Seasonal variations of Zn/Cu ratios in acid mine water from Iron Mountain, California

Time-series data on Zn/Cu weight ratios from portal effluent compositions [(Zn/Cu)_water] at Iron Mountain, California, show seasonal variations that can be related to the precipitation and dissolution of melanterite [(Fe^II,Zn,Cu)SO₄·7H₂O]. Mine water and actively forming melanterite were collected from underground mine workings and chemically analyzed. The temperature-dependent solubility of Zn-Cu-bearing melanterite solid solutions was investigated by heating-cooling experiments using the mine water. Rapid kinetics of melanterite dissolution and precipitation facilitated reversed solubility experiments at 25°C. Non-reversed solubility data were obtained in the laboratory at 4° and 35°C and at ambient underground mine conditions (38° and 42°C). Copper is partitioned preferentially to zinc into melanterite solid solutions at all temperatures investigated. During the annual dry season, values of (Zn/Cu)_water in the Richmond portal effluent increase to values between 8 to 13, consistent with formation of melanterite during this period. During the annual wet season, the onset of high discharge from the mine portals is characterized by a significant decrease in (Zn/Cu)_water to values as low as 2. This phenomenon may be caused by dissolution of melanterite with values of (Zn/Cu)_solid ranging from 1.5 to 3.5.