northeast. The distance between veins varies from.
Tayoltita Block
The Tayoltita Block, also known as the East Block, is limited to the west by the Guamuchil Fault and to the east by the Arana Fault. It covers an area of 1,800 m in the northeast–southwest direction and 3,500 m in the southeast–northwest direction. Within the block, a total of 43 veins have been identified.
The largest vein in this block was San Luis vein, which was mined out in the past and was the only vein known to have a north–south orientation. The remaining known veins are northeast–southwest oriented. The highest vein in elevation is San Luis, located at 1,900 masl; and the lowest is Vein-36, which is at 470 masl. The strike length varies from 80–1,800 m and the average thickness is 1.5 m. The distance between veins varies from.