Contrasting evolution of fluorine- and boron-rich tin systems

Mineral. Deposita 22, 315-321 (1987) MINERALIUM DEPOSITA

© Springer-Verlag 1987

Contrasting evolution of fluorine- and boron-rich tin systems P. J. Pollard 1, M. Pichavant 2 and B. Charoy 2

1 Department of Geology, James Cook University of North Queensland, Townsville, 481 l, Australia 2 Centre de Recherches Petrographiques et Geochimiques, BP 20, 54501 Vandoeuvre Les Nancy, France

Abstract. Individual Sn provinces or regions within provinces are sometimes enriched in fluorine or boron, giving rise to fluorine-rich and boron-rich environments. The structural styles of mineralisation within these environments are similar except that hydrothermal intrusive breccia pipes are more common in boron-rich environments and apogranite/massive greisen systems are more common in fluorine-rich environments. The increased solubility of H20 in B-bearing magmas compared to F- bearing magmas may play a role in the structural evolution of the mineralising systems. The greater mechanical energy produced during crystallisation of B-rich magmas provides a mechanism for breccia pipe and stockwork formation, while the more passive crystallisation of F-rich magmas often results in the formation of disseminated mineralisation. The partitioning of boron toward the aqueous fluid phase and the enhanced solubility of silica in the fluid phase frequently results in tourmalinisation and silicification of the wall-rocks in B-rich environments. In contrast, feldspathic and sericitic alterations usually predominate in F-rich environments.

Tin and associated rare-element mineralisation (W, Nb, Ta, Be) is frequently associated with granites which are enriched in F, B and Li, and the deposits themselves contain mineral species such as fluorite, topaz, tourmaline and Li-micas. Phosphates and carbonates are also signifi- cant components in some tin systems, although their formation frequently postdates cassiterite deposition. Re- cent experimental studies have assessed the important roles of F and B in phase equilibria in silicate magmas, and have also clarified their effects on fluid phase com- positions in equilibrium with molten or crystalline silicate phases. The purpose of this paper is to discuss various features of F- and B-rich tin systems in relation to the contrasting behaviour of F and B in experimental phase equilibria.

General features of tin systems

Tin deposits display a wide range of structural and mineralogical types (Taylor 1979a) and are generally spatially related to granite intrusions which are emplaced at shallow levels (1-4 km) in the crust. These intrusions

occur in several different tectonic settings, are composed of different magma types (peralkaline or peraluminous), and are chemically specialised (Tischendorf 1977). Components such as B, F and Li are generally concentrated in evolved, late-stage magmas, and this enrichment is often considered to be one of the most crucial factors in the formation of associated ore deposits. Fluorine and boron are involved in a number of processes in the magmatic and postmagmatic evolution of tin systems, including fractionation/crystallisation, fluid phase evolution, wall-rock alteration, metal transport and deposition.

Tin mineralisation occurs in a variety of deposit styles within and around the apical zones of the late-stage intrusions. The most frequent occurrence within the granites is as vein and pipe-like bodies, as massive, irregular and sheet-like bodies, and as breccia pipes. In the exo- contact zones the brittle styles (veins, stockworks, pipes) predominate, together with replacement deposits related to the reactive nature of the host-rocks (carbonate-rich and mafic units).

Tin deposits are associated with a variety of alteration styles (see Taylor 1979a) including feldspar-, mica-, chlorite- and silica-rich assemblages, almost invariably associated with minerals such as F and/or Li-rich micas, fluorite, topaz and tourmaline.

Individual regions or districts within tinfields are often enriched with respect to certain components, and two main types can be recognised, boron-rich environments and fluorine-rich environments. These two environments can occur at the province scale (eg. the boron-rich province of southwestern Thailand or the fluorine-rich provinces of Nigeria and the Erzgebirge) or as small components within provinces of mixed character (eg. the SW England and Herberton (Australia) provinces). This enrichment in fluorine and boron can be expressed at the magmatic stage by the crystallisation of topaz, fluorite, tourmaline, Li-mica, etc. and/or at the postmagmatic stage by the development of a similar suite of hydrothermal minerals. Other components (eg. CO2, C1 and to a lesser extent CH4, N~ etc.) may in addition be present in F- and B-rich environments, as well as in other, non B- and F-rich tin environments. However, since these are essentially components of the fluid phase at both the magmatic and postmagmatic stage (Charoy 1979) they are not considered separately at this time.

The reasons for the enrichment of individual regions in F and B as expressed by the development of corresponding

316

Table 1. Major features of fluorine-rich and boron-rich tin systems. Note that clay alteration is an important component of many tin deposits, particularly those in boron-rich environments

Fluorine Boron

Major granite type peralkaline peraluminous peraluminous

Tectonic se t t ing nonorogenic postorogenic postorogenic

Principal mineral- disseminated breccia pipe isation styles in (apogranite- stockwork/vein granitic host-rocks massive greisen) disseminated

stockwork/vein

Main alteration feldspars tourmaline minerals in granitic muscovite silica host-rocks topaz muscovite

silica feldspars chlorite

Sn, W

Cu, As (Ta, Nb)

Principal metals

Associated elements

Examples

Sn, W, Nb, Ta

Li, Be, (Zr, REE Zn, As, Cu)

NE Tasmania (Australia) Nigeria

Erzgebirge Mexico

Cooktown (Australia) SW Tasmania (Australia) Southern Bolivia SW Thailand

mineral species at the magmatic and hydrothermal stages are still poorly understood. Processes which may lead to this enrichment probably include partial melting of mate- rial of appropriate composition (boron-rich metasediments (Charoy 1979; Renard et al. 1985), B-Li-F enriched metasediments of evaporitic environments (Waters and Moore 1985)), emplacement of magmas in environments where partial assimilation of similar materials may occur, and concentration of F and B via fractionation processes occurring during magma evolution and crystallisation (eg. Christiansen etal. 1983). The general features of Sn environments dominated by F or B are outlined below and summarised in Table 1.

Boron-rich environments

Boron-rich tin systems generally have received little detailed research attention and some of the mineralisation styles remain poorly documented, the general features of boron-rich districts are outlined in table 1 and figure 1.

Boron-rich environments are characterised by the presence of tourmaline. Other boron-rich minerals such as axinite are minor and generally occur in calcic skarn environments. Although clearly a magmatic mineral in some felsic, peraluminous granites (Pichavant and Man- ning 1984; Renard et al. 1985), tourmaline has been found to crystallise during the late-magmatic to post-magmatic stage in many granite types (Charoy 1982).

Mineralisation styles in boron-rich tin environments are generally dominated by brittle fracture, and include hydrothermal intrusive breccias, stockwork and vein deposits. Tourmaline-rich, hydrothermal intrusive breccias may result from fluid overpressures generated during the

HYDROTHERMAL~ /@/~'/~//~ VEINS AND INTRUSIVE • /////////SHEETED VEINS

BRECCIA PIPE ~ GRANITE WITH / I + + " /I/~ / ~ PERVASIVE / ~ ~ / / / ~ /INCIPIENT

j l ~ / ~ + ~ i i ~ + + + ÷ + + ~ T I O N

FINER GRAINED GRANITE WITH PERVASIVE,

INCIPIENT ALTERATION

PEGMATITE VEINS MUSCOVITE- TOURMALINE- / MUSCOV,TE GTO %A

BIOTITE GRANITE

Fig. 1. A, B. General features of boron-rich tin environments. A breccia pipe and vein systems (common), B disseminated style (rare) - adapted from Sirinawin et al., 1986

crystallisation of boron-rich magmas (Charoy 1979, 1982; Allmann-Ward etal. 1982). Some large examples are virtually barren, while many others contain disseminated cassiterite mineralisation (Grant et al. 1977, 1980; Goode and Taylor 1980; Wells 1978).

Perhaps the most common styles of tin mineralisation in boron-rich regions are the vein and stockwork deposits (eg. Taylor 1979b; Hosking 1973; Rivas 1979; Jackson 1979). The major alteration styles associated with this fracture controlled mineralisation include tourmaline-, silica-, muscovite- and chlorite-rich assemblages. Argillic alteration is frequently an important alteration style in boron-rich systems and is sometimes enhanced by weath- ering (eg. Cooktown, Australia; SW Thailand). In a given district several stages of tourmaline precipitation may be recognised with different texture, composition and mineral associations, and cassiterite may not be related to all stages (Charoy 1979, 1982).

More rarely, tin mineralisation is related to tourmaline- bearing pegmatites (SW Thailand (Nutalaya et al. 1979); Broken Hill, Australia (Katz and Tuckwell 1979)), and sometimes occurs disseminated through the upper portions of partially albitised, tourmaline-muscovite granites (SW Thailand (Nutalaya et al. 1979)). Although not well-re- presented in most boron-rich provinces, these two styles of mineralisation produce major alluvial concentrations and low-grade primary deposits, particularly in southwestern Thailand.

Fluorine-rich environments

Fluorine-rich granites are characterised by the presence of fluorite and/or topaz, and include peralkaline and per-

aluminous varieties. Many fluorine-rich granites are also enriched in lithium, which is reflected by the development of Li-micas such as zinnwaldite, protolithionite or lepi- dolite. Topaz occurs in strongly evolved, high-silica rocks such as ongonites (Kovalenko et al. 1971), topaz rhyolites (Burt et al. 1982) and late-stage granites (Manning and Exley 1984; Stemprok and Skvor 1974), while fluorite is widely developed as a late-stage mineral in tin-bearing granites.

Mineralisation styles in fluorine-rich tin environments include brittle fracture, vein and stockwork systems, and apogranite/massive greisen systems (table 1, figure 2). Ma- jor mineralised breccia pipe systems are rare, although topaz occurs in the intrusive breccias in the boron-rich system at Ardlethan (Scott 1981). Topaz is a major component of a barren breccia pipe at Schneckenstein in the Erzgebirge.

The apogranite/massive greisen systems are characterised by the development of massive, irregular or sheet-like alteration zones in the apical parts of granite cupolas which commonly show a vertical zonation of rock types (figure 2). There is possibly a complete gradation between the massive greisen systems where greisenisation is the main alteration (Groves and Taylor 1973; Baumann et al. 1974) and the apogranite systems where feldspathic alteration predominates (Pyatenko et al. 1967; Beus 1982).

Effects of F and B in granite and fluid phase evolution

Tin-bearing granites are generally chemically specialised, with high contents of SiO2, alkalis, Sn, Li, Rb, W, Mo, Be, B and F, and low contents of CaO, MgO, Ba and Sr (Tischendorf 1977). The role of components such as H20, F, C1, B and Li during the evolution of these granitic magmas has been partially resolved recently by experimental work. A detailed review of these data is given by Manning and Pichavant (1987) and Pichavant and Man- ning (1984) while the present work attempts to relate these experimental studies to natural tin mineralising systems.

Magmatic Evolution

Components which are readily soluble in silicate magmas (H20, F, B203, Li20) may directly affect their processes of crystallisation. The role of F, B203 and Li20 has been clarified recently in the model haplogranite-H20 system at 1 kb. The addition of both F (1-4 wt%) and B203 (1-4.5 wt%) causes the ternary minimum composition to be shifted toward the Ab apex (Manning 1981; Pichavant 1984) while the addition of Li20 causes a slight shift of the minimum composition towards the Qz-Ab join (Martin 1983). Fractional crystallisation type processes in a pro- gressively F-, B- and Li-enriched system will produce (assuming their effects to be additive) residual magmas rich in normative albite. Both liquidus and solidus temperatures are strongly depressed (Pichavant and Manning 1984).

The application of these experimental results to natural systems will largely depend on the F, B and Li content of natural magmas. Chemically specialised granitic liquids contain up to 3.5 wt% F (Kovalenko and Kovalenko 1976), 2 wt% Li20 (Stewart 1978) and up to 1 wt% B203 (Picha-

317

~ - ' ~ " ~ VEINS

MICROCLINISED GRANITE

PEGMATITE PODS ~/~~__~ ~EINS

ALBITISED GRANITE ~ \ / \ ~ ~'~/

MICROCLINISED GRANITE

Fig. 2. A, B. General features of fluorine-rich tin environments. A greisen systems (common), B apogranite (feldspathic) system

vant and Manning 1984). Due to the rather low B content of natural magmas and the small effect on phase equilibria at low concentrations (Pichavant 1984), B-bearing residual magmas are not expected to have their major element composition significantly modified compared to similar, B- free magmas. On the other hand, given the high concentration of fluorine in many natural magmas and the important effect on phase equilibria (Manning 1981), the presence of fluorine is of major importance concerning the composition of residual magmas. Sodic trends have been noted in a number of F-rich tin-bearing granites (Manning 1981; Cuney etal. 1985; Hudson and Arth 1983).

The water content of residual magmas is also of critical importance concerning their normative composition. F- rich magmas are often believed to have a relatively low water content (Christiansen et al. 1983) compared to B- rich magmas (Pichavant and Manning 1984). Limited experimental evidence shows that an isobaric increase of H20 content in the melt shifts the minimum liquidus composition towards the Qz-Ab side in the normative Qz- Ab-Or diagram (Pichavant and Ramboz 1985). This mechanism will also produce residual magmas enriched in normative albite. On the other hand, the presence of components such as CI and CO2 which lower the activity of water, would produce an opposite effect. Thus C1 and CO2 which are virtually insoluble in silicate melts may have an indirect effect on phase relations through affecting aH20.

Another critical effect of the presence of boron and fluorine in granite magmas is in the reorganisation of melt structure (eg. Manning 1981). The presence of these

318

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.-= 20-

u-u_ 10-

'~ 0

. . . . closed system

open system

6.3 wt % H20

i I

.:3 .4 .5 .; Melt fraction

I

.9

Fig. 3. Change in B and F concentration of the fluid phase during crystallisation assuming a constant melt water content of 6.3 wt.% H20. Open system - the exsolved fluid phase is separated from the magma undergoing crystallisation; closed system - the exsolved fluid phase remains in equilibrium with the crystallising magma. Note that 6.3 wt.% corresponds to the H20 solubility value at 2 kbars (Burnham, 1979). The partition coefficients for F and B between melt and vapour are given in the text

components promotes depolymerisation and the creation of new complex species in the melt. In this process, the number of structurally favourable sites for the incorpora- tion of lithophile elements such as tin increases, resulting in enhanced transport of these elements. Studies on ongonites, which are subvolcanic analogues of Li-F granites (Antipin et al. 1982) show that tin accumulates in the residual magma and may reach concentrations high enough to promote cassiterite precipitation. Other examples of this accumulation include the Beauvior Granite (Cuney et al. 1985), an example of a Li-F granite containing disseminated cassiterite, and the Macusani glasses (Pichavant et al. 1987), which are examples of B-F-Li-rich residual magmas containing approximately 200 ppm Sn.

Fluid phase composition at the magmatic stage

The presence of F and B does not dramatically narrow the silicate melt - aqueous fluid immiscibility domain. Thus in B- and F-bearing magmas, the exsolution of an aqueous fluid phase should occur at a stage dependent on the solubility of H20 in the melt. The addition of B leads to an increase of H~O in the melt (Pichavant 1981), whereas there is no evidence for any increase of H20 in F-rich systems (Manning 1981). For similar degrees of fractionation and/or crystallisation, boron-bearing magmas should therefore retain a higher H20 content than F-bearing magmas. This higher water content has important implications for the physical and mechanical processes of vapour phase exsolution (fracturing).

The composition of the fluid phase in equilibrium with the magma differs in B-bearing and F-bearing systems (Pichavant 1981; Dingwell and Scarfe 1983). In the boron- bearing system the magmatic aqueous fluid phase is rich in silica and alkalis (mainly sodium) and poor in aluminium, whereas in fluorine-bearing systems, it is strongly enriched in aluminium. Large differences are also apparent in the partitioning behaviour of boron and fluorine. Fluorine partitions toward the melt (KD=5 to 10, Hards 1976) while boron is partitioned towards the vapour phase (KD=0.3, Pichavant 1981). Consequently, B may be pro- gressively extracted from crystallising magmas whereas the partitioning of F to the aqueous phase will take place only during the final stages of crystallisation (if F is not bound

in crystalline phases). The extraction o f F and B from crystallising magmas is illustrated graphically in figure 3.

In the presence of both fluorine and boron, the silicate solute content of the fluid phase in equilibrium with the magmas is significantly increased in comparison with the pure H20 system (Pichavant 1981; Dingwell and Scarfe 1983). Thus boron- and fluorine-bearing aqueous magmatic fluids ("pegmatitic fluids") may have elevated concentrations of dissolved silicates and appreciably higher densities than H20 systems under the same P-T conditions.

Data are still lacking concerning the influence of boron and fluorine on the partitioning of tin between silicate melt and vapour.

Fluid phase composition at the postmagmatic stage

In some special circumstances, high concentrations of tin may occur at the magmatic stage (Antipin et al. 1982). However tin deposits are most frequently associated with hydrothermal alteration involving alkali metasomatism (albitisation, K-feldspathisation) and/or a variety of other alteration types (greisenisation, tourmalinisation, chloriti- sation, topazisation, etc.).

The transport of tin in hydrothermal solutions is a function of a variety of parameters including temperature, pH, oxygen fugacity, bulk salinity and the nature and importance of complexing ligands. Subsequent precipitation of cassiterite will result from changes in these physico- chemical variables, and these changes can sometimes be constrained by observations on the hydrothermal alteration/infill mineral assemblages.

Numerous studies in the Russian literature favour tin transport as hydroxy-fluoro-stannate complexes or as complexes with sodium and fluorine (Barsukov 1957; Barsukov and Kuril'chikova 1966; Barsukov and Sush- chevskaya 1973). However, Jackson and Helgeson (1985) have calculated that fluoride complexing of Sn 2+ (the likely tin species in the main range of temperature, oxygen fugacity and pH for the quartz-muscovite-cassiterite as- sociation) is negligable compared to chloride complexing, even if topaz or fluorite are present in the paragenesis. Destruction of complexes and subsequent precipitation of cassiterite from NaC1 solutions can be initiated by a drop in oxidation state, temperature, salinity and/or an increase in pH (Jackson and Helgeson 1985; Eugster and Wilson 1985; Eadington 1982).

Ion exchange (Na +, K +, H +) reactions between alkali feldspars and fluid phases are useful in modelling feldspathic (Orville 1963; Lagache and Weisbrod 1977) and phyllic (Montoya and Hemley 1975; Burt 1981) alteration in C1- and/or F-bearing hydrothermal systems. In the presence of C1 anions, K/Na (atomic) ratio of the aqueous fluid buffered with two feldspars is temperature dependent (Lagache and Weisbrod 1977). Whereas C1 is one of the most important anionic species in hydrothermal fluids, there is also evidence for the presence of fluoride, borate and carbonate anions due to the presence of topaz, fluorite, tourmaline and carbonates in alteration assemblages produced in tin-bearing granites. Barsukov and Ryabchikov (1977) suggest that F predominates over C1 in fluids associated with high-temperature, topaz-bearing, quartz-cassiterite deposits.

319

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z + . 4 - v

,v, .3

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I I I I

I I I I I I

C I

I I

0 .1 .2 .3

I I 1 I I

O 5 0 0 C

I 1 I I I I

.4 .5 .6 .7 .8 .9

K / ( K + N a ) f l u i d (at.)

Fig. 4. Alkali distribution curves between alkali feldspars and aqueous fluid phase at 500 °C. Sources of data: C1- Orville (1963), Pichavant (1983); F and B - Pichavant (1983). Pressure is 1 kbar, except Orville's data (2 kbar)

A B . . . . . . . . . . . :'5 I ' . ' . ' . ' . ' . ' . " . - ~ " , t - - - N a " " '~ '~ . . . . • " ' ' " - . . . . . . . . . " J . . . . . . . . . . .

N a - . ' ~ - . . . - . . . . . . . . . I ' . ' . " . ' . ' . ' . " . ~ . : . 4 ~ - - K - - . - ~ . ~ ' • " " " . " . ' . " . " . . . . . . . . . . . i . . . . . . . . . . .

l . a .g~ _ K • . ' . ' , ' . ' , ' . ' . ' . • • . . ' . ' . ' . ' . ' . ' , I . " " . - . ' . ' . ' . ' . ' , . . l i . . . . . . . . . . , - . I

:z!z:iii:ziz: ~'~'~" ::iizzzzzzzzll filflf!fiff :l .(;; z:ii=zizzzzlz:i . . . . . . . . . . . - - ~ a ~ - - . . . . . . . . . . . N a - - ~ . . . . . . . I" . ' . ' . ' . ' . ' . " , ' . ' d ' . " . ' . " . ' , ' . ' , " . ' . I . . . . . . . . . . . . . . . . . . " . ,

. . . . . . . . '.~:'. , - - K - - -~.. . . . . . . , . . . . . . . . . ," K * - . . . . . . . . . . .

ii!ili::iif ii!!i::ii::::::ii::ii::! i::iiiii!i!ii::iiiil vein vein

' • exchange react ion ~ granite - - - - - equ i l ib r ium

Fig. 5A, B. Effects of (A) decreasing temperature and percolation of borate-bearing fluids, and (B) decreasing borate concentration on the distribution of alkalis between granite and aqueous fluid phase

Pichavant (1983) documents a change in the K/Na atomic ratio in the fluid phase in equilibrium with feldspars at 400-500 °C, 1 kb in the presence of B, F and C1 anions - (K/Na) c] > (K/Na) B > (K/Na) F (Fig. 4). How- ever this change in the composition of the fluid phase is not accompanied by any change in feldspar composition. Figure 5 illustrates how the anonic composition of the fluid phase will affect alkali distribution between granite and hydrothermal fluid phase. For example, the percolation of borate-bearing hydrothermal fluids will lead to potassic alteration of the wall-rocks previously equili- brated with chloride-bearing solutions. On the other hand, dilution or mixing of borate-bearing hydrothermal fluids with chloride-bearing fluids will promote albitisation of the wall-rocks.

It should be stressed that the results obtained for fluoride-bearing solutions (Fig. 4) probably do not re- present feldspar-fluid exchange equilibrium reactions be- cause of the limited solubility of NaF in high-temperature aqueous fluids. This limited solubility indicates that most fluorine in solution will be present as species other than NaF (eg. as HF). This will be important for alkali partitioning in fluorine-bearing systems.

Discussion

The almost constant occurrence of B, F and/or Li-rich minerals associated with magmatic-hydrothermal Sn mineralisation is more than coincidence. However, despite the accumulation of relevant experimental results, the precise role of F and B in promoting the formation of Sn deposits remains poorly understood.

Similarly, the detailed textural and chronological relationships between F- and B-bearing mineral species and ore minerals are lacking in many descriptions of the deposits themselves.

The dominant role played by C1 in the transport of tin in hydrothermal fluids over the temperature and composi- tional ranges most commonly encountered in tin deposits naturally raises questions as to the role of F and B in these systems.

Fractionated peraluminous granites which are not enriched in F or B are seldom associated with tin mineralisation, and enrichment in these components appears to be a critical factor in producing specialised granites which ultimately give rise to tin deposits. Perhaps one of the most important roles for these elements at the magmatic stage is in extending the temperature range of crystallisation of silicate magmas, and allowing residual magmas to crystallise at temperatures down to 600°C (Pichavant and Manning 1984). Melt viscosities decrease and diffusion rates increase in fluorine-bearing magmas (Dingwell and Scarfe 1983; Dingwell etal. 1985) and structural modifications in the melt promote the accumulation of ore elements in residual magmas through fractionation processes.

One of the important contrasts between F-bearing and B-bearing magmas is in the water content, with higher solubility of H20 promoted in B-bearing magmas. This water content is an important factor in determining the amount of mechanical energy released during crystallisation of residual magmas (Burnham 1979). The relatively high mechanical energy developed in B-bearing magmas during crystallisation is consistent with the common occurrence of hydrotherrnal intrusive breccia pipes in boron- rich environments. On the other hand, the common occurrence of disseminated mineralisation in F-rich environments may reflect the relatively low amount of mechanical energy released during crystallisation of H20- poor, F-bearing magmas.

The increased solubility of silica in boron-bearing aqueous phases and the preferential partitioning of boron to the aqueous phase may be an important factor in determining the characteristics of associated hydrothermal alteration. The most important early-stage alteration in B- rich environments is frequently an intense tourmaline- silica alteration of the host-rocks.

Fluid inclusion and stable isotope studies of magmatic- hydrothermal Sn systems generally show an evolution

320

from high temperature, magmatic-derived fluids associated with early alteration and mineralisation events to lower temperature, meteoric-dominated fluids associated with later stages (Grant et al. 1980; Eadington 1983; Eadington and Sun 1982). The thermodynamic evolution of the hydrothermal fluid phase (liquid, vapor, critical, homo- geneous, heterogeneous) is of prime importance with respect to the physico-chemical behaviour of the solute species (metal complexes). Unmixing by boiling (Phillips 1973; Ramboz etal . 1982) or progressive dilution by external meteoric waters may occur during decreasing temperature. Sharp drops in pressure due to repeated opening of fractures, decrease in temperature and oxidation state, and decrease in acidity aid in the breakdown of metal complexes and precipitation of ore minerals (Eug- ster and Wilson 1985). The influence of one variable on the others is still poorly understood, and different mecha- nisms of ore precipitation and wall-rock alteration may dominate in different deposits depending on their structural evolution and the nature of the complexing species.

An example of this complexity is illustrated by Sn systems which contain both fluorine and boron in signifi- cant amounts. These systems sometimes exhibit a zonal sequence where fluorite- and /o r topaz-rich alteration predominates in the granite, while tourmaline- and/or chlorite- rich alteration predominates in the host metasediments (Tischendorf 1973). It is sometimes assumed that boron is derived from the metasediments (Tischendorf 1973), however this has yet to be systematically documented. At the Zaaiplaats mine, South Africa, sheet-like and massive alteration-mineralisation zones in the upper portion of the granite contain abundant fluorite, while crosscutting pipe- like orebodies frequently contain abundant tourmaline and fluorite (Strauss 1954). These crosscutting orebodies do not extend outside the granite and there thus appears to be no external source for the boron. An alternative explanation for the apparent spatial dissociation of fluorine and boron in some cases may be that their behaviour in solution is quite different in variable ranges of P and T, and possibly that host-rock composition also plays a role in the precipitation of tourmaline, topaz and fluorite.

Where hydrothermal fluids are channelled out o f the granite, mineral-fluid equilibria become much more complex due to the reactive nature of some rock types. For example, carbonate-rich rocks frequently act as a host for tin mineralisation, and a wide variety of skarn and replacement deposits may be developed (Einaudi et al. 1981). In fluorine-rich environments however, laminar magnetite-fluorite-vesuvianite skarns are frequently developed (Kwak and Askins 1981a, b), while in boron-rich environments granular and laminar skarns containing magnesium and calcium tin-borates and /o r borosilicates (Alexandrov 1974, 1975; Jackson 1979) may be developed.

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Received: June 18, 1986 Accepted: May 19, 1987

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Contrasting evolution of fluorine- and boron-rich tin systems