Selwyn Basin Research Articles

Geochemical Constraints on the Genesis of the Scheelite Dome Intrusion-Related Gold Deposit, Tombstone Gold Belt, Yukon, Canada

The Scheelite dome intrusion-related gold deposit, western Selwyn basin, Yukon, is hosted in hornfelsed metasedimentary strata that lie adjacent to the exposed apices of a monzogranite to quartz monzonite plutonic complex of the mid-Cretaceous Tombstone-Tungsten magmatic belt, Tintina gold province, Alaska and Yukon. A variety of mineralization styles occur throughout a 10- × 3-km east-trending corridor and include reduced Au- and W-rich skarns, Au-, W- and Ag-Pb-Zn-Sb–rich quartz tension-vein arrays, and multiphase fault veins and isolated zones of Au-rich sericite-carbonate altered rock. Integrated U-Pb SHRIMP data for magmatic zircon and Ar-Ar data for magmatic and hydrothermal biotite indicate that gold mineralization occurred within 1 to 2 m.y. of magma emplacement. Fluid inclusion, oxygen isotope, and arsenopyrite geothermometry data indicate that hydrothermal minerals formed at depths of 6 to 9 km over a temperature range from 550°C. High-temperature Au-rich skarns formed at >400°C, whereas vein-hosted mineralization formed at 280° to 380°C. In skarns, Au is strongly associated with enrichments of Bi, Te, W, and As, whereas a variety of Au-rich veins occur, with As-rich (type 1), and Te- and W-rich (type 2) end members. Silver-Pb-Zn-Sb veins are typically Au poor and represent the latest and lowest temperature phase in the hydrothermal paragenesis. The fluid inclusion data indicate that all mineralization styles were formed from low-salinity (≤4 wt % NaCl equiv) aqueous-carbonic fluids, consistent with the composition of fluid inclusions within infilled miarolitic cavities in the intrusive rocks. However, the nonaqueous fluid was predominantly CH 4 in skarn, CO 2 in Au-Te and Au-W veins, and a fluid with roughly equal amounts of CO 2 , CH 4 , and N 2 in Au-As and Ag-Pb-Zn-Sb veins. Oxygen isotope data are consistent with a mineralizing fluid of predominantly magmatic origin that was variably modified to more positive δ 18 O values during interaction with 18 O-enriched metasedimentary strata. Sulfur isotope data suggest two possible sources of sulfur, a magmatic source characterized by δ 34 S values of approximately −5 to 0 per mil and sulfur from the metasedimentary country rocks characterized by more negative δ 34 S values of approximately −15 to −10 per mil. Collectively the data indicate that gold at Scheelite Dome was deposited from a magmatic-hydrothermal system. Interaction of magmatic fluids with graphitic hornfels rocks resulted in reduction of the ore fluids, higher CH 4 /CO 2 ratios, and modification of the oxygen and sulfur isotope values of the ore fluids toward those of the metasedimentary hornfels. Progressive reduction and cooling of hydrothermal fluids, in addition to phase separation in vein-hosted mineralization, were the mechanisms for gold deposition. Compared to other intrusion-related gold deposits associated with the Tombstone-Tungsten magmatic belt magmatism, exposed mineralization at Scheelite Dome is predominantly hosted by hornfelsed metasedimentary rocks. This results in more diverse mineralization styles and a greater spread of isotope and fluid inclusion data.

Nitrogen and carbon partitioning in diagenetic and hydrothermal minerals from Paleozoic Black Shales, (Selwyn Basin, Yukon Territories, Canada)

Selected mineralized black shales of Devonian age from the Selwyn Basin, Northwest Territories (Canada) were analyzed by Nuclear Reaction Analyses (NRA) and electron microprobe for nitrogen and carbon in silicates, sulfides, phosphates and organic matter in order to give new insights on nitrogen and carbon fractionation processes during diagenesis and hydrothermal infiltration. Hydrothermal feldspars show tri-modal composition: albite, high nitrogen-bearing K-feldspar (∼56 mol% buddingtonite (NH 4AlSi 3O 8·1/2H 2O, hydrous ammonium-feldspar, ∼51 mol% orthoclase) and hyalophane (∼32 mol% celsian). Barium-rich feldspars (hyalophane) contain lowest nitrogen contents. Potassium and nitrogen are positively correlated, while nitrogen and barium are negatively correlated due to the replacement of monovalent NH 4 + by divalent Ba 2+. The Ba-rich K-feldspar rim shows penetrative textures towards an internal K–N-rich core that is interpreted as diffusive overgrowth. These feldspars are interpreted to be deposited from hot hydrothermal Ba-bearing fluids. The second important nitrogen carrier is organic matter (from 0.6 to 0.66 wt.%). Hydrothermal quartz (N=527 ppm), diagenetic biogenic F-rich apatite (conodonts: N=468 ppm,), biogenic Fe–Ni sulfides (N=380–620 ppm) and abiogenic Ni–Fe sulfides (N>440 ppm) contain homogeneously distributed nitrogen with amounts 10-fold lower than those measured in organic matter. A two-step nitrogen-release model is suggested to explain the nitrogen-partitioning in these minerals. Primary organic matter breakdown is considered to liberate nitrogen, phosphate and sulfur to pore fluids and the water column, providing nutrients for vent fauna growth. Sulfurization, due to microbial sulfate reduction, and silicification of the vent fauna releases nitrogen in a second step. Minor nitrogen was trapped as organic molecules in conodonts, while the majority was transported by hydrothermal fluids and was incorporated as ammonium in feldspars substituting for potassium.

Superimposed Neoproterozoic and Early Tertiary alkaline magmatism in the La Biche River area, southeast Yukon Territory

Abstract Three distinct intrusive phases have been identified in the Pool Creek map area (95C/5), southeast Yukon, during geological mapping as part the Central Foreland NATMAP project: a nepheline syenite and associated dykes (Pool Creek syenite), a biotite syenite, and an intrusive breccia. All intrusive units are exposed in the core of a large anticline in the transition zone between Early Paleozoic MacDonald Platform carbonates to the southeast and Selwyn Basin basinal shales to the northwest. U-Pb isotopic dating of zircons from the different intrusive phases resulted in a cluster of ages of 640-650 Ma for the Pool Creek syenite and associated dykes, and an age of 51.8 ± 0.2 Ma for the biotite syenite. An age could not be assigned to the intrusive breccia on the basis of the analytical data. Similar-age Neoproterozoic igneous rocks are rare in the North American Cordillera. The Pool Creek syenite may be related to prolonged syn-Winderemere age rifting on the western margin of Laurentia. The superposition of Eocene and Neoproterozoic intrusive activity is rare and cannot be readily explained, but may be related to focused magmatism along the Liard Line, which is interpreted as a Neoproterozoic transfer fault structure.

Biostratigraphical and ecostratigraphical significance of Tremadoc (Ordovician) graptolite faunas from the Misty Creek Embayment and Selwyn Basin in Yukon and Northwest Territories

Graptolite zones of early and late Tremadoc ages are described from the Elmer Creek Formation in the Selwyn Basin and from the Duo Lake Formation in the Misty Creek Embayment. In ascending order, these are: Staurograptus dichotomus, Anisograptus matanensis, Adelograptus cf. A. tenellus and Aorograptus victoriae Biozones. The uppermost zone embraces the Adelograptus antiquus Biozone, Kiaerograptus? pritchardi Biozone, and the Paradelograptus kinnegraptoides Biozone in the Richardson Trough. Twenty five species are recorded and fourteen species are described, including the first record of the Rhabdinopora fauna in the early Tremadoc of northwest Canada and Kiaerograptus magnus Williams and Stevens and Paratemnograptus isolatus Williams and Stevens in the late Tremadoc. When modifications to Cooper's (1999) ecostratigraphic model are made based on data from Yukon and Northwest Territories, the upper slope biotope and lower slope – ocean-floor biotope become so similar that there is little basis for recognising both biotopes.

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

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

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

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

Geology and geochemistry of the Clear Creek intrusion-related gold occurrences, Tintina Gold Province, Yukon, Canada

The Clear Creek gold occurrences lie within deformed lower greenschist-facies rocks of the western Selwyn basin. They consist of auriferous, sheeted quartz veins that cut six Cretaceous stocks and their hornfels. The veins contain 1–2% combined pyrite and arsenopyrite, with lesser pyrrhotite, bismuthinite, and scheelite, as well as 2–5 g/t Au. New 40Ar/39Ar geochronology of hydrothermal micas indicates that the veins formed within 1–2 million years of granitoid emplacement. Fluid inclusion microthermometry defines a parent ore fluid of ~81 mol.% H2O, 14 mol.% CO2, 4 mol.% NaCl ± KCl, and 1 mol.% CH4, which unmixed into a low- and high-salinity immiscible pair. This suggests a more saline parent fluid and a greater degree of fluid unmixing relative to the other occurrences in the eastern Tintina Gold Province. Inclusions trapped in As- and Bi-rich, high-gold grade veins have homogenization temperatures of 300–350°C, whereas inclusions found in more Ag- and Pb-rich veins are characterized by temperatures of 250–300°C. Fluid inclusion geobarometry suggests hydro-fracturing and gold deposition at 5–7 km depth. The δ18O values of quartz samples range from 13–16‰ (per mil) and δ34S for sulfides are also consistent between –3.0‰ and 0‰, with the exception of some outliers from the Contact Zone of the Pukelman stock that indicate a lower temperature second phase of mineralization. It remains uncertain as to whether the Clear Creek ore fluids were exsolved from magmas at depth or from devolatilization reactions within the contact metamorphic aureoles of the intrusions.

Conodont biostratigraphy of the Lower to Middle Devonian Deserters Formation (new), Road River Group, northeastern British Columbia

A new Lower to Middle Devonian basinal unit of the Road River Group, herein formally named the Deserters Formation, contains argillaceous, crinoidal limestone and black shale deposited in a linear sub-basin of the Ospika Embayment, southern Kechika Trough. The abrupt lateral facies changes in the region, facies thickness, and occurrence of volcanics indicate a period of extensional tectonism. A total of 53 (4–5 kg each) samples from the Deserters Formation yielded 7766 conodont elements assigned to 14 genera representing 33 species. The formation ranges from the Lochkovian (eleanorae Zone, or lower part of the delta Zone of the Cordilleran Region) to Eifelian (australis Zone). The temporal constraints established by conodont biostratigraphy allow correlation to the Grizzly Bear Formation, a regionally restricted Lower to Middle Devonian unit in the Selwyn Basin to the north.

New and biostratigraphically useful thelodonts from the Silurian of the Mackenzie Mountains, Northwest Territories, Canada

ABSTRACT Two new species of thelodont are described. A new thelodontiform is based on articulated skeletons and isolated scales from late Telychian (Llandovery) to earliest Sheinwoodian (Wenlock) shales of the Avalanche Lake sections, Mackenzie Mountains, N.W.T., Canada. The head is blunt; it and the anterior trunk are broad and dorsoventrally flattened, while the posterior trunk and tail are more slender and laterally compressed. Unlike most other articulated thelodonts, there is no evidence of fins apart from the caudal fin. The hypocercal tail displays no evidence of rays or intermediate secondary lobes. The scales are robust, with large bases, large pulp cavities, and strong crown ridges in a stellate pattern; scale crowns are almost unornamented along the presumed ventral margin of the trunk and tail. An articulated specimen from the uppermost Telychian of Baumann Fiord, Ellesmere Island, Canadian Arctic Archipelago, is considered conspecific because of similarities in scale size and ornament, and extends the stratigraphic range of the species from the late Telychian griestoniensis-sakmaricus Graptolite Zone to the early Sheinwoodian centrifugus-insectus Graptolite Zone/Pterospathodus procerus Conodont Superzone, and the geographic range from the Selwyn Basin of the Yukon and N.W.T. to the Franklinian Basin of the Canadian Arctic Archipelago, Nunavut. A new phlebolepidiform is based on scales only. It also occurs both in the Mackenzie Mountains and the Canadian Arctic Archipelago. The two new species contribute significantly to a preliminary vertebrate faunal succession scheme for both areas and are therefore considered to be biostratigraphically useful.

00/00155 Behaviour of selected minerals in an improved ash fusion test: quartz, potassium feldspar, sodium feldspar, kaolinite, illite, calcite, dolomite, siderite, pyrite and apatite

Multiple occurrences of bedded barite (BaSO4) are preserved in mudstones of the upper-most Canol Formation (Frasnian) of the Selwyn Basin (Canada). Samples of barite were collected from eight sites (seven measured sections), and examined using petrographic and isotopic techniques (87Sr/86Sr, δ34S and δ18O values) to determine whether the barite has a diagenetic or hydrothermal origin.Barite at all locations occurs in two morphologies: (1) laminated barite composed of thin layers of microcrystalline barite with lesser amounts of pyrite and hyalophane (Ba-K-feldspar) interlaminated with mudstone, and (2) nodular barite consisting of barite, hyalophane, cymrite (hydrous Ba-silicate) and minor pyrite. At the Cowan occurrence, one sample has a mineral assemblage comprising barite and nodular barium carbonates (witherite, norsethite).The δ34S and δ18O values of the laminated barite co-vary, and range between +24.1 and +35.3‰ and +14.8 to +18.3‰ respectively, with the exception of one sample with δ34S and δ18O values of +32.9‰ and −0.5‰. The laminated samples have an average 87Sr/86Sr of 0.7085 (n = 8), which is consistent with Frasnian seawater (0.7081) that has undergone some minor water sediment interaction during early diagenesis.The nodular barite has highly variable δ34S (+4.8 to +56.5‰) and δ18O (+9.2 to +19.3‰) values, which developed from more evolved diagenetic pore fluids with limited sulphate re-supply (i.e., as a relatively closed system). In these conditions sulphate reduction likely occurred at a slower rate, and the sulphate in the barite records both kinetic (δ34SSO4) and equilibrium (δ18OSO4) isotope effects. One sample of nodular barite has a 87Sr/86Sr ratio of 0.7086. The single sample with a barite and barium-carbonate assemblage at Cowan has a 87Sr/86Sr ratio of 0.7133, a δ34S value of +23.9‰, and a δ18O value of +10.7‰ and may represent a local hydrothermal event at this site.Our data supports the interpretation that the barite horizon is a stratigraphic marker throughout the Canol Formation. Laminated and nodular barite formed as a result of diagenetic processes, which recycled Ba from organic matter in the sediment below, and reprecipitated it above, the sulphate methane transition zone in the sediments.The results of this study are similar to those obtained for other Late Devonian sediment-hosted barite occurrences in North America (Yukon, Nevada, New York State), meaning these occurrences likely formed by similar processes and diagenetic pore water conditions within oceanic sediments that covered the North American continent during the Late Devonian. We suggest that the combination of high biological productivity and low seawater sulphate at this time contributed to the efficient cycling of Ba from seawater into the sediments on the continental margin.

The Importance of Oxidized Brines for the Formation of Australian Proterozoic Stratiform Sediment-Hosted Pb-Zn (Sedex) Deposits

A two-fold subdivision for stratiform sediment-hosted Pb-Zn (sedimentary exhalative, sedex) deposits is proposed, based on fundamental differences in the chemistry of the mineralizing brines. The type of sedimentary basin from which the ore fluids are derived, and the lithologies contained within the basin, control these differences in fluid chemistry. The two discrete brine types capable of transporting Zn and Pb are oxidized brines and reduced, acidic brines. McArthur-type deposits (e.g., McArthur River, Mount Isa, Hilton) precipitate from oxidized (SO42—predominant), acidic to near-neutral brines that evolve from sedimentary basins dominated by carbonates, evaporites, and hematitic sandstones and shales. Selwyn-type deposits (e.g., Sullivan, Rammelsberg, sedex deposits of the Selwyn basin) precipitate from acidic, reduced (H2S-predominant) connate brines that evolved in reduced siliciclastic and shale basins. Temperature decrease and dilution (fluid mixing), addition of H2S, and pH increase can all be effective depositional processes for Zn and Pb from reduced (Selwyn-type) brines. In contrast, sulfate reduction and/or addition of H2S (via fluid mixing or interaction with earlier formed pyrite) may be the important processes for sphalerite and galena deposition from oxidized (McArthur-type) brines. McArthur-type sedex deposits are intimately associated with siderite or ferroan carbonate alteration halos and most likely precipitate from lower temperature brines than Selwyn-type deposits. The redox state of the mineralized brines (sulfate or sulfide predominant) is important for controlling minor element associations in the two classes of sedex deposits. Weakly acidic to weakly alkaline oxidized brines can precipitate siderite but are incapable of carrying significant gold, tin, and barium in solution, and as such, McArthur-type deposits do not contain anomalous concentrations of these elements. Reduced, acid brines can carry high concentrations of barium, explaining the common association with barite in these deposits. If reduced sulfur concentrations were sufficient in the mineralizing brines, individual Selwyn-type deposits may contain anomalous or ore-grade gold. If the brines were highly reduced (pyrrhotite-stable), they may have carried high concentrations of tin (e.g., Sullivan). The lack of sulfide-bearing feeder systems in McArthur-type deposits and their common occurrence in Selwyn-type deposits probably also relate to the redox state of the brines. From a mineral exploration perspective, oxidized sedimentary brines are more likely to produce large tonnage Zn-Pb-Ag deposits that have siderite or ankerite alteration halos and commonly lack barite lenses and vent complexes. By contrast, deposits that form in reduced siliciclastic and shale-dominated basins are more likely to be lower tonnage and to contain barite, vent complexes and may have minor gold or tin credits.

Trace-element geochemistry of metabasaltic rocks from the Yukon-Tanana Upland and implications for the origin of tectonic assemblages in east-central Alaska

We present major- and trace- element geochemical data for 27 amphibolites and six greenstones from three structural packages in the Yukon-Tanana Upland of east-central Alaska: the Lake George assemblage (LG) of Devono-Mississippian augen gneiss, quartz-mica schist, quartzite, and amphibolite; the Taylor Mountain assemblage (TM) of mafic schist and gneiss, marble, quartzite, and metachert; and the Seventymile terrane of greenstone, serpentinized peridotite, and Mississippian to Late Triassic metasedimentary rocks. Most LG amphibolites have relatively high Nb, TiO2, Zr, and light rare earth element contents, indicative of an alkalic to tholeiitic, within-plate basalt origin. The within-plate affinities of the LG amphibolites suggest that their basaltic parent magmas developed in an extensional setting and support a correlation of these metamorphosed continental-margin rocks with less metamorphosed counterparts across the Tintina fault in the Selwyn Basin of the Canadian Cordillera. TM amphibolites have a tholeiitic or calc-alkalic composition, low normalized abundances of Nb and Ta relative to Th and La, and Ti/V values of <20, all indicative of a volcanic-arc origin. Limited results from Seventymile greenstones indicate a tholeiitic or calc-alkalic composition and intermediate to high Ti/V values (27-48), consistent with either a within-plate or an ocean-floor basalt origin. Y-La-Nb proportions in both TM and Seventymile metabasalts indicate the proximity of the arc and marginal basin to continental crust. The arc geochemistry of TM amphibolites is consistent with a model in which the TM assemblage includes arc rocks generated above a west-dipping subduction zone outboard of the North American continental margin in mid-Paleozoic through Triassic time. The ocean-floor or within-plate basalt geochemistry of the Seventymile greenstones supports the correlation of the Seventymile terrane with the Slide Mountain terrane in Canada and the hypothesis that these oceanic rocks originated in a basin between the continental margin and an arc to the west.

Early Paleozoic (Cambrian to Early Devonian) Tectonic Framework, Canadian Cordillera

Uppermost Proterozoic to Lower Devonian rocks in the Cordillera of western Canada accumulated within two ancestral continental margin systems--the Canadian Cordilleran and southwestern Franklinian miogeoclines. These miogeoclines are separated by the Ogilvie Arch. Prominent features of the southwestern Franklinian Miogeocline are the Richardson Trough, a failed arm or aulacogen opening northward into the outer Franklinian Miogeocline, and, west and east of it, the Porcupine and Lac des Bois platforms. The Canadian Cordilleran Miogeocline formed in Late Proterozoic as the result of rifting, and although the newly formed margin was Atlantic-type, it continued to have distinct periods of rifting and extension with volcanism, throughout its history. The Canadian Cordilleran Miogeocline is divided by numerous northeast-trending linear features interpreted to reflect the position of ancestral strike-slip or transfer faults. More than six major block segments are recognized. From south to north they are: the Montana Block, bordered north by the Moyie-Dibble Creek fault systems; the Alberta Block, bordered north by the Hay River and Great Slave Lake fault systems, and divided into the Peace and Southern Prairies sub-blocks along the Rimbey Magnetic-Snowbird Tectonic zones; the Macdonald Block bordered north by the Liard Line; the Blackwater Block, bordered north by the Fort Norman Line; and the Mackenzie Block, bordered north by the southwestern Franklinian Miogeocline. Many facies changes occur across these blocks and their bounding ancestral transfer or strike-slip fault zones, and these or associated faults are likely keys to independent tectonic histories, such as that demonstrated by the Peace River Arch. The most prominent and well known northeast-trending lineament is the Liard Line. Apparent on geological maps of Canada, or the Canadian Cordillera, is a northeastward swing of structural trends north of this feature. The Canadian Cordilleran Miogeocline to the north shows extensive preservation of lower Paleozoic (to Lower Devonian) strata northeastward to the Canadian Shield, whereas to the south the same rocks are generally preserved in only the Cordilleran deformed belt. Although Lower Paleozoic rocks in the south have been substantially cut by younger erosional events, facies and thickness patterns, in fringing Lower Paleozoic strata, indicate that the pattern of erosion and nondeposition is as much due to Lower Paleozoic tectonics and paleogeography as it is due to younger exposure or erosion. This profound change, and the connection to the Franklinian, divides the Canadian Cordilleran Miogeocline into three fundamental parts. The Liard Line is interpreted to have been an ancient transfer fault zone across which there was a reversal in the direction of asymmetry within the master separation fault system that formed the ancestral continental margin in Late Proterozoic time. The wide area north of the Liard line has a large outer miogeoclinal basin (Selwyn Basin), discrete rift systems (Misty Creek Embayment, Mackenzie Trough, Root Basin), a peculiar zone of high crustal attenuation and sediment accumulation (Liard Depression). In addition, it is complicated by several different arches or highs (Mackenzie, Redstone, McConnell, Keele, Bulmer Lake, Niddery, etc.). This northern half of the Canadian Cordilleran Miogeocline has the characteristics of a lower plate margin and is here defined as the Northern Cordilleran Lower Plate Zone, comprising the Blackwater and Mackenzie blocks. The Canadian Cordilleran Miogeocline south of the Liard Line is narrowly preserved in the Cordillera and consists of the White River, Robson and Kechika troughs, and their marginal Macdonald and Bow platforms. The eastern miogeocline is dominated by Lower Paleozoic paleotopographic highs (Montania, West Alberta Ridge, Peace River Arch and Macdonald High). This southern zone has the characteristics of an upper plate margin and is here defined as the Southern Cordilleran Upper Plate Zone, comprising the Montana, Alberta and Macdonald blocks. A previous upper-lower plate model of the Canadian Cordilleran Miogeocline is reviewed and its criteria of width, structural complexity and uplift-subsidence are found to be useful as plate-type indicators. However, the use of gneisses to indicate lower crust, transitional gradients base on isopleths, and Cambrian zero edges are found to be equivocal. The Canadian Cordilleran Miogeocline non passive character is compared to that of Mesozoic-Tertiary continental margins, which are also fundamentally non passive, suggesting that models for plate tectonic drifting phases need to be revised.

Sulphur isotope composition of the Brunswick No. 12 massive sulphide deposit, Bathurst Mining Camp, New Brunswick: implications for ambient environment, sulphur source, and ore genesis

The Brunswick No. 12 massive sulphide deposit occurs within a Middle Ordovician bimodal volcanic and sedimentary sequence that is thought to have formed in a continental back-arc rift covered with a thick succession of carbonaceous hemipelagic and turbiditic sedimentary rocks. The deposit consists of three en echelon lenses that are zoned from Vent Complex to Bedded Ore and Bedded pyrite facies. The Bedded Ore facies has the lowest average δ34S values (14.2[Formula: see text]), but are only slightly less positive than laminated pyrite in footwall sedimentary rocks (δ34Smean = 15.1[Formula: see text]). δ34S values for the bedded sulphides show an upward increase from 14.2[Formula: see text] in Bedded Ore to 16.5[Formula: see text] in Bedded Pyrite. Average δ34S values for Vent Complex (15.8[Formula: see text]) and underlying stringer sulphides (16.1[Formula: see text]) are consistently more positive than those for Bedded Ore. In carbonaceous shales and siltstone of the Patrick Brook Formation that underlie the deposit, δ34S values that range between 13.8 and 25.6[Formula: see text], and the similarity of these values to those of the Brunswick No. 12 deposit indicate major bacterial reduction of sulphate to sulphide under closed or partly closed conditions, and that most of the S in the deposit originated from ambient sulphidic bottom waters. Furthermore, the average δ34S value for Brunswick No. 12 bedded ores lies on the Selwyn Basin pyrite evolutionary curve and indicates that anoxic conditions within the Tetagouche back-arc basin reflect a global anoxic episode. The Brunswick No. 12 deposit probably formed, therefore, by the mixing of hydrothermal metals with dissolved sulphide of seawater origin during periods of ocean anoxia. The increase of δ34S values towards the Vent Complex may reflect the addition of isotopically heavy S formed by the inorganic reduction of seawater sulphate.

Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline

The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and NbIY and low ZrINb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group 11 rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher ZrINb, and lower NbIY. Group I11 rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and I1 rocks, and contain elevated Ba contents. Groups 1-111 are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and I1 basalts. Groups I-111 are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline

The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and Nb/Y and low Zr/Nb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group II rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher Zr/Nb, and lower Nb/Y. Group III rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and II rocks, and contain elevated Ba contents. Groups I–III are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and II basalts. Groups I–III are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

Reconnaissance geochemistry and its environmental relevance

Abstract In Canada, geochemical reconnaissance using lake and stream sediments and waters has been a cooperative program between federal and provincial geological survey organizations since the 1970's. Similar methods of sampling, analysis and quality control were established to ensure comparable data across the country. As of 1992, 2.5 million km2, or approximately one quarter of Canada's landmass has been covered. The original purpose of this National Geochemical Reconnaissance (NGR) was to assist mineral exploration. However, with time, the relevance of the data to environmental issues has become increasingly apparent. The concentrations of metals in the natural environment vary widely and in some areas unaffected by human activity may reach levels that elsewhere have been considered to have an effect on the ecosystem. Environmental agencies need to incorporate the natural distribution of metals into assessments of ecosystem health and anthropogenic pollution. Sediment quality guidelines, standards or clean-up criteria, are required for the assessment and remediation of contaminated areas under a variety of programs in Canada. These standards need to take into account the natural background of metals in the affected areas. For instance, under the Canadian Environmental Protection Act, a sediment Cd concentration of 0.6 ppm is used to screen ocean dumping permits and yet 20% of the lake and stream sediment samples collected by the NGR exceed this level naturally. We present Canada-wide maps illustrating the regional variability of As, Cd, Cu, Hg, Ni, Pb, Zn and U in sediments, together with more detailed maps for selected areas. Cumulative frequency distribution plots for these elements plus Cr have been constructed based on up to 250,000 samples. At the scale that the maps are presented, anomalies a few km2 or tens of km2 in extent, which may be associated with individual mineral deposits, are not visible. What may be observed are large, regional anomalies hundreds or thousands of km2 in extent. In some cases these may mark important mineral districts, such as U around the Athabasca and Elliot Lake uranium districts and the suite of metals that are enriched in stream sediments from the Selwyn Basin, Yukon that contains a variety of Sedex, Mississippi Valley and intrusion-related mineral deposits.

New geophysical data from the northern Cordillera: preliminary interpretations and implications for the tectonics and deep geology

In this paper we analyze recently acquired geophysical data from the northern Cordillera and their relation to the mapped geology. A prominent gravity high (> −45 mGal (1 Gal = 1 cm/s2)) coincides with a magnetic low and an aseismic region in west-central Yukon where the underlying geology is dominated by quartzo-feldspathic rocks having moderate densities. Extension (~15%), magmatic underplating, and accretion of the anomalous region onto oceanic crust are three possible explanations.Magnetic, gravity, and seismicity data all show significant differences in the physical state of the crust on either side of the Tintina Fault and, together with geological data indicating large offset, suggest it was once a major crustal-scale strike-slip fault. The new gravity data also delineate an arcuate zone of steep gradients (up to 1.4 mGal/km) in the miogeocline, which may correlate with a west-dipping Proterozoic basement ramp mapped on deep seismic sections farther to the north and a transition from thin (east) to thick sediment cover (west). Seismicity data show that current tectonic activity is concentrated along the Pacific – North America plate margin in southwestern Yukon and adjacent Alaska and, although there is a marked decrease in activity inland of this margin, notable concentrations occur along the Denali Fault System and in the eastern miogeocline. There is a distinct absence of earthquakes in parts of the Selwyn Basin and in the northern Yukon–Tanana Terrane. Limited field studies suggest activity is confined to the upper 10–15 km of the crust.

Late Ordovician mass extinction in the Selwyn Basin, northwestern Canada: geochemical, sedimentological, and paleontological evidence

We present a detailed study of the trace element and stable isotope geochemistry, sedimentology, and fossil distributions in two Avalanche Lake (AV4B, AV1) Ordovician–Silurian boundary sections in the Selwyn Basin. Trilobites and conodonts indicate a profound extinction at the end of the Ordovician, which is constrained stratigraphically within a <60 cm interval at AV4B. Facies analysis suggests that the extinction interval coincides with the maximum shallowing (low stand of sea level), which was probably caused by a galcioeustatic regression induced by the Late Ordovician Gondwanan glaciation. The extinction crisis is also signalled by the change in carbonate δ13C: a sudden "Strangelove ocean" δ13C excursion (>3‰ in magnitude) is recorded in the extinction interval. Iridium abundances (<0.051 ppb) in the extinction interval are low and fail to provide evidence for an impact. The highest Ir abundance is found to be associated with reduced sedimentation in a condensed horizon. Cerium anomalies indicate a short period of basin ventilation in the otherwise anoxic Selwyn Basin. The extinction occurred during the time of this basin ventilation, which was probably caused by the cold climate during the glaciation. The ventilation may have triggered upwelling of the deep water through vertical advection, bringing up toxic material, poisoning the upper-water photic zone, and causing the extinction.

Contractional nature of Devonian-Mississippian Antler tectonism along the North American continental margin

Regional tectonic analysis suggests that the entire Cordilleran continental margin, from Nevada to the Yukon, underwent contractional orogeny involving emplacement of deep basinal, submarine-fan, and mafic volcanic strata over autochthonous continental margin strata during Late Devonian to Mississippian time (Antler orogeny). Displaced parautochthonous strata are present from Nevada to east-central British Columbia, and perhaps within the Yukon-Tanana terrane of Yukon and eastern Alaska. Manifestations of the orogenic event in the northern Cordillera are preserved primarily by normal faults and related coarse clastic strata of the Selwyn basin. We propose that these features formed due to flexural extension during foreland deformation, rather than mid-Paleozoic rifting or transtension.