Beate Orberger

Se, As, Mo, Ag, Cd, In, Sb, Pt, Au, Tl, Re traces in biogenic and abiogenic sulfides from Black Shales (Selwyn Basin, Yukon territories, Canada): A nuclear microprobe study

Abstract Black shales can be an important source of rare metals such as Cu, Ag, As, Mo, Se, Tl, Cd, platinum group elements (PGE) and gold (Pasava et al., Econ. Geol. 91 (1996) 63). However, especially in the case of noble metals, carriers such as discrete minerals could not be identified. This nuclear microprobe study brings new data on the partition of rare metals in Fe-, Fe (Ni)-, Ni- (Fe)-, Ni- and Zn-sulfides from the Ni-Zn-Mo-PGE mineralized marin black shales (Selwyn Basin, Yukon Territories). Sulfides are present as mineralized vestimentiferan tube-worms which are pyritized. Fluid-mineral interaction led to transformation of these biogenic Fe-sulfides successively to Fe–Ni- and Ni–Fe-sulfides which is indicated by increasing Se contents. Ni-sulfides which are interstital to the tube worms represent remobilized material, they are richest in Se. Several tens to thousands ppm of Cu, Zn, As, Se, Mo were located in the biomineralized and transformed tube worms. Antimony was located in Fe–Ni and Ni-, and Zn sulfides and Ag was present in Fe–Ni and Ni–Fe-sulfides. Pt and Au can also be related to submicron-sized inclusions in Fe–Ni sulfides. Hydrothermal fluids (at least 250 °C) precipitated silica around tube worms, intercalating submicron-sized sphalerites. Larger Zn-sulfides contain about 5000 ppm Cd and locally about 60 ppm In. Ba-rich K-feldspar contains Cl-rich Ag–Cd alloys.

Biogenic nitrogen and carbon in Fe-Mn-oxyhydroxides from an Archean chert, Marble Bar, Western Australia

To quantify and localize nitrogen (N) and carbon (C) in Archean rocks from the Marble Bar formation, Western Australia, and to gain insights on their origin and potential biogenicity, we conducted nuclear reaction analyses (NRA) and carbon and nitrogen isotope ratio measurements on various samples from the 3460-Myr-old Fe-rich Marble Bar chert. The Marble Bar chert formed during the alteration of basaltic volcanoclastic rocks with Fe- and Si-rich hydrothermal fluids, and the subsequent precipitation of magnetite, carbonates, massive silica, and, locally, sulfides. At a later stage, the magnetite, sulfides, and carbonates were replaced by Fe-Mn-oxyhydroxides. Nuclear reaction analyses indicate that most of the N and C resides within these Fe-Mn-oxyhydroxides, but a minor fraction is found in K-feldspars and Ba-mica dispersed in the silica matrix. The N and C isotopic composition of Fe-oxides suggests the presence of a unique biogenic source with δ15NAIR values from +6.0 ± 0.5‰ to 7.3 ± 1.1‰ and a δ13CPDB value of −19.9 ± 0.1‰. The C and N isotope ratios are similar to those observed in Proterozoic and Phanerozoic organic matter. Diffusion-controlled fractionation of N and C released during high combustion temperatures indicates that these two elements are firmly embedded within the iron oxides, with activation energies of 18.7 ± 3.7 kJ/mol for N and 13.0 ± 3.8 kJ/mol for C. We propose that N and C were chemisorbed on iron and were subsequently embedded in the crystals during iron oxidation and crystal growth. The Fe-isotopic composition of the Marble Bar chert (δ56Fe = −0.38 ± 0.02‰) is similar to that measured in iron oxides formed by direct precipitation of iron from hydrothermal plumes in contact with oxygenated waters. To explain the N and C isotopic composition of Marble Bar chert, we propose either (1) a later addition of N and C at the end of Archean when oxygen started to rise or (2) an earlier development of localized oxygenated environments, where biogeochemical cycles similar to modern ones could have developed.

Biogenic and abiogenic hydrothermal sulfides: controls of rare metal distribution in black shales (Yukon Territories, Canada)

Abstract The black shales of the Selwyn basin contain relicts of worm tubes which are sulfurized and silificified. Nuclear microprobestudies on biogenic Fe Ni sulfides indicate the presence of several tenths to thousands ppm of Cu, Zn, Se, Mo, As, Sb and Tl. Several tenths of ppm of Ag. Cd and In were observed in hydrothermal sphalerite, and Pt and An of several hundreds of ppm occur as microsize inclusions in framboidal pyrite, rather than being incorporated in the sulfide structures. Mineralogical and chemical investigations prove the presence of hydrothermal fluids of acidic composition, pH of 8–8.5 and temperatures of about 260 °C which carried at least part of the metals. Nuclear reaction analyses indicate the presence of nitrogen mainly in hydrothermal feldspar.

Nuclear microprobe analysis of serpentine from the mid-Atlantic ridge

Abstract At mid-ocean ridges, ultramafic rocks are serpentinized by interaction with seawater-derived fluids. Elements, dissolved in large quantities in seawater, e.g., Na, K, Cl, Br, Ca and Sr, can be, in small amounts, incorporated as traces into the crystal structure of the various serpentine minerals (Mg 3 Si 2 O 5 (OH) 4 ). These trace elements can be used to track the composition of the reacting fluids and to constrain physico–chemical conditions. This paper represents the first application of particle-induced X- and γ-ray emission (PIXE/PIGE) analysis to serpentine using the nuclear microprobe at the Laboratoire Pierre Sue (CEA-CNRS). Three types of serpentine, belonging to two different serpentinization generations, have been analysed in samples collected from the Mid-Atlantic Ridge (14°45′N/45°W) that exposes serpentinized peridotites on which the Logachev black smoker is placed. The trace elements Cl, F, S, Cu, Zn, Ca, K, Ni, Cr and Mn were detected from several tens to several thousands of ppm. Bromine, As and Sr are close to the detection limit of about 5 ppm. The trace element concentrations and interelement relationships in serpentines vary (a) with the serpentine type and (b) with the geographic location to the black smoker. Chlorine and in part S originated from seawater, whereas Cu, Zn, Ca, K, Ni, Cr and Fe and the major amount of S were mobilized from the unaltered host rock and partitioned between the serpentine and the aqueous solution.

Diagenetic evolution of Cretaceous siltstones from drill core MAR 501 (South-Eastern France)

Abstract A mineralogical, petrological and chemical study on samples issued from drill core MAR 501 documents the diagenetic evolution of Upper Cretaceous siltstones at the western margin of the South-Eastern French basin. The main detrital grains (quartz, feldspars, muscovite, ilmenite), bioclasts and organic matter are partly altered by carbonate fluids, liberating Si, Ba, K, Ca, Mg, Sr, Fe, S, P and Ti. Early diagenetic glauconite incorporated Fe and K. During burial, glauconite is transformed into clay minerals with liberation of Fe and K. The diagenetic fluids precipitated framboidal pyrite, Ti oxides in clay filled cataclastic fissures of glauconite, collophane which traps P and F and part of the S in its structure and precipitation of the excess S as Fe-sulphides. Zircon, rutile and tourmaline were not altered. Quartz overgrowth around detrital quartz grains was rarely observed. Two major types of calcite are present. Large Fe-rich calcite II replaced Fe-poor micritic calcite I. Minor dolomite, Fe-dolomite and Sr-bearing baryte were contemporaneous to calcite II. Calcite II of poikilotopic texture embedded detrital quartz and formed scattered centimetric patches leading to a heterogeneous porosity along the drill core.

Contribution to the petrogenesis of the platinum palladium-bearing black dunites of the Acoje block, Zambales ophiolite, Philippines: selenium and sulfur-contents, clinopyroxenes, initial weathering

Abstract Black serpentinites (BS), hosting the NiCu sulfide, platinum and palladium mineralisation in the Acoje ophiolite block in contact with chromitite concentrations, were investigated in comparison to the non-mineralised green serpentinites (GS), concerning their selenium and sulfur bulk rock contents, their secondary clinopyroxene, pyroaurite and hematite formation. Se/S-ratios in the black serpentinites range from 3.4 × 10 −3 to 4.8 × 10 −4 . They are in the same order of magnitude as Se/S-ratios analysed in individual sulfides (Orberger and Taxel 1991) and consistent with the suggested mantle Se/S-ratios of 10 −4 (Eckstrand R., Goldschmidt Conference, Baltimore, 1988). A positive correlation of selenium and platinum/palladium is found for this ophiolitic environment. This correlation is proposed for high PtPd potential magmatic sulfide deposits in layered intrusions (Eckstrand 1988); thus, this study confirms the magmatic character of the NiCu-sulfide mineralisation proposed by Abrajano and Pasteris ( Tectonophysics 168 , 65–100, 1989). Two types of diopsides characterise mainly the black serpentinites. Diopside I is present in green and black serpentinite and shows CaO contents of about 21 wt%, and Cr 2 O 3 contents of 0.54–0.92. Diopside II is mainly present in black serpentinite. It is extremely CaO-rich (24.0–25.4 wt% CaO) and significantly lower in Cr 2 O 3 (0.04–0.54 wt%). Initial weathering is mainly observed in BS and is expressed in the formation of pyroaurite and locally Fe-oxide and -hydroxide, surrounding the base metal sulfides, and the formation of Fe-rich “serpentines” (up to 11 wt%), forming large black platy aggregates in the clay fraction of BS. Pyroaurite-formation buffers pH and retains PtPd during weathering, whereas hematite and goethite formation provide sufficiently oxidising conditions for a PtPd-transport. The black platy aggregates in the clay fraction of the black serpentinites might be responsible for its black color.

Preface for thematic issue on “Supergene ore deposits”

Supergene ore deposits such as Ni-laterite, Al-bauxite, or Pb-Zn oxide deposits are typically mined by open pit methods. Open pit mining has a considerable impact on the environment, such as dust generation of partly toxic materials, changes in water table elevation, and impact on fauna and flora and urbanization. Furthermore, lower ore grades and multiple metal carriers in supergene ores (silicates, carbonates, oxyhydroxides) create beneficiation and metallurgical processing challenges. Both environmental impact, mining, and processing costs can be reduced through a better and early understanding of the complex evolution of these supergene deposits and applying highresolution analytical techniques. This thematic issue contains six original contributions on Ni-laterite deposits from two major mining regions, the Carribean islands (Cuba and Dominican Republic) and New Caledonia (Koniambo), which were presented at the 13th SGA Biennial Meeting (2015) in Nancy, France. One paper discusses karstic bauxite, an important source for aluminum, which has new uses in the cement industry. Two papers document supergene (Cu) Pb-Zn deposits. It is shown that in many laterite profiles and Pb-Zn carbonate-oxide deposits, ascending hydrothermal fluids at the base of the alteration profile and descending or laterally infiltrating meteoric water, can lead to a complex metal distribution even at the mineral scale. This has implications for tracing high-tech elements such as Ge and V (Pb-Zn deposits) and platinum-group elements (PGEs) (Ni-laterite deposits). About 70% of the current global Ni production is from laterite deposits. Nickel laterites were extensively studied in the 1980s and 1990s. Research restarted about 15 years ago, as typical Ni-laterite grades decreased from 2 to 4% down to 0.8% (cut-off), and ore processing became more complex. Ni laterites were classically mined for garnierite in veins, or disseminated in the saprolite zone, or at the interface between serpentinite and saprolite. Currently, several Ni laterite ores are characterized by multiple Ni-carrier minerals finely dispersed in the soil matrix. Advanced analytical techniques (micro-Raman spectroscopy, infrared spectroscopy) coupled with EMPA and XRD instruments and thermodynamic and numerical modeling yield new insights in the complex processes and mechanisms of laterite evolution. So far, the Carribean deposits were little studied, and this volume presents remarkable contributions on laterite from the Dominican Republic and Cuba by the Carribean Lithosphere Study group (http://caribbeanlithos.com/), covering the mineralogy and geochemistry of Ni, but also of PGE/PGM along laterite profiles. Serpentine, usually considered having formed through hydrothermal processes, occurs in saprolite as Niand Fe-rich varieties. These varieties are, in fact, a combination of lizardite-nepouite and Fe-lizardite, a mineral association resulting from weathering and Ni leaching from goethite (Villanova-deBenavent et al. this issue). Domènech and coworkers (this issue) study limonite-dominated Ni laterite with a thin saprolite zone at the base (Punta Gorda, Cuba; 1.0–1.6% Ni), similar to 60% of Ni-laterite deposits worldwide. They show, on the basis of mineralogical analyses and 1D Editorial handling: G. Beaudoin