Major and trace element composition of copiapite-group minerals and coexisting water from the Richmond mine, Iron Mountain, California

Abstract

Copiapite-group minerals of the general formula AR 4 (SO 4 ) 6 (OH) 2 d nH 2 O, where A is predominantly Mg, Fe 2+ , or 0.67Al 3+ , R is predominantly Fe 3+ , and n is typically 20, are among several secondary hydrous Fe sulfates occurring in the inactive mine workings of the massive sulfide deposit at Iron Mountain, CA, a USEPA Superfund site that produces extremely acidic drainage. Samples of copiapite-group minerals, some with coexisting water, were collected from the Richmond mine. Approximately 200 mL of brownish pore water with a pH of À0.9 were extracted through centrifugation from a 10-L sample of moist copiapite-group minerals taken from pyritic muck piles. The pore water is extremely rich in ferric iron (Feand has a density of 1.52 g mL À1. The composition of the pore water is interpreted in the context of published phase relations in the Fe 2 O 3 -SO 3 -H 2 O system and previous work on the chemistry of extremely acid mine waters and associated minerals in the Richmond mine. Two distinct members of the copiapite mineral group were identified in the samples with coexisting water: (1) abundant magnesiocopiapite consisting of platy crystals 10 to 50 Am and (2) These results suggest that magnesiocopiapite is the least soluble member of the copiapite group under the prevailing conditions. Micro-PIXE analysis indicates that the copiapite-group minerals in this sample sequester Zn (average 1420 ppm), with lesser amounts of Cu (average 270 ppm) and As (average 64 ppm).

Jarosite versus Soluble Iron-Sulfate Formation and Their Role in Acid Mine Drainage Formation at the Pan de Azúcar Mine Tailings (Zn-Pb-Ag), NW Argentina

“…Magnesiocopiapite precipitated in a drift a few meters away from K-H 3 O jarosites, where there was less evaporation; there the water pH values were ~2 (because of the addition of cool, dilute groundwater from the adjacent host felsic metavolcanic rocks), and had lower Fe and SO 4 concentrations. In this case, jarosite and iron sulfate salt precipitation followed a sequence similar to the one proposed by Merwin and Posnjak [10,27,33]. [33] in Jambor et al [2].…”

“…This phase diagram was used by Jamieson et al [10] to understand jarosite and soluble iron-sulfate formation under different pH, Fe, SO 4 concentrations and evaporation rates at the Richmond mine [1,27]. The authors indicate that throughout the mine, where the formation of iron-sulfate salts prevailed, pH values were low in co-existing water (i.e., pH = −0.9 ± 0.2 (magnesiocopiapite); pH = −2.5 (rhomboclase-römerite); pH = −3.6 (rhomboclase)).…”

“…There is also a group of OH-bearing minerals known as the copiapite group with the formula AR 2 (SO 4 ) 6 (OH) 2 ·20H 2 O. Minerals from this group are some of the most common soluble salts derived from the oxidation of sulfides [2,10]. The less soluble sulfate minerals are divided into two groups; the poorly crystalline oxy-hydroxysulfates of Fe and Al, such as schwertmannite, ferrihydrite, gibbsite, alunogen, etc.…”

Preserved Filamentous Microbial Biosignatures in the Brick Flat Gossan, Iron Mountain, California

“…Iron-sulfate salts that precipitated, such as copiapite group minerals (white tabular crystals, Supplementary Material), halotrichite group minerals (fibrous efflorescent crystals, described above), and jarosite group minerals require very specific pH, redox, temperature, and relative humidity conditions. For example, copiapite group minerals are known to precipitate at Iron Mountain from fluids with pH = -0.9 ( Jamieson et al, 2005), and ferricopiapite was estimated to be stable in the pH range 0-1 (Majzlan et al, 2006). In the Iron Mountain gossan, mixed-valence copiapite group mineral salts precipitated first, followed by mixed-valence halotrichite group salts.…”

Climate Change Adaptation in Acid Sulfate Landscapes

“…Exposure of sulfidic rocks on the land surface could be part of the natural geomorphic evolution, leading to acid generation and translocation to downstream areas (Grant and Newell, 1993;Neubert et al, 2011). However, human intervention such as mining and large construction activities can dramatically facilitate these processes (Jamieson et al, 2005;Johnson and Hallberg, 2005;Nieto et al, 2007).…”