Constraining the provenance of the Stonehenge 'Altar Stone': Evidence from automated mineralogy and U–Pb zircon age dating

The Altar Stone at Stonehenge is a greenish sandstone thought to be of Late Silurian-Devonian (‘Old Red Sandstone’) age. It is classed as one of the bluestone lithologies which are considered to be exotic to the Salisbury Plain environ, most of which are derived from the Mynydd Preseli, in west Wales. However, no Old Red Sandstone rocks crop out in the Preseli; instead a source in the Lower Old Red Sandstone Cosheston Subgroup at Mill Bay to the south of the Preseli, has been proposed. More recently, on the basis of detailed petrography, a source for the Altar Stone much further to the east, towards the Wales-England border, has been suggested. Quantitative analyses presented here compare mineralogical data from proposed Stonehenge Altar Stone debris with samples from Milford Haven at Mill Bay, as well as with a second sandstone type found at Stonehenge which is Lower Palaeozoic in age. The Altar Stone samples have contrasting modal mineralogies to the other two sandstone types, especially in relation to the percentages of its calcite, kaolinite and barite cements. Further differences between the Altar Stone sandstone and the Cosheston Subgroup sandstone are seen when their contained zircons are compared, showing differing morphologies and U-Pb age dates having contrasting populations. These data confirm that Mill Bay is not the source of the Altar Stone with the abundance of kaolinite in the Altar Stone sample suggesting a source further east, towards the Wales-England border. The disassociation of the Altar Stone and Milford Haven undermines the hypothesis that the bluestones, including the Altar Stone, were transported from west Wales by sea up the Bristol Channel and adds further credence to a totally land-based route, possibly along a natural routeway leading from west Wales to the Severn estuary and beyond. This route may well have been significant in prehistory, raising the possibility that the Altar Stone was added en route to the assemblage of Preseli bluestones taken to Stonehenge around or shortly before 3000 BC. Recent strontium isotope analysis of human and animal bones from Stonehenge, dating to the beginning of its first construction stage around 3000 BC, are consistent with the suggestion of connectivity between this western region of Britain and Salisbury Plain.This study appears to be the first application of quantitative automated mineralogy in the provenancing of archaeological lithic material and highlights the potential value of automated mineralogy in archaeological provenancing investigations, especially when combined with complementary techniques, in the present case zircon age dating

Identification of the source of dolerites used at the Waun Mawn stone circle in the Mynydd Preseli, west Wales and implications for the proposed link with Stonehenge

A Neolithic stone circle at Waun Mawn, in the Mynydd Preseli, west Wales, has been proposed as the original location of some dolerite megaliths at Stonehenge, including one known as Stone 62. To investigate this hypothesis, in-situ analyses, using a portable XRF, have been obtained for four extant non-spotted doleritic monoliths at Waun Mawn, along with two weathered doleritic fragments from a stonehole (number 91). The data obtained have been compared to data from spotted and non-spotted dolerite outcrops across the Mynydd Preseli, an area known to be the source of some Stonehenge doleritic bluestones, as well as data from in-situ analysis of Stone 62 (non-spotted dolerite) and ex-situ analysis of a core taken from Stone 62 in the late 1980′s. Recently, Stone 62 has been identified as coming from Garn Ddu Fach, an outcrop some 6.79 km to the east-southeast of Waun Mawn. None of the four dolerite monoliths at Waun Mawn have compositions which match Stonehenge Stone 62, and neither do the weathered fragments from stonehole 91. Rather the data show that the Waun Mawn monoliths, and most probably the weathered stonehole fragments, can be sourced to Cerrig Lladron, 2.37 km southwest of Waun Mawn, suggesting that a very local stone source was used in construction of the Waun Mawn stone circle. It is noted that there is evidence that at least eight stones had been erected and subsequently removed from the Waun Mawn circle but probability analysis suggests strongly that the missing stones were also derived, at least largely, from Cerrig Lladron

Direct evidence of fluid mixing in the formation of stratabound Pb–Zn–Ba–F mineralisation in the Alston Block, North Pennine Orefield (England)

The North Pennine Orefield Alston Block has produced approximately 4 Mt Pb, 0.3 Mt Zn, 2.1 Mt fluorite, 1.5 Mt barite, 1 Mt witherite, plus a substantial amount of iron ore and copper ore from predominantly vein-hosted mineralisation in Carboniferous limestones. However, a significant proportion of this production (ca. 20%) came from stratabound deposits. Though much is known about the vein mineralisation, the relationship between the veins and the stratabound mineralisation is not well-understood. New petrographic, isotopic and fluid inclusion data derived from samples of stratabound mineralisation allow us to present a unified model that addresses the genesis of both the vein and stratabound styles of mineralisation. The mineralisation can be considered in terms of three episodes: 1. Dolomitisation and ankeritisation Limestones in the vicinity of the stratabound mineralisation were pervasively dolomitised/ankeritised, and developed vuggy porosity in the presence of a high-salinity brine consistent with fluids derived from adjacent mud and shale-filled basins. 2. Main stage fluorite–quartz–sulphide mineralisation Metasomatism of limestone was accompanied by brecciation, dissolution and hydrothermal karstification with modification of the existing pore system. The open space was filled with fluorite, galena, sphalerite, quartz and barite, formed in response to mixing of lowsalinity sodic groundwater with high-salinity calcic brine with elevated metal contents (particularly Fe up to 7,000 ppm) relative to “normal” high total dissolved solids sedimentary brines. 3. Late-stage barite mineralisation paragenetically appears to represent either the waning stages or the distal portions of the main hydrothermal circulation system under cooler conditions

Permeability of rock discontinuities and faults in the Triassic Sherwood Sandstone Group (UK): insights for management of fluvio-aeolian aquifers worldwide

Fluvio-aeolian sedimentary successions host groundwater aquifers at shallow depths (<~0.15 km), which overlie geothermal and shale-gas reservoirs, and nuclear waste repositories at intermediate depths (~0.15–2.0 km). Additionally, such deposits represent petroleum reservoirs at greater depths (~2.0–4.0 km). The need to improve conceptual understanding of the hydraulic behaviour of fluvial-aeolian sandstone successions over a large depth interval (~0–4 km) is important for socio-economic reasons. Thus, the hydraulic properties of the Triassic Sherwood Sandstone aquifer in the UK have been reviewed and compared to similar fluvio-aeolian successions. The ratio between well-scale and core-plug-scale permeability (Kwell-test/Kcore-plug) acts as a proxy for the relative importance of fracture versus intergranular flow. This ratio (which typically varies from ~2 to 100) indicates significant contribution of fractures to flow at relatively shallow depths (<~0.15 km). Here, permeability development is controlled by dissolution of calcite-dolomite in correspondence of fractures. The observed ratio (Kwell-test/Kcore-plug) decreases with depth, approaching unity, indicating that intergranular flow dominates at ~1 km depth. At depths ≥ ~1 km, dissolution of carbonate cement by rock alteration due to groundwater flow is absent and fractures are closed. Aeolian and fluvial deposits behave differently in proximity to normal faults in the Sherwood Sandstone aquifer. Deformation bands in aeolian dune deposits strongly compartmentalize this aquifer. The hydro-structural properties of fluvio-aeolian deposits are also controlled by mineralogy in fault zones. A relative abundance of quartz vs. feldspar and clays in aeolian sandstones favours development of low-permeability deformation bands