Climatic and compositional controls on secondary arsenic mineral formation in high-arsenic mine wastes, South Island, New Zealand

Figures & data

Table 1  List of arsenic minerals in this study, and their formulas

Mineral	Formula	Identification	Paragenesis	Colour
Arsenopyrite	Fe^2+(AsS)^2-	XRD, optical	Primary in veins	Silvery white–grey
Arsenolite	^+O3	XRD, XRF	Roasted concentrate	white
Scorodite	Fe^3+As^5+ 2H2O	XRD, XRF, microprobe	Secondary in waste	Pale olive green–greenish grey (10Y 5/2)
Kankite	Fe^3+As^5+ 3.5H2O	XRD, XRF, microprobe	Secondary in waste	Blue–green
(5GY 8/6)
Pharmacosiderite	^+(As^5+O4)3 6H2O	XRD	Secondary, in waste & adits	White
Bukovskyite	^+(As^5+O4)(SO4)(OH)^.7H2O	XRD	Secondary, in waste (S-rich) (10YR 8/8)	Yellow–brown
Zykaite	^+(As^5+O4)3(SO4)(OH)^.15H2O	XRD, XRF	Secondary, in waste (S-rich)	Pale yellow–cream (10Y 9/2)

Fig. 1  Digital elevation model of the South Island of New Zealand (downloaded, with permission, from www.geographx.co.nz), showing the topography of the Southern Alps that form the island backbone and induce differing climatic regimes as indicated. Localities mentioned in the text are shown, with the secondary As minerals (Table 1) indicated. Localities are described in Table 2.

Table 2  Localities mentioned in the text (see Fig. 1), and their environmental settings

	Mine age	Climatic regime	Rainfall (mm/year)	Altitude (m)	Microclimate	Precursor minerals	Secondary As minerals	pH	K^+, mg/L	mg/L
Macraes mine, historic adits	Closed 1940	Semiarid, temperate, rain shadow	600	540	Vadose zone	Arsenopyrite, pyrite	Scorodite; ferihydrite	4–7.5*
Macraes mine	Currently active (since 1990)	Semiarid, temperate, rain shadow	600	540	Sunny exposed slopes, windy, dry	Arsenopyrite, pyrite	Arsenical ferrihydrite; scorodite	1–11*	10–90	1000–5000
Golden Point battery	Closed 1940	Semiarid, temperate, rain shadow	600	330	Sunny valley floor	Arsenopyrite, pyrite	Scorodite; bukovskyite;
arsenical ferrihydrite	2–3^#
Barewood mine	Closed 1911	Semiarid, temperate, rain shadow	600	320	Damp adits	Arsenopyrite, pyrite	Arsenical ferrihydrite; pharmacosiderite	7–7.5*	5–14	5–20
Phoenix battery	Closed 1907	Alpine, rain shadow	900	700	Steep shady valley, damp and sunless in winter	Arsenopyrite, pyrite	Scorodite; kankite; zykaite; arsenical ferrihydrite	3–6^#
Globe-Progress, Coal Adit	Closed 1920	Warm temperate, moist	2300	360	Wet adit discharge	Arsenopyrite, pyrite	Arsenical ferrihydrite; pharmacosiderite	7–7.5*	11	500
Globe-Progress mine	Closed 1920, reopened 2007	Warm temperate, moist	2300	500	Vadose zone	Arsenopyrite, pyrite	Scorodite; ferrihydrite
	7–7.5*
Prohibition Mill	Closed 1951	Warm temperate, moist	2300	540	Exposed hilltop, periodically dry	Arsenolite, hematite	Scorodite	3–5^#*	0.2–1	12–14
Snowy River battery	Closed 1938	Warm temperate, moist	2300	260	Damp shady valley, periodically dry	Arsenolite, hematite	Scorodite	3–6^#
The pH values are water* and paste pH^#.

Fig. 2  Ternary diagrams, showing the relative compositions of ideal secondary arsenic minerals in this study (black squares). Broadly related common secondary iron minerals in mine wastes are indicated with open circles. (A) Diagram showing iron, arsenate and sulfate variations. (B) Diagram showing iron arsenate, iron sulfate, and hydration variations.

Fig. 3  The concentrations of sodium and potassium in groundwaters and mine waters in the Reefton and Macraes areas, South Island. Mine water compositions are affected by mining and processing activity.

Fig. 4  Electron backscatter images of some secondary arsenic minerals and textures in South Island mine wastes. (A) Crystalline kankite (K) encrustations with scattered quartz (Q) grains, Phoenix battery. (B) Massive fine-grained kankite (K) cementing mine waste grains of quartz (Q) and muscovite (M). (C) Precipitated material from the Globe-Progress mine lower tunnel apron (Table 2). Amorphous ferrihydrite (black) and amorphous ferrihydrite with abundant admixed arsenate (Fe,As; middle grey) dominate this material, which is cut by a veinlet of secondary pharmacosiderite (P; pale grey). (D) Scorodite (S; middle grey) forms a cement in mine wastes dominated by dark grey quartz (Q) and muscovite (M). A relict arsenolite grain (white, A) has been partially plucked during specimen preparation, leaving a black hole.

Fig. 5  Compositions of South Island mine waters on a dissolved As versus solution pH diagram. Black triangles are for Otago (east of mountains) (Craw & Pacheco 2002). Circles are for mines west of the mountains (DAME database); open circles=gold mines; grey circles=coal mines. Scorodite solubility curves are from Krause & Ettel (1988) and Langmuir et al. (2006).

Fig. 6  Relative stabilities of As minerals discussed in this study, as deduced from empirical observations of South Island mine sites. (A) Eh-pH diagram, after Vink (1996) and Craw et al. (2003), for total dissolved As of c. 10^-4 moles/L, Fe/As ≥1, and dissolved K⁺ of c. 5 mg/L. Secondary As minerals are commonly accompanied by arsenical ferrihydrite (grey ornament). The boundary between pharmacosiderite and scorodite-like minerals has not been quantified thermodynamically. (B) Schematic phase diagram showing qualitative controls on iron arsenate mineral formation. The boundaries separating the different minerals in B have not been quantified thermodynamically, but reflect differences in mineral formulae (Table 1) and observed occurrences (Table 2). The ‘persistent moisture’ axis reflects microclimates as outlined in the text.

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