Published on July 22, 2018


Midas Gold Idaho wants to keep the community informed about the work we are doing at the Stibnite Gold Project site. The “Ask Midas” blog series gives the experts in our company a chance to answer some of the community’s most frequently asked questions and help clear up any misconceptions around the project.

As CEO and President of Midas Gold Idaho, I hear a lot of different questions in the community about the work we are doing. This week, I want to address a question I was asked around acid rock drainage.

Is acid rock drainage a concern at Stibnite?

The Stibnite area has been mined off and on for almost 100 years and, throughout that time, acid rock drainage (also known as acid mine drainage) has never occurred. Simply put, the geochemical composition of the substantial majority of rocks at site, including those that we propose to mine, make acid rock drainage improbable. It is not a concern for our project. While the rocks at site have low levels of sulfides, which can generate acid, they also have significant quantities of carbonate and other minerals that neutralize any acid generation potential.[i]

We understand why people ask about this issue. Acid rock drainage is a naturally occurring process of sulfide minerals oxidizing, combining with water and leaching metals and sulfuric acid. This process has been the cause of a major water quality issues at a number of former mine sites across the western United States. While acid rock drainage is not a concern for our project, we are concerned about ongoing metal leaching up at site, which can occur under neutral pH conditions.[ii]We believe the existing water quality issues at site stem from a combination of natural leaching of untouched areas and leaching of decades old waste and tailings, the latter stored in unlined facilities.

Currently, there are elevated levels of arsenic and antimony in the ground and surface water around our site. This is caused by metals leaching from undisturbed bedrock and the millions of tons of tailings and spent ore that were left behind from past mining projects without being properly stored.[iii]Unless these materials are properly handled and stored, water quality issues will persist. As part of our proposed Stibnite Gold Project, we have a plan to pick up spent ore, reprocess the unlined tailings to remove metals and safely store them in engineered facilities.

Midas Gold has gone to great lengths to understand the water quality issues up at site. Starting in 2011, we hired SRK Consulting to conduct a geochemical characterization program for our project site. The study was designed to determine the environmental characteristics of different rock types and mining products. This information helps predict future water quality, specifically the potential for metal leaching, and allows us to make the appropriate plans to mitigate that risk. We want to make sure we have as much information around water quality as possible, so the geochemical characterization program is still ongoing, eight years after it first started.[iv]

Under this program, we collected representative samples of rock, historical mine wastes and future mine tailings materials. In total, we collected more than 50,000 samples, including 400 drill core samples, from across the site to provide an accurate representation of the rocks’ geochemical behavior. These samples were tested following industry standard static and kinetic geochemical testing methods as approved by the United States Forest Service (USFS). Static testing is designed to characterize the chemistry of samples and determine their overall potential to generate or neutralize acid. Kinetic testing occurs over multiple months and is designed to accelerate natural weathering processes to determine any potential rates of acid production and metal release from samples. The results of these laboratory tests are used to assess the potential for metal leaching and acid rock drainage.[v]

Based on the geochemical testing completed over multiple years, SRK Consulting has concluded that development rock associated with the project has a negligible to moderate potential to generate acid and limited potential to leach metals and metalloids from freshly mined rock, with the exception being arsenic, antimony, aluminum, manganese and sulfate. With this knowledge in hand, we are working with regulatory agencies through the NEPA process to revise the development rock storage facility designs to mitigate any potential for metal leaching and degradation of water quality.[vi]

For those that are interested, here is a summary of our geochemical characterization program.

As we go through each step of the testing process, our results must be verified by the USFS. They check the data to ensure the reports we deliver to them line up with our science. This is an ongoing process of improvement, so we design the best project possible using the latest scientific findings.

If you have a question you would like us to answer, please email it to [email protected].


[i]Carbonate minerals in the Stibnite Gold Project mineral deposits occur within certain carbonate rock formations (limestone and dolomitic marbles) in the metasedimentary hosted West End deposit, and also within hydrothermally altered igneous intrusive rocks as calcite, dolomite and ankerite vein fill within the intrusive-hosted Yellow Pine and Hangar Flats deposits.

[ii]Some metals and metalloids are more soluble under acidic conditions such as zinc, aluminum, iron copper and manganese while others, including antimony, arsenic, cadmium, and molybdenum, can remain dissolved in water at high concentrations under circum-neutral pH conditions. The terms neutral metal leaching, neutral mine drainage and mining-influenced waters are used synonymously to distinguish these conditions from acid rock drainage or acid mine drainage.

[iii]Elevated metal concentrations in surface and groundwater at Stibnite can be attributed to both natural and mining related sources. While the legacy tailings and spent ore are well known to contribute metals to surface water, elevated arsenic and antimony also occur in areas of known bedrock mineralization which are unimpacted by prior mining activities, leaching out through seeps and springs all over the site.

[iv]In addition to predictive testing and modeling, Midas Gold initiated baseline surface and groundwater quality monitoring programs in 2011 to fully understand water quality issues on the project site. Baseline data collection programs consist of quarterly water quality monitoring at over 60 surface sites and groundwater wells and are currently ongoing 8 years later.

[v]The geochemical kinetic test program entailed humidity cell testing (HCT) in which rock samples are exposed to alternating cycles of dry and moist air with deionized water flushing to simulate geological weathering processes at the laboratory scale. The baseline characterization program included 17 HCTs which were run for 144 to 168 weeks at McClelland Labs, much longer than a standard 40-week test.

[vi]Midas Gold is investigating the use of impermeable covers on some facilities to reduce infiltration of rainfall and snowmelt through the facilities as well as underdrain design options to reduce contact with groundwater.


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