San Jose Mine, Mexico

Overview

The San Jose Mine is located in the Taviche Mining District, Oaxaca, Mexico. The mineralization at San Jose is mainly silver with some gold, contained within epithermal veins. The ratio of silver to gold is around 100:1 so San Jose is considered to be a silver deposit as the value of silver mined surpasses the value of gold.

The San Jose mine is operated by Compania Minera Cuzcatlan S.A. de C.V., a Mexican subsidiary 100% owned by Fortuna Silver Mines Inc. Fortuna is listed on the New York Stock Exchange (FSM), Toronto Stock Exchange (FVI), Bolsa de Valores de Lima (FVI) and Frankfurt Stock Exchange (F4S.F). The company has its headquarters in Vancouver, Canada and has offices in Lima, Peru.

Regional Geology

The host rocks for the mineralisation in the San Jose area are intermediate volcanic rocks.  These lava flows and ash layers were deposited during the Paleogene period, 65 to 23.8 million years ago. Overlying these rocks are younger silica-rich volcanic ash layers and material called ignimbrite which are derived from extremely violent volcanic eruptions between 23.8-5.32 million years ago (Ma), during the Miocene period. The basement rock underlying these volcanic sequences consists of  highly metamorphosed granulite-facies rock of the Zapoteco terrane. This rock unit is very old and is believed to be of Mesoproterozoic age, which is 1.6-1.0 billion years old. To the east of the mine, the basement rock changes to Cuicateco terrane, which is a west-dipping fault-bounded mass of strongly deformed Jurassic (200-145 Ma) and Cretaceous (145-65 Ma) oceanic and arc volcanic rocks.

The volcanic rocks of the San Jose Mine area are thought to have been extruded during a period of subduction along the convergent southern Mexico plate boundary. Localized extension in the area resulted in normal faulting and rifting and the formation of pull-apart basins. During this extensional phase, hydrothermal fluids containing precious metals were injected into host structures.

Deposit Geology

The mineralisation is hosted within a low sulfidation epithermal vein system contained within a thick sequence of intermediate volcanic host rocks, ranging in composition from andesite to dacite.

The host rocks have been displaced along major N-NW striking extensional faults. This extension has resulted in the opening up of conduits and pathways within the rock where hot mineralized fluids containing silver and gold have been injected. In places, this forceful injection of fluids has locally broken up the rock and formed breccias during a number of pulses. Quartz-carbonate-sulfide veins containing silver-gold were also formed during the same episodes.

The main zone of mineralization in the area is structurally controlled and extends for more than 3 km in a N-S direction. The mineralization has been subdivided into the northern Trinidad deposit area and the southern San Ignacio area.

Discussion

The San Jose Mine is slated to close later in 2024, a victim of rising costs and depleted reserves. Only 0.7 Mt ore remain. This is representative of a long term trend in silver mining, which is increasingly produced as a byproduct of gold, copper, and other metals. As dedicated silver mines run dry.

Despite the fact that silver is more desirable than ever and prices are forecast to rise significantly, it may be too late for San Jose. While drilling has recently yielded some promising results Fortuna is still planning to close the mine, although they have said a final decision on closure won’t be made until the second half of 2024.

Further Reading

• Fortuna Press Release Reviews 
• Fortuna Silver Mines Inc. (Corporate Site)