Durango State, Mexico
Two underground Ag/Au mines & two mills (oxide & sulfide)
Silver and gold; lead and zinc byproducts
Continuing work regarding potential 2022-2023 re-start of production
Current Activities & Work
Final testing for optimum BIOX processing; then underground test mining
Current and complete
The Velardeña Properties in Durango State, Mexico contain two past-producing underground silver and gold mines and two processing mills. Mining was suspended in November 2015 when a combination of low metals prices, mining dilution and metallurgical challenges rendered operations unprofitable. We subsequently leased one of Velardeña’s two mills to Hecla Mining Company - beginning late 2015 and ending in November 2020 - and this lease arrangement provided cash flow that supported Golden's exploration activities. The oxide mill at Velardeña is currently being used to process gold-silver material from our Rodeo mine. We installed a second ball mill at Velardeña's oxide mill in April 2021 that is used in processing Rodeo material.
In recent years we have evaluated and tested various mining methods and processing alternatives that could potentially enable sustainable profitable operations at Velardeña. In late 2019, Golden announced it had achieved successful results from testing Velardeña gold concentrate material using Finnish firm Outotec’s bio-oxidation or “BIOX” process. BIOX is a unique and sustainable technology that was developed to pre-treat refractory ores and concentrates ahead of conventional cyanide leaching. The gold in these types of mineralized material, such as those found at Velardeña, is encapsulated in pyrite and arsenopyrite which prevents the gold from being successfully cyanide leached. BIOX utilizes bacteria to oxidize these sulfide materials, thereby exposing the gold for subsequent cyanide leaching and increasing overall gold recoveries. Golden Minerals believes BIOX technology is key to unlocking successful and sustainable value from production at Velardeña. Indeed, 2019 BIOX testing of Velardeña material achieved gold recoveries of 92% from pyrite concentrates, compared to sub-30% gold recoveries realized when Golden last operated Velardeña in 2015.
During 2021, we worked on optimizing Velardeña's mine plan and processing details, and conducted bulk sample test-mining and processing, all in advance of making a decision to potentially restart commercial production at Velardeña and install the bio-oxidation circuit. In June 2021, we began limited scale mining activities in the underground mines to obtain further bulk samples for use in final optimization of the bio-oxidation plant design and for use in additional flotation separation studies that will indicate how we can best separate the gold-bearing minerals into the pyrite-arsenopyrite concentrate that is proposed for processing in the bio-oxidation circuit.
As of late March 2022, we are in the final stages of testing pyrite concentrates for optimum processing using bio-oxidation technology in South Africa, and we estimate we will receive final results in April. These results will enable us to complete the detailed design of the proposed bio-oxidation plant. We are also conducting test mining underground this spring to ensure that with our chosen mining subcontractor we can control mining dilution, which is critical to the ultimate success of the project. Upon completion of test mining, we plan to make the formal decision on resumption of mining activities around mid-year. If we make the decision to re-start mining activities at Velardeña, construction of the BIOX plant is expected to take roughly one year, running into mid-year 2023. The timing of production start-up would depend in part on metals prices. It is conceivable that we could begin mining activity with a three to four-month ramp-up period and begin to sell silver-lead and zinc concentrates while stockpiling the pyrite concentrates which contain most of the gold while the BIOX facility is being completed.
In March 2022, the independent firm of Tetra Tech completed a Technical Report Summary ("TRS") for the Velardeña Properties. The purpose of the report is to summarize the results of an Initial Assessment for the property as defined under the U.S. Securities and Exchange Commission’s Regulation S-K 1300. This is the first TRS prepared for the Project under S-K 1300 guidelines.
Two economic models were prepared for the TRS: one includes Inferred Mineral Resources in the analysis (MII model), and the second excludes the Inferred material (MI model). The economic model results are based on Mineral Resources that, unlike Mineral Reserves, do not have demonstrated economic viability. Results indicate mining is potentially economically viable both with and without the Inferred Resources. The MII case has a mine life of 11 years and a pre-tax NPV of $119M with an IRR of 114%. The MI case has a mine life of 4 years and reports a pre-tax NPV of $48M with an IRR of 101%. Both cases were discounted at 8%.
For both economic analyses, reclamation costs are assumed to be canceled by salvage value and are therefore not included. The TRS assumes prices of $1,744/oz gold, $23.70/oz silver, $0.97/pound (“lb”) lead (“Pb”) and $1.15/lb zinc (“Zn”). Mineral resources were calculated having an effective date of February 28, 2022, as diluted to a minimum of 0.7 meters and are reported at a $175 NSR cutoff. A federal precious metal royalty of 0.5% is assumed.
The MII model includes Measured, Indicated, and Inferred resources. Reclamation costs are assumed to be canceled by salvage value and are therefore not included. The LOM is 11 years, with a pre-tax NPV of $119 million using a discount rate of 8%.
Under the MII model, payable production is estimated at just under 2 million silver equivalent oz/year, based on gold and silver only. (Lead and zinc production is not included in this figure.) The composition is estimated at 54% silver and 46% gold at assumed recoveries.
The MII model includes Measured, Indicated and Inferred resources. The LOM is 11 years, with a pre-tax NPV of $119 million and an IRR of 114%, using a discount rate of 8%.
Capital cost estimates - MII Model
Operating cost estimates - MII Model
MII Plan Sensitivity - A 10% increase in operating costs results in a 16% reduction in project NPV.
The MI model includes only Measured & Indicated resources. The LOM is 4 years, with a pre-tax NPV of $48 million and an IRR of 101%, using a discount rate of 8%.
Under the MI model, payable production is estimated just under 2 million silver equivalent oz/year, based on gold and silver only. (Lead and zinc production is not included in this figure.) The composition is estimated at 57% silver and 43% gold at assumed recoveries.
Economic Model Results - MI Model
Capital cost estimates - MI Model
Operating cost estimates - MI Model
The Velardeña Properties are comprised of two underground silver and gold mines (Velardeña and Chicago) and two processing plants, located within the Velardeña mining district approximately 65 kilometers southwest of the city of Torreón and 140 kilometers northeast of the city of Durango. The properties are accessed by a seven-kilometer road from the village of Velardeña, which is reached by highway from Torreón and Durango.
Power is provided through substations connected to the national grid. Water is provided by wells located in the valley adjacent to the Velardeña Properties. We hold title to three wells located near our sulfide plant and certificates of registration to three wells located near our oxide plant. We are licensed to pump water from all six wells up to a permitted amount.
The Velardeña Properties are subject to the Mexican ejido system requiring us to contract with the local communities, or ejidos, surrounding our properties to obtain surface access rights needed in connection with our mining and exploration activities. We currently have contracts with two ejidos to secure surface rights for the Velardeña Properties: one that provides surface rights to certain roads and infrastructure through 2021, and a second that provides exploration access and access rights for roads and utilities until 2038.
We hold 29 mineral concessions covering 557 hectares in the Velardeña Properties. We also own the land surface rights to 144 hectares that contain the oxide plant, tailings area and access to the Chicago mine, along with surface lands that may be required for potential plant expansions.
Exploration and mining in the Velardeña district extends back to at least the late 1500s or early 1600s, with large scale mining beginning in 1888 with the Velardeña Mining and Smelter Company. In 1902, the mining properties were acquired by ASARCO, who mined the property until 1926 when the mines were closed. For the next 35 years, the mines were operated from time to time by small companies and local miners. The property was nationalized in 1961, and in 1968 the sulfide processing plant was built by the Mexican government. In 1994, William Resources acquired the concessions comprising the Velardeña Properties. In 1997, ECU Gold (the predecessor to ECU Silver Mining Inc.) purchased from William Resources the subsidiaries that owned the concessions and the oxide processing plant. The sulfide processing plant was acquired in 2004.
Golden Minerals Company mined oxide and mixed oxide/sulfide material from the Velardeña and Chicago mines during the years 2011 to 2013. Between November 2014 and November 2015, we mined from the Velardeña mine’s San Mateo, Terneras, and Santa Juana vein systems, as well as from the Chicago mine. We suspended mining activities in November 2015 when a combination of low metals prices, dilution and metallurgical challenges resulted in continued operating losses. We placed the mine and sulfide mill on care and maintenance to enable a re-start when mining and processing plans and metals prices indicate mining activities may be conducted profitably, or until we are able to locate and develop alternative mineral sources that may be economically mined and transported to the Velardeña Properties for processing.
We continued to search for and evaluate other oxide and sulfide feed sources, focusing on sources within haulage distance of our sulfide and oxide mills at Velardeña. Additionally, as noted above we have evaluated and tested various mining methods and processing alternatives that could result in sustained profitable operations. In 2020 we released results of an updated PEA that incorporated refinements to the resource model and a bio-oxidation processing methodology designed to greatly improve the recovery of gold from pyrites and arsenopyrites.
We own a 300 tonne per day flotation mill situated near the town of Velardeña, which accounted for 100% of our revenue from saleable metals during 2014 and 2015. The mill includes three flotation circuits in which we can process the sulfide material to make lead, zinc and pyrite concentrates. Most of the silver and gold sold in 2014 and 2015 was contained in the lead concentrate. During 2015 we processed all our mined material through the sulfide plant.
We also own a conventional 550 tonne per day cyanide leach mill with a Merrill-Crowe precipitation circuit which is located adjacent to our Chicago mine. We previously used this mill to process oxide and mixed sulfide/oxide material from the Velardeña Properties. Subsequently, the mill was leased to Hecla Mining Co between late 2015 and November 2020. The mill is presently being used to process Rodeo's gold-silver material (as of January 2021).
For additional details see the Technical Reports page
The precious metal deposits in the Velardeña and Chicago mines are narrow epithermal to mesothermal quartz-calcite to quartz-sulfide veins that follow three distinct trends. In the Velardeña mine east-west striking steeply north-dipping veins include the Terneras and San Mateo vein groups and the north-west striking, steeply northeast dipping Santa Juana veins. In the Chicago mine, the veins strike north-east and dip steeply to the southeast. Most veins carry recoverable lead and zinc values in galena and sphalerite as well as silver either in galena or in freibergite and gold hosted in pyrite and arsenopyrite.
The Velardeña district is located at the boundary between the Sierra Madre Oriental to the east and the Mesa Central sub-province to the west. The regional geology is characterized by a thick sequence of limestone and calcareous clastic sediments of Cretaceous age, intruded by Tertiary plutons. During the Laramide Orogeny, the sediments were folded into symmetrical anticlines and synclines that were modified into a series of asymmetrical overturned folds by a later stage of compression.
A series of Tertiary stocks have intruded the Cretaceous limestone over a distance of approximately 15 kilometers along a northeast to southwest trend. The various mineral deposits of the Velardeña mining district occur along the northeast southwest axis and are spatially associated with the intrusions and their related alteration.
Several types of Tertiary intrusive rocks are present in the Velardeña district. The largest of these intrusions outcrops on the western flank of the Sierra San Lorenzo and underlies a portion of the Velardeña Properties. It forms a northeast oriented, elongated body of diorite composition that outcrops over a distance of about 2.5 kilometers. The intruded limestone has been altered by contact metamorphism (exoskarn), and locally the intrusive has been metamorphosed (endoskarn).
The Santa Juana, Terneras, and San Mateo vein groups on the Velardeña property are hosted by Cretaceous Aurora Formation limestone, a diorite intrusion and related skarn.
Two main vein systems are present in the Velardeña mine. The first is the northwest striking, steeply north-dipping Santa Juana group of veins which include over 20 individually named veins, the most important of which are the A1, A4, CC, C1, and Santa Juana veins. The second vein system is east-west trending and steeply north-dipping and is represented by the Terneras and San Mateo vein groups. The Terneras group of veins includes the Roca Negra, Terneras Norte, Terneras Sur, Hiletas, and San Juanes veins. The San Mateo group includes San Mateo Oeste and San Mateo Este.
The most extensive of these is the Terneras vein, which was mined in the past over a strike length of 1,100 meters. All of these veins are observed to have extensive strike lengths and vertical continuity for hundreds of meters. The mineralogy of the east west system is somewhat different in that it contains less arsenic than the Santa Juana veins.
Vein widths in minable portions of the vein systems vary from 0.2 meters to 4 meters but average widths for most veins is 0.3 to 0.5 meters. Vein widths generally decrease where veins cut skarn.
On the Chicago property, the oldest rocks outcropping are Cretaceous limestone of the Aurora Formation which are highly folded. This limestone is locally metamorphosed by the intrusion of the Tertiary dioritic stocks and dykes. The general geology of the Chicago property is very similar to the geology of the Velardeña property. The Chicago veins strike northeast and dip steeply southeast. The main veins of interest at Chicago are the Chicago and Escondida veins. Chicago veins tend to be higher in lead and zinc than the Santa Juana or Terneras veins. Vein widths at Chicago are variable and tend to be narrower than at the Santa Juana deposit, especially in the skarn host.
Velardeña 2014 Drilling Results
Tetra Tech, Velardeña Project, Technical Report Summary. March 2022. Prepared under SEC S-K 1300 guidelines.
Tetra Tech, Preliminary Economic Assessment & NI 43-101 Technical Report, May 8, 2020. Prepared under Canadian NI 43-101 guidelines.
Tetra Tech, NI 43-101 Technical Report - Preliminary Economic Assessment, Feb. 20, 2015. Prepared under Canadian NI 43-101 guidelines.