Selwyn Basin Research Articles

The pre-Mississippian "Neruokpuk Formation," northeastern Alaska and northwestern Yukon: review and new regional correlation: Reply

Since the early 1900's, regional reconnaissance in Alaska and the Yukon has failed to resolve the stratigraphy and structure of the pre-Mississippian Formation. Its age and distribution have been defined and redefined as new data have slowly accumulated. In most recently published reconnaissance maps of the Yukon, the Neruokpuk includes nearly all of the pre-Mississippian strata in the British Mountains and is assigned a Precambrian age. In contrast, approximately half of contiguous strata in adjacent Alaska are interpreted as early Paleozoic in age and are excluded from the Neruokpuk. Recent detailed studies in the Firth River area of the Yukon have documented intense small-scale imbrication of fossiliferous Lower Cambrian to Devonian(?) units that were previously mapped as Precambrian Neruokpuk.A remarkable similarity between the lithologies of lower Paleozoic rocks in the British Mountains and the Selwyn Basin 1000 km to the southeast is strengthened by biostratigraphic ties in Lower Cambri...

Stratigraphy and tectonic significance of Lower Paleozoic continental margin strata in northeastern Washington

Lower Paleozoic eugeoclinal strata in the Kootenay Arc in northeastern Washington and southeastern British Columbia are transitional between autochthonous lower Paleozoic miogeoclinal strata and outboard volcanic arc terranes of uncertain paleogeographic affinity. They provide a record of lower Paleozoic continental margin depositional and tectonic processes oceanward of the continental shelf. The southernmost stratigraphic unit, the Covada Group, is divided into two formations, the Daisy Formation, a mid fan sequence of arkosic and subarkosic wacke and arenite, and the Early Ordovician Butcher Mountain Formation, consisting of alkalic(?) pillow basalt and tuff of within‐plate affinity. Another unit, formerly part of the Covada Group, is excluded and informally named the Bradeen Hill assemblage. It contains chert, chert‐quartz arenite, quartz arenite, chert pebble conglomerate, shale, and basalt, and may be Ordovician to Devonian on the basis of stratigraphic evidence and regional correlations. The Covada Group and Bradeen Hill assemblage record (1) deposition of continentally derived sediments in a submarine fan setting, (2) relatively quiescent starved basin conditions, (3) local faulting; and (4) intermittent periods of volcanism, perhaps reflecting local extension. They can be correlated with other stratigraphic units in the Kootenay Arc and resemble units as far north as the Selwyn basin in northern Canada and as far south as the Roberts Mountains allochthon in central Nevada. This unites the stratigraphic record and implies a high degree of synchroneity of tectonic events along over 2500 km of the outer continental margin during early Paleozoic time.

The pre-Mississippian "Neruokpuk Formation," northeastern Alaska and northwestern Yukon: review and new regional correlation

Since the early 1900's, regional reconnaissance in Alaska and the Yukon has failed to resolve the stratigraphy and structure of the pre-Mississippian Neruokpuk Formation. Its age and distribution have been defined and redefined as new data have slowly accumulated. In most recently published reconnaissance maps of the Yukon, the "Neruokpuk" includes nearly all of the pre-Mississippian strata in the British Mountains and is assigned a Precambrian age. In contrast, approximately half of contiguous strata in adjacent Alaska are interpreted as early Paleozoic in age and are excluded from the Neruokpuk. Recent detailed studies in the Firth River area of the Yukon have documented intense small-scale imbrication of fossiliferous Lower Cambrian to Devonian(?) units that were previously mapped as Precambrian Neruokpuk.A remarkable similarity between the lithologies of lower Paleozoic rocks in the British Mountains and the Selwyn Basin 1000 km to the southeast is strengthened by biostratigraphic ties in Lower Cambrian, Lower Ordovician, and Lower Silurian strata. This correlation between basin facies suggests that shelf and slope facies of upper Proterozoic through lower Paleozoic strata may also be correlatable between the two areas. The paleogeographic implications of these correlations indicate that pre-Mississippian strata in Arctic Alaska and the Yukon are part of a single Arctic–Pacific continental margin.The Neruokpuk Formation name should be restricted in Canada to the quartzite-dominant unit, contiguous with similar strata to which the restricted Neruokpuk definition applies in Alaska. The current broad definition, based on an assumed Proterozoic age but including many rock types, should be discontinued.

Bedded barite deposits in the United States, Canada, Germany, and China; two major types based on tectonic setting

Bedded barite deposits can be divided into two types based on stratigraphic succession, tectonic setting, and geochemistry. The first, herein termed the margin has abundant chert in the host sediments, is deposited in small ocean basins lying between a subduction zone and a passive continental margin, and has a geochemical signature that suggests an appreciable influence of oceanic crust. The barite is deposited on or adjacent to the passive margin side of the basin. Subduction leads to consumption of the ocean floor of the basin, collision of the volcanic arc with the passive margin, and thrusting of the barite-bearing sedimentary sequence onto the stable platform. This group of deposits lacks economic accumulations of Pb and Zn. Examples are the Fancy Hill and Chamberlain Creek deposits in the Ouachita Mountains of Arkansas and the East Northumberland Canyon deposit in central Nevada. The second type, for which we propose the term rift is hosted by coarse clastics of turbidity current origin and is deposited in intracratonic rifts with strong local relief; the host rocks have a chemistry suggesting entirely continental affinities. Volcanic rocks, where present, tend to be alkalic basalts. Most barite deposits of this type are accompanied by economic accumulations of Pb and Zn. Examples include the Meggen and Rammelsberg deposits in Germany and the Anvil and Howard's Pass deposits in the Selwyn basin of western Canada.The barite deposits of south China can be assigned tentatively to these two types. Those of the Qinling district were deposited in a chert-dominated section on oceanic crust in a closing, remnant-ocean basin, placing them in the continental margin type. Those of the Jiangnan region are chemically more like the cratonic rift-type deposits and seem to reflect a similar tectonic setting. If this assignment is correct, the Jiangnan area should be considered prospective for Pb and Zn mineralization near the barite deposits.

Pine Point; a case history of lead isotope homogeneity in a mississippi valley-type district

The lead isotope compositions of 32 galena samples from 20 different deposits in the Pine Point district are essentially identical within error (avg 206 Pb/ 204 Pb = 18.175 + or - 0.008) and indicate a remarkable uniformity of isotope composition over a mineralized area of several hundred square kilometers. The isotope homogeneity could result from an homogeneous source or from thorough mixing during extraction and subsequent precipitation of lead. The homogeneity of the lead isotope data allows an interpretation in terms of a single-stage lead-evolution model and calculation of a model age of mineralization of about 290 Ma (Late Pennsylvanian). This age is substantially younger than the Middle Devonian host rocks (375 Ma), but much earlier than the post-Early Cretaceous ( approximately 100 Ma) tectonic events along the western margin of the Mackenzie basin that have been credited with initiating the regional hydrologic regime that resulted in mineralization. More complex multistage models are possible, but cannot be more precisely defined by present data.The homogeneity and nonradiogenic composition of Pine Point leads is in marked contrast to those reported for Devonian carbonate-hosted Pb-Zn deposits of the Mackenzie platform. This discrepancy may be due in part to the different paleogeographic positions of the two groups of deposits. The more radiogenic, inhomogeneous group is situated on the western edge of the platform adjacent to deep-water shales of the Selwyn basin. Pine Point, in contrast, lies more than 500 km to the east within the carbonate platform.

Graptolites as indicators of regional maturity in lower Paleozoic sediments, Selwyn Basin, Yukon and Northwest Territories, Canada

Variations in reflectance and bireflectance of graptolites and bitumen are documented from lower Paleozoic rocks of Howard's Pass (XY) Zn–Pb deposit and adjacent portions of the Selwyn Basin. Two types of graptolite morphologies are recognized in these rocks: a nongranular type, comprising blocky and lath-shaped varieties, and a granular type. The nongranular graptolites generally display higher reflectance and bireflectance than granular graptolites. Bitumen (epi-impsonite to meso-impsonite) is common in these rocks and displays a wide range of reflectance and bireflectance values.At the Howard's Pass (XY) property, graptolite-reflectance values display a slight increase with depth, and anomalously high values (7.46–10.25% [Formula: see text]) are documented in the sulphide-rich, upper Active Member. In surrounding regions of the Selwyn Basin, remote from Howard's Pass, graptolite-reflectance values are lower (3.02–6.26% [Formula: see text]). Conodont colour-alteration-index values reported by others show no detectable variation in the same rocks, which indicates that graptolite-reflectance measurements are more sensitive to variations in thermal maturity, at least for overmature sediments, than conodont colour-alteration index.The anomalously high reflectance values of both the graptolites and the bitumen from the Howard's Pass (XY) property, particularly in samples from the upper Active Member, indicate that these rocks have experienced elevated temperatures compared with regional values. This study demonstrates that graptolite-reflectance measurements may be used to delineate thermal anomalies. The integration of graptolite-reflectance and biostratigraphic studies in lower Paleozoic sequences is potentially useful for determining episodes of high heat flow.

Lingulid brachiopods and Lingulichnus from a Silurian shelf–slope carbonate sequence, Delorme Group, Mackenzie Mountains, Northwest Territories

Structures assigned to the ichnogenus Lingulichnus, commonly containing the burrow-forming brachiopod, were collected from an argillaceous unit in a Silurian shelf dolostone sequence (Delorme Group). The sediments, benthic fauna, and inclined orientation of all lingulid burrows indicate the presence of a bottom current and restricted conditions on the margin of the Selwyn Basin. High-abundance–low-diversity faunas of lingulids and Lingulichnus are indicative of limiting conditions at the substrate surface. Under low-energy conditions, they seem to be indicative of the Zoophycos ichnofacies.

Ammonium silicates associated with sedimentary exhalative ore deposits: A geochemical exploration tool

Abstract The fixed-ammonium (NH 4 + ) concentration in sediments hosting sedimentary exhalative Zn-Pb-(Ag) sulfide deposits was examined at the Howards Pass, Clear Lake, Tom and Jason deposits in the Selwyn Basin, Yukon Territory, Canada. A few samples from the McArthur River deposit, Australia, were examined for comparison. Infrared spectrophotometry shows that NH 4 + concentrations in alkali-bearing mineral phases increase near ore bodies not diluted by the influx of abundant detrital material. In whole-rock samples from the Selwyn Basin, NH 4 + concentrations reach as high as 2000 ppm above ore, against a background of approximately 467 ± 70 ppm. In McArthur River's Emu Basin, the concentration of NH 4 + in two samples located adjacent to sulfides reaches over 2000 ppm whereas no NH 4 + was detected in any other samples taken from one core through the deposit. All samples for this study were taken from well below the weathered zone. X-ray diffraction of clay minerals does not indicate NH 4 + substitution for K + in interlayer sites. Instead, a comparison of NH 4 + concentrations with X-ray diffraction estimates of major mineral constituents of the samples suggests that NH 4 + substitutes for K + and Na + in feldspars. The substitution is highest in the stratigraphic hanging wall at both the Howards Pass and Clear Lake deposits. At Clear Lake, high NH 4 + values (up to 1800 ppm) are found near stringer mineralization, presumably where the venting of hydrothermal fluids occurred. The Tom and Jason deposits contain low or background NH 4 + concentrations throughout the stratigraphic section, probably as a result of detrital dilution of the host sediments from slumping within a local graben coincident with mineralization. High concentrations of ammonia (NH 3 ) may be incorporated in hydrothermal solutions by breakdown of organic matter in the sedimentary pile. As hot, mineralizing fluids rise into a starved, stagnant basin of low pH and reducing conditions, NH 4 + would become the dominant nitrogen species at low temperature and may substitute for K + or Na + in authigenic minerals.

Anoxic stratified oceans as a source of sulphur in sediment-hosted stratiform Zn [sbnd]Pb deposits (Selwyn Basin, Yukon, Canada)

Three periods of stagnation, viz. Middle Ordovician to Early Silurian, Early Devonian, and Late Devonian, have been recognized in the Selwyn Basin by extremely positive δ 34S-values in pyrite. These δ 34S-values in pyrite reflect almost complete bacterial sulphate reduction of sulphate to sulphide within a restricted anoxic layer of a stratified water column. W.C. Shanks and coworkers have reported a fourth anoxic event in the basin during the Early Cambrian. Each period of basin stagnation coincides with times when sedimentary Zn Pb-sulphide and barite deposits formed, while intervening periods correspond with the formation of stratiform barite deposits. The stratigraphic distribution of δ 34S-values for pyrite are similar across the basin, whether near to or remote from mineralization, indicating a common seawater source of S. The generally parallel tracking of isotopic curves for sphalerite and galena with pyrite indicates that this sulphide also came from the bacterial reduction of sulphate within a stratified water column. This interpretation is supported by data on hydrothermal fluids that produced Zn Pb mineralization, which are near-neutral pH and range in temperature between 100° and 350°C. In such solutions, it is thermodynamically difficult to transport appreciable sulphide in the presence of Pb and Zn. Since these conditions apply to mineralizing fluids of many sedimentary-exhalative Zn Pb deposits, it follows that they too were likely S-poor, requiring an external seawater source of sulphide. This source is most effectively supplied by the sulphidic bottom layers of a stratified water column. This explains the correlation between anoxic events and Zn Pb sedimentary deposits, both within the Selwyn Basin and world-wide during the Phanerozoic. The latter observation also implies that the anoxic events within the Selwyn Basin were not controlled locally, but instead reflect periods of worldwide ocean stagnation.

Sulfur and lead isotope studies of stratiform Zn-Pb-Ag deposits, Anvil Range, Yukon; basinal brine exhalation and anoxic bottom-water mixing

Five stratiform Zn-Pb-Ag deposits are known in Early Cambrian metapelitic rocks along a curvilinear trend in the Anvil Range, central Yukon. The Anvil Range deposits occur along the southwestern boundary of the Selwyn basin in the stratigraphic transition zone between metapelites of the Mt. Mye unit and calcareous phyllites of the overlying Vangorda unit. The massive sulfides are associated closely with anomalously thick graphitic phyllites, apparently related to a second-order basin. A typical Anvil cycle of mineralization begins with a ribbon-banded graphitic-quartzitic-pyritic unit. This grades upward into sulfide-bearing quartzite, quartzitic massive sulfide, massive sulfide, and finally a baritic massive sulfide horizon. Sericitic alteration envelopes irregularly encompass each deposit and locally are developed best in footwall rocks.Detailed sulfur isotope studies have been carried out on the DY and Grum deposits and on one representative drill hole from the Faro deposit. The delta 34 S values of sulfide minerals generally range from 10 to 22 per mil and are similar in all three deposits. The delta 34 S values of pyrite in unmineralized samples from the district exhibit a wider range, from 6 to 34 per mil, and show distinct upward stratigraphic increase due to a stagnation cycle in the basin.The delta 34 S values of barite samples are strongly dependent on bottom-water conditions and mode of mixing during brine exhalation. They range from 22 to 26 per mil in the Faro deposit to 36 to 42 per mil in the DY deposit. These variations are due to mixing of isotopically light sulfate (18-20ppm) in Ba-bearing ore fluid and isotopically heavy residual sulfate (30-60ppm) in anoxic seawater.Thirty-eight samples of galena from the DY, Grum, Faro, SB, and Swim deposits have been analyzed for lead isotope ratios. In general, the lead isotope ratio data indicate an upper crustal lead source, with the Proterozoic Grit unit which is inferred to underlie the district being the most likely source rock. A small component of mantle leads from mafic igneous rocks or due to source rock inhomogeneity is also indicated.Graphitic host lithologies, lack of stratiform iron oxides, delta 34 S values of sedimentary pyrite, and sulfide sulfur-organic carbon variations in unmineralized cores indicate formation of the Anvil deposits in strongly reduced bottom water related to a previously unknown Early Cambrian anoxic event.

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.

Summary of the Geologic Mapping Program by the Division of Geological and Geophysical Surveys (DGGS) in McGrath Quadrangle, Alaska: ABSTRACT

From 1977-84, DGGS completed 1:63,360-scale geologic mapping of the McGrath quadrangle in western Alaska. Rock units in the study area range in age from lower Paleozoic to recent. The geologic history of this region has integrated stratigraphy, igneous activity, and mineral resources. From Cambro-Ordovician to Middle Devonian time, the Nixon Fork and Dillinger assemblages were deposited as coeval platform continental margin facies. Upper Devonian through Lower Jurassic shallow to deep-water sedimentary-volcanic facies of the Mystic assemblage overlie both older sequences. All three may have been offset right laterally from the Selwyn basin in northwestern Canada along the Tintina and other faults in central Alaska. Upper Jurassic to Lower Cretaceous flysch of the Black Clastic sequence was structurally juxtaposed against the Dillinger and Mystic assemblages, probably during mid-Cretaceous compression related to terrane accretion in southern Alaska. Upper Cretaceous Kuskokwim Group flysch was deposited over Nixon Fork assemblage rocks in an extensional environment in the westernmost portion of the quadrangle. A series of calc-alkaline intrusive and extrusive rocks ranging in age from Late Cretaceous to middle Tertiary intrude and overlie older, layered lithologies. Significant metalliferous epigenetic, stockwork, and skarn deposits are related to these igneous rocks near Farewell and McGrath. Middle Tertiary coal-bearing arenaceous sediments have been deposited in structural troughs related to the Farewell fault system. Offset solutions along the Farewell fault suggest up to 60 km of right-lateral offset since the Cretaceous. At least four Pleistocene and one late Tertiary(?) glaciations have advanced northward from the Alaska Range. The DGGS efforts have resulted in the release of eight geologic maps and an extensive geochemistry survey of the entire quadrangle. A comprehensive summary of the geology and mineral resources of the entire quadrangle is in process. The poster session will also summarize university graduate studies of mineral deposits, igneous petrology, and stratigraphy. End_of_Article - Last_Page 658------------

Ocean stagnation and ventilation defined by δ34S secular trends in pyrite and barite, Selwyn Basin, Yukon

Within the epicratonic Selwyn Basin, at least three cycles can be recognized for Paleozoic time when the water column alternated from open and ventilated to closed and stratified conditions. These cycles are recorded by δ34S values in pyrite that exceed those for coeval seawater during periods of stagnation and decrease markedly during periods of greater circulation. The marked increase in δ34S values for pyrite formed in stratified seas is controlled by the high percentage of sulfate bacterially reduced to sulfide, coupled with the removal of isotopically lighter sulfur from a closed system during pyrite sedimentation. The δ34S curve for barite during this time has a shape similar to the mean evaporite curve except that it is displaced positively, particularly in Frasnian time. This pronounced increase in δ34S values for barite of Frasnian age coincides with the mixing of isotopically heavier sulfur, accumulated earlier in a stratified water column, with surface waters during a ventilation event.

Stratiform sulfide and barite–fluorite mineralization of the Vulcan prospect, Northwest Territories: exhalation of basinal brines along a faulted continental margin

Laminated and massive sulfide (pyrite, sphalerite, galena) mineralization and massive barite – fluorite–galena lenses occur in Upper Silurian – Lower Devonian Road River Formation shales of the Vulcan property along the eastern flank of the Selwyn Basin, Northwest Territories, Canada. The 5 km thick stratigraphic section, ranging in age from Hadrynian through Mississippian, offers insight into the nature of the Mackenzie Platform – Selwyn Basin transition. Abrupt facies changes, synsedimentary faults, debris flows, local unconformities, and the presence of high-potassium mafic flows indicate extensional tectonics during deposition of the Road River Formation. Mineralization resulted from heated, metal-rich basinal brines that vented on the sea floor up normal faults.Sulfur-isotope studies indicate that both sulfate and sulfide were derived from the exhaling metalliferous brine. Sulfur-isotope data also suggest that reduction of sulfate in the brine occurred as a result of organic decomposition, possibly during thermal maturation of hydrocarbons at temperatures greater than 80 °C. Fluid inclusion observations indicate that the brine salinity reached 26 wt.% NaCl for at least a portion of the evolution of the hydrothermal system.Cooling of the brine during venting into bottom waters caused initial rapid precipitation of fine-grained barite, resulting in a baritic buildup above vent areas. Continued percolation of the brine through the baritic mound caused recrystallization of the barite and then the deposition of interstitial fluorite and galena. In other areas the dense ore fluid collected in topographic depressions, or brine pools, in which sulfide minerals accumulated under anoxic conditions. Location of hydrothermal vents, paleotopography, and intensity of hydrothermal activity were the main controls on the thickness, distribution, and grade of Vulcan mineralization.

Zonation of chalcophile elements about the howard's pass (XY) Zn-Pb deposit, Selwyn Basin, Yukon

ABSTRACT Goodfellow, W.D., Jonasson, I.R. and Morganti, J.M., 1983. Zonation of chalcophile elements about the Howard's Pass (XY) Zn-Pb deposit, Selwyn Basin, Yukon. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 503–542. The Selwyn Basin comprises those rocks of Paleozoic age that were deposited in a fault-controlled epicratonic basin or aulacogen bounded by the Mackenzie Platform on the east and north, and the Cassiar Platform to the west. Boundaries between sedimentary facies within the Selwyn Basin during Ordovician to Middle Silurian time were parallel to and symetrically disposed about the long axis of that basin, consisting of chert in the central and deeper zone and shales in the flanking zones. Conodont dates indicate that the Howard's Pass (XY) Zn-Pb deposit formed during Llandoverian time in a local third-order basin within the shale facies. Various geological features such as abrupt facies changes, local thickening and slumping of sedimentary units suggest that growth faults played an important role in basin development as well as serving as conduits for the discharge of ore-forming fluids. The only sulphides recognized to date at the XY deposit are sphalerite, galena, chalcopyrite and pyrite. Sphalerite and galena can occur as concordant, fine laminations, or remobilized into sedimentary dewatering structures, or transposed and remobilized into a tectonic (slaty) cleavage. Pyrite most commonly forms diagenetic microframboids which postdate the laminated sulphides but predate the sphalerite and galena emplaced in dewatering structures. Within the main ore-bearing unit, i.e. the Active Member, the Pb/(Pb+Zn) ratio decreases stratigraphically upwards and laterally towards the margins of the deposit. This zonation accompanies a change in host-rock composition from cherty limestone at the base to carbonaceous chert towards the top of the Active Member. In addition to Zn and Pb, the other chalcophile elements concentrated in the Active Member are Cd and Hg, both of which are bound isomorphously in sphalerite. Both ore-forming and ore-associated elements are zoned stratigraphically and laterally in the host rocks of the XY deposit. The footwall carbonaceous mudstones are enriched in Ni, Cu, Co, Mo, As, Sb, Zn, Cd, Hg and Pb relative to rocks of similar age and composition located remote from mineralization. Ni, Cu, Co, Mo, As and Sb now reside in diagenetic pyrite but in whole rock chemical analyses show a strong positive correlation with organic C. On the other hand, Zn, Cd, Hg and Pb are bound in sphalerite or galena. By comparison, the overlying phosphatic chert is enriched in Ni and Cu only, both of which correlate positively with organic C. The zonation of these elements reflects a decrease in pyrite content in the rocks and also a decrease in trace-element contents of pyrite itself away from the XY deposit. On the basis of extremely low S/C ratios, the low pyrite content outside the XY deposit is thought to be due to a lack of sufficient Fe2+ to fix S2- generated by bacterial reduction during diagenesis. The higher content of Fe near the XY deposit probably reflects Fe debouched onto the seafloor during hydrothermal activity. The increased trace-element content of pyrite in rocks hosting the XY deposit is due most likely to elevated levels of these elements accumulated by dead organic matter from ore-forming fluids during sedimentation. A period of Devonian hydrothermal activity in the Howard's Pass area can be recognized by elevated levels of Zn, Hg, Sb, Ag, As, Mo and Ni in highly carbonaceous and calcareous rocks. The spatial association of these Devonian metalliferous mudstones with the underlying XY deposit suggests that structures which served as conduits for the discharge of ore-forming fluids during the Lower Silurian were reactivated in Devonian time.

The MacTung W-Cu(Zn) contact metasomatic and related deposits of the northeastern Canadian Cordillera

The MacTung deposit is the largest of the economically important contact metasomatic deposits of the northeastern Canadian Cordillera which are characterized by the ore element assemblage W-Cu(Zn). The deposits occur within the thermal aureoles of Late Cretaceous felsic intrusions which were emplaced into a dominantly pelitic sequence along the eastern margin of the Selwyn basin. The mineralization occurs in skarn bodies which are comprised dominantly of pyroxene, amphibole, garnet, pyrrhotite, biotite, clinozoisite, plagioclase, and quartz, with economically important amounts of scheelite and chalcopyrite and minor sphalerite and ferberite. The skarn bodies tend to occur in relatively fiat-lying limestone or marble beds. Although the skarns are heterogeneous, there are clear patterns in the preferred associations and nonassociations of minerals on all scales, and an orderly sequence of alteration facies of skarn can be distinguished. The distribution, and compositional and mineralogical, relationships of alteration facies suggest that they formed as a result of the progressive reaction of an infiowing hydrothermal solution with a generally uniform limestone protolith, and that the dominant effect of this interaction was the progressive removal of Ca and addition of other constituents to the altered rocks. Scheelite was deposited in the zonal sequence where the product of the activity of the WO (super -2) 4 being added, and of the Ca (super +2) being extracted, exceeded the solubility product of scheelite. Concurrently with the alteration of limestone, interbedded pelites were progressively altered to Ca-rich assemblages and leached of sulfides and graphite in a broad zone surrounding the ore-bearing skarns. This raises the possibility that much of the material added to the limestones was derived from the pelites and, similarly, much of what was added to the pelites was derived from the limestones in the ore-forming process, although there is compelling evidence that the ore-forming fluids at least migrated through, and probably were partly derived from, the associated felsic intrusions.

Average lead isotope growth curves for shale-hosted zinc-lead deposits, Canadian Cordillera

Average lead isotope growth curves for shale-hosted zinc-lead deposits have been constructed empirically for a large but well-defined tectonic segment of the Canadian Cordillera. The model defined is applicable to the dating of galena-lead from shale-hosted deposits in the Selwyn basin and in the Ogilvie, Richardson, Wernecke, and Mackenzie Mountains, in the Eastern fold belt, and in much of the Omineca crystalline belt. This area of the cordillera generally had a coherent geochemical evolution. Lead for deposits was extracted from sediments which had been deposited along the western margin of the North American craton and which were derived largely from upper crustal crystalline rocks that formed the western part of the Canadian Shield.The average shale growth curves involve three stages of evolution. The third (most recent) stage of the three-stage model, starts at 1.89 m.y. and evolves to 0 m.y. with mu of 12.16 and omega of 49.09. The first two stages follow the two-stage model of Stacey and Kramers (1975). The model allows precise model age estimates, with an error generally less than 0.05 m.y., to be made for deposits of known age. This is particularly useful in property evaluation during exploration because in many cases the certainty of a syngenetic or epigenetic origin can control future philosophy about exploration and/or evolution of a deposit or an area. Model ages, in concert with stratigraphic information, commonly indicate the likelihood of epigenetic or syngenetic origins.

A re-examination of the type area of the Devono-Mississippian Cariboo Orogeny, central British Columbia

Three tectonostratigraphic successions are established from remapping of the area near Barkerville and Cariboo River. The first, of Late Proterozoic to Cambrian sediments, was deposited on the shallow to moderately deep platformal shelf west of and derived from the exposed North American craton. The second is an unconformably overlying Ordovician to Permian sequence of sedimentary and volcanic rocks representing a basinal environment with periodic highs. These packages of sediments were deposited on the North American craton and its western transitional extensions. The third succession, composed of oceanic chert and basalt of the Permo-Pennsylvanian Antler Formation, was thrust eastward over the other two during the early Mesozoic. The three successions were folded, faulted, and metamorphosed during the mid-Mesozoic Columbian Orogeny. The Devono-Mississippian Cariboo Orogeny, which was thought to have affected all of the first sequence and part of the second, could not be documented in its type locality. The geology of the Barkerville – Cariboo River area has many similarities with that of Selwyn Basin and Cassiar platform of northern British Columbia and Yukon.

Ordovician Formations and Faunas, Southern Mackenzie Mountains

Ordovician strata of the South Nahanni River area occur as a western fine clastic facies with graptolitic faunas (Road River Formation) and an eastern carbonate facies with shelly faunas (Broken Skull, Sunblood, Esbataottine, and Whittaker Formations). The lithofacies are controlled by major tectonic features. In the early Ordovician the Selwyn Basin received fine clastic sedimentation (Road River) while carbonate sedimentation (Broken Skull, Sunblood, Esbataottine) was confined to the Root Basin and the fringes of the emergent Redstone Arch. Later in the Ordovician a pronounced transgression caused a marked eastwardly shift of facies belts and resulted in the flooding of the Arch and deposition of a widespread carbonate sheet (Whittaker) in the northeastern part of the study area.The carbonate facies of the Root Basin and south of the Redstone Arch contains a complete record of shelly faunas of Canadian to, probably, Maysvillian age. The Broken Skull Formation consists of craggy weathering, variably sandy carbonates and contains, in its highest parts, the Hesperonomia fauna (Canadian). The Sunblood Formation consists of generally resistant, red-orange-buff weathering limestones and minor dolostones and contains two faunas; the Orthidiella-'Goniotelina' fauna (Whiterockian) and the Bathyurus sp. 1 fauna (?Chazyan). The Esbataottine Formation (proposed) comprises moderately recessive, grey-buff weathering limestones and contains two faunas; the Mimella fauna (Chazyan) and the Doleroides fauna (Blackriveran). The lower Whittaker Formation (Trentonian to ?Maysvillian) comprises three lithosomes containing discrete faunas which appear to represent coeval communities: The Strophomena-Ceraurus fauna in thinly bedded, argillaceous limestones; the Bighornia-Thaerodonta fauna in well bedded, nonargillaceous limestones; and the Hesperorthis-Oepikina fauna in well bedded dolostones. A unique fauna of deep water aspect, the Cryptolithus-Anataphrus fauna, occurs in shales and black limestones of the Road River Formation and is, in part, equivalent to the faunas of the lower Whittaker Formation. The Sunblood, Esbataottine, and Whittaker Formations contain abundant and well preserved silicified trilobites; many of which are reported for the first time from western and northern Canada.

Environments of Cordilleran Depositional Basins: ABSTRACT

The stratigraphy of the cordillera can be interpreted in terms of stratigraphic assemblages that are unique in distribution, gross lithology, and lateral facies variations. Models of depositional basins in which these assemblages accumulated are essential in exploration for mineral deposits whose distribution is controlled by stratigraphy. End_Page 1437------------------------------ Proterozoic and lower Paleozoic strata in the cordillera comprise an assemblage of clastic and carbonate with minor volcanic rocks that appears to represent a continental-terrace wedge built along the margin of an earlier Precambrian continent. All units show a distinct polarity of facies distribution and thickness relative to the source area. This assemblage contains most of the known stratiform mineral deposits of gypsum, iron, copper, zinc, and lead in the cordillera. The distinctive elements of a Late Devonian and Early Mississippian assemblage suggest, at least in the northern cordillera, the presence of a foredeep and related source areas in the west and northwest. In the southern and eastern parts of the cordillera, however, the rocks reflect a continuing shelf-platform environment linked to the craton. The mineral potential of these rocks has been considered low but needs further study in view of an important zinc-lead deposit in eastern Selwyn basin. Distinctive rocks of oceanic character ranging in age from Mississippian to Middle Triassic underlie parts of the cordilleran intermontane belt. The important mineral deposits in these rocks include asbestos deposits in ultramafic rocks in the northern cordillera. Generally, however, mineral discoveries, other than those in ultramafic rocks have been few. In the eastern cordillera a shelf environment prevailed. The association of copper with volcanic rocks of Late Triassic and Early Jurassic ages is well known. The volcanic rocks, together with spatially and temporally associated plutons, are thought to outline a system of evolving island arcs probably roughly coincident with the mapped distribution of these rocks. Between the arcs and the craton, strata were deposited in a marginal basin with little or no evidence of the volcanism that occurred farther west. The remaining stratigraphic units, ranging in age from Early Jurassic to the Cenozoic, are described as successor-basin and foredeep assemblages whose distribution and lithology reflect a close relation to bounding uplifts of metamorphic and plutonic terrains. Because they are a late-stage phenomenon in the evolution of the cordillera these assemblages have potential for a variety of placer deposits. They also contain all of the known coal reserves of the region. End_of_Article - Last_Page 1438------------