R. Garth Platt

Nepheline-bearing rocks from the Poohbah Lake complex, Ontario: Malignites and Malignites

Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di71 Hd18Ac11 to Di63Hd22Ac15, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or88 to Or95. Perthites are absent.Inconsistancies in the usage of the terms malignite and juvite are discussed and it is considered that a non-genetic petrographic classification of nepheline syenites leads to the obscuration of a group of potassic nepheline syenites, characterized by the presence of nepheline plus orthoclase which are typically associated with saturated to over-saturated alkaline rocks, contain pseudo-leucite or nepheline-orthoclase intergrowths, are emplaced in mobile belts and are not associated with rocks of the ijolite-carbonatite suite.A genetic classification of nepheline syenites is suggested and it is proposed that; (1) mafic-rich nepheline syenites be referred to as mela-nepheline syenites (sensu lato) rather than as malignites; (2) the term malignite be used for magmatic potassic nepheline syenites characterised by the presence of nepheline plus a single potassium-rich feldspar (orthoclase or microcline) and devoid of exsolution perthite under subsolvus conditions; (3) the metasomatic malignites and juvites of ijolite-carbonatite complexes be referred to as varieties of fenites.

TRANSITION METAL RUTILES AND TITANATES FROM THE DEADHORSE CREEK DIATREME COMPLEX, NORTHWESTERN ONTARIO, CANADA

Abstract The main mineralized zone of the West subcomplex of the Deadhorse Creek diatreme complex, northwestern Ontario possesses an exotic mineralogy. Mineralization involves the first-order transition metals (principally Sc, Ti, V, Cr, Mn, and Fe), the second-order transition metals (principally Zr and Nb), the lanthanides, the actinides (principally Th and U), Be, Ba and Sr. Minerals include phenacite, zircon, uraninite, thorite, monazite-(Ce), xenotime-(Y), barylite, thortveitiite, hollandite, tyuyamunite, a number of unknown and as yet undescribed species, and those minerals more specifically described in this paper. These are Cr-V-Nb rutile, V-rich members of the crichtonitc series, and a titanate of general composition (Cr,V3+,Fe3+)2(Ti,V4+,Nb)O5. Similar to rutiles reported from alkaline rocks in general, the Deadhorse Creek rutiles are enriched in Cr and Nb, with the latter element attaining some of the highest recorded values. V contents are also unusually high and this element is thought to exist in both the tri- and tetravalent states. The V-rich crichtonites are essentially vanadium analogues of crichtonite and lindsleyite. M-site Nb and V are the highest yet recorded. A-site cations are dominated by Ba and Sr with an inverse relationship together with lesser but significant amounts of Ca and Pb. Although not of upper mantle origin, they plot in the upper mantle LIMA quadrant of TiO2 vs. FeO + Fe2O3 + MgO (Haggerty, 1991). (Cr,V3+,Fe3+)2(Ti,V4+,Nb)O5 is thought to be a member of an homologous series of type (Cr,V3+,Fe3+)2p(Ti,V4+,Nb)2p+qO5p+4q with p = 1 and q = 0 and a V3O5 -type structure. Whether this structure is ultimately derived from that of rutile or from α-PbO2 by crystallographic shear is not known. The rutiles and titanates discussed here are thought to have formed from hydrous alkaline solutions which have scavenged the necessary elements from a mafic/ultramafic source. The origin of the solutions is not specifically known although the magmatic activity associated with the spatially related Coldwell alkaline complex and/or the Prairie Lake complex are both potential sources. Both complexes contain the necessary mafic/ultramafic rocks.