Macmillan Pass Research Articles

Paleoredox conditions, hydrothermal history, and target vectoring in the Macmillan Pass base-metal district, Yukon, Canada: 2 – Pyrite paragenesis and mineral chemistry

ABSTRACTThe MacMillan Pass district in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Pyrite-bearing drill core samples were collected from seven drill holes that intersected sulfide mineralization and time-stratigraphically equivalent rocks at varied spatial distances extending up to 3 km away from the deposits to assess the relative timing of pyrite mineralization and the chemistry of pyrite paragenesis. There are four pyrite morphologies: framboids and polyframboids (Py1), subhedral to euhedral inclusion-free crystals (Py2a), silicate inclusion-bearing nodules with serrated edges (Py2b), and euhedral idiomorphic overgrowths on preexisting pyrite morphologies (Py3). These morphological varieties correspond in time from syngenetic to earliest diagenetic growth (Py1), early to late diagenetic growth (Py2a, Py2b), and metamorphic crystallization and/or recrystallization of previous textural varieties (Py3). A representative subset of pyrite grains was analyzed for trace element contents and distributions by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses by LA-ICP-MS reveal that each textural variety of pyrite has a distinct trace element composition that also varies depending on stratigraphic unit. A suite of clastic sediment-hosted sulfide mineralization-related elements was incorporated into Py2 within sulfide mineralized units at greater abundances than that in unmineralized units (e.g., Zn, As, Pb, Tl, Bi). Lead abundances and Pb/Se and As/Mo values in pyrite are the most robust vectoring tools documented. The timing for clastic sediment-hosted Zn-Pb mineralization was syn and/or post late diagenesis (Py2b).A Ba-enriched horizon was identified in rocks and this is interpreted to be the distal time-stratigraphic equivalent unit to Zn-Pb mineralization. The Ba-enriched horizon contains Py2 with anomalous metal (Tl, Co, Mn, Cd, Zn, Sb) contents and abundant macroscopic baryte, and it is interpreted to represent the distal expression of sulfide mineralization-forming hydrothermal activity. Four genetic models for mineralization are reviewed; however, the only model that is consistent with our whole rock and pyrite geochemistry involves venting of buoyant hydrothermal fluid, mixing with ambient seawater, and remaining or sinking into unconsolidated sediments, with lateral migration up to 2–3 km from the vent source.

Paleoredox conditions, hydrothermal history, and target vectoring in the MacMillan Pass base-metal district, Yukon, Canada: 1 – Lithogeochemistry of proximal and distal shales

ABSTRACT The MacMillan Pass District in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Bulk geochemical paleoredox proxies (Eu/Eu*, Ce/Ce*, Mo, Re/Mo, and Ni/Co) indicate anoxic–dysoxic water column and sulfidic porewater conditions persisted during the Late Devonian deposition of the Lower Earn Group host rocks. Positive Eu/Eu* anomalies (up to 3.31) in sulfide mineralization at the Tom deposit are consistent with relatively high temperature (probably >250 °C), reducing, acidic hydrothermal fluids that infiltrated laterally through unconsolidated sediments proximal to the hydrothermal upflow zone and/or exhaled at the seafloor as moderate- to high-density brines. Molybdenum and U enrichment factors (relative to upper continental crust) and Mo/organic C values are consistent with a moderately restricted basin; Mo/C values fall between those of the Black Sea (highly restricted) and the Framvaren Inlet (moderately restricted). A Ba-rich shale was identified in rocks that are distal and time-equivalent units to the Pb-Zn mineralization; based on the bulk chemical compositions and on previous S and Sr isotope studies, we interpret the baryte in this unit to be largely hydrothermal in origin and perhaps remobilized and reprecipitated during hydrothermal base-metal mineralization.

A NEW SUBSEAFLOOR REPLACEMENT MODEL FOR THE MACMILLAN PASS CLASTIC-DOMINANT Zn-Pb ± Ba DEPOSITS (YUKON, CANADA)

Abstract Sedimentary exhalative (SEDEX) deposits are a subset of sediment-hosted massive sulfide deposits and provide our dominant resource of Zn. In the SEDEX model, base metals (Zn, Pb, Fe) are hydrothermally vented into sulfidic (euxinic) seawater and deposited coevally with the organic-rich mudstone host rock, resulting in laterally extensive layered mineralization. In the Selwyn Basin (Canada) at Macmillan Pass, two deposits (Tom, Jason) are well preserved in a succession of Upper Devonian mudstones and are considered type-characteristic examples of the SEDEX deposit model. As with a number of SEDEX deposits, at Macmillan Pass barite is abundant in the succession hosting hydrothermal mineralization. Early work presented a hydrothermal model for barite formation, in which barite coprecipitated with base metal sulfides in a redox-stratified water column. Recently, however, studies have both proposed an alternative diagenetic model for barite formation and provided more precise constraints on the chemistry of the hydrothermal fluid that entered the vent complexes. Here, we present a new model for Macmillan Pass in which sulfide mineralization occurred entirely within the subsurface. The introduction of hot (300°C) hydrothermal fluids into the shallow subsurface (<1-km depth) resulted in the thermal degradation of organic matter and generated CO2; this promoted barite dissolution, which both provided a source of sulfate for thermochemical sulfate reduction and increased the porosity and permeability within the system. Importantly, there was clear potential for the development of positive feedbacks and self-organization between diagenetic and hydrothermal processes, resulting in highly efficient ore-forming systems. In contrast to the SEDEX model, alteration footprints will be controlled by the mass transfer involved in (barite) replacement reactions rather than hydrothermal venting, and exploration criteria at a district scale should strongly favor highly productive continental margins.

The Mineralogical Evolution of the Clastic Dominant-Type Zn-Pb ± Ba Deposits at Macmillan Pass (Yukon, Canada)—Tracing Subseafloor Barite Replacement in the Layered Mineralization

Abstract Clastic dominant-type massive sulfide deposits are well preserved in Upper Devonian carbonaceous mudstones in the Macmillan Pass district (Yukon, Canada). The Macmillan Pass deposits have been considered to be type examples of sedimentary exhalative Zn-Pb mineralization, whereby sulfides precipitated when hydrothermal fluids were vented into a euxinic (H2S-bearing) water column. We propose a new mineralization model, documenting the mineralogical evolution of layered mineralization. We show that textures previously interpreted to form via depositional processes actually formed by subseafloor replacement of diagenetic barite. Mineral assemblages associated with barite dissolution in the layered mineralization include (1) barium carbonate phases (witherite, barytocalcite, and norsethite), which are intergrown with Zn-Pb sulfides and represent localized Ba mass transfer, and (2) barium feldspar (celsian, hyalophane) that is abundant in the mudstones surrounding the layered mineralization. The barium feldspar formed following transport of Ba in low-sulfate fluids on the margins of the subseafloor replacement system. This resulted in whole-rock Ba enrichments (up to 5 wt %) in mudstones 15 m below and above the layered mineralization. High Ba in these surrounding mudstones is coupled with decreasing K/Al ratios, indicative of secondary illite and kaolinite. The source(s) of fluids related to the diagenetic (barite, barytocalcite) and hydrothermal (ankerite) assemblages can be constrained using Sr isotopes. Whereas highly radiogenic 87Sr/86Sr values (>0.714) in ankerite correspond with host-rock alteration within the vent complex, the overlying barite and barytocalcite preserve lower 87Sr/86Sr values (<0.714), providing evidence of mixing between a radiogenic fluid (likely a formation water) and Late Devonian seawater. The complex mineralogy and paragenesis contained within the layered mineralization are linked to a protracted history of diagenetic and hydrothermal fluid events, all of which took place in and peripheral to a subseafloor replacement hydrothermal system.

Links between seawater paleoredox and the formation of sediment-hosted massive sulphide (SHMS) deposits — Fe speciation and Mo isotope constraints from Late Devonian mudstones

Many models of sediment hosted massive sulphide (SHMS) deposit formation invoke basin restriction events that resulted in long-term stagnation and anoxia, in which sulphidic (euxinic) conditions ultimately prevailed. Euxinic conditions are then thought to provide a chemical trap for hydrothermally exhaled base metals. Here, we present Fe speciation and Mo isotope data for organic-rich mudstones from two drill-holes intersecting Upper Devonian strata, deposited along the passive margin of ancestral North America. One drill-hole intersects a 35 m thick sequence of SHMS mineralisation, while the other intersects correlative, un-mineralised strata. All samples have high FeHR/FeT values (>0.38), indicating water-column anoxia. For the majority of samples in the un-mineralised drill-hole, the levels of pyritisation fall below the threshold typically used to define euxinic conditions (FePY/FeHR ≤ 0.70). In contrast, higher levels of pyritisation in the mineralised drill-hole (median FePY/FeHR = 0.86) likely developed via diagenetic pyrite enrichment. Whereas Pb and Zn are negatively correlated with Mo, Mo-U co-variation is consistent with Fe (oxyhydr)oxide particulate shuttling in the water-column. In addition, a weak correlation between TOC/P and Mo provides further evidence that Mo was sourced via authigenic, rather than hydrothermal, processes. The δ98Mo values (+0.66 to +1.02‰) are uniform between both drill-holes, and substantially lower than constraints for Late Devonian seawater (+1.5 to +2.0‰), consistent with Mo adsorption to Fe (oxyhydr)oxides. Collectively, the data provide evidence that local seawater was dominantly ferruginous (anoxic, non-sulphidic) at Macmillan Pass. Regional variability in the extent of ferruginous (low TOC/P) and euxinic (high TOC/P) conditions likely contributed to a balance between P regeneration and P enrichment that maintained nutrient availability and productivity in the Selwyn Basin during the Late Devonian. We argue that high primary productivity and enhanced organic carbon burial are key variables for promoting sulphate reduction in the sub-surface. Moreover, how such conditions are maintained over long periods of basin evolution is more important for producing effective metal traps in SHMS systems, rather than a specific, localised redox condition of seawater (i.e. euxinia).

The thermal and chemical evolution of hydrothermal vent fluids in shale hosted massive sulphide (SHMS) systems from the MacMillan Pass district (Yukon, Canada)

At Macmillan Pass (YT, Canada), the hydrothermal vent complexes beneath two shale-hosted massive sulphide (SHMS) deposits (Tom, Jason) are well preserved within Late Devonian strata. These deposits provide a unique opportunity to constrain key geochemical parameters (temperature, salinity, pH, fO2, ΣS) that are critical for metal transport and deposition in SHMS systems, and to evaluate the interaction between hydrothermal fluids and the mudstone host rock. This has been achieved using a combination of detailed petrography, isotopic techniques (δ34S, δ13C and δ18O values), carbonate rare earth element analysis (LA-ICP-MS), fluid inclusion analysis (microthermometry, gas analysis via incremental crush fast scan mass spectrometry), and thermodynamic modelling.Two main paragenetic stages are preserved in both vent complexes: Stage 1 comprises pervasive ankerite alteration of the organic-rich mudstone host rock and crosscutting stockwork ankerite veining (±pyrobitumen, pyrite and quartz) and; Stage 2 consists of main stage massive sulphide (galena-pyrrhotite-pyrite±chalcopyrite-sphalerite) and siderite (±quartz and barytocalcite) mineralisation. Co-variation of δ18O and δ13C values in ankerite can be described by temperature dependent fractionation and fluid rock interaction. Together with fluid inclusion microthermometry, this provides evidence of a steep thermal gradient (from 300 to ∼100°C) over approximately 15m stratigraphic depth, temporally and spatially constrained within the paragenesis of both vent complexes and developed under shallow lithostatic (<1km; 250 bars) to hydrostatic (<400m; 40 bars) conditions. There is evidence of mixing between diagenetic and hydrothermal fluids recorded in chondrite-normalised rare earth element (REE) profiles of ankerite and siderite. Middle REE enrichments and superchondritic Y/Ho ratios (>28), characteristic of diagenetic fluids, are coupled with positive europium anomalies and variable light REE depletion, which are more consistent with chloride complexation in hot (>250°C) hydrothermal fluids. In this shallow sub-seafloor setting, thermal alteration of organic carbon in the immature, chemically reactive mudstones also had an important role in the evolution of fluid chemistry. Reduced sulphur generation via thermochemical reduction of Late Devonian seawater sulphate produced positive δ34S values in sulphide minerals (+7.5‰ to +19.5‰), coupled with a suite of volatile components (CO2, CH4, C1–C4 hydrocarbons, N2) trapped in Stage 2 quartz.Many of these geochemical features developed during the final stages of fluid ascent, in a system where the fluid cooled close to the site of mineralisation. Using this information, we have modelled the metal transporting capacity of the deep hydrothermal fluid, which even at modest salinities (6wt.% NaCl) was high (≫100ppm Pb, Zn), owing to the combined effects of high temperature and low pH (⩽4.5). Therefore in SHMS systems, enhanced geothermal gradients and rapid fluid ascent (with minimal fluid cooling) are considered to be the most important factors for transporting high concentrations of base metals to the site of mineralisation.

Open system sulphate reduction in a diagenetic environment – Isotopic analysis of barite (δ34S and δ18O) and pyrite (δ34S) from the Tom and Jason Late Devonian Zn–Pb–Ba deposits, Selwyn Basin, Canada

Highly positive δ34S values in sulphide minerals are a common feature of shale hosted massive sulphide deposits (SHMS). Often this is attributed to near quantitative consumption of seawater sulphate, and for Paleozoic strata of the Selwyn Basin (Canada), this is thought to occur during bacterial sulphate reduction (BSR) in a restricted, euxinic water column. In this study, we focus on drill-core samples of sulphide and barite mineralisation from two Late Devonian SHMS deposits (Tom and Jason, Macmillan Pass, Selwyn Basin), to evaluate this euxinic basin model. The paragenetic relationship between barite, pyrite and hydrothermal base metal sulphides has been determined using transmitted and reflected light microscopy, and backscatter electron imaging. This petrographic framework provides the context for in-situ isotopic microanalysis (secondary ion mass spectrometry; SIMS) of barite and pyrite. These data are supplemented by analyses of δ34S values for bulk rock pyrite (n=37) from drill-core samples of un-mineralised (barren), siliceous mudstone, to provide a means by which to evaluate the mass balance of sulphur in the host rock.Three generations of barite have been identified, all of which pre-date hydrothermal input. Isotopically, the three generations of barite have overlapping distributions of δ34S and δ18O values (+22.5‰ to +33.0‰ and +16.4‰ to +18.3‰, respectively) and are consistent with an origin from modified Late Devonian seawater. Radiolarian tests, enriched in barium, are abundant within the siliceous mudstones, providing evidence that primary barium enrichment was associated with biologic activity. We therefore propose that barite formed following remobilisation of productivity-derived barium within the sediment, and precipitated within diagenetic pore fluids close to the sediment water interface. Two generations of pyrite are texturally associated with barite: framboidal pyrite (py-I), which has negative δ34S values (−23‰ to −28‰; n=9), and euhedral pyrite (py-II), which has markedly more positive δ34S values (+8‰ to +26‰; n=86). We argue that stratiform pyrite and barite developed along diagenetic redox fronts, where the isotopic relationships (δ34Spyrite≈δ34Sbarite) are explained by anaerobic oxidation of methane coupled to sulphate reduction (AOM-SR). Furthermore, the relatively narrow distribution of δ34Sbarite values is consistent with an open system model of sulphate reduction, in which reduced sulphur generation occurred with a reduced isotopic fractionation (ε34S=<15‰) linked to higher rates of sulphate reduction and AOM-SR. Importantly, hydrothermal sulphides (pyrite, sphalerite and galena) all post-date this diagenetic barite-pyrite assemblage, and textural and mineralogical evidence indicates barite replacement to be an important process during hydrothermal mineralisation. Neither the textures nor the documented isotopic relationships can be produced by processes operating in a euxinic water column, which represents a major departure from the conventional model for SHMS formation at Macmillan Pass. We suggest that positive δ34S values in sulphides, a common feature of SHMS systems both in the Selwyn Basin and throughout the geologic record, could be linked to AOM-SR. At Macmillan Pass, positive δ34Spyrite values developed during open system diagenesis, which was critical for rapid sulphur cycling and the development of an effective metal trap.

The Importance of Siliceous Radiolarian-Bearing Mudstones in the Formation of Sediment-Hosted Zn-Pb ± Ba Mineralization in the Selwyn Basin, Yukon, Canada

Sediment-hosted massive sulfide (SHMS) mineralization occurred in three main episodes during the evolution of the Selwyn Basin, Yukon, and Northwest Territories, Canada. In the Macmillan Pass district (Tom and Jason deposits), the Upper Devonian strata of the Earn Group host the youngest, least deformed examples of SHMS mineralization. The Earn Group comprises gray to black, homogeneous mudstones, which have been deformed in the Cordilleran fold-and-thrust belt, are weathered and poorly exposed; this makes stratigraphic correlations and the recognition of prospective units very challenging. The host rock (siliceous mudstone) and mineralized intervals at the Tom and Jason Zn-Pb ± Ba deposits have been sampled from drill core. Major element chemistry and total organic carbon (TOC) data are presented for the host-rock samples from two drill holes (TYK-5, n = 18; 76-17, n = 19). All mudstones contain between 2 and 5 wt % TOC and 67 to 83 wt % SiO2. Subtle alteration, marked by potassium loss (K2O/Al2O3 50% radiolarians by volume) within the mineralized horizons and in barren, time equivalent strata, and therefore are a useful stratigraphic marker unit in otherwise homogeneous, featureless fine-grained mudstones. The presence of highly siliceous, carbonaceous mudstone units (with preserved radiolarians) suggests that in the Late Devonian there is a link between biological productivity in the water column, the resultant sedimentation, and the SHMS deposits. These biosiliceous lithologies may be particularly favorable host rocks to SHMS mineralization for the following reasons: (1) high porosities during early diagenesis preserve volume and enhance permeabilities in partially lithified sediments, which are important parameters for subseafloor replacement mineralization; (2) these units are rich in organic materials due to their deposition in productive settings, and have a high capacity for the generation of sulfide via sulfate reduction. Therefore, we suggest there is a link between times of high biological activity and productivity in oceans, sediments derived from those processes, and SHMS deposits.

Natural acid rock drainage associated with black shale in the Yukon Territory, Canada

Investigated herein are water and sediment geochemistry, and metal attenuation processes associated with natural acid rock drainage originating from black shale formations in the Macmillan Pass area, Clear Lake prospect and Engineer Creek by the Dempster Highway in the Yukon Territory, Canada. The most metalliferous water having pH 3.0, 150 mg/L Zn, 39 mg/L Ni, 2.8 mg/L Cu and 9.1 mg/L As was found in a tributary stream of Engineer Creek with no known mineral deposits occurring in the vicinity. For all three study areas, the water and sediment geochemistry is significantly affected by the local lithology and prevailing metal attenuation processes. Despite their anomalous acidity and metal contents, the natural acid streams contribute only a small fraction of the contaminant loadings to the major water courses because of their low flows. Dilution, neutralization, sorption and co-precipitation are identified as the major mechanisms attenuating aqueous transport of potentially deleterious metals. However, microbial mediation in metal attenuation is also evident in low-flow systems. The wide variation of water and sediment geochemistry along a flow path renders the establishment of background metal values difficult. In assessing environmental impacts, it may be more practical to consider metal loadings on a watershed scale than to rely on a comparison with operationally defined background concentrations.

Natural attenuation of aqueous metal contamination by an algal mat

Naturally occurring mechanisms of attenuation for metals in the environment are important for understanding and remediating acid rock drainage. A filamentous green algae was found forming an extensive mat below an outflow of acidic, metal-laden groundwater, at Macintosh Creek, MacMillan Pass, Yukon Territory, Canada. Emerging waters had a pH of 3.3 and the following dissolved metal concentrations in milligrams per litre: Al = 156, Fe = 298, Mn = 1.1, Cd = 0.13, Ni = 2.52, Cu = 0.69, and Zn = 5.0. In contrast, waters that had coursed over the algal mat had the following concentrations in milligrams per litre: Al = 26.4, Fe = 29.3, Mn = 0.18, Cd = 0.03, Ni = 0.4, Cu = 0.2, Zn = 0.88. In addition, the concentration of dissolved As declined from 32.9 to 9.3 µg·L-1. Thus, the concentrations of potentially deleterious elements were typically reduced by between 5- and 10-fold. Scanning electron microscopy (SEM) indicated that individual filaments of the algae were encrusted with mineral precipitate. Microprobe analyses indicated that the coatings were predominantly composed of Fe with other metals, in the presence of S and P, the latter possibly associated with the algal biomass. While culturing methods indicated the presence of 104-106 Thiobacillus ferrooxidans·mL-1 in the water, epifluorescence microscopy observations using DAPI and SYTO 9 nucleic acid stains did not reveal bacteria in association with the algal filaments. Hydrated samples were also observed using confocal scanning laser microscopy (CSLM) with FITC-conjugated lectin staining, autofluorescence, and reflection imaging. These observations indicated that the algal filaments had an extensive exopolysaccharide surrounding the filaments and that mineralization occurred within the matrix. This suggested that factors such as the Eh and pH proximal to the algae may be playing an important role in mineral production.Key words: heavy metals, exopolymers, acid drainage, confocal microscopy.

Use of summer habitat by caribou on the north slope of a mountain near the Macmillan Pass, N.W.T.

Habitat use by woodland caribou was investigated by counting pellet-groups, sampling phytomass, and evaluating topography in nine habitat-types on the north slope of an unnamed mountain near Macmillan Pass, N.W.T. Caribou pellets were most abundant in high elevation habitat-types, and pellet density was greatest in an alpine Lichen-Grass habitat-type with a slope of &amp;lt;1&amp;deg;. The high density of pellets in alpine areas may have resulted from of the use of cool, windy, alpine habitats by caribou seeking relief from insect harassment. There were no apparent relationships between pellet abundance, and phytomass of mosses, lichens, or graminoids, possibly as a result of caribou feeding and defecating in different habitats. The occurrence of pellets with a coalesced morphology in the barren Lichen-Grass habitat-type provided indirect evidence in support of a feeding cycle, whereby caribou visit lush habitats to feed, and return to open, alpine habitats to rest and ruminate.

Palsa Formation in Floating Peat and Related Vegetation Cover as Illustrated by a Fen Bog in the Macmillan Pass, Yukon Territory, Canada

Floating islands of vegetation-covered peat which include palsas are observed in spring-fed fens within a string bog in the MacMillan Pass, Yukon Territory, Canada. The fens are dammed by stable an...

Palsa Formation in Floating Peat and Related Vegetation Cover as Illustrated by a Fen Bog in the Macmillan Pass, Yukon Territory, Canada

Palsa Formation in Floating Peat and Related Vegetation Cover as Illustrated by a Fen Bog in the Macmillan Pass, Yukon Territory, Canada

The Selection of Snowpatches as Relief Habitat by Woodland Caribou ( Rangifer Tarandus Caribou ), Macmillan Pass, Selwyn/Mackenzie Mountains, N.W.T., Canada

The Selection of Snowpatches as Relief Habitat by Woodland Caribou ( <i>Rangifer Tarandus Caribou</i> ), Macmillan Pass, Selwyn/Mackenzie Mountains, N.W.T., Canada

The Selection of Snowpatches as Relief Habitat by Woodland Caribou (Rangifer tarandus caribou), Macmillan Pass, Selwyn/Mackenzie Mountains, N.W.T., Canada

Woodland caribou (Rangifer tarandus caribou Gmelin) occupy the Mackenzie and Selwyn Mountains of the Yukon and N.W.T, Canada. During the warm weather months they move into alpine areas and are frequently observed to utilize late-lying snowpatches. Microclimatic observations confirm that at 0.5 m height the snowpatch is consistently 3?C cooler than adjacent areas. Insect trap data indicated that significant differences in numbers and species occur between snowpatches and adjacent snow-free areas at the same and lower elevations. Indices of harassment correlated with biting insect numbers and with higher air temperatures (11.3 to 20.4?C, shielded temperatures) and below-average wind speeds (<2.5 m s-1). Group size on snowpatches was significantly greater than on adjacent snow-free areas. While on snowpatches animals ingested snow and otherwise spent most of their time standing or lying. These activities we interpreted as behavioral thermoregulation. However, based on our data it was not possible to separate definitively microclimatic conditions from insect harassment as cause for snowpatch selection as relief habitat.

The use of lithogeochemical patterns in wall rock as a guide to exploration drilling at the Jason lead-zinc-silver-barium deposit, Yukon Territory

The Macmillan Pass shale-hosted mineral district is located 400 km northeast of Whitehorse, Yukon Territory. On the Jason property, surface exploration based primarily on geological mapping and soil geochemistry has led to the discovery of three massive to laminated deposits rich in lead, zinc, silver, barium and iron. To date, diamond drilling has outlined geological reserves of 14.1 million tonnes grading 7.09% Pb, 6.57% Zn and 79.9 g/t Ag. One of these deposits, known as the South Zone, exhibits classical zonation; a high-grade massive to thick-banded lead-rich facies is interpreted as being proximal to a vent source. A well-laminated facies consisting of layers of chert, barite and sulphides is more distal from the vent. Pb/(Pb+Zn) ratios decrease outward from the presumed vent area. A program of systematic sampling of selected drill cores from the wall rocks of the South Zone was instituted to identify geochemical halos which could be attributed to ore emplacement and which could be used as a predictive guide to new ore. Significant copper and arsenic concentrations are restricted to the footwall rocks in the immediate vicinity of the presumed vent. A manganese deficiency in hanging-wall rocks immediately above the sulphide-rich portion of the South Zone is noted. Barium concentrations well above the regional background occur in hanging-wall rocks in excess of 50 m above the distal or pooling facies of the South Zone. The shape of the barium diffusion curve into the hanging wall can be used as a guide to the source of the barium-rich solutions, and to the direction of the greatest brine pooling.

Geochemistry, mineralogy, and geology of the Jason Pb-Zn deposits, Macmillan Pass, Yukon, Canada

Three separate stratiform Pb-Zn deposits occur on the Jason property in the Macmillan Pass area of the Yukon, Canada. The deposits are contained within chert conglomerates and finer grained clastic rocks of Middle to Late Devonian age. The deposits are on the eastern margin of the Cambro-Ordovician Selwyn basin and are associated with a fault-bounded, second or third-order subbasin, referred to as the MacPass graben.Three different facies of mineralization and associated sedimentation have been defined on the basis of field relationships, drill core descriptions and correlations, and detailed petrography, Facies A comprises massive to thick-bedded galena and sphalerite with abundant pyrrhotite and pyrite. This facies is extensively crosscut by galena-bearing quartz siderite and quartz ankerite veins and has the highest combined Pb-Zn-Ag concentrations. Facies A is proximal to the hydrothermal source represented by the veins. Facies B has a mineralogic assemblage and textures common to facies A but is considerably thinner. Abundant soft sediment deformation features indicate facies B formed from slumping of unstable accumulations of material from facies A. Facies C contains well-bedded sphalerite, chert, and barite laminae and is distal to the hydrothermal source. Secondary alteration and pore-filling minerals found in facies C include celsian, kaolinite, ankerite, and a complex suite of rare Ba carbonates.Fluid inclusions in quartz, siderite, and ankerite within veins crosscutting facies A indicate that the ore-forming fluids were NaCl brines (9 equiv. wt %) with temperatures of approximately 250 degrees C. Sulfur isotope data indicate that the sulfides within the three separate deposits had a common sulfur source, probably reduced seawater sulfate. Barite in the laminated facies C has delta 34 S values consistent with coeval Devonian seawater. The sulfide minerals are approximately 10 per mil lighter than coeval seawater.A maximum water depth of 420 m for venting of hydrothermal fluids was determined by combining fluid inclusion data with boiling curves for H 2 O-NaCl. Brine densities indicate the hydrothermal plume may have reached the seawater surface and dispersed laterally to form the barite in facies C. Accumulations, on unstable slopes, of thick deposits of sulfide slumped to cause thinning and soft sediment deformation of facies B. Since facies B is a sedimentary feature, not necessarily proximal or distal to the vent, it may or may not contain barite.

Growth and decay of palsas and peat plateaus in the Macmillan Pass–Tsichu River area, Northwest Territories, Canada

Macmillan Pass, at 1350 m asl (above sea level), is located in the Selwyn Mountains at the Yukon–Northwest Territories border (63 °15′N, 130°02′W). This area lies within the discontinuous but widespread permafrost zone. Palsa–peat plateau complexes cover 0.7% of the 235 km2 study area and are found in bog and fen depressions at elevations from 1285–1690 m. Palsa heights range from 0.15–9.75 m and diameters from 3.25–75.0 m; peat plateaus have maximum heights of 2.5 m and maximum diameters of 225 m. Both features are vegetated by Cladina-Betula glandulosa, Cladina-Polytrichum-Cetraria, and crustose lichens-Polytrichum plant communities.Palsas and peat plateaus are windswept during winter. On surfaces which support recumbent (5–15 cm tall) plant communities there was an average of only 7.5 cm of snow during late winter 1978. Snow cover was thinner by a ratio of 1:4 compared to control areas.These permafrost features have formed since the White River volcanic ash fall of 1220 BP. On palsas and peat plateaus this ash occurs at an average depth of 21 cm and has an average thickness of 11.6 cm.Shrinkage and (or) total decay of palsas and peat plateaus has occurred during the past 34 years. In one palsa field this represents a 34% reduction of area whereas in two others, 100%. The areal extent of some peat plateaus has also been reduced.

Barite nodules of possibly late diagenetic origin from Twitya River area, Mackenzie Mountains, Northwest Territories

Conspicuous spherical and subspherical barite nodules occur in Devonian(?)/Mississippian carbonaceous shales and argillaceous dolomitic limestones in Twitya river area (western foothills of the Backbone Ranges, Mackenzie Mountains, N.W.T.).The majority of barite nodules are spherulitic, composed of grey radiating barite crystals. The microscopic study of barite and of sedimentary fabrics in its host rocks has demonstrated that the crystallization of nodules was a late diagenetic event postdating the formation of carbonate concretions in shales, dolomitization, and the early phase of fracturing, but predating the late phase of fracturing and tectonic deformation. The nodules have clearly developed by replacement of their carbonate host.The middle to late Paleozoic, black shale sequence that occurs on both sides of the Yukon – Northwest Territories boundary at the latitude of Macmillan Pass (Canol road) appears to be in a distinct barite province with numerous occurrences of mostly bedded barite recorded. The presently available evidence indicates that the introduction of barium into the sedimentary basin had probably been syndepositional, followed by diagenetic concentration and barite emplacement well before the 'Laramide Orogeny' and the emplacement of Cretaceous quartz monzonite stocks.