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The Misery syenitic intrusion in northern Quebec is host to a potentially important, recently discovered rare earth element (REE)-Zr-Nb prospect, containing significant concentrations of both light and heavy REEs, and is conspicuous on aeromagnetic maps as a well-defined, ring-shaped anomaly. Felsic syenite composed of idiomorphic perthite with interstitial mafic minerals, comprising fayalite, hedenbergite, ferropargasite and annite dominates the exposed outer part of the intrusion (the core is covered by Misery Lake). This unit is accompanied by ferrosyenite containing up to 50 vol % mafic minerals, including cumulate fayalite. The ferrosyenite, which occurs as amoeboid-like inclusions in the felsic syenite, is interpreted to have formed by fractional crystallization of ferromagnesian minerals, leaving behind a residual magma which produced the felsic syenites. This latter magma was progressively enriched in alkalis and silica, and only became saturated in ferromagnesian minerals at a very late stage of crystallization. The bulk of the REE mineralization was initially concentrated in pods and layers of fluorapatite that accumulated as a result of gravitational settling. The fluorapatite crystals were partly or completely replaced by brithohte-(Ce), which further enriched the rocks in LREEs. Locally, thin quartz-fayalite dikes cut the felsic syenite. These dikes contain up to 10 vol % fergusonite-(Y) and 8 vol % zircon, and are interpreted to be products of an immiscible FeO-SiO2-rich magma into which Zr, Nb, and HREEs were preferentially fractionated from the conjugate felsic syenite magma. [ABSTRACT FROM AUTHOR]

The Siscoe mine was one of the richest in the Val d'Or mining camp, producing 27.5 tonnes (t) of gold and 9.5 t of silver from 1929 to 1949. The C quartz-tourmaline vein at the Siscoe mine, which is the youngest auriferous vein on the property, is an excellent example of a high-grade Archean lode gold deposit; it contains an average of 45 g/t Au and locally up to 221 g/t Au. This vein cuts all dikes, the main regional foliation (S2), and several auriferous quartz-carbonate veins, and is interpreted to be a shear structure that formed during the development of a late- to post-D2 reverse fault. The vein comprises mainly alternating tourmaline- and quartz-rich layers, and gold occurs commonly in late fractures cutting the vein. The visible wall-rock alteration is characterized by the occurrence of tourmaline, pyrite, and calcite. In this study, we report the Pb isotope compositions of hydrothermal pyrite separates taken from the C quartz-tourmaline vein and its wall rock. Pyrite is abundant in the wall rock, where it occurs as grains with inclusions of chalcopyrite, gold, tetradymite, pyrrhotite, rutile, silicates, and carbonates. In the vein, it forms euhedra generally devoid of inclusions. Locally, the euhedra are fractured, and the fractures are filled with native gold, calcite, and tetradymite. Textural relationships suggest that pyrite precipitated synchronously with and/or after gold in the wall rock, but predated gold in the vein. Pyrite separates from the vein and its wall rock yielded similar 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (13.63-13.83, 1.4.58-14.62, and 33.42-33.76, respectively), which are more radiogenic than those reported for volcanic supracrustal and syntectonic plutonic rocks of the southern Abitibi subprovince, but are very similar to those of the late- to posttectonic S-type granites in the Pontiac subprovince and Lacorne block. This suggests that the Pb originated from a reservoir similar to that of the magmas which produced these granites, or alternatively, that it was leached from the latter. The Pb ratios yield a model age of 2.57 ± 0.07 Ga, which is interpreted to date the cooling of the hydrothermal system that introduced and/or remobilized gold into the C quartz-tourmaline vein. These results, when considered in the context of the evolution of the southern Abitibi, indicate that widespread hydrothermal circulation occurred between 2.64 and 2.55 Ga, after the collision of the Pontiac and Abitibi subprovinces, and was caused by dewatering and S-type magmatism in the thickened unstable continental crust. This long-lived hydrothermal system promoted remobilization of gold from earlier veins and possibly introduction of gold from deeper zones, enhancing gold resources in the Siscoe deposit. [ABSTRACT FROM AUTHOR]

Presents a study on the composition and structural state of columbite-tantalite rocks located in rare-element granitic pegmatites from the Preissac-Lacorne batholith, Quebec, Canada. Experimental procedure; Characteristics of pegmatites; Types of pegmatites; Nomenclature.

Although it is well known that A-type granites are enriched in the rare earth elements (REE) and other high field strength elements (HFSE), the magmatic processes that concentrate these elements are still poorly understood. The 1.24 Ga Strange Lake pluton in northern Québec-Labrador provides an extraordinary example of hyper-enrichment in the REE, Zr, and Nb in a peralkaline A-type granite. The pluton consists of two hypersolvus granite units (southern and northern) and a transsolvus granite, all of which contain perthitic alkali feldspar as the earliest major mineral; the transsolvus granite also contains separate albite and microcline crystals. Arfvedsonite, a sodic amphibole, occurs exclusively as phenocrysts in the transsolvus granite, whereas in the hypersolvus granite it is present as a late, interstitial phase. The primary HFSE minerals are zircon, monazite-(Ce), gagarinite-(Ce) and the pyrochlore group minerals. Magma evolution was monitored by the alumina content in the bulk rock, which decreases from the southern to the northern hypersolvus granite and is lowest in the transsolvus granite. Alkalinity indices and bulk Si, Fe, Rb, REE, Zr, Nb concentrations show the opposite trend. Alkali feldspar compositions mirror the trend shown by the bulk rock, i.e., decreasing Al contents are accompanied by increasing Si, Fe 3+ , REE, Zr and Nb contents. The major driving forces for the evolution of the hypersolvus magma prior to emplacement were the early separation of a fluoride melt from the silicate melt and the crystallization of alkali feldspar and HFSE-rich phases (zircon, monazite-(Ce), pyrochlore group). An alkali feldspar-rich crystal-mush containing LREE-fluoride melt droplets was emplaced as the least evolved southern hypersolvus granite. Massive fractionation of alkali feldspar led to a sharp increase in ƒH 2 O and F − activity in the magma chamber that triggered the crystallization of arfvedsonite and was followed by emplacement of the northern hypersolvus granite, which contained a higher proportion of LREE-fluoride melt droplets. Further evolution in the magma chamber led to a transition from a miaskitic to an agpaitic composition. The transsolvus granite was intruded in the form of a low viscosity crystal mush of alkali feldspar, quartz, arfvedsonite (after appreciable crystallization of arfvedsonite) and LREE-fluoride melt droplets. Upon emplacement, arfvedsonite (and gagarinite-(Ce)) crystals segregated as cumulates in response to a combination of flow differentiation and gravity settling. The immiscible fluoride melt accumulated in a volatile-rich residual silicate magma, which migrated to the top of the pluton where it formed the F-REE-rich cores of highly mineralized pegmatites. [ABSTRACT FROM AUTHOR]