Kevin M. Ansdell

Plutonism, deformation, and metamorphism in the Proterozoic Flin Flon greenstone belt, Canada: Limits on timing provided by the single-zircon Pb-evaporation technique

The authors have used the single-zircon Pb-evaporation technique to determine the age of granitoids in the western Flin Flon domain of the Trans-Hudson orogen in Canada and to constrain the timing of molasse sedimentation, deformation, metamorphism, and mesothermal gold mineralization. The plutons were intruded between 1,860 and 1,834 Ma, about 30 m.y. after volcanism, and provide further evidence of a major period of intrusive activity throughout the Trans-Hudson orogen at that time. The older plutons (1,860 to 1,848 Ma) are synchronous with P2 deformation and the early stages of peak metamorphism, whereas the younger plutons are synchronous with or postdate the P3 deformation event. Molasse sedimentary units are deformed and metamorphosed; thus, deposition must have begun at about 1,860 Ma. Mesothermal gold mineralization postdates all plutons and may be related to high-grade peak metamorphism in adjacent terranes at about 1,815 Ma. This study also provides further support for the use of the Pb-evaporation technique as a suitable method for dating single zircons and indicates that the {sup 208}Pb/{sup 206}Pb ratio of zircons may be a useful tool in detecting the presence and source of xenocrystic zircons.

UPb geochronology and tectonic development of the southern flank of the Kisseynew Domain, Trans-Hudson Orogen, Canada

Abstract The southern flank of the Kisseynew Domain, northern Manitoba, underwent polyphase deformation and metamorphism during the Palaeoproterozoic Trans-Hudson Orogen, and represents a lithological, metamorphic and structural transition from greenschist-grade volcano-plutonic rocks of the Flin Flon Domain to amphibolite-grade paragneisses of the central Kisseynew Domain. The timing of geological events has previously been constrained using widely dispersed sample sites, but this study has integrated UPb geochronology with detailed lithological and structural mapping in the Cleunion Lake area. Pink gneisses, interpreted as Missi supracrustal rocks, were deposited after 1855 Ma, but prior to ∼ 1835 Ma, and thus are similar in age to fluvial sedimentary rocks in the lower-grade Flin Flon Domain. All the zircons in the pink gneisses appear to be inherited, or resorbed during later amphibolite-grade metamorphism, and a late Archaean component is also indicated. Corundum-bearing gneisses in the Missi Group and leucosomes in Burntwood River Group rocks are deformed by F1 and are indicative of an early high-T, low-P metamorphic event (M1) that may have been synchronous with the intrusion of 1835 to 1827 Ma intrusions, and related to regional crustal extension. The above rocks are folded during a continuous compressional deformation event (D2) related to terminal collision between the Palaeoproterozoic rocks of the Trans-Hudson Orogen and the Superior Province. Three main fold generations (F1, F2, F3) have been identified, which occurred during D2. A 1818 Ma granitic vein is approximately synchronous with F2 and peak metamorphic (M2) conditions in the Cleunion Lake area. Cooling, uplift, and intrusion of pegmatites during F3 at ∼ 1800 Ma provides a time limit on the waning stages of compressional deformation. The main phase of compressional deformation (D2) in the southern flank of the Kisseynew Domain thus lasted for about 30 million years.

Coniform Stromatolites from Geothermal Systems, North Island, New Zealand

Abstract Coniform stromatolites are found in several sites in the Tokaanu and Whakarewarewa geothermal areas of North Island, New Zealand. At Tokaanu, silicification of these stromatolites is taking place in Kirihoro, a shallow hot spring-fed pool. At Whakarewarewa, subfossil silicified coniform stromatolites are found on the floor of “Waikite Pool” on the discharge apron below Waikite Geyser, and in an old sinter succession at Te Anarata. The microbes in the coniform stromatolites from Tokaanu, Waikite Pool, and Te Anarata have been well preserved through rapid silicification. Nevertheless, subtle differences in the silicification style induced morphological variations that commonly mask or alter morphological features needed for identification of the microbes in terms of extant taxa. The coniform stromatolites in the New Zealand hot-spring pools are distinctive because (1) they are formed of upward tapering (i.e., conical) columns, (2) neighboring columns commonly are linked by vertical sheets or bridges, (3) internally, they are formed of alternating high- and low-porosity laminae that have a conical vertical profile, and (4) Phormidium form more than 90% of the biota. As such, they are comparable to modern coniform mats and stromatolites found in the geothermal systems of Yellowstone National Park and ice-covered lakes in Antarctica. Formation of the coniform stromatolites is restricted to pools that are characterized by low current energy and a microflora that is dominated by Phormidium. These delicate and intricate stromatolites could not form in areas characterized by fast flowing water or a diverse microflora. Thus, it appears that the distribution of these distinctive stromatolites is controlled by biological constraints that are superimposed on environmental needs.

Paleoproterozoic arc magmatism imposed on an older backarc basin: Implications for the tectonic evolution of the Trans-Hudson orogen, Canada

Compositional data from 72 granitoids and associated mafic rocks in the Paleoproterozoic Kisseynew domain, Trans-Hudson orogen, Canada, were used to evaluate the tectonic setting of magma generation between 1840 and 1820 Ma. These data thus provide insights into the development of the Trans-Hudson orogen just prior to amphibolite-grade metamorphism and the final stages of continental collision. The granitoid rocks are hornblende- or orthopyroxene-bearing, medium- to high-K calc-alkalic, and weakly peraluminous. The trends on Harker variation diagrams are consistent with crystal-fractionation processes, although the range in values is related to the temporal and spatial distribution of samples. The extent of LREE (light rare earth element) enrichment increases with increasing SiO 2 , as does variability in HREE (heavy rare earth element) fractionation and magnitude of Eu anomalies. Most samples have (Gd/Yb) n < 10 (suite Ia), although five have (Gd/Yb) n = 17-24, suggesting the presence of garnet residua (suite Ib). The range in rock types, calc-alkalic character, and negative Nb and Ti anomalies are all characteristic of arc environments. Magma generation within the Trans-Hudson orogen, including the Kisseynew domain, between 1.84 and 1.82 Ga was related to interaction between the Sask craton and the Flin Flon-Glennie complex. A decrease in subduction angle likely resulted from northward-directed subduction of more buoyant crust, with associated changes in thermal conditions leading to melting over a wide area. Most magmas formed in a slab-dehydration-dominated regime, although localized slab melts may have been generated. Ultimately subduction ceased, and the slab foundered into the mantle, potentially resulting in the generation of alkalic and potassic magmas within the Rae and Hearne cratons.

Trace analysis of single zircons for rare-earth elements, U and Th by electrothermal vaporisation-inductively coupled plasma-mass spectrometry (ETV-ICP-MS)

Abstract A method is described for the trace analysis of single zircons for Y, La, Ce, Nd, Sm, Yb, Th and U by electrothermal vaporisation-inductively coupled plasma-mass spectrometry (ETV-ICP-MS). Zircons are cleaned by an abrasion process and dissolved in HNO 3 and HF in a pressure vessel. Following conversion to chlorides and evaporation to dryness, the dissolved zircon residue is re-dissolved in 500 μl of high-purity 2.5 M HNO 3 . Analyte concentrations were measured with a precision of ∼ ±6%. Agreement between found and reference values for BCS- 388 zircon reference material was excellent. Limits of detection for the analysis of a 10-μg zircon were 150 ng g −1 for Y, 90 ng g −1 for La, 115 ng g −1 for Ce, 65 ng g −1 for Nd,180 ng g −1 for Sm, 22 ng g −1 for Yb, 190 ng g −1 for Th and 80 ng g −1 for U. Absolute limits of detection for a 10-μl solution aliquot ranged from 4 to 36 fg (10 −15 g). Zircon solutions were analysed using external calibration by aqueous standards with the addition of a mixed component carrier (NASS-3 open ocean seawater). No matrix or spectroscopic interferences were observed from major-element matrix components. The analysis of a typical set of single zircons gave concentration levels well above the limit of detection for all elements except La.

The nature and origin of pegmatites in a fluorine-rich leucogranite, East Kemptville Tin Deposit, Nova Scotia, Canada

Pegmatites at the East Kemptville Sn-Zn-Cu-Ag deposit occur within a F- and P-rich, 370 Ma topaz-muscovite leucogranite, the most evolved phase of the chemically zoned Davis Lake Pluton. Structural observations and geochronology indicate that the leucogranite intruded into an active shear/fault zone environment. Pegmatites are preferentially located near the roof zone of the intrusion and include: (1) quartz-feldspar pods (≤ 1–2 m) aligned parallel to a foliation in the leucogranite. Such pegmatites have marginal aplites or may be cored by aplites; (2) aplite-pegmatite zones with well-developed crenulate and unidirectional solidification textures; and (3) muscovite-bordered miarolitic cavities lined with quartz-feldspar ± cassiterite. Associated with areas of pegmatites are irregular- to cuspate-shaped zones enriched in muscovite. Mineralogically the pegmatites are simple, consisting of clear to black quartz, albite, microcline perthite and muscovite; topaz may be enriched in aplites coring pegmatites and coarse euhedral cassiterite is rare. Chemically the leucogranites marginal to pegmatites are similar to the host leucogranite, but some aphanitic felsic dykes indicate extreme differentiation, as reflected by REE depletion and P-enrichment. Enrichment of pegmatitic feldspar in P 2 O 5 (to 1 wt. %) indicates that melts contained 2–3 wt. % P 2 O 5 , whereas muscovite chemistry (to 5 wt. % F) reflects the F-rich nature of the melt. Trace and REE contents of pegmatitic feldspars are consistent with local segregation of volatile-rich melts to form pegmatites rather than extreme crystal-chemical fractionation. Fluid inclusion studies of pegmatitic quartz and cassiterite indicate the presence of a highly saline brine (40 wt. % eq. NaCl; Na > K > Fe > Mn > Ca > Sr) of magmatic origin. Isochores modelled for a 40 wt. % eq. NaCl fluid constrain pegmatite formation at 550–600°C, thus the depressed solidus is consistent with the volatile-rich (F, P, H 2 O) nature of the melt. Stable isotopes demonstrate that a magmatic fluid (δ 18 O H 2 O = +5·5 to +10‰, δD H 2 O = −33 to −41‰ for 450–500°C) equilibrated with the pegmatites and the system cooled abruptly at c . 425°C. The low volume of pegmatite at East Kemptville suggests that the melt was not near volatile saturation, instead pegmatite generation is interpreted to have resulted from rapid decompression related to the active shear zone setting of the granite. Although feldspar chemistry reflects a local segregation model, the chemistry of aphanitic dyke rocks indicates that the leucogranite did evolve into a more fractionated melt. The local presence of cassiterite in pegmatites and miarolitic cavities indicates that locally, saturation of Sn occurred, but not throughout the EKL where elevated Sn contents are attributed to infiltration of mineralising fluids (i.e. deuteric alteration).

Geochemical and Nd isotopic compositions of the metasedimentary rocks in the La Ronge Domain, Trans-Hudson Orogen, Canada: implications for evolution of the domain

Abstract Metasedimentary rocks of greenschist to amphibolite facies occur in various belts in the Paleoproterozoic La Ronge Domain of the Trans-Hudson Orogen. Metasedimentary rocks in the Central Metavolcanic greenstone belt are low to intermediate in SiO2 contents, have ϵNd values of +2.3 to +4.5, with two rare earth element (REE) patterns of (La/Yb)n 8.5, respectively. The predominant component, meta-arkoses, of the overlying McLennan Lake belt, have distinctly high K2O and SiO2 contents, high (La/Yb)n ratios (11–46), large negative Eu anomalies (Eu/Eu*=0.1–0.58) and uniformly low ϵNd values (−0.8 to +0.2). The MacLean Lake belt is comprised of a variety of metasedimentary gneisses. The volumetrically minor amphibole- and diopside-rich gneisses have high ferromagnesian element (Sc and Co) concentrations, high ϵNd values of +4.1 to +5.7, and flat REE patterns with no Eu anomalies. The predominant lithological unit in the MacLean Lake belt, the psammitic gneisses, is geochemically more variable. The mafic psammitic gneiss samples in the MacLean Lake belt and the metasedimentary rocks in the Central Metavolcanic belt are similar in trace and major element geochemistry and Nd isotope composition, whereas those more felsic psammitic gneisses are similar to the McLennan Lake meta-arkoses. Geochemical data, combined with lithological and geochronological evidence, indicate that sediments in the Central Metavolcanic belt were derived mainly from contiguous volcanic debris, with minor addition of felsic components from exotic sources. In contrast, the McLennan Lake meta-arkoses were derived from a distinct felsic source, whereas the psammitic gneisses in the MacLean Lake belt are mixtures of volcanic materials in the Central Metavolcanic belt and meta-arkoses-like felsic components. Nd isotopic data and ages of detrital zircon suggest that Proterozoic juvenile materials were dominant sources for metasedimentary rocks in the La Ronge Domain. Comparison of geochemical signatures of the metasedimentary rocks with those of the felsic plutonic rocks in the Central Metavolcanic belt suggests that the felsic materials in the McLennan Lake and MacLean Lake belts could not have been derived predominantly from the plutonic rocks, implying more distal sources. Topographic changes during collision may have resulted in a shift from mixing of both local and distal sources for the rocks in the MacLean Lake belt, to the dominance of a more distal source for the McLennan Lake meta-arkoses.

Development of a transverse to orogen parallel extension lineation in a complex collisional setting, Trans-Hudson Orogen, Manitoba, Canada

Abstract In the southeastern Trans-Hudson Orogen (THO), the wide dispersion in orientation of the L3 extension lineation from orogen-parallel to transverse is considered primary as it cannot be accounted for by simple overprinting of a single precursor orientation. Interpretation of the origin of this extension lineation is complicated by the fact that its gently northeast-plunging orientation at the north end of the study area is inconsistent with the evidence for sinistral, east side up movement of the steep oblique slip D3 faults. The hypothesis presented here to explain the complicated lineation pattern is based on evidence that the preceding period D2 southwest-directed transport, overlapped temporally and spatially with D3 northwest-southeast transpressional shortening. D2 and D3 are attributed to successive, but overlapping, collisional events. D3 transpressional shortening is interpreted to have propagated westward through the THO resulting in a prolonged and complex transition between D2 and D3 deformation regimes. The dispersion of the L3 extension lineation is interpreted to reflect the competing effects of both deformation regimes which appear to have been partitioned in the crust with D2 effects best developed in higher metamorphic grade rocks and D3 effects best developed in lower metamorphic grade rocks.

Graphic-textured inclusions in apatite: Evidence for pegmatitic growth in a REE-enriched carbonatitic system

Graphic texture, most common in felsic pegmatites, is documented here in an unusual setting, within apatite of the Hoidas Lake light rare earth element (LREE) deposit, an inferred distal pegmatitic-hydrothermal counterpart of a carbonatite in northern Saskatchewan, Canada. The graphic texture is displayed by vermicular and cuneiform LREE-rich inclusions which define primary growth zones in apatite. Integrated electron microprobe analysis and scanning electron microscope–energy dispersive X-ray spectroscopy imaging document the unusual and heterogeneous compositions of the inclusions, which vary between Ce2O3 + SiO2 (+ ThO2)–dominant and La2O3 + Nd2O3 (+ F)–dominant end members. Here we discuss the formation of these graphic-textured inclusions in the context of models suggested for the formation of graphic quartz–K-feldspar intergrowths in granitic pegmatites, based on the similarity of these features.

Geochemical footprint of the Millennium unconformity-type uranium deposit, Canada: implications for vectoring new targets

The Millennium deposit, a c. 650-m deep monomineralic uranium deposit, is located in the southeastern Athabasca Basin, Saskatchewan, a region containing numerous high-grade unconformity-type U deposits. 3D modelling of the whole-rock lithogeochemistry of sandstones above the deposit reveals a distinct footprint with select major and trace elements showing increased concentrations towards mineralization. Molar Mg/K ratios increase from background levels 10 km north of the deposit, along the B1 conductive trend, whereas Mo, Co, Rb, and Ga exhibit elevated concentrations immediately above the deposit, extending vertically from the unconformity to surface. Lead, Ag, Bi, Sb, REE, and Y exhibit elevated concentrations up to 650 m above the main mineralized body. Increasing Mg/K values indicate the transition from diagenetic to hydrothermal alteration with shifts from illitic (K-dominant) to chloritic and dravitic (Mg-dominant) alteration, with the latter increasing more proximal to mineralization. Trace element enrichment patterns highlight that fractures and faults were conduits for fluid flow from the basement into the basin, both during ore formation and through tectonically driven post-depositional remobilization. Key indicators such as molar element ratios (Mg-K-Al) and trace elements related to redox reactions provide scalable vectors at the Millennium deposit that are likely applicable to similar unconformity-type U deposits elsewhere.