Jan M. Peter

Determination of selenium isotopic ratios by continuous-hydride-generation dynamic-reaction-cell inductively coupled plasma-mass spectrometry

The goal of this study was to develop an accessible method for the determination of selenium isotopes within selenium-rich geological samples and examine the influence of sample introduction, instrumental parameters, column separation and the application of standard-sample bracketing for mass bias and drift correction. Quantitative selenium separation and enrichment of samples was achieved by a column separation using 0.2 g thioglycollic impregnated cotton fiber prior to introduction of the sample to an ICP-DRC-MS. 0.6 ml min−1 premixed Ar (95%) + H2 (5%) was favored over CH4 and NH4 as a reaction cell gas and was used within the DRC coupled with optimized DRC rejection parameters RPa (0) and RPq (0.65) to effectively reduce the signal to background ratio of all measured selenium isotopes (m/z 76, 77, 78, 80 and 82). Ion signal intensity of all measured selenium isotopes were increased 100 fold over classic nebulization by mixing of 1% NaBH4 and acidified sample digestions in a membrane-less computer-controlled continuous hydride generator. Transient hydride ion signals were time-averaged for five readings and three replicates to produce an in-run precision (2σ) of ±0.45‰ δ82/76SeMerck (relative to a Merck titrosol ICP-MS standard) and ±0.85‰ δ82/76SeMerck over an 18 month period. In the absence of a selenium isotopic standard, the accuracy of the method was determined using four interlaboratory solutions and five geological standard reference materials covering 0 to −4.5‰ δ82/76SeMerck. Our results indicate excellent reproducibility within method precision. The minimum mass of Se required for isotopic ratio determination was 3 μg (>100 000 cps at m/z 82 and 76).

U–Pb geochronology and Pb isotope characteristics of the Chahgaz volcanogenic massive sulphide deposit, southern Iran

The Chahgaz Zn–Pb–Cu volcanogenic massive sulphide (VMS) deposit occurs within a metamorphosed bimodal volcano–sedimentary sequence in the south Sanandaj–Sirjan Zone (SSZ) of southern Iran. This deposit is hosted by rhyodacitic volcaniclastics and is underlain and overlain by rhyodacitic flows, volcaniclastics, and pelites. Peperitic textures between rhyodacite flows and contact pelites indicate that emplacement of the rhyodacite occurred prior to the lithification of the pelites. The rhyodacitic flows are calc-alkaline, and show rare earth and trace elements features characteristic of arc magmatism. Zircons extracted from stratigraphic footwall and hanging-wall rhyodacitic flows of the Chahgaz deposit yield concordant U–Pb ages of 175.7 ± 1.7 and 172.9 ± 1.4 Ma, respectively, and a mean age of 174 ± 1.2 Ma. This time period is interpreted to represent the age of mineralization of the Chahgaz deposit. This Middle Jurassic age is suggested as a major time of VMS mineralization within pull-apart basins formed during Neo-Tethyan oblique subduction-related arc volcano-plutonism in the SSZ. Galena mineral separates from the layered massive sulphide have uniform lead isotope ratios of 206Pb/204Pb = 18.604–18.617, 207Pb/204Pb = 15.654–15.667, and 208Pb/204Pb = 38.736–38.769; they show a model age of 200 Ma, consistent with the derivation of Pb from a Late Triassic, homogeneous upper crustal source.

The Lake St. Martin bolide has a big impact on groundwater fluoride concentrations

The majority of residents of Manitoba (Canada) outside of the capital, Winnipeg, rely on groundwater for their drinking water. Between lakes Winnipeg and Winnipegosis, most aquifers occur in Paleozoic carbonate lithologies. Proximal to the town of Gypsumville, however, lithologies associated with the Lake St. Martin impact structure and younger basin-filling red bed and evaporite (gypsum/anhydrite) sedimentary rocks complicate the hydrology and hydrochemistry. Here, domestic wells have elevated salinities (up to 8000 mg/L total dissolved solids), elevated sulfate (up to 4000 mg/L), and elevated fluoride concentrations that are in excess of health limits (F − up to 15.2 mg/L, with 20% over 1.5 mg/L). Groundwaters with elevated fluoride occur exclusively within the impact structure. The impact melt rocks and younger red beds consistently have the highest fluoride abundances, up to 2160 ppm. Groundwater pH values are alkaline, ranging up to 10.7, with highest groundwater pH from wells in the impact melt rocks. The spatial associations of impact melt rocks and red beds with elevated fluoride, strong positive correlations between fluoride and pH, sodium, chloride, sulfate, boron and lithium, greater Fe 2 O 3 and Al 2 O 3 concentrations of the host rocks, and cation exchange capacity (CEC) all indicate that fluoride concentrations in groundwaters are enhanced as a result of anion exchange wherein OH − and \(CO_{3}^{2{-}}\) displace F − adsorbed onto Fe- and Al-oxyhydroxide surfaces. Thus, the elevated fluoride contents of groundwaters at Gypsumville are a consequence of the composition of the impact melt rocks and enhanced permeability and grain-size reduction produced by bolide impact.

Non-hydrothermal origin of apatite in SEDEX mineralization and host rocks of the Howard’s Pass district, Yukon, Canada

Abstract The Howard’s Pass district (HPD) comprises 14 Zn-Pb sedimentary exhalative (SEDEX) deposits and is located within the Selwyn basin, Yukon, Canada. Although the HPD is renowned for its large accumulation of base-metal sulfides, in places the Late Ordovician to Early Silurian host rocks also contain abundant carbonate-bearing fluorapatite (CBFA). This mineral is present stratigraphically below, within, and above the SEDEX deposits and occurs as fine-grained layers that are interbedded with cherty carbonaceous mudstone. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometric analysis reveal that mineral compositions and rare earth element-yttrium (REE-Y) systematics, respectively, are remarkably similar throughout the stratigraphic succession. North American Shale Composite (NASC)-normalized La/Sm and La/Yb ratios indicate that the original REE compositions in CBFA have undergone only minor compositional modification subsequent to deposition. Uniformly negative Ce anomalies indicate that the mineral formed in analogous manner to modern and ancient sedimentary phosphorites under suboxic bottom-water conditions. Europium anomalies are mostly absent, indicating that reduced, slightly acidic high-temperature hydrothermal fluids were not a major source of REE-Y to CBFA. The chemical homogeneity of the mineral irrespective of its stratigraphic position indicates that a common process was responsible for its deposition within the sedimentary rocks of the HPD. On the basis of the similarity of the REE patterns to modern and ancient phosphorites, and the absence of positive Eu anomalies, we conclude that the CBFA is of hydrogenous origin, and not hydrothermal as suggested by previous workers. As such, phosphorite formation in the HPD is casually related to SEDEX Zn-Pb deposit formation.

Till geochemical signatures of volcanogenic massive sulphide deposits: an overview of Canadian examples

Volcanogenic massive sulphide (VMS) deposits are an important source of Cu, Pb, Zn and Ag in Canada. These deposits formed on the paleo-seafloor in volcanic or volcano-sedimentary rocks. On the glaciated landscape of Canada, the bulk composition of till (so-called ‘till geochemistry’) is an important exploration method for VMS deposits. Herein we provide an overview of the 50-plus year history of the application of till geochemical methods to VMS exploration in Canada, and summarize best practices, including appropriate size fractions of till to analyse, analytical techniques and indicator and pathfinder elements. Till geochemical methods are now well-developed and used widely, most commonly employing the <0.063 mm till fraction and an extensive suite of VMS indicator elements Cu, Pb, Zn, Au, and Ag, as well as pathfinder elements Ag, Au As, Bi, Cd, Ga, Ge, Hg, In, Sb, Se, Sn, Tl. In order to detect the clastic glacial dispersal signal down-ice of a VMS deposit, unoxidized to weakly oxidized till should be collected and analysed, and not B-horizon soil. Case studies and examples of glacial dispersal patterns associated with known VMS mineralization in major mining camps and deposits across Canada are highlighted.

Geochemical constraints on the tectonic setting of basaltic host rocks to the Windy Craggy Cu-Co-Au massive sulphide deposit, northwestern British Columbia

Windy Craggy is an approximately 300 Mt Cu-Co-Au volcanogenic massive sulphide (VMS) deposit in northwestern British Columbia, Canada. The Windy Craggy deposit is hosted by the Middle Tats Volcanics (MTV), a Late Triassic volcano-sedimentary sequence of intercalated mafic pillowed to massive volcanic flows and sills and calcareous argillite that are part of the Alexander terrane. The host footwall and hangingwall flows and sills are predominantly alkalic basalts (Nb/Y > 0.70). MTV alkali basalts at Windy Craggy are enriched in light rare earth elements (LREEs) >100X chondrite compared to chondrite, have steep REE patterns [(La/Yb)cn = 7.1–25.4], and generally lack the Ta and Nb depletions relative to primitive mantle (e.g. [Nb/Th]pm = 0.68–1.94) characteristic of arc environments, although most have [Nb/La]pm < 1. By contrast, volcanic rocks away from the deposit (and regionally; Lower Tats Volcanics, LTV) as well as late dikes that cross-cut all lithologies including metamorphic and deformational fabrics are sub-alkalic tholeiitic to calc-alkaline basalts and basaltic andesites that are less enriched in the LREEs (10–100X chondrite), have less steep REE patterns [(La/Yb)cn = 0.41–10.6], and show well-developed Ta and Nb depletions (arc signatures; [Nb/Th]pm = 0.20–0.79), consistent with formation in an oceanic arc environment. The co-occurrence of tholeiitic/calc-alkaline arc rocks with alkalic rocks indicates that the LTV (former) and MTV (latter) formed from melts that were influenced to varying degrees by subducted oceanic crust, and likely formed within a back-arc basin setting formed on a rifted oceanic arc. There is no geochemical or isotopic evidence for major involvement of continental crust. The LTV basalts likely were produced by progressive depletion in the source by partial melting of mantle overlying the subducting oceanic crust. The presence of the MTV alkalic Windy Craggy rocks overlying the LTV is consistent with the presence of a slab-window, perhaps related to subduction of a spreading centre, which allowed more enriched magmas to reach the surface with only minimal interaction with subduction-modified mantle. The presence of this slab-window might have provided the mechanism for the generation of anomalously high heat flow close to the seafloor, which initiated and sustained vigorous, long-lived hydrothermal activity necessary for the precipitation of large accumulations of massive sulphide. To our knowledge, this is the first example of a large VMS deposit associated with a slab-window.

Transport and deposition of selenium in felsic volcanic-hosted massive sulfide deposits of the Finlayson Lake District, Yukon Territory, Canada

The mobility and hydrothermal transport of selenium are a function of the fluid pH, fO2 and temperature. At high temperatures (>200°C) and relatively acidic (pH <6) conditions, H2Se is the dominant aqueous species. At ambient seafloor temperatures, reduced forms of selenium are relatively immobile, but the element becomes increasingly mobile under oxidized conditions. Deposition of selenium from a hydrothermal fluid results when relatively hot, acidic, reduced hydrothermal fluid mixes with cold (1-10°C), oxidized ambient seawater. The selenium content of the precipitating sulfide is controlled by the reaction: MtSe2 + H2S(aq)=MtS2+H2Se(aq), where Mt is a transition metal, and therefore H2Se/H2S approximates the Se/S of the fluid at the time of sulfide precipitation. However, in modern and ancient volcanic-hosted massive sulfide (VHMS) systems the final residence site of transition metals (and Se), or their redistribution, is ultimately a function of the solubility product (log Q/K) and temperature of precipitation of the sulfide minerals, which produce metal zoning in sulfide mounds as a result of hydrothermal fluid interaction/ dissolution with previously precipitated sulfide minerals (a.k.a., zone refining). Observations and data from ancient VMS deposits in the Finlayson Lake District (FLD), Yukon, Canada show a strong positive correlation between high-temperature (<300°C), copper-rich sulfide assemblages at the base of VHMS and the selenium content of all sulfide minerals. Thermodynamic calculations for selenide and sulfide minerals indicate significantly higher temperatures and lower solubility indices for the formation of selenide versus sulfide pairs, which is consistent with petrographic observations and can explain the distribution of selenium in zone-refined VMS deposits.

Metavolcanic host rocks, mineralization, and gossans of the Shaib al Tair and Rabathan volcanogenic massive sulphide deposits of the Wadi Bidah Mineral District, Saudi Arabia

ABSTRACT The Wadi Bidah Mineral District of Saudi Arabia contains more than 16 small outcropping stratabound volcanogenic Cu–Zn–(Pb) ± Au-bearing massive sulphide deposits and associated zones of hydrothermal alteration. Here, we use major and trace element analyses of massive sulphides, gossans, and hydrothermally altered and least altered metamorphosed host rock (schist) from two of the deposits (Shaib al Tair and Rabathan) to interpret the geochemical and petrological evolution of the host rocks and gossanization of the mineralization. Tectonic interpretations utilize high-field-strength elements, including the rare earth elements (REE), because they are relatively immobile during hydrothermal alteration, low-grade metamorphism, and supergene weathering and therefore are useful in constraining the source, composition, and physicochemical parameters of the primary igneous rocks, the mineralizing hydrothermal fluid and subsequent supergene weathering processes. Positive Eu anomalies in some of the massive sulphide samples are consistent with a high temperature (>250°C) hydrothermal origin, consistent with the Cu contents (up to 2 wt.%) of the massive sulphides. The REE profiles of the gossans are topologically similar to nearby hydrothermally altered felsic schists (light REE (LREE)-enriched to concave-up REE profiles, with or without positive Eu anomalies) suggesting that the REE experienced little fractionation during metamorphism or supergene weathering. Hydrothermally altered rocks (now schists) close to the massive sulphide deposits have high base metals and Ba contents and have concave-up REE patterns, in contrast to the least altered host rocks, consistent with greater mobility of the middle REE compared to the light and heavy REE during hydrothermal alteration. The gossans are interpreted to represent relict massive sulphides that have undergone supergene weathering; ‘chert’ beds within these massive sulphide deposits may be leached wall-rock gossans that experienced silicification and Pb–Ba–Fe enrichment from acidic groundwaters generated during gossan formation.

Combined hyperspectral and lithogeochemical estimation of alteration intensities in a volcanogenic massive sulfide deposit hydrothermal system: A case study from Northern Canada

The most intense hydrothermally altered rocks in volcanogenic massive sulfide (VMS) deposit systems occur in the stratigraphically underlying feeder zone and rocks immediately adjacent to mineralization. This alteration zone is typically much larger than the mineralization itself, and hence the ability to detect such alteration by optical remote sensing can be invaluable for mineral exploration. Our investigation focuses on assessing the applicability of hyperspectral data to determine trends in hydrothermal alteration intensity in and around the Izok Lake VMS deposit in northern Canada. To this end, we linked hydrothermal alteration intensity information based on two indices, the Ishikawa (AI) and chlorite-carbonate-pyrite (CCPI), to hyperspectral field and laboratory data in three dimensions. Our results suggest that chlorite group minerals display variable chemical composition across the study area that broadly correlates with hydrothermal alteration intensity.

An appreciation of Daniel Robert Boyle, and an introduction to the special issue of Geochemistry: Exploration, Environment, Analysis in his honour

Daniel Robert Boyle,our friend and colleague, died on 29 June 2000, in Ottawa, Canada, at the age of 52 after a courageous battle with cancer. Dan was a leader at the Geological Survey of Canada (GSC) in the application of lithogeochemistry, and groundwater geochemistry for the study of mineral deposit genesis, mineral exploration, environmental and health issues. He was the son of the (recently deceased: 5 August 2003) internationally renowned and respected geochemist Bob(Robert W.) Boyle, who was also employed at the GSC (Garrett 2004). Growing up in the 1950s and 1960s, Dan spent many happy summers travelling with his family to places across Canada, notably New Brunswick and Nova Scotia, where his fathers fieldwork took him. These experiences instilled in Dan and his sister Heather a love for the sciences and the outdoors that shaped both their future careers: Dan following in his fathers footsteps as a geochemist, while Heather, the family radical, became a biochemist (Carleton University, Ottawa, Canada and Victoria University, Wellington, New Zealand). Dan completed a BSc honours degree at Queens University at Kingston, Canada, with a thesis entitled “Geochemical environmental study over a Cu–Mo porphyry deposit, British Columbia”. He continued his education at the Imperial College of Science and Technology, University of London, London, UK. He graduated with a PhD in 1976, and his thesis was entitled “The geochemistry of fluorine and its application in mineral exploration“. During his studies in the UK, he completed several mineral exploration consulting contracts in Spain, France, Ireland, Turkey, and Great Britain. During the latter part of his PhD studies, he became a member of the Mineral Industry Research Organization review board of Great Britain. While at Imperial College, Dan became an associate professor, and later (1975) head of the mineral exploration research section of the applied geochemistry …