The Pemberton Hills study site is situated in northern Vancouver Island, British Columbia, and forms part of a northwest trending belt of porphyry deposits and prospects within the Wrangellia Terrane. The study site is underlain by late Triassic to middle Jurassic Bonanza Group volcanic rocks, with porphyry-related alteration and mineralization thought to be related to the emplacement of the Jurassic Island Plutonic Suite (181 – 141 Ma).

Whole-rock geochemistry, high precision geochronology combined with trace element and Hf isotope analysis of zircons from various intrusive units has led to an improved understanding of the magmatic evolution of plutonic rocks at Pemberton Hills. The oldest unit is a granodiorite dated at 172.34 ± 0.11 Ma. Following this is a quartz diorite with an age of 172.01 ± 0.17 Ma, which overlaps with a tonalite dyke sample dated at 171.77 ± 0.13 Ma. A second quartz diorite sample yielded the youngest reported age at 171.44 ± 0.25 Ma. These results imply that felsic and intermediate plutonism were broadly synchronous at Pemberton Hills.

Zircon trace element geochemistry reveals contrasting conditions in the felsic and intermediate plutonic magmas, despite broadly similar emplacement ages. The granodiorite and tonalite dyke samples are characterized by higher, overlapping Eu/Eu* (~0.2-0.3) and lower, overlapping Dy/Yb (~0.15-0.2) ratios, whereas the quartz diorite samples show low and increasing Eu/Eu* ratios (~0.1-0.17) as well as high and decreasing Dy/Yb ratios (0.25-0.45) over time. These results suggest that conditions remained stable throughout the emplacement history of the felsic plutonic magmas, but varied during the emplacement of the intermediate magmas.  Zircon Hf isotope compositions of the granodiorite (+11.4), tonalite dyke (+11.2), and one of the quartz diorite samples (+9.5) suggest these rocks were derived from a juvenile mantle source that experienced minimal crustal contamination. Together, zircon chemistry and Hf isotope compositions suggest that the granodiorite and tonalite dyke magmas were the most fertile, meaning that felsic plutonic rocks have the most potential to be associated with a larger and more mineralized porphyry system at Pemberton Hills.

Field mapping combined with detailed petrography have shown propylitic alteration assemblages to be present throughout most of the porphyry environment at Pemberton Hills.  Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analyses of epidote, chlorite, and pyrite have identified that major and trace element variations in these minerals vary systematically across the study site, and can be used to discriminate multiple porphyry-related alteration events at Pemberton Hills more effectively than whole-rock geochemistry.

Distal pathfinder elements displayed the most variation in epidote. Elements such as As and Sb in epidote were highest in a cluster of samples from the center of study site to the northeast of the lithocap. A small group of samples proximal to the southwestern margin of the quartz diorite pluton also displayed high As in epidote. Chlorite displayed clear systematic spatial distribution patterns, illustrated by increasing Ca and Sr concentrations in chlorite from samples moving towards the east from the southeast margin of the quartz diorite pluton. Fertility assessment plots of Sb vs. As in epidote and Zn vs. Mn in chlorite were used to discriminate spatially distinct groups of samples interpreted to relate to discrete porphyry alteration events at Pemberton Hills.

Pyrite mineral chemistry combined with pyrite element maps illustrate a complex paragenetic history at Pemberton Hills. Pyrite in samples located around the southeastern margin of the quartz diorite pluton are enriched in elements such as Au, As, Sb, Te, and Ag, whereas pyrite from samples overprinted by phyllic alteration assemblages are enriched in Mn, V, and Zn, consistent with abundant silicate micro-inclusions in these pyrites. Pyrite in samples located along the surficial margin of the lithocap are enriched in Re, likely reflecting the overprinting of fluids derived from the lithocap.

Trace element mineral chemistry of epidote, chlorite, and pyrite have proven to be effective at vectoring towards potential porphyry targets in an area with limited geologic context, and as a useful method to discriminate samples from different porphyry alteration events. The results of this study demonstrate the application of mineral chemistry as a tool for exploration in greenfield porphyry environments.

URI:

https://knowledgecommons.lakeheadu.ca/handle/2453/4881

The Guichon Creek Batholith (GCB) is a Late Triassic calc-alkaline intrusive complex approximately 60km x 35km in size and located approximately 54km southwest of Kamloops, British Columbia, Canada. The Guichon Creek Batholith forms part of the Quesnel Terrane, comprised of stacked volcanic-arc assemblages and associated sedimentary units, which in addition to the Stikine and Cache Creek terranes, comprise the Intermontane Belt of central British Columbia. The Guichon Creek Batholith intrudes the western volcanic belt of the Upper Triassic Nicola Group and is partially overlain by volcanic and sedimentary rocks belonging to the Kamloops Group and are of Jurassic age and younger.

The Guichon Creek Batholith is host to the Highland Valley Cu-Mo porphyry system comprised of at least five known significant porphyry centers (Valley, Lornex, Highmont, Bethlehem and J.A.). It is one of two mineralized calc-alkaline batholiths that form a Late Triassic belt parallel to younger mineralized, alkaline and calc-alkaline belts to the east. Most of this belt is buried under a thin veneer of Jurassic and younger cover. Understanding the petrogenesis and composition of the Guichon Creek Batholith is important in developing exploration strategies to locate similar buried porphyry deposits along this belt.

The Guichon Creek Batholith consists of six concentrically zoned intrusive facies ranging from diorite in the core to granodiorite in the margin. Field relationships indicate that the facies young towards the centre of the batholith although observable contacts are rare in outcrop. The six intrusive facies are from the margin inward: 1) Border facies; Highland Valley facies [subdivided into the 2) Guichon sub-facies; and 3) Chataway sub-facies]; 4) Bethlehem facies; 5) Skeena facies; and 6) Bethsaida facies. 

The marginal Border facies is the most heterogeneous facies and contains numerous autoliths near the contact between Guichon Creek Batholith and the host Nicola Group basalts which are brecciated and intruded by the Border facies. Rocks belonging to the Border facies are olivine-bearing leuco-gabbros to diorites with equigranular, phaneritic textures. 

The Highland Valley facies is comprised of the Guichon and Chataway sub-facies. The composition of both sub-facies is similar, varying from quartz monzodiorite to granodiorite compositions and the Guichon sub-facies is most prominent to the northeast of the batholith whereas the Chataway sub-facies is most prominent to the southeast. The key difference between both sub-facies is the presence of conspicuous pink K-feldspar in the Guichon sub-facies and white K-feldspar in the Chataway sub-facies. Optically continuous, interstitial, sub-ophitic amphibole, K-feldspar and quartz are characteristic of all rocks belonging to the Highland Valley facies. 

The three youngest facies are all similar in composition (granodiorite) and show a progressive increase in quartz and K-feldspar, decrease in total mafic minerals (with an increase in biotite relative to hornblende) and nearly constant plagioclase contents progressing from the Bethlehem facies to the Bethsaida facies. The Bethlehem facies is characterized by ophitic amphibole phenocrysts that poikilitically enclose smaller plagioclase chadacrysts, whereas the Bethsaida facies is characterized by biotite and amoeboid quartz phenocrysts. The Skeena facies is texturally and compositionally intermediate to the Bethlehem and Bethsaida facies, lacking both amphibole and biotite phenocrysts but containing finer-grained amoeboid quartz phenocrysts than those present in the Bethsaida facies. 

Weak chlorite-epidote-sericite alteration is ubiquitous across the batholith, and even least altered samples typically contain biotite and amphibole crystals that have been affected by <5 to 60% alteration to chlorite and epidote and plagioclase that has been weakly to moderately sericitized. Alteration is most prominent in samples of the Bethsaida facies, particularly those closest to the mineralized porphyry centers. 

The Guichon Creek Batholith is a magnesian, calcic to calc-alkalic (MALI = -5 to 7.3) and metaluminous to weakly peraluminous (ASI = 0.77 to 1.28; AI = 0.02 to 0.13) batholith. Major and trace element geochemistry are consistent with fractional crystallization of the predominant minerals observed petrographically and plots of Zr vs. molar Al/Ti and Al2O3 vs. TiO2 are effective at discriminating between the Border, Highland Valley and Bethlehem-Skeena-Bethsaida facies. 

The petrographic and geochemical characteristics of the Guichon Creek Batholith in addition to cross-cutting relationships between facies and recent U-Pb ages suggest that the batholith was emplaced as at least two, but possibly three different magma pulses. 

High Sr/Y, low La/Yb, fractionated LREE and HREE relative to MREE and concave primitive-mantle-normalized multi-element diagrams indicate fractional crystallization of hornblende and clinopyroxene in a deep crustal magma reservoir. These geochemical signatures also preclude a significant role for garnet in magma genesis, either as a restite phase left behind by adakite melts of eclogite-facies subducted slabs or by assimilation and contamination by garnetiferous metamorphic rocks in the deep crust. This is consistent with Sm-Nd systematics (∈Nd(T) = +6.7 to +7.5) which are consistent with <2% contamination of primitive mantle melts by partial melts of subducted sediment, although 87Sr/86Sri values of 0.703367 to 0.703493 suggest minor contamination of the Border and Guichon facies by radiogenic Sr derived from Nicola Group limestones or contaminated Nicola Group basalts. 

Plots of Sr/Y, Al2O3 and V/Sc vs. SiO2 suggest that the parent magmas for the Guichon Creek Batholith were hydrous and oxidized, two criteria key for the production of porphyry deposits. Amphibole chemistry indicates temperature, pressure and ƒO2 conditions of crystallization were 712 ± 23.5 to 846 ± 23.5°C, 2.5 ± 0.3 to 0.9 ± 0.1 kbar (equivalent to 7.4 to 2.6km depth) and ΔNNO = 0.03 to 1.81, respectively. Pressure estimates are consistent with gentle tilting to the northeast, and imply that the Valley deposit may have formed at pressures where a single-phase supercritical fluid would have been stable, possibly leading to mineralization styles and an alteration footprint that are atypical of porphyry environments.

The Rainy River gold mine is located in northwestern Ontario and has been operating since 2015. The deposit has three major high-strain zones striking NW to SE across the main ore zones. Thin sections taken from sampling transects across each zone provide evidence that these high strain zones are ductile shear zones. Microstructural evidence including strong preferred mineral orientation, folded quartz veins and grainsize reduction indicate dislocation creep as the ductile deformation mechanism in these zones. These shear zones also seem to influence the shape of each mapped ore zone, seeming to bound each ore zone and separating them from each other. It is likely that these major structures are responsible for the multiple ore zones and the shape of each ore body being mined today.

New roadside outcrops along the Trans-Canada Highway 11/17 near Pass Lake, ON, expose the basal unconformity between Archean basement rock and the Proterozoic Gunflint Formation. Shear fractures, joints, and the Blende Lake fault damage zone seen in these two outcrops record the brittle deformation history of the area before and after the Gunflint Formation was deposited.  The unconformity represents a temporal gap in the stratigraphic sequence of at least 800 million years.  The Archean rock belongs to the Wawa subprovince of the Superior province, in close proximity to the Wawa-Quetico subprovince boundary.  Structural measurements, stereographic projections, and qualitative observations have allowed deeper insight and analysis of how structural controls have changed over an approximately 2.7 billion year-old geological history.  The Archean basement unit underneath the unconformity is a coarse-grained amphibolite that contains accessory epidote and biotite, and is homogeneous throughout the length of both high-standing outcrops.  This is interpreted to be a mafic pluton that has undergone amphibolite-facies metamorphism.  The amphibolite records a scatter in orientation of joints and shear fractures, but some trends align well with data from Mackenzie River granite plutons, including an overall east-northeast and west-southwest strike.  Later Proterozoic features, including the Blende Lake fault, have a common strike of east-northeast, which aligns with the orientation of the 1.1 Ga Mid-Continent Rift in Thunder Bay.  This similarity is further reflected by the Blende Lake fault being oriented subparallel to silver veins related to the Mid-Continent Rift.  Similarities between orientations of brittle structures in the amphibolite and Gunflint Formation suggest that the Mid-Continent Rift in Thunder Bay may have reactivated some Archean-aged orogenic-related faults and shear fractures.  Minor folding in the Gunflint Formation truncated by the Blende Lake fault, as well as reverse reactivation along the plane, may be evidence of compression during the later stage of the Mid-Continent Rift.

Differences between deeper water and shallow water iron-formation (IF) deposited during the Archean are poorly understood. Sedimentology, bulk rock geochemistry combined with laser ablation geochemistry, X-ray fluorescence false-colour mapping and 34S isotopes were utilized to compare a variety of IFs deposited at different water depths. These are the deeper water Morley sulfides, Deloro, Temagami, and Soudan IFs; the intermediate depth IF at Red Lake; and the shallow water Lake St. Joseph and Beardmore IFs. The Morley sulfides are massive to laminated sulfides associated with hydrothermal venting deposited at depth. The Deloro IF contains both oxide-facies IF (OIF) and sulfide-facies IF (SIF) and was deposited within a deeper water volcanic assemblage. The Temagami IF is entirely OIF sitting atop volcanics and banded chert with overlying siliciclastic rock. The Soudan IF is located within the Ely greenstone containing both volcanics and siliciclastics. The intermediate depth IF at Red Lake was deposited on the basinward slope of the oldest known carbonate platform on Earth. The shallow water IFs at Lake St. Joseph and Beardmore were deposited within deltaic successions, associated with the distributary channel mouth bars and proximal prodeltas.

The Morley massive and laminated sulfides have the same REE(PAAS) patterns as overlying mudstones, indicating that REE signatures within these sulfides are derived from what was scavenged on clay minerals. Small negative Y anomalies are present in the sulfides and mudstones, with the resulting Y/Ho ratios almost exclusively subchondritic (<28). 34S isotopes of the Morley and nearby sulfide deposits show heavier 34S values in laminated and mudstone associated sulfides compared with the lighter, hydrothermal-magmatic range of 34S within the massive sulfides, indicative of inorganic sulfate reduction deep in the Archean ocean.

The deeper water IF of the Deloro, Temagami, and Soudan are similar geochemically but vary greatly in jasper content and magnetite layer thickness. REE(PAAS) patterns are HREE enriched, containing large Eu and Y anomalies in each IF, with Y/Ho ratios from 28 to 55.  Enrichments in V and low Th/U ratios are present in each deeper water IF, indicating scavenging of V and U from at least somewhat oxidizing sub-aerial weathering conditions and adsorption of these elements onto the iron-hydroxides in the deep ocean. P/Fe ratios range from 0.001 to 0.006, overlapping with shallow water IF but greater than the Morley sulfides by up to a factor of 30. Magnetite within these deeper water IFs appears to contain trace amounts of Ti (<0.1% TiO2) within its structure, which was most likely supplied by detrital material.  δ34S values within the Deloro SIF are very light, suggesting biologic reduction of sulfate rather than a reduced hydrothermal source of the sulfur. However, SIF at Temagami shows 34S values similar to the Morley massive and laminated sulfides, indicating a dominantly hydrothermal source of S.

The IF present at Red Lake was deposited at intermediate depths of a few hundred meters yet resembles deeper water IF with HREE enriched REE(PAAS) patterns and similar ranges for Y anomalies. However, Eu anomalies are smaller by up to a factor of 2 and there are some samples with larger Y/Ho ratios (up to 75) than the highest deeper water values. V and Cr appear enriched within the Red Lake IF and Th/U ratios are lower than in both deep and shallow IFs. P/Fe ratios are generally much lower than the deep and shallow water IF, owing to P sequestration from the water into organisms on the carbonate platform.

The Beardmore and Lake St. Joseph IFs show flat REE(PAAS) patterns with very small Eu and few Y anomalies, due to significant amounts of siliciclastic contamination. Y/Ho ratios are near chondritic due to this as well. Laser ablation REE(PAAS) show Beardmore to have a HREE enriched signatures with small Eu anomalies. P/Fe ratios are comparable to deeper water.

The major difference between deep and shallow IF is chert content, which is much greater in deeper water IF. REE(PAAS) patterns are similarly HREE enriched at all depths but this is masked by siliciclastic contamination in the shallow water IF. Eu anomalies decrease from deep to shallow water, while Y/Ho ratios appear highest in intermediate depths. Redox elements such as V, Cr, and U appear enriched in deep and intermediate IFs, rather than in shallow IFs.

Acid mine drainage (AMD) is one of the greatest challenges facing the mining industry globally. Methods for addressing this issue have been widely studied; however, few studies have addressed sites with a less common water quality problem resulting from AMD: neutral pH, metal-poor, and sulphate-rich water. The Steep Rock Iron Mine site in Atikokan, Ontario is utilized as a case study where AMD-affected waters have acidity neutralized by carbonate rocks, and metals precipitate out of solution as the pH rises. This process alleviates major environmental hazards associated with acidic waters and toxic metal concentrations; however, sulphate is not removed and presents toxic conditions for aquatic fauna. These sites are also a risk to human health, and can potentially contaminate drinking water supplies. Funding for the remediation of abandoned mine sites is limited, and innovative solutions utilizing passive treatment mechanisms are needed in order to deliver efficient and effective remediation. The goals of this project were: 1) to assess the capability of a permeable reactive barrier (PRB) system to remediate sulphate-rich, pH neutral, metal-poor water, 2) to assess nutrient balance within the system to ensure the availability of nutrients is not a rate-limiting factor for sulphate reduction, and 3) to improve reactive substrate selection procedures by determining which assessment tools are most useful in selecting substrates effective at stimulating sulphate reducing bacteria (SRB).

Candidate reactive substrates including cow, horse, poultry, rabbit, and sheep manures, as well as leaf compost and hay, were assessed according to their concentrations of vital nutrients for SRB, including carbon, nitrogen, and phosphorous. Additionally, their relative degradability was tested via a procedure known as easily available substances (EAS). This testing determined how readily a given substrate could be broken down by bacteria, as well as the change in concentration of desired nutrients in the substrate before and after EAS testing, which gives an indication as to the availability of those specific nutrients. Plant and manure substrates were tested, with one of each type used in each reactive mixture. Based on this testing, poultry and sheep manures were selected as the most likely manure substrates to provide effective nutrition for SRB. In contrast, there was no significant difference found between hay and leaf compost. Poultry consistently performed the best in each test, with a C:N ratio of 11, a C:N:P ratio of 1772:160:1, and an EAS mass loss of 71%.

Eight flow-through reactors were constructed and operated for a period of 23 weeks. Six of these reactors contained organic materials to stimulate SRB, while two were controls. Of the six reactors using organic materials, three mixtures were used, each containing a different combination of the four substrates. One control reactor assessed the impact of zero-valent iron which was also added to all of the organic reactors, while an additional control simulated the natural environment and contained only creek sediment and silica sand. Reactors 3 and 7 were the most effective at sustaining high rates of sulphate removal, with >80% sulphate removal maintained for the first 14 weeks. These reactors utilized a mixture of poultry manure and hay, validating the measures which indicated poultry manure as the most effective manure-based substrate. However, poultry manure was also used in reactors 1 and 5 in combination with leaf compost, and were not as effective for sulphate removal. These results indicate that hay was a more effective substrate than leaf compost. Comparing this finding against the original substrate testing presents two differences between hay and leaf compost; the C:N:P ratio and the availability of phosphorus in EAS testing both had stronger results for hay. This result indicates that phosphorus is a critical nutrient for SRB, and that tests considering phosphorus should be an integral part of reactive substrate selection procedures in systems attempting to stimulate SRB. The control reactors found that the addition of only zero valent iron did not have a significant impact on sulphate removal, as performance in this reactor was similar to the natural aquifer conditions control reactor. Eh/pH conditions supported the activity of SRB; but did not support the stability of sulphide produced by SRB, and it is unclear of SRB were in fact active within the flow-through reactors.

Significantly reduced sulphate concentrations in reactor effluent initially appeared to indicate the sulphate reduction was occurred as intended. However, in post-experiment analysis there was no evidence of iron sulphide formation that would confirm sulphide production by SRB, and Eh/pH conditions were not supportive of sulphide stability. Furthermore, saturation index calculations using PHREEQC determined that iron sulphides were highly under saturated in effluent waters. In contrast, sulphate minerals including barite, gypsum, and jarosite were slightly oversaturated, and present a viable sink for the sulphate removed from solution. Following experiment completion it was found that the reactors with the greatest sulphate removal also had the most significant declines in nutrient concentration, with 52-64% C, 45-58% N, and 24-62% P losses in reactors 3 and 7. This is strong evidence of a bacterially driven process for sulphate mineral precipitation. The reduced availability of these nutrients may have played a role in the decline of sulphate removal over time.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4277

The White Pine Fork Mo porphyry has an estimated resource of 16 Mt of Mo at 0.1% Mo.  The host intrusions are K-feldspar- and quartz-porphyritic monzo- to syeno-granites characterised by LREE enrichment, fractionated HREEs and negative Nb, Ta, Sr and Ti anomalies, consistent with a subarc mantle melt source.  The granites also have an adakitic geochemical signature.  A U-Pb age for zircon from a clast in the mineralized breccia pipe at White Pine yielded an age of 26.52±0.42 Ma, which falls within the error of the White Pine intrusion age of 26.61±0.24 Ma.  Re-Os ages for duplicate samples of the Mo-mineralised quartz breccia are 30.21±0.14 and 29.84±0.15 Ma, which correlate with the age of the Little Cottonwood stock rather than the host intrusion and may represent Re-Os inheritance.  In contrast, Buckingham Mo (-Cu) porphyry has an estimated resource of 1,000 Mt of Mo at 0.1% MO and is hosted in Cretaceous K-feldspar- and quartz-porphyritic granites.  Four feldspar- and quartz-porphyritic granites in the area were dated using LA-ICP-MS U-Pb of zircon and yielded ages of 38.68±0.53, 39.28±0.58, 40.76±0.41, and 40.81±0.51 Ma and therefore unrelated to the Buckingham deposit, and instead are correlated with Tertiary magmatism associated with Au skarns in the nearby Battle Mountain district.  The Tertiary intrusions are feldspar- and quartz porphyritic granites.  Primitive mantle-normalized geochemistry of both suites of intrusive rocks have LREE enrichment, fractionated HREE, negative Nb, Ta and Ti anomalies and a slight enrichment of Zr and Hf, consistent with a subarc mantle source for both suites.

The most prominent alteration in both systems is phyllic alteration comprising an assemblage of white micas, quartz and pyrite.  Potassic alteration was also observed at the White Pine Fork Mo breccia pipe and kaolinite and chlorite observed in SWIR data suggest advanced argillic alteration around the Buckingham system.  These petrographic observations are substantiated by the whole rock geochemistry.  The potassic, phyllic, and possible advanced argillic alteration were mapped out by the absolute values of trace elements.  The trace element geochemistry of quartz and pyrite can be used to fingerprint deposit types and as vectors toward mineralisation in alteration systems around ore deposits.  At White Pine Fork, the hydrothermal quartz is characterised by higher Ti and As than the igneous quartz.  The Li content of hydrothermal quartz is greater near the centre of the White Pine Fork deposit than in its margins.  At Buckingham, quartz in the breccia cement at the centre of the deposit shows the highest concentration of Al, Li, K, Ca, As, and Sb, and metals (i.e. Cu, Fe, Zn, and Pb), whereas the igneous and sedimentary quartz shows the highest Ti values.  The high values indicate that the primary Ti contents were not subjected to recalibration during hydrothermal alteration.  The Al and Sb contents of quartz decrease away from the centre of deposit at Buckingham.  This trend was not observed at White Pine Fork.

At White Pine Fork, pyrite occurs in a domain extending more than 1.5 km from the breccia pipe and is partially weathered to Fe-oxides and Fe-hydroxides.  LA-ICP-MS analyses showed Ni and Co compositional zoning within the pyrite grains.  The Ni content in the pyrite increases away from the centre of the White Pine Fork deposit.  At Buckingham, the pyrite was almost completely altered to Fe-oxides and Fe-hydroxides in most of the surface samples studied here.  The trace element concentrations of Au, Cu, and Cd in pyrite decreases away from the centre of the deposit.  Ni, Co and Pb zonations were also seen in the cores of the pyrite grains at Buckingham.  LA-ICP-MS element maps in most of the surface samples studied here showed that the pyrite at Buckingham is depleted in trace elements relative to the weathering rinds which are enriched in Au, Ag, Cu, As, Sb and Mo.  The increased Au concentration in the weathering rind suggests either an overprinting mineralised rind similar to the proximal skarns or Carlin-style sediment hosted Au.  The rind was then weathered during supergene alteration.  Another possibility is that the cores of the sulphides acted as traps for the precipitation of precious metals from the hydrothermal event associated with the proximal Eocene granite intrusions which were subsequently concentrated in the weathering rind during supergene alteration.

The comparison the White Pine Fork and Buckingham Mo porphyries has refined the processes associated with hydrothermal alteration around Mo porphyry deposits and the applicability of trace element chemistry of alteration minerals as exploration tools.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4331

The Gunflint and Biwabik Formations comprise the middle units of the Paleoproterozoic Animikie Group that crop out near the northwestern shore of Lake Superior.  Stromatolitic units were investigated and sampled at 9 outcrop sites, 4 mine sites and in 12 drill-cores.  Controversy exists as to whether the stromatolitic units that are now composed of chert were deposited as a silica gel or represent carbonate replaced by chert.  There is also debate in the literature as to whether the iron oxides that coat many microfossils are primary or deposited during later diagenesis.  Younger examples of siliceous stromatolites are very rare and iron hydroxides coating bacteria are only known for limited modern environments.  Thus, understanding the primary composition of the Gunflint microbialites will provide information on ocean chemistry during this interval.

Three stromatolitic members have previously been described within these formations: the Lower Stromatolitic Member containing stromatolites that grow directly on the peneplained Archean basement or on the conglomerate which forms the base of the Animikie Group, a Middle Member 45 to 50 meters above the base, and the Stromatolitic Limestone Member at the top of the formation directly beneath the Sudbury impact layer.  Based on this work, a new Upper Stromatolitic Member 50 meters below the Limestone Member in the Gunflint Formation was recognized.  This member contains abundant iron-silica-carbonate-manganese oncolites, and is correlative to stromatolitic units described from the Biwabik.  The oncolites represent a primary-to-very early authigenic, precipitated mineral assemblage.  The accretion of lacustrine ferromanganese nodules at the sediment-water redox boundary provides a modern analogue for the development of the oncolites described in this work.

The presence of exceptionally well-preserved, hematite coated microfossils, encased in chert in the relatively unmetamorphosed Middle Stromatolitic Member together with positive cerium and europium anomalies indicates that these sediments were deposited at a redox boundary in ocean waters near saturation with respect to silica and iron.  Conversely, precipitates, containing carbonaceous filaments in the Lower Stromatolitic Member contain a distinct negative cerium anomaly, indicating deposition in an oxidized environment.

Alternating manganese- and iron-rich laminae within the Upper Stromatolitic Member indicate a fluctuating redoxcline and oxygen levels in the shallow subtidal to peritidal environments within the Animikie Basin.  The fluctuation reduces the Fe:Mn ratio, allowing for the less readily oxidized manganese to precipitate.  The presence of dehydration cracks which cross-cut original ooid and stromatolitic laminae indicate that deposition of the silica and iron most probably occurred as amorphous gels of opal-A and iron hydroxides.

This study strongly indicates the Gunflint and Biwabik stromatolites were originally siliceous and formed by a different precipitation mechanism than that of Proterozoic carbonate stromatolites or modern agglutinated forms.

URI
https://knowledgecommons.lakeheadu.ca/handle/2453/4126

The 1.878 Ga Gunflint and equivalent iron formations of the Animikie Basin were deposited during the period after the rise in atmospheric oxygen during the Great Oxidation Event, which began at approximately 2.4 Ga. The atmospheric composition, and especially oxygen content, during this period after the rise in atmospheric oxygen, is not well known. The Gunflint and correlative formations, having been deposited in a shallow-marine setting, might provide information on oxygen levels present in the atmosphere-ocean system at this time. This is further made possible as the rocks that make up these formations are excellent records of transgression and regression of the sea. This record is reflected in up-section changes in lithology of the rocks that make up these formations. Further, the strata show evidence of exposure indicative of maximum regression. This exposure would have allowed the sediments and rocks to interact chemically with the atmosphere and meteoric water at the time. 

Stromatolites developed in the near-shore deposits and the rocks upon which the stromatolites grew showevidence of exposure during the repeated relative sea level low stands. These rocks have an alteration pattern that depends on their lithology. The rocks that comprise the Archean crystalline basement below the Gunflint show an intense alteration pattern that includes the formation of large corestones as well as increases in Fe and Mn content. Their geochemistry is largely the result of alteration by basin derived fluids. The exposure surfaces in the grainstones that make up the Gunflint and equivalent formations exhibit cementation and brecciation of the grainstones directly beneath the stromatolites.

Geochemical analysis performed on these rocks targeted the concentration of redox sensitive oxides and elements; Fe₂O₃, MnO, V, Cr, Mo and U.  Also considered were the Ce and Eu anomalies present. These oxides and elements react differently in oxidizing vs. reducing fluids. Subsequently, this analysis showed a shift in the Ce anomaly throughout the stratigraphy. The change from negative Ce anomalies in the basal samples to the positive Ce anomalies in the middle and upper lowstand layers indicate a shifting redox boundary, where the lowest stromatolite layers had produced enough oxygen to balance the flux of Fe²⁺- rich seawater in a position away from them, whereas the middle and upper stromatolite areas had redox boundary at the surface of the stromatolite themselves. Enrichment in Cr and V, as well as depleted Eu anomalies in strata associated with lowstands, points to the mixing of oxic freshwater,  probably a combination of surface runoff from the craton and diffusion of groundwaters, and the anoxic seawaters filling the basin.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/758