Carbonatite Dikes Research Articles

Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior

The Precambrian rocks of the Slate Islands exhibit shock features that have been attributed to meteorite impact. The islands occur at the intersection of two major regional faults, one of which controls the location of late Precambrian alkalic magmatism. Alkalic intrusive events north of Lake Superior occurred on the Slate Islands. These alkalic intrusions are represented by a carbonatite dike in the southeast corner of the island and a set of alkalic diabase dikes exposed at several locations on the island. Diatreme breccias cut these alkalic rocks and are interpreted to be late-stage phases of this volatile-rich alkalic magmatism. Shatter cones appear spatially related to the diatremes, and quartz grains displaying deformation lamellae are present in the matrix of the breccias. Shatter-cone structures and deformation lamellae are considered to be indicative of shock events. On the basis of field observations and data, the shock and diatreme events can be correlated and related to volcanism or alkalic magmatic processes associated with major regional fractures and not to meteorite impact.

Mineralogical and petrological notes on the McCloskey's Field carbonatite dikes, Ottawa, Canda.

Mineralogical and petrological notes on the McCloskey's Field carbonatite dikes, Ottawa, Canda.

Sulphur isotope geochemistry of carbonatites

Sulphur isotopic data for sulphides and barite from several carbonatites (Mountain Pass, Oka, Magnet Cove, Bearpaw Mountains, Phalabora) show that individual carbonatites have different mean sulphide or barite isotopic compositions which deviate from the meteoritic mean δ 34S(0‰) . Classification of carbonatites in terms of T,ƒO 2 and pH during formation of the sulphur-bearing assemblages indicates that with decreasing T and increasing relative ƒO 2 the mean δ 34 S sulphide becomes increasing negative relative to the mean magma δ 34 S. Only barite-free high temperature carbonatites (Phalabora) in which the mean δ 34 S sulphide approaches the mean magma δ 34 S as a consequence of the paucity of oxidized anionic sulphur species in the magma can be used to directly estimate the mean isotopic composition of the source material. Barites from the Mountain Pass carbonatite show an increase in δ 34 S with sequence of intrusion of the carbonatite units; dolomitic carbonatite (mean δ 34 S, + 5.4‰), calcitic carbonatite (+ 4.8%.), silicified carbonatite (+ 6.9‰), tabular carbonatite dikes (+ 8.7‰), mineralized shear zones (+ 9.5‰). Within each of these units a spread of 6.8%. is evident. Isotopic trends in this low temperature (300°C) carbonatite are evaluated by treating the system as a hydrothermal fluid. The observed isotopic variations can be explained by removal of large amounts of sulphur from a fluid whose mean δ 34 S is 0 to + 1‰

Phlogopitization of pyroxenite; its bearing on the composition of carbonatite magmas

SummaryPhlogopitization of pyroxenite is common in contact zones between clinopyroxenites and carbonatite dikes of the Cargill ultramafic rock—carbonatite complex near Kapuskasing, Ontario. The most typical development is a mica zone 1–10 cm wide but phlogopite is also developed in a more pervasive manner throughout the groundmass of several types of ultramafic rock. Fenitization is most commonly thought of as a process whereby aegirine and riebeckitic amphiboles are formed in the host rock while feldspar is recrystallized and silica progressively removed. Phlogopitization of pyroxenite can properly be referred to, however, as a type of fenitization. It is clearly related to the intrusion of carbonatite into pyroxenite and is further testimony to the fact that many carbonatite magmas are initially alkalic but lose alkalies to the surrounding rocks and crystallize as calcitic and dolomitic carbonatite with alkali contents restricted to the amounts that could be fixed as micas, pyroxenes or amphiboles. This in turn is controlled by the silica and alumina activity of the carbonatite magma. Abundant evidence for considerable amounts of fluorine in carbonatite magmas suggests that alkalies may be transported into the country rocks as fluorides. It is further suggested that late-stage feldspathization in carbonatite complexes is explained by the abstraction of potassic halide solutions from the crystallizing carbonatite magma. The conclusion seems inescapable that alkali carbonatite magmas, far from being the curiosity thought by many petrologists, are in fact very common during the evolutionary history of carbonatites. The common calcitic and dolomitic carbonatites have not generally crystallized from a magma of the same composition but are the residue remaining after the abstraction of an alkali-rich aqueous fluid. Consequently, there is a need to redesign the experimental phase equilibrium approach to problems of carbonatite genesis in order to take account of the presence of alkalies in most carbonatite magmas.

Thorium Veins in the United States

Thorium-bearing veins present the largest known potential resource of thorium in the United States in deposits that can be easily exploitable in the near future. Veins of this type are principally of quartz, feldspar, and iron oxides and are known from at least 13 widely scattered areas: (1) Lemhi Pass, Montana-Idaho; (2) Diamond Creek, Idaho; (3) Hall Mountain, Idaho; (4) Wet Mountains, Colorado; (5) Powderhorn, Colorado; (6) Laughlin Peak, New Mexico; (7) Capitan Mountains, New Mexico; (8) Gold Hill district, New Mexico; (9) Quartzite district, Arizona; (10) Cottonwood area, Arizona; (11) Monroe Canyon, Utah; (12) Mountain Pass, California; and (13) Wausau, Wisconsin. Veins in these areas range in length from a few feet to at least 5,000 feet and in thickness from thin seams to 50 feet. Many are 200 to 600 feet long and 0.5 to 3.0 feet thick. The veins occupy shear zones and the shearing occurred before, during, and after vein formation. Fracturing in hard, brittle rocks appears unrelated to either the contacts of units or the earlier geologic structures. Fifty-one different minerals have been identified in the veins in the 13 areas. Thorite is by far the most abundant thorium mineral. Monazite is also commonly present and in a few veins there is brockite and allanite. The rare-earth minerals, bastnaesite, xenotime, cenosite, florencite, and synchisite, are all extremely rare except in the veins at Mountain Pass, which contain substantial amounts of bastnaesite. Quartz is the most abundant gangue mineral and is commonly accompanied by microcline. Calcite is common in some areas. Other gangue minerals that are commonly found in moderate to minor amounts in some areas include muscovite, biotite, chlorite, barite, apatite, and fluorite. Iron oxide minerals, especially limonite and hematite, are characteristic of these veins and they commonly stain most of the rock. Over 20 other accessory and secondary minerals of which pyrite and rutile are the most common have been found. The thorium content of samples from various veins ranges from 0.001 to 21.0 percent ThO 2 . The rare-earth content of the veins is substantial and, depending on demand, may be economically as important as the thorium. Although rare earths commonly are present in the same minerals as thorium, there may be little correlation between thorium and rare-earth content in a sample.In eight of the 13 areas thorium veins are associated with alkalic rocks or carbonatites, or both. Carbonatite dikes are generally found near the center of an alkalic rock intrusion whereas thorium veins are at greater distances from the center. The thorium veins are believed to have been derived from a volatile late-stage phase of the magma that formed the alkalic rocks. The low-viscosity fluids traveled long distances in major fractures and slowly formed veins at low temperature, generally in minor shears.

Carbon and oxygen isotopic composition of carbonatite dikes and metamorphic country rock of the Haast Schist terrain, New Zealand

A lamprophyre dike swarm intruding the Haast Schist terrain of Southern New Zealand includes Ba-, Sr-, and REE-rich carbonatites with carbon and oxygen isotopic compositions quite distinct from those of the country rock. In the light of more recent literature data it is argued that the field of carbon and oxygen isotopic compositions defined by Taylor et al. (1967) as pertaining to “primary igneous carbonatites” (PIC), is still relevant. The Haast carbonatite isotopic compositions are readily derived from the PIC field, if assimilation of and isotopic exchange with the known country rock by the magma is allowed for. The Haast Schist terrain is very poor in carbonate rocks and may directly overlie oceanic crust. Hence δC13 values typical of lower crust or upper mantle sources are preserved in the Haast carbonatites, and only δO18 has been modified during intrusion.

Carbonatite-kimberlite relations in the Cane Valley Diatreme, San Juan County, Utah

The Cane Valley diatreme is a kidney-shaped structural depression about 1.1km long that contains small composite kimberlite and carbonatite dikes. It is unquestionably related to nearby larger kimberlite-bearing pipes and dikes and may represent an early stage of diatreme development. Heavy minerals concentrated from the Cane Valley carbonatite include chrome-rich pyrope and chrome-rich chlorite. The overall suite of minerals is similar to the kimberlite at Moses Rock dike. Microprobe data on carbonatite and kimberlite minerals indicate (1) the layer silicate minerals in Cane Valley carbonatite are very similar to those in Moses Rock kimberlite and (2) Al/Cr and Cr/Mg ratios of the layer silicate minerals are very similar to chrome- and aluminum-rich diopsides observed in kimberlite and spinel-peridotite (lherzolite) xenoliths at Moses Rock dike. We suggest that chlorite and phlogopite originate by reaction of diopside in mantle rock with volatiles; carbonatite is produced in the reaction. Carbonate inclusions in pyropes from the nearby Garnet Ridge and Red Mesa diatremes, reported here, suggest the existence of mantle carbonate. The kimberlite and carbonatite are believed to be emplaced directly from the upper mantle as a system consisting of a single supercritical volatile-rich phase with suspended phlogopite, chlorite, and xenocrysts of comminuted mantle rock (e.g., olivine, diopside, orthopyroxene, spinel, pyrope). The transport of such a system to the surface is believed to occur over a short time, of the order of 1 hour. Significant cooling occurs only as the system approaches the surface. Carbonate would crystallize from such an erupting medium as its temperature fell below about 630°C; this might occur on chilled margins or in the main flow at about 1 km depth approximately at the present erosional level of the Cane Valley diatreme. Kimberlite, carbonatite, and minette (alkali-lamprophyre) were all emplaced during the mid-Tertiary, possibly directly from the upper mantle. The process probably reflects localization or liberation of volatiles in the upper mantle at that time. We believe the presence of volatiles may have played a role in the other major Tertiary events of the province.

Carbonatites and Fenitization Associated with a Lamprophyric Dike-Swarm Intrusive into Schists of the New Zealand Geosyncline

At Haast River, South Westland, a lamprophyric dike swarm with carbonatitic affinities intrudes schists of the New Zealand Geosyncline. The dike swarm comprises a series of consanguineous members ranging in composition from ultramafic hornblende-mica peridotite, through camptonitic lamprophyre and mafic sodalite-nepheline microsyenite, to sodalite-tinguaite, potassic trachyte, and carbonatite. One carbonatite dike, 1.25 m thick, consists predominantly of the Ba-Mg carbonate, norsethite (Ba 0.96 Mg 0.79 Fe 0.14 Sr 0.02 Mn 0.03 Ca 0.01 (CO 3 ) 2.02 ) with subordinate magnesian siderite, aegirine, albite, pyrite, sphalerite, galena, and monazite. Other carbonatites contain siderite, norsethite-ankerite, and calcite-dolomite-barite assemblages. Metasomatic alteration of the country-rock schist, likened to fenitization, is most pronounced at the margins of alkalic and carbonatite dikes. It involves the introduction of Na 2 O, CO 2 , S, Nb, and Th into the country rock, with possible depletion in K 2 O. A progressive fenitization sequence from quartzofeldspathic schist to aegirine-crossite-albite schist is petrographically described and illustrated. An analysisis presented of crossite, typical of the strongly pleochroic blue amphibole produced during fenitization. The Haast River dike swarm intruded a geosynclinal terrain in immediately postorogenic times, postdating widespread metamorphism and intensive deformation of the Rangitata orogeny by at most about 10 m.y. Similarities between the Haast River occurrence and other carbonatite complexes from geosynclinal areas are briefly discussed.

Dikes of the McClure mountain — Iron mountain alkalic complex, Fremont county, Colorado, U.S.A.

The McClure Mountain — Iron Mountain alkalic complex, the largest of three Precambrian alkalic complexes in south-central Colorado, is an irregularly ovoid body about 8×6.5 kilometers in plan. It contains major petrologic units (in age sequence): 1) peridotite and ilmenite-magnetite lenses, 2) gabbro, 3) hornblende syenite, 4) ijolite and 5) nepheline syenite, whose internal arrangement is irregular. Dikes are numerous and varied both within the complex and in the surrounding Precambrian metasediments and meta-volcanics (Idaho Springs formation), granitoids and migmatites to a distance of 27 kilometers from the complex. Fenitization has only locally affected these wall rocks at their contacts with the complex. Dikes within the complex are chiefly light colored and of syenitic or nepheline syenitic composition. Texturally they range from aphanitic, through porphyritic with an aphanitic to fine-grained matrix, to uniformly medium-grained. Most of the intracomplex dikes (save the few carbonatites) are represented by compositionally equivalent major units within the complex. In contrast, the extracomplex dikes are chiefly lamprophyres and carbonatites, none of which has a compositionally similar unit within the complex. The carbonatite dikes are younger than the lamprophyres; where the two types co-occupy the same fracture, the lamprophyre has been carbonatized. The lamprophyres are represented by an extraordinary number of textural varieties, involving phenocrysts and phenocryst combinations of augite, barkevikite, olivine and plagioclase in varying matrix combinations of olivine, augite, barkevikite, biotite, magnetite, apatite, plagioclase and orthoclase. Many of the dikes can be classed as camptonites or olivine camptonites. The Goldie lamprophyres contain local marginal phases spectacularly marked by concentrically zoned ovoids. The Cabin dike is studded by megaphenocrysts of augite. The carbonatites are of three main types: Many carbonatites and Th-RE veins emit, when broken, a fetid gas consisting of a mixture of fluorinated hydrocarbons of the C5 and C6 types along with F2, HF, and F2O. The fluorine has been derived from structurally degraded radioactive fluorite; the hydrocarbon gases appear to represent primary inclusions. Within the dike halo are two small breccia pipes consisting of fenitized, locally derived Precambrian gneiss blocks with minor fragments of transported biotitized lamprophyre in a subordinate calcitic matrix also containing aegirine, crocidolite, potash feldspar and hematite. The abundance of the lamprophyres, the great width of the dike halo, the lack of ring structure within the complex, the restriction of carbonatite to dikes, and the subordinate fenitization effects all combine to suggest that the McClure Mountain — Iron Mountain complex is now exposed at a relatively deep niveau, possibly near the mesozone — catazone boundary.

Carbonatite and related alkalic rocks at Powderhorn, Colorado

The pear-shaped outcrop of the alkalic rock complex at Powderhorn, Gunnison County, covers approximately 12 square miles. About 70 percent is underlain by pyroxenite which is intruded and/or replaced by concentric suites of nepheline and melilite-bearing rocks, magnetite-perovskite concentrations, biotite-rich zones, and by carbonatite. The central carbonatite occupies an area of about two and one-half square miles; radial and concentric carbonatite dikes are spatially related to it. The granitic country rock is partially fenitized in proximity to the complex. The complex is located within a region characterized by the presence of thorium-bearing veins. The central carbonatite constitutes a major reserve of columbium contained in the mineral pyrochlore. Columbium is also contained in the magnetite-perovskite concentrations in the pyroxenite, and the mineral columbite has been identified in thorium veins close to the complex. In addition, a substantial reserve of titanium is contained in the pyroxenite, principally in the minerals perovskite and sphene. Significant quantities of rare earth elements occur mostly in the apatite of the carbonatite and in the magnetite-perovskite rocks. Drilling indicates that the pyroxenite is conical in shape and that the carbonatite was asymmetrically emplaced along the inclined pyroxenite-country rock contact. Faulting has exposed two structural levels within the complex, the upper characterized by magnetite-perovskite concentrations, and the lower by melilite-bearing rocks and carbonatite.