Mafic Syenite Research Articles

Lithium isotopes at gold deposits: Insights from the giant Kirkland Lake Gold Deposit, Canada

The Archean world-class Kirkland Lake Au deposit hosts characteristics of intrusion-related- and orogenic-type deposits and the origin of Au in this giant deposit has remained elusive. Here, the Li isotopic composition of hydrothermal alteration is combined with bulk compositions and petrographic observations to shed light on the genesis of the deposit. The Li isotopic composition was measured along a drill core cutting through comagmatic Timiskaming alkali tuff, Kirkland Lake mafic syenite and syenite porphyry, and the crosscutting fault zone (the ’04 Break). A combined increase in δ7Li, Li, Au, and major/trace elements with distance measured along the drill core in the lower alkali tuff was related with increased amounts of hydrothermal K feldspar and resulted from the injection of a magmatic-hydrothermal fluid, conferring δ7Li values of ~ +4‰ to the tuff. Kinetic isotopic fractionation resulted in a smooth decrease of δ7Li values from ~ +4‰ to ~ +2‰ down the Li concentration gradient and it is proposed that Au followed Li as colloidal particles along the interconnected network of fluid pathways. The upper syenite porphyry shows no variation of major/trace elements or mineralogy with drill hole distance but ranges in δ7Li values from ~ +5‰ to ~ +2‰, resulting from magmatic- or metamorphic-hydrothermal alteration and the recrystallization of white mica. A V-shape pattern is observed for δ7Li, Li and Au through the ’04 Break, with a low focused on the central veinlet and values increasing on both sides. Enhanced permeability at the fault centerline allowed the circulation of fluids, which leached Au and Li from the rock, triggered the recrystallization of white mica, and decreased the δ7Li values. Anomalously high δ7Li values of + 14.0‰ and + 16.4‰ were obtained for an auriferous (0.91 ppm Au) quartz-carbonate-pyrite veinlet, where the high δ7Li values are ascribed to the preference for 7Li of quartz. The Li isotopic values presented here are in agreement with previous studies suggesting that the auriferous hydrothermal alteration at Kirkland Lake was sourced from magmatic fluids and agree that metamorphic fluids and groundwaters were also involved in the hydrothermal alteration of the system.

Geology of Pulikonda and Dancherla alkaline complexes, Andhra Pradesh

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively. Dancherla syenite body has yielded a six point Rb-Sr whole rock isochron age of 2211±110 Ma with low initial 87Sr/86Sr ratio of 0.7004±0.00046; MSWD = 5.8.The low initial Sr(i) ratio suggests mantle derived juvenile additions to the crust. The Pulikonda syenite body yielded a six point whole rock isochron age of 1500 ±100 Ma with initial 87Sr/86Sr ratio of 0.7085±0.0047 and MSWD = 2%. The higher initial Sr(i) value for this body suggests metasomatised source. Resetting of Rb-Sr systematics has resulted younger age for Pulikonda syenite though it is supposed to be older as per the geological setting and this anomaly is interpreted interms of repeated deformation and reactivation of shear zone, emplacement of alkali granite into Pulikonda syenite and the role of Rudravaram shear zone (basement tectonics) in the formation of the Nallamali fold belt of the Cuddapah Basin. Alkaline magmatism was initially triggered along microplate margin tectonic setting and later confined to the intraplate tectonic setting.

Características Isotópicas (Nd e Sr), Geoquímicas e Petrográficas da Intrusão Alcalina do Morro de São João: Implicações Geodinâmicas e Sobre a Composição do Manto Sublitosférico

The Morro de São João Alkaline Complex (MSJ) is located in the NE part of Rio de Janeiro State and shows pronounced topography, with the following lithology: coarse-grained felsic syenites (K-feldspar, nepheline, hornblende, titanite and pseudoleucite), and mafic syenites (with K-feldspar, hornblende and piroxene). Petrography and lithogeochemistry data suggest a bimodal suite. Felsic and mafic magma mixing and mingling textures are locally observed indicating the presence of coeval magmas. Nd and Sr isotope compositions correspond to EMI (Enriched Mantle I) mantle reservoir signatures, suggesting an asthenospheric enriched mantle source with low 87Sr/86Sr values (from 0.7049 to 0.7061) and low 143Nd/144Nd (from 0.512361 to 0.512428). The εNd values range from -4.03 to -5.54, indicating an anomalous enriched mantle reservoir, confirmed by the high T DM values between 730 - 830 Ma, very different from the 72 - 56 Ma K-Ar cooling ages. The comparison of Sr and Nd signatures with related hotspot traces (Trindade, St. Helena and Tristan da Cunha) indicates that they are similar to those of Tristan da Cunha, but somewhat different from those of Trindade and St. Helena.

Brasiliano (Pan-African) granitic magmatism in the Pajeú-Paraíba belt, Northeast Brazil: an isotopic and geochronological approach

The Neoproterozoic Brasiliano/Pan-African Orogeny in the Borborema Province (BP) NE Brazil, is characterized by intense granitic magmatism spatially associated with continental scale shear zones and metamorphism under high temperature conditions. U/Pb zircon and Rb–Sr whole rock data for 14 granite intrusions from the Pajeú-Paraíba Belt have ages that suggest more than 100 Ma of intrusive magmatism, which can be divided in four events: the oldest granites (between 620 and 600 Ma) are medium-to-slightly high-K calc-alkaline I-type granitoids, intruded into metagreywackes and gneiss-granites of 0.9–1.0 Ga; they are related to the peak of metamorphism and to the development of a flat lying foliation. The youngest intrusions (540–520 Ma) have geochemical signature of A-type, post-orogenic, extension-related granites, and are associated with sub-volcanic bimodal magmatism, probably contemporaneous with deposition of small sedimentary basins in the North (Iara, Jaibaras graben and Saíri) and Central (Fatima, Betânia and Carnaúbeira) Tectonic Domains of the Borborema Province; they reflect post-tectonic relaxation of the Brasiliano Orogeny. Between these episodes, two other intrusive events were identified: (1) high-K calc-alkaline granitoids and shoshonitic granitoids associated with mafic syenites, meladiorites and hornblende – biotite diorites, intruded between 590 and 581 Ma, associated with a transcurrent deformation event, and (2) alkaline post-collisional granitoids having U/Pb zircon ages of ca. 570 Ma, marking the final stage of the Brasiliano Orogeny. In the South Domain of the Borborema Province, migmatization took place between 610 and 600 Ma. Although similar ages were not found in the granitoids of the Central Tectonic Domain, field evidence suggest that migmatization followed the intrusions of the oldest (620–600 Ma) granitoids.

Geochemistry and Petrogenesis of Kunduru Betta Calc-Alkaline Ring Complex in the Dharwar Craton, Southern India

The Kunduru Betta Ring Complex (KRC), at the southern margin of Dharwar craton, South India, comprises metaluminous sub-solvus syenites and quartz monzonite with a concentric disposition younging towards the center. An outer mafic syenite (of lamprophyric affinity) is followed by porphyritic monzonite, quartz monzodiorite and finally a quartz monzonitic stock at the centre. SiO 2, Al 2O 3 and Na 2O increase from the primitive lamprophyric mafic syenite to the quartz monzonite through the intermediate members, while CaO, MgO, Fe 2O 3 T, TiO 2, P 2O 5 and MnO show an opposite trend suggesting fractionation of hornblende, clinopyroxene, biotite, apatite, sphene, and iron oxide minerals. Rb, Th and U increase with a complementary decrease in Sc, V, Cr, Co, Cu, Sr and Ba from the outer mafic syenite to the inner quartz monzonite. Y, Zr and Hf decrease from lamprophyric mafic syenite to quartz monzodiorite and the trend is reversed in the final quartz monzonite phase. However, the suite is characterised by a compositional gap between quartz monzodiorite and quartz monzonite. Total REE gradually decrease from the mafic syenite to quartz monzonite and the REE distribution patterns show LREE-enriched and HREE-depleted parallel distributions with negligible Eu anomalies. The geochemical data suggest that the rock types were formed as products of progressive differentiation by crystal fractionation of calc-alkaline lamprophyric parent magma which was derived by partial melting of metasomatically enriched mantle in the Kabini lineament. Although the quartz monzonites conform to the trend of differentiated Kunduru Betta suite, the compositional gap between them and the quartz monzodiorite precludes their origin by simple differentiation. It is suggested that convective liquid fractionation might have resulted in the discrete body of quartz monzonite.

Geochronology (U-Pb/Pb-Pb) and isotopic signature (Rb-Sr/Sm-Nd) of the Paleoproterozoic guanambi batholith, southwest Bahia state (Ne Brazil)

The Guanambi batholith (GB) is located in south-western Bahia, Brazil, where it forms part of the Urandi-Paratinga mobile belt. The batholith extends over an area of almost 6,000 km 2 and is mainly composed of syenites and monzonites, with subordinate granites, mafic syenites and lamprophyric dykes. Two main units can be recognized: the multiple intrusions, being divided into four regions (Paratinga, Laguna, Igapora and Guanambi) and the so-called late-intrusions forming the Cara Suja, Cerafma and Estreito massifs. U-Pb and Pb-Pb isotopic data demonstrate that the batholith was emplaced 2.05 Ga ago. GB has (i) highly negative initial a Nd (-7.4 to -10.6), pointing to an enriched source, and (ii) Sr initial ratios between 0.704 and 0.707. Both chemical and isotopic signatures suggest that GB formed by fractional crystallization of a lamprophyric magma derived from a Paleoproterozoic enriched mantle source.

GEOCHRONOLOGY (U-PB/PB-PB) AND ISOTOPIC SIGNATURE (RB-SR/SM-ND) OF THE PALEOPROTEROZOIC GUANAMBI BATHOLITH, SOUTHWEST BAHIA STATE (NE BRAZIL)

The Guanambi batholith (GB) is located in south-western Bahia, Brazil, where it forms part of the Urandi-Paratinga mobile belt. The batholith extends over an area of almost 6,000 km 2 and is mainly composed of syenites and monzonites, with subordinate granites, mafic syenites and lamprophyric dykes. Two main units can be recognized: the multiple intrusions, being divided into four regions (Paratinga, Laguna, Igaporã and Guanambi) and the so-called late-intrusions forming the Cara Suja, Ceraíma and Estreito massifs. U-Pb and Pb-Pb isotopic data demonstrate that the batholith was emplaced 2.05 Ga ago. GB has (i) highly negative initial ε Nd (-7.4 to -10.6), pointing to an enriched source, and (ii) Sr initial ratios between 0.704 and 0.707. Both chemical and isotopic signatures suggest that GB formed by fractional crystallization of a lamprophyric magma derived from a Paleoproterozoic enriched mantle source.

Allard stock, La Plata Mountains, Colorado—an alkaline rock-hosted porphyry copper – precious metal deposit

The Allard stock, which hosts a porphyry Cu–Ag–Au–Pt metals deposit, is an epizonal Laramide pluton at the southwest end of the Colorado mineral belt. The stock and associated mineralization evolved by a complex interaction of igneous and metasomatic processes as follows. Stage 1: forceful emplacement of syenite plutons. Stage 2: intense hydrothermal argillization of stage 1 syenites. Stage 3: intrusion of mafic syenite plutons, some of which are layered, along shear zones. Stage 4: violent escape of volatiles, producing intrusive breccia pipes. Stage 5: intense K-metasomatic alteration of the older stock units with production of a fenitelike alteration halo extending 1000 m beyond the stock and simultaneous intrusion of carbonate-bearing alkali feldspar dikes. Stage 6: emplacement of trachyte, syenite pegmatite, and mafic pegmatite dikes. Stage 7: deposition of metallic minerals, sanidine, quartz, calcite, and fluorite as veinlets and disseminated replacement masses from hydrothermal solutions.The stock shows enrichment of volatiles (CO2, F, and S), high concentrations of Ba and Sr, low concentrations of Rb, high concentrations of Cu, Ag, Au, Bi, Te, Pt, and Pd, with little or no enrichment in Nb, U, Y, and Zr.The Allard stock and its associated mineralization are unlike the calc-alkaline porphyry copper deposits of the western United States and Canada. Although the Allard syenites are silica saturated (quartz-normative), some aspects of Allard alteration and mineralization show affinities with copper and (or) precious metal deposits hosted by alkaline silica-undersaturated (nepheline- or leucite-normative) plutons and carbonatite complexes including the "alkaline suite" porphyry copper deposits of British Columbia, the Palabora, South Africa, copper deposit, and the Cripple Creek, Colorado, gold telluride deposit. Computer-assisted evaluation of major-element analyses of igneous rocks suggests that Allard-type Cu – precious metal deposits may occur in other North American alkaline rock provinces. Epithermal gold telluride deposits in subvolcanic alkaline rocks may grade downward into alkaline porphyry Cu – precious metal deposits.

Upper Mantle and Crustal Fragments in the Ithaca Kimberlites

Xenocrysts and xenoliths from three Ithaca kimberlite localities are consistent with derivation from mantle depths of less than 150 km. Mantle mineral assemblages reconstructed from xenocrysts and rare 2-4 mineral fragments include spinel and garnet peridotites, and garnet pyroxenites. The Cr-rich garnets, pyroxenes, and spinels and the subcalcic diopsides commonly found in mantle xenoliths from other kimberlites are apparently absent. Secondary pargasitic amphibole attributed to mantle metasomatism is found associated with pyrope garnet. Al-rich pyroxenes and spinels suggest that Al-spinel lherzolites of the type commonly found in alkali basalts, but rarely found in kimberlites, occur. Some of the spinels are the most aluminous ever reported from kimberlites. These Al-spinel lherzolites suggest either a fossil high geothermal gradient or a very undepleted mantle in the Ithaca region. Fragments of presumed crustal origin include a suite of Mg-rich minerals which may belong to a granulite carbonate assemblage, small mafic syenite xenoliths, and clinopyroxene and garnet xenocrysts similar to minerals found in basic to intermediate composition granulites. The Ithaca basement is probably an extension of the Grenville metamorphic terrain, and the lower crust may be in the granulite facies. The Ithaca kimberlites are kimberlites on the basis of their whole rock compositions and mineral components, yet they lack the deep mantle fragments found in other kimberlites. The Ithaca kimberlites may have been finally emplaced by explosive eruptions from depths of less than 150 km.

Proterozoic mafic syenites from Baker Lake, N.W.T., Canada

The petrography and major-element chemistry of the Martell Syenite, a Paleohelikian sub-volcanic intrusion, and of some flows of the Christopher Island Formation of the Dubawnt Group, south of Baker Lake, are described. The chemical analyses were carried out using standard X-ray fluorescence techniques. The major-element chemistry of these rocks shows them to be only mildly alkalic. Elemental ratios K 2O/Na 2O, K/Rb, Rb/Ba, and elemental contents of CaO, Rb, Ba, Sr, Zr, U and Th suggest that the syenite has shoshonitic affinities. This is supported by the stratigraphic and structural relationships of the syenite with a volcanic sequence containing mafic trachyte and latite flows. It is concluded that the Martell Syenite is part of a shoshonitic association of volcanic rocks which erupted during the late stages of the Hudsonian Orogeny as the crust thickened and became stable. The chemical characteristics of these rocks suggest that they have been derived by partial melting of an ultramafic source material at mantle pressures. The rising magma may have become enriched in incompatible elements by the process of zone refining. It is suggested that the field relationships and composition of this rock suite indicate that processes similar in kind, but different in degree, to modern plate-tectonic processes were operational from the early Proterozoic.

Petrology of the Older Series lavas from Mauritius, Indian Ocean

The Older Series volcanics of Mauritius form a widely differentiated transitional basalt suite, in which two distinct stages of activity can be recognized. The first stage, shield-building, is composed principally of alternating picrite-basalt flows and agglomerate; this is followed by a second, evolved stage composed of feldsparphyric basalt, hawaiite, mugearite, and high-level trachytic intrusive rocks. Three suites of nodules, each with specific associations, occur in the volcanic rocks; these are probably derived from layered subvolcanic cumulates. The nodular suites are: (a) dunite and wehrlite (restricted to the picrite basalt); (b) Bytownitic anorthosite (found only in the feldsparphyric basalt); and (c) mafic syenite (exclusive to the trachytic intrusive rocks). There is some evidence, from the extremely calcic nature of the plagioclase in the anorthosite nodules and feldsparphyric basalt and from the presence of kaersutite in evolved lava, that a hydrous period of crystallization developed - probably in the later stages of activity. Variation diagrams indicate clearly the close control of phenocryst mineralogy on bulk-lava chemistry, compositions with more than 5 or 6 percent MgO lying along a pronounced olivine -+- clinopyroxene control line and more evolved lava along a trend developed from fractionation of olivine + clinopyroxene + plagioclase + titanomagnetite. Trachytic compositions show pronounced trace-element trends, probably controlled by anorthoclase fractionation, but it can be demonstrated that they are unlikely to represent successive differentiates from a common trachytic magma. A distinct “silica gap” within the series, reflected in the absence of benmoreite, is tentatively ascribed to secondary boiling phenomena.

Timiskaming volcanism in the Kirkland Lake area, Ontario, Canada

The petrography and chemistry of the Timiskaming volcanics of the Kirkland Lake area reveal three distinct yet intimately associated suites within a composite volcanic-sedimentary sequence. The initial phase of Timiskaming volcanism was characterized by calc-alkaline lavas, which are transitional between the later alkaline volcanics of the Timiskaming and the underlying calcic Keewatin greenstones. Flows of alkalic mafic trachytes and a variety of leucitic lavas and pyroclastic rocks are the latest differentiates represented. This occurrence of leucitic volcanics is the first reported anywhere in the Precambrian. Deposition of sedimentary rocks and intrusions of syenite and mafic syenites were contemporaneous with the period of volcanism, but intrusions of feldspar porphyry were post-Timiskaming.The major mineral constituents are augite, biotite, plagioclase, leucite, and their alteration products. Potash feldspar and hornblende occur in subsidiary amounts, and magnetite and apatite are the common accessory minerals. The chemical composition of this volcanic sequence is marked by high alkalies, Al2O3, and low SiO2, P2O5, and TiO2. The calc-alkaline and alkaline lavas are rich in Na2O and poor in K2O and BaO, whereas the leucite lavas are high in K2O and BaO, and comparatively low in Na2O.The Kirkland Lake sequence is compared with similar exotic, but less altered, lavas of more recent age in order to clarify further the nature of Timiskaming volcanism. The alkalic association of the Kirkland Lake area is similar to those occurring at a late stage in the volcanic cycle of modern volcanic arcs and along orogenic belts marginal to major continental masses.

Igneous rocks of the Highwood Mountains, Montana: Part IV. The Stocks

The largest of the stocks of the Highwood Mountains is about 3 miles long. Most of the stocks are made up of shonkinite and mafic syenite. The stock of East Peak is a mafic pseudoleucite-augite syenite. Small bodies of leucite-rich rocks—missourite and niligongite—are present in the Shonkin stock. Leucite malignite makes several small bodies. In the stocks, augite is the predominant mineral, sanidine is the chief felsic mineral, altered nepheline, olivine, and biotite are in moderate amounts, and plagioclase, hornblende, and leucite are rare. The early magmatic sanidine is high in barium. The Highwood Peak stock is made up chiefly of felsic syenite and monzonite.