Oka Carbonatite Complex Research Articles

3D Bayesian Inversion of Potential Fields: The Quebec Oka Carbonatite Complex Case Study

Potential fields methods, based on the exploitation of gravity and magnetic fields, are among the most important methods to recover fundamental information on the Earth crust structure at global, regional and local scales. The bottleneck for this kind of geophysical methods is often represented by the development of ad-hoc techniques to fully exploit the available data. In fact, each different technique can observe the effect of a single property of the subsurface and when we want to estimate this property from the observed field (the so-called inverse problem), several problems such as non-uniqueness and instability arise. A possible solution to these problems consists in jointly inverting, in a consistent way, different observed fields, possibly also incorporating all the available geological constraints. In the current work, we present an innovative Bayesian algorithm aimed at performing a full 3D joint inversion of gravity and magnetic fields constrained by geological a-priori qualitative information. The algorithm is tested on a real-case scenario, namely, a local study to estimate a complete 3D model of the Oka carbonatite complex. This complex is a composite pluton in Quebec (Canada), important for mining operations related to critical raw material such as Niobium and other rare earth. This example shows the reliability of the developed inversion algorithm and gives hints on the fundamental role that potential fields can play in mining activities.

A new Chinese national reference material (GBW04481) for calcite oxygen and carbon isotopic microanalysis

Carbonate oxygen (O) and carbon (C) isotopes are widely used as proxies for tracing the processes and physicochemical conditions of many geological events and environmental changes in Earth Science. In particular, O and C isotopic variations at micrometer scales revealed by modern microbeam analytical techniques such as SIMS and NanoSIMS are robust archives for reconstructing palaeoenvironment and paleoclimate changes at annual and seasonal resolution or even higher temporal resolution. Widespread application of carbonate O and C isotopic microanalysis in Earth Sciences, however, has been restricted due to limitation of high‐quality carbonate reference materials for O and C isotopic microanalysis. We introduce in this paper a new calcite reference material for calcite O and C isotopic microanalysis. This calcite is collected from a drill‐core of the Oka carbonatite complex (Quebec, Canada). We demonstrated that the Oka calcite is fairly homogeneous in O and C isotopic compositions at micrometer scales based on homogeneity test by hundreds of SIMS O and C isotopic analyses. Precise determinations by using conventional gas‐source IRMS yield the recommended value of δ18OVPDB = −23.12 ± 0.15‰ (1SD) and δ13CVPDB = −5.23 ± 0.06‰ (1SD) for the Oka calcite, which has been certified as the first class of Chinese national certified reference material (GBW04481) for O and C isotopic microanalysis.

Mineral chemistry of melanite from calcitic ijolite, the Oka carbonatite complex, Canada: Implications for multi-pulse magma mixing

Ti-rich garnet is found within calcitic ijolite from the Oka carbonatite complex in Canada, which is characterized by 58%–73% andradite component (2.12 wt.%–4.18 wt.% TiO2) and classified as melanite. The garnet displays complex zoning and contains abundant high field strength elements (HFSEs) and rare earth elements (REEs). Three groups (I, II, III) have been identified based on their petrographic nature. Compared to groups II and III, Group I garnet cores contain higher TiO2, MgO, HFSE, and REE and lower SiO2 abundances. The distinct chemical and petrographic signatures of the investigated garnets cannot be attributed to simple closed system crystallization, but they are consistent with the multi-pulse magma mixing. Combined with previously reported U-Pb ages for apatite from the calcitic ijolite, at least three stages of magma evolution and subsequent mixing have been involved in the generation of calcitic ijolite at Oka. The early-formed melt that generated Group I garnet core was later mixed with at least two small-volume, more evolved melts. The intermediate stage melt formed the remaining garnet along with some pyroxene, calcite, nepheline, and apatite at 127±3.6 Ma. The youngest, most evolved melt generated the majority of pyroxene, calcite, nepheline, and apatite within the calcitic ijolite at 115±3.1 Ma.

Isotopic (Pb, Sr, Nd, C, O) evidence for plume-related sampling of an ancient, depleted mantle reservoir

The exact mantle source for carbonatite melts remains highly controversial. Despite their predominant occurrence within continental (lithospheric) domains, the radiogenic isotope data from young (< 200 Ma) carbonatite complexes worldwide overlap the fields defined by present-day oceanic island basalts (OIBs). This feature suggests an intimate petrogenetic relationship with asthenospheric mantle. New Pb, Sr, C, and O isotopic data are reported here for constituent minerals from the Oka carbonatite complex, which is associated with the Cretaceous Monteregian Igneous Province (MIP), northeastern North America. The Pb isotope data define linear arrays in Pb–Pb isotope diagrams, with the corresponding Sr isotope ratios being highly variable (0.70314–0.70343); both these features are consistent with open system behavior involving at least three distinct mantle reservoirs. Compared to the isotope composition of known mantle sources for OIBs and carbonatite occurrences worldwide, the least radiogenic 207Pb/204Pb (14.96 ± 0.07) and 208Pb/204Pb (37.29 ± 0.15) isotopic compositions relative to their corresponding 206Pb/204Pb ratios (18.86 ± 0.08) reported here are distinct, and indicate the involvement of an ancient depleted mantle (ADM) source. The extremely unradiogenic Pb isotope compositions necessitate U/Pb fractionation early in Earth's history (prior to 4.0 Ga ago) and growth via a multi-stage Pb evolution model. The combined stable (C and O) and radiogenic isotopic compositions effectively rule out crustal/lithosphere contamination during the petrogenetic history of the Oka complex. Instead, the isotopic variations reported here most likely result from the mixing of discrete, small volume partial melts derived from a heterogeneous plume source characterized by a mixed HIMU–EM1–ADM signature.

Evidence for the Multi-Stage Petrogenetic History of the Oka Carbonatite Complex (Québec, Canada) as Recorded by Perovskite and Apatite

The Oka complex is amongst the youngest carbonatite occurrences in North America and is associated with the Monteregian Igneous Province (MIP; Québec, Canada). The complex consists of both carbonatite and undersaturated silicate rocks (e.g., ijolite, alnöite), and their relative emplacement history is uncertain. The aim of this study is to decipher the petrogenetic history of Oka via the compositional, isotopic and geochronological investigation of accessory minerals, perovskite and apatite, using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The new compositional data for individual perovskite and apatite grains from both carbonatite and associated alkaline silicate rocks are highly variable and indicative of open system behavior. In situ Sr and Nd isotopic compositions for these two minerals are also variable and support the involvement of several mantle sources. U-Pb ages for both perovskite and apatite define a bimodal distribution, and range between 113 and 135 Ma, which overlaps the range of ages reported previously for Oka and the entire MIP. The overall distribution of ages indicates that alnöite was intruded first, followed by okaite and carbonatite, whereas ijolite defines a bimodal emplacement history. The combined chemical, isotopic, and geochronological data is best explained by invoking the periodic generation of small volume, partial melts generated from heterogeneous mantle.

A COMBINED GEOCHEMICAL AND GEOCHRONOLOGICAL INVESTIGATION OF NIOCALITE FROM THE OKA CARBONATITE COMPLEX, CANADA

This study is the first to report a detailed geochemical investigation and in situ U-Pb ages for niocalite, which occurs within carbonatite from the Bond Zone area of the Oka Carbonatite Complex (Canada). Niocalite is a Nb-disilicate member of the lavenite–cuspidine group. The major element composition of the niocalite studied here is relatively homogeneous with the average formula of: (Na 0.34 Fe 0.06 Mn 0.19 Mg 0.09 Ca 13.40 REE 0.15 Ti 0.02 ) ∑14.25 Nb 2.12 Ta 0.06 (Si 2 O 7 ) 4 O 7.65 F 2.44 . Niocalite is enriched in minor and trace elements [ i.e. , Ta, Ti, and rare earth elements (REEs) up to 4.35 wt.%], with double- and triple-valenced elements ( i.e. , Sr, Y, REEs) substituting at different Ca-occupied lattice sites. The chondrite-normalized REE patterns for niocalite are LREE-enriched (~10 4 times chondrite) and negatively sloped. In addition, niocalite has higher HREE contents compared to those for co-existing apatite from the identical carbonatite sample. This result is expected based on bond valence data because of differing cation sizes. In situ U-Pb ages for niocalite from three carbonatite samples obtained by LA-ICP-MS define a wide range of ages, between ~111 and ~133 Ma. Niocalite from one carbonatite sample yields a bimodal distribution with weighted mean 206 Pb/ 238 U ages of 110.1 ± 5.0 Ma and 133.2 ± 6.1 Ma, and overlap those of co-existing apatite for the same sample. Two other samples of carbonatite yield weighted mean 206 Pb/ 238 U ages of 110.6 ± 1.2 Ma and 115.0 ± 1.9 Ma. The geochronological data for niocalite reported here suggests a protracted petrogenetic history for Oka, which corroborates the relatively large range of ages (~20 Myr) previously determined for apatite from the Oka complex. In situ 87 Sr/ 86 Sr isotope ratios for niocalite obtained by LA-MC-ICP-MS are variable and range between 0.70314 (±7) and 0.70343 (±8). Based on the combined U-Pb ages, major and trace element abundances, and Sr isotope ratios, the history of niocalite formation cannot be attributed to either simple closed-system fractional crystallization or carbonate-silicate liquid immiscibility. Rather, these are consistent with periodic carbonatite melt derivation from a heterogeneous mantle source, and suggest that some of niocalite investigated here represent cognate crystals.

In-situ determination of major and trace elements in calcite and apatite, and U–Pb ages of apatite from the Oka carbonatite complex: Insights into a complex crystallization history

This study reports in-situ determination of major and trace element compositions of calcite and apatite, and U–Pb ages of apatite for carbonatites and associated okaites and ijolites from the Oka carbonatite complex (Canada). Both apatite and calcite are rare earth element (REE)-enriched (total REEs >4wt.%), with REE and SiO2 contents that correlate positively in apatite. This result supports the previously proposed substitution scheme of REE3++Si4+=Ca2++P5+. With the exception of Ba, Sr, and Mn, the other trace elements investigated preferentially partition into apatite relative to calcite. Trace element abundances for apatite and calcite both within and between different samples exhibit large variations, which cannot be reconciled with equilibrium or fractional crystallization from a single parental melt using available partition coefficient data. In-situ U–Pb ages determined by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) on individual apatite grains (total of 215 analyses) from carbonatites (n=7), okaites (n=4), and ijolites (n=2) define a relatively large range, from ~111 and ~129Ma. Evidence for protracted magmatic history is also present within individual samples. Apatite from one ijolite defines two distinct ages; an older age of 127±3.6Ma is recorded in apatite inclusions within early-formed melanite, whereas apatite within matrix calcite is characterized by a younger age of 115±5.1Ma. U–Pb ages obtained for all apatite from carbonatites, okaites and ijolites yield bimodal distributions with peaks at ~113 to ~117 and ~125 to ~127Ma. Our multi-faceted investigation of individual minerals has documented, for the first time, a bimodal age distribution and simultaneous emplacement of the carbonatites and associated okaites and ijolites at the Oka carbonatite complex. The combined large variations in major and trace element compositions recorded by both apatite and calcite, and petrographic textural evidences are consistent with a complex crystallization history involving mixing of discrete partial melts.

U–Pb dating of perovskite by LA-ICP-MS: An example from the Oka carbonatite, Quebec, Canada

The potential for U–Pb dating of the mineral perovskite, using laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), has been evaluated. Perovskite can have relatively elevated U and Th contents, making it a potentially useful geochronometer. Kimberlite, lamprophyre and carbonatite intrusions are prone to alteration and typically contain abundant xenocrysts, thus standard techniques for dating the magmatic emplacement of these rocks becomes highly problematic. Perovskite, however, is commonly present as a primary magmatic mineral in these rocks and has been used to constrain the timing of kimberlite and lamprophyre emplacement. The main limitations to the production of accurate LA-ICP-MS U–Pb ages for perovskite are the lack of suitable standards coupled with the large common-Pb correction required. 204Hg can produce large isobaric interference on the non-radiogenic 204Pb isotope and methods based on 208Pb/ 206Pb and U/Th ratios are hampered by the enhanced Th content present in most perovskite crystals. After careful calibration using zircon reference materials, common-Pb corrected 206Pb/ 238U ages can be determined on each individual LA-ICP-MS analysis by using the measured 207Pb/ 206Pb ratio which is dominated by the common-Pb proportion in phanerozoic rocks. This method assumes that the perovskite crystals analyzed are cogenetic and have the same initial-Pb ratios. The LA-ICP-MS method has been applied to data collected on two large, single perovskite crystals from the Oka carbonatite complex, Quebec, Canada. The weighted mean 206Pb/ 238U age of 131 + /−7 Ma ( N = 25, MSWD = 0.42) is consistent with published geochronology data from the region and suggests that LA-ICP-MS dating of perovskite could be a useful, low-cost alternative to compliment existing techniques.

The nature of orthomagmatic hydrothermal fluids in the Oka carbonatite, Quebec, Canada: Evidence from fluid inclusions

Fluid inclusions in apatite, monticellite, and calcite from the Oka carbonatite complex generally comprise either aqueous liquid + vapour or aqueous liquid + vapour + halite. Some inclusions also contain a variety of trapped solids, including calcite, strontianite, nahcolite, and mirabilite. Microthermometry and Raman spectroscopy indicate that CO 2 is not an important constituent of these fluids. Microthermometric data also indicate that the hydrothermal fluids liberated from the carbonatitic magma had a wide range of salinities (4.2 to 49 eq. wt% NaCl) and densities (0.77 to 1.26 g/cm 3). Most of the fluids had relatively low concentrations of SO 4 2− and CO 3 2− (<6 wt%); however, the nature of the trapped minerals and the phase behaviour of some inclusions indicate that some fluids may have had higher concentrations of SO 4 2−, CO 3 2− and/or HCO 3 − Textural relationships indicate that the crystallization sequence for the host minerals was monticellite → apatite → calcite, which, when combined with fluid inclusion data from the three minerals, indicates that fluid salinity decreased and fluid density increased during progressive crystallization of the magma. The density increase is thought to have been mainly caused by decreasing temperature. Isochoric projections indicate that the fluids were exsolved at minimum pressures of 4 to 8 kb. These data provide a basis with which to assess the involvement of orthomagmatic aqueous fluids in the formation of hydrothermal rare element mineralization associated with carbonatites.

Lead and strontium isotope relationships in the Oka carbonatite complex, Quebec

Abstract Lead isotopic compositions are presented for 8 calcite, 1 pyrite and 1 melilite separates from the Oka, Quebec, carbonatite complex. Pyrite, melilite and two calcite samples with small in situ decay corrections yield similar initial isotope ratios, averaging 206 Pb 204 Pb = 19.65 , 207 Pb 204 Pb = 15.56 , 208 Pb 204 Pb = 39.00 . This composition shows a close affinity for Pb isotope ratios from ocean island basalts (OIB) and indicates an origin of the Pb in large-ion lithophile (LIL)-element depleted mantle sources. The Pb probably results from a mixture from a LIL-element depleted source and possibly, metasomatic fluids. Four calcites fail to yield accurate initial ratio information due to large in situ decay corrections. In addition three samples exhibit high initial 207 Pb 204 Pb ratios indicative of probable crustal contamination. The silicate fraction of one okaite and a sample of country rock gneiss near Oka contain Pb with isotopic compositions that illustrate the kinds of contamination that could be involved. Sr isotope data are reported for 2 whole-rock sovites, plus 1 nepheline, 4 calcite and 2 apatite separates. All samples yield nearly identical isotopic compositions, with a mean 87 Sr 86 Sr ratio of 0.70331 ± 0.00002 at the 2 sigma level. This result verifies an earlier value reported by Bell et al. (1982) and supports their suggestion of a LIL-element depleted mantle source for the Sr. The Sr data do not show the apparent crustal contamination detected by some of the Pb data, presumably because of the high Sr contents of the carbonate samples. The Oka data are compared with results of a similar study by Lancelot and Allegre (1974) on carbonatite complexes from eastern Uganda. We conclude that the African data, as presently known, are compatible with the model developed for the mantle-derived Oka suite.

The Ile Bizard intrusion, Montreal, Quebec—kimberlite or lamprophyre?

The Ile Bizard intrusion is a composite diatreme consisting of a pipe-like main body with cross-cutting pipes and dikes. The rock types include an intrusive breccia consisting of a kimberlite-like matrix bearing many xenoliths, a later tuffisite breccia that contains abundant autoliths, and two phases of xenolith-poor tuffisite breccia. All kimberlitic intrusive phases are more aluminous, iron-rich, and less magnesian than typical kimberlites, but more closely resemble kimberlites than alnoites.Xenolith rock types include sedimentary rocks, basement rocks, alloclastic fragments, autoliths, and ultramafic rocks. Phenocrysts and (or) xenocrysts of clinopyroxene, phlogopite, olivine, and magnesian titanomagnetite are common. Ultramafic xenoliths include websterites, orthopyroxenites, dunites, harzburgites, garnet and garne–spinel lherzolites, garnet websterites, and garnet clinopyroxenites. Banded xenoliths represent a cumulate phase in the upper mantle; the remaining ultramafic xenoliths are samples of unmelted mantle material. Temperature and pressure estimates for the garnet lherzolites, ranging from 880 to 980 °C and 2.1 to 2.8 GPa, do not outline a paleogeotherm.Mineralogical and chemical characteristics of the intrusive rocks and the xenolith suite are compatible with those of true kimberlites. The Ile Bizard diatreme is, however, closely related in space and time to the Oka carbonatite complex and Monteregian alkaline activity, and provides an example where a clear distinction between true kimberlites and lamprophyres of so-called central complex kimberlites cannot be made. This intrusion represents the deepest level of generation of Monteregian igneous activity in the form of potassic–utramafic (or kimberlitic) magmatism.

Abundance and distribution of the rare-earth elements and yttrium in the rocks and minerals of the Oka carbonatite complex, Quebec

are-earth (REE) and yttrium abundances were determined, by an ion-exchange-X-ray fluorescence procedure, for whole-rock (14) and mineral (87) samples from the Oka carbonatite complex. Whole-rock and mineral data indicate a trend of total REE + Y enrichment, and relative enrichment in light REE, in the order: ultrafenites < ijolites < okaites. The sövites may show wide variations in total REE + Y concentrations, but relative REE abundance patterns will be similar. The greatest REE and Y concentrations occur in apatite, niocalite, perovskite and pyrochlore. Many of the minerals show europium anomalies (both positive and negative), and these are believed to be the result of closed system competition between the various minerals for divalent Eu. The partition coefficients for mineral pairs are quite variable, indicating that the Oka rocks were emplaced through a wide-range of physicochemical and/or nonequilibrium conditions. A reasonable model for the origin of the complex involves a limited partial melting of mantle material, emplacement of the melt in a magma chamber, crystallization of mafic minerals resulting in a residual liquid which produced ijolite and subsequently okaite, and crystallization of the carbonatites from a volatile-rich, possibly immiscible, phase.

Scandium, cobalt and iron geochemistry of the Fen alkaline complex, southern Norway

Whole rock abundances of Sc, Co and total Fe are presented for the Fen carbonatite complex. Average Sc and Co abundances are as follows: urtites (Sc = 1.8 ppm; Co. 5.5 ppm), ijolites (6.6; 25), silicocarbonatites (3.2; 12.2), søvites (3.5, 8.6), damtjernites (24; 57), vipetoites (5.2, 36), rauhaugites (15, 26), rødberg (61, 28). With in the ijolite-søvite series Sc decreases with degree of differentiation due to removal of pyroxene and garnet. In the damtjernite-rødberg series Sc increases with differentiation due to concentration by volatiles involved in the metasomatic conversion of damtjernite to rødberg. Data for the ijolite-søvite series is opposite to trends for this series found in the Oka carbonatite complex.

カナダOka carbonatiteの生成温度について

The Oka, carbonatite complex is located 32km west of Montreal, Quebec, Canada, and is situated west end of “Monteregian Hills alkaline rock petrographic province”.The St. Lawrence Columbium and Metals Corporation Mine is situated near the center of the Oka carbonatite complex. The main ore mineral is pyrochlore, and the annual production is about 2250 tons of pyrochore concentration containing about 52% Nb2O5.Many of polyphase inclusions having very small size are found in calcite, apatite and biotite from the Oka carbonatite.The filling temperatures of fluid inclusions in calcite and apatite from carbonatite were determined under heating stage. From these results, the range of the filling temperatures of calcite is from 395°C to 275°C. 320°C in average, the range of apatite, from 350°C to 260°C, 300°C in average.The solid phases in polyphase inclusions in calcite and apatite show an interference color between crossed nicols, however, it does not melt at upper temperature than filling temperature of fluid inclusions. Mineral species of solid phase are not yet been determined.The analyses of experimental data allow to assume that the range of formation temperature of the Oka carbonatite is considered from 800v to 600°C.

The carbon and oxygen isotopic composition of carbonates from the Oka carbonatite complex, Quebec, Canada

The Oka complex is a carbonatite belonging to the alkaline intrusive suite of the Monteregian petrographic province. The carbon and oxygen isotopic composition of carbonates from this complex were studied and found to vary by 7%. for carbon and about 13%. for oxygen. The mode of the δC 13- δO 18 distribution for the carbonatite lies at δC 13 = − 5% . vs. PDB and δO 18 = 7.4%. vs. SMOW. The isotopic composition variability could not be related to weathering or hydrothermal alteration. The frequency distributions (650 samples) of carbon and oxygen isotopic compositions are highly skewed toward increasing C 13 and O 18 concentrations. The carbon isotopic composition frequency distribution may be interpreted as the result of a Rayleigh fractionation process taking place during the formation of the carbonatite. The variations in O 18 content are greater than would be expected from such a process; part of the increased variability may be due to isotopic exchange with or the incorporation of the intruded gneisses. Relationships between isotopic composition and rock type could be demonstrated in several cases. With respect to sövites, rauhaugites show a distinct, and carbonates from the alkaline ultrabasic rocks a slight enrichment in C 13. The oxygen isotopic compositions of sövites and rauhaugites are consistently lighter than those of carbonates from the alkaline ultrabasic rocks, and the carbonates from fenites and silicate rocks, for which a derivation by ultra-fenitization is suggested on petrographic grounds, show the highest average O 18 concentrations. Small gradients in isotopic composition exist in cross sections through the carbonatite complex.