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.
- 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.
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.