Linking derived debitage to the Stonehenge Altar Stone using portable X-ray fluorescence analysis

The Altar Stone at Stonehenge in Wiltshire, UK, is enigmatic in that it differs markedly from the other bluestones. It is a grey-green, micaceous sandstone and has been considered to be derived from the Old Red Sandstone sequences of South Wales. Previous studies, however, have been based on presumed derived fragments (debitage) that have been identified visually as coming from the Altar Stone. Portable X-ray fluorescence (pXRF) analyses were conducted on these fragments (ex situ) as well as on the Altar Stone (in situ). Light elements (Z<37) in the Altar Stone analyses, performed after a night of heavy rain, were affected by surface and pore water that attenuate low energy X-rays, however the dry analyses of debitage fragments produced data for a full suite of elements. High Z elements, including Zr, Nb, Sr, Pb, Th and U, all occupy the same compositional space in the Altar Stone and debitage fragments, and are statistically indistinguishable, indicating the fragments are derived from the Altar Stone. Barium compares very closely between the debitage and Altar Stone, with differences being related to variable baryte distribution in the Altar Stone, limited accessibility of its surface for analysis, and probably to surface weathering. A notable feature of the Altar Stone sandstone is the presence of baryte (up to 0.8 modal%), manifest as relatively high Ba in both the debitage and the Altar Stone. These high Ba contents are in marked contrast with those in a small set of Old Red Sandstone field samples, analysed alongside the Altar Stone and debitage fragments, raising the possibility that the Altar Stone may not have been sourced from the Old Red Sandstone sequences of Wales. This high Ba 'fingerprint', related to the presence of baryte, may provide a rapid test using pXRF in the search for the source of the Stonehenge Altar Stone

The Stonehenge Altar Stone was probably not sourced from the Old Red Sandstone of the Anglo-Welsh Basin: time to broaden our geographic and stratigraphic horizons?

Stone 80, the recumbent Altar Stone, is the largest of the Stonehenge foreign “bluestones”, mainly igneous rocks forming the inner Stonehenge circle. The Altar Stone's anomalous lithology, a sandstone of continental origin, led to the previous suggestion of a provenance from the Old Red Sandstone (ORS) of west Wales, close to where the majority of the bluestones have been sourced (viz. the Mynydd Preseli area in west Wales) some 225 km west of Stonehenge. Building upon earlier investigations we have examined new samples from the Old Red Sandstone (ORS) within the Anglo-Welsh Basin (covering south Wales, the Welsh Borderland, the West Midlands and Somerset) using traditional optical petrography but additionally portable XRF, automated SEM-EDS and Raman Spectroscopic techniques. One of the key characteristics of the Altar Stone is its unusually high Ba content (all except one of 106 analyses have Ba &gt; 1025 ppm), reflecting high modal baryte. Of the 58 ORS samples analysed to date from the Anglo-Welsh Basin, only four show analyses where Ba exceeds 1000 ppm, similar to the lower range of the Altar Stone composition. However, because of their contrasting mineralogies, combined with data collected from new automated SEM-EDS and Raman Spectroscopic analyses these four samples must be discounted as being from the source of the Altar Stone. It now seems ever more likely that the Altar Stone was not derived from the ORS of the Anglo-Welsh Basin, and therefore it is time to broaden our horizons, both geographically and stratigraphically into northern Britain and also to consider continental sandstones of a younger age. There is no doubt that considering the Altar Stone as a ‘bluestone’ has influenced thinking regarding the long-held view to a source in Wales. We therefore propose that the Altar Stone should be ‘de-classified’ as a bluestone, breaking a link to the essentially Mynydd Preseli-derived bluestones.</p

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

A review of the mineralisation at Driggith and Sandbed mines, Caldbeck Fells, Cumbria

Low temperature lead-zinc-copper veins are developed within a NE-SW trending fracture system in rocks of the Eycott Volcanic Group at Driggith and Sandbed mines in the Caldbeck Fells, Cumbria. The lead veins were worked by a succession of mining companies from the 18th century onward, first at Driggith mine and later at Sandbed mine. East-west trending baryte-quartz-carbonate veins were worked for baryte in the mid-20th century. The dumps from these mines surround and in some cases cover those of the earlier lead workings. Several episodes of primary mineralisation can be distinguished. The primary lead vein mineralisation comprises major quartz, calcite, sphalerite, galena and chalcopyrite. Sparse antimony-rich sulphide mineralisation is present as inclusions in the galena. There is rich, localised, iron sulphide-arsenopyrite mineralisation, which pre-dates the lead-copper-zinc vein mineralisation; sparse nickel- and cobalt-bearing mineralisation of unknown affinity, and baryte mineralisation with quartz and carbonates that post-dates the lead-copper-zinc vein mineralisation. The later baryte is present in distinct E-W trending veins and as a later stage of mineralisation in the fractures that host the lead-copper-zinc vein mineralisation. Supergene oxidation is extensive. About sixty supergene minerals have been identified. Distinctive specimens of curved green arsenate-rich pyromorphite on hackly quartz are well known from the opencut above Driggith mine. Bayldonite, cerussite, mimetite and malachite are widespread and abundant. Anglesite, aurichalcite, hemimorphite, Iinarite and philipsburgite are relatively common in micro-crystalline specimens. The arsenate minerals bariumpharmacosiderite, beudantite, mimetite, segnitite and scorodite form distinct localised gossans, which are closely associated with primary arsenopyrite. A variety of sulphate minerals including brochantite, langite, linarite, serpierite and schulenbergite have been formed by post-mining oxidation. Minerals discovered for the first time at the Sand bed and Driggith mines as a result of this study include arsendescloizite, annabergite, brianyoungite, djurleite, erythrite, gersdorffite, kottigite, lanarkite, lavendulan, olivenite, mawbyite, parnauite, philipsburgite, pyrrhotite, redgillite, strashimirite, yarrowite and zalesiite. Of these, annabergite, brianyoungite, gersdorffite, lavendulan, parnauite, strashimirite, yarrowite and zalesiite are reported for the first time in the Caldbeck Fells and mawbyite for the first time in the British Isles. The published mineralogy of the Driggith and Sandbed mines is complicated by the claims of Arthur Kingsbury, many of which are fraudulent. Rare species claimed by Kingsbury, which should be removed from the list of minerals from Driggith and Sand bed include allophane, chalcanthite, conichalcite, chlorargyrite, plancheite, plumbogummite, phosgenite, pseudomalachite, turquoise and wulfenite. In addition, many fine specimens in the Kingsbury collection which are labelled from Driggith or Sandbed mine, including common species such as adamite, brochantite, cerussite, leadhillite, Iinarite and malachite, are fraudulent, although undoubted specimens (almost always of much poorer quality) are well known at both localities