Aplite Dikes Research Articles

Magmatic evolution and sources of metals and fluids in the large granite-related Xiasai vein-type Ag–Pb–Zn(–Sn) deposit, northern Yidun terrane, SW China

The Xiasai Ag–Pb–Zn–(Sn) deposit in the northern Yidun terrane (eastern Tethys), hosted in Late Triassic metasandstones, is genetically associated with Late Cretaceous granites. We present chemical and Sr–Nd–Pb–Li–B isotope data of Late Cretaceous granites and their mafic microgranular enclaves (MME), late aplites, and Late Triassic metasedimentary rocks. The narrow range of 87Sr/86Sr99 (0.70627–0.70953) and εNd99 (–6.1 to –5.2) values of most Late Cretaceous granites reflects a mixed source with contributions from the mantle and Paleoproterozoic felsic crust. These granites have high SiO2 contents (>69 wt%) and strong depletions in Sr, Ba, and Eu. Major and trace element contents correlate with the fractionation index 1/TiO2, indicating that they experienced extensive fractionation. Local late aplite dikes were derived from similar source rocks, but seem to be younger.The Pb isotopic compositions reflect that the metal Pb was derived from the granite with variable contributions from the Late Triassic sedimentary rocks. Weakly and moderately altered metasandstones have similar δ7Li and δ11B values as most granites (δ7Li: 0.00 to 2.68 ‰; δ11B values: –14.93 to –12.36 ‰), indicating magmatic fluid sources. The magmatic fluids resulted in the initial enrichment of Sn, Cu, Pb, and Zn. Strong alteration of feldspar and biotite resulted in the metasandstones in negligible change of Li concentrations, significant loss of Sr, Na2O, CaO, MgO, and Fe2O3, but significant addition of B. This alteration lowered δ7Li from 1.01 to –3.65 ‰ and lowered δ11B from –9.43 to –19.18 ‰. The low δ7Li values and extremely low δ11B values reflect external fluids from the Late Triassic sedimentary wall rocks, which interacted with granites and mobilized metals (e.g., Sn, Ag, Pb, and Zn) from the granite and/or the sedimentary wall rocks. The location of the Xiasai deposit is controlled by regional NNW-trending faults induced by Late Cretaceous extension.

Bureya−Jiamusi−Khanka superterrane linked to the Kuunga-Pinjarra interior orogen of East Gondwana and its drift toward Northeast Asia

There is general agreement that a series of East Asian blocks has always lain outboard of both India and Australia along the North Indo−Australie peripheral orogen. However, whether the East Asian blocks were involved in the interior orogens of East Gondwana remains equivocal. The geochronology and geochemistry of Neoproterozoic−Late Triassic rocks in the Russian Far East, together with existing paleontological and detrital zircon data, offer an opportunity to determine the tectonic origin and drift history of the Bureya−Jiamusi−Khanka superterrane. Biotite and amphibole 40Ar/39Ar dating results define a distinctive episode of Late Pan-African (ca. 550 Ma) metamorphism and a local Late Triassic (ca. 219−200 Ma) episode of deformation for the Bureya−Jiamusi−Khanka superterrane. Zircon U−Pb ages and whole-rock geochemical data indicate that the Early Ordovician (483 ± 3 Ma) highly fractionated I-type monzogranites were emplaced in a post-collisional setting linked to the collapse of a Late Pan-African orogen, while the Late Triassic (ca. 234−223 Ma) A-type quartz syenites and I-type granite aplite dikes were formed in a slab-pull−induced passive continental margin of the subducting Mudanjiang oceanic plate. These crucial archives, complemented by data from the literature, reveal that the Bureya−Jiamusi−Khanka superterrane made up the northernmost Kuunga-Pinjarra interior orogen during the final assembly of East Gondwana. As a result of Devonian rifting after Early Ordovician orogen collapse, the Bureya−Jiamusi−Khanka superterrane escaped from the Kuunga-Pinjarra interior orogen and subsequently migrated to Northeast Asia by the Late Triassic to Jurassic due to the subduction and closure of the Paleo-Tethys and Paleo-Pacific oceans.

Titanite geochemistry and textures: Implications for magmatic and post-magmatic processes in the Notch Peak and Little Cottonwood granitic intrusions, Utah

Abstract Textural and compositional variations in titanite constrain the roles of magma mixing and hydrothermal alteration in two plutons in central Utah: the Jurassic Notch Peak and the Oligocene Little Cottonwood stocks. In the Notch Peak intrusion, magmatic titanite grains usually have oscillatory zones combined with BSE-bright sector zones, in some cases surrounding simple unzoned cores. These grains are frequently overprinted by hydrothermal titanite with low concentrations of high field strength elements (HFSE). Magmatic titanite has an average δ18O of 6.0‰ and post-magmatic titanite is 6.2‰, as analyzed by SIMS. Average Zr-in-titanite temperatures are also similar, with 718 °C for magmatic and 711 °C for hydrothermal titanite. These observations indicate simple magmatic growth, followed by hydrothermal alteration by magmatic fluids. Titanite in aplite dikes and sills has lower concentrations of all trace elements except F. Many titanite grains in the aplites have late overgrowths of high-Fe titanite. This high-Fe titanite has δ18O of 6‰ and an average Zr-in-titanite temperature of 718 °C and likely precipitated from a last flush of exsolved magmatic water enriched in Cl and Fe. Titanite in the Little Cottonwood stock typically has distinct patchy cores with rounded and embayed ilmenite inclusions. Mafic enclaves have abundant titanite that is similar in texture and δ18O (5.1‰) to titanite in the host (δ18O = 4.9‰), but it has a slightly higher average Zr-in-titanite temperature (731 vs. 717 °C). The patchy cores in the enclaves have the highest average Zr-in-titanite temperature (759 °C) and distinctive REE patterns. The textural and compositional data indicate that a hotter, more reduced, ilmenite-bearing mafic magma mixed into an oxidized felsic magma, destabilizing existing ilmenite and allowing crystallization of titanite. In the granodiorite and in the enclaves, hydrothermal growth of titanite is evidenced by distinct narrow rims as well as anhedral titanite that grew between sheets of chloritized biotite. Secondary hydrothermal titanite typically has lower concentrations of most HFSE, but is relatively enriched in F, Mg, Mo, and U, and it has higher Nb/Ta and lower Th/U ratios. Post-magmatic titanite also has strikingly different REE patterns than magmatic titanite, including the absence of pronounced Eu anomalies and lower REE abundances. These chemical features are controlled by element solubilities in aqueous fluids. In most cases, hydrothermal titanite has δ18O values similar to magmatic titanite, indicating alteration and recrystallization from exsolved magmatic fluids. The involvement of meteoric water with low δ18O is evident locally; individual spots have δ18O as low as 1.7‰ in the Little Cottonwood stock. Titanite compositions and textures provide important insights into the origins of granitic rocks and can be used to distinguish separate batches of magma, gauge the evolution of magmatic rocks, assess mixing processes, and infer compositions of mixing components. Because titanite also forms hydrothermally, it retains hints about the composition, temperature, and oxygen fugacity of the hydrothermal fluids and reveals details about titanite-forming reactions. However, the Al-in-titanite geobarometer does not yield realistic pressures of crystallization and the use of titanite as a geochronometer is compromised by the development of U-rich hydrothermal titanite.

Rock textures and mineral zoning – A clue to understanding rare-metal granite evolution: Argemela stock, Central-Eastern Portugal

The small leucocratic strongly peraluminous P, F, Li-rich granitic system at Argemela, central-eastern Portugal, is an ideal object for the study of relations between chemical composition of granitic rocks, rock-forming minerals and rock and mineral textures, and the associated Sn, Ta and W mineralization. To define the rock and mineral composition and to formulate an evolutionary model, traditional methods of bulk-rock chemical analyses, and EMPA and LA ICP-MS analyses of rock-forming minerals were combined with the study of macrotextures, TIMA-automated mineralogical mapping of typical rock samples, and CL study of internal texture of quartz and mica crystals. The Argemela stock forms a steep cylindrical body 1 km deep, with an elliptical outcrop 250 × 180 m in size, crosscutting Cambrian schists. The stock comprises two intrusions: a geochemically moderately evolved equigranular facies (0.3 wt% F, 1.2 wt% P2O5, 1850 ppm Li, 310 ppm Sn, 24 ppm Ta, 5 ppm W) composed of albite, quartz, muscovite and minor montebrasite followed by a rather inhomogeneous, more evolved porphyritic facies (0.25–1.2 wt% F, 1.3–1.8 wt% P2O5, 1600–4900 ppm Li, 600–1000 ppm Sn, 40–85 ppm Ta, 3–6 ppm W) composed of quartz, mica and subordinate K-feldspar phenocrysts embedded in an albite-quartz-mica-amblygonite matrix. The later facies is rimmed by stockscheider at the contact with slates, and with an up to 1 m thick layered zone with unidirectional solidification textures along contact with the equigranullar facies. The NE-part of the stock is crosscut by numerous thin quartz veinlets ±K-feldspar, phosphates and wolframite, while several aplitic dikes and thicker quartz+phosphate veins were found in two boreholes NW of the granite. Porphyritic facies, the most voluminous part of the system, contains strongly zoned phenocrysts of mica (phengite→lepidolite) and quartz (Ti-enriched to Al, Rb-enriched) indicating crystallization in two magmatic stages, while the equigranular facies and extragranitic aplitic dikes crystallized in one magmatic stage only. Hydrothermal muscovite enriched in Sn and Ta forms late overgrowths on some mica flakes in the porphyritic facies and in aplitic dikes. Based on a combination of chemical and textural observations, a four-stage evolutionary model of the Argemela stock was formulated: (i) an early magmatic stage comprises intrusion and in situ crystallization of aplitic dikes and the equigranular facies, and crystallization of quartz and mica cores in the deeper reservoir prior to intrusion of the porphyritic facies; (ii) in the late magmatic stage, crystal mush + evolved residual melt from the reservoir intruded upwards crystallizing the porphyritic facies including the stockscheider and layered zone. A small part of evolved melt formed rounded enclaves crystallizing as a phenocrysts-free ball facies. (iii) In the early (high-temperature) hydrothermal stage, magmatic fluid caused weak pervasive muscovitization of the porphyritic facies and aplitic dikes accompanied by the crystallization of disseminated cassiterite and columbite. After the transition from ductile to brittle deformation, quartz veinlets also developed, mainly in the NE part of the body. (iv) In the late, low-temperature stage, schist-derived fluid enriched in Ca, Sr and Ba caused local alteration of granite, forming crandallite–goyazite–gorceixite solid solution.

Geology of the Macao Special Administrative Region (China)

ABSTRACT A 1:12,000 geological map of the Macao Special Administrative Region has been produced through detailed field work supported by petrographic, mineralogical, geochronological and geochemical data obtained in previous studies. This map aims to represent a reliable tool to understand the geological evolution of the region and for management of the territory. The geology of Macao is dominated by two groups of Jurassic granitic rocks belonging to an intrusive suite located along the coast of Southeast China: Macao Group I (MGI: 164.5 ± 0.6 to 162.9 ± 0.7 Ma) and Macao Group II (MGII: 156.6 ± 0.2 to 155.5 ± 0.8 Ma), including the associated microgranite, aplite and pegmatite dikes and quartz veins. Remnants of the metasedimentary wall-rock are present as Devonian xenoliths enclosed within the granites. Younger Jurassic to Cretaceous andesite to dacite dikes (150.6 ± 0.6 to <120 Ma) intrude the granitic rocks. Additionally, Quaternary sedimentary deposits cover the older lithologies.

Age Benchmark of Granulite Metamorphism in the Angara–Kan Block, South Yenisei Range: Evidence from Zircon Dating of Postcollisional Granite–Aplite Dikes

The paper presents isotope–geochronological data on zircons from a granite–aplite dike that cuts garnet–hypersthene gneisses of the Kuzeyev unit of the Neoarchean–Paleoproterozoic Kan Group in the southern extension of the Ishimba–Tatarka regional shear zone, Kan River. The morphology and internal structure of most of the zircon grains indicate that the zircons are metamorphic and xenogenic and were captured by the acidic magma from the host rocks when the magma intruded the granulites. Based on these data, the obtained Paleoproterozoic U–Pb age of 1784 ± 6 Ma can be interpreted as the age of granulite metamorphism. The interpretation of available geological and isotopic data confirms a postcollisional age of the aplite, which was emplaced nearly synchronously with the formation of anorogenic leucogranites of the late phase of the Tarak complex, during the final stage of the formation of the Nuna supercontinent.

Petrology, Geochemistry, and Sr-Nd-S Isotopic Compositions of the Ore-Hosting Biotite Monzodiorite in the Luanjiahe Gold Deposit, Jiaodong Peninsula, China

The Jiaodong Peninsula is one of the most important Au ore provinces in China. There is an ongoing debate on the correlation between ore formation and magmatism in this province, because few intrusive rocks exhibit a clear association with ore deposits. A mineralized biotite monzodiorite (BM) stock, with disseminated ore, pervasive phyllic alteration, and no deformation, was found in a borehole in the footwall of the Zhaoping fault within the Luanjiahe Au deposit, which may shed light on this debate. The biotite monzodiorite contains explosion breccias, miarolitic cavities, skeletal and dendritic quartz, and late-stage evolved aplite dikes, and the in-situ δ34S values of the disseminated pyrite which is associated with Au mineralization are -1.7‰ to 7.3‰ (mean=3.5‰), indicative of a magmatic-hydrothermal system. These findings, combined with the reported age of 123 Ma, show that the intrusion has close spatial, temporal, and geochemical relationships with Au mineralization in the area. The biotite monzodiorite is metaluminous, high-K calc-alkaline and shoshonitic, with enrichment in light rare earth elements (REEs) and large-ion lithophile elements (LILEs), depletion in high-field-strength elements (HFSEs), and enriched Sr-Nd isotopic compositions. The intrusion may be the product of partial melting of enriched lithospheric mantle with a small lower crustal component. The hydrous, Au-bearing, enriched mantle source, and the strongly oxidized magma that was generated, created favorable conditions for Au mineralization.

Elemental substitutions and compositional evolution during tourmaline formation in metasomatic schistose rocks of Sikait area in the Southern Eastern Desert, Egypt

Tourmaline deposit in Sikait area (Southern Eastern Desert of Egypt [SEDE]) is closely associated with metapelitic schistose rocks that belong to Neoproterozoic time. The distribution of tourmaline was mostly controlled by Nugrus shearing, along with schist leucocratic rock varieties (leucogranite, pegmatite, and aplite dikes), which are syntectonically dissected by hydrothermal quartz veins. Local concentrations of tourmaline occur either as discontinuous tourmalinite bands/pockets or as disseminated isolated clusters of crystals within the metapelitic schistose rocks, pegmatites, and quartz veins. Chemical and mineralogical changes accompanying with tourmaline formation have been observed within the schistose rocks in contact with a pegmatitic bodies in Sikait area. Four mineral assemblages were identified on approaching to the pegmatite contact: (1) Quartz-Biotite-Potassium Feldspar-Chlorite assemblage (Q-B-K-C); (2) Quartz-Biotite-Tourmaline-Chlorite assemblage (Q-B-T-C); (3) Tourmaline-Quartz-Muscovite assemblage (T-Q-M); and (4) Tourmaline-Quartz assemblage (T-Q). Toward the pegmatite contact, K, Li, Rb, SiO2, CaO, MgO, Sr, and Ba are decreased, while Al2O3, B2O3, Na2O, Fe (tot), Zr, Y, and Th are moderately increased. Variations in the mineral assemblages and whole-rock chemistry within the four alteration zones appear to be controlled by boron metasomatism and potash leaching. The normalized rare earth element patterns of rock samples from the four alteration zones suggest a partial alteration of the original sedimentary patterns by the metasomatic-magmatic fluids. The alteration could be either a single phase of interactions between pegmatite fluid and schists with B-, Li-, Rb-carrying fluid or a couple of phases (B-, Mn-, Be-rich fluid and Rb-, Li-, K-rich fluid). In all cases, boron from the pegmatite-associated fluids reacts with the surrounding schistose rocks to breakdown of sheet silicate which acts as “traps” for Rb, Sr, Li, Ba, and K and hence leads to tourmaline-rich assemblages.

Aplite diking and infiltration: a differentiation mechanism restricted to plutonic rocks

The Half Dome Granodiorite in Yosemite National Park, California, contains multiple crosscutting generations of high-silica dikes that vary in texture but share the distinctive trace element characteristics of fine-grained aplite dikes of the region (e.g., low Y and middle REE). Dikes of the youngest generation are typical fine-grained, sugary aplites with sharp planar contacts, whereas progressively older dikes are increasingly coarse-grained. The oldest ghostly forms recognizable as dikes are similar in grain size to the host granodiorite and their contacts are irregular at the grain scale, making them easy to overlook in the field. Mineral compositions and microstructures of felsic dikes indicate that all dikes, including fine-grained aplite, have undergone significant recrystallization. We propose that older dikes were originally fine-grained aplites that, after emplacement, recrystallized together with the host to a typical granitic texture. Extraction, transport, and redistribution of aplitic melt by various mechanisms explains major and trace element variation in the pluton and likely was the dominant differentiation mechanism at the level of emplacement. This late-stage differentiation process can only occur in a largely crystallized host capable of sustaining tensile cracks, and therefore cannot play a role in the differentiation of volcanic rocks. Predicted geochemical effects of aplite redistribution are evident in trace-element geochemical patterns of Circum-Pacific plutonic rocks but are absent from corresponding volcanic rocks. This indicates that aplite infiltration may be an important and widespread late-stage process of pluton differentiation.

High‐Resolution Numerical Modeling of Heat and Volatile Transfer from Basalt to Wall Rock: Application to the Crustal Column beneath Long Valley Caldera, CA

AbstractWe present a high‐resolution numerical model of the thermal evolution of the crustal column beneath Long Valley caldera, California, from which &gt;800 km3 rhyolite erupted over the last 2.2 Myr. We examine how randomly emplaced basaltic sills of variable thickness (10, 50, or 100 m) at various depth intervals (10–25 km) and at variable emplacement rates (5–50 m/kyr) gradually heat the crust and lead to a variably mixed crustal lithology (solidified mafic sills and preexisting granitoid). We additionally explore the time scales over which dissolved water (~3 wt%) in a newly emplaced basaltic sill exsolves during crystallization and is transferred to adjacent wall rock that is undergoing partial melting. We employ a finite‐difference‐based technique, with variable spatial (≥1 m to ≥10 km) and temporal (&lt;100 and &gt; 106 years) resolution, that enables dense analysis within and directly adjacent to a newly emplaced sill. Our results show that once ambient crustal temperatures reach ~500–600 °C, subsequent injections of basaltic sills lead to significant partial melting of adjacent wall rock (granitoid and solidified mafic sills) on time scales (101–102 years) that match those of exsolution of H2O‐rich fluid from basaltic sills. Large volumes of fusion (&gt;10%) during fluid‐undersaturated partial melting, combined with the preexisting occurrence of aplite dikes, facilitates the development of melt‐filled fractures that exceed the critical length for self‐propagation. The advection of wall‐rock partial melts (with a combined mantle‐derived and crustal geochemical signature) to shallower depths will alter both the thermal and compositional profile of the middle‐upper crust.

The origin of the Pailin Crystalline Complex in western Cambodia, and back-arc basin development in the Paleo-Tethys Ocean

The Pailin Crystalline Complex, which consists of amphibolites, metagabbros, and felsic igneous rocks, is located in western Cambodia. We used the geochemistry of the amphibolites and the zircon UPb ages of felsic igneous rocks to constrain the origin of the Pailin Crystalline Complex and to gain insights into the tectonic framework of Southeast Asia. Geochemical analyses indicate that the amphibolites are normal (N-type) mid-ocean ridge basalts, similar to the Sa Kaeo Back-arc Basin basalts of southeastern Thailand. The zircon UPb ages are 283 ± 2 Ma (anorthosite dike), 280 ± 2 Ma (aplite dike), and 275 ± 2 Ma (plagiogranite). The geochemistry and detrital zircon UPb ages of sandstones adjacent to the Pailin Crystalline Complex suggest that they are sediments that filled the Sa Kaeo Back-arc Basin during the Triassic. We infer that the Pailin Crystalline Complex is part of a dismembered ophiolite related to the back-arc basin, and that the igneous rocks formed during the Early Permian. Most of the metamorphism occurred during continental collision in Southeast Asia. However, the amphibolites of the Pailin Crystalline Complex also experienced ocean-floor metamorphism at a back-arc spreading center, in a tectonic setting similar to that of the mafic metamorphic rocks of the Nan Back-arc Basin, northern Thailand. We conclude that the Pailin Crystalline Complex is the southernmost extension of a back-arc basin related to subduction of Paleo-Tethys oceanic crust and the Sukhothai Arc.

Highly fractionated leucogranites in the eastern Himalayan Cuonadong dome and related magmatic Be–Nb–Ta and hydrothermal Be–W–Sn mineralization

Leucogranitic rocks, including two-mica granite, muscovite granite, and granitic pegmatite (aplite), crop out in the eastern Himalayan Cuonadong dome, southern Tibet. These rocks have high SiO2 (∼73 wt.%), Al2O3 (∼16 wt.%), and total alkali contents (∼9 wt.%), and are peraluminous with A/CNK ratios of 1.0–1.1. Whole-rock major and trace element data and mineral compositions reveal strong differentiation and fractionation trends for the two-mica granite, muscovite granite, and pegmatite–aplite dikes. Albite, muscovite, and garnet contents increased as the leucogranitic magma evolved, whereas K-feldspar and biotite contents decreased. Muscovite in aplite has higher Rb2O (∼0.4 wt.%), Nb (∼219 ppm), and W + Sn (∼737 ppm) contents, and lower Nb/Ta ratios (∼6) than muscovite in the two-mica and muscovite granites. Garnet varies in composition from almandine in the leucogranites to spessartine in the aplites. The muscovite granite and pegmatite are associated with rare-metal mineralization. Rare-metal-bearing minerals include beryl, columbite-group minerals, tapiolite, Nb–Ta-bearing rutile, cassiterite, and wolframite. Skarn and altered (i.e., chloritized) rocks along the margins of the leucogranites are the products of hydrothermal reaction between leucogranite and marble. Beryl, scheelite, and cassiterite occur in the skarn and altered rocks. Rare-metal mineralization in the Cuonadong dome can be divided into two types: magmatic Be–Nb–Ta mineralization related to pegmatite–aplite dikes and hydrothermal Be–W–Sn mineralization hosted in the skarn and associated with the two-mica and muscovite granites. Monazite U–Th–Pb dating reveals two episodes of leucogranitic magmatism in the Cuonadong dome at ca. 20 and 17 Ma. Columbite U–Pb dating indicates two stages of rare-metal mineralization at ca. 17 and 14 Ma (including tapiolite). It is inferred that magmatism was related to the exhumation of the Higher Himalayan Crystalline Basement, with the degree of differentiation of the associated leucogranites being the key factor controlling the type of mineralization.

Li–Nb–Ta mineralization in the Jurassic Yifeng granite-aplite intrusion within the Neoproterozoic Jiuling batholith, south China: A fluid-rich and quenching ore-forming process

The Neoproterozoic Jiuling batholith in northwestern Jiangxi Province, south China, is one of the largest batholiths in the Nanling Range, and it is intruded by Mesozoic granitic intrusions associated with mineralization. The Jurassic Yifeng granitic pluton intrudes to the southern part of the Jiuling batholith and comprises Li-rich muscovite (Li-Mus) granite, trilithionite (Tri) granite, and aplite dikes. The granitic rocks are enriched in rare metals (Li–Rb–Cs–Nb–Ta). We conducted whole-rock geochemical analyses, undertook columbite U–Pb dating, and examined the textures and chemistry of feldspars, micas and the accessory minerals montebrasite, columbite-group minerals (CGMs), microlite, cassiterite, and zircon. All three granitic rock types have high Li contents (up to ~6200 ppm), and the δ7Li values of the Li-Mus granite and aplite are −3.55 ± 0.14‰ and −5.34 ± 0.04‰, respectively. The isolated grains of CGMs and cassiterite in the aplite are magmatic products, but intense fluid activity in the later stages and quenching resulted in unique mineral aggregates of cassiterite + CGMs + zircon. The Nb–Ta mineralization of the Li-Mus granite took place at 144 ± 5 Ma according to columbite U–Pb dating, and this age is similar to that of the neighbouring Yichun granitic pluton to the south. Moreover, both the Yifeng and Yichun plutons contain P-rich rare-metal-bearing Li–F granitic rocks that contain P-rich feldspars, montebrasites (amblygonites), Cs-rich lepidolites, CGMs close to the Mn endmember, and Hf-rich zircons. It is inferred that a metallogenic district characterized by Li–Nb–Ta mineralization exists in northwestern Jiangxi Province, thus providing a reliable guide for further exploration.

New U–Pb zircon ages for the Glenroy Complex and McKnee Intrusives, southeast Nelson: strengthening links with northern Fiordland and constraining the timing of metamorphism

ABSTRACTThe Glenroy Complex and McKnee Intrusives, southeast Nelson, have previously been correlated with similar rocks in northern Fiordland: the Worsley Pluton of the Western Fiordland Orthogneiss and George Sound Paragneiss, and Mistake Diorite and Darran Leucogabbro, respectively. Two samples of the Glenroy Complex (Woodham Orthogneiss) and one from the McKnee Intrusives yielded laser ablation inductively-coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon ages of 123.7 ± 1.4 and 121.9 ± 1.1 Ma, and 138.8 ± 1.3 Ma, respectively. These new ages fall within the ranges previously published for the Glenroy Complex, McKnee Intrusives and their northern Fiordland counterparts and, thus, provide further evidence for their correlation. An age of 116.3 ± 2.4 Ma was also determined for a post-metamorphic pegmatite–aplite dike, constraining metamorphism of the Woodham Orthogneiss to within 2.0–9.0 Ma following magmatic crystallisation. The time gap between crystallisation and metamorphism is similar to that of the Worsley Pluton (3.7–8.9 Ma), lending additional support for their direct correlation.

Mineral compositions of magmatic dikes cutting across Khe Phen granites (Huong Tra, Thua Thien Hue, Central Vietnam)

The Khe Phen granite quarry located in Huong Tra district (Thua Thien Hue province) has been confirmed as a part of the Ba Na granitoid complex (G/K2bn), mostly composed of two-mica granite and porphyritic granite. Field survey data show that the granites here are cut across by five distinct narrow dikes (about 50–70 cm wide), including granite pegmatite, granite aplite, aplite, granodiorite, and lamprophyre. The mineral compositions of the granite pegmatite and aplite dikes are similar to those of the host granite, which is mainly comprised of quartz (27–35%), orthoclase (45–58%), plagioclase (4–15%), biotite (1–3%), and a few opaque minerals. Meanwhile, the granodiorite and lamprophyre dikes are melanocratic and compositionally much more mafic, particularly lamprophyre, evidenced by a presence of hornblende (50–55%), plagioclase (33–40%), quartz (3–15%), calcite (5–17%), etc. The origin and emplacement age of the latter dikes have not been reported so far and thus, are needed for further studies based on geochemical and isotopic data.

Contribution of remote sensing to mapping In-Abeggui gold deposit (Central Hoggar, South Algeria)

The area concerned with the application of remote sensing is the In-Abeggui gold deposit in the Eastern part of Laouni terrane (Central Hoggar, South Algeria). This deposit includes a system of gold-bearing quartz veins marked by intense hydrothermal alteration. The gold veins are hosted in a big gabbro-diorite massif, cut by microgranite and aplite dike veins, all situated in a Neoproterozoic volcanogenic basin. The aim of this paper is to use LANDSAT 7ETM+ satellite images for mapping and characterizing gold-bearing veins, faults, and host rocks in the studied area and especially comparing the results with our traditional field work. All of the techniques used, such as colored compositions (CC), spectral band ratio, principal component analysis (PCA), and directional filtering, allowed a better lithological discrimination of the different formations. Moreover, they highlighted a hierarchical distribution of faults and emphasized their preferential directions. As regards lithology, the visualization with colored composition (RGB) of the three principal components (PC1, PC2, PC3) enabled us to clearly distinguish gabbros, diorites, volcanogenic rocks, and detrital rocks. Besides, we could distinguish between various faults, white aplitic dikes, and various gold-bearing quartz veins.

The Jurassic tourmaline–garnet–beryl semi-gemstone province in the Sanandaj–Sirjan Zone, western Iran

ABSTRACT Deposits of semi-gemstones tourmaline, beryl, and garnet associated with Jurassic granites are found in the northern Sanandaj–Sirjan Zone (SaSZ) of western Iran, defining a belt that can be traced for about 400 km. Granitic magmas strongly interacted with or were derived from melts of continental crust and/or sediments. Based on morphologies, size, mineral assemblage, and contact relationships with host granite and associated metamorphic aureoles, these deposits are categorized into six types: (1) garnet in skarns, (2) tourmaline, beryl, and garnet in pegmatite and aplitic dikes, (3) disseminations and patches of tourmaline in leucogranites, (4) quartz-tourmaline veins in granite, (5) tourmaline and garnet in metamorphic aureoles, and (6) tourmaline orbicules in aplite. Tourmalines are mostly schorl and dravite, and garnets are mostly almandine, spessartine, and grossular. Tourmaline, beryl, and garnet from pegmatites in the contact aureole of Jurassic granites reflect segregations of Be, B, Mn, and Al bearing melts from the Jurassic peraluminous granites. Quartz-tourmaline veins and hydrothermal garnets in skarns reflect fluids exsolved from the surrounding metasediment and pegmatite melt. In contrast, tourmaline patches and orbicules developed from boron-rich aqueous fluids exsolved from cooling granitic magma. Distribution of semi-gemstones in the SaSZ shows that these are mostly related to pegmatites associated with Jurassic granitic intrusions. Mineral equilibrium considerations indicate that SaSZ semi-gemstones crystallized at P = 3.5–7.5 kbar (11.5–25 km deep) and temperatures of 550–650°C. SaSZ pegmatites fall in the muscovite (MS) and MS-rare element classes. They are Lithium Cesium Tantalum (LCT)-type pegmatites. Fluids responsible for gem mineralization were exsolved from cooling granite bodies and released by metamorphosed sediments. Further studies are needed to better understand the northern Sanandaj–Sirjan tourmaline–garnet–beryl semi-gemstone Province.

Correction to: Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within aplite dikes

In the version of the paper originally published the current address of author Jeremy T Ross was given incorrectly as Untied Status Marine Corp, 3351 Onslow Drive, Camp Lejeune, NC 28547, USA.

Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within aplite dikes

Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, eNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N 100 and absence of an Eu anomaly in both the aplites and segregations. These data are consistent with the aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher eNd1.42Ga values for the aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.

Hydrothermal evolution of granitoid-hosted gold mineralization in gidami area: An example for orogenic-gold deposits in Egypt

The Gidami gold ore is a unique form for the orogenic-gold deposits in Egypt. The gold lode is found in the form of auriferous quartz veins and aplitic dikes, cut through hosting granitic rocks and probably extend into the neighbor metavolcanic rocks. The geochemical data revealed that the granitic rocks are of tonalite-granodiorite composition, show calc-alkaline nature and related to active continental margin (syn-orogenic). The ore body is controlled by NNW trending shear/fault zones, occupying fracture zones and fissures and dipping generally to East direction. Microscopic examination accompanied with Electron Microprobe Analyses revealed that, the existed sulfide minerals are (pyrite, sphalerite, chalcopyrite, molybdenite, pyrrhotite, covellite, Galena, and pentlandite). Ilmenite and goethite are the main oxide minerals. Native gold commonly occurs as small inclusions within pyrite and goethite, and also detected as tiny grains scattered within quartz vein (near the sulfides). Well-developed alteration zones occur associated with auriferous quartz veins; two types of wallrock alteration are distinguished: a) Inner (phyllic) alteration zone, and b) Outer (carbonatized) alteration zone. The outer alteration zone always exhibits higher gold content than the inner zone.Three types of fluid inclusions are recognized at the core and rim of the auriferous quartz vein as follows: 1) Two phases aqueous (H2O-NaCl ± KCl), 2) Two phases aqueous (H2ONaCl ± MgCl2), and 3) Three phases aqueous-carbonic (H2O–NaCl-CO2±CH4) fluid inclusions. Isochores intersection from fluid inclusions data outlines that, the Gidami gold deposits are of the mesothermal type formed at temperatures range between (270–300 °C), and pressure (750–850 bar) corresponds to a depth around 3 km (assuming pure lithostatic pressure). The ore-bearing fluids are supposed to be sourced mainly by fluids of metamorphic and meteoric origin. However, criteria of partial mixing with water of magmatic signatures has been proposed during fluid evolution. The transportation of gold was in form of bisulphide compounds in moderately reduced, neutral to weakly alkaline environment and then the precipitation occurred as a result of changing in Eh and pH conditions.