GEOCHEMICAL CHARACTERIZATION OF COEXISTING PRECIPITATES AND WATERS FROM HIGH-SULFIDATION EPITHERMAL GOLD DEPOSITS

Water management and treatment at closed mine sites are major concerns for mining companies due to their costly nature, potential commitment of decades of water treatment and impact on the environment. The long term objective of this research investigates methods of extracting metals from acid mine waste, minimizing the environmental impact on the drainage quality and decreasing the financial burden. However, understanding the mechanism of element mobility in mine waste requires a comprehensive assessment of the composition of the water and coexisting minerals.

Samples were collected from El Indio, Chile, and Lagunas Norte and Pierina, Peru. The presence of secondary precipitates stands as evidence that sulfide oxidation at these mine sites is ongoing. Coexisting waters have an acidic pH between 2.2 and 4.5. Analysis by inductively coupled plasma mass spectrometry and inductively coupled plasma optical emission spectrometry showed that the waters were rich in Ca, Al, and Fe, and SO₄ is the major anion. Minor elements included Cu, Mg, Mn, Zn, K, Na and trace elements consisted of As, Co, Ni, Sr, Pb and Cd. This study identified solid phases using an environmental scanning electron microscope, X-ray diffraction and synchrotron-based microanalysis. These included previously unreported phases and may be categorized into 3 broad groups: 1) hydrous Na, Mg, Ca, Cu and other metal-sulfates such as blodite group minerals, hexahydrite and gypsum; 2) Fe-oxides, oxyhydroxides and oxyhydroxysulfates such as ferrihydrite, goethite, scorodite and jarosite; and 3) other poorly crystalline Al and Fe phases. Some of these minerals demonstrate paragenesis formed by evaporation in an arid environment. Changes in redox potential, humidity and temperature are coincident with changes in secondary mineralogy observed at these sites. Synchrotron-based microanalysis showed that these solid phases sequester Cu, Mn, Zn, Cd, As, Ni and other trace elements. No other study has provided a detailed analysis of the secondary solids at these mines or their role in storing trace elements. This research has improved our understanding of the mobility of elements by identifying the trace elements associated with solid phases, their temporary storage sites, and by characterizing the relationship between chemistry of precipitates and solution.