The Bolañitos Silver-Gold Mine is situated in Mexico. (Credit: Ashley Dace/
A truck at work on the site. (Credit: omid roshan on Unsplash)
Endeavour Silver acquired the project in 2007. (Credit: James St. John/Flickr)

Bolañitos is an underground gold and silver mine located in the state of Guanajuato in Central Mexico.

The project was acquired by mid-tier precious metals mining company Endeavour Silver in 2007. The company maintained low level of production at the project until it discovered the first new orebody at Lucero in 2008.

Endeavour Silver eventually discovered ten new orebodies, of which eight have been mined. The current production is from three orebodies at Bolañitos- La Luz, Lucero, and San Miguel.

A technical report with updated mineral resource and reserve estimates for the Bolañitos Project was reported in November 2022.

In 2022, the mine produced 622,892 ounces of silver and 21,813 ounces of gold. In 2023, plant throughput at Bolañitos is expected to be between 1,150 tonnes per day (tpd) and 1,250 tpd.

Location and Site details

The Bolañitos Project is located 12km northwest of the Guanajuato city in Central Mexico.

The project consists of three operating mines- the Bolañitos, Asuncion, and Lucero mines. These mines are located near the La Luz Town, about 12km of the northeast of Guanajuato.

The Bolañitos and other mines are accessible by paved and gravel roads.

Bolañitos Project History

In 1968, Fresnillo Company acquired claims and then incorporated Peregrina Mine and Negociación Minera Santa Lucía (now Cebada).

A photogeological study in the Cebada Mine holdings was completed by contracting company Tormex in 1973.

Production at the Cebada Mine started in 1976.

In 2003, the Torres, Peregrina, Sirena, and Apolo mines were closed by Grupo Guanajuato while Bolañitos, Golondrinas, Cebada, and Asunción mines continued production.

Endeavour acquired the Bolañitos Project in 2007. The acquisition included Mina Cebada, Mina Bolañitos, Mina Golondrinas, and Mina Asunción and few other closed mines.

Geology and Mineralisation

The Bolañitos Mine is located in the eastern part of Guanajuato mining district.

The district features classic, high-grade silver-gold, epithermal vein deposits with low sulfidation mineralisation and adularia-sericite alteration.

Guanajuato has three major mineralised fault systems- the La Luz, Veta Madre, and Sierra.

Only the Esperanza and La Luz formations occur in the Bolañitos mine area with mineralisation occurring primarily in the La Luz formation.

The mineralisation at Bolañitos is directly related to faulting and occurs as open space fillings in fracture zones in locally porous wall rock. The mining primarily targets veins formed in relatively open spaces.

The mineralised veins consist of the classic banded and brecciated epithermal variety.

Silver is found in dark sulphide rich-bands within the veins with little mineralisation within the wall rocks.

The major metallic minerals reported are pyrite, electrum, argentite, ruby silver, some galena and sphalerite deeper in the veins generally.

The mineralisation is associated with phyllic (sericite) and silicification alteration forming haloes around the mineralising structures.

Overall, the mineralisation style is pinch-and-swell with some flexures resulting in closures and others generating wide sigmoidal breccia zones.

Mineral reserves

As of May 2022, Endeavour reported total measured and indicated mineral resource estimate in the project is 453.5 kilo tonnes (kt) containing 4,132 thousand oz of silver equivalent (AgEq) at a grading of 283g/t, 1,601 thousand oz of silver (Ag) at a grading of 111g/t, and 30 thousand oz of gold (Au) at a grading of 2.3g/t.

The total (proven and probable) mineral reserves stand at 534kt containing 4,556 thousand oz of AgEq at a grading of 326g/t, 1,168oz of Ag at a grading of 101g/t, and 42.6oz of Au at a grading of 2.8g/t.

Mining and Ore processing at Bolañitos

At Bolañitos, conventional drill and blast methods are used for extracting ore, with ramps and audits to access mining areas.

A conventional bottom-up cut and fill mining method was used while waste rock was brought in using diesel loaders.

According to the technical report of 2022, the mine has gradually transitioned to a long hole method over the last eight years.

Cut and fill mining involved upholes usually less than 2m, while the hole varies from 6m to 16m in long hole mining.

Stope production can start once sill development is completed and ore limits have been defined.

The processing of ore is done in Bolañitos plant.

The Run-of-mine ore is hauled by 10 and 20 tonnes trucks and a hydraulic hammer is used for breaking oversize rock greater than 14-inches.

The undersize material is fed to a primary jaw crusher, following which it is stored in one of two coarse ore bins.

The ore is then transferred to a vibratory screen and the undersized product is transferred to the fine ore bins.

The oversized material is fed to a 4.25-inch standard head Symons secondary cone crusher for additional crushing.

The secondary crusher product is screened by a vibratory screen and sent then to fine ore bins. The oversized product is transferred to tertiary cone crusher for crushing.

Finely crushed ore is stored in two ore bins with 250tonne and 350tonne storage capacities.

The grinding circuit consists of two ball mills and are fed independently from respective ore bins.

From grinding, the pulp passes to the flotation circuit.

The final concentrate flows by gravity to a thickener to produce 60% solids before being pumped the filter press for dewatering to 13-17% moisture.

The filtered concentrate is then stored, loaded to trucks, and then shipped to concentrate traders.

Contractors involved

Hard Rock Consulting was associated with the preparation of the technical report ‘Updated Mineral Resource and Reserve Estimates for the Bolañitos Project’, effective November 2022.

Campos Hernandez Contratistas Mineros was selected as the mining contractor of the Bolañitos Project.

SETRAMEX Transportes was the freight concentrate contractor for the project.

The contract of security and surveillance services was awarded to ‘SERVIAX’ Alineacion Estratégica de Capital Humano y Seguridad Privada.

Isabel Camarillo and Jose Vicente Morales Zarate won the ore haulage contract, while J. Guadalupe Huerta Ortega was selected as the water haulage contractor, and road watering and water for plant.