Central Mineral Belt Research Articles

Geochemical characterization of the Central Mineral Belt U ± Cu ± Mo ± V mineralization, Labrador, Canada: Application of unsupervised machine-learning for evaluation of IOCG and affiliated mineral potential

The Central Mineral Belt (CMB) in Labrador, Canada, hosts multiple U (±base ± precious metal) showings, prospects and deposits in metamorphosed and variably hydrothermally altered Neoarchean to Mesoproterozoic, igneous and sedimentary rocks. Previous work has recognized U mineralization locally associated with Fe-Ca and alkali metasomatism typical of metasomatic iron oxide and alkali-calcic alteration systems (IOAA) that host iron oxide-copper-gold (IOCG) and affiliated critical metal deposits. However, the type, extent and temporal or genetic relationships between the diverse Fe, Ca and alkali metasomatism and the regionally distributed U mineralization remains poorly understood.Combined unsupervised machine-learning and classification of alteration from a large geochemical dataset distinguish the main alteration phases in the CMB, identify compositional changes related to U mineralization, and infer lithological/mineralogical information from samples with censored (i.e., missing), limited and/or inaccurate metadata. Weak to intense Na and Na + Ca-Fe (Mg) metasomatism in the southwest (Two-Time and Moran Lake areas) and eastern (Michelin area) portions of the CMB pre-dates U mineralization and Fe-oxide breccia development, similar to albitite-hosted U and IOCG deposits globally.Rare earth elements and spider diagrams highlight both preservation and disruption of normally immobile elements. Principal component and cluster analysis indicate significant variations in Fe-Mg ± Na contents in the rocks from combinations of Na, Ca, Fe, and Mg-rich alteration, while protolith REE signatures can be locally preserved even after pervasive albitization-hematization. Cluster analysis identifies mineralized felsic and mafic rocks in the Michelin deposit and Moran Lake area, facilitating inference of relevant lithological/mineralogical information from samples lacking or with limited meta-data.The methods outlined provide rapid and relatively inexpensive means to optimize identification of mineral systems within large geochemical datasets, verify drill core or field observations, highlight potentially overlooked alteration, and refine economic mineral potential assessments. Based on our results and previous work, we suggest the mineral potential of the southwestern and eastern CMB needs to be re-assessed with modern exploration models for IOAA ore systems and their iron oxide-poor variants.

Uraninite chemistry of the Central Mineral Belt, Labrador, Canada: Application of grain-scale unsupervised machine-learning

The Central Mineral Belt (CMB) in Labrador hosts several enigmatic U ± base ± precious metal showings, prospects, and deposits. Multiple mineralization styles occur within different host rocks, which has led to a variety of ore system models to be proposed. Here, unsupervised machine-learning (principal component (PCA) and targeted cluster analyses) applied to quantitative LA-ICP-MS trace element maps of uraninite were used to understand genesis the U systems in the CMB.Trace element mapping, PCA and cluster analysis indicate that: i) the largest source of data variance in uraninite from the occurrences is related to Th, REE, Zr, Hf, As, V and Ba contents, and ii) there are distinct uraninite compositions at the Two-Time deposit, Near Miss and Anomaly No. 7 mineral occurrences, not recognized from petrographic studies or major element chemistry.Trace element chemistry indicates that uraninite precipitated from fluids under (1) high-temperature magmatic and/or metasomatic conditions (> 350 °C; Dandy prospect; U/Th = 107 and ΣREE = 0.9 wt%), (2) low-temperature (< 350 °C) and locally oxidizing hydrothermal vein-type environments (e.g., Two-Time and Anomaly No. 7; U/Th ≥104 and ΣREE ≤0.1 to 3.6 wt%), and (3) complex environments were precursor uraninite was overprinted by presumably lower temperature hydrothermal fluids (e.g., Jacques Lake and Nash deposits; U/Th = 102 to 105 and ΣREE ≤0.1 to 1.4 wt%). Hydrothermal alteration caused LREE enrichment and/or increased U/Th ratios of the primary uraninite and locally, U remobilization into microfractures. Recognizing high- and low-temperature uraninite from the same mineral occurrences provides new evidence for multiple stages of hydrothermal fluid influx. Therefore, our results further support previous studies indicating significant variation in the trace element contents of uraninite due to hydrothermal alteration.In addition to the genetic constraints, PCA results and normalized REE patterns also show that the CMB uraninites have distinct geochemical signatures. Combining petrographic studies, trace element mapping, and unsupervised machine-learning unravelled cryptic chemical variations in key minerals that can guide future mineral exploration in the district. In the CMB, these chemical signatures provide insights on the evolution of U-rich fluids, in particular the presence of multiple fluid sources that evolved through a complex tectono-magmatic history.

Shear-Hosted Uranium Deposits: A Review

A group of uranium deposits is described that is hosted within polyphase shear zones. The group is economically significant, collectively containing over 500,000 tonnes of uranium and several examples have been or are being mined. Over a hundred individual deposits are known widely spread over many countries. It is proposed that this group be assigned to a new shear-hosted uranium deposit category. Uranium deposition was superimposed upon intense and extensive feldspathic alteration formed during ductile deformation. This intense alteration has led to the alternative albitite-type or metasomatite-type nomenclature. The evidence is clear that in most cases uranium mineralization postdates regionally extensive feldspar alteration and is associated with a range of alteration assemblages which overprint early albite or K-feldspar dominant alteration. Abundance of hydrothermal zirconium and phosphate minerals is a common characteristic of this group which implies high activity of F and P during mineralisation, but the source of hydrothermal fluids remains uncertain. Also uncertain is the geodynamic setting of uranium mineralisation which is a consequence of absolute mineralisation age being poorly defined. Data from three of the four major districts are suggestive that mineralisation was a consequence of fluid migration along shears during regional compression. This paper reviews key aspects of the group in a mineral systems context, focussing on the four major districts of Kropyvnytskyi (Ukraine), Lagoa Real (Brazil), Mount Isa (Australia) and the Central Mineral Belt (Canada).

The evolution of metasomatic uranium ore systems in the Kitts-Post Hill belt of the Central Mineral Belt, Labrador, Canada

Uranium mineralization in the Kitts-Post Hill belt of the Central Mineral Belt (CMB) of Labrador, Canada is concentrated in the upper portion of the Paleoproterozoic Post Hill Group, along a structurally complex shear zone that developed during ca. 1860–1800 Ma orogenic events. Recent research in the CMB highlights regional-to deposit-scale features such as Na-, Ca-Fe-, K- and K-Fe- alteration and hydrothermal breccia zones characteristic of iron oxide and alkali-calcic alteration systems that host iron oxide-copper–gold (IOCG) and affiliated deposits. Potential genetic links between the CMB uranium district and metasomatic systems hosting IOCG deposits was explored through petrography and trace element chemistry of uraninite. The least-altered uraninite grains from mineral occurrences in the Kitts-Post Hill belt contain elevated Th, Zr, Y and rare earth element (REE) contents, implying primary uranium mineralization from high temperature, alkaline fluids capable of transporting high field-strength elements – similar to deposits in iron oxide and alkali-calcic alteration systems. Quantitative LA-ICP-MS mapping of uraninite at the Kitts deposit demonstrates remobilization of U, Pb and light-REE along grain boundaries and fractures, coupled with zones of elevated Zr, and lower Th and REE contents that record element leaching during alteration and/or recrystallization. In chondrite-normalized REE plots of uraninite, contrasting REE patterns define three populations of uraninite grains that formed under different temperature and physico-chemical conditions. Leaching of light-REE from uraninite grains support recent studies that indicate light-REE signatures of uraninite can be disturbed under certain conditions, whereas the primary signatures of heavy-REE remain preserved. The inferred contrasting fluid sources revealed by the major and trace element composition of uraninite suggest that metasomatism and mineralization involved a complex and evolving fluid history, complicated by post-ore magmatism and metamorphism.

Radon (Rn) Induced Health Risk Assessment: Need To Re-Examine Current Policy And To Act Beyond Disciplinary Boundaries

Introduction Radon (Rn) is formed by uranium (U) decay &amp; its exposure is the 2nd leading cause of lung cancer after smoking in North America. Natural U contents in rocks &amp;soils are quite variable in nature. In 2009-11, Health Canada survey showed that 7% homes had indoor Rn levels above the guideline value (200Bq/m3). However, the survey had no scope to link elevated concentrations of U in local bedrock to indoor air Rn levels. We wanted to assess Rn induced health risks in such micro-geological settings. We also intended to take a fresh look at the current recommendations concerning keeping Rn detectors at home. Methods Makkovik, a remote Inuit (Eskimo) community in Labrador was selected for study. It lies within the Central Mineral Belt which contains potentially economically exploitable concentrations of U. We selected 25 homes (20% of the total) by convenience sampling. We kept electret ion chamber Rn monitoring devices in the lowest levels of the home (basement/crawlspaces) for 5 months (Oct. 2013 – Mar. 2014). Health Canada guidelines, however, suggest keeping detectors in the normal occupancy (&gt;4 hrs/day) areas at the lowest lived-in level of the home. We obtained lung cancer data (1993-2013) of all local Inuit communities. Results 28% homes (7) had Rn levels above the guideline value (249-574Bq/m3), higher than average local health region (i.e. 3%). 2 homes previously tested normal levels by Health Canada (the devices were kept in bedroom) also included in our study &amp; both had higher Rn level (335 &amp; 501Bq/m3). Makkovik had 70% higher lung cancer rate than rest of the Inuit communities. Discussion Geological profile of any community is very important for proper assessment for indoor Rn-related health risks. Testing at basement/crawlspace provides more accurate assessment of health risk. Any report showing low Rn level in normal occupancy rooms might give a false sense of security if Rn remains trapped for time being in basement/crawlspace area due to intact floor.

Use of surficial geochemical methods to locate areas of buried uranium mineralization in the Jacque’s Lake area of the Central Mineral Belt, Labrador, Canada

Surficial geochemical methods were applied to delineate zones of anomalous uranium and related element concentrations in areas overlain by an extensive blanket of glacial sediments and dense vegetation cover in the Jacque’s Lake area of the Central Mineral Belt, Labrador, Canada. The study involved sampling and analyses of vegetation including black spruce twigs and bark, Labrador tea shoots, and humus. Ash derived from the vegetation samples was analyzed using inductively coupled plasma – mass spectrometry (ICP–MS) following ignition at 450 °C, and humus was analyzed with delayed neutron counting (DNC) for a suite of 35 elements. B-horizon soil was analyzed using aqua regia digestion to investigate potential chemical signatures of bedrock mineralization at the surface. Uranium concentration in humus varied from 0.05 to 885 ppm. Zones exhibiting anomalous U responses were associated with areas proximal to anoxic peat and sphagnum bogs where mobile U species were sequestered. Uranium and pathfinder element (e.g., Pb, V, Sr, and Mo) concentrations were low in the &lt;250 μm fraction of B-horizon soils. The soil geochemistry delineated bedrock U mineralization in areas with &lt;15 cm of overburden and U concentration varied from 50 to 405 ppm. Biogeochemical signatures of the bedrock mineralization in black spruce twigs produced greater anomaly to background contrasts for U and pathfinder elements (e.g., Be, Ag, Pb, Ca, and Sb) and correlated more precisely with the detected radiometric U/Th anomaly than did those of soil and humus. Principal component analysis of spruce twig data discriminated three major components, including plant nutrients, ore-related elements, and a mobile species. Uranium concentration varied from below detection limit to 23 ppm U in black spruce bark and from below detection limit in ∼40% of samples to 18 ppm in Labrador tea stem.

U–Pb age and Hf-isotope geochemistry of zircon from felsic volcanic rocks of the Paleoproterozoic Aillik Group, Makkovik Province, Labrador

U–Pb zircon SHRIMP geochronology and Lu–Hf zircon LA–MC–ICPMS isotope geochemistry are used to constrain the ages of Paleoproterozoic felsic magmatism and their crustal sources in the Aillik Group, Makkovik Province of Labrador. This information is important for understanding the extent of crustal reworking vs. juvenile crustal growth in subduction-related magmatism during the Paleoproterozoic and improves framework knowledge for uranium exploration in the Central Mineral Belt of Labrador. U–Pb SHRIMP zircon geochronology from four felsic tuff samples collected from two different areas of the Aillik Group yield magmatic 207Pb/206Pb ages of 1852±7Ma (2σ), 1854±7Ma (2σ), 1861±7Ma (2σ), and 1862±7Ma (2σ). The ages fall within the range of previous U–Pb zircon dates reported for felsic volcanic rocks of the Aillik Group. Altogether these dates indicate that felsic volcanism spanned at least 30m.y. and was perhaps most intense during the last 10m.y. of that interval. A foliated syn-deformational monzogranite in the Aillik Group (Cross Lake granite) yields a 207Pb/206Pb zircon date of 1805±6Ma (2σ), which indicates that development of a pervasive planar fabric associated with northwestward thrusting of the Aillik Group continued after this time.Initial εHfst values for magmatic zircons from the four felsic tuffs (−4.8 to −1.9, −1.7 to +2.2, −11.8 to +3.8, −9.0 to −4.6) and monzogranite (−4.8 to −1.5) exhibit significant variability, which indicates heterogeneous sources. Most of the zircon grains have negative initial εHfst values and calculated crust formation Hf-isotope TDM model ages point to substantial contributions from melts of ancient crust, ranging in age from ca. 2200 to 2900m.y. and clustering at ca. 2500m.y. Inherited zircon grains in the felsic tuffs and monzogranite, mainly have U–Pb ages of ca. 1880–1920Ma, though others are older, up to ca. 2743Ma. Hf-isotope TDM model ages for the inherited zircons are similar to those of the magmatic zircons. The Hf-isotope results indicate that subduction-related felsic magmatism in the Makkovik Orogen involved significant crustal reworking (∼80%) during formation of the Aillik Group.

Metallogenic significance of trace element and U–Pb isotope data for uraninite-rich mineral separates from the Labrador Central Mineral Belt

Thirteen concentrates of uraniferous material were prepared from uranium occurrences in the Central Mineral Belt of Labrador. Host rocks to these occurrences include granitoid rocks of the Archean basement, ca. 2000 Ma metasedimentary rocks of the Lower Aillik Group, and 1860 Ma felsic volcanic rocks of the Upper Aillik Group. Common lead corrected Pb isotope data from inductively coupled plasma mass spectrometry analyses define 207Pb/206Pb ages ranging from 1805 to 1697 Ma for all but one sample, with a mean age of 1752 ± 27 Ma (1 σ). Ages calculated for individual samples are similar to those derived by previous workers using standard analytical techniques. Eleven of these samples define linear trends that intersect the U–Pb concordia at 1741 ± 23 Ma and a Tera-Wasserburg curve at 1740 ± 21 Ma, respectively. These data suggest that the occurrences are epigenetic with respect to host rocks and possibly related to a common metallogenic event, therefore resolving a long-standing controversy about the timing and mode of occurrence of the widespread uranium mineralization in this part of the belt. These ages broadly correlate with a period of migmatization, metamorphism, and granitoid plutonism, as defined by U–Pb zircon geochronological data for regional units. Rare earth element data for uraninite from all concentrates resemble those of uraninite in granite-related deposits. One sample has a distinctly different calculated 207Pb/206Pb age of 495 Ma, indicative of a later remobilization of the ca. 1741 Ma mineralization. The geochemical and geochronological data collectively suggest that the Central Mineral Belt uranium occurrences were related to posttectonic granite magmatism and have no direct genetic relationships with nongranitoid host rocks.

Origins of stratiform and stratabound Fe–Cu–Zn horizons in the Lower Proterozoic Moran Lake Group, Labrador Central Mineral Belt

Zn, Cu, and Fe are concentrated as stratiform and stratabound sulphide-rich beds in the Lower Proterozoic Warren Creek Formation, Moran Lake Group, central Labrador. Upper Member sedimentary rocks have a hydrothermal-like Fe enrichment but a dominantly hydrogenous signature as indicated by high Al2O3 relative to SiO2, and high Al and Fe relative to Mn. The Upper Member shales and sulphide-rich beds were deposited as Fe-rich pelagic sediments. The paucity of Mn and abundance of Fe in typical shale samples and lack of Cu, Pb, and Zn fractionation in stratiform massive sulphide beds that contain up to 4702 ppm Zn, 533 ppm Cu, and 15 ppm Pb suggest that deposition occurred in restricted brine pools (i.e., Cu and Zn were precipitated rapidly and were not fractionated). Stratabound sphalerite mineralization containing &gt; 3.7% Zn and 121 ppm Cu (but no Pb) was deposited in a porous lithology at the top of the Warren Creek Formation and represents a unique style of metal concentration.The stratiform deposits probably formed by advection of low-temperature connate waters in a situation typical of sediment-hosted exhalative mineralization (SEDEX). The potential for ore-grade metal concentration is apparently low because metal associations (Fe,Cu,Pb,Zn,Ba) are unlike those of sediment-hosted massive sulphide deposits, the sediments have a dominantly hydrogenous rather than hydrothermal signature, and the absolute grades of known occurrences are very low. The stratabound Zn deposit was probably formed by converting Zn-rich brines (≤ 200 °C) trapped during development of a hydrothermal convection system during a period of increased geothermal gradient. The potential for this type of occurrence in the Warren Creek area to reach economic grade is limited because the convection cells were shallow, ephemeral, and without the metal associations of sediment-hosted massive sulphide deposits.

Metallogenic and tectonic implications of Pb isotope data for galena separates from the Labrador Central mineral belt

Galena separates were collected from three Proterozoic supracrustal sequences comprising the Labrador Central mineral belt and also from minor occurrences in the Archean granitoid basement. Galena occurs in two environments: epigenetic quartz veins, and veins associated with uraninite occurrences. Fourteen samples from seven vein occurrences along the unconformity between shales of the ca. 2000-Ma Moran Lake Group and Archean basement and other spatially related rocks define a secondary isochron with a t 2 of 1163 Ma and a t 1 of 2473 Ma. These model ages suggest that Pb was mobilized from the Archean basement and deposited during the Grenvillian orogeny. Scatter in 208 Pb/ 206 Pb ratios reflects differing mixtures of source components. Separates from the Big Bight and Ford9s Bight showings in coastal exposures of the 1860-Ma Upper Aillik Group have Pb isotope ratios which reflect a source region with higher U/Pb and Th/Pb values than source regions for the Moran Lake Group galenas. In uraninite-associated Pb occurrences linked to post-tectonic granites elsewhere in the Upper Aillik Group, Pb isotope ratios reflect mixtures of country-rock Pb from low U/Pb sources (similar to that of the Moran Lake Group) and uranogenic Pb. Mobilization of the Pb from uraninite may have been the product of Grenvillian tectonism. The Moran Lake Group and uraninite-associated leads were derived from Archcan crust in the Nain province and this crust isotopically resembles Nuk gneiss of southern Greenland. The Big Bight-Ford9s Bight Pb isotope ratios indicate a more radiogenic Pb source to the southeast of the Central mineral belt and therefore the influence of another cratonic block outboard from the Nain province.

A major Aphebian–Helikian unconformity within the Central Mineral Belt of Labrador: definition of new groups and metallogenic implications

The Central Mineral Belt of Labrador consists of a belt of supracrustal rocks that occupies the northern foreland region of the Grenville Province of the Canadian Shield. Recent mapping in this belt has shown that the Proterozoic Croteau Group consists of two distinct sequences separated by an observed angular unconformity. It is therefore proposed that the name Croteau Group be abandoned and that the lower, Aphebian, marine sequence of sandstone, dolostone, slate, argillite, and mafic volcanic rocks be named the Moran Lake Group and that the upper, Helikian, continental sequence of conglomerate, tuffaceous sandstone, and a calc-alkalic volcanic assemblage be named the Bruce River Group.The Moran Lake Group underwent polyphase deformation, which has been assigned to the Hudsonian Orogeny, prior to deposition of the Bruce River Group around 1474 Ma. The Bruce River Group was intruded by a large granitic batholith, the Otter Lake Granite, for which a preliminary Rb–Sr isochron age of 1445 Ma has been obtained; this age correlates with the Elsonian magmatic event, an event well documented in northern Labrador. The Seal Lake Group, a Neohelikian (1278 Ma) sequence of quartzites, conglomerates, and intercalated mafic lava flows, was unconformably deposited upon the Bruce River Group and the Otter Lake Granite. During the Grenvillian Orogeny, the Bruce River and Seal Lake Groups were deformed together into a major easterly trending syncline. Deformation and metamorphism decrease across these groups to the north.The Bruce River Group forms part of the Labrador uranium area and hosts 14 known uranium occurrences. Occurrences are concentrated in the basal sandstones and conglomerates of the group, above the Aphebian–Helikian unconformity, and in ignimbrites and acid tuffs near the top of the group. No uranium occurrences are known from the Moran Lake Group except in fault-related fractures below the unconformity.