Journal of Petrology

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Journal of Petrology Volume 41 Number 9 Pages 1439-1453 2000
© Oxford University Press 2000

Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England

B. J. WILLIAMSON^1,*, J. SPRATT², J. T. ADAMS², A. G. TINDLE³ and C. J. STANLEY²

¹DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, WILLS MEMORIAL BUILDING, QUEEN’S ROAD, BRISTOL BS8 1RJ, UK
²DEPARTMENT OF MINERALOGY, THE NATURAL HISTORY MUSEUM, CROMWELL ROAD, LONDON SW7 5BD, UK
³DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, MILTON KEYNES MK7 6AA, UK

Received January 26, 1999; Revised typescript accepted February 7, 2000

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY ANALYTICAL TECHNIQUES TOURMALINE CHEMISTRY DISCUSSION CONCLUSIONS AND WIDER... REFERENCES

 
Hydrothermal fluid evolution north of the St Austell granite, southwest England, has been studied through geochemical analysis of tourmaline from a fault breccia of <2 cm width within massive quartz–tourmaline rocks at Roche. Brecciated tourmaline grains have overgrowths of <400 µm width [Fe/(Fe + Mg) = 0·31–0·99] with four chemically distinct zones (1–4, towards the margins). Variations in overgrowth composition were caused by episodic mixing between Mg-, Al-rich magmatic hydrothermal fluids (dominant in zone 1), with an increasing component of more oxidizing, Fe-rich formation waters (zones 2 and 4). More oxidizing conditions are supported by high Sn contents in zone 2 (<0·35 wt %), with Sn probably present as Sn⁴⁺ rather than Sn²⁺, the usual form in hydrothermal fluids. From X-ray maps, zones 1 and 3 occur exclusively as overgrowths on pre-existing grains, indicating that overgrowth formation was kinetically favoured over tourmaline nucleation. In zones 2 and 4, nucleation and growth occurred, possibly as a result of supersaturation with respect to tourmaline during increased mixing with formation waters. Tourmaline is associated with the main episode of mineralization in many important mineral deposits, often unaffected by alteration. This method of studying hydrothermal fluid evolution may therefore have uses in exploration, particularly for tourmaline-breccia-hosted ores in Cu-porphyry deposits.

KEY WORDS: breccia; Cornwall; hydrothermal; tin; tourmaline

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY ANALYTICAL TECHNIQUES TOURMALINE CHEMISTRY DISCUSSION CONCLUSIONS AND WIDER... REFERENCES

 
The nature and origin of hydrothermal fluids responsible for tin–tungsten mineralization in SW England have been the subject of numerous mineralogical (e.g. Farmer et al., 1991; London & Manning, 1995; Smith & Yardley, 1996; Williamson et al., 1997) and fluid inclusion studies (Jackson et al., 1982; Alderton & Rankin, 1983; Shepherd et al., 1985; Bottrell & Yardley, 1988; Wilkinson et al., 1994, 1995; Smith et al., 1996; Smith & Yardley, 1996). However, there are many aspects of their evolution that remain poorly understood. One of the most crucial of these is the source and behaviour of elements involved in mineralization, particularly whether they had a magmatic source or were scavenged from crustal rocks by convective circulation of magmatic hydrothermal fluids and/or formation waters. What is also poorly understood is the strong association between tourmalinization and cassiterite mineralization. No direct role has been identified for B and F in the formation of the Sn deposits (Pollard et al., 1987; Taylor & Wall, 1993) although it has been suggested that their presence in granite magmas lowers the solidus temperature to 650°C (Pichavant & Manning, 1984), allowing time for more extensive fractionation and partitioning of ore-forming elements into hydrothermal fluids (Pollard et al., 1987).

Tourmaline is a particularly useful mineral in the study of ore-forming fluids as it occurs in a wide variety of ore deposits (e.g. Slack, 1982, 1996; Slack et al., 1993; London & Manning, 1995; Griffin et al., 1996). It is also stable during most types of weathering processes and potentially up to upper amphibolite facies [reviewed by Henry & Dutrow (1996)], limiting problems with later alteration, which can be extremely extensive in mineral deposits. In addition, it has a complex chemistry, with a number of different site occupancies (e.g. Burt, 1989; London & Manning, 1995; Hawthorne, 1996; Jiang et al., 1998), which can accommodate a chemically varied suite of ore-forming elements. Its trace element composition may therefore provide a tool in exploration [see review by Slack (1996)]. This was recently demonstrated by a world-wide study of massive sulphide deposits, which identified a good correlation between base metal proportions in tourmaline and the bulk composition of ore being mined (Griffin et al., 1996).

There have been several previous studies of tourmaline in SW England. These have mainly focused on the discrimination of different genetic environments using ‘major element’ variations (Manning, 1991; London & Manning, 1995) and on the source of B using B isotopes (Smith & Yardley, 1996). There have also been useful trace element studies on tourmaline from different settings in SW England. Li concentrations in tourmaline (and other minerals) from granites and a wide variety of other rock types [including Roche massive quartz–tourmaline (MQT) rock] have been determined by ion microprobe (Wilson & Long, 1983). Power (1968) has provided trace element data (including Sn) for a similar variety of rocks.

There have, however, been no detailed studies on major and trace element variations within zoned hydrothermal tourmalines and the bearing of this on the evolution of ore-forming fluids. Hydrothermal breccias at Roche are ideal for such a study as they formed within MQT rocks, which consist almost entirely of quartz and tourmaline, the latter showing strong mechanical and chemical stability (Slack, 1982; Henry & Dutrow, 1996). This will have greatly limited the extent of wall-rock reaction, which may otherwise have altered the chemical and physical nature of the hydrothermal fluids. The compositions of hydrothermal vein minerals in the breccia are therefore likely to yield valuable information on the nature of the fluids from which they formed.

	GEOLOGICAL SETTING

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY ANALYTICAL TECHNIQUES TOURMALINE CHEMISTRY DISCUSSION CONCLUSIONS AND WIDER... REFERENCES

 
Roche Rock forms an isolated stock of MQT rock within Lower Devonian, mainly calcareous slate, Meadfoot Bed country rocks (Fig. 1). It lies 500 m north of the northern margin of the St Austell granite, the smallest of the five main apophyses of the Hercynian batholith of SW England. The St Austell granite is a highly complex body consisting of six main granite types, many of which show extensive late magmatic and hydrothermal alteration (Manning et al., 1996). Granite magmatism in the area occurred over an extended time period. The main biotite granites have yielded an age, by Rb–Sr whole-rock isochron, of 285 ± 4 Ma and the Brannel elvan, a nearby late granite porphyry dyke, an age of 270 ± 9 Ma (Darbyshire & Shepherd, 1985). Present outcrop is considered to have been close to the roof of the intrusion as indicated by the presence of numerous pegmatites occurring as sheets and containing abundant miarolitic cavities carrying quartz, tourmaline, zinnwaldite, topaz and a wide range of other phases (Manning, 1983; Weiss, 1989). The St Austell granite contains abundant cross-cutting tourmaline fissure veins and stockworks, which are often mineralized, containing amongst others cassiterite, stannite and wolframite. The veins are commonly bordered by greisens of <10 cm width, which have been dated in the Goonbarrow pit, 2 km SE of Roche, at 273·6 ± 1·1 Ma (K–Ar, Bray & Spooner, 1983) and 276·8 ± 1·2 Ma (⁴⁰Ar/³⁹Ar, Chesley et al., 1993).

View larger version (27K): [in this window] [in a new window]   Fig. 1. Map showing the location of Roche Rock and the main granite varieties of the St Austell granite [after Exley & Stone (1982) and Manning et al. (1996)].

 

MQT rocks mainly crop out at the margins of the western part of the St Austell granite. In the past, probably from observations made in long since disused underground workings, these bodies have been described as forming elongate, tin-bearing stripes proceeding from the main mass of St Austell granite (De La Beche, 1839; Collins, 1878). Today, their contacts with the granites and surrounding country rocks cannot be seen. The origin of MQT rocks, and particularly whether they formed from highly evolved silicate liquids or magmatic hydrothermal fluids, remains problematic despite numerous studies (see London & Manning, 1995). Badham (1980) pointed out the lack of replacement textures in the MQT rocks and therefore suggested that they crystallized from a quartz–tourmaline magma that rose above the roof of the main mass of granite. Charoy (1982) considered that MQT rock is undoubtedly of magmatic origin and formed by unmixing of a phase rich in B, alkalis and silica. Smith & Yardley (1996) came to a similar conclusion from B isotope studies, that the formation of MQT rocks was due to crystallization of volatiles produced by the degassing of granite silicate liquids. There is therefore little argument that MQT rocks formed from magmatic ‘fluids’ of some kind, rather than from formation waters. This is supported by the MQT rock tourmaline being more similar in composition to magmatic tourmaline from SW England rather than breccia and replacement deposit tourmaline in the country rocks (London & Manning, 1995).

The hydrothermal breccia being studied forms a narrow (<2 cm width) vein in a large boulder of MQT rock at the southwestern margin of Roche Rock. The MQT rock immediately around the breccia is generally tourmaline rich (70%) and contains a number of narrow ‘quartz’ veins with <5% acicular tourmaline. From the elongate nature of the breccia, it is fault related rather than representing an intrusive hydrothermal body, such as those at Wheal Remfry, in the western part of the St Austell granite (Allman-Ward et al., 1982, fig. 1).

	PETROGRAPHY

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The breccias contain 50–90% angular, fractured tourmaline grains and abundant cavities, up to 0·5 cm in diameter, with tourmaline extending into the voids. The tourmaline shows no embayments or other textures that may indicate partial resorption, and often has concentrically zoned hydrothermal overgrowths. The overgrowths, which are developed parallel to the c-axis and almost exclusively at one end of each tourmaline grain, are dark blue–purple () to light blue or pale tan () (Fig. 2a and f). The outermost part of the overgrowths commonly consists of fine (<10 µm width) feathery fibres (Fig. 2). Tourmaline also occurs as fine-grained needles and sprays of feathery crystallites throughout the quartz matrix. The angular tourmaline fragments commonly contain rounded inclusions of quartz and more rarely inclusions of zircon, apatite and monazite.

View larger version (68K): [in this window] [in a new window]   Fig. 2. (a) Photomicrograph in plane-polarized light of brecciated tourmaline (light brown cores with pale blue rims) with light blue to black zoned overgrowths. T, tourmaline; Qz, quartz. The matrix is quartz containing fine tourmaline needles and abundant fluid inclusions. (b–e) X-ray element maps for Mg, Fe, Sn and F for the same field of view as in (a). The map for Sn is overlaid (in white) with the outline of four discrete zones in the overgrowths (1–4) and the brecciated MQT tourmaline (0) determined from the maps for Fe, Mg and F. (f) Photomicrograph in plane-polarized light for a second area of the breccia showing a brecciated tourmaline grain with secondary hydrothermal overgrowth. (g, h) X-ray element maps for Fe and Sn for the field of view in (f) and an overlay for the different zones on the map for Sn.

 

Quartz in the breccia is granoblastic and considerably more coarse grained (2–3 mm) than the tourmaline. Large, optically continuous crystals commonly enclose a number of broken tourmaline crystals. In tourmaline-rich areas, quartz occasionally forms narrow veins (<100 µm width), which cross-cut tourmaline aggregates and commonly contain acicular tourmalines. The quartz contains abundant fluid and mineral inclusions, the latter being mainly haematite, and more rarely zircon, monazite and apatite. Many of the fluid inclusions contain halite crystals and fluids associated with the formation of the breccias are, therefore, likely to have been relatively saline. Fluid inclusion studies on Sn–W veins from SW England by Alderton & Harmon (1991) indicated moderate salinities of between 5 and 15 wt % NaCl equivalent, with homogenization temperatures of between 230 and 400°C. More detailed fluid inclusion studies on the breccias could not be carried out because when the quartz was viewed in cathodoluminescence, it was found to be made up of a complex ‘mosaic’ of angular (brecciated) fragments cemented by several generations of quartz. It would therefore be extremely difficult to identify fluid inclusions in quartz cement, formed during the brecciation process, from those in the breccia tourmaline inherited from the MQT rocks.

The MQT rocks at Roche are for the most part granoblastic and homogeneous. They are medium grained (<1 to 3 mm) and consist almost entirely of tourmaline (30–70%) and quartz. They show very little in the way of internal structure apart from the presence of rare, dark, commonly elongate, tourmaline-rich patches, ‘quartz veins’ (containing <10% tourmaline) and numerous tourmaline- and quartz-lined vugs between <500 µm and >50 cm in diameter. Tourmaline from the MQT rocks commonly has cores showing relatively strong pleochroism from dark brown () to pale tan–yellow () with rims that are mid-blue () to light blue–colourless (). Some finer-grained tourmaline is entirely pale blue () to colourless ().

	ANALYTICAL TECHNIQUES

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY ANALYTICAL TECHNIQUES TOURMALINE CHEMISTRY DISCUSSION CONCLUSIONS AND WIDER... REFERENCES

 
Quantitative microprobe analysis was carried out using an automated WDS Cameca SX50 microprobe with an accelerating voltage of 20 kV, a beam current of 25 nA and a spot size of 3 µm, identical conditions to those used by London & Manning (1995). The following standards were used: jadeite for Na, wollastonite for Si and Ca, olivine for Mg and Fe, pure manganese for Mn, rutile for Ti, corundum for Al, synthetic KBr for K, synthetic BaF for F, synthetic cassiterite for Sn. Detection limits (3 SD) were <0·03 wt % (except for F, <0·1 wt %). Two traverses, parallel to the c-axis, were carried out across the contact of MQT fragments and secondary overgrowths. The positions of the traverses are shown in Fig. 2. Traverse 1 contains 25 points (length 190 µm) compared with 49 points (length 284 µm) in traverse 2, the latter being considerably more detailed. The data were recalculated to atoms per formula unit (a.p.f.u.), based on 24·5 O, which assumes fixed B at 3 a.p.f.u. and fixed (OH, F) at 4 a.p.f.u. The data have been plotted on tourmaline ‘discrimination diagrams’ in Fig. 3, and with distance along each traverse in Fig. 4. Data for individual points from traverse 1, with an average for the MQT tourmaline, are presented in Table 1. The recalculation procedure used produced slightly high values (<6·1) for Si, above the maximum T site occupancy of 6 a.p.f.u., mainly in zones 2 and 4 (especially for traverse 2). Extensive checks were carried out to demonstrate that this was not an analytical problem, as is also evident from its effects being strongest in certain zones. The high Si values are therefore considered to be an artefact of the recalculation procedure used to express the data in formula units, particularly the assumptions of total Fe as Fe²⁺, (OH, F) fixed at 4 a.p.f.u. and B fixed at 3 a.p.f.u., although the latter seems likely from recent structural refinement studies (Bloodaxe et al., 1999).

View larger version (36K): [in this window] [in a new window]   Fig. 3. Variation diagrams for points in traverses 1 and 2 (except MQT rocks, which are shown as averages for each traverse) of (a) Mg/(Fe + Mg) vs Xvac/(Na + Xvac), (b) Fe vs Mg, (c) Al vs Fe and (d) Fe vs Xvac (in a.p.f.u.). Xvac (X vacancy) = 1 - (Na + K + Ca). Exchange vector arrows and reference lines important in this study are indicated (, vacancy). Also shown are data from SW England for tourmalines from granites and pegmatites, MQT rocks, hydrothermal veins, breccias within granites, and replacements and breccias in country rocks [from London & Manning (1995)].

 

View larger version (37K): [in this window] [in a new window]   Fig. 4. (a, b) Microprobe traverses from within the MQT tourmaline and across the overgrowths. The locations of traverses 1 and 2 are shown in Fig. 2. The units on the left-hand axis are in a.p.f.u.

 

View this table: [in this window] [in a new window]   Table 1: Tourmaline compositions for each point in traverse 1 (1–18 towards the margin) and an average for the MQT tourmaline (point 0)

 

X-ray element maps were generated for small (0·5 mm²) areas of the sample surface using a moving stage rastering program on an automated Cameca SX50 microprobe. The maps were obtained using an accelerating voltage of 15 kV, a beam current of 100 nA, a spot size of 1 µm, a spot interval of 2 µm and a count time of 200 ms per spot. Counts per second gain for each element were measured and plotted on separate maps using a colour scale, shown in the right-hand margin, from black to red representing low to high counts, respectively. X-ray element maps for Mg, Fe, Sn and F are shown in Fig. 2.

	TOURMALINE CHEMISTRY

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Tourmaline chemistry is complex with the possibility for a wide range of substitutions (e.g. Burt, 1989; London & Manning, 1995; Hawthorne, 1996). A great deal of research has been undertaken on tourmaline structure and chemistry, which has recently culminated in a redefined chemical classification scheme (Hawthorne & Henry, 1999). The basic formula for tourmaline is: XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W with common site occupancies being:

• X = Ca, Na, K or vacant;
  
• Y = Li, Fe²⁺, Mg, Mn, Al, Cr³⁺, Fe³⁺, V³⁺, Ti⁴⁺;
  
• Z = Mg, Al, Fe³⁺, V³⁺, Cr³⁺;
  
• T = Si, Al, B;
  
• B = B, (vacant);
  
• V = OH, O;
  
• W = OH, F, O.
  

The chemistry of tourmaline from the breccia is, however, ‘relatively simple’ because of its restricted contents (in a.p.f.u.) of Ca <0·06, K <0·01, Mn <0·04 and Ti <0·06 (Table 1). The main end-member compositions between which the tourmalines in this study may fall have the general formula XY₃Z₆(Si₆O₁₈)(BO₃)₃V₃W with [after Hawthorne & Henry (1999)]:

View this table: [in this window] [in a new window]

 
For ease of description of chemical variations in the tourmalines studied, the overgrowths have been divided into four zones and the MQT tourmaline has been described separately. The overgrowths can be clearly distinguished from the MQT tourmaline on the X-ray element maps from their much lower F contents and from their strong chemical zonation [Fe/(Fe + Mg) = 0·31–0·99]. The zones were delimited on the basis of broad geochemical trends, identified from the X-ray element maps (Fig. 2), and their outlines have been overlaid on the X-ray element maps for Sn (Fig. 2d and h). They have also been given different symbols in the variation diagrams in Fig. 3 and are marked on the traverses in Fig. 4.

In the Xvac vs Mg/(Fe + Mg) discrimination diagram in Fig. 3a, the MQT tourmalines lie just within the Schorl field, close to the boundary with Foitite. Because of their high F contents (>0·5 a.p.f.u.), they may be described as F-Schorl (D. J. Henry, personal communication, 1999). The MQT tourmalines show a limited range in compositions [Fe/(Fe + Mg) = 0·92–0·99], similar to MQT tourmaline from Roche Rock, which has a mean value of 0·937 ± 0·043 (3 SD; London & Manning, 1995). Their high Fe/(Fe + Mg), F and Al are more similar to magmatic than hydrothermal tourmaline in the Cornubian batholith and are very different from tourmaline formed in replacement deposits and breccias in the country rock (Fig. 3).

Zone 1 contains considerably higher Mg and lower F compared with the MQT tourmaline. From a comparison of Figs 2, 3 and 4, the inner part of zone 1 (two points in traverse 1 and one point in traverse 2) is particularly Mg rich, and lies within the Dravite field of Fig. 3a. The remainder of zone 1 straddles the Mg-Foitite, Foitite and Schorl fields, showing a poorly defined trend of decreasing Mg/(Fe + Mg) and Xvac in the direction of the exchange vectors (Fe²⁺Fe³⁺)(MgAl)_-1, (NaFe²⁺)(Al)_-1 or (Fe)(Mg)_-1, or more probably a combination of these vectors (Fig. 3c and d). Zones 2, 3 and 4 fall within the Schorl field with generally lower Xvac and Mg/(Fe + Mg). From zone 2 to 3 there is a marked increase in Xvac and Al and a slight increase in Mg/(Fe + Mg), indicating the substitution (Al)(NaFe²⁺)_-1. The trend within zone 3 is parallel to the exchange vector (Al)(NaMg)_-1 (Fig. 3c and d). Zone 4 shows higher Fe and lower Al compared with zone 3 and similar Al, Fe and Mg values to zone 2. The main difference between zone 2 and 4 is the considerably higher Xvac in zone 4. Because of the similarities in Al, Mg and Fe contents, this cannot be explained by the substitution (Al)(NaMg)_-1 or (Al)(NaFe²⁺)_-1 and is more likely to be due to (Fe³⁺)(NaFe²⁺)_-1.

Li contents in the tourmaline have been estimated using the technique of Burns et al. (1994) (see Table 1). Calculated Li values are relatively low, with Li₂O <0·53 wt % (<1231 ppm) in the hydrothermal overgrowths. This is consistent with the hundred to several thousands of ppm Li found during ion probe studies of tourmaline from Cornish hydrothermal breccias (Wilson & Long, 1983).

Zones 2 and 4 contain high Fe (>3 a.p.f.u.) and low Mg (<0·5 a.p.f.u.), and lie above the Schorl–Dravite vector in Fig. 3b. The Y site has a maximum occupancy of 3 a.p.f.u. and the ‘excess’ Fe must therefore be contained within a different site. It is highly unlikely to substitute into the X site, in nine-fold co-ordination (Henry & Dutrow, 1996), as its ionic radius is too small and because Fe increases with Na towards the maximum Na site occupancy in X of 1 a.p.f.u. (Table 1). The most likely site for the ‘excess’ Fe is as Fe³⁺ in Z, because of the substitution FeAl_-1 (London & Manning, 1995; Dyar et al., 1998). This is supported by the most Fe-rich compositions showing proportionately large negative values for Al in the Y site (Fig. 4), which indicates a ‘deficit’ of Al in Z allowing the substitution of Fe³⁺ (London & Manning, 1995). It is also possible that a component of Fe in Y is contained as Fe³⁺ towards the composition of Buergerite.

Tourmaline from the breccia and MQT tourmaline were also analysed for a variety of ore-forming elements, Sn, W and Cu. The only element to be found above detection limits (dl) was Sn (<0·35 wt %; dl = 0·03 wt %), and this was only in the overgrowths. Both the X-ray element maps and traverses show significant concentrations of Sn in zone 2 and to a lesser extent in zones 1 and 4. The homogeneous distribution of Sn throughout zone 2 is likely to indicate that the Sn is ‘structurally bound’ rather than being contained as discrete mineral or amorphous inclusions.

A comparison of the photomicrograph in plane-polarized light (Fig. 2a and f) with the X-ray element maps shows that there is some correspondence between the chemical composition of the tourmaline and its colour, although this is not always the case in tourmaline from SW England (London & Manning, 1995). The more Fe-rich zones are dark blue and the more Mg-rich zones are pale blue. The brecciated MQT tourmalines have high F and are generally colourless–pale blue with light brown cores.

	DISCUSSION

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Constraints on fluid evolution from tourmaline chemistry
The relative contributions from magmatic waters, defined as having equilibrated with a silicate melt (Bottrell & Yardley, 1988), and formation waters in hydrothermal fluids are notoriously difficult to establish (Manning, 1985). A dominantly magmatic source for early mineralizing fluids in SW England is indicated by the close spatial and temporal association of mineralized veins to granite plutons (Jackson, 1979; Bray & Spooner, 1983; Chen et al., 1993) and from the high concentrations of transition metal chlorides in fluid inclusions from tourmaline–topaz–quartz rocks at St Mewan Beacon (Fig. 1; Bottrell & Yardley, 1988). Fluid inclusion and isotopic studies suggest that Sn–W deposits at Cligga Head, Cornwall, were largely derived from magmatic fluids (Smith et al., 1996). Boron in mineralizing fluids is also likely to have had a dominantly magmatic source, as indicated from B isotope studies of tourmalinized veins at Cligga Head, 30 km west of Roche (Smith & Yardley, 1996). A contribution from formation waters is, however, difficult to disprove, as they can commonly have B contents up to 300 ppm (Leeman & Sisson, 1996), or even higher if associated with evaporites, although these have not been found in the local country rocks (Smith & Yardley, 1996).

Evidence for a magmatic component during the initial formation of zone 1 is the relatively high levels of Al (Table 1, Fig. 4). Al is particularly soluble in low-pH (May et al., 1979), B- (Li + F)-rich aqueous fluids (Morgan & London, 1989). This type of fluid is likely to be of magmatic origin in the Roche area as formation waters will have originated in, or at least passed through, local country rock calcareous slates. Al is considered to be largely immobile in ‘typical’ aqueous crustal fluids [Al_(aq) = 5 x 10^-5 to 10^-4 m, at 400–700°C and 1 kbar; Ragnarsdottir & Walther, 1985]. There is no source of Al in the MQT rocks as MQT tourmaline, the only major local Al-bearing phase, shows no embayments or other resorption textures. Under normal circumstances, for tourmaline formed within the granites or country rocks, tourmaline will have derived its Al from the breakdown of local minerals such as feldspar and micas.

Further evidence for a magmatic fluid component is from the relatively reduced nature of the hydrothermal fluids during the initial formation of zone 1. The change in composition from the inner to outer part of zone 1 is along the exchange vector (Al)(NaMg)_-1, combined with a more minor component of (Fe)(Mg)_-1, with no evidence for substitutions involving Fe³⁺. Magmatic waters will have been relatively reducing as the granites from which they were derived show low magnetite abundance, low magnetic susceptibility (1 x 10^-5 e.m.u./g; Willis-Richards & Jackson, 1989) and have been classified as belonging to the ilmenite series of Ishihara (1977). This argument is supported by the presence of detectable concentrations of Sn in the inner part of zone 1 (0·06 wt %, Fig. 4). Sn transport in aqueous fluids requires low pH and reducing conditions (Taylor & Wall, 1993), far more likely for magmatic than formation fluids, the latter having originated in or passed through calcareous slate country rocks.

Substantial chemical changes in the hydrothermal fluid are reflected by variations in tourmaline composition through zones 1–4. The main trend in zone 1 is parallel to one or a combination of the exchange vectors (Fe²⁺Fe³⁺)(MgAl)_-1, (NaFe)(Al)_-1 or (Fe)(Mg)_-1 (Fig. 3c and d). The field for zone 2 lies roughly along the trend of zone 1, but is offset to higher Fe and lower Xvac. Within zone 2, there is a poorly defined trend, roughly in the direction of the exchange vector (NaFe²⁺)(Al)_-1. Fe contents in zone 2 exceed the maximum that can be accommodated in the tourmaline Y site (3 a.p.f.u.). From the proportionate decrease in Al in Z (Fig. 4), it is probable that the ‘excess’ Fe is accommodated in the Z site as Fe³⁺, indicating relatively oxidizing conditions during the formation of zone 2. Slightly higher Mg contents in zone 3 are probably due to the substitution (MgAl)(Fe²⁺Fe³⁺)_-1 whereas the trend within zone 3 is parallel with the exchange vector (Al)(NaMg)_-1. Zone 4 shows almost identical compositions to zone 2, in terms of Fe, Mg and Al, but with a considerably higher Xvac. This is likely to be due to the substitution (Fe³⁺)(NaFe²⁺)_-1, in addition to (Fe²⁺Fe³⁺)(MgAl)_-1, which caused the main compositional shift from zone 3 to 4. This may be indicating even more oxidizing conditions in zone 4 than in zone 2. Relatively oxidizing conditions are indicated from the presence of haematite in matrix quartz, which is likely to have formed contemporaneously with, or soon after, zone 4.

The more oxidizing conditions could have been caused by either mixing with more oxidizing fluids or boiling. The latter was suggested for the formation of tourmalines under relatively oxidizing conditions in the Larderello Geothermal Field, Italy (Cavarretta & Puxeddu, 1990). Boiling causes the loss of H into the vapour phase and the resulting oxidation of Fe in the aqueous phase (Drummond & Ohmoto, 1985). However, boiling is considered unlikely to have caused the apparent increase in fO₂ because F concentrations in the overgrowths increase in zones 2 and 4 (Table 1). This is the reverse of what would be expected during boiling, as, above 250°C, HF is preferentially partitioned into the vapour phase (Reed & Spycher, 1985; Smith, 1995).

The increase in Fe and apparent increase in Fe³⁺/Fe²⁺ through zone 1, which peak in zones 2 and 4, suggest open-system behaviour during breccia formation, with an episodic but generally increasing contribution from relatively oxidizing formation waters. A significant component of formation waters in the hydrothermal fluids of SW England has been suggested from numerous stable isotope determinations and fluid inclusion studies (e.g. Jackson et al., 1982; Wilkinson, 1990; Alderton & Harmon, 1991; Wilkinson et al., 1995).

One of the limiting factors for the formation of tourmaline in the breccia may have been a decrease in Al in the hydrothermal fluid, as indicated from its general decline through zones 1–4. From previous discussion (above) it is likely that formation waters were relatively depleted in Al and that magmatic fluids were the main source for this element. Another limiting factor may have been an increase in pH as a result of progressively higher degrees of mixing with formation waters from local calcareous slates. The amount of B required to stabilize tourmaline increases with increasing pH, with tourmaline becoming unstable at pH > 6·5–7 (Morgan & London, 1989). Other ‘ingredients’ for tourmaline formation, mainly Fe, Na, OH and Si, were abundant in the hydrothermal fluids at this time, as indicated from the presence of inclusions of haematite in matrix quartz and high-salinity fluid inclusions. High hydrothermal fluid Fe contents are evident in the St Austell area from the occurrence of bleached, biotite-depleted aureoles around hydrothermal quartz–tourmaline veins in biotite granites (London & Manning, 1995). Similar bleached zones are also commonly seen in country rock metasediments.

Open-system formation of hydrothermal tourmaline, involving mixing between magmatic (providing B, Al, Mg and Fe) and formation waters (probably providing further Fe), is in agreement with the findings of London & Manning (1995) from geochemical studies of many different types of tourmaline from across SW England.

Ore-carrying potential of the hydrothermal fluids
In a comprehensive microprobe study of tourmaline compositions from many different occurrences in Cornwall, none were found to contain Sn (London & Manning, 1995). Why Sn was found in the overgrowths is not clear. It may simply be due to the refractory nature of the MQT rocks in which the breccia formed, with there having been little cassiterite crystallization because of limited wall rock reaction. Alternatively, Sn concentrations in the hydrothermal fluid may have been below cassiterite saturation.

Sn concentrations show marked variations through the overgrowths. In zone 1, Sn shows oscillatory zoning from 0·09 to 0·18 wt %, at a scale of 20 µm, independent of any other element (Fig. 4, traverse 2). Zone 2 shows a peak for Sn of 0·35 wt %, which correlates with the highest peak for Fe. Zones 3 and 4 show a decrease in Sn to similar levels as in zone 1. The origin of the oscillatory zoning in zone 1 is uncertain, as this process can be caused by a number of mechanisms, which are often difficult to identify (e.g. Yardley et al., 1991; Lanzirotti, 1995), particularly in minerals formed in open systems [reviewed by Shore & Fowler (1996)]. These mechanisms are either extrinsic, caused by external changes such as episodic fluid flow, variations in pressure, temperature, pH or fO₂, or intrinsic, being caused by ‘feed-back between crystal growth and solute diffusion or surface effects as a source of non-linearity in the crystal-growth kinetics’ (Shore & Fowler, 1996). The effects of extrinsic mechanisms can be recognized between zones in the overgrowths as it is likely that each was formed from different mixtures of fluids. The oscillatory zoning for Sn in zone 1 is better explained by intrinsic mechanisms, as this is the only element to show such regular oscillatory trends (Fig. 4).

How Sn was incorporated within the tourmaline structure in zone 1 is problematic. From previous discussion, zone 1 (particularly the inner part) is likely to have been formed under relatively reducing conditions. The Sn was therefore probably contained in the hydrothermal fluid as Sn²⁺. Sn is dominantly thought to be transported as Sn²⁺ in low redox hydrothermal fluids responsible for the formation of Sn deposits (commonly in the form of chloride and hydroxy chloride complexes; Jackson & Helgeson, 1985; Wilson & Eugster, 1990; Taylor & Wall, 1993). Sn²⁺ has an extremely large ionic radius of 0·122 nm in eight co-ordination (Shannon & Prewitt, 1969), 0·093 nm in six co-ordination (see Taylor, 1979). It is therefore highly unlikely to substitute into an octahedral tourmaline site for Fe²⁺ (0·078 nm), Mg²⁺ (0·072 nm) or Al³⁺ (0·0535 nm) (six co-ordination; Shannon & Prewitt, 1969). The most likely substitution is within the X site, which is a nine co-ordinated trigonal antiprism located along the three-fold axis of symmetry (Henry & Dutrow, 1996). This site is usually occupied by Na (0·102 nm), Ca (0·10 nm), with minor K (0·138 nm), or can be vacant.

The increasing concentrations of Sn in zone 2 can be explained either by an increase in Sn²⁺ concentrations in the hydrothermal fluid or by more oxidizing conditions, with Sn⁴⁺ being more easily accommodated within the tourmaline structure than Sn²⁺. The first possibility is unlikely, as Sn in zone 2 is much higher than normally found in tourmaline from SW England or mineral deposits world-wide. Southwest England tourmaline was found to have maximum Sn contents of <500 ppm from emission spectrography studies on 48 samples by Power (1968). Proton microprobe studies of 32 samples of tourmalines from massive sulphide deposits and tourmalinites world-wide indicated Sn concentrations of <745 ppm (Griffin et al., 1996). Nemec (1973) found a maximum of 500 ppm in 80 tourmaline samples from different settings, Sn-mineralized and non-mineralized, world-wide. This suggests maximum Sn contents in tourmalines much less than 1000 ppm, which is likely to reflect the relatively reducing conditions that usually prevail during tourmaline crystallization and the dominance, and incompatible nature, of Sn²⁺ in hydrothermal fluids, especially where associated with Sn deposits.

The increase in Sn concentration in zone 2 to anomalously high levels is far more likely to be due to Sn⁴⁺, with an ionic radius of 0·069 nm [six co-ordination; see Taylor (1979)], being more easily accommodated within the tourmaline structure than Sn²⁺. This provides further evidence in support of more oxidizing conditions during the formation of zone 2. Which site it occupies is unclear, as again there are a variety of possible coupled substitutions that could occur. Assuming that the high Sn⁴⁺ content of the tourmaline reflects high concentrations in the hydrothermal fluid, its presence in the fluid is likely to be a transient phenomenon, as it would usually be expected to precipitate out, forming cassiterite. It is possible that Sn partitioning within tourmaline was kinetically favoured over cassiterite nucleation. Alternatively, Sn concentrations may have been below cassiterite saturation, although this seems unlikely because of the rare occurrence of such high tourmaline Sn contents. Unfortunately, as the amount of Sn in the tourmaline probably relates to its valence state during tourmaline crystallization rather than to its concentration, no firm conclusions can be reached on its magmatic or formation water source. An interesting observation, however, is that the Sn content of zone 4 is very much lower than that of zone 2, whereas (with the exception of Xvac) these zones are almost identical in all other aspects of their geochemical compositions. Both zones were apparently formed under relatively oxidizing conditions with the hydrothermal fluid being dominated by formation waters. The most likely explanation for this is that the fluids were depleted in Sn by the time zone 4 was formed. Whether this was because of a decrease in cassiterite solubility in the magmatic fluid over time or was due to the diluting effect of mixing with formation waters is unclear.

Breccia formation
The X-ray element maps in Fig. 2 give an indication of the timing of tourmaline nucleation and growth. Zones 1 and 3, which show low Fe/Mg, are present only as overgrowths on, or fracture fills within, brecciated MQT tourmaline. Nowhere can they be seen to have formed separate nucleation sites. Tourmaline nucleation appears to have occurred only during the formation of zones 2 and 4, with evidence for the latter being the common occurrence of feathery filaments of high-Fe tourmaline in the quartz matrix. The lack of nucleation during the formation of zones 1 and 3 was probably due to crystallization on pre-existing tourmaline being kinetically favoured. Nucleation during the formation of zones 2 and 4 coincides with the proposed peak in mixing with formation waters and may therefore have been due to this process having led to supersaturation of the fluid with respect to tourmaline.

	CONCLUSIONS AND WIDER IMPLICATIONS OF THE STUDY

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY ANALYTICAL TECHNIQUES TOURMALINE CHEMISTRY DISCUSSION CONCLUSIONS AND WIDER... REFERENCES

 
Compositional variations across secondary tourmaline overgrowths in breccias at Roche have provided detailed information on the evolution of mineralizing fluids. Four distinct zones have been recognized in the overgrowths, zones 1–4 towards the margins. The overgrowths are interpreted as having initially crystallized from relatively Mg-, Al-rich hydrothermal fluids of magmatic origin, which progressively mixed with more oxidized, Fe-rich formation waters, dominant during the formation of zones 2 and 4. All four zones contain detectable concentrations of Sn, which reach a peak in zone 2. This peak is considered to be due to more oxidizing conditions during the formation of zone 2, with Sn being more easily accommodated within the tourmaline structure as Sn⁴⁺ than as Sn²⁺. The presence of Sn⁴⁺ in hydrothermal fluids is likely to be a transient phenomenon, as it would normally be expected to be precipitated out as cassiterite.

The identification of chemically distinct zones in the overgrowths has also yielded valuable information on the timing of tourmaline nucleation and growth. During the formation of zones 1 and 3, tourmaline crystallization was restricted to overgrowths on pre-existing brecciated MQT tourmaline. During the formation of zones 2 and 4 there was also widespread tourmaline nucleation away from the MQT tourmaline. These nucleation events are likely to relate to periods of supersaturation with respect to tourmaline, probably as a result of a sudden increase in mixing with formation waters.

The study of just a few grains of tourmaline has revealed details of the evolution of mineralizing fluids in the area. It is hoped that this will encourage more extensive studies of chemical variations within single tourmalines as a tool in mineral exploration and in petrogenetic studies. Tourmalinization is commonly associated with mineralization in many of the world’s most economically important mineral deposit types, particularly in breccias related to Cu porphyry deposits. In the Los Bronces Cu porphyry, for example, which is one of the largest known of its type in the world, mineralization is concentrated within a tourmaline breccia, with tourmalinization having occurred before, during and after the main episode of mineralization (Warnaars et al., 1985). Many of these types of deposit are highly altered from the ingress of later fluids, which commonly produce multiple generations of fluid inclusions and complex mineral assemblages, which hamper the study of fluid chemistry related to primary mineralization. Tourmaline is commonly unaffected by later alteration and so can preserve original information on the conditions of mineralization. Its chemistry also often directly reflects that of related ores (Griffin et al., 1996). The study of individual tourmaline grains will complement the recently more popular approach of comparing the compositions of single or a few tourmalines from many deposit types to identify genetic criteria to be used in exploration and genetic studies (e.g. Clarke et al., 1989; London & Manning, 1995; Griffin et al., 1996; reviewed by Slack, 1996). The type of study presented here is likely to become increasingly worth while with the development and routine use of more sophisticated methods of ‘point analysis’, with progressively lower detection limits, such as laser ablation ICP-MS and ion probe, particularly for B and ore-forming elements. Methods for studying the valency of Sn and Fe in situ (and non-destructively) in polished sections of mineral samples are also eagerly awaited.

	ACKNOWLEDGEMENTS

 
All analyses were carried out at the Natural History Museum, London, through internal funding. The authors wish to thank Terry Williams and Tony Wighton for technical help, and Martin Smith, Darrell Henry, David Manning and an anonymous referee for comments on the manuscript.

	FOOTNOTES

 
^*Corresponding author. Telephone: +44-117-954-5235. Fax: +44-117-925-3385. e-mail: Ben.Williamson{at}bristol.ac.uk

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