Lamprophyre Sill Research Articles

1U-Pb SIMS and Ar-Ar geochronology, petrography, mineralogy and gold mineralization of the late Mesozoic Amga alkaline rocks (Aldan shield, Russia)

Late Mesozoic Upper Amga alkaline rocks are located in the Amga tectonic mélange zone within the Aldan-Stanovoy shield, Russia. The main phases of the Upper Amga complex consist of alkaline syenite intrusions as well as the lamprophyre dikes and sills. The mineralogical features indicate that the lamprophyre may belong to minette. Recent geological works revealed the linear and area-specific geochemical gold anomalies in association with late Mesozoic alkaline magmatism in the Upper Amga gold district. The age of the syenite intrusions is 131.4 ± 1.5 (2.9) Ma, and the lamprophyre sills and dikes were formed at 134.9 ± 1.6, 132.3 ± 1.5 and 117.7 ± 3.4 Ma. The mineralization is represented by the REE-Ti-Th, polymetallic (sulfide) and telluride-gold types in the quartz-calcite-chlorite alteration of the main magmatic phases of the alkaline rocks. The fluid inclusions study shows that the sulfides were formed from hydrothermal fluids of CO2-N2-NaCl-H2O composition at temperatures ranging from 390 to 340 °C, with 44–22.7 wt% NaCl-eq, whereas the epithermal telluride-gold mineralization originated from low-temperature CO2 ± N2-NaCl-H2O fluids, with the homogenization temperature of 230–210 °C and a of 9.2–3.3 wt% NaCl-eq.

Petrology and geochemistry of dolerite and lamprophyre sills in Mesozoic successions of Khanozai–Muslim Bagh area, northwestern Pakistan

In Khanozai–Muslim Bagh area, NW Pakistan, the sills are intruded into mainly Triassic–Jurassic successions of Indian platform sediments. They are petrographically identified as dolerite and lamprophyre. The dolerite is a normal dolerite and olivine dolerite and is altered compared with lamprophyre. Lamprophyre is classified as sannaite, camptonite, minette, and damtjernite. The geochemical signature of dolerite, olivine dolerite, and lamprophyre suggests that these rocks belong to alkali series by classification and may be alkaline in nature. Normal mid-oceanic ridge basalts (NMORB)–normalized plots of dolerite, olivine dolerite, and lamprophyre show higher enrichment of large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and marked positive anomalies on Nb which confirm their origin from an enriched mantle source. The ocean island basalt (OIB)–normalized plots of these rocks exhibit patterns almost alike to those of OIB suggesting a source similar to OIB. The tectono-magmatic discrimination plots of dolerite, olivine dolerite, and lamprophyres plot them in the OIB field and indicate that they are alkaline rocks in nature. The petrogenesis and tectonic setting of these rocks suggest that they are OIB in nature and may represent a Late Cretaceous magmatic activity that erupted as hotspot fluid through the crust of Indian Plate during the Late Cretaceous. It is much similar to the other hotspot-related rocks that intruded into the Parh Group and Bela ophiolite melange. Much like the magmatism of Deccan Traps and the Chagos-Laccadive Ridge, these rocks may be the melt of a hotspot possibly Reunion that intruded into the Indian Plate margin when it had passed over it during the Late Cretaceous.

Thermal controls of lamprophyre sill on hydrocarbon generation outlook of shale beds in Raniganj basin, India

The present work highlights the role of lamprophyre sill on the potential of hydrocarbon generation of shale beds. Here, the authors had systematically collected seven shales for detailed investigations of thermal impact provided by lamprophyre sill both along the dip and opposite to the dip directions of the shale beds. Proximate, ultimate, Rock Eval pyrolysis, organo-petrographic, vitrinite reflectance, Fourier Transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and X-ray diffraction (XRD) analyses were performed. Authors have also introduced new parameters like index of aliphaticity (IAL), index of aromaticity (IAR) and index for hydrocarbon generation (IHG) for precise quantification of FTIR spectra of shales, which in turn helpful in indicating the influence of thermal aureole on shale beds. The results indicate more distinct thermal alterations in the heat-affected shales collected along the dip direction of the shale beds owing to more pronounced and extensive thermal impact of lamprophyre sill in the dip direction. The estimated vitrinite reflectance (EVRo) and mean random vitrinite reflectance (VRo) show maximum difference in the heat-affected shales. Low genetic potential (GP), IHG indicate poor potential for heataffected shales along the dip due to intense thermal influence. In contrast, the fairly-affected shales collected opposite to the dip of the bed have higher GP, IHG and hence considered to be excellent source material. The mesopores present in the fairly-affected shales held good amount of hydrocarbons and thus suitable for hydrocarbon exploration and exploitation. The igneous intrusion improves the overall the general maturity of the area although at the expense of the reduction of the hydrocarbon generation potential of the proximal samples.

Petrology of Late Jurassic allochthonous ultramafic lamprophyres within the Batain Nappes, Northeastern Oman

ABSTRACTLate Jurassic ultramafic lamprophyre (UML) sills and dikes occur as 3 km-long intrusions within the allochthonous Whara Formation of the Batain nappes, eastern Oman. The sills and dikes comprise macrocrystic phlogopite and spinel-bearing aillikite and damtjernite. Aillikite is a light grey, massive fine-grained tuffaceous rock with euhedral laths of mica, while damtjernite is a dark grey, medium- to coarse-grained rock with abundant pelletal lapilli and globular segregationary textures. Both lithologies are composed of calcite, phlogopite, apatite, magnetite, spinel, diopside, and richterite. Orthoclase occurs only within damtjernite. The rocks are strongly silica undersaturated (17.6–33.7 wt.% SiO2), with low MgO (4.7–10.2 wt. %) and high Al2O3 (3.5–8.6 wt.%). The aillikites are distinguished from the damtjernites by their lower SiO2, Al2O3, and Na2O abundances, and their higher MgO, CaO, and P2O5 contents. The rare earth element (REE) patterns of both rock types are similar and show strong light REE (LREE) enrichment. Both are enriched in Ba, Th, U, Nb, and Ta, with normalized concentrations of up to 1000 times those of primitive mantle. Relative depletions are apparent for high REE (HREE), K, Rb, Pb, Sr, P, Zr, and Hf. The rocks have initial 87Sr/86Sr ratios of 0.70435–0.70646, whereas initial 143Nd/144Nd ratios vary between 0 · 512603 and 0 · 512716 (εNdi 2.6–3.2). Pb isotopic ratios are more varied among the aillikites and damtjernites: 208Pb/204Pbi = 38.97–39.39 and 207Pb/204Pbi = 15.35–15.58, 206Pb/204Pbi = 18.08–18.96. The abundance of phlogopite, apatite, and rutile and enrichment in LREEs, Ba, Th, U, Nb, and Ta in the Sal UMLs suggest metasomatic enrichment of these rocks following a low degree of partial melting of a depleted source region. Ar–Ar age dating of phlogopite macrocrysts from the aillikites and damtjernites (154 and162 Ma, respectively) correlates with large-scale tectonic events recorded in the proto-Indian Ocean at 140–160 Ma.

An occurrence of coked bitumen, Raton Formation, Purgatoire River Valley, Colorado, U.S.A.

Numerous examples of coke produced by igneous intrusion into coal have been reported in the Spanish Peaks region of south central Colorado. However, in a recent study of an intruded section of the Raton Formation (Upper Cretaceous-Paleocene) along the Purgatoire River near Medina Plaza, CO, coked bitumen has been observed. This material occurs in “fingers” (hexagonally jointed bodies) in a shaley xenolith within a lamprophyre sill and in carbonaceous Type III shale directly below the sill. The coke fingers are characterized by a remarkable flow mosaic texture, high vitrinite/coke reflectance (average random reflectance between 8% and 9%, but with maximum readings around 14–15%), high anisotropy, abundant devolatilization vacuoles, and an absence of inertinite inclusions. Within the underlying shales, the coked bitumen occurs as pore, void, and fracture linings and fillings. Geochemically, the coke in the fingers has low S1 and S2 values (<0.2 and <3mg HC/g, respectively), and low HI and OI values (<6mg HC/g TOC and <3mg CO2/g TOC, respectively). Within the fingers, there is evidence for multiple stages of accumulation, including coarse-grained circular or ribbon coke frequently containing pores edged by vapor-deposited carbon (pyrolytic carbon), layers of pyrolytic carbon, clusters of spherulitic pyrolytic carbon, and layers of highly porous coke. This coked bitumen is quite different from coked coal from the same locality. The coked coal has a medium-grained circular mosaic texture that is consistent with the high volatile bituminous rank of unaltered coal in the area. The coal-derived coke has similar devolatilization vacuoles but also has numerous inclusions of inertinite macerals such as fusinite and secretinite. A previously reported coal “dike” that had flowed through a sill in the same area also showed circular mosaic texture.These observations suggest that the coke found in this study was not formed by the direct coking of coal, but from a mobile phase (bitumen or pitch) that was subsequently coked by the intrusion. This bitumen was either derived from the abundant coal within the section above the sill, or from the organic matter contained within the Type III carbonaceous shales, or both. Unaltered Type III shale sampled at this site has a HI of ~350mg HC/g TOC suggesting some, albeit limited, capacity to generate petroleum. We suggest that bitumen was generated prior to or during the time of intrusion and was subsequently coked by the high temperatures of the emplaced sill. As the bitumen pooled adjacent to the sill, rapid heating led to the development of mesophase, resulting in a highly anisotropic ribbon mosaic texture on cooling, and vaporization of gases that subsequently condensed along pores, vacuoles, and fractures as pyrolytic carbon and spherulitic pyrolytic carbon. Bitumen within the underlying silty shales was also coked, but to a lesser degree. The texture of this coked bitumen looks very much like commercially produced petroleum coke. This coked bitumen differs from other reports of coked bitumen in its mode of occurrence (coke fingers in a xenolith), ribbon mosaic structure, and extremely high anisotropy and reflectance.

Clinopyroxene and amphibole crystal populations in a lamprophyre sill from the Catalonian Coastal Ranges (NE Spain): A record of magma history and a window to mineral-melt partitioning

A Cretaceous sill of alkaline lamprophyre (camptonite) cropping out in the northern Catalonian Coastal Ranges includes macrocrysts and microcrysts of clinopyroxene and amphibole with complex zoning patterns. The in-depth petrographic, compositional and barometric study of the different crystal populations, including statistical evaluation of the compositional dataset by principal component analysis, provides insights into the magmatic plumbing system. Macrocryst cores are inherited antecrysts crystallised in a deep (500–800MPa) magma chamber or conduit. Their reverse zoning patterns reveal repeated injection and mixing of batches of a more primitive and water-rich magma with the resident magma in the magma chamber. Macrocryst rims and microcrysts, in contrast, crystallised during the ascent and shallow emplacement of the magma below 50MPa pressure. They define normal zoning patterns that can be related to progressive fractionation of the magma. This study shows that an apparently simple porphyritic rock can actually contain a significant amount of recycled crystals, reflecting an open-system magmatic behaviour. Only the groundmass of the sill represents a closed-system, where microphenocryst cores are in chemical equilibrium with the bulk composition of the groundmass. Using trace element compositions obtained by LA–ICP–MS and geochemical modelling, a complete and consistent partition coefficient dataset is obtained for the first time for clinopyroxene and amphibole in camptonite melts. Both minerals show a similar partitioning behaviour for most trace elements. Clinopyroxene-melt and amphibole-melt partition coefficients are overall consistent with data from basic rocks, although clinopyroxene partition coefficients are generally higher than data from basalt and basanite melts. The results from this research support the use of naturally occurring mineral-melt equilibrium pairs for partitioning studies but emphasise the need for a detailed study of the phases used to define the partition relationships.

The influence of crystal settling on the compositional zoning of a thin lamprophyre sill: A multi-method approach

We have studied a visibly zoned, thin (< 0.5 m) lamprophyre sill that crops out in the Catalonian Coastal Ranges (NE Spain). The sill is a camptonite composed of large abundant crystals, mainly of clinopyroxene and amphibole, set in a fine-grained groundmass. The mineral chemistry of the different crystal populations indicates that the large crystals are inherited antecrysts incorporated into the magma before emplacement. The major and trace element whole rock profiles are S-shaped, with the development of a marginal reversal in the lower chilled margin. These profiles cannot be explained by normal fractionation of the magma inwards. Instead, the whole rock zoning is controlled by the presence of antecrysts. This is proven: 1) quantitatively, through a trace element model which evaluates the contribution of the antecrysts to the overall composition of the rock, and 2) statistically, through a principal component analysis on the complete trace element data set. The mineral and groundmass compositions show rectilinear compositional profiles, indicating that the magma was emplaced in a single pulse. The accumulation of the antecrysts towards the bottom of the sill, together with the calculation of settling velocities for clinopyroxene and amphibole and cooling velocities for the magma, indicate that the settling of antecrysts during cooling is responsible for the varying proportions of antecrysts and therefore for the whole rock compositional zoning. This study proves that crystal settling is a significant process in triggering compositional zoning of igneous intrusions even at the cm-scale, provided that the magma carries large crystals upon emplacement.

Late Cretaceous alkaline sills of the south Tethyan suture zone, Pakistan: Initial melts of the Réunion hotspot?

During the closure of the Tethyan Ocean, Triassic to early Jurassic marine sediments containing alkali dolerite and ultramafic lamprophyre sills were accreted to the margin of Greater India. The 69.7 ± 0.2 Ma weighted mean 40Ar- 39Ar plateau age of the sills, their location, and their incompatible element and Nd–Pb–Sr isotopic composition, which is similar to that of recent Réunion shield lavas, are consistent with their derivation from the Réunion hotspot. Mantle melt modelling suggests that the alkali dolerites are the result of < 3% melting of a moderately incompatible element-depleted, garnet-bearing source. However, some of the ultramafic lamprophyres have lower heavy rare-earth element, Nb and Zr contents that cannot be explained by melting of the same mantle source as the alkali dolerites. This unusual feature can be explained by small-degree melting of a metasomatised mantle source containing small amounts (< 0.02 vol.%) of residual zircon. Isotopic signatures of the ultramafic lamprophyres are similar to those of the alkali dolerites, implying that metasomatism was not significantly older than the melting event that formed the sills. The metasomatism may have been due to small-degree melting and re-solidification at the leading edge of the starting head of the Réunion plume. This alkaline magmatism may represent the arrival of the Réunion plume at the base of the Indian lithosphere several million years before the onset of the main phase of Deccan flood basalt volcanism.

Petrology of the ultramafic lamprophyre and associated rocks at Coral Rapids, Abitibi River, Ontario

An ultramafic lamprophyre sill and dikes, and an olivine–melilite-rich dike rock intrude Lower to Middle Devonian sediments and low- to high-grade Archean metamorphic rocks on the west bank of the Abitibi River, Coral Rapids, Ontario. Although previously considered to be kimberlitic, all these rocks contain olivine + clinopyroxene + phlogopite ± melilite, and hence are ultramafic alkaline rocks. The ultramafic lamprophyre can be distinguished from the dike by its lower SiO2, Na2O, Al2O3, and higher MgO and FeO. In contrast the olivine–melilite dike rock has a more uniform composition, characteristically contains melilite, and has higher Cr and Ni contents. Enriched light rare earth element (LREE) chondrite-normalized patterns are similar for all rocks.Olivine, clinopyroxene, and phlogopite have Mg# (Mg# = 100 Mg/(Mg + Fe) mol) typical of minerals in primitive alkaline rocks. Melilite composition is similar to that of igneous melilites. Phlogopites in all rock types are enriched in Ba and F and the degree of enrichment is distinct for each rock type. Accessory minerals include apatite, carbonates, chlorite, sericite, and sodalite (only in the olivine–melilite-bearing rock).The mineralogy and chemistry of the Coral Rapids rocks suggest that they are derived from a primitive olivine melilitite magma that may have evolved by fractionation of small amounts of olivine and clinopyroxene to form these alkaline ultramafic magmas.Xenoliths in the ultramafic lamprophyre sill and in lesser abundance in the olivine–melilite dike rock include olivine, phlogopite, and clinopyroxene-rich mantle-derived assemblages. The similarity between these xenoliths and their host rocks at Coral Rapids and those from southwest Uganda and West Eifel, Germany, suggests that the Coral Rapids rocks may be derived from magmas that originated from metasomatized mantle sources.

High-Viscosity ‘Conglomerate’ Channel Deposits in Tertiary Lamprophyre Sill, Sacramento Mountains, New Mexico

Three “conglomerate” channels are exposed in an approximately 12-m-thick (40 ft) Tertiary (44.2 ± 2.2 m.y.) lamprophyre sill in the northern Sacramento Mountains. The sill is intruded into the Pennsylvanian Gobbler Formation as two intrusions, the conglomerate occurring in troughs within the lenticular upper sill. The lamprophyre contains abundant fine phenocryst (0.25 to 8 mm) of hornblende, augite, and plagioclase (An40–45) in a groundmass of plagioclase (An35–40), chlorite, magnetite, and minor orthoclase. The bifurcating lenticular conglomerate is ca. 90 to 210 m (300 to 700 ft) wide and scours up to 5 m (15 ft) into the lower sill. The conglomerate has a chaotic and often nongrain-supported texture of poorly sorted xenoliths and phenocrysts in a groundmass of lamprophyre. The xenoliths consist of fragments of Precambrian schist, gneiss, and granite, Paleozoic sedimentary rock, and clasts eroded from the lower sill; they range in size from less than 1 cm to 0.5 m, with a mean size of 6 to 8 cm. The conglomerate channels are the result of local erosion along the top of the lower sill by a dense xenolith-laden magma current during intrusion of the upper sill. The chaotic texture of the conglomerates and the lack of any axial migration of the xenoliths and phenocrysts indicate transportation and deposition were by a highly viscous flow similar to a pebbly mudstone deposit.

Coal dikes that intrude lamprophyre sills; purgatoire river valley, Colorado

Fluidization of coal by heat from adjacent lamprophyric sills, injection by lithostatic pressure into fractures in partially cooled sills, metamorphism of dike subsequent to injection

Petrological characteristics of barakar coal seams, metamorphosed by lamprophyre sill in the Jharia coalfield, Bihar

In the Jharia Coalfield, Bihar, dykes and sills of mica lamprophyre are common and have thermally metamorphosed the coal seams in contact into natural coke. Petrological study of the metamorphosed zones of the coal seams in the south-eastern extremity of the coalfield has been carried out. Four petrologically distinct zones designated as natural coke, coked coal, affected coal and unaffected coal are traced, when studied away from the contact. Chemically, a marked increase of volatiles and decrease of ash is recorded away from the seam-igneous contact. Phosphorus content is quite high in the natural coke ash.

Petrological characteristics of Barakar coal, metamorphosed by lamprophyre sill, in Jharia coalfield, India

Four petrologically distinct transitional zones are recognized in the metamorphosed Barakar coal seams (Permian)--natural coke, coked coal, affected coal, and unaffected coal. Distance from the contact to the unaffected coal is similar to the thickness of the sill. Metamorphism obliterated the volatile-rich organic layers, destroying the banding and forming vacuoles. Inorganic matter, principally apatite and calcite, was introduced from the sills.