Geochemistry of iron ochres and mine waters from Levant Mine, Cornwall

Applied Geochemistry,Vol.

10, pp. 237-250, 1995 Elsevier Science Ltd Printed in Great Britain. 0883-2927/95 $9.50 + 0.00

Pergamon

0883-2927(94)00036-0

Geochemistry of iron ochres and mine waters from Levant Mine, Cornwall R. J. B o w e l l 1 Department of Mineralogy, Natural History Museum, London, SW7 5BD, U.K. and I. B r u c e 59 Exeter Road, Okehampton, Devon, U.K. (Received 17 November 1993; accepted in revised form 28 June 1994)

Abstract--The geochemistry of metal-rich mine waters and mineral precipitates from the Levant mine, Cornwall, has been examined. Sulphide oxidation at Levant mine has produced a wide range of secondary sulphides, oxides, chlorides, sulphates and carbonates in a gossan environment, The mine waters display a wide variation in alkalinity, pH, chloride, sulphate, sodium, potassium and heavy metal content which can be explained by variable degrees of mixing between acidic, metal-rich, rock drainage waters and neutral to alkaline sea waters. Transition metals are soluble in the acidic mine waters with concentrations up to 665 mg/l Cu, 41 mg/l Zn, 76 mg/l Mn, 6 mg/l Co and >2500 mg/l total Fe. The production of acid rock drainage and leaching of metals can be related to sulphide oxidation. Where these metal-rich acidic waters mix with infiltrated sea water, neutralization occurs and some metals are precipitated (principally Cu). Where pools of mine drainage are stagnant native copper and euprite are precipitated, frequently observed replacing iron pipes and rail tracks and wooden shaft supports, due to electrode potential differences. In these solutions, dissolved copper species are also reduced by interaction with wood-derived organic species. Precipitation of iron oxyhydroxides, caused by a pH increase, also occurs and leads tO coprecipitation of other metals, including Cd, Co, Pb, Mn, Ag and Zn, thus limiting the release of dissolved metals in solution from the mine. However, the release of suspended metal-rich ochres in mine discharge waters (with high Pb, Zn, Cd, Mn, Ni, Sn, Sb, As, Bi, Cu, Co and Ag) will still present a potential environmental hazard.

INTRODUCTION Aqueous discharge from mines can seriously affect local groundwater quality. Oxidation of sulphide ores, particularly those rich in pyrite, introduces high concentrations of metals, hydrogen ions and sulphate ions into waters fed by mine discharge (Lowson, 1982; Nordstrom, 1982; Fuge et al., 1993). These acidic waters affect aquatic life, and in some instances, the quality of drinking water supplies. Effective management and remediation of acid mine drainage is possible only if the processes that influence metal release and transportation are fully understood. Fluctuations of metal concentrations carried in surface waters, in areas of mining or former mining activity, have been related to both the flocculation of ochres (Duniker, 1980) and the production of acid rock drainage (Fuge et al., 1993). Several studies have shown the importance of cation adsorption by iron and manganese oxyhydroxides in limiting metal discharge from mine workings (Langmuir and Whittemore, 1971; Nordstrom, 1982; Johnson, 1986; Bowell et al., 1994). Further laboratory studies have 1present address: 7 Sheldrake Gardens, Hordle, Hampshire, U.K. 237

investigated these adsorption processes in well defined media (Sigg and Stumm, 1980; Thornber and Wildman, 1984). However, the geochemistry of metal behaviour in mine drainage from which many of the ochres originate are less well known. The St Just district, in western Cornwall, has a long history of mining stretching back to pre-Roman times (Dewey, 1923; Dines, 1956; Noall, 1972). The most extensively mined metals were tin, copper and arsenic, and the main period of activity lasted from the middle of the 19th to the early 20th centuries. As a result many abandoned mines with old spoil tips litter the orefields and represent a potential environmental hazard. A t the Levant mine, situated on the coast (Fig. 1), copper and arsenic sulphide-rich veins are exposed to alteration from infiltrating sea water, rich in anions, and from acidic groundwaters. Under these conditions sulphide oxidation occurs leading to the leaching of potentially toxic elements, such as arsenic, either as dissolved species or adsorbed onto iron oxyhydroxide ochres. Within the oxidized zone of the Levant mine, atmospheric oxygen diffuses into the subsurface where sulphide oxidation reactions occur. Additionally, in the submarine sections of the mine, the infiltration of sea water increases the dissolved salt

238

R.J. Bowell and I. Bruce

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km into the aureole. Five styles of mineralization have been recognized (Jackson et al., 1982). In chronological order these are: metasomatism; barren pre- and post-joint pegmatites; mineralized sheeted vein systems; mineralized fissure systems; and irregularly shaped replacement bodies. At Levant the major ore minerals, in the hypogene quartz (chlorite-hematite-carbonate) veins, are in order of decreasing abundance; cassiterite, chalcocite, chalcopyrite, arsenopyrite, bornite, pyrite, wolframite, scheelite, stannite and bismuthinite. Tropical weathering of the Cornish lodes during the Permo-Trias resulted in the formation of a range of secondary arsenate, phosphate, carbonate and oxide minerals in gossans (Greg and Lettsom, 1858; Embrey and Symes, 1987; Wolloxall, 1989; Bowell, 1992). Where sea water has breached the submarine parts of the nearby Botallack mine (Fig. 1) a range of secondary oxyhalides, sulphates, carbonates and phosphates have been formed by alteration of the sulphide assemblage (Bowell, 1992).

1000 i

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METHODOLOGY BOSTRAZE

::::::::::::::::::St:JUST

":'"~':"':~'~'-

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c o n t e n t of the g r o u n d w a t e r s . A n a l y s e s of m i n e drainage waters p r o v i d e i n f o r m a t i o n o n the c o n c e n t r a t i o n a n d d i s t r i b u t i o n of dissolved c o m p o n e n t s of the m i n e waters whilst analyses of precipitates indicate t h e n a t u r e of the m a j o r s e c o n d a r y p h a s e s a n d the e x t e n t of dissolution a n d s o r p t i o n processes.

GEOLOGY Geologically the St Just mining district can be divided into two units (Fig. 1): an eastern unit, formed by the Land's End granite pluton and a western unit of basalts and sedimentary rocks in the contact aureole. The northern part of the pluton consists of medium to coarse-grained two-mica granites. Hydrothermal alteration has taken place leading to sericitization of feldspars and chloritization of biotite. The contact aureole consists of interbedded sequences of sediments and basalt lavas of possible Devonian age (Miller and Mohr, 1964). Laminated argillites are the most abundant rocks but calcareous, tuffaceous and fine-grained argillites, cherts and arenites are also present. Deformation of the volcano-sedimentary sequence during the Hercynian orogeny resulted in the formation of NE-trending, Nplunging major and minor folds. During emplacement of the granites, the host rocks were thermally metamorphosed in the albite-actinolite and amphibole-hornfels facies. The development of skarns rich in magnetite and garnet occurred early in the mineralizing sequence, with boron in these fluids contributing to the formation of axinite and tourmaline (Dines, 1956; Jackson et al., 1982). Extensive fracturing of both the pluton and the hornfels contact yielded conduits which were subsequently utilized by hydrothermal fluids. Veins and replacement deposits were developed in the roof and contact zones of the pluton and up to 2

Samples of ochres and mine drainage waters were collected from level 12 (approximately 300 m deep) of the Levant mine, Cornwall, together with samples of fresh and partly altered sulphides. Three separate water samples were collected at each site; one was filtered through a Durapore 0.45 /~m filter and acidified with 1 M HNO3; one was acidified with 1 M HNO3; and the third was collected unprocessed. All water samples were stored in air-tight containers, as were samples of mine ochres, to minimize atmospheric contamination. Rock samples were stored in sealed air-tight plastic bags in order to minimize further mineral breakdown. Redox potential, pH, dissolved oxygen content and conductivity were measured using an Oakton Water Test kit, calibrated with the standards supplied. Mineral identification was carried out by optical and electron microscopy and confirmed by X-ray diffraction and Fourier transform infra-red spectroscopy (Table 1). Trace elements were extracted from 5 g of ochre by digestion with 15 ml of nitric acid (70% w/v) and 15 ml of perchloric acid (70% w/v). Chemical analysis of the ochres was carried out by inductively coupled plasma atomic emission spectrometry (ICPAES) and the detection limits are given along with results in Table 2. Precision of the technique was checked against known standards and found to be +2%. Mine and sea waters were analysed by ion chromatography (Dionex-300) using, for anions, an AS4A-AMMS column with Na2CO3(1.8 mM) eluent at a flow rate of 2.5 ml/min, and, for group I/If cations a CS 12 column with methane sulphonic acid eluent (20 mM) at a flow rate of 2 ml/min. Detection in both cases, was by a pulsed electrochemical detector in Whitconductivity mode. Transition metal determinations were accomplished with a Dionex CS 5 column using pyridine-2, 6-dicarboxylic acid eluent and 4(2-pyridylazo)resorcinol post-column derivitization with measurement by a variable wavelength detector operated at 525 nm. Detection limits are shown along with the results in Table 3. Geochemical modelling of metal speciation was achieved using the computer code HSC-CHEMISTRY (Roine, 1992).

RESULTS Sulphide oxidation Microscopic e x a m i n a t i o n of sulphide minerals rev e a l e d the general o r d e r of susceptibility to a l t e r a t i o n as b e i n g t e n n a n t i t e > sphalerite > chalcopyrite >

Ochres and mine waters, Levant mine, Cornwall

239

Table 1. Secondary minerals described from the Levant mine Cornwall (Greg and Lettsom, 1858; Dewey, 1923; Dines, 1956; Embrey and Symes, 1987; this study)

Cu3SO4(OH)4 (Ag,K)Fe3(SO4)z(OH)6

Antlerite

Argentojarosite Atacamite Botallackite Brochanite Calcite Chalcocite Copper Cornubite Cornwallite Covellite Cuprite Devilline Ferrihydrite Goethite Gypsum Halite Hematite Jarosite Kaolinite Lepidocrocite Natron Olivenite Paratacamite Scorodite Smectite Smithsonite Tenorite Woodwardite

Cu2CI(OH)3 Cu2CI(OH)3

Cu4804(OH)6 CaCo 3 Cu2S Cu Cus(AsO4)2(OH)4

Cu5(AsO412(OH)4.H20 CuS Cu20 CaCu4(SO4)2(OH)6.3H20 FeO(OH) a-FeO(OH) CaFO4.2H20 NaCI Fe203 KFe3(SO4)z(OH)6 A12Si203(OH)4 7-FeO(OH) NazCO 3.10H20 Cu2AsO4(OH) Cu2CI(OH)3 FeAsO4.2H20 (NaCa)o.3(AL,Fe,Mg)2Si40 jo(OH)2.nH20 ZnCO 3 CuO

Cu4AIz(SO4)(OH)12.2-4H20

bornite = arsenopyrite > pyrite > chalcocite > covellite, as could be predicted from published work ( T h o r n b e r and Taylor, 1992). Alteration is concentrated along grain boundaries and fractures producing typical boxwork textures, with precipitation of secondary phases m o r e abundant along sulphidegangue interfaces than along sulphide-sulphide interfaces (Fig. 2). In areas of low p H ( < 5 ) mine water, alteration assemblages are dominated by ferrihydrite, goethite and covellite, with lesser amounts

of lepidocrocite, antlerite, chalcocite, cuprite, olivenite and scorodite. A characteristic feature is the trellis-like covellite fringes developed on most sulphides (Fig: 2). W h e r e the pH of the mine waters is higher or in areas impacted by seawater (pH range of 5-8) then other secondary salts are also present. In this study these secondary minerals consist of atacamite, argentojarosite, botallackite, brochantite, copper, cornwallite, cornubite, devilline, hematite, paratacamite,

Table 2. Trace metal geochemistry of Levant mine ochres and limonites. All analyses by 1CPAES. All concentrations in mg/kg Sample

Mn

Lv01 22.7 Lv02 18.9 Lv03 12.6 Lv04 17.1 Lv05 19.2 Lv06 15.9 Lv07 13.1 Lv08 11.9 Lv09 40.2 Lvl0 41.2 Lvll 39.3 Lvl2 41.7 Lvl3 45.2 Lv14 38.9 Lvl5 36.1 Lvl6 34.2 Detection limit 0.007 AG IO:Z-H

Co

Ni

Cu

Zn

As

Mo

Ag

Cd

Sn

Sb

Pb

Bi

0.39 0.11 0 0.06 0.14 0 0 0 2.76 3.23 2.52 3.39 4.11 2.39 2.11 2.05 0.05

0.22 0.09 0 0 0.12 0 0 0 1.31 1.52 1.18 1.76 2.19 0.86 0.61 0.49 0.06

0.43 0.29 0.14 0.26 0.33 0.21 0.18 0.12 0.98 1.04 0.95 1.08 1.22 0.91 0.81 0.76 0.05

2.49 1.14 0.71 0.96 1.25 0.83 0.79 0.62 3.68 4.23 3.39 4.86 6.95 2.95 2.7 2.55 0.02

11.8 14.3 29.8 16.5 13.8 18.5 27.2 34.9 6.8 6.3 7.1 5.9 5.4 7.5 8.5 8.2 0.2

0.42 0.51 0.68 0.54 0.48 0.59 0.65 0.71 0 0 0 0 0 0.22 0,36 0.31 0.2

0 0 0 0 0 0 0 0 0.12 0.14 0.11 0.16 0.23 0.09 0.05 0.03 0.02

0 0 0 0 0 0 0 0 0.16 0.18 0.11 0.23 0.26 0.08 0.06 0.03 0.03

0 0 0 0 0 0 0 0 0.28 0.34 0.26 0.37 0.48 0.21 0 0 0.2

0.27 0.33 1.42 0.49 0.33 0.69 1.08 1.69 0.22 0 0.21 0 0 0.29 0.31 0.31 0.2

0 0 0 0 0 0 0 0 0.68 0.73 0.62 0.92 1.28 0.51 0.49 0.47 0.2

0 0 0 0 0 0 0 0 0.48 0.57 0.38 0.61 0.69 0.29 0.25 0.22 0.2

Water pH 4.4 3.6 2.5 3.3 3.6 2.8 2.6 2.1 6.9 7.3 6.8 7.9 8.1 6.5 5.7 5.8

R. J. Bowell and I. Bruce

240

Table 3. Geochemistry of mine and sea water. All element and anion concentrations in mg/l

Sample

Eh (V)

pH

Lvl 4.4 Lv2 3.6 Lv3 2.5 Lv4 3.3 Lv5 3.6 Lv6 2.8 Lv7 2.6 Lv8 2.1 Lv9 6.9 Lvl0 7.3 Lvll 6.8 Lv12 7.9 Lvl3 8.1 Lvl4 6.5 Lv15 5.7 Lvl6 5.8 SWI 7.1 SW2 7.3 Seawater 7.5 Detection limit

0.5 0.55 0.61 0.57 0.55 0.58 0.61 0.69 0.21 0.18 0.21 0.13 0.11 -0.21 0.48 0.48 0.1 0.11 nd

Na ÷ Lvl 70 Lv2 80 Lv3 85 Lv4 95 Lv5 80 Lv6 80 Lv7 55 Lv8 75 Lv9 81 Lvl0 17550 Lvll 710 Lvl2 15260 Lvl3 16250 Lvl4 2150 Lv15 4650 Lvl6 69 SW1 15200 SW2 14500 Seawater 10770 Detection limit 0.1

Alkal

F-

CI-

18.9 17.2 14.3 16.5 17.1 11.2 10.1 15.7 58.5 5 63.9 5.2 5.3 11.9 159 15.9 4.9 153 142 1

0 0 0 0 0 0 0 0 0.3 1.5 0.5 1.4 1.3 0.1 0.3 0.2 1.6 1.5 1.28 1

139 109 118 126 133 141 136 28 349 16230 349 11650 12990 395 228 142 20300 20700 19300 0.1

K+ 7.7 8.2 9.1 8.7 11 8.5 7.2 11 11 486 271 437 422 216 196 12.9 487 256 399 0. t

Ca 2+ 275 195 205 235 235 250 195 435 275 400 281 396 379 186 283 215 405 398 412 0.1

Br

I

1.2 0 0 0 0 0 0 0 0 10.1 0 0 6.9 3.6 0.5 0 20.2 19.9 21.9 0.1

0 0 0 0 0 0 0 0 0 5.59 0 3.1 2.9 0 0 0 6.1 5.62 7.2 0.1

SO] 2660 2570 2380 2510 2570 2570 8550 11220 955 587 858 851 652 915 595 2250 750 510 2712 0.01

Mg 2+

A13+

Si4+

65 75 75 85 80 85 90 70 65 2280 795 1990 2100 225 315 63 2380 2020 1294 0.1

80 110 100 105 110 110 175 215 245 20 43 12 10 51 69 58 <0.1 <0.1 nd 0.1

45 45 45 50 45 50 45 40 45 10 15 8 6 22 43 40 <0.1 <0.1 nd 0.1

$2O2

SO~-

HSO~-

HS-

0 0 0 0 0 0 0 0 3.6 5.2 3.4 14.2 15.9 2.9 1.7 8.9 0 0 nd 0.1

0 0 0 0 0 0 0 0 1.2 0.87 0.9 8.7 9.6 1.2 3.1 2.6 0 0 nd 0.3

3.1 6.5 23.5 19.6 6 4.2 21.2 25.2 0 0 0 0 0 0 0 0 0 0 nd 0.2

0 0 0 0 0 0 0 0 0 0 0 0.1 0 0.36 0 0 0 0 nd 0.2

Fe 2÷ 16 2.2 0.05 0.05 0.42 0.4 2.65 21 15 0.02 0.3 0.1 0.2 69.5 1.7 1.2 <0.05 <0.05 nd 0.05

Fe 3+

Total Fe

240 22 1.1 0.6 8.9 8.5 10 340 460 0.51 96 425 625 115 29 23.6 <0.01 <0.01 nd 0.01

351 429 623 487 426 591 608 825 285 271 296 237 229 173 329 322 0.02 0.03 nd 0.01

PO43 0.36 0.29 0 0.26 0.3 0 0 0.1 0.49 0.21 0.36 0.4 0.48 0.4 0.31 0.42 0.21 0.18 0.2 0.1 AsO~ 0.49 0.37 0 0.12 0.39 0.1 0 0 0.35 0 0.39 0.21 0.46 0.42 0.36 0.58 0 0 nd 0.2

Alkal = alkalinity. Seawater data from Riley and Chester, 1971 and Millero et al., 1993. nd = not detected.

s m i t h s o n i t e , t e n o r i t e a n d w o o d w a r d i t e ( T a b l e 1). From EDX semi-quantitative analysis, jarosite hosts u p to 2 w t % P b a n d 8 w t % A g f o r m i n g a r g e n t o j a r o site. H e m a t i t e a n d c h a l c o c i t e a r e p r e s e n t in t h e a l t e r a t i o n a s s e m b l a g e s b u t a r e m o s t likely o f h y p o g e n e o r i g i n w h i c h a r e a b l e to c o - e x i s t w i t h t h e s u p e r gene phases. Halite, gypsum and natron were obs e r v e d as a c o a t i n g o n w a l l r o c k w h i c h h a s r e a c t e d with sea water.

Mineralogy and chemistry o f iron ochres S u s p e n d e d i r o n - r i c h f l o c c u l a t e s w e r e p r e s e n t in m o s t o f t h e w a t e r s a m p l e s c o l l e c t e d at L e v a n t (Fig. 3). T h e o c h r e s a r e f i n e - g r a i n e d , p o o r l y c r y s t a l l i z e d

mixtures of iron oxides and oxyhydroxides, jarosite, g y p s u m , s m e c t i t e a n d q u a r t z ( T a b l e 4). I n t h e l o w p H (<5) waters the ochres consist of goethite, ferrihydrite, lepidocrocite and quartz. In near-neutral (pH 58) g r o u n d w a t e r s j a r o s i t e , g y p s u m , s m e c t i t e a n d h e m a t i t e a r e also p r e s e n t , l e p i d o c r o c i t e is n o t as a b u n d a n t a n d f e r r i h y d r i t e is a b s e n t . F r o m m i n e r a l o g y o f t h e o c h r e s it w o u l d a p p e a r t h a t w i t h t i m e f e r r i h y d r i t e a n d l e p i d o c r o c i t e a l t e r to g o e t h i t e ( L a n g m u i r a n d W h i t t l e m o r e , 1971; B i g h a m et al., 1990), e x c e p t in s a m p l e s L v 7 a n d L v 8 w h i c h a r e c h a r a c t e r i z e d b y h i g h A s c o n t e n t ( T a b l e 2). T h i s w o u l d b e explained by the greater disorder of ferrihydrite than g o e t h i t e , l e a d i n g to g r e a t e r c a p a c i t y to a d s o r b AsO43- .

Ochres and mine waters, Levant mine, Cornwall

Fig. 2. Photomicrographs of sulphide alteration at the Levant mine, Cornwall. All micrographs photographed in reflected light. (a) Minor replacement of chalcopyritc by chalcocitc and covcllitc along grain boundaries and fracturcs. Level 12, South Lode, Levant mine, Cornwall. Field of view 1 ram. (h) Extensive replacement of chalcopyritc, chalcocitc and pyrite by antleritc, brochantitc, gocthite, jarosite, paratacamitc and covcllite. Note the greater proportion of secondary minerals at the junt:tion with carbonate ganguc. Level 12, South Lode, Levant mine, Cornwall. Field of view 2 mm. (c) Boxwork pscudomorph texture and covellitc-trcllis texture produced by replacement along grain boundaries trod fractures in arsenopyritc. Level 12, South Lode, Levant mine, Cornwall. Field of view 0.1 ram.

24

242

R.J. Bowell and 1. Bruce

Fig. 8. Copper-cupritc pseudomorphs from Levant mine. (a) Copper-cupritc replacement of a stce pipe. Level 12. (b) Copper-cuprite partial pscudomorph of a wooden shaft support pillar, Level 12 (c) Photomicrograph of coppcr-cupritc intergrowth pscudomorphing a steel pipe. Lcvel 12.

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1500

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Cornwall.

The variation in mineralogy has an influence on metal sorption by the ochres. In the ochres of the low pH (<5) waters As, Sb, Mo and Zn are strongly adsorbed (Table 2). At higher pH (5-8) Zn is present at an order of magnitude greater, and Cu, Pb, Ag, Mn, Bi and Cd are also adsorbed but As, Sb and Mo adsorption is much less than at low pH. The total trace metal content of the ochres is greatest at higher pH (Fig. 4). This effect is related to the nature of the ions in solution and their solubility, the surface properties of the ochre phases, and the mineralogy of the ochres at higher pH (see below).

Table Sample Lvl Lv2 Lv3 Lv4 Lv5 Lv6 Lv7 Lv8 Lv9 LvlO Lvll Lvl2 Lv13 Lvl4 Lvl5 Lvl6

Calcite

x x xx

xxx--abundant,

mine,

Cornwall

243

Mine water chemistry

g

aaoo o

Levant

4.

Ferrihydrite

Goethite Gypsum

x xx xx xx xx xx xx xxx

xxx xxx xxx xxx xxx xxx xxx xxx xxx xx xxx xx xx xxx xxx xxx

xx--accessory, x--trace.

x xx x xx xxx

Mineralogy Hematite

x xx x xx xxx xx x x

The chemistries of the mine waters are shown in Table 3. The acidic mine waters contain low concentrations of Ca, Mg, Sr and carbonate content. The transition metal content of these waters is high, both in the suspended load (for iron) and in solution (other metals). The concentration of transition metals in true solution (i.e. passes through a 0.45 a m filter) increases as pH decreases while total metal concentration in the suspended load increases as pH increases (Fig. 5). Iron speciation determinations in the waters were possible by ion chromatography. The ratio of Fe 3+ to Fe 2+ varies from 0.27 in Lvl4 (reduced water) up to 302 (Lv8), with most waters showing a ratio greater than 6 confirming that Fe3+ is the dominant form of Fe. The alkalinity of the waters varies from 5 to 64 mg/l and the high alkalinity may be a reflection of sea water contamination (Millero et al., 1993). Total sulphur in solution, like total metal concentration, increases with decreasing pH. Sulphate is the most dominant sulphur species in all waters analysed with minor amounts of HSO 4 and HS- present at low pH (pH<5). In higher pH waters metastable sulphur species sulphite and thiosulphate were detected (Table 3). The proportion of metastable species increases with increasing pH (Fig. 6). It is possible that a proportion of the metastable sulphur species present are the result of post-collection alteration of H2S. Although atmospheric contamination was minimized during collection and subsequent storage (over 2 days) and analysis there is still a possibility that H2S oxidation resulted in the generation of thiosulphate and sulphite (Goldhaber, 1983). However, given the high concentration of these metastable oxidized species, relative to H2S in all samples, the trend in S speciation is still valid. From the data a division can be made of waters collected underground at the Levant mine. Acidic mine waters are characterized by low carbonate and

of mine Jarosite

ochres

Lepidocrocite

Quartz

xx xx xx xx x x xx x

xxx xxx xxx xxx xxx xx xx xx xx x xx

x x x

x xx xx

and limonite

x x x

xxx xxx xxx

Smectite

Kaolinite Cuprite

X

X

XX

X

X

X

XX

X

X

X

Copper

X

XX

X

X

XX

X

XX

X

XX

XX

XX

244

R.J. Bowell and I. Bruce 1400-

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5

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pH

pH Ag

+

Cd

-

Sn

:~

Pb

]

25O

700O

A

E 2o0

._=

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&

5000

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2000-

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pH

pH [ A

CO

+

CU

"

Zn

:~

As]

I ,&

Ni

÷

MO

°

S In

::

8i

Fig. 4. Trace element geochemistry of in situ iron ochres vs associated water pH.

waters with a pH above 7, the Na:C1 ratio is 1:1.08, suggesting a seawater origin. The increased proportions in solution of K and Ca and decrease in Mg and alkalinity are probably due to mineral-water interactions during infiltration. The influx of sea water into the mine, and its mixing with groundwaters has produced a higher p H (pH 6.8--8), low transition metal content water with a high C1-/SO~4ratio and higher cation (Na, Ca, Mg) and carbonate

base cation contents, high transition metal and sulphur contents, and low C1-/SO~- ratio (Fig. 7). From a comparison of average sea water composition (Table 3, from Riley and Chester, 1971; Dickson, 1993; Millero et al., 1993), Cornish sea water and Levant waters the similarities in molar proportions of Na: C1 also imply seawater infiltration. In the average seawater and those analysed off the Cornish coast the Na : C1 ratio is approximately 1:1. In the Levant mine

"

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3

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•

s

F,=.

-

e

~.3+

:I

02

e

7

- 1~F,

°

"

;:

•

o °

°

°

oo "3

.

S

.o II

.

E. . . . . . .

I

..

7

;

e

l

i 6

I

omu~,~c,

" -i -

co

e

I

,,s!°

~o iz

4o~- *

s

I •

o,l=.,~ic~

o

?

-

'r~,ud~

J

i!

::t.

:":! 12

O.l J oJ

.1

; *

•

s

*

** °%..

.

7

"o

i

i'*--'''""

Fig. 5. Trace element geochemistry of in situ mine waters vs pH.

I

Ochres and mine waters, Levant mine, Cornwall 12"

Table 5. Microprobe analysis of cuprite and copper intergrowths

10"

E

6 4 A

A

A

A

3

4

A

5

4~,•A

A A

7

B

6

pH

I •

Sulphate I

25

Zn

0

Total

Copper Copper Copper Copper

99.85 99.92 99.76 99.21

0.11 0.11 0.12 0.39

0.12 0.1 0.18 0.21

-----

100.08 100.13 100.06 99.8l

Cuprite Cuprite Cuprite Cuprite

88.79 89.11 88.12 88.62

0.15 0.11 0.18 0.14

--0.22 0.15

11.19 10.89 11.88 12.03

100.13 100.11 100.4 100.94

20 15

,~ lO t

n

.............

-~ ~ , = - -

3

4

5

6

~,.

7

8

pH

i •

Fe

Analysis by Cameca Camebax SX 50 electron microprobe. Voltage 30 kV. Current 2.6 × 10 A. Standards-pure elements and hematite. Radiations measured: Cu-Ka; Fe-La; Zn-Ka.

30 ¸

~

Cu

8"

~

245

Hydrogen

sul

m

Thiosulphate

~

Sulphite

~

Bisulphide

Fig. 6. Sulphur speciation in Levant mine waters.

contents. In these mixed waters, metal precipitation is high with the F e - C u redox couple and hydrolysis reactions described by the equations: Cu 2+ + F e ~ C u

° + F e 2+

(followed by the reaction Fe 2÷ ~ Cu 3÷ + e - ) Cu 2+ + 2Fe 2+ ~ Cu ° + 2Fe 3+ and subsequent Fe hydrolysis Fe 3+ + 3H20 --, Fe(OH)3 + 3H ÷. This leads to precipitation of iron oxides and oxyhydroxides (with high trace metal contents), cuprite and copper. Despite the presence of other

DISCUSSION

1000~

,

- 2 2 --2,

10000

t '~

Seawater ,

1000

,¢ 1C //Mixed i

o=

.J • 0.

0.01

trace elements in solution cuprite and copper show almost ideal compositions (Table 5). The iron originates not only from oxidizing sulphides but also from oxidation of steel rail tracks, nails and other mining implements. This leads to pseudomorphs of mining relics in copper, cuprite and goethite (Fig. 8). Where wooden relics are present the reduction of copper in solution can occur by reaction with organic matter in the wood, similar to reactions reported in laboratory experiments (Ephraim and Marinsky, 1986). Some mine waters do not appear to be either particularly acidic or highly saline and are characterized by low Eh-high pH (6-7), low cation concentration and a low C1-/SO]- ratio and high alkalinity (Table 3). These waters are possibly the result of acid neutralization or could have been produced by cathodic reactions related to electrochemical oxidation of the sulphides (Thornber and Taylor, 1992). Concentrations of dissolved oxygen are highest in the low pH mine waters and sea waters and lowest in the low E h high pH waters (Table 3). The positive correlation between total sulphur in solution and dissolved oxygen content (Fig. 9) confirms that the extent of sulphide oxidation is directly related to dissolved oxygen content and that the primary source of S species in the mine waters is from sulphide oxidation.

Acid mine

r ~ A A

A t

waters

waters A

/

I

~ • .-~---

0.001" pH

Fig. 7. CI/SO4 vs pH.

/

i

The mineralogy of mineral precipitates and the composition of mine drainage waters are dependent on the composition of the primary sulphides and the extent of oxidation. The major elements affected by sulphide oxidation are Fe and S and their behaviour in drainage waters can be related directly to that process as can that of some trace elements such as Cu and As. However, other elements not contained in the sulphide assemblages but present in associated gangue minerals (such as A1 in aluminosilicates) are indirectly released by the action of sulphide oxidation and generation of acid mine drainage (Dubrovsky et al., 1984; Blowes and Jambor, 1990).

246

R.J. Bowell and I. Bruce and Taylor, 1992). The separation of the cathodic reaction from the anodic reaction will have a major influence on the mineralogy of the resulting assemblage. Where a wide separation between cathode and anode reactions occur (scale of metres) then a wide range of alteration products can be produced along the generated concentration gradient between the two reactions. However, where sulphides are more disseminated the cathodic and anodic reactions are close together so neutralization is more rapid and alteration assemblages are more compressed between the oppositely charged electrodes (Sato and Mooney, 1960; Thornber, 1975). Water-table fluctuations will also affect sulphide oxidation by lowering the water-table leading to greater oxygen access and thus greater oxidation. The nature of the alteration and retention of metals is also affected by pH. Which, in turn, is controlled by the composition of the initial sulphide assemblage (Thornber and Taylor, 1992). The sulphide assemblage at Levant is dominated by pyrite which yields acidic weathering solutions by reactions such as:

4.5" 4' E £ 3.5

&

~ 2.5 ~5

2

1.5-

pH

5 4.5 4 3.5

i

2.5 2

FeS2 + 8H20 = Fe 2÷ + 2SO 2- + 16H+ + 14e-. 1,5

i

~

Sulphate, mg/I

~

1'0

12

('Thou~nde)

Fig. 9. Dissolved oxygen (% saturation) vs (a) total S and (b) pH in Levant mine waters.

A major control on the development of a well defined concentration gradient and consequent production of reaction products is the separation of the cathodic oxygen-consuming, alkali-producing reactions (Thornber and Taylor, 1992), which can be generalized by a reaction such as 02 + 2H20 + 4 e - ~ 4 O H from the anodic, oxidizing, acid-producing reactions, such as MS + 4H20 ~ M 2+ + S O ] - + 8H + + 8 e (where M is a divalent metal, Thornber and Taylor, 1992). The ore-type studied in the Levant lodes can be described as a matrix-sulphide ore, that is an ore in which sulphide material is sufficiently abundant to form a continuous matrix leading to good electrical conductivity throughout the ore (Fox, 1830). Since good electrical contacts can be maintained throughout the lode a wide separation can occur between cathodic and anodic reactions with variable groundwater E h - p H (Baas-Becking et al., 1960; Sato and Mooney, 1960; Bailey and Peters, 1976). The rate of sulphide oxidation will be controlled by the rate at which oxygen is supplied (and reduced at the cathode-solution interface) and by p H (Thornber

The ferrous ion produced is rapidly oxidized to ferric ion which is strongly hydrolysed to form Fe hydroxide precipitates (Thornber, 1975): Fe 3+ + 3H20 = Fe(OH)3 + 3H +. Only a small proportion of the base metals present in low pH waters will be retained, sorbed onto the Fe hydroxide, but elements such as As, Sb, W and Mo will be retained, adsorbed as oxyanions. This behaviour is observed in the trend of trace element geochemistry of the iron ochres shown in Fig. 4. Leaching of elements (AI, K, Na, Ca) from the wallrock and gangue minerals at low pH and co-precipitation with metals, by neutralization, leads to the formation of devilline, woodwardite and jarosite at Levant. These minerals have also been reported from elsewhere in the Cornish mines (Greg and Lettsom, 1858; Emburey and Symes, 1987; Wolloxall, 1989). Where sulphide/gangue ratios are low, higher pHs are maintained (pH > 6), particularly if carbonates are present which act to buffer solution pH. However, a lower pH would still be expected immediately around weathering sulphides. The occurrence of quartz in the limonite fabric is due to leaching of SiO 2 from wallrock silicates which will precipitate on acidification. During sulphide oxidation there is a tendency for Fe and base metals to diffuse towards the surface of the gangue minerals and precipitate by hydrolysis reactions, lowering pH and continuing the leaching process (Thornber et al., 1981). These reactions occur some distance from the leaching surface to form Fe oxyhydroxide crusts. Precipitated Fe oxyhydroxides can adsorb substantial concentrations of liberated metals from the mine waters depending on solution pH (Thornber, 1975; Johnson, 1986).

Ochres and mine waters, Levant mine, Cornwall Many oxide surfaces including those of goethite, are positively charged at low pH and become negatively charged at higher pH due to the increased binding of hydroxyl groups (Parks and DeBruyn, 1962; Parfitt, 1980). Consequently at low pH, oxyanions like arsenate will be adsorbed while with a pH increase metal ions will be adsorbed when the mineral surface charge is negative. The point at which this change occurs, i.e. the point at which the surface has a net zero charge can be used as a measure of the adsorption potential at another particular pH. This is shown by the increase in base metals and decrease in As adsorbed by the Fe ochres analysed with increasing pH (Fig. 4). The presence of smectite and kaolinite in the flocculated material could explain the lower retention of base metals, even in very acidic waters, observed in the data (Table 2) than has been recorded in ochres from other mine sites (Dubrovsky etal., 1984; Johnson, 1986; Fuge etal., 1993). Both of these clay minerals can have a pHzpc (zero point charge) as low as 2.2 and up to 5 (Parks and DeBruyn, 1962). Consequently they could provide negatively charged Helmholtz layers at their surfaces, leading to favourable conditions for sorption of positively charged base metals at pHs as low as 2.2. In the open system of the Levant mine waters, the surface properties of the different minerals will also be affected by other influences such as the presence of organic acids which can reduce metal adsorption by competitive adsorption and by complexation (Tipping and Cooke, 1982; Rashid, 1985; Xu et al., 1988; Bowell, 1994). Although dissolved organic acids were not measured in this study, other authors have commented on the presence of 'peaty waters' in the mines of the St Just area (Noall, 1972). Also humic substances have been recorded and are known to be present in seawater (Rashid, 1985).

Iron geochemistry in the mine waters Iron is the principal cation in the sulphide lodes at Levant. The speciation of Fe is dependent on redox potential and pH and was estimated, from measured water chemistry data, thermodynamic relationships based on the Nernst equation held by the geochemical code HSC (Roine, 1992) and activities given in Table 6. For much of the E h - p H field measured at

Levant, pyrite is the stable phase in the F e - S - H 2 0 system. However, in waters of higher Eh/pH Fe 3+ or Fe(OH)3 are more stable. A n experimental study has shown that iron can be precipitated at low pH to yield ferrihydrite and ferric oxyhydroxide sulphates (Bigham et al., 1990). With time these phases transform to the more stable phases goethite, lepidocrocite and jarosite. A t higher pH (pH > 7), hematite is more stable than the ferric hydroxides. The leaching of base cations from gangue or wallrock silicates increases pH leading to precipitation of the ferric hydroxysulphate jarosite. Goethite is ubiquitous in the ochres at Levant and the high adsorption capacity of this mineral would explain the high concentrations of Zn, Cu and Mn sorbed by the ochres over a wide E h - p H range.

Sulphur geochemistry in the mine waters The oxidation of sulphides can be thought of as the production of sulphate and the generation of sulphuric acid (Nordstrom, 1982a). The stable sulphur species present at Levant are SO ] - , H2S, HSO~- and elemental sulphur. The stability field of sulphate encompasses that of the metastable species, thiosulphate, sulphite and polythionite, all of which are potential intermediate species in the conversion of sulphide to sulphate (Fig. 10). The reaction kinetics of the oxidation of pyrite has been reported to be fractional to first order, with respect to oxygen (Lowson, 1982; McKibben and Barnes, 1986). The wide range of reported activation energies, from 39 to 88 kJ/mol over a temperature range of 25-250°C, further reinforces the concept that oxidation occurs through a series of bond making and breaking steps. Although only transient, these intermediate species are present and have been recorded in the Levant mine waters. Of these, the most important are thiosulphate and sulphite, which can be produced by alkaline hydrolysis of pyrite at pH 7-9 (Goldhaber, 1983). These experimental data on pyrite oxidation agree with the field data reported here (Fig. 6). It is Eh (Volts)

S - H 2 0 - System at 25,00 C

5.0

.

.

.

.

.

.

.

.

.

.

~2UI-1.5

1.0

Table 6. Ion activities used to construct Figs 10-11 and 13-14. All activities were calculated using the mean dissolved (<0.45 pro) concentration of each species for all mine waters analysed

247

HSO, 1

0.5

[

SO,?

0.0 -0.5

Species

Activity coefficient

Calculated activity -i.0

S Fe As A1 Cu CI

0.66 0.74 0.99 0.74 0.57 0.90

2.27 x 5.37 × 1.16 × 2.41 × 1.51 × 5.17 ×

10-z 10-3 10-6 10-3 10-3 10-2

H2S

HS

~

-1.5 -2.0

i

i 2

i

i 4

i

i 6

L

i 8

I

L - 10

i

i 12

Fig. 10. Eh-pH diagram for the system S-O-H.

S~ 14 pH

248

R.J. Bowell and I. Bruce

worth noting that phototropic S-bacteria and colourless S-bacteria, such as Thiobacilli readily reduce sulphur species and may also participate in sulphide oxidation at Levant (Lundgren and Silver, 1980; Nordstrom, 1982a).

waters (due to sea water infiltration) could explain the change in behaviour. However, further experimental work would be necessary to confirm this.

Copper geochemistry in the mine waters Arsenic geochemistry in the mine waters

The distribution and speciation of As in the mine waters and ochres is controlled by the oxidation of arsenopyrite and tennantite and the subsequent behaviour of the oxyanion, arsenate. In this study the only arsenic species identified in the groundwater was arsenate, although arsenite and organoarsenides were also sought. Both arsenite and organoarsenides have been reported from other areas of acid mine drainage in Cornwall and elsewhere (Haswell et al., 1985; Bowell et al., 1994). The low As concentrations in solution and high concentrations in the ochres at low pH is due to adsorption of arsenate by Fe oxyhydroxides. In the less acidic solutions arsenate is not as strongly adsorbed by the ochres but the concentration in solution is still low. This is probably due to the precipitation of metal arsenates (such as olivenite, cornubite, scorodite and cornwallite).

Aluminium geochemistry in the mine waters

Initial oxidation of copper sulphides occurs with the replacement of bornite, chalcopyrite and tennantite, to covellite, copper, cuprite, copper arsenates, chlorides and sulphates either directly or via chalcocite. But it should be noted that chalcocite is most likely a late-stage hypogene phase and not formed by supergene processes. Thermodynamic predictions for the Levant mine waters suggest that copper sulphides, except covellite, are unstable (Fig. 12). In acidic oxidizing environments, only Cu 2+ was found, but with a slight pH increase then cuprite and copper are formed, which are stable with Fe oxyhydroxides. Antlerite and brochantite are also precipitated where pH and sulphate concentration are favourable (Fig. 13). From the stability relationships of the principal copper sulphates (Pollard et al., 1992) it can be observed that chalcanthite would not be stable in the

Eh(Volts)

Aluminium occurs in solution due to leaching of gangue silicates and wallrock by acid mine drainage. The concentration of A1 in low pH waters may be controlled by aluminium sulphate minerals, such as alunite (Nordstrom, 1982b). Below pH 4 (Fig. 11) a case could be made that the species A1SO4(OH ) (jurbanite) controls A1 solubility although the only secondary A1 minerals observed in this study, associated with mine drainage, were smectite, kaolinite, devilline and woodwardite. Above pH 4 the relationship is lost, which is also observed in other studies (Dubrovsky et al., 1984; Blowes and Jambor, 1990). The high concentration of CI, Br and I in the ground-

I~.

Cu2 '"

I.O 0.5

C

~

~

0.0

........

-0.5

_~

-1.6 -1~

CuFeS2

-2.0

2

4

6

8

10

12

14 pH

Eh(Volts) 2.0

-2.2 • -2.4-

•

•

D.51'OI~

I~

CuO

-2.6 -

t~

-2.8-

9.0 r

~

~

-3-

am

4.2-

~

1.0 Cu~S2 1.5

-3.6

2

,

3

,

4

,

i

1.0

5

6

7

8

pH

Fig. 11. aAj VSpH for Levant mine waters (diagram construction from Nordstrom 1982b).

0

2

4

6

8

10

12

14 pH

Fig. 12. Eh-pH diagram for the (a) Cu-Fe-S-O-H system and (b) Cu-Fe-S-CI-O-H system.

Ochres and mine waters, Levant mine, Cornwall -1

•

•

Brochantite

Antlefite

• ~

A•

Tenmite

•

{-2 ~

A

0

i

i

3

4

s

e

7

OH

Fig. 13. aso 4vs pH showing stability fields of the main basic copper(II) sulphates (after Pollard et al., 1992).

aso;-pH range of the Levant mine waters but antlerite, brochantite and tenorite (an oxide) would (Fig, 13). Where sulphate content is very high, such as at a sea water breach a wide range of copper sulphates, not necessarily in equilibrium, can be formed (Bowell, 1992). The presence of atacamite, botallackite, and paratacamite in the seawater breaches implies a high a a - n e u t r a l pH of the precipitating liquor and that precipitation was very recent, since with time paratacamite is the most stable polymorph and the other polymorphs will reorder to paratacamite, the most stable form (Pollard et al., 1989).

CONCLUSIONS Sulphide oxidation has occurred at the Levant mine due to the destabilization of the sulphides in the presence of dissolved oxygen and water. The composition and quantity of the sulphides in the lodes, the nature of gangue phases and textural associations, and groundwater chemistry have influenced the nature of oxidation and the characteristics of the alteration assemblages. In low pH mine waters, produced by anodic reactions, positively charged base metal ions are soluble while oxyanions, such as arsenate, are adsorbed onto mineral surfaces. At higher pH due to a change in the charge of the Helmholtz layer at the mineral surfaces, base metal adsorption is possible along with precipitation of metal salts, such as olivenite. A major control on solution pH is the extent of pyrite oxidation and Fe hydrolysis to form Fe ochres. Sulphate is the most common S species with lesser amounts of metastable species, dependent on solution pH, Eh and time at ambient temperatures. Copper geochemistry in the waters is influenced by the release of Cu from sulphides, formation of stable species in solution and precipitation or adsorption of Cu in response to a change in solution chemistry. As a result of coupled reaction between Fe metal and dissolved Cu, cuprite and copper pseudomorphs of steel mining implements have been formed as well as pseudo-

249

morphs of iron sulphides. Organic matter in wooden pillar supports can also reduce Cu in solution leading to the precipitation of copper and cuprite on wood. The extent of environmental contamination from the mine will depend on the neutralizing effect of surface waters, the stability of metal-rich Fe ochres, and the amounts and forms of metals released. The precipitation of iron oxyhydroxides, at pH above 4, leads to co-precipitation of other metals, including As, Sb, Cd, Co, Pb, Mn, Mo, Ag and Zn thus reducing metal concentrations in solution. However. the release of suspended metal-rich ochres in mine discharge waters (with high As, Pb, Zn, Cd, Mn, Co and Ag) presents a potential environmental hazard as will subsequent reaction of the ochres with acidic waters. Acknowledgements--M. Mount and D. Hudson (Geevor mine) are thanked for assistance with fieldwork and with information on the Levant mine. Vic Din is thanked for assistance with chemical analysis at the Natural History Museum. X-ray diffraction measurements were made by J. Francis and T. Greenwood of the Natural History Museum and M. Gill of Imperial College. The manuscript was greatly improved by discussion and review by R.F. Symes, A. Clark, and R. Fuge, D, Runnells, and another referee. Editorial handling: Ron Fuge.

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