Summary:
The silver-gold deposits at the San Jose Mine are typical of low-sulfidation epithermal deposits according to the classification of Corbett (2002), having formed in a relatively low temperature, shallow crustal environment.
Precious metal mineralization at the San Jose Mine is hosted by hydrothermal breccias, crackle breccias, quartz-carbonate veins and zones of sheeted and stockwork-like quartz-carbonate veins emplaced along steeply dipping north and north-northwest trending fault structures.
The mineralized structural corridor extends for greater than 3 km in a north–south direction and has been divided into two sectors. The Trinidad Deposit area located between 1846500N and 1847800N, and the San Ignacio area located between 1845000N and 1846500N. The Victoria mineralized zone is located approximately 350 m to the east of the Trinidad Deposit.
The major mineralized structures or vein systems recognized in the Trinidad Deposit area are the Trinidad and Bonanza structures and the Stockwork zones. In addition to the major mineralized structures, secondary vein systems are present between the Trinidad and Bonanza systems and locally in the hanging wall to the Bonanza system and also in the footwall to the Trinidad system. To-date, drilling has defined the Trinidad and Bonanza mineralized structures over a strike length of approximately 1,300 m and to depths exceeding 600 m from the surface, with average thicknesses of the veins ranging from 1.5 m up to 50 m in some areas of the main Stockwork Zone.
Trinidad vein system
The Trinidad vein system (Tv) is emplaced in the footwall fault zone of the extensional system hosting the mineralized vein systems at the San Jose Mine. The Trinidad vein system strikes 355 degrees and dips 70 to 80 degrees to the east–northeast. The vein system ranges from less than one meter to locally over 15 m in true width, with higher grade mineralization generally being present in zones with greater widths. Significant portions of the Trinidad structure are characterized by late, black matrix, silicified fault breccias with only trace to weak mineralization. Higher-grade precious metal zones in the Trinidad vein system vary from 200 g/t to as much as 1,300 g/t Ag Eq across the width of the vein. Combined copper, lead and zinc values are generally less than one percent but locally higher concentrations are present. At approximately 1,100 masl in the central portion of the Trinidad Deposit, four drill holes intercepted higher-grade base metal mineralization. Fault gouge seams are commonplace at the footwall and hanging wall of the Trinidad vein system. The Trinidad hanging wall splays and the Trinidad footwall veins are considered to be part of the Trinidad mineralized structure.
Since late 2017 it has been observed that fluorine levels are not consistent throughout the Trinidad vein, with levels varying from 500 ppm in the central and lower portions of the vein system to above 5,000 ppm and even 10,000 ppm in certain areas in the north. High fluorine concentrations are generally, but not always, related to low silver–gold mineralization, and are thought to be related to a late stage of mineralization. Stopes where the highest fluorine levels have been encountered include J, R, S, G1 and the northern part of H1, located between elevations 1000 to 1300 masl.
Bonanza vein system
The Bonanza vein system (Bv) is emplaced in the hanging wall zone of the structural corridor hosting the mineralized vein systems in the Trinidad Deposit. The Bonanza vein system generally strikes 350 degrees and dips steeply to sub-vertical to the east. The Paloma vein (Pv) is considered to be part of the Bonanza vein system. Mineralization within the Bonanza vein system is present in the form of shoots plunging shallowly to moderately to the north-northwest, reflecting the dominant dip-slip movement of the controlling fault structures. Combined copper, lead and zinc values for the Bonanza vein range from negligible in the upper portions of the vein system to approximately 0.1 to 0.5 % at depth.
Stockwork Zone
The main Stockwork Zone (Swk) is located between 1846550N to 1847200N and 1,000 masl to 1,300 masl, being situated in an extensional environment between the principal Bonanza and Trinidad structures. The main Stockwork Zone is present over 650 horizontal meters and 300 vertical meters being elliptical in shape, with a variable thickness ranging to greater than 50 m.
The primary silver-bearing mineral in the main Stockwork Zone is acanthite, usually in association with traces of pyrite. Secondary minerals accompanying the acanthite are silver-rich electrum, fine-grained galena, sphalerite, chalcopyrite and gangue minerals including hyaline quartz, white quartz, and calcite, together with minor concentrations of adularia and fluorite.
In addition to the main Stockwork Zone, exploration has identified the Stockwork 2 (Swk2) and Stockwork 3 (Swk3) zones, located in the north of the Trinidad Deposit area between the Trinidad and Bonanza veins. Definition drilling has demonstrated that the Stockwork 2 and Stockwork 3 zones are similar to the main Stockwork Zone and appear to be interconnected.
Fortuna vein system
The Fortuna vein (Fv) strikes north–south and, in contrast to the other major veins in the Trinidad Deposit, dips steeply to the west. The Fortuna vein has been extensively mined on levels 2, 3 and 4 of the historic mine workings with vein widths ranging from approximately 2 to 5 m.
Other Trinidad vein systems
A number of other veins have been intersected by exploration and definition drilling, as well as contributed to production. These include the Bonanza Hangingwall (Bhws), Trinidad Footwall (Tfw), Trinidad Footwall 2 (Tfw2), Trinidad Footwall 3 (Tfw3), Trinidad Hanging Wall (Thws4), and Paloma veins.
Victoria mineralized zone
The Victoria mineralized zone is located approximately 350 m east of the Trinidad vein and north of the current underground operations of the San Jose Mine. It is structurally related to the same extensional regime that dominates the Trinidad Deposit with a similar style of mineralization, corresponding to a low sulfidation epithermal deposit formed in a shallow crustal environment with a relatively low temperature resulting in the precipitation of silver and gold mineralization. Formation temperatures are believed to be on average less than 250°C with salinities less than 1.8 %wt NaCl. Mineralization is hosted in breccias and quartz–carbonate veinlets with a general northwesterly direction and an approximate dip of 65 degrees to the northeast. The dominant alteration within the structural system is argillic, grading to propylitic towards the periphery. The hosting lithology is related to effusive volcanoclastic facies and flows of andesitic/dacitic rocks possibly of Paleogene age.