Fluids and mineralisation in the Scottish Dalradian

Fluid inclusion studies of orogenic vein-type gold deposits show a strong genetic association with high-temperature, low-salinity, and volatile-rich fluids. Although fluid models for this type of mineralisation are well established, they commonly ignore several important facts that indicate a role for low temperature fluids. Visible gold is invariably fracture-controlled and is hence related to fluid processes represented by later (secondary) fluid inclusions. In the limited number of cases where these have been studied, a multiple fluid history involving low-temperature brines (<250ºC) is commonly observed. Gold is often associated with a suite of minerals characterised by sulphosalts, tellurides, sulpho-tellurides and rare gold alloys. Some of these are only stable below 275ºC, indicating an association with low-temperature processes. This study collates fluid inclusion and mineralogical data representing the major styles of mineralisation (e.g. Cononish and Calliachar Burn [Au–quartz]; Lagalochan [porphyry–Cu±Au]; Tomnadashan [igneous related Cu]; Tyndrum, Castleton and Inverneil [Pb–Zn]; Stronchullin and Corrie Buie [Pb–Zn±Au]) and regional fluids (e.g. Loch Lomond) in the Scottish Dalradian. Six fluid types are identified: 1. High-salinity (halite-bearing: NaCl>35 wt %) and high-temperature (>300ºC) fluid inclusions. These are typical of porphyry copper deposits world-wide and were recorded from samples at Lagalochan, Tomnadashan and Comrie. 2. High-temperature (250–400ºC), volatile-rich (major CO2+CH4+N2: 15–25 wt % NaCl eq) and moderate salinity (7–10 wt % NaCl eq) fluids inclusions. In the Dalradian, this fluid is ubiquitous. It occurs in veins and breccias, associated with igneous intrusions. It is also one of the major fluid-types in regional metamorphic quartz-veins and is recorded at nearly all mineralised localities. Elsewhere, this fluid-type is associated with orogenic gold mineralisation. However, in the Scottish Dalradian, its presence is not indicative of gold mineralisation. 3. Moderate to high-temperature (200–350ºC) and moderate salinity (7–10 wt % NaCl eq) fluids containing volatiles (minor CO2+CH4+N2: 10–15 wt %% NaCl eq). These have the same distribution and associations as Fluid 2. 4. Low to moderate-temperature (150–250ºC) low-salinity brines (<10 wt % NaCl eq) with little or no volatile component. This is analogous to fluids associated with epithermal gold mineralisation. It occurs in both igneous and metasedimentary rock-hosted mineralisation, and is present in a number of metamorphic quartz veins. Its presence as primary fluid inclusions in sphalerite, at Stronchullin, shows it plays a significant role in mineralisation. 5. Low-temperature (<150ºC) high-salinity (c. 20 wt % NaCl eq) brines. This fluid is typical of Mississippi Valley Type Pb–Zn deposits world-wide. It is present in most gold mineralised localities, but inclusions are low in abundance. Its role in Au metallogenesis is unclear, but a similar type of fluid is associated with gold-free base metal mineralisation (e.g. Tyndrum Pb–Zn). 6. A low temperature (monophase) aqueous fluid. This could be a low temperature equivalent of either Fluid 4 or Fluid 5. This fluid is sparsely distributed over a wide area. Types 1 and 2 and possibly 3 represent prograde fluids, which have a deep crustal (magmatic and metamorphic) origin, and are probably responsible for introduction of metals into the system. Then initiation of extensional tectonics permitted a major ingress of meteoric–basinal fluids (Types 4, 5 and 6). Fluid 4 and/or 5 remobilises earlier Fe–Cu–(Mo)–As–Au–S mineralisation and results in a base metal–gold overprint at many of the localities. Late-stage (Devonian-Carboniferous?) basin development is a possible source for the high-salinity low-temperature brines (Type 5 fluid). Fluid 6 could be responsible for the localised dickite–kaolinite mineralisation in the Highland Boundary Fault Zone and supergene alteration seen in mineralised localities (e.g. Calliachar Burn). Although the volatile-rich fluids play a major role in metallogenesis in the Scottish Dalradian, it is clear that low-temperature brines played a significant role in gold mineralisation. In terms of understanding the deposit-scale distribution of gold, it is of prime importance to know how these late-stage fluids interacted with pre-existing mineralised structures. Also, for exploration, there is a need to develop new technologies to predict where and how these fluids have acted, as they are probably responsible for the erratic distribution of gold-grades that characterise many orogenic gold deposits.