Acid rock drainage

Acid generation

Acid producing rock, Coromandel. The grey patches are rich in pyrite, and rain water running off this outcrop has pH near 2.

• Iron sulphide minerals, pyrite and marcasite (FeS₂), readily decompose in oxygenated waters at the surface.
• One of the products of this decomposition reaction is sulphuric acid.
• This acid lowers the pH of the immediate environment, and can spread downstream (= "acid rock drainage")
• Environmental pH <1 is possible in this situation.

Acid neutralisation

• The acidification can be limited if the surrounding rocks contain reactive minerals, especially calcite (calcium carbonate).
• There is sufficient calcite in schist to neutralise acid from pyrite in schist-hosted gold deposits, which almost invariably yields environmental pH between 6 and 8.
• There is insufficient calcite in most volcanic rock related gold deposits to neutralise acid from pyrite oxidation, and acid rock drainage is common.

Metal mobility

Neutral pH water seeping from a historic gold mine, Reefton, has elevated arsenic and antimony. Arsenic and antimony are also adsorbed by the brown iron-rich precipitate.

• Low pH encourages environmental mobility of cadmium, copper, lead, and zinc in streams and groundwater.
• Very low pH (<2) facilitates mobility of aluminium, iron and manganese in streams and groundwater.
• Arsenic and antimony are more soluble at neutral pH than at low pH.
• Hence, volcanic-related gold deposits typically yield acid waters with elevated Cd, Cu, Pb and Zn, with some As.
• Schist-hosted gold deposits can yield waters with very high As and Sb.