Experimental Metalloid Mobilisation from a New Zealand Orogenic Gold Deposit

Abstract

Elevated dissolved antimony (Sb) and arsenic (As) are common environmental issues around orogenic gold mines. The metalloids occur principally in stibnite (Sb 2 S 3 ) and arsenopyrite (FeAsS), and are typically accompanied by pyrite (FeS 2 ). Samples of arsenopyrite-rich (&2.5 wt% As) and stibnite-rich (&10 wt% Sb) ore were collected from the Globe-Progress mine in the Reefton goldfield, New Zealand. Crushed samples (\5 mm particles) were placed in kinetic leach columns and monitored for a year. Leachate was collected and analysed monthly. After 12 months, only a small portion (\1 %) of the total As and Sb had leached from the ore samples. Over the course of each month, the pH of all leachates decreased from &7 to &3 due to pyrite oxidation. Dissolution of CaFe-Mg carbonates in the host rock was insufficient to neutralize the leachate. Dissolved As concentrations from the arsenopyrite-rich ore sample were initially 16 mg/L, but decreased to &2 mg/L over 12 months. Dissolved Sb concentrations from the stibnite-rich ore were [7 mg/L throughout the experimental period, with maxima of 12 mg/L. SEM analysis after 6 months showed secondary arsenolite (As 2 O 3 ) and Ca-Al-Mg sulphates on arsenopyrite-rich ore surfaces, but an absence of secondary minerals, including Sb oxides, on stibnite-rich ore surfaces.These experiments document geochemical and mineralogical processes associated with short-term (days to weeks) water-rock interactions that yield relatively high concentrations of dissolved metalloids ([5 mg/L) where localised (1-10 m) acidification and limited oxidation occur in sulphide-rich rocks.

Morphology, Mineralogy, and Chemistry of Ocherous Precipitate Aggregates Downstream of an Abandoned Mine Site

“…However, the mass of As extracted by the 6 M HCl reagent positively correlated with the Sb extracted by the reagent (Figure 8). In addition, the mass of As and Sb extracted by the reagent was equivalent to four parts in the aggregate, suggesting the occurrence of ultra-fine silt to clay-size ore particles, such as arsenic-containing Sb oxides, as an impurity that has moved from the surrounding area and/or secondary minerals, including Sb leached from Sb-bearing ores that have been transported by groundwater and surface water [2,23].…”

Mineralogy and Geochemistry of Biologically-Mediated Gold Mobilisation and Redeposition in a Semiarid Climate, Southern New Zealand

“…Most (>95%) primary hydrothermal gold in the Otago Schist goldfield is encapsulated in sulfide minerals, pyrite, and arsenopyrite, as micron scale particles [25][26][27][28][29]. These gold particles are too small to form significant placers during erosion.…”

“…Most such coatings also include evaporative calcite and/or halite (Figures 2b and 5a). Sulfate minerals also locally coat sulfide minerals at the micron scale where the sulfides have undergone partial oxidation [27,29].…”

“…Oxidation of the arsenopyrite beneath the regional unconformity has yielded widespread scorodite (Fe III As V O 4 •2H 2 O) as direct pseudomorphous replacement of arsenopyrite crystals and as evaporative coatings on joint surfaces (Figure 2b) [25,27]. Incipient oxidative alteration of arsenopyrite can also result in minor amounts (micron scale) of arsenolite (As III 2 O 3 ) as an intermediate oxidation product (Figure 2b), typically on the surface of arsenopyrite particles [27,29]. For example, detrital arsenopyrite derived from rapid erosion of mineralised rocks with immediate sedimentary burial below the water table also has minor amounts of arsenolite as an intermediate oxidation product on particle surfaces [27,29].…”

“…Incipient oxidative alteration of arsenopyrite can also result in minor amounts (micron scale) of arsenolite (As III 2 O 3 ) as an intermediate oxidation product (Figure 2b), typically on the surface of arsenopyrite particles [27,29]. For example, detrital arsenopyrite derived from rapid erosion of mineralised rocks with immediate sedimentary burial below the water table also has minor amounts of arsenolite as an intermediate oxidation product on particle surfaces [27,29].…”