Matthew I. Leybourne

Hf isotopic evidence for small‐scale heterogeneity in the mode of mantle wedge enrichment: Southern Havre Trough and South Fiji Basin back arcs

Magmas from SW Pacific back-arc basins have geochemical and isotopic signatures indicating variable mantle and subduction-derived components. Basalts from South Fiji Basin (SFB) are little influenced by subduction, but come from variably enriched mantle, resulting from mixing between enriched mantle, like FOZO, and depleted mantle, like DMM. The same components are present in the Havre Trough mantle, but Havre Trough basalts come from a mantle wedge to which a greater proportion of subduction-derived components are added. Their slab-derived components are isotopically similar to locally subducting sediment, with variable Sr and Pb from altered oceanic crust. Their compositional diversity correlates with morphology, previously described as contrasting Arc-type and Rift-type back-arc regimes. Geochemical modeling indicates that material is added as both supercritical fluids and slab melts below the back-arc, is locally distinct, and correlate with differences in the predicted slab-surface pressure and temperature conditions. Deeper slab surfaces correspond to higher-temperatures at a given distance from the volcanic front, but not necessarily with an increase in the amount of slab-derived material. Slab fluxes for rift-type basalts are consistent with predicted slab-surface temperatures at or below the water-saturated solidus. However, some are consistent with melting in equilibrium with residual rutile, zircon, and monazite, so melting may have occurred by fluid fluxing of the slab surface, requiring external fluids from within the slab. Arc-type basalts are explained by thermal anomalies in the mantle wedge, which may correspond to locally hotter slab-surface temperatures and more fractionated slab-derived component signatures in their source.

Geochemical evolution of Monowai volcanic center: New insights into the northern Kermadec arc subduction system, SW Pacific

We present trace element and Sr-Nd-Pb isotope data on volcanic rocks recovered from the submarine Monowai volcanic center, which marks the midpoint of the ∼2500 km long Tonga-Kermadec arc. The center consists of a large (12 × 9 km) partly hydrothermally active caldera and a 12 km diameter ∼1500 m high volcanically and hydrothermally active stratovolcano. The stratovolcano lavas are tholeiitic basalts, which show variable evidence for plagioclase (±pyroxene) accumulation. The caldera lavas range from basalt to andesite, with the compositional variation being consistent with fractional crystallization as the dominant process. The mafic Monowai magmas were generated by relatively high degrees (12%–20%) of partial melting of a previously depleted MORB-type spinel-peridotitic mantle, metasomatized by slab-derived fluids. Strongly fluid mobile 87Sr/86Sr and 207Pb/204Pb ratios of the Monowai basaltic lavas are similar to those from the Putoto, Raoul, and Macauley volcanic centers 200–400 km to the south, suggesting derivation largely from subducted sediment. In contrast, variably fluid immobile 143Nd/144Nd ratios suggest an isotopically heterogeneous mantle along this segment of the arc. Higher 206Pb/204Pb in Monowai lavas imply some influence from the nearby subducting Louisville seamounts in melt generation. The formation of one of the Earths largest submarine mafic calderas can best be explained through drainage of a single magma reservoir and subsequent collapse caused by trench-perpendicular extension, probably via southward progressive rifting of the northern Havre Trough.

Groundwater in geochemical exploration

ABSTRACT Groundwater is an important medium for geochemical exploration of many different styles of mineralization, including porphyry copper, volcanogenic massive sulphide (VMS), sandstone uranium, and gold. Groundwater recharges to depth, resulting in greater likelihood of interacting with buried mineralization compared to surface geochemical methods, and thus providing a three-dimensional perspective. Advances in the understanding of ore formation processes, water-rock interaction and element transport/attenuation in the secondary environment are enhancing the efficacy of groundwater geochemical exploration. This paper describes key techniques and methodologies for sampling, analysis and interpretation of groundwater geochemical data, and provides two different approaches for use by industry: routine exploration and research approaches. New advances in analytical methods are providing new isotopic systems and improving the cost and speed of traditional isotopic techniques, which can greatly aid in interpretation of water sources, water-rock reactions and fingerprinting of ore sources. Case studies are presented for the use of groundwater geochemistry around a porphyry copper deposit in the hyperarid Atacama Desert of Chile, and VMS mineralization in a mature mining camp in Canada. This paper also summarizes key elemental associations for successful utilization of aqueous geochemistry in mineral exploration. The most successful aqueous-phase indicators of mineralization are those that are associated with the ore and are mobile in solution.

Kimberlites and the start of plate tectonics

We want to know when plate tectonics began and will consider any important Earth feature that shows significant temporal evolution. Kimberlites, the primary source of diamonds, are rare igneous features. We analyze their distribution throughout Earth history; most are young (∼95% are younger than 0.75 Ga), but rare examples are found as far back as the Archean (older than 2.5 Ga). Although there are differing explanations for this age asymmetry (lack of preservation, lack of exposure, fewer mantle plumes, or lack of old thick lithosphere in the Archean and Proterozoic), we suggest that kimberlite eruptions are a consequence of modern-style plate tectonics, in particular subduction of hydrated oceanic crust and sediments deep into the mantle. This recycling since the onset of modern-style plate tectonics ca. 1 Ga has massively increased mantle CO2 and H2O contents, leading to the rapid and explosive ascent of diamond-bearing kimberlite magmas. The age distribution of kimberlites, combined with other large-scale tectonic indicators that are prevalent only in the past ∼1 Ga (blueschists, glaucophane-bearing eclogites; coesite- or diamond-bearing ultrahigh-pressure metamorphic rocks; lawsonite-bearing metamorphic rocks; and jadeitites), indicates that plate tectonics, as observed today, has only operated for <25% of Earth history.

Geochemical anomalies in northern Chile as a surface expression of the extended supergene metallogenesis of buried copper deposits

ABSTRACT Supergene enrichment played a critical role in making northern Chile the most productive copper province of the world. This occurred over a long period from 45 to 9 Ma in a semi-arid climate. Subsequently, many deposits were covered by thick alluvial gravels, such that exploration is now focused on deposits buried beneath these gravels. Well-defined saline+metal anomalies are present at the gravel surface above deposits. Work, starting in 1999, characterized these anomalies and ascribed their origin to pumping of mineralized saline waters to the surface during earthquakes. Atacamite, a copper hydroxychloride, and an important ore mineral, had previously been considered to be of primary supergene origin, i.e. 45 to 9 Ma. However, it is readily soluble in meteoric water and could not have been stable either during supergene enrichment by meteoric water, or later, when stream waters carrying alluvial gravels penetrated oxide zones. Atacamite must be younger than 9 Ma. Studies at the University of Chile have established that: (a) atacamite has been found at the surface along faults, 300 m above copper mineralization; (b) salinities of fluid inclusions in atacamite are the same as local saline groundwater; (c) 36Cl analyses of atacamite show that it could not have formed prior to 1.5 Ma; (d) U-Th disequilibrium dating of gypsum from atacamite-gypsum intergrowths give Pleistocene ages that range from 237 ka for Chuquicamata in the east to 75 ka for Michilla on the Pacific coast. We propose that anomaly formation at the surface and saline metasomatism of supergene oxides were parts of the same process, which occurred after the climate became hyper-arid. Tectonic de-watering of the forearc basin sent saline waters through oxide zones to replace pre-existing minerals with atacamite and continued to the surface to create geochemical anomalies; these waters, having first modified the deposits, were rich in indicator elements. Thus, although perceptions are that geochemical anomalies above thick gravel sequences should be weak, selective leach anomalies in northern Chile have clear anomaly/background contrast.

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.

Lead isotopes in ground and surface waters: fingerprinting heavy metal sources in mineral exploration.

ABSTRACT We collected ground and surface water samples, as well as samples of host felsic and mafic volcanic lithologies and massive sulphide ore from the Restigouche deposit in the Bathurst Mining Camp, New Brunswick, Canada. These water and rock samples were analysed for their Pb isotope composition in order to investigate the utility of Pb isotope analyses of waters for mineral exploration and environmental tracing. Waters proximal to the near-surface Zn–Pb Restigouche deposit have Pb isotope compositions indistinguishable from galena from a number of volcanogenic massive sulphide deposits in the camp (206Pb/204Pb = 18.18 – 18.34; 207Pb/204Pb = 15.63 – 15.68; 208Pb/204Pb = 38.10 – 38.21). These deposit proximal waters are interpreted to have derived the dissolved (< 0.45 μm) Pb primarily from oxidation of massive sulphide minerals (galena, sphalerite). Groundwaters more distal from the Restigouche deposit have more radiogenic isotopic compositions, with Pb derived by weathering of U- and Th-rich minerals in the felsic volcanic and volcaniclastic country rocks that host the deposit. There is no correlation between dissolved Pb concentration and isotopic signature owing to the low solubility of Pb in water. The Pb isotopic composition of the dissolved loads of waters is therefore a powerful tool in fingerprinting Pb sources and has important implications for mineral exploration and environmental baseline studies.

Geochemical and mineralogical dispersal in till from the Mount Polley Cu-Au porphyry deposit, central British Columbia, Canada

The Quesnel terrane in the Interior Plateau of British Columbia, Canada, is highly prospective for locating new porphyry deposits; however, the bedrock in this region is obscured by a nearly continuous blanket of till, making mineral exploration challenging. Located within the Quesnel terrane is the Mount Polley deposit. It is an alkaline, silica-undersaturated, Cu-Au porphyry deposit mined by Imperial Metals Corporation. Eighty-six basal till samples were collected for geochemical and mineralogical analyses in the region of this deposit. Ore elements (Ag, Au and Cu), as well as pathfinder element (Hg and Zn) contents in till reflect detrital glacial dispersal from the Mount Polley deposit. The distribution of anomalous mineral counts of andradite garnet, apatite, chalcopyrite, epidote, jarosite and native gold also reflect glacial dispersal from the deposit. Outcrop-scale ice-flow indicators indicate a dominant ice-flow event to the NW that was preceded by a southwestward glacial advance. The element and mineral signatures of the Mount Polley Cu-Au porphyry deposit are dispersed in sub-glacial surface tills up to 12 km in the down-ice (NW) direction. We demonstrate that till geochemistry and mineralogy can serve for mineral exploration of Cu-Au porphyry mineralization in drift covered areas. Supplementary material: Complete data set for element and mineral results and ice-flow measurement is available at www.geolsoc.org.uk/SUP18828

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.

Hydrogeochemical and geophysical evidence for borehole oxidation of massive sulphides below the water table and generation of self-potential anomalies

ABSTRACT Groundwater was sampled with a straddle-packer system at the Stratmat Main Zone deposit, New Brunswick, Canada, to better understand the origin of self-potential (SP) anomalies associated with massive sulphide mineralization and to investigate processes of sulphide oxidation below the water table. Groundwater displays a marked increase in total dissolved solids (TDS; 200 to 1600 mg/l) through the sulphide zone (84–89 m in depth). Groundwater ranges from low TDS CaHCO3-type water at shallow depth to higher TDS CaSO4-type water within the massive sulphides. The massive sulphides are coincident with a large SP anomaly (−600 mV) compared to the host rocks with an associated positive temperature anomaly of 0.05°C. With increasing depth through the massive sulphide zone, base metal (Cu, Zn, Mo, Pb, Cd) and Fe concentrations increase (e.g. Fe increases from 40 to 2300 mg/l). Stable isotopic compositions in groundwater also vary with depth in the sulphide zone with δ18OVSMOW increasing from −12.32 to −12.00‰ and δ13CVPDB from −13.55 to −10.31‰. The stable isotopes, TDS and straddle-packer recovery time data indicate that groundwater flow is greatest in the upper parts of the massive sulphides. The large SP anomaly is interpreted to result from decoupled sulphide oxidation occurring between oxygenated waters impacted by the borehole and non-pyritic sulphide phases within the sulphide mass. Electrons pass from sulphide phases (e.g. pyrrhotite) through intervening pyrite to dissolved oxygen in the borehole, which renders a cathodic character to the wall of the borehole and an anodic character to the interior of the sulphide. The higher hydraulic conductivity of the upper massive sulphide has allowed penetration of dissolved oxygen, which produces a similarly decoupled oxidation reaction between the upper and lower sulphides, at least proximal to the borehole. The increasingly anodic nature of the sulphide mass with increasing depth is demonstrated by the chemical evolution of groundwater with depth. Increasing SO42−, H+, Fe2+ and Zn2+ concentrations are evidence of the oxidation reactions occurring in deeper sulphides, whereas the higher pH in the upper massive sulphide is evidence of O2 reduction. Increased δ13CVPDB with depth probably results from enhanced carbonate mineral dissolution as acid is produced. The unusual positive temperature anomaly associated with the borehole may be partly a result of exothermic oxidation of the sulphide but is more likely due to the higher thermal conductivity of massive sulphide relative to country rock. The sulphide appears to be acting as a conduit for heat from deeper areas, which suggests that a large sulphide mass extends beneath the intersected section.