Granitic Intrusions Research Articles

Происхождение и структурная позиция Камчатского срединного массива по данным глубинных геолого‑геофизических исследований

The article shows the results of deep research along the profile of the settlement of Nizhnyaya Oblukovina – the city of Andrianovka, crossing the northern part of the Kamchatka median massif. A geological and geophysical model of the structure of the earth’s crust and upper mantle has been constructed, where the structural position of the object under study is presented and an assumption is made about its origin. The model highlights a fragment of paleosubduction (slab), which was part of the most ancient convergent boundary in western Kamchatka. The final stage of subduction blocking and its displacement to the east at a distance of ~60 km in the Early Eocene is associated with the entry into the accretionary complex of a terrane in the form of an island-arc plate 6–9 km thick. At the site of the maximum inflection of the subsequent slab, an extension zone was formed – a rift zone, along which the rise of mantle material and high-temperature fluid occurred. Approximately 52 million years ago, the processes of metamorphism, focal melting and intrusion of granites into the upper layers of the crust took place. As a result, in the eastern part of the plate and its flanks, a granitoid massif was formed with a rock density of 2.58 g/cm3, which is significantly lower than the environmental density. Density deficiency led to a violation of isostatic equilibrium and, as a result, to a rise in this part of the structure. The most intense uplift occurred at the end of the Oligocene, as a result of which a ledge was formed, which the authors recommend giving the name: “Middle Kamchatka ledge” instead of the rooted “Kamchatsky median massif”. The genetic relationship of the Shanuch ore region with the features of the deep structure of the lithosphere has been revealed. The results of the research indicate a hidden (buried) distribution of the island-arc plate beyond the boundaries of the mapped outcrops of metamorphids. Intrusions of the main composition, promising for the opening of sulfide copper-nickel ores, are located in the marginal parts of the ledge.

Graphite, Sheelite and Cassiterite Mineralized Skarns of Frag Al Ma, Sidi Bou Othmane District, Jebilet, Morocco

Sidi Bou Othmane region in the Central Jebilet is formed by a Devonian schistose series. The polyphase Hercynian deformation in this area is characterised by penetrative, subvertical, synmetamorphic schistosity (S1), accompanied by folding. Syntectonic granitic intrusions, hidden in the Sidi Bou Othmane region, is marked by a contact of metamorphism, which responsible for the replacement phenomena observed essentially in the limestones of the region which are transformed into graphitic Sn-W exoskarns. Syn-tectonic, skarnification is polyphase : (i) prograde stage (T= 500- 750°C) : with graphite, garnets, idocrase, pyroxene, wollastonite, quartz, scheelite and molybdenite, crossed by a network of successive veins (garnet, idocrase, wollastonite, quartz and calcite; (ii) retrograde stage (T= 400-500°C): with garnet, idocrase, pyroxene wollastonite, quartz, calcite, epidote scheelite and cassiterite; (iii) hydrothermal alteration stage (T ≤400°C) : with sericite and clay accompanied by abundant scheelite and cassiterite, favoured by the high oxidation rate. Therefore, they would have formed in a reduced environment, in relation to large batholiths or stocks of quartz diorites and/or granites in syn, late- to post-orogenic to anorogenic contexts, in the absence of any cogenetic volcanism. The economic mineralisation of graphite was formed mainly during prograde stage, under reducing conditions. It was formed by devolatilization of a bituminous origin linked to the reef limestones and organic matter, trapped in the shales according to the following reaction:

Распространение и формирование подземных содовых вод на водосборной площади Верхней Колымы

In the Upper Kolyma basin, mostly hydrocarbonate and sulfate waters are distributed with a predominance of alkaline earth metal cations in the composition. Hydro-carbonates with alkali metal cations (Na+ + K+), distinguished as soda, are found only in local areas in fault zones and those of granite intrusion contacts. They are the least mineralized (no more than 100 mg/dm3) in the seasonally thawed layer and river valley taliks, but even there, in separate foci at the elevation of above 1600 m, soda water springs with the mineralization exceeding 200 mg/dm3 occur. Soda waters in Cretaceous coal-bearing deposits within intermountain artesian basins are the most mineralized (up to 3000 mg/dm3). The association of these waters with deep fault zone has been assumingly traced. The sharp increase of the dissolved carbon dioxide and the carbonic acid anions in waters at local river valley sites is offered as the indicator of the current ingress of carbon dioxide into underground waters. Soda waters with mineralization exceeding 600 mg/dm3 , contained in some samples, are referred to balneal. Hydrogeochemical methods are proposed for further study of the role of deep faults in soda waters formation as well as for the possible use of them for curing and preventing diseases.

Geochemical composition of apatite from the Zhuxi tungsten granite and the Zhenzhushan granite porphyry in the Jiangnan porphyry-skarn tungsten belt, China

Apatite is increasingly used as a tracer for petrogenetic and metallogenic processes. We present the cathodoluminescence (CL) texture, and composition of igneous apatite from the tungsten-bearing Zhuxi biotite granite and the nearby Zhenzhushan granite porphyry in the Late Mesozoic Jiangnan porphyry-skarn tungsten belt in South China. Most apatite grains show homogeneous CL-dark texture, but some apatite grains have CL-dark cores and CL-bright rims, or CL dark-bright-dark-bright (from core to rim) growth zoning bands. The CL-bright bands are Mn depleted. Apatite from the Zhuxi biotite granite has elevated W contents (mean: 0.16 ppm; n = 75) compared to the Zhenzhushan granite porphyry (mean: ≤0.02 ppm; n = 32). The CL-bright rims of apatite from Zhuxi have more W than the CL-dark cores, i.e. average 0.58 ppm W versus average 0.07 ppm W, respectively, implying that the late-stage magmatic system was enriched in W. The chondritic Y/Ho ratios (22−33) of Zhuxi apatite, and the similar or even slightly more elevated REE contents of the CL-bright rims, compared to the cores, rule out hydrothermal effects. The Zhuxi apatite samples have distinctly negative δEu, F-rich and sulfur-depleted features, which are interpreted as signature of igneous apatite from magmatically evolved granitic intrusions associated with W deposits, while the inverse trend applies to apatite from Cu porphyry systems. In-situ microanalysis of apatite may be a useful exploration tool.

Monazite and xenotime petrochronologic constraints on four Proterozoic tectonic episodes and ca. 1705 Ma age of the Uncompahgre Formation, southwestern Colorado, USA

Abstract The Proterozoic tectonic evolution of the southwestern USA remains incompletely understood due to limited constraints on the timing and conditions of the tectono-metamorphic phases and depositional age of metasedimentary successions. We integrated multi-scale compositional mapping, petrologic modeling, and in situ geochronology to constrain pressure-temperature-time paths from samples of Paleoproterozoic basement gneisses and overlying quartzites in southwestern Colorado, USA. Basement gneiss from the western Needle Mountains records metamorphic conditions of 600 °C at 0.75 GPa at 1764 ± 9 Ma and ~575 °C at 1741 ± 10 Ma. Gneiss sampled from drill core near Pagosa Springs, Colorado, records conditions of 700 °C at 1748 ± 9 Ma, 800 °C at 1.1 GPa at 1650 ± 40 Ma, 540 °C at 1570 ± 36 Ma, and 440 °C at 1424 ± 12 Ma. The Uncompahgre Formation was deposited at ca. 1705 Ma, as constrained by detrital monazite (1707 ± 8 Ma) and xenotime (1692 ± 40, 1725 ± 50 Ma), metamorphic xenotime (1650 ± 10 Ma), and published 40Ar/39Ar and detrital zircon data. Compositions of ca. 1705 Ma detrital monazite and xenotime are consistent with derivation from a garnet-bearing source in the Yavapai orogenic hinterland. The Vallecito Conglomerate and Uncompahgre Formation record macroscopic folding and greenschist-facies metamorphism at 1650 ± 10 Ma and temperatures of 270 °C to >570 °C at 1470–1400 Ma. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon geochronology yielded dates of 1775 ± 18 Ma from the Twilight Gneiss and 1696 ± 7 Ma from the Bakers Bridge Granite, supporting previous isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates. The Eolus Granite yielded a date of 1463 ± 6 Ma, which is older than previous 1.44–1.43 Ga ID-TIMS dates. The newly dated granite of Cataract Gulch is 1421 ± 12 Ma. In situ analysis of detrital and meta-morphic monazite and xenotime, igneous zircon, and quantitative thermobarometry, integrated with previously published constraints, indicate multiple tectonic episodes after the emplacement of 1800–1760 Ma arc-related rocks. The region experienced greenschist- to amphibolite-facies metamorphism (M1) from 1760 Ma to 1740 Ma, which was followed by the intrusion of granites at 1730–1695 Ma and deposition of the Uncompahgre Formation at ca. 1705 Ma, contemporaneous with the Yavapai orogeny. Metamorphism at 1680–1600 Ma was characterized by greenschist-facies conditions near Ouray, Colorado, and granulite-facies conditions near Pagosa Springs (M2) during the Mazatzal orogeny. From 1470 Ma to 1400 Ma, greenschist- to amphibolite-facies metamorphism (M3) and largely granitic plutonism occurred during the protracted Picuris orogeny. These results demonstrate the power of monazite and xenotime analyses to constrain depositional ages, provenance, and pressure-temperature-time (P-T-t) paths to resolve the compound orogenic history that is characteristic of most mountain belts.

The genetic association between vein and skarn type tungsten mineralization in the Yaogangxian tungsten deposit, South China: Constraints from LA-ICP-MS analysis of individual fluid inclusion

The large-scale Yaogangxian W deposit, located in central of the Nanling metallogenic belt, South China, comprises independent quartz vein-type and skarn-type W mineralization. Wolframite-bearing quartz veins occur invariably in metasandstone and slate to the northwest of the Yaogangxian granitic intrusion, whereas scheelite-skarn are mainly hosted by the limestone to the east. The vertical extension of single wolframite-bearing quartz vein can reach up to 1000 m and we focused on the vein samples collected from the deepest level (26 level) which likely recorded early magmatic fluids. For skarn mineralization, two scheelite precipitation stages were recognized, namely scheelite-1 in retrograde stage and scheelite-2 from sulfide-stage. To reveal the fluid sources and characteristics of two mineralizing systems, detailed petrography and microthermometry analyses were carried out on fluid inclusion assemblages (FIA) in scheelite, wolframite and their coexisting quartz. Fluid compositions were obtained from individual fluid inclusion (FI) in ore and gangue minerals via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. These data aid elucidation on the source and nature of fluids in the two distinct W mineralization styles. The chemical compositions of FIs hosted in wolframite and scheelite points to a common magmatic source. The fluid in the scheelite ore section exhibits lower As and B than the wolframite ore section, indicating that boiling may have occurred at early skarn stage. Fluid evolution in the two mineralizing systems suggest that formation of diverse W mineralization styles is mainly controlled by different host lithology and fluid processes. In the wolframite ore section, ore-forming fluid recorded in the deepest level of vein show elevated As, B, K and Rb contents than previously reported ore-forming fluid obtained from the tip of the vein. This likely indicate absence of intensive fluid immiscibility in the deep during wolframite formation, which may rather be accompanied by fluid-rock interaction. In the scheelite ore section, constant injection of meteoric water is revealed. FI hosted in scheelite-1 and 2 exhibit similar compositions, and the lower K content in scheelite-2 fluid is thought to be caused by sericitization in the retrograde stage skarn. The Mo content of scheelite decreased from the retrograde stage to sulfide-stage, indicating a decrease in oxygen fugacity. Based on these conclusion and previous geological and geochronological studies, we propose a metallogenic model of the coupled wolframite-quartz vein and scheelite-skarn mineralization.

Indium mineralization and genesis of the Bainiuchang Ag–Sn–polymetallic deposit in southeast Yunnan, China: Evidence from mineral chemistry and U–Pb geochronology

The Bainiuchang Ag–Sn–polymetallic deposit is located in the world-calss Southeast Yunnan Indium Metallogenic Belt (SYIMB). The deposit contains approximately 1000 tons of In, but the mechanisms of mineralization and hosts of In in this deposit remain unclear. To clarify these ambiguities, we carried out fieldwork, major- and trace-element analyses of In-bearing minerals, and cassiterite and zircon U–Pb geochronology. Five ore zones were identified, which, from contact of the granitic intrusion upwards, are the In-poor magnetite–cassiterite ore zone, the In-poor Cu ore zone, the In-rich Zn–Sn ore zone, the In-rich Sn–Pb–Zn ore zone, and the In-poor Pb–Zn ore zone. Cassiterite and zircon U–Pb geochronology demonstrate that mineralization at Bainiuchang occurred at ca. 85.80±0.45 Ma (MSWD = 1.5), which is similar to the 85.34±0.42 Ma (MSWD = 1.7) age of the granitic intrusion. This temporal relationship suggests that there is a genetic link between In mineralization and granitic magmatism in this deposit. Ore petrography and mineral chemical analyses indicate that sphalerite, chalcopyrite, and stannite are main In-bearing minerals. From a mass balance perspective, sphalerite is the most important In-bearing mineral, accounting for ∼85% of the In budget at Bainiuchang. Indium is incorporated into sphalerite via the coupled substitution 2Zn2+ ↔ Cu+ + In3+ and/or 3Zn2+ ↔ Ag+ + Sn2+(+In3+). Two generations of sphalerite occur in the deposit, SpI and SpII, which formed during the cassiterite–sulfide and carbonate–sulfide stages, respectively. Based on textural relationships, SpI is further subdivided into SpIa, SpIb, and SpIc. These temporally and texturally distinct sphalerite have variable In contents, with SpIa having the highest In content (0.08–0.44 wt%), followed by SpIb (0.02–0.39 wt%), SpIc (187–236 ppm), and SpII (0.64–7.14 ppm). It is suggested here that the variations in In contents of the sphalerite reflect the evolution of the magmatic–hydrothermal fluids from which the sphalerite precipitated under different fS2 and temperature. By comparing the Bainiuchang deposit to other In-bearing deposits in the southwestern Yangtze Block, it is evident that that the nature and source of the associated granites, and the degree of fractionation of the granitic magmas are likely important controls on In mineralization in the SYIMB and other regions.

Origin and Evolution of Ore-Forming Fluids at the Small-Sized Gold Deposits in the Khudolaz Area, Southern Urals

Lode gold deposits are widespread in orogenic belts of various ages and are a valuable gold source, but their genesis remains debatable. The close relationship between native gold and quartz was considered a reason to search for acid-magmatic sources of heat and fluids (i.e., granite intrusions), while small gabbro bodies were often ignored. Six minor gold deposits associated with NE-strike faults were studied in the Khudolaz area of the South Urals (Tukan, Bilyan-Tau, Fazly-Tau, Muildy-Tamak, Alasiya-II and Isyanbet-I). It was established, for the first time, that all of the studied deposits are similar geologically but differ in mineralogical diversity of ore-bearing quartz veins, which is due to the different composition of host rocks and ore-bearing intrusions of the Khudolaz (325–329 Ma, U-Pb) and the Ulugurtau (321 ± 15 Ma, Sm-Nd) ultramafic-mafic complexes. Results of the geochemical study of quartz veins (ICP MS) and their fluid inclusions (microthermometry, gas chromatography) showed that native gold was mostly precipitated at temperatures of 230–330 °C from a low- to moderate-saline (8–12 wt.% NaCl-eq.) H2O–CO2–CH4-bearing fluid, when weakly oxidized or near-neutral conditions, were replaced by reducing ones. No significant differences between barren milky white and ore-bearing brownish quartz veins were defined, which indicates their common formation settings and an impulse pattern of vein injection. The stable pattern of the fluid salinity, along with low hydrocarbon and N2 contents, as well as a narrow range of δ18O values, indicate a prevailing magmatogenic source with a certain influence of host rocks but without the influence of meteoric waters. Based on the presented data, the studied deposits were attributed to the epizonal orogenic type. This study shows the formation of lode gold deposits is possible without the participation of granite massifs.

Insights into the formation of Shihuiyao Ta-Nb deposit in southern Great Xing'an Range, NE China: Evidence from chronology and fluid inclusion

Shihuiyao is a typical granite-type Ta-Nb deposit in the southern Great Xing'an Range (SGXR), Northeast China. The ore field is comprised of several granitic intrusions that were emplaced into the Early Permian Linxi Formation during the Late Jurassic (ca. 145 ∼ 150 Ma). The main Ta-Nb mineralization (stage 1–2) is found within the leucogranite, with minor identified in the porphyritic granite in the deposit. Four distinct stages in the metallogenic process can be identified: weakly mineralized magmatic stage (stage 1), strongly mineralized magmatic stage (stage 2), post-magmatic hydrothermal stage (stage 3) and low-temperature hydrothermal stage (stage 4). Cassiterite samples collected from stage 2 were dated to be 147 ± 3.5 Ma and 147.7 ± 1.9 Ma, providing evidence for the latest Jurassic Ta-Nb metallogenic event at Shihuiyao. To acquire a more profound comprehension of the properties and behaviors of the fluids, fluid inclusions within quartz, albite, amazonite and fluorite were analyzed among all stages. The results showed that the Shihuiyao fluid inclusions were relatively homogeneous and predominantly comprised liquid-rich inclusions, with occasional occurrences of CO2-type, vapor-rich type, and solid-type inclusions. From stage 1 to stage 4, the temperature of the ore-forming fluids decreased gradually, while the salinity showed increasing during stage 4, which indicates a possible contribution from surrounding strata sourced fluids. Raman analysis of the inclusions at each stage revealed that the vapor components were primarily H2O, CO2, and CH4, with additional N2 and CH4 appearing in the late stage, pointing to mixing between the metallogenic fluids and the surrounding strata source materials. Notably, the simultaneous occurrence of liquid-rich and solid-type inclusions under low pressure conditions (0.6 ∼ 1.5 km) in the early ore-forming stage (stages 1–2) suggests a fluid boiling process. We argue that the fluid boiling and rapid chemical quenching of the granitic melt play a crucial role in changing the physicochemical conditions of the ore-forming fluids, ultimately resulting in the enrichment and precipitation of niobium and tantalum. The fluid mixing and intense water–rock interaction in the late stages (stages 3–4) may have also contributed to the minor mineralization.Drawing upon our investigations utilizing petrographic features and composition analysis of individual fluid inclusions, we have prognosticated on the Sn mineralization potential within the Shihuiyao deposit. Our results indicate the Shihuiyao granites are comparable to the tin granites worldwide. Genesis of cassiterites is linked to both magmatic and hydrothermal processes, exsolution from tin-rich fluids (up to 233 ppm) and interaction with country rocks attribute to the probable economic tin mineralization. Consequently, the Shihuiyao deposit has been noteworthy prospectively for further tin mineralization.

Geochemical and remote sensing integrated with satellite gravity data of Darhib and Atshan talc deposits, South Eastern Desert, Egypt

The current contribution conducted new geochemical, remote sensing integrated with gravity detailed studies of talc deposits to identify the talc protolith as well as its extension, depth, and structures. There are two examined areas, distributed from north to south, Atshan and Darhib and both belong to the southern sector of the Egyptian Eastern Desert. They occur as individual lenses or pocket bodies in ultramafic-metavolcanics following NNW-SSE and E-W shear zones. Geochemically, among the investigated talc, Atshan samples have high contents of SiO2 (av. 60.73 wt.%), and higher concentrations of transition elements such as Co (av. 53.92 ppm), Cr (781 ppm), Ni (av. 1303.6 ppm), V (av. 16.67 ppm), and Zn (av. 55.7 ppm). Notably, the examined talc deposits contain low contents of CaO (av. 0.32 wt.%), TiO2 (av. 0.04 wt.%), SiO2/MgO (av. 2.15), and Al2O3 (av. 0.72 wt.%), which is comparable with ophiolitic peridotite and forearc setting. False color composite (FCC), principal component analysis (PCA), minimum noise fraction (MNF), and band ratio (BR) have been used to distinguish talc deposits in the investigated areas. Two new proposed band ratios were created to separate talc deposits. FCC band ratios (2/4, 4/7, 6/5) and (4 + 3/5, 5/7, 2 + 1/3) have been derived to focus on talc deposits in two case studies, Atshan and Darhib areas. The application of regional, residual, horizontal gradient (HG), and analytical signal (AS) techniques to gravity data are used in interpreting the structural directions of the study area. The analysis of this technique displays several notable faults trending in NW–SE, NE–SW, NNW–SSE, and E–W directions. Two techniques of gravity depth calculation were applied in the study areas, namely source parameter image (SPI), and Euler deconvolution (EU). The analysis of these techniques reflects that the depth of subsurface sources ranges between 383 and 3560 m. Talc deposits may be attributed to greenschist facies metamorphism or to a magmatic solution that is (associated with granitic intrusions) interacted with the surrounding volcanic rocks forming metasomatic minerals.

Geochronology, petrogenesis, and magmatic oxidation state of the Mangling intrusive complex, Northern Qinling Belt, Central China: Implications for magma fertility and tectonic setting

The Mangling intrusive complex has different dioritic to granitic phases and is spatially and temporally related to molybdenum deposits in the Qinling Orogen. Zircon U-Pb dating of the Mangling intrusive complex indicates that dioritic rocks (biotite diorite and biotite diorite enclave; ca. 150−147 Ma) formed earlier than granitic rocks (medium- to fine-grained and fine-grained monzogranite and K-feldspar granite; ca. 145−141 Ma). The Mangling dioritic rocks exhibit large ion lithosphere element (e.g., Rb) and light rare earth element enrichment and high field strength element (e.g., Nb, Ta, and Ti) depletion. They have low to moderate SiO2 (51.33−58.16 wt%), high MgO (3.10−4.75 wt%) and Mg# (48−60), and negative zircon εHf(t) values (−11.6 to −6.8), suggesting origination from the continental lithospheric mantle that may have been metasomatized by previous sediment-derived melts and slab-derived fluids constrained by high Nb/Y, Th/Yb, and Rb/Y ratios. The Mangling granitic rocks are I-type granites and have high SiO2 (67.90−81.88 wt%) and low MgO (0.16−0.74 wt%). They have low and negative zircon εHf(t) values (−18.7 to −1.9) and old zircon Hf two-stage model ages (2334−1287 Ma), as well as similar mineral fractionation (e.g., hornblende, biotite, sphene, and apatite) with the Mangling dioritic rocks, indicating that they were derived from the remelting of old crustal rocks (e.g., Xiong’er and Kuanping groups) by the evolved underplated mafic magma. Compared with the Taoguanping mineralized monzogranite in the Northern Qinling Belt, zircon geochemistry (e.g., Ce4+/Ce3+, Eu/Eu*, and ΔFMQ [relative fayalite-magnetite-quartz buffer]) indicates that magma of the Mangling intrusive complex (except the biotite diorite) has high oxygen fugacity and small fractionated granitic intrusions, which are coeval with the biotite diorite enclave or younger than the Mangling granitic rocks, may have potential for generating porphyry molybdenum mineralization. The combination of this study and previous studies corroborates that the Qinling Orogen underwent an intracontinental orogenic evolution in a post-collisional compression to extension transitional setting during the Late Jurassic to Early Cretaceous, affected by far-field Paleo-Pacific slab subduction.

Emplacement and accumulation of magmatic products in contractional structures: How does it occur? Insights from Central Andes of northern Chile

Structural and stratigraphic data demonstrate that the magmatism in northern Chile was accompanied by shortening related to the development of fold and thrust belts. However, how magmatic products were emplaced and accumulated in the contractional structures is an unresolved question. We, therefore, integrated field and seismic data from the regions of the Pampa del Tamarugal, Salar de Punta Negra Basin, and the Hornitos Fold and Thrust Belt to determine the main structures that facilitated the magma accumulation. The syn-kinematic granitic intrusions in the Hornitos Fold and Thrust Belt were preferably emplaced along fold-related thrusts. In this region, magma migrated along thrust ramps and finally accumulated into the core of the anticlines and broken synclines. Subsidiary and vertical tensional fractures controlled the magma accumulation in the hinge zone and at the axis of the folds. The seismic data from the Pampa del Tamarugal and Salar de Punta Negra Basin present evidence of buried volcanoes on inversion anticlines. During the tectonic inversion of the pre-existing Late Paleozoic to Mesozoic half-graben structures in the regions, magma migrated along the master faults and finally accumulated into the core of the inversion anticlines as sills or was expulsed to the surface as volcanoes, as evidenced by the Punta Negra Monogenetic Volcano.

Composition and evolution of ore-forming fluids in the Sansheng porphyry W-Mo deposit, Inner Mongolia, NE China: Evidence from LA-ICP-MS analysis of fluid inclusions

Unveiling the composition and evolution of ore-forming fluids in porphyry W-Mo mineralization systems is key to understanding their formation processes, but detailed quantitative studies are still limited. The Sansheng porphyry W-Mo deposit (17,285 t WO3 @ 0.569 % and 24,361 t Mo @ 0.226 %), located in southern Great Xing’an Range W belt, NE China, is characterized by quartz vein- and veinlet-disseminated-type W-Mo ore bodies mainly hosted in the cupolas of Early Cretaceous granitic intrusion. In this study, we provide an elaborate study that integrates detailed petrographic, laser Raman, microthermometric, and LA-ICP-MS analyses of fluid inclusions (FIs) to reconstruct the fluid evolution history of the Sansheng W-Mo mineralization system and to bridge the aforementioned knowledge gap. Four stages of hydrothermal veins are identified: stage 1 is the dominant stage of W mineralization, and characterized by the development of wolframite, quartz, K-feldspar, muscovite, and minor amounts of molybdenite; stage 2 is the main stage of Mo mineralization, and featured with the mineral assemblages of molybdenite, quartz, sericite together with minor amounts of pyrite; stage 3 involves Cu-Pb-Zn mineralization, and is characterized by the occurrence of polymetallic sulfides, quartz, and sericite; and stage 4 is dominated by barren carbonate-quartz veins. In wolframite and quartz, the FIs are classified as liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and pure vapor mono-phase FIs (PV-type) in types. Microthermometric results reveal decreasing homogenization temperatures in primary FIs from stages 1 to 4. Stage 1 fluids are characterized by high homogenization temperatures (320–420 ℃), moderate salinities (7.9–14.8 wt% NaCl equiv.), highly variable W contents (17–7251 ppm) and constant Mo contents (22–288 ppm). FIs in wolframite and coexisting quartz share similar homogenization temperatures and salinities, suggesting that they were formed concurrently. Stage 2 fluids have similar salinities (7.0–13.9 wt% NaCl equiv.) to stage 1 fluids but lower homogenization temperatures (250–340 ℃), and contain highly variable Mo contents (21–11176 ppm) and low W contents (5–23 ppm). Stage 3 fluids display moderate homogenization temperatures (220–300 ℃) and low salinities (3.1–10.1 wt% NaCl equiv.). The contents of W and Mo in stage 3 fluids decrease dramatically, marking the termination of W-Mo mineralization. Stage 4 fluids are characterized by moderate homogenization temperatures (200–270 ℃), low salinities (0.2–5.0 wt% NaCl equiv.), and the lowest contents of W (1–4 ppm) and Mo (8–16 ppm). All stage fluids contain high concentrations of B and Mn, and high ratios of Li/Na, K/Na, Rb/Na, Ba/Na, Pb/Na, and Zn/Na, similar to magmatic-hydrothermal fluids in composition and against typical basinal brines. In addition, they also have high Rb/Sr and low K/Rb ratios, which are equivalent to those of the Sansheng ore-bearing granite. In combination with the constant Cs/(Na + K) and Cs/Rb ratios of all stage FIs, we propose that the multistage hydrothermal fluids originated from a common source–the highly evolved and geochemically uniform Sansheng granitic magma, and that the derivation of ore-forming fluids was a continuous fluid exsolution process. Fluid boiling and fluid-rock interaction are collectively responsible for the deposition of wolframite and molybdenite, while the role of fluid cooling may be subordinate. The meteoric water began to enter the hydrothermal system in stage 3, and fluid cooling and dilution due to the fluid mixing process played a critical role in the deposition of Cu-Pb-Zn sulfides. Our findings highlight the significance of in-situ FI analyses in revealing the composition and evolution of multistage fluids in porphyry W-Mo deposits, which is critical for elucidating detailed ore-forming processes of intrusion-related W-Mo mineralization systems in NE China and globally.

Redefinition of the Petersburg batholith and implications for crustal inheritance in the Dinwiddie terrane, Virginia, USA

Field relations as well as geochemical and petrologic studies of meta-igneous rocks assigned to the Pennsylvanian–Permian Petersburg batholith identify at least two distinct rock types: foliated metagranitoid gneiss and massive to porphyritic granite. Foliated metagranitoid gneiss of mostly granodioritic composition is geochemically distinct from associated massive and porphyritic granitic rocks. These gneissic rocks yield radiometric ages from ca. 425 Ma to ca. 403 Ma and document that many of the rocks assigned to the late Paleozoic Petersburg batholith are 100 m.y. older than the youngest portions of the composite batholith and are part of an earlier infrastructural terrane. Two samples of massive equigranular granite southwest of Petersburg, Virginia, yield ages of ca. 321 Ma and ca. 317 Ma, which are 15–20 m.y. older than ca. 300 Ma ages for porphyritic granite, massive granite, and monzodiorite near Richmond, Virginia. Geologic mapping shows that the Early Pennsylvanian granite southwest of Petersburg is separated from Late Pennsylvanian to early Permian granite near Richmond by a map-scale septum of Silurian–Devonian foliated metagranitoid gneiss, referred to herein as the informal Pocoshock Creek gneiss. Laser ablation–inductively coupled plasma–mass spectrometry data from one sample of a quartz-muscovite felsic schist xenolith show a peak age mode of ca. 529 Ma that we interpret to be the maximum depositional age. Inherited zircons from foliated metagranitoid gneiss and massive equigranular granite range from ca. 631 Ma to ca. 376 Ma, but many are Cambrian. Neoproterozoic–Cambrian quartz-muscovite felsic schist and amphibolite, Silurian–Devonian Pocoshock Creek gneiss, and Pennsylvanian–Permian granite comprise a fault-bounded terrane referred to herein as the Dinwiddie terrane. Ages of inherited cores in zircon from igneous rocks and limited detrital zircon geochronology suggest the terrane is of peri-Gondwanan affinity. U/Pb ages of healed fractures in zircon grains from foliated metagranitoid gneiss indicate low-grade deformation of the gneiss at ca. 378–376 Ma, while ca. 320–280 Ma rims on many grains record intrusion of late Paleozoic granite. The temperature-time-deformation history of the Dinwiddie terrane is distinct from the adjacent Goochland and Roanoke Rapids terranes. Orogen-scale dextral transpression likely translated the Dinwiddie terrane southward during the Alleghanian orogeny, at which time they were intruded by Pennsylvanian to Permian granite.

A review of the geophysical knowledge of the Borborema Province, NE-Brazil, and tectonic implications

This review summarizes the results of nearly 50 years of application of geophysical data to study tectonic problems in Borborema Province (BP), Northeastern Brazil. Much information has been obtained from the interpretation of magnetic, gamma-ray spectrometry, gravity, seismological, magnetotelluric, deep seismic and heat flow data. The resulting picture is that, in comparison with other Brazilian geological domains, BP has a thin crust, a lithosphere with small effective elastic thickness, the presence of predominantly positive free-air gravity anomalies, besides the most expressive geoid anomaly, in addition to relatively high heat flow and intraplate seismicity. These facts reinforce the hypothesis that the BP lithosphere suffered a strong impact in Gondwana fragmentation. Predominantly positive free-air gravity anomalies reveal that part of the province has not yet reached isostatic equilibrium. Gravity data established the boundaries of the BP crustal block with neighboring blocks; in particular, the crustal boundary with the São Francisco Craton is located in the innermost regions of the marginal fold belt, tens of kilometers at the north of the conventional limit. Magnetic data allowed to infer the continuity of basement structures beneath sedimentary basins, in particular that BP continues to the Transbrasiliano Lineament, underneath the sedimentary of the Parnaíba Basin. In addition, enhanced magnetic maps made it possible assessing the amount of deformation associated with the major shear zones and identified a continental-scale Cretaceous dike system crossing the BP. Gamma-ray spectrometry data allowed extensive geological mapping of metasedimentary sequences, granitic intrusions and basement complexes. Seismological data allowed to recognize that major shear zones in BP have lithospheric significance. Deep seismic data evidenced a well-defined contrast between the upper and lower crust in BP (Conrad discontinuity). Magnetotelluric data revealed several deep geoelectrical anomalies in BP, some of them interpreted as relics of subduction zones. Intraplate seismicity studies allowed inferring that the current stress state in the BP upper crust is predominantly transcurrent. The list of open problems in BP is also expressive. Notable examples are: a) it has not yet been possible to define which type of orogeny (accretionary or intracontinental?) was dominant in the Neoproterozoic; b) the cause of the expressive geoid anomaly is unknown; c) the tectonic significance of the Transbrasiliano Lineament (collisional suture? Just an intracontinental shear zone?) is also unknown. Regarding economic applications, the use of geophysical data to prospect new ore deposits is still relatively poor. We hope that this review can contribute to increase the familiarity and acceptance of geophysical results by the geological community.

The occurrence of anthophyllite–phlogopite-bearing schist in the Seridó belt, northeastern Brazil: Geological significance and potential guide of prospecting for gold and emerald deposits

In the Bonfim W–Mo and Au–Bi–Te skarn deposit, Northeastern Brazil, Archean metamafic–ultramafic rocks are similar to those found in greenstone belt sequences worldwide. The Bonfim emerald deposit comprises metric layers of lenticular-shaped phlogopite-rich schist developed at the symmetrical contacts between metamafic–ultramafic rocks and Be-rich granite intrusions. They formed near these rocks by metasomatic fluids emanating from granitic magma. Metasomatism also transformed the marble and schists into skarns at the marble–schist contact and within the marble. This contribution describes the drill cores containing Mg-rich schist in the Neoproterozoic metapelitic sequence. Detailed petrographic observations and geochemical analysis reveal that it contains phlogopite, anthophyllite, fuchsite, and anomalous Cr–Ni–V–Co–Sc contents. These characteristics contrast those of the common schists at Bonfim or regionally and indicate a probable komatiitic affinity. The anthophyllite–phlogopite-bearing schist contains a metamorphic paragenesis of phlogopite + anthophyllite + garnet + tourmaline + zircon + monazite + plagioclase + quartz, followed by two successive retrograde metasomatic parageneses comprising (i) clinozoisite–zoisite + apatite and (ii) white mica (±fuchsite) + clinochlore + quartz. It exhibits a peculiar texture containing prismatic pseudomorphs (1–7 mm long) composed of anthophyllite, clinozoisite–zoisite, and white mica. The relict orthogonal cleavage suggests that they could represent preexistent Mg-rich pyroxene. Similar to the common schist in Bonfim, the anthophyllite–phlogopite-bearing schist near the skarns is metasomatically converted in places into actinolite–anthophyllite–phlogopite-bearing schist. The anthophyllite–phlogopite-bearing schist contains phlogopite and biotite with low Fe/(Fe + Mg) (between 0.2 and 0.4) and anthophyllite with a composition close to Mg5Fe2Si8O22(OH)2 (MgO approximately 22 wt% and Al2O3 <0.7 wt%). In the actinolite–anthophyllite–phlogopite-bearing schist, the metasomatically formed amphibole has a composition varying from actinolite to magnesiohornblende with a high MgO (>16.5 wt%). The whole-rock geochemical analyses of three representative samples indicate that the anthophyllite–phlogopite-bearing schist contains high MgO (approximately 15%), Cr (up to 574 ppm), and Ni (up to 237 ppm) contents, besides V (up to 198 ppm), Co (up to 51 ppm), and Sc (up to 25 ppm). The anthophyllite–phlogopite-bearing schist has higher MgO, K2O, S, Cr, Co, Ni, and Rb contents, and lower Na2O, P2O5, Zn, Sr, Zr, and Pb contents than the average composition of the Seridó regional schist. The 87Sr/86Sr ratio in two representative samples of the Bonfim schists revealed that the anthophyllite–phlogopite-bearing schist has higher ratio (0.89455) than the enclosing common schist (0.74514), indicating distinct ages of the source rocks for the sedimentary protolith of these two schists, with evidence for an older age of the former source. The original sediments of the anthophyllite–phlogopite-bearing schist could have had an Archean komatiitic source (e.g., komatiitic basalt, peridotite, websterite or pyroxenite), as indicated by the Mg-rich metamorphic minerals (phlogopite and anthophyllite), Cr–K-rich metasomatic ones (fuchsite), and the peculiar MgO, Cr, Ni, Co, Sc, and K2O contents. This source could be part of an Archean greenstone belt now dismantled and/or removed by tectonic and erosion processes. Alternatively, the Mg-rich schist could be a remnant of the upper sedimentary unit of such an ancient rock sequence. The unusual schist could potentially guide gold and emerald deposit prospecting in the region.

Amalgamation in the Central African Shield (CAS) by the Kibaran orogen: New hypothesis and implications for the Rodinia assembly

The Central African Shield has been built in the Mesoproterozoic time coevally with the “Rodinia assembly”. By the beginning of the Neoproterozoic, this Rodinia supercontinent was dispersed to a new assembly (the Gondwana supercontinent) built during the Neoproterozoic by the Brasiliano-Pan-African belts. During the Rodinia assembly, the western and eastern previous blocks (Congo Craton, Tanzanian Craton and Bangweulu Block) were amalgamated by the Kibaran Belt. The aim of this paper is to better understand the collage of blocks (or cratons), which constitute the Central African Shield during the Kibaran orogeny, which leads to the Kibaran belts by the Middle and Upper Mesoproterozoic time (1400-1000 Ma). The cratons involved in this Mesoproterozoic collage are: the Congo Craton, the Tanzanian Block and the Bangweulu Block themselves made by the amalgamation of Archean and Paleoproterozoic terranes. However, this collage was the result of a long geodynamic evolution with at least three Kibaran orogenic events (ca.1400-1375, 1375-1180 Ma and 1180-1000 Ma). Each orogenic event comprised: deposition of sediments, tectono-metamorphic events with granitic intrusions and extensive to compressive tectonic events with molassic deposits. The early event (ca.1400-1375 Ma) leads to a “triple junction rifting” hypothesis. Consequently, this geodynamic history has a regional impact on the Rodinia evolution, on the following Neoproterozoic belts and likely on the geometry of the African rift.

Petrogenesis of granitic pegmatite veins: Perspectives from major element and B isotope in tourmalines, Chakabeishan, Northern Tibetan Plateau

The petrogenesis of regionally zoned granitic pegmatite veins remains debated. Because of the economic significance, we carried out a study on the Chakabeishan (CKBS) pegmatite-type Li-Be deposit, eastern North Qaidam Tectonic Belt, Northern Tibetan Plateau, by means of in-situ major element and B isotope compositions of tourmalines in the beryl-bearing and spodumene-bearing pegmatite veins. Tourmalines (Tur-Be) from the beryl-bearing pegmatite are homogeneous schorl with low Mg/(Mg + Fe), high Na/(Na + Ca) and YAl, suggesting that they are of magmatic origin. Two generations of tourmalines (Tur-Li) from the spodumene-bearing pegmatite are identified: (i) the crystal cores (mostly elbaite and Li-rich schorl with subordinate schorl) are consistent with being of magmatic origin crystallized at the magmatic stage; (ii) the crystal rims (schorl) are best understood as the overgrowth at the later hydrothermal stage. Tur-Be and Tur-Li show an obvious difference in core-to-rim B isotopic variation trend with δ11B decrease in Tur-Be and increase in Tur-Li. The core-to-rim δ11B decrease in Tur-Be results from degassing during its host pegmatitic melt evolution, whereas the core-to-rim δ11B increase in Tur-Li is related to fluid exsolution. The estimated δ11B values for the initial melts of the beryl-bearing and spodumene-bearing pegmatites are −10.46‰ and −10.78‰, respectively, indicating that they most likely originate from protracted fractional crystallization/differentiation of granitic intrusions rather than partial melting of metapelite. Both Mg/(Mg + Fe) ratios and Li abundances in the cores of Tur-Be are lower than those of Tur-Li, suggesting that Tur-Li crystallizes from chemically more evolved melts.

Zircon xenocrysts obscured the zircon date for the lower Koras Group, southern Africa

Abstract The Koras Group is a bimodal volcanosedimentary group located in post-tectonic grabens in a foreland thrust complex in the Kaaien Terrane of the Mesoproterozoic Namaqua-Natal Province of southern Africa. It contains two sequences of mafic and felsic volcanic rocks with an unconformity between them, only the lower sequence being slightly folded. The Koras Group was long regarded as having formed at the end of the 1 210 to 1 000 Ma Namaqua Orogeny, because it lacks the severe deformation and metamorphism of the underlying rocks, with igneous minerals preserved in many samples. Following years of unsuccessful attempts to precisely date the volcanic rocks, the first two ion probe U-Pb zircon studies both reported ages of ~1 172 Ma for the Swartkopsleegte Formation felsic lava in the slightly folded lower sequence (based on relatively few dated zircons) and ~1 100 Ma for the Leeuwdraai Formation rhyolite in the undeformed upper sequence. Thus a major 70 m.y. hiatus seemed apparent between the lower and upper sequences despite their similar geochemistry and rift-related setting. This gave rise to models which envisaged the Kaaien Terrane being unaffected by the syn- to late-tectonic deformation, migmatisation and granite intrusions, documented between 1 200 and 1 150 Ma in the adjoining Namaqua-Natal terranes to the west. A high-pressure (10 kbar) metamorphic event, recognised in the Kaaien Terrane basement just south of hardly-deformed Koras Group exposures and dated at 1 150 Ma, is inconsistent with such models. A re-investigation and microbeam dating campaign on the Koras Group confirms the 1 101 ± 2 Ma (n = 6) age for felsic volcanic rocks of the upper sequence, but establishes a new reliable age of 1 114 ± 4 Ma for the lower one (n = 2). The 1 170 ages obtained in the earlier two studies were revisited and are now considered to reflect the age of zircon xenocrysts from the source rocks, which dominate the zircon population of some Swartkopsleegte Formation samples. Several criteria to distinguish autocrystic (magmatic) from antecrystic (age-overlappping xenocrystic) data points were investigated. One sample had high Th levels in only the younger zircons, but histograms of sufficiently precise 207Pb/206Pb ages provided the main criterion. Calculations of zircon crystallisation temperature intervals were not useful in predicting the abundance or proportions of magmatic and antecryst zircons. A multi-episode model of magmatic generation and crystallisation events is probably appropriate. In cases when felsic volcanic samples yield few zircons, care must be taken to avoid the problem exposed in this study. The Koras Group sediments have similar detrital zircon U/Pb age distributions to those of the Rehoboth Basement Inlier. This supports the concept that the Kaaien Terrane originated as the southern part of the Rehoboth Province.

Mapping Structurally Controlled Alterations Sparked by Hydrothermal Activity in the Fatira–Abu Zawal Area, Eastern Desert, Egypt

AbstractThe Eastern Desert of Egypt suffered a protracted period of deformation triggered by cratonization of the new juvenile crust known as the Arabian Nubian Shield (ANS), which has been proposed for potential gold discoveries associated with the corresponding tectonic event. The Fatira area, on the border of Egypt's Northern and Central Eastern Deserts, is covered with metavolcanic rocks twisted by a dextral relocation of the Fatira Shear Zone (FSZ) relative to the Barud magmatic body. The recent study evaluated many deformed post‐orogenic granitic intrusions and felsite dikes associated with promising mineralization localities, notably orogenic gold deposits. The combination of various field observations and remote sensing data, followed by the analysis of aeromagnetic enhanced maps, allowed the differentiation of distinct lithologies, structural features, and hydrothermal alterations in the study area. Additionally, the integrated results obtained from the different interpretation techniques are utilized to identify and confirm the previously supposed mineralized localities in the Fatira and Abu Zawal areas and predict other matched localities. The final ASTER, Sentinel 2 hydrothermal alteration, and orientation entropy heat maps demonstrate the association between these mineralized regions and major structures related to the FSZ late stage of deformation rather than other structures studied throughout the area of interest.