ENVIRONMENTAL AND HEALTH CONSEQUENCES ASSOCIATED WITH THE WEATHERING OF ARSENOPYRITE FROM OROGENIC GOLD DEPOSITS

Although orogenic gold mine tailings are typically low in sulfide, they generally contain significant concentrations of arsenopyrite. The fate of arsenic from weathered tailings and consequent risks to human and ecosystem health are sensitive to the primary sulfide-carbonate ratio. Geochemical and mineralogical studies of mine waste from gold deposits in Nova Scotia and Yellowknife, Canada have revealed the following patterns.

Surface and ground waters associated with orogenic gold mine tailings tend to be pH-neutral, reflecting the effectiveness of carbonate in neutralizing the acid produced by oxidizing sulfide minerals Aqueous arsenic is relatively mobile under these conditions and may exceed environmental quality guidelines.

In Nova Scotia, variations in tailings composition and weathering conditions have led to different secondary mineralogy, water chemistry and environmental consequences. Weathered sulfide concentrate without carbonate is transformed into arsenate-rich hardpan with high total arsenic hosted in scorodite (FeAsO₄•2H₂O), amorphous iron arsenate and residual arsenopyrite. These materials have little neutralization potential and are associated with acidic, arsenic-rich pore water (average 25 mg/L). However, the bioaccessibility of arsenic in scorodite and arsenopyrite, as measured with a simulated gastric fluid, is very low (<1%), reducing the risk associated with accidental ingestion of tailings dust. This is consistent with sequential extraction analysis and the stability of scorodite in acid environments.

In contrast, tailings that originate from the same orebody, but contain sufficient carbonate, are associated with pH-neutral pore water and lower dissolved arsenic. Weathering products of arsenopyrite in Ca-rich tailings include yukonite (Ca₇Fe₁₂(AsO₄)₁₀(OH)_2O•15H₂O) or amorphous Ca-Fe arsenate, which are significantly more bioaccessible under gastric conditions than scorodite and arsenopyrite.

Carbonate-bearing gold mine tailings produce near-neutral drainage water but represent a higher risk to human health if fine-grained secondary minerals are ingested. Liming could provide a labile source of calcium at neutral pH and lead to alteration of iron-arsenates to calcium-iron arsenates with a corresponding increase in arsenic bioaccessibility.