TORONTO (miningweekly.com) – TSX-V-listed Atlanta Gold on Monday reported that two events last month caused arsenic and iron discharge from the water treatment facility at the Atlanta gold project, in Elmore County, Idaho, to exceed US Environmental Protection Agency standards.

The Toronto-based firm, which was currently busy with a test-mining programme at the project, said high spring runoff and an uncontrolled release in water flow caused by a ground collapse in the cross-cut behind the bulkhead in the 900-level adit, resulted in water cresting the bulkhead and flushing sediment from the floor of the crosscut and carrying it downstream through the prepond solids filters to the containment ponds.

Evidence of this could be seen as a 0.3 m orange line above the normal high water mark on May 17. A further surge in water raised the pond surface water over 0.61 m on Tuesday.

From the containment ponds, the water pushed over and through the filters in the water treatment facility, depositing silt throughout. The silt restricted flow through most of the filters in the system. The company said it was busy cleaning the water treatment facility.

Atlanta reported that it had ordered new filter material that would be installed within the next two weeks at a cost of about $30 000. Portions of the water treatment facility had been taken off line for cleaning, rebuilding and maintenance.

These were the first incidents where pollutants exceeded prescribed limits in six months, the company said, adding that it planned to install a plug in the crosscut that would be necessary to manage water flows and treatment.

Atlanta was required to make long-term upgrades to the water treatment system in terms of a July 19, 2012, US District Court order for the state of Idaho. The court imposed a penalty of $2-million and ordered the company to implement measures to comply with the National Pollutant Discharge Elimination System permit.

The court’s decision earlier in 2012 held that Atlanta was not in compliance with the Clean Water Act with regard to its operation of the passive water filtration system at the 900-level adit, which was located on property owned by the Bureau of Land Management and administered by the US Forest Service.

Atlanta said the arsenic and iron removal could be improved with a modification to the final filter, by installing a solids removal filter between the existing settlement ponds and the final filter, instead of increasing the pond capacity.

At the end of last month, the company reported that successful bench-scale testing had confirmed that gold occurred both as free gold and gold in association with arsenic sulphides at the project.

The programme had enabled Atlanta to draw up a process flow sheet that would separate gold mineralisation from the rock and concentrate it for shipment off site for further processing.

A process using only water and gravity produced a concentrate that, based on a production rate of 800 t/d and a feed grade of 0.075 oz/t, would result in a concentration rate of 50:1 and produce a concentrate of 15.7 t at a grade of 3.178 oz/t gold. The concentrate would contain 49.9 oz of gold, which would be equal to a recovery of 82.8% of total gold.

The next step was to apply this technology to process a larger bulk sample.

A National Instrument 43-101-compliant resource estimate completed in January 2012 had calculated an inferred resource of 785 000 gold equivalent ounces in the indicated category and 397 300 gold equivalent ounces in the inferred category, at an average grade of 3.86 g/t.

There were 752 000 oz of gold in the indicated category and 385 900 gold in the inferred category. The technical report assumed a gold price of $1 419/oz and $28.18/oz of silver.