VANCOUVER — The latest drill results from Viscount Mining’s (TSXV: VML; US-OTC: VLMGF) Silver Cliff project are casting a new light on the historic Hardscrabble silver-lead-zinc mining district, 64 km southwest of the city of Pueblo, Colorado.
The prospect generator reported a spread of intercepts between 6.1 metres of 53.8 grams silver per tonne to 6.1 metres of 1,778.5 grams silver from nine twin drill holes targeting the project’s Kate deposit, a flat-lying sheet of supergene silver-manganese mineralization near surface.
Other top intercepts include 18.3 metres of 203.4 grams silver, 13.7 metres of 390.9 grams silver and 17.7 metres of 265.9 metres.
“There aren’t a lot of silver projects in the U.S. with assays like the ones we are getting at Silver Cliff,” president and CEO of Viscount, Jim Mackenzie, tells The Northern Miner over coffee in downtown Vancouver. “Now that we’ve proved there’s still silver in the ground, the next step is to bring the historical resource into compliance and continue exploration on the property.”
Most information about Silver Cliff’s exploration and mining history is hard to find, being either lost or buried in government and university archives. Mackenzie says the company has dug out some remnants of data, including a preliminary economic study for Silver Cliff completed by Tenneco (NYSE: TEN) in the 1980s.
Mackenzie says Tenneco and Hecla were about to spend US$30 million to build a mill and put Silver Cliff in production, but then Tenneco was bought out by a Belgian conglomerate that had little interest in the resource sector, and the project was dropped.
Corazon Gold (TSXV: CGW), a junior explorer that worked briefly on Silver Cliff in 2013 tabulated that the Kate deposit had proven and probable reserves (not compliant with National Instrument 43-101 standards) of 3.8 million tonnes grading 91.9 grams silver for 11.1 million oz. silver, using a 34.3-gram silver cut-off.
Between Kate and two other deposits at Silver Cliff, total resources historically reported were 50 million oz. silver, Mackenzie says, with drill intercept grades ranging from zero to a high of 2,125 grams silver over 13.4 metres, whereas gold ranged from zero to a high of 9.1 grams gold per tonne over 1.2 metres.
“When we acquired the project three years ago what attracted us was the historical resource and the high grades,” Mackenzie says. “The mineralization occurs between 15 and 53 metres below surface, so it’s attractive from a mining standpoint, and there’s potential to find stacked horizons beneath the deposits, or potentially high-grade epithermal veins. It just needs to be explored with more modern techniques and drilled more extensively.”
The first wave of exploration hit the region in 1870, when explorers discovered silver in rocks surrounding an ancient volcano that has been extinct for 26 million years.
Mineralization crept its way into the crust during breaks in volcanism, driven by heat and pressures from the underlying magma chambers, according to documents in archives kept by the Colorado Geological Survey.
Hydrothermal fluids deposited silver and manganese in vugs and open spaces within carbonate reefs, whereas other fluids deposited silver and base metals within faults, fissures and other cracks in the rocks.
During the later stages of volcanism, an explosive eruption blasted through the overlying rock, and the fluids deposited metals in chimneys of rubble. Eventually the upper elevations of the volcano collapsed, creating a caldera, and bodies of magma beneath slowly solidified to become large, granitic plutons.
Both Silver Cliff and the adjacent Rosita Hills district were recognized for their shallow mineralization, and many small mines were established. Mining reached its peak in 1940, with 10 producing mines in the Silver Cliff district, and four producing mines in the Rosita Hills district. As a whole, they produced 4.7 million oz. silver, 41.9 million lb. lead, 1.9 million lb. zinc and 627,000 lb. copper.
Most of the historical silver production at Silver Cliff and Rosita Hills came from fault-hosted veins, replacement zones and breccia pipes — the latter reported as having grades up to 24,000 grams silver at their core, surrounded by a lower-grade halo.
In the Kate deposit area, metallurgical test work by Tenneco showed that the manganese-rich ores posed a challenge for recoveries.
When Corazon was evaluating the property in 2013, it reported that Tenneco’s cyanide extraction test work only achieved 45–50% silver recovery.
Corazon collected a 20-kilogram surface sample for more testing, with assays of the sample returning 240 grams silver with 10.9% manganese. But the company dropped the property — and a number of other projects — a couple of months later during a phase of corporate restructuring, and the results of the bulk sample were never released.
“The recoveries weren’t that big of problem for Tenneco because they were going to put the project into production,” Mackenzie says. “And that was back in the 1980s, so a lot has changed since then, and there are new techniques to address that. But we’ll make metallurgical tests part and parcel for the upcoming program, as we investigate the project further.”
Viscount is also advancing its Cherry Creek silver-gold-tungsten property, 48 km north of the city of Ely in White Pine County, Nev., with partner Sumitomo.
The Japanese giant signed on in February last year, agreeing to spend US$10 million on the project over eight years to earn 75% interest, in addition to producing a bankable feasibility study at an estimated cost between US$30 million and US$40 million.
“We’re fortunate to have partnered with a company like Sumitomo,” Mackenzie says. “We’re their only exploration play in North America, and that speaks volumes about the potential of Cherry Creek.”
The property hosts 20 historic mines that produced silver, gold and tungsten between 1872 and 1940, and primarily exploited mineralization entombed in veins.
But it’s the potential for Carlin-style gold deposits that caught Sumitomo’s interest, Mackenzie says.
“In 2015 we brought in geologists from Snowden to complete work on the area, and a number of them who used to work at Newmont’s Long Canyon discovered the same type of geology at the Flint Canyon prospect on our property,” he says.
The partners began a 7,300-metre Flint Canyon drill program in September, targeting a prospective contact between highly dissected fault blocks of shale and underlying limestone units. In earlier work, the companies found weakly mineralized jasperoid outcrops across the prospect, which are typical vectors to Carlin-style mineralization.
Assay results from the recent drill program are pending.
“The Cherry Creek property has never been explored properly because of the fractured land ownership,” Mackenzie says. “We’re the first company to consolidate the ground and apply modern exploration techniques, so we look forward to seeing the results as we advance this project forward.”