John Still

Mobility of the high field strength, rare earth elements and yttrium during late diagenesis

Authigenic titanite occurs as a secondary, pore-filling cement in the Permian sandstones of the Cock of Arran, Scotland. The titanite is apparently not associated with in situ alteration of a specific mineral phase or phases, suggesting it crystallized from fluids migrating through these porous and permeable sandstones. The titanite contains significant concentrations of Zr, Nb, P, Ce, Nd, Gd and Y, implying that these elements were also carried in solution, probably in the form of halogen or carbonate complexes associated with hydrothermal circulation related to the intrusion of the adjacent Northern Granite.

Sulfur isotope signatures for rapid colonization of an impact crater by thermophilic microbes

In the 23-km-diameter Haughton impact structure, Canadian High Arctic, in sulfate-rich bedrock, widespread hydrothermal sulfide mineralization occurred in breccias formed during the impact. The sulfides exhibit extreme sulfur isotopic fractionation relative to the original sulfate, requiring microbial sulfate reduction by thermophiles throughout the crater. This evidence of widespread microbial activity demonstrates that colonization could occur within the lifetime of a moderately sized, impact-induced hydrothermal system. The pyrite was subsequently oxidized to jarosite, which may also have been microbially mediated. The successful detection of evidence for microbial life suggests that it would be a valuable technique to deploy in sulfate-rich impact terrain on Mars.

Redox-controlled selenide mineralization in the Upper Old Red Sandstone

Synopsis Selenium concentrations occur in organic-rich lithologies such as coal, and as secondary accumulations from the oxidative mobilization, migration and subsequent concentration upon encountering reductants such as organic matter or H2S. Here we assess the redox mineralization of copper and lead selenide phases in the cores of reduction spheroids in Devonian sedimentary red beds. We propose that the selenide mineralization occurred as descending meteoric fluids, supplied with selenium from Carboniferous coal seams, migrated through a major fault plane into underlying sandstones where localized, microbially-induced reducing conditions prevailed. These findings and other occurrences of selenide mineralization between Devonian and Carboniferous strata on the British Isles suggest a widespread selenide mineralization system between strata of that age. Supplementary material: Detailed quantitative methodologies and supplementary data are available at http://www.geolsoc.org.uk/SUP18770.

The occurrence of Zr-bearing amphiboles and their relationships with the pyroxenes and biotites in the teschenite and nepheline syenites of a differentiated dolerite boss, Islay, NW Scotland

Abstract The Cnoc Rhaonastil minor dolerite intrusion on Islay, NW Scotland represents a single body of alkali olivine basalt magma which differentiated in situ, from leucodolerite, through teschenite to minor nepheline syenite. The syenites occur as isolated nests and pegmatitic schlieren within the leucodolerite, and schlieren of gabbroic pegmatite also occur at the margin of the teschenite. The differentiated rocks contain pyroxene, amphibole and biotite of variable compositions which reflect both primary fractionation processes and late-stage deuteric alteration and reaction.Mafic phases within the gabbroic pegmatite, teschenite and syenite are typically rimmed and speckled with biotite, the composition of which is controlled by the local environment of crystallization. The nepheline syenites contain primary ferro-kaersutite which, where in contact with interstitial patches, has been altered to arfvedsonite, which occasionally contains up to 1.2 wt.% ZrO2. The occurrence of Zr-arfvedsonite (and of Zr-aegirine) in interstitial patches suggests that variably trace element-enriched domains existed within the residual melts on very small scales.

Roscoelite associated with reduction spots in Devonian red beds, Gamrie Bay, Banffshire

Synopsis Roscoelite, a vanadium mica, occurs in the cores of reduction spots within the Lower Old Red Sandstone that crops out in Gamrie Bay (Moray Firth). The finely crystallized mineral is seen both as a cement in the sandstone and replacing plagioclase feldspar in a lithic clast of granitic composition. It is suggested that roscoelite was formed by a complex reaction involving a change in the oxidation–reduction potential of the groundwater and the presence in the sandstone of sensitive associations of minerals containing vanadium-rich iron and titanium oxides. The reduction spots formed by diffusion as part of the same process.

Cadmium sulfide in a Mesoproterozoic terrestrial environment

Abstract Cadmium sulfide mineralization occurs in grey-black shales of the late Mesoproterozoic Stoer Group, Torridonian Supergroup, northwest Scotland. Cadmium is strongly redox-controlled, and normally concentrated in anoxic marine sediments or epigenetic mineralization involving organic matter. However the Stoer Group was deposited in a terrestrial environment, including lacustrine deposits of shale. At the limited levels of atmospheric oxygenation in the Mesoproterozoic (~10% of present), the near-surface environment could have fluctuated between oxic and anoxic, allowing fractionation of Cd from Zn, and the formation of Cd sulfide rather than Cd-bearing sphalerite. This occurrence emphasizes the importance of the Stoer Group as a record of the Mesoproterozoic terrestrial environment.

Gold in Devono- Carboniferous red beds of northern Britain

Authigenic gold occurs in Devono-Carboniferous red beds in northern Britain. Red beds exhibit concentrations of gold and pathfinder elements for gold mineralization including tellurium and mercury, at redox boundaries. Detailed studies of samples from Millport, Isle of Cumbrae, Firth of Clyde, show particles of native gold up to 10 µm size, typically with less than 15 wt% silver. Their context indicates that the gold was concentrated during diagenesis, in rocks that had not experienced regional temperatures above 100°C. These occurrences add to other evidence of a role for red beds in the genesis of gold mineralization.