Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

Abstract

The Bohaun gold deposit in County Galway, western Ireland is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks, which have only reached the late diagenetic zone of low-grade metamorphism.The mineralization consists of undeformed veins, stockworks and breccias. Quartz dominates all the veins and commonly displays multiple growth stages and a variety of classic epithermal textures, including comb, plumose and banded, indicative of low confining pressures. The veins have a simple mineralogy comprising quartz-sericite-chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite.Vein quartz is associated with two fluids compositions: Type I with moderate to high salinity (8.0-23.6 wt % NaCl equiv) and Type II with low to moderate salinity (0.0-7.8 wt % NaCl equiv). CO 2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126 to 257 o C and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175 and 245°C.The geodynamic setting of Bohaun, which comprises a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine type fluid, in a similar fashion to red-bed Au-Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic grade gold mineralization, independent of other fluids.3

The genesis of gold mineralisation hosted by orogenic belts: A lead isotope investigation of Irish gold deposits

“…Alongside deposit scale studies of other regionally important discoveries, such as Croagh Patrick (Aherne et al, 1992;Wilkinson and Johnston, 1996) and Cregganbaun (Thompson et al, 1992), all have indicated orogenic style mineralisation. However Lusty et al (2011) suggest a role for low temperature Cl-rich basinal brine-type fluids for the mineralisation at Bohaun (Co. Galway), similar to those invoked for the final phases of mineralisation at Curraghinalt (Parnell et al, 2000) and red-bed Au-Pd mineralisation.…”

“…6) indicates a second source of Pb, more radiogenic in nature, was involved in the mineralisation together with source 2. The radiogenic Cregganbaun end member has similar isotopic ratios to vein gold from Bahaun, Co. Galway (also the Mayo subterrane), and Lusty et al (2011) proposed an association with post-Caledonian (Carboniferous or later) Cl-rich basinal brine-type fluids, similar to those involved in red-bed Au-Pd mineralisation (see Chapman et al, 2009) and one of the four mineralisation phases at Curraghinalt (Parnell et al, 2000). However such gold typically contains very low Ag, has alloys in the Au-Pd-Hg system, and is characterised by an inclusion suite containing selenides and tellurides and absent of sulphides.…”

“…However such gold typically contains very low Ag, has alloys in the Au-Pd-Hg system, and is characterised by an inclusion suite containing selenides and tellurides and absent of sulphides. In contrast, gold from Bohaun has none of these features, and it contains up to 40 mass % Ag (Lusty et al, 2011). The mineralisation instead appears similar to relatively shallow 'epizonal' orogenic gold from the Yukon, Canada (Allan et al, 2013), where mineralisation was ascribed to telescoping of an orogenic system during rapid uplift, one of the scenarios specified by Yardley and Cleverly (2013) as a potential setting for orogenic gold formation through metamorphic processes.…”

Tectono-magmatic controls of post-subduction gold mineralisation during late Caledonian soft continental collision in the Southern Uplands-Down-Longford Terrane, Britain and Ireland: A review

“…Reactivation of D3 faults during these extensional events could have remobilised gold. This is supported by evidence from the distribution and character of alluvial gold in the Thornhill area (Leake and Cameron, 1996;Leake et al, 1998) and by atypical oxide-related lode-gold mineralisation in Western Ireland (Lusty et al, 2011). It is likely that regional hydrothermal activity and Pb-Zn-Cu-Ag vein mineralisation was related to this tectonic reactivation (Baron and Parnell, 2005;Wilkinson et al, 1999).…”

“…δ 34 S values for sulphide minerals from a gold-bearing vein within the Fleet pluton near Talnotry (Orchars Vein; Figure 1) are between -12 and -5 ‰; comparable with those for Pb-Zn veins at Leadhills-Wanlockhead (Anderson et al, 1989), and therefore are considered to reflect low temperature meteoric remobilisation during a significantly later episode of structural reactivation (Anderson et al, 1989;Baron and Parnell, 2000;Lowry et al, 1997;Lusty et al, 2011;Samson and Banks, 1988).…”