Abstract. The sedimentary sequences of the Southern Uplands of Scotland host numerous lead-zinc-copper-silver vein deposits, the genesis of which has never ...
MINERALIUM DEPOSITA
Mineral. Deposita 23, 1-8 (1988)
© Springer-Verlag 1988
Epithermal base-metal vein mineralization in the Southern Uplands of Scotland: Nature and origin of the fluids I. M . S a m s o n i , and D. A. B a n k s 2
1 Scottish Universities Research and Reactor Centre, East Kilbride, Scotland, UK, G75 OQU 2 Department of Applied Geology, University of Strathclyde, Glasgow, Scotland, UK, G 1 1XJ Abstract. The sedimentary sequences of the Southern Uplands of Scotland host numerous lead-zinc-coppersilver vein deposits, the genesis of which has never been adequately explained. Fluid inclusion and stable isotope analysis of vein minerals from these deposits indicate that, for the vein stages studied, the mineralizing fluids were low temperature ( < ~ 1 5 0 ° C ) , high salinity (~19 to 30 equiv, wt. % NaCI+CaCL) modified meteoric waters. A consideration of the availability of such fluids throughout the geological history of the Southern Uplands suggests a Lower Carboniferous (Dinantian) age for the mineralization.
With the exception of those in the Leadhills-Wanlockhead district (hereafter referred to as Leadhills), most of the PbZn-Cu-Ag vein systems (Fig. 1) hosted by the sedimentary sequences of the Southern Uplands are relatively small. At Leadhills, productivity from the 70 or so veins, which outcrop over an area of 12km 2, was approximately 500,000 tons of galena and sphalerite and 1 million oz of silver (Wilson and Flett 1921). In terms of their geological setting and mineralogy, these deposits resemble the much larger Irish base metal deposits which occur along strike within the Caledonian orogen, and to which they have been genetically related (Russell 1976). The purpose of this paper is to clarify the nature and origin of the mineralizing fluids responsible for these veins as deduced from fluid inclusion and stable isotope data. G e o l o g y and m i n e r a l i z a t i o n
Most of the rocks underlying the Southern Uplands of Scotland comprise sequences of Silurian and Ordovician continental margin and abyssal greywackes, shales, cherts and volcanics (Fig. 1). These sequences occur in thrustbounded accretionary wedges which accumulated above a northwardly dipping subduction zone on the northern margin of the Iapetus ocean (Leggett et al. 1979). Veins are typically hosted by the more competent greywacke units and, in the case of those in the Leadhills district and at Pibble, within NW- to NE-trending wrench * Present address: Department of Geology, University of Windsor, Windsor, Ontario, Canada, N9B 3P4
faults (Temple 1956; Cook 1976). The exception to this is the Knipe antimony vein which is hosted by a small, late Caledonian, granite stock. The sedimentary sequence has been intruded by numerous Devonian (late Caledonian) granitic plutons. The three largest exposed intrusives are the Loch Doon, Cairnsmore of Fleet and Criffel stocks (Fig. 1), which have been dated at 408 ± 2, 392 +__2 and 397 +_2 Ma respectively (Halliday et al. 1980). Some veins, notably those in the southwest of the Southern Uplands, have a close spatial relationship with these plutons. This fact and the presence of a rough elemental and mineral zonation away from the Fleet pluton led Cook (1976) to propose a genetic link between the granite and the mineralization. The base metals occur as galena, sphalerite and chalcopyrite in a gangue of calcite, dolomite, ankerite, baryte, quartz and pyrite. These minerals are commonly the matrix to a variety of vein breccia types and may themselves be brecciated. Several of the samples of quartz from the spoil heaps at Leadhills were associated with hematite and magnetite, the latter mineral being previously unreported from the area. The Knipe antimony vein contains stibnite in a gangue of quartz and chert. Some of the veins, notably at Leadhills and Pibble, are highly oxidized in their upper portions, presumably as a result of reaction with postmineralization ground-waters. Other, rarer, minerals include niccolite, rammelsbergite, cobaltite, witherite, chalcocite and native silver (Temple 1956; Cook 1976). In addition to having been a base-metal and silver mining area, the LeadhiUs district was also a prominent alluvial gold mining area, with approximately 100,000 oz of gold recovered from the strams around Leadhills and Wanlockhead. The source of the gold has never been found. Quartz is normally late in the paragenetic sequence of the main mineralizing event at Leadhills (Temple 1956) and often occurs as euhedral crystals in cavities. Therefore, although some of the samples studied have base-metal sulphides intergrown with the quartz, and hence may be considered part of the mineralizing process, they may only represent the later stages of mineralization; a scenario supported b y the observations of Temple (1956) who reports two distinct base-metal stages at Leadhills. In the other deposits studied, quartz may also be found as euhedral crystals in cavities or may be more massive. At least in the case of Pibble, quartz was deposited during the main stage mineralization (Cook 1976).
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In addition to the quartz associated with the main base metal mineralization at Leadhills, an earlier set of massive, milky quartz veins are also present. The earlier veins carry lesser amounts of calcite, muscovite and feldspar, and minor amounts of pyrite, pyrrhotite, pentlandite, chalcopyrite, marcasite and sphalerite. Temple (1956) suggests that these early veins were formed during the end stages of the Caledonian orogeny. K-Ar age dates, obtained using clay gouge (mixtures of illite, chlorite and kaolinite) from the the foot- and hanging-walls of veins at Leadhills mostly fall in the range 280 to 320 Ma, with one value at 265 Ma (Ineson and Mitchell 1974). This would suggest that, for Leadhills at least, the mineralization is mid to upper Carboniferous in age, and therefore unrelated to Devonian plutonism. These dates support the proposal of Dunham (1952), that the mineralization is of Hercynian age. Although it is possible that the veins close to the granites in the southwest represent an earlier mineralizing event related to the granites, their similarities in style, mineralogy and structural setting to the Leadhills mineralization would suggest that they are the same age. Sampling and analysis
Because mine workings were inaccessable and exposure is poor in the area, samples used in this study were mostly obtained from spoil heaps, except those from most of the early quartz veins from Leadhills, which were collected insitu. Fluid inclusion thermometry was performed on doubly polished wafers (Crosbie 1981) using a Linkam TH 600 stage. Maximum errors in temperature measurements between 30 and 300°C are + 3 ° C . At =< 30°C, the horizontal thermal gradient that develops when using this stage is the main source of error (1.5 °C over the viewing area, MacDonald and Spooner 1981); analytical and calibration errors are only _+0.3 °C. Oxygen and hydrogen isotopic analyses were performed using standard extraction and mass spectrometric techniques (Samson and Russell 1987; Borthwick and Harmon 1982). Mass spectrometric analytical precision is -< + 1 per mil for hydrogen and -< +0.05 per rail for oxygen with average sample reproducabilities o f - 2.7 and +0.27 per rail for hydrogen and oxygen respectively.
Fig. 1. Simplified geological map of the Southern Uplands of Scotland showing the location of the base metal deposits studied. Faults: SUF, Southern Uplands fault; StF, Stinchar fault. Post-Lower Paleozoic basins: S, Sanquhar; T, Thornhill; D, Dumfries. Base metal deposits: L, Leadhills-Wanlockhead, W, Woodhead, K, The Knipe, CB, Coldstream Burn, B, Blackcraig; P, Pibble, E, Enrick. Granites: LD, Loch Doon; Cr, Criffel; CF, Cairnsmore of Fleet. TGF = Tweedale granite anomaly (Lagios and Hipkin, 1979)
Results are reported in the standard '6' notation relative to the SMOW standard. After thorough cleaning with concentrated HC1 and deionised water, ,-~ 1 mm sized, hand picked mineral chips were decrepitated and subsequently leached with 3M nitric acid for quartz, or deionised water for sphalerite and carbonate. Leachates were analysed by flame emmision spectrometry for Na, K and Ca and by atomic absorption spectrometry for Mg. Laser-exited Raman spectroscopy was performed using a Jobin-Yvon microprobe spectrometer, fitted with a Spectra Physics 5 W ionized argon laser. Physical and chemical nature of the fluids
Fluid inclusion types Calcite and clear, euhedral quartz from Leadhills, Woodhead, Pibble, Coldstream Burn and Enrick generally contain primary and pseudosecondary fluid inclusions that are either liquid only (L) or liquid-vapour (LV) with high degrees of fill (Fig. 2). These inclusions commonly show evidence of necking, making homogenization temperature measurements somewhat unreliable. At Leadhills, Pibble and Woodhead, some samples contain LV inclusions with lower, but inconsistent, degrees of fill. In the cases of Pibble and Woodhead, these inclusions may be associated with L and VL (vapour-rich) inclusions. The VL inclusions observed in quartz from Pibble had consistent phase ratios in spatially related inclusions. This may be taken to suggest an origin through boiling rather than necking. Unfortunately, most samples did not contain many fluid inclusions or were inclusion free, thus limiting the amount of data that could be collected.
Fluid chemistry Final melting temperatures of ice (TmICE) for L and LV inclusions with high degrees of fill in Leadhills quartz and calcite lie mostly in the range -17.8 to -40.2 °C, with three values at > -4 °C (Fig. 3). On warming some frozen inclusions, a solid persisted above TmICE to between -12.5 and + 5.7 °C (Fig.4). This solid is interpreted as a salt-hydrate. Because the most abundant solute in fluid inclusions is normally NaC1, the commonest hydrate observed in fluid inclusions is hydro-
halite (NaC1.2H20). The upper temperature limit of hydrohalite stability in the NaC1-HaO system is 0.1°C (Crawford 1981). The low TmlCE values and leachate data (see below; Table 1) indicate the presence of significant CaC12 concentrations in these inclusions. This would lower the hydrohalite stability limit, probably to -5 °C and below. Hydrohalite metastability above the true melting temperature is a possible explanation, however, this type of behaviour is generally not observed (Roedder 1984), and because the observed solid consistently melted at > 0 °C, even when very slow heating rates were used, we propose an alternative explanation. Furthermore, if the solid is hydrohalite, fluid compositions must lie within the hydrohalite stability field of complex salt-water systems (Crawford 1981). Particularly in the Ca-bearing systems, this field covers very restricted compositional ranges. The large spread of TmICE values indicates that these are not compositionally restricted fluids and one might expect some fluids to lie in the juxtaposed halite stability field, an expectation that is not bourne out by the fluid inclusion phase assemblages. The solid is also unlikely to be a CO2clathrate as CO2 was not detected in LV inclusions using Raman spectroscopy, and the optical properties are inconsistent with it being this phase. Carbonates and baryte are common vein minerals at Leadhills, and solid anhydrite inclusions have been identified in quartz using Raman spectroscopy (Fig. 5). Anhydrite has not been reported as a vein mineral from Leadhills, presumably because, if present, it was dissolved by post-mineralization groundwater. It is therefore suggested that the observed solid phases are carbonate- or sulphate-hydrates, a conclusion that is consistent with phase behaviour in CO3- and SQ-bearing systems (Linke 1965; Samson and Russell 1987). Salinities, as estimated from TmICE values in inclusions where a hydrate was not observed, fall in the range 19.5 to 29.3 equiv, wt. % NaCI+CaC12 (Linke 1958). Leachate analyses (Table 1) indicate significant proportions of calcium with subordinate potasium in the inclusion fluids. Sodium is the dominant cation. In addition, species such as magnesium and/or iron have to be present to explain the observed eutectic temperatures of as low as -60 °C. Eutectic temperatures range from -40 to -60 °C. Some inclusions from Enrick and Pibble show the same low temperature phase behaviour as the inclusions described above from Leadhills (Figs. 3 and 4). TmICE values are typically higher at between -6.2 and -26.5 °C. With the exception of some very low salinity inclusions from Woodhead, inclusions from Woodhead and Coldstream Burn also fall within this range (Fig. 3). Some inclusions have very low salinities of 0 to 3.1 equiv, wt. % NaC1) (calculated using the equation of Potter et al. 1978). It is interesting to note that these inclusions appear to have lower degrees of fill than the high salinity inclusions. The only VL inclusions from which TmICE values were obtained were from Woodhead and have salinities of 0.2 to 1.4 equiv, wt. % NaC1 for four inclusions. These low salinities support an origin through boiling (vapour condensation) but are too high to have been in equilibrium with the observed liquids at 100 to 150°C (see below) (Sourirajan and Kennedy 1962). The low salinity LV inclusions could also be condensates, or could contain younger groundwater. Further study is required to clarify the origin of both these inclusion types.
Primary LV inclusions in the early-stage quartz from Leadhills have salinities of 2 to 8 equiv, wt. % NaC1 and inclusions in quartz from a quartz-stibnite vein from The Knipe have salinities of 5.2 to 7.6 equiv, wt. % NaC1 (Fig. 3) (Potter et al. 1978).
Fluid temperatures As was previously noted, L and LV inclusions commonly show evidence of necking. However, where measured, homogenization temperatures range from 5 to 134°C (Fig. 6), which, along with the common occurrence of liquid-only inclusions, indicates that precipitation occurred at low temperatures. The peak at 60-80 °C for Leadhills is composed mainly of results from magnetite-hematite bearing samples. Inclusions with lower than average degrees of fill typically had inconsistant phase ratios and may have leaked. However, some such inclusions from Pibble (Fig. 2) gave relatively consistent temperatures of 140 to 164 °C (Fig. 6). There is no direct way of estimating the depths at which these veins formed. If we consider what is probably an extreme case, that the veins formed at pressures of 50 to 75 MPa ( = depths of 1.9-2.9 km under lithostatic conditions and 5.1 to 7.6 km under hydrostatic conditions), the pressure correction required for a 75 °C inclusion with a 25 wt % fluid is 45 to 62 °C (Potter 1977). This still results in relatively low fluid temperatures of 120 to 137 °C. Primary LV inclusions in the early-stage quartz from Leadhills yielded homogenization temperatures of 187 to 236 °C and homogenization temperatures of LV inclusions in quartz from The Knipe range from 168 to 213°C (Fig. 6).
Isotopic results Most dD values for fluid inclusion waters range from -40 to -70 per rail (Fig. 7). The exceptions to this range are the early Leadhills quartz (sample GC-3) and one calcite sample, also from Leadhills (MH-2). The oxygen isotopic compositions of the fluids (6180) range from -7.5 to + 6.5 per mil with one abnormally high calcite value at + 13.0 per mil (Fig. 7). These values have been calculated from mineral values 1 using published mineral-water fractionation factors (Matsuhisa et al. 1979; O'Neil et al. 1969; Northrop and Clayton 1966). Oxygen isotope fractionation factors have been corrected for salinity using the experimental data of Truesdell (1974). One calculated value (sample GC-4) lies to the left of the meteoric water line and is therefore probably unrealistic, given that few natural waters have isotopic compositions in this region. If it is assumed that the minimum 6~80 value that this fluid can have is -7.51 per rail (on the meteoric water line at 6D = - 5 0 . I per mil) the minimum allowable precipitation temperature is ,-~111 °C (calculated using the quartz-water fractionation factor of Matsuhisa et al. (1979)). It is worth noting that this temperature is consistant with fluid inclusion temperatures. The lack of temperature and salinity data on Blackcraig does not allow the calculation of accurate fluid 61sO values. However, using the upper temperature range ob1 available from authors on request
4
tained for the other deposits of 140 to 160°C, the calculated fluid values for Blackcraig ( + 3.9 to + 6.2 per mil) lie in the upper range of values obtained for the other veins. The one 6D value obtained (-51A per mil) lies well within the range obtaind for the other deposits.
Discussion
The distinct temperatures and salinities of the early Leadhills quartz vein fluids supports Temple's (1956) suggestion that these veins are a pre-mineralization event related to
Fig. 2. A-J. Representative fluid inclusions from Southern Uplands vein quartz. A, B 3-dimensional array of primary liquid-vapour inclusions (same group in both cases) (Pibble). C, D Growth zone primary liquid-only inclusions (Pibble). Crystal grew from right to left in C. E Pseudosecondary liquid-only inclusions (Leadhills). F, G, H, I Typical liquid-vapour inclusions with high degrees of fill, J Liquidvapour inclusions with slightly lower than average degrees of fill (Pibble). Scale bars: A, C = 100 microns; B, D, E, G, H= 20 microns; F, H, I = 10 microns
the end stages of the Caledonian orogeny. The small amounts of sulphides hosted by these veins were probably introduced at a later date: secondary inclusions in these veins are similar to the late-stage fluids. The fluids from the Knipe are similar to this early vein set at Leadhills, but whether this links the antimony mineralization to a late Caledonian metamorphic event or to the granite which hosts the vein is unclear. The similarities in fluid chemistry (including isotopic composition) and temperature between the various deposits within the Southern Uplands, including Leadhills, suggests that the same highly saline fluids were principally responsible for the mineralization. These fluids were low temperature (< 150°C), highly saline, and possibly had relatively high concentrations of CO3 or SO4. This conclusion is supported by the similarities in the style of mineralization, mineralogy and structural setting of the deposits.
The isotopic composition of the fluids rules out sea water as a source; assuming that it had similar D / H and 180/160 ratios to modern seawater. On a 6D-6180 plot, most values lie between the compositions of unmodified meteoric waters and magmatic or metamorphic waters. Given their geological setting, the possible fluid origins are: (1) meteoric water that has been isotopically modified through water-rock interaction; (2) mixed meteoric and magmatic waters; or (3) mixed meteoric and metamorphic waters. Unless temperatures were higher than those estimated, which would reduce the calculated fractionations, these fluids cannot be simply magmatic or metamorphic fluids. Several factors suggest that metamorphic fluids were not involved in the base-metal mineralization: (1) the very low 6D values of some fluids; (2) the K-Ar age dates; (3) the difference in fluid chemistry and temperatures be-
6 Table 1. Leachate analysis of Leadhills minerals Sample
Mineralogy
GC- 1 GC-2 GC-3* GC-4 IMS-5 WC-1 IMS-8c IMS-8s WC-3
Concentration (ppm)
Q Q, Cp Q, C Q, P Q, G Q, C C, S S, C S
Atomic ratios
Na
K
Ca
Mg
K/Na
Ca/Na
Mg/Ca
12.3 4.5 4.4 2.4 7.7 15.7 2.5 3.0 2.6
2.0 1.2 10.9 < 0.5 4.5 2.7
