After more than 13 years of exploration in the Chilkat Valley, Constantine issued a press release this week with highlights from its preliminary economic assessment, the first step towards establishing feasibility for a potential mine.

The company is required to provide the report to shareholders to inform their investment decisions. The report predicts a mine life of 11 years. It would generate an estimated 10,000 metric tonnes of primarily zinc, copper and barite, worth $266 million after taxes, based on today’s prices.

Conceptual mine plans include no permanent surface storage of toxic waste rock, a dry stack tailings method, and about a 260-person local and non-local workforce including contractors for trucking, barging and crew transportation.

Constantine’s vice president of external affairs, Liz Cornejo, said that the report addresses major concerns of community members. “We know that the key concerns are water quality and acid rock drainage and we knew we had the potential to put this plan together, but it took several years and several million dollars in studies to compile like this,” she said.

Cornejo explained that the mining process will circulate all material through four sorting cells to extract saleable metals and dangerous minerals.

In the first two floatation cycles, copper and zinc concentrates will be separated for sale. In the third cell, pyrite sulfide will be extracted into a concentrate and sent in a cement flurry through a piping system back underground.

The fourth cell will pull off barite for sale, a mineral used for oil drilling listed by the United States Geological Survey as one of 23 critical minerals essential to economic and national security.

The leftover tailings are classified as non-potentially acid-generating, the report states. Seventy eight percent of tailings will be used as backfill, or placed back underground. The remaining tailings that won’t fit underground will be desulfurized and stored in what’s known as a dry-stack tailings method, where water is removed and the waste is stored in a compacted structure.

Geochemist and consultant for Greens Creek mine in Juneau, Peter Condon, said that dry stack mining has “significant advantages” over tailings dams.

Kennecott Greens Creek Mining Company has operated an underground silver and gold mine utilizing the dry stack method since 1989. Last year, it produced eight million ounces of silver and about 500,000 ounces of gold. As of 2017, the company employed about 430 people, according to the Alaska Department of Environmental Conservation.

Fifty percent of their tailings are used as backfill, and the remaining are dry stacked.

Cornejo said Greens Creek mine is similar, but the production rate in the Chilkat Valley would be slightly higher, with a slightly smaller workforce.

The dry stack method typically has a smaller footprint than a traditional dam, and provides more stability against toxic breech into nearby waterways, Condon said.

“Despite higher operational costs relative to slurry-tailings disposal, the decision to produce dewatered tailings provided economic and environmental advantages,” Condon wrote in a geochemical report of Greens Creek mine. “It reduced the ultimate footprint of the facility, lowered closure and water treatment costs, improved pile stability and reduced environmental liability by allowing half of the tailings to be returned underground for use as structural backfill.”

Some potentially acid generating waste rock uncovered will be temporarily stored at the surface in separate lined facilities until space underground is available, the report said.

Local environmentalists and fisherman have critiqued Constantine’s wastewater management plan when they applied for state permits last month for not accounting for environmental conditions that could cause release of toxic materials into downstream salmon spawning grounds.

Despite Constantine’s preliminary plan for what geochemists call a more environmentally friendly methods of mining, there is no certainty that a potential mine purchaser would follow its plans. “There is no certainty that PEA (preliminary economic assessment) results will be realized,” the report says.

It’s hard to comment on the current plan without a full technical report detailing operations, said Dave Chambers, president of the Center for Science in Public Participation’s –a group that provides technical assistance on mining and water quality.

Constantine will release a full technical report within 45 days.