Is it true that metagreywackes do not contain arsenic?
The gold is contained within quartz veins that are interlayed with slates and metagreywackes. Both these rock types may contain a mineral called Arsenopyrite, which is an arsenic sulphide. The significant of this is that the arsenic has a high probability of leaching out of these piles and in the tailings ponds.
I know from personal experience that slates and metagreywackes do contain arsenopyrite and in fact in some cases the arsenopyrite is auriferous, i.e. it contains gold. However, in order to answer this question with respect to the Cochrane Hill location, we sought to find descriptions of drill core from past drilling activities at the Cochrane Hill Gold Mine. An August 2014 report entitled “Technical Report of the Cochrane Hill Gold Project, Nova Scotia. Prepared for Spur Ventures Inc by FSSI Consultants (Aust) Pty Ltd by: Neil Schofield MS Applied Earth Sciences.“ was reviewed. (Unfortunately this report is no longer available on line.)
Section 7.4.2 of the report: Type and Character of Cochrane Hill Mineralisation notes that “The gold mineralisation in the Cochrane Hill Deposit occurs both within the quartz veins and within the biotite schist (after argillite) and metagreywacke host rock, in association with sulphides, particularly arsenopyrite, pyrite and pyrrhotite but also with traces of base metal sulphides particularly galena and sphalerite. The arsenopyrite occurs as porphyroblasts to 1cm diameter and more and as thin lenses or stringers of sulphide aggregates with pyrite and pyrrhotite, ….“. Simply, a porphyroblast is a large mineral crystal in a rock which has grown within the finer grained portion of the rock. So the answer to the question is yes arsenic containing minerals have been identified at the Cochrane Hill Gold Mine location. This has been recently confirmed by Atlantic Gold Feb. 6, 2019 in a drilling update “Gold mineralization, including frequently visible gold specks, is associated with pyrrhotite and arsenopyrite. Arsenopyrite will be in mine tailings and any rock pile that remains on the ground, be this the lower grade ore pile or the mine waste pile.
One of the largest hazards often found in gold mining tailings is acid mine drainage. Due to inefficiencies in the extraction process, large volumes of sulphides often end up going into the tailings. Over time, the sulphides start to oxidize as they’re exposed to wind, snow and temperature changes, creating sulphuric acid. The sulphuric acid mixes with rainwater and eventually works its way out of the tailings and into waterways, where it can change the pH of the water and precipitate harmful heavy metals.
To prevent this, mining operations use a variety of storage methods to contain toxic tailings, and focus on treating water for contamination, though this process can be costly and resource intensive. “You can treat the water and you can treat the discharge, but you’ll be doing that for 100 years and that’s not very effective,” said Ross Orr, CEO of BacTech Environmental Corporation. Further discussion on mine tailings management will be continued in an upcoming tailings pond blog …stay tuned.
A request has been submitted via e-mail to Atlantic Gold to review the tailings pond and water management system at the Moose River Touquoy Mine location. This will allow us to speak more realistically of what may be expected at the Cochrane Hill location.
As a quick aside, the above linked Scofield report, is worth reading as it contains information up to 2014, on the history of the mineral claim ownership over the years, royalties owing to third parties and it refers to a Memorandum of Understanding (MOU) with the Assembly of Nova Scotia Mi’kmaq Chiefs. “The MOU contemplates the negotiation and conclusion of a Mutual Benefits Agreement between the parties to engage further and specifically in terms of employment, training, provision of services and other opportunities and undertakings to the benefit of both parties“. Whether an agreement was signed or not it it provides an example of what a Community Benefit Agreement could contain between Atlantic Mining NS Corp. and the Municipality of the District of St. Mary’s(MODSM). MODSM will release the results of their commissioned report on the Economic Analysis proposed Cochrane Hill Gold Mine.
Is there was a process that can treat the rock to decrease this potential hazard?
You can spread calcium carbonate (such as lime) on the surfaces of rock piles, roads, etc to keep the pH up, preferably above 5. This will prevent bacteria from munching on the pyritic metals, causing acidity which then elevates the dissolved metals in the water. They become very active when the pH is less than 4.2.
This process of liming needs of course to be monitored over the long term to ensure the pH remains higher, much like adding fertilizer to your lawn. This was/is the process used to combat acid rain effects in rivers and streams.Spreading lime on exposed ground surface is relatively easy. What is more difficult to prevent is ARD coming into the Open Pit via groundwater flowing through the blast halo zone. As mentioned in my presentation, this is anticipated to start in about 14 years at Beaverdam.
It’s not easy to spread lime in the forested area above the blast halo. Therefore, ARD has to be treated as it enters the Open Pit or outflow from the pit after it fills up. One method to treat the water as it enters the pit is to place limestone around the base of the pit to add carbonate. Again, how long this will delay ARD entering the pit is very difficult to predict.
Treating water with lime as it leaves the pit is difficult. One needs a lime treatment plant operated continuously like any other water treatment plant. When this was done at the HFX Airport back in the mid 1980s, what I observed was metal laiden lime sludge settling in the bottom of the receiving stream. Every time there was a huge rainfall, the increased water flow in the stream stirred up the sediment, causing the sediment to resolubilize, releasing metals back into the stream again. This defeated the purpose of liming the effluent from the site. The outcome, fish kills in downstream lakes.
Answer courtesy of Peter Lund.