Stephen J. Piercey

Boninitic magmatism in a continental margin setting, Yukon- Tanana terrane, southeastern Yukon, Canada

Mid-Paleozoic mafic rocks in the Finlayson Lake region of the Yukon-Tanana terrane, southeastern Yukon, Canada, have the diagnostic geochemical signatures of boninites: high MgO, Cr, Ni, and Co contents, intermediate SiO 2 contents, high Mg#9s (MgO/ (MgO+FeO*), Al 2 O 3 /TiO 2 , and Zr(Hf)/middle rare earth element (REE) ratios; low TiO 2 , REE, and high-field-strength element contents; and U-shaped primitive mantle–normalized trace element patterns. However, unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms, those from the Finlayson Lake region are part of a mid-Paleozoic continental margin arc-backarc magmatic system. We propose a model in which the boninitic rocks from the Finlayson Lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust. In the oceanic segment, upwelling asthenosphere induced melting of a subducted-slab metasomatized refractory mantle source to form boninitic magmatism. In the continental sector, upwelling asthenospheric mantle, and/or the melts derived thereof, induced crustal melting, which explains the large volume of temporally equivalent felsic volcanic and intrusive rocks.

Mid-Paleozoic initiation of the northern Cordilleran marginal backarc basin: Geologic, geochemical, and neodymium isotope evidence from the oldest mafic magmatic rocks in the Yukon-Tanana terrane, Finlayson Lake district, southeast Yukon, Canada

The Fire Lake formation of the Yukon-Tanana terrane in the Finlayson Lake region, Yukon, Canada, consists primarily of Late Devonian (ca. 365–360 Ma) mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions. In this paper, field, geochemical, and Nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting, the magmatic processes involved in their formation, and their roles in Cordil-leran crustal growth. The mafic rocks of the Fire Lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites: (1) back-arc-basin basalt, (2) enriched mid-oceanic-ridge basalt (E-MORB), (3) oceanic-island basalt (OIB), (4) Th-rich OIB, (5) boninite, (6) island-arc tholeiite, and (7) light rare earth element (LREE)–enriched island-arc tholeiite. The diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric (MORB-type) mantle, subarc mantle wedge, and lithospheric (OIB-type) mantle with or without elemental contributions from the subducted slab and/or continental crust. These suites of rocks are also associated with fine-grained basinal sedimentary facies, variations in metavolcanic and metasedimentary unit thickness, extensional synvolcanic faults, and apparent extensional-fault–controlled emplacement of mafic intrusive rocks and hydrothermal volcanic-hosted massive sulfide mineralization. The suites also exhibit a broad spatial distribution; those with “arc” signatures (Nb/Thmn < 1; mn—normalized to primitive mantle values) are located primarily in the western parts of the formation, and suites with “nonarc” signatures (Nb/Thmn ≥ 1) are located primarily within the eastern parts of the formation. Collectively, these geologic and geochemical attributes are interpreted to stem from the transition from arc magmatism to the initiation of an extensional backarc basinal environment associated with an east-dipping subduction zone. The initiation of backarc-basin magmatism recorded in the Fire Lake formation was part of a much larger Late Devonian backarc basinal system forming along the western edge of the margin of North America. The Fire Lake formation is interpreted to represent (1) the commencement of Yukon-Tanana arc rifting and separation from the North American cratonic margin, and (2) the initiation of a marginal (backarc) basin (now the Slide Mountain terrane) inboard of the Yukon-Tanana arc system. This tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for Yukon-Tanana arc activity or as a result of the propagation of the Slide Mountain backarc-basin spreading ridges into the Yukon-Tanana arc system. This Yukon-Tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the North American craton. The geochemical and isotopic signatures of magmatic rocks in the Fire Lake formation have some features similar to intraoceanic arc rocks (e.g., boninites, island-arc tholei-ites), and many have juvenile Nd isotope signatures (i.e., ϵNd( t ) > 0; most have ϵ Nd( t ) > +5), suggesting that the pericratonic terranes of the northern Cordillera have a significant juvenile component. If this is the case throughout the Yukon-Tanana terrane, then the pericratonic terranes may have contributed much more juvenile material to Cordil-leran crustal growth in the Phanerozoic than has previously been considered.

Composition and provenance of the Snowcap assemblage, basement to the Yukon-Tanana terrane, northern Cordillera: Implications for Cordilleran crustal growth

The Yukon-Tanana terrane of the northern Cordillera comprises a basement of metamorphosed continental margin sedimentary rocks of pre–Late Devonian age (Snowcap assemblage) and overlying subduction-generated Late Devonian to Permian arc and backarc facies igneous rocks. While preliminary analytical data have suggested that the Yukon-Tanana terrane originally formed as part of the western peri-Laurentian margin, its position outboard of an upper Paleozoic oceanic terrane (Slide Mountain) and the lack of information from its basement, the Snowcap assemblage, continue to raise questions about its original paleogeographic location along the margin. We describe here the geological relationships, geochemical and Nd-Hf isotopic compositions, and the detrital zircon signature of the Snowcap assemblage. Geochemical and Nd-Hf isotopic data for most siliciclastic rocks suggest derivation from evolved, upper crustal material, with Paleoproterozoic Nd-Hf depleted mantle model ages and detrital zircon data with major peaks in age ca. 1870 Ma and ca. 2720 Ma, and secondary peaks ca. 2080 Ma and ca. 2380 Ma. Minor juvenile contributions to some metaclastic rocks are more likely related to coeval, rift-related mafic alkalic magmatism than to younger arc magmatism in the terrane, as previously suggested. The detrital zircon signature of the Snowcap assemblage confirms a northwestern Laurentian cratonic source, similar to that of the adjacent Cordilleran miogeocline, and provides a local source in the Yukon-Tanana terrane for evolved signatures and Paleoproterozoic-Archean zircons (both detrital grains and xenocrystic cores) in younger mid- to late Paleozoic rocks of the terrane, at times when the Laurentian craton was probably not available as a direct source. Mafic alkalic rocks of the Snowcap assemblage were the products of low degree partial melting of incompatible element–enriched lithospheric mantle sources, most likely related to one of several Neoproterozoic–early Paleozoic rifting events recorded along the western margin of Laurentia. Marble and calc-silicate rocks have trace element compositions similar to modern seawater and juvenile Nd-Hf isotopic signatures similar to the mafic rocks, implying coeval carbonate sedimentation and magmatism. The overall character and composition of the Yukon-Tanana terrane suggest that crustal recycling processes dominated its evolution. Its accretion to the western margin of North America in early Mesozoic time contributed only a limited amount of juvenile crustal material to the Cordillera.

An overview of petrochemistry in the regional exploration for volcanogenic massive sulphide (VMS) deposits

ABSTRACT Volcanogenic massive sulphide (VMS) deposits are important global sources of base and precious metals. Igneous geochemistry (petrochemistry) of mafic and felsic rocks associated with VMS deposits is extremely useful in delineating potentially fertile ground for VMS mineralization. In mafic-dominated, juvenile environments (e.g. mafic, bimodal mafic and mafic-siliciclastic VMS-types) VMS deposits are associated with boninite and low-Ti island arc tholeiite, mid-ocean ridge basalt, and back-arc basin basalt. These rocks are ultimately sourced from either depleted arc mantle wedge (e.g. boninite, low Ti island arc tholeiite) or upwelling depleted, mid-ocean ridge or back-arc asthenospheric mantle (e.g. MORB and back-arc basin basalt). In evolved environments, those associated with continental crust and typically dominated by felsic magmatism (e.g. bimodal felsic and felsic-siliciclastic VMS-types), VMS-associated mafic rocks have alkalic (ocean island basalt-like) and/or mid-ocean ridge/back-arc basin basalt-like signatures. In these environments alkalic basalt and mid-ocean ridge/back-arc basin basalt-like mafic rocks overlie felsic rocks and mineralization and represent melts derived from lithospheric and asthenospheric mantle sources, respectively. Felsic rocks in Archean sequences are typically tholeiitic, have elevated high field strength elements (HFSE) and rare earth elements (REE), and FIII affinities (low Zr/Y and La/Ybn, flat chondrite-normalized rare earth element profiles). In post-Archean evolved environments, felsic rocks associated with VMS deposits have HFSE- and REE-enrichment and within-plate signatures on discrimination diagrams, like their Archean counterparts, but are more calc–alkalic in composition and commonly have FII affinities. Felsic rocks associated with VMS deposits in post-Archean mafic-dominated, juvenile substrates are associated with trace element depleted rhyolites with tholeiitic to boninite-like signatures and M-type and FIV affinities on discrimination plots. Using mafic or felsic rocks in isolation may lead to erroneous assignments of prospectivity for terrains; however, when mafic and felsic rocks are used in tandem with geological context they are powerful tools in outlining potentially prospective regions. Within VMS-hosting environments there are specific petrochemical assemblages of mafic and felsic rocks. Petrochemical assemblages are specific lithogeochemical associations between mafic and felsic rocks that are common to VMS forming environments and are useful in identifying two key ingredients required to form prospective VMS belts: (1) rifting; and (2) high temperature magmatism.

Analysis of powdered reference materials and known samples with a benchtop, field portable X-ray fluorescence (pXRF) spectrometer: evaluation of performance and potential applications for exploration lithogeochemistry

Powdered international reference materials and samples with previously obtained conventional geochemical data were analysed using a benchtop portable X-ray fluorescence (pXRF) spectrometer to test the abilities of pXRF in silicate rock lithogeochemistry. Results from international reference materials illustrate that pXRF can provide very precise data for many major, minor, and trace elements, generally with RSD values of <7.5 % and many <5 %, except at very low concentrations (i.e. approaching the limit of detection). Despite good precision, accuracy is highly variable and ranges from excellent to reasonable for many major and minor elements (±15–20 % relative difference, RD, for Al2O3, SiO2, K2O, CaO, Fe2O3, TiO2, and MnO±S), base metals (±20 % for Cu, Zn), the low field strength (LFSE) and high field strength elements (HFSE) (±15 % RD for Rb, Ba, Zr; ±20 % RD for Nb). Poor accuracy was obtained for MgO, P2O5, and the transition elements (V, Cr, Ni); Sr shows variable accuracy. Comparison of pXRF results to independent samples with data from conventional analyses illustrates very poor correlation for MgO, P2O5, V, Cr, and Ni, suggesting they have poor accuracy by pXRF. Aluminum (Al2O3), SiO2, and Zn have r2 values of c. 0.6–0.7 illustrating reasonable correlation, whereas most other elements (S, K2O, CaO, TiO2, MnO, Fe2O3, Co, Cu, Pb, Rb, Sr, Ba, Zr, Nb, U, As, and Mo) have very good to excellent correlation between pXRF data and conventional analysis (i.e. r2 >0.80). In addition, many of the elements with r2 >0.8 have slopes that are close to 1 or within 20 % of ideal, indicating that pXRF is replicating the results of conventional analyses and likely within ±20 % of what can be obtained by conventional methods. Down-hole profiles of pXRF data and element ratios replicate the geometry of the profiles from conventional analyses and illustrate the ability of the pXRF to discriminate rock type, alteration, and mineralization in unknown samples. Portable XRF can provide fit-for-purpose data that is useful in discriminating lithogeochemical variations related to lithology, alteration, and mineralization. However, pXRF should be considered a preliminary screening tool for sample selection and not a substitute for conventional lithogeochemical methods (e.g. XRF, fusion ICP-ES and ICP-MS), particularly when important economic decisions are to be made using such data (e.g. NI-43-101 resource calculations). Supplementary Material: Collated data for repeat analyses of reference materials in Mining Plus (Table 1) and Soil 3 Beam (Table 2) modes. Tables 3-9 contain plots comparing results from pXRF to accepted values for various reference materials. All tables are available at at www.geolsoc.org.uk/SUP18735

Episodic arc-ophiolite emplacement and the growth of continental margins: Late accretion in the Northern Irish sector of the Grampian-Taconic orogeny

In order to understand the progressive growth of continental margins and the evolution of continental crust, we must first understand the formation of allochthonous ophiolitic and island-arc terranes within ancient orogens and the nature of their accretion. During the early Paleozoic closure of the Iapetus Ocean, diverse sets of arc terranes, oceanic tracts, and ribbon-shaped microcontinental blocks were accreted to the passive continental margin of Laurentia during the Grampian-Taconic orogeny. In the northern Appalachians in central Newfoundland, Canada, three distinct phases of arc-ophiolite accretion have been recognized. New field mapping, high-resolution airborne geophysics, whole-rock and Nd-isotope geochemistry, and U-Pb zircon geochronology within the Tyrone Volcanic Group of Northern Ireland have allowed all three episodes to now be correlated into the British and Irish Caledonides. The Tyrone Volcanic Group (ca. 475–469 Ma) is characterized by mafic to intermediate lavas, tuffs, rhyolite, banded chert, ferruginous jasperoid, and argillaceous sedimentary rocks cut by numerous high-level intrusive rocks. Geochemical signatures are consistent with formation within an evolving peri-Laurentian island-arc/backarc, which underwent several episodes of intra-arc rifting prior to its accretion at ca. 470 Ma to an outboard peri-Laurentian microcontinental block. Outriding microcontinental blocks played a fundamental role within the orogen, explaining the range of ages for Iapetan ophiolites and the timing of their accretion, as well as discrepancies between the timing of ophiolite emplacement and the termination of the Laurentian Cambrian–Ordovician shelf sequences. Accretion of the Tyrone arc and its associated suprasubduction-zone ophiolite represents the third stage of arc-ophiolite emplacement to the Laurentian margin during the Grampian-Taconic orogeny in the British and Irish Caledonides.

Mid- to late Paleozoic K-feldspar augen granitoids of the Yukon-Tanana terrane, Yukon, Canada: Implications for crustal growth and tectonic evolution of the northern Cordillera

Potassic feldspar-bearing augen granitoids are a fundamental component of the architecture of the Yukon-Tanana terrane and the ancient Pacific margin of the northern Cordillera. These augen granitoids form a belt that extends from Alaska to southeast Yukon Territory, vary in age, and provide probes of the crustal evolution and tectonic history of the Yukon-Tanana terrane and ancient Pacific margin of North America in the Paleozoic. We present results of an integrated field mapping, geochemical, Sm-Nd tracer isotopic, and U-Pb zircon geochronologic study of the augen granitoids in the Stewart River area in an attempt to understand their role in the crustal evolution and tectonic history of the Yukon-Tanana terrane and ancient Pacific margin of North America. Augen granitoids of the Stewart River area are of three distinct ages: Late Devonian, early Mississippian, and Permian. U-Pb zircon geochronology of these augen granitoids has yielded ages of 362.1 ± 2.7 Ma (Stewart River augen granite), 347.5 ± 0.7 Ma (Mount Burnham augen granite), and 264.8 ± 3.7 Ma (Wounded Moose augen granite). All of the augen granitoids, regardless of age, have negative ϵ Ndt values (−2.0 to −15.3) and Proterozoic-Archean depleted-mantle model ages (T DM = 1.37–2.56 Ga). These geochemical and isotopic attributes, coupled with the presence of inherited zircon with Precambrian ages, suggestthatthesegranitoidsaretheproductof crustal melting and crust-mantle mixing during three different cycles of arc magmatism in the Paleozoic. Furthermore, these granitoids represent net crustal recycling along the ancient Pacific margin of North America in the Paleozoic. Importantly, however, there are minor secular variations in crustal recycling, and the younger Permian augen granitoids exhibit higher ϵ Ndt , Nb/Ta, V/Yb, and Sc/Yb, consistent with a greater juvenile component in their genesis. This juvenile component is probably due to assimilation of underplated mafic material derived from older early Mississippian Yukon-Tanana terrane arc magmatism and/or a greater mantle component due to enhanced infiltration of underplated mafic material into augen granitoid magma chambers through rheologically weak crust associated with Permian subduction. The older Late Devonian and early Mississippian augen granitoid suites represent two pulses of Yukon-Tanana terrane arc magmatic activity that developed in response to east-dipping subduction along the western edge of the North America craton in the mid-Paleozoic. This east-dipping Yukon-Tanana terrane arc system continued to evolve throughout the Mississippian to Early Permian and was coincident with the development of the Slide Mountain backarc basin that formed between the Yukon-Tanana terrane arc system and the North American craton; this east-dipping arc-backarc system continued until ca. 275 Ma. After ca. 275 Ma, the east-dipping arc and backarc magmatism ceased and was replaced by ca. 270–269 Ma high-pressure metamorphism and the establishment of a new subduction zone that formed in response to the closure of the Slide Mountain backarc basin. The Permian augen granitoids from the Stewart River are the magmatic record of this new west-dipping subduction zone. Although there are subtle variations, the petrogenetic and tectonic histories of the three suites of augen granitoids in the Stewart River area are remarkably similar and attest to the constancy of magmatic and tectonic processes that occurred along the ancient Pacific margin of North America in the Paleozoic.

Erratum to: Variations of sulphur isotope signatures in sulphides from the metamorphosed Ming Cu(−Au) volcanogenic massive sulphide deposit, Newfoundland Appalachians, Canada

The Ming deposit is an early Ordovician, bimodal-mafic Cu–Au volcanogenic massive sulphide (VMS) deposit in the Newfoundland Appalachians that was metamorphosed to upper greenschist/lower amphibolite facies conditions and deformed in the Silurian and Devonian. The Ming deposit consists of several spatially proximal ore bodies of which the 1806 Zone, 1807 Zone, Ming South Up Plunge and Down Plunge and the Lower Footwall Zone are the focus of this paper. The ore bodies have similar stratigraphic sequences. The ore bodies can be divided into (1) a silicified horizon that caps the massive sulphides, (2) semi-massive to massive sulphides and (3) sulphide mineralization in a rhyodacitic footwall. Sulphide mineralization in a rhyodacitic footwall includes (a) sulphide stringers immediately below the semi-massive to massive sulphides and (b) chalcopyrite–pyrrhotite–pyrite stringers distally from semi-massive to massive sulphides in the Lower Footwall Zone. Pyrite, chalcopyrite, pyrrhotite, arsenopyrite and galena were analysed by in situ secondary ion mass spectrometry (SIMS) for sulphur isotope compositions. The isotopic signatures of pyrite, chalcopyrite, pyrrhotite and arsenopyrite fall within a limited range of 2.8 to 12.0 ‰ for semi-massive to massive sulphides and sulphide mineralization in the footwall. The silicified horizon capping the semi-massive to massive sulphides has higher δ 34S (5.8–19.6 ‰), especially for pyrrhotite (mean, 17.2 ± 2.2 ‰, n = 8). The sulphur isotope composition of galena is more heterogeneous, especially within semi-massive to massive sulphides and sulphide stringers, ranging from 0.8 to 17.3 ‰ (mean, 6.1 ± 4.3 ‰, n = 35) and 7.6 to 17.1 ‰ (mean, 13.7 ± 5.3 ‰, n = 3), respectively. Geothermometric calculations give insufficient formation and metamorphism temperatures for neighbouring mineral pairs, because sulphides were not in isotopic equilibrium while deposited in early Ordovician or re-equilibrated during Silurian–Devonian metamorphism, respectively. Therefore, original isotopic compositions of sulphides at the Ming deposit have been preserved. Modelling of the source of sulphur shows that: (1) reduced seawater sulphate and (2) sulphur leached from igneous wall rock and/or derived from magmatic fluids are the main sources of sulphur in the Ming deposit. The influence of igneous sulphur (igneous wall rock/magmatic fluids) increases with temperature and is an important sulphur source for the semi-massive to massive sulphides and footwall mineralization, in addition to a contribution from thermochemical sulphate reduction (TSR) of seawater. It is difficult to distinguish between sulphur leached from igneous rocks and magmatic fluid-related sulphur, and it is possible that both sources contributed to the ores at the Ming deposit. In addition to igneous sulphur, the heavy isotopes of the silicified horizon are consistent with the sulphur in this horizon being derived only from thermochemical sulphate reduction of early Ordovician seawater sulphate.

Lithosphere-asthenosphere mixing in a transform-dominated late Paleozoic backarc basin: Implications for northern Cordilleran crustal growth and assembly

The Slide Mountain terrane is part of a North American Cordillera–long backarc basinal assemblage that developed between the ensialic arc terranes (Yukon-Tanana and affiliated pericratonic terranes) and the North American craton in the middle to late Paleozoic. The Slide Mountain basin started to open in the Late Devonian, and spreading continued through the late Paleozoic in an oblique (transform-dominated) manner such that the pericratonic terranes were translated into southerly latitudes. The basin closed, also in an oblique manner, by the Early Triassic, resulting in the reaccretion of the Yukon-Tanana terrane to the northwestern Laurentian margin. Both the opening and closing likely involved hundreds to possibly thousands of kilometers of intra-ocean and/or intra-arc strike-slip displacement, sinistral during the ocean9s Late Devonian to mid-Permian opening and dextral during its Late Permian closing. In southeastern Yukon, Canada, the Early Permian Slide Mountain terrane is dominated by mafic and ultramafic volcanic and plutonic rocks of the Campbell Range Formation. These rocks are narrowly distributed, for over 300 km, on either side of the Jules Creek–Vangorda fault, a fault that separates Slide Mountain terrane from Yukon-Tanana terrane. The Campbell Range basaltic volcanic and high-level intrusive rocks have geochemical and isotopic signatures that vary systematically across the Jules Creek–Vangorda fault: ocean-island basalt (OIB) and enriched mid-ocean ridge basalt (E-MORB) suites with lower eNd t occur exclusively south of the fault, whereas north of the fault they have normal mid-ocean ridge basalt (N-MORB) and backarc basin basalt (BABB) signatures with higher eNd t values. The eNd t values are inversely correlated with Nb/Th pm and Nb/La pm , suggesting that the lower eNd t values present in the E-MORB and OIB are mantle source features of these basalts and not due to continental crustal contamination. Isotopic and multi-element mixing calculations illustrate that the OIB-like basalts were derived primarily from enriched continental lithospheric mantle, whereas the N-MORB and BABB suites were sourced primarily from the upwelling backarc asthenospheric mantle; E-MORBs represent mixtures of depleted asthenospheric and enriched lithospheric mantle. The geochemical and isotopic variations in the Campbell Range Formation across the Jules Creek–Vangorda fault is attributed to formation in different parts of an extending continental-backarc basin and then their subsequent juxtaposition by continued displacement along the fault. Despite the juvenile isotopic signatures present in the Slide Mountain terrane, they occur as thin klippe atop rocks of recycled continental crustal affinity, suggesting that they were likely only minor contributors to Cordilleran crustal growth.

Multiple sulphur and lead sources recorded in hydrothermal exhalites associated with the Lemarchant volcanogenic massive sulphide deposit, central Newfoundland, Canada

Metalliferous sedimentary rocks (mudstones, exhalites) associated with the Cambrian precious metal-bearing Lemarchant Zn-Pb-Cu-Au-Ag-Ba volcanogenic massive sulphide (VMS) deposit, Tally Pond volcanic belt, precipitated both before and after VMS mineralization. Sulphur and Pb isotopic studies of sulphides within the Lemarchant exhalites provide insight into the sources of S and Pb in the exhalites as a function of paragenesis and evolution of the deposit and subsequent post-depositional modification. In situ S isotope microanalyses of polymetallic sulphides (euhedral and framboidal pyrite, anhedral chalcopyrite, pyrrhotite, galena and euhedral arsenopyrite) by secondary ion mass spectrometry (SIMS) yielded δ34S values ranging from −38.8 to +14.4 ‰, with an average of ∼ −12.8 ‰. The δ34S systematics indicate sulphur was predominantly biogenically derived via microbial/biogenic sulphate reduction of seawater sulphate, microbial sulphide oxidation and microbial disproportionation of intermediate S compounds. These biogenic processes are coupled and occur within layers of microbial mats consisting of different bacterial/archaeal species, i.e., sulphate reducers, sulphide oxidizers and those that disproportionate sulphur compounds. Inorganic processes or sources (i.e., thermochemical sulphate reduction of seawater sulphate, leached or direct igneous sulphur) also contributed to the S budget in the hydrothermal exhalites and are more pronounced in exhalites that are immediately associated with massive sulphides. Galena Pb isotopic compositions by SIMS microanalysis suggest derivation of Pb from underlying crustal basement (felsic volcanic rocks of Sandy Brook Group), whereas less radiogenic Pb derived from juvenile sources leached from mafic volcanic rocks of the Sandy Brook Group and/or Tally Pond group. This requires that the hydrothermal fluids interacted with juvenile and evolved crust during hydrothermal circulation, which is consistent with the existing tectonic model that suggests a formation of the Tally Pond belt volcanic rocks and associated VMS deposits in a rifted arc environment upon crustal basement of the Ediacaran age Sandy Brook Group and Crippleback Intrusive Suite. Combined S and Pb isotope data illustrate that sulphides within the deposit that are proximal to the vent contain a higher proportion of sulphur derived from thermochemical sulphate reduction (TSR), because hydrothermal fluids are enriched in H2S derived from TSR. They also have lower radiogenic Pb contributions, than sulphides occurring distal from mineralization. Hence, the TSR S and non-radiogenic Pb composition may provide an exploration vector in exhalites associated with similar VMS environments.