2020 Vol. 11, No. 6

Research Paper
Quantitative study on hydrocarbon expulsion mechanism based on micro-fracture
Kaiming Su, Jungang Lu, Huanxu Zhang, Shijia Chen, Yong Li, Zhenglu Xiao, Wen Qiu, Meimei Han
2020, (6): 1901-1913. doi: 10.1016/j.gsf.2020.05.013
Abstract(191) HTML PDF(25)
Abstract:
The significance of source rocks for oil and gas accumulation has been indisputably acknowledged. Moreover, it has been gradually realized that there is difference between hydrocarbon generation capacity and hydrocarbon expulsion capacity, and this has prompted research on hydrocarbon expulsion efficiency. However, these studies dominantly highlight the results of hydrocarbon expulsion, and investigation into the corresponding process and mechanism is primarily from a macroscopic perspective. Despite its wide acceptance as the most direct hydrocarbon expulsion mode, hydrocarbon expulsion through micro-fractures is still not sufficiently understood. Therefore, this study obtains observations and performs experiments on two types of source rocks (mudstones and shales) of the Chang 7 oil group of the Yanchang Formation in Ordos Basin, China. Microscopy reveals that organic matter is non-uniformly distributed in both types of source rocks. Specifically, mudstones are characterized by a cluster-like organic matter distribution, whereas shales are characterized by a layered organic matter distribution. Thermal evolution simulation experiments demonstrate that the hydrocarbon generation process is accompanied by the emergence of micro-fractures, which are favorable for hydrocarbon expulsion. Moreover, based on the theories of rock physics and fracture mechanics, this study establishes micro-fracture development models for both types of source rocks, associated with the calculation of the fracture pressure that is needed for the initiation of fracture development. Furthermore, the relationship between the fluid pressure, fracture pressure, and micro-fracture expansion length during micro-fracture development is quantitatively explored, which helps identify the micro-fracture expansion length. The results indicate that the development of micro-fractures is commonly impacted by the morphology and distribution pattern of the organic matter as well as the mechanical properties of the source rocks. The micro-fractures in turn further affect the hydrocarbon expulsion capacity of the source rocks. The results of this study are expected to provide theoretical and practical guidance for the exploration and exploitation of tight oil and shale oil.
P-T path of metamorphism and U-Pb monazite and zircon age of the Kitoy terrane: Implication for Neoarchean collision in SW Siberian Craton
Vasiliy P. Sukhorukov, Valentina B. Savel’eva, Yingde Jiang, Zhiyong Li
2020, (6): 1915-1934. doi: 10.1016/j.gsf.2020.05.012
Abstract(161) HTML PDF(8)
Abstract:
The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane (SW Siberian Craton). Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution. The first stage of regional metamorphism (M1) occurred at high-amphibolite facies conditions at T ¼ 780–800 C and P ¼ 8–9 kbar. The second stage (M2) belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600–750 C and pressure 2–4 kbar. Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses. Both of them correspond to the Neoarchean age: the age of M1 falls into the interval of ca. 2489–2496 Ma, the age of M2 – ca. 2446–2456 Ma. The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton.
Testing final closure time of the Paleo-Asian Ocean along the Solonker suture by a transition of compressional and extensional setting
Haidong Zhang, Jianchao Liu, Jinkun Yang, Jiakun Ge, Jinya Wang, Zhen Li
2020, (6): 1935-1951. doi: 10.1016/j.gsf.2020.05.020
Abstract(272) HTML PDF(8)
Abstract:
The giant Central Asian Orogenic Belt is an extensive accretionary orogen, of which the Solonker suture, as a major regional suture, coincides closely with an early Permian paleobiogeographical boundary. This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane. Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian–Early Triassic, uncertainty remains because of a lack of typical collisionrelated features (e.g., high-grade regional metamorphism and well-developed fold–thrust structures) and a scarcity of outcrops. The present study reports Early Permian foliated gabbros and dikes (288–275 Ma) and Middle-Late Permian undeformed layered gabbros, strongly peraluminous granites, and I-type granites (265–254 Ma) in the Xinhure area along the northern margin of the North China Craton. The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton. In contrast, the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle–upper crust, respectively, triggered by asthenospheric upwelling. Therefore, a transition from an end-compressional to an extensional environment according to a transition from collision termination to postcollision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma. This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.
Petrology and geochemistry of South Mid-Atlantic Ridge (19S) lava flows: Implications for magmatic processes and possible plume-ridge interactions
Haitao Zhang, Xuefa Shi, Chuanshun Li, Quanshu Yan, Yaomin Yang, Zhiwei Zhu, Hui Zhang, Sai Wang, Yili Guan, Renjie Zhao
2020, (6): 1953-1973. doi: 10.1016/j.gsf.2020.06.007
Abstract(129) HTML PDF(7)
Abstract:
The South Mid-Atlantic Ridge (SMAR) 19S segment, approximately located along the line of Saint Helena volcanic chain (created by Saint Helena mantle plume), is an ideal place to investigate the issue whether the ridgehotpot interaction process affected the whole MAR. In this study, we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19S segment. Major oxides compositions show that all samples are tholeiite. Low contents of compatible trace elements (e.g., Ni ¼ 239–594 ppm and Cr ¼ 456–1010 ppm) and low Fe/Mn (54–67) and Ce/Yb (0.65–1.5) ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle source. Using software PRIMELT3, this study obtained mantle potential temperatures (Tp) beneath the segment of 1321–1348 C, which is lower relative to those ridges influenced by mantle plumes. The asthenospheric mantle beneath the SMAR 19S segment starts melting at a depth of ~63 km and ceases melting at ~43 km with a final melting temperature of ~1265 C. The extent of partial melting is up to 16%–17.6% with an average adiabatic decompression value of 2.6%/kbar. The correlations of major oxides (CaO/Al2O3) and trace elements (Cr, Co, V) with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface. 87Sr/86Sr (0.702398–0.702996), 143Nd/144Nd (0.513017–0.513177) and 206Pb/204Pb (18.444–19.477) ratios of these lavas indicate the mantle source beneath the SMAR 19S segment is composed of a three-component mixture of depleted MORB mantle, PREMA mantle, and HIMU mantle materials. The simple, binary mixing results among components from plume-free SMAR MORB, Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19S segment may be polluted by both Saint Helena and Tristan plume enriched materials. The abovementioned mantle potential temperatures, together with the low Saint Helena (<10%) and Tristan (<5%) components remaining in the asthenospheric mantle at present, show that the physically ridge-hotspot interactions at SMAR 19S segment may have ceased. However, the trace element and Sr- Nd-Pb isotopically binary mixing calculation results imply that these lavas tapped some enriched pockets left when Saint Helena and/or Tristan plume were once on the SMAR during earlier Atlantic rifted history.
Early Paleozoic geodynamic evolution of the Eastern Central Asian Orogenic Belt: Insights from granitoids in the Xing’an and Songnen blocks
Xin-Lu Hu, Shu-Zhen Yao, Cheng-Yin Tan, Guo-Ping Zeng, Zhen-Ju Ding, Mou-Chun He
2020, (6): 1975-1992. doi: 10.1016/j.gsf.2020.05.018
Abstract(169) HTML PDF(6)
Abstract:
The early Paleozoic tectonic framework and evolutionary history of the eastern Central Asian Orogenic Belt (CAOB) is poorly understood. Here we present zircon U–Pb geochronology, whole rock geochemistry, and Sr-Nd- Hf isotope data of the early Paleozoic granitoids in eastern CAOB to investigate the petrogenesis and geodynamic implications. The early Paleozoic granitoids from the Songnen Block yield zircon U–Pb ages of 523–490 Ma, negative εNd(t) values of –6.7 to –0.8, and εHf(t) values of –8.6 to 7.1, indicating they were generated by partial melting of ancient crustal materials with various degrees of mantle contribution. They generally show affinities to A-type granites, implying their generation from an extensional environment after the collision between the Songnen and Jiamusi blocks. In comparison, the early Paleozoic granitoids from the Xing’an Block have zircon U–Pb ages of 480–465 Ma, εNd(t) values of –5.4 to 5.4, and εHf(t) values of –2.2 to 12.9, indicating a dominated juvenile crustal source with some input of ancient crustal components. They belong to I-type granites and were likely related to subduction of the Paleo-Asian Ocean. The statistics of TDM2 Hf model ages of the granitoids indicate that the Erguna and Jiamusi blocks contain a significant proportion of Mesoproterozoic crystalline basement, while the Xing’an Block is dominated by a Neoproterozoic basement. Based on these observations, the early Paleozoic evolutionary history of eastern CAOB can be divided into four stages: (1) before 540 Ma, the Erguna, Xing’an, Songnen, and Jiamusi blocks were discrete microcontinents separated by different branches of the Paleo-Asian Ocean; (2) 540–523 Ma, the Jiamusi Block collided with the Songnen Block along the Mudanjiang suture; (3) ca. 500 Ma, the Erguna Block accreted onto the Xing’an Block along the Xinlin–Xiguitu suture; (4) ca. 480 Ma, the Paleo-Asian Ocean started a double-side subduction beneath the united Erguna–Xing’an and Songnen–Jiamusi blocks.
Physics informed machine learning: Seismic wave equation
Sadegh Karimpouli, Pejman Tahmasebi
2020, (6): 1993-2001. doi: 10.1016/j.gsf.2020.07.007
Abstract(505) HTML PDF(51)
Abstract:
Similar to many fields of sciences, recent deep learning advances have been applied extensively in geosciences for both small- and large-scale problems. However, the necessity of using large training data and the ‘black box’ nature of learning have limited them in practice and difficult to interpret. Furthermore, including the governing equations and physical facts in such methods is also another challenge, which entails either ignoring the physics or simplifying them using unrealistic data. To address such issues, physics informed machine learning methods have been developed which can integrate the governing physics law into the learning process. In this work, a 1- dimensional (1D) time-dependent seismic wave equation is considered and solved using two methods, namely Gaussian process (GP) and physics informed neural networks. We show that these meshless methods are trained by smaller amount of data and can predict the solution of the equation with even high accuracy. They are also capable of inverting any parameter involved in the governing equation such as wave velocity in our case. Results show that the GP can predict the solution of the seismic wave equation with a lower level of error, while our developed neural network is more accurate for velocity (P- and S-wave) and density inversion.
Apatite U–Pb dating and geochemistry of the Kyrgyz South Tian Shan (Central Asia): Establishing an apatite fingerprint for provenance studies
Stijn Glorie, Samantha March, Angus Nixon, Fun Meeuws, Gary J. O’Sullivan, David M. Chew, Christopher L. Kirkland, Dmitry Konopelko, f, Johan De Grave
2020, (6): 2003-2015. doi: 10.1016/j.gsf.2020.06.003
Abstract(158) HTML PDF(6)
Abstract:
This paper presents an apatite U–Pb and geochemistry archive for exposed plutons and metamorphic rocks of the Kyrgyz South Tian Shan (STS) within the Central Asian Orogenic Belt. Apatite U–Pb dates and trace-element geochemistry are provided for 17 samples from late Carboniferous – early Permian I-type granites in the Terktinsky complex and A-type granites in the Kokshaal Range; early Devonian granites in the Kembel complex; Cryogenian granitoids and tuffs from the Middle Tian Shan and gneisses from the Atbashi metamorphic complex. These samples form a comprehensive selection of igneous and metamorphic rocks within the cores of Mesozoic basement highs that supplied detritus to adjacent basins such as the Tarim, Ferghana and Yarkand-Ferghana Basins. Generally, the granitoid samples preserve primary igneous apatite U–Pb ages that are within uncertainty of previously published zircon U–Pb dates. The apatites from the Atbashi metamorphic complex record anomalous Ordovician dates with large uncertainties that are interpreted as mixing ages between Cryogenian protolith formation and Carboniferous metamorphism. Principal component analysis discriminates apatite samples from the different bedrock terranes in the Kyrgyz STS based on their geochemical fingerprint and categorizes the samples with respect to an extensive apatite geochemical archive. The combined apatite-zircon archive provides a novel framework for provenance studies on the Meso–Cenozoic sedimentary history of the Central Asian Orogenic Belt.
Petrogenesis and tectonic significance of the early Paleozoic Delenuoer ophiolite in the Central Qilian Shan, northeastern Tibetan Plateau
Zhen Ma, Jinrong Wang, Liangliang Zhang, Yixin Liu, Yunyun Gao, Xiang Zhang, Sheng Yu, Cheng Zhang
2020, (6): 2017-2029. doi: 10.1016/j.gsf.2020.06.002
Abstract(180) HTML PDF(4)
Abstract:
The newly discovered early Paleozoic Delenuoer ophiolite, in the western margin of the Central Qilian Shan, is composed of serpentinized peridotite, cumulate gabbro, diabase, massive basalt, and pillow basalt. This study presents geochronological and geochemical data for the cumulate gabbro and basalt. LA–ICP–MS U–Pb dating of zircons from the cumulate gabbro yielded a magmatic crystallization age of 472  4 Ma. The basalts have normal mid-ocean ridge basalt (N-MORB) compositions and a narrow range of εNd(t) values (þ4.5 to þ5.3), which indicates they were derived from a depleted mantle source. On the basis of regional geological constraints, it is proposed that the Delenuoer ophiolite is a westward extension of the South Ophiolite Belt (Yushigou– Youhulugou–Donggou–Dongcaohe Ophiolite Belt) in the North Qilian Shan. The Delenuoer ophiolite, along with the Gulangxia–Delenuoer fault, defines the westernmost part of the tectonic boundary between the North and Central Qilian Shan. This ophiolite may have formed during southward subduction of the Qilian Ocean slab during the early Paleozoic.
Source-to-sink of Late carboniferous Ordos Basin: Constraints on crustal accretion margins converting to orogenic belts bounding the North China Block
Anqing Chen, Hao Zou, James G. Ogg, Shuai Yang, Mingcai Hou, Xiuwei Jiang, Shenglin Xu, Xiaoxing Zhang
2020, (6): 2031-2052. doi: 10.1016/j.gsf.2020.05.008
Abstract:
The Upper Carboniferous Benxi Formation of the Ordos Basin is the lowest strata overlying Middle Ordovician above the major ca. 150-myr sedimentary gap that characterizes the entire North China Block (NCB). We apply an integrated analysis of stratigraphy, petrography, and U–Pb dates and Hf isotopes on detrital zircons to investigate its provenance and relationships to the progressive collisions that formed the Xing’an-Mongolia Orogenic Belt to the north and the Qinling Orogenic Belt to the south. The results show that, in addition to regional patterns of siliciclastic influx from these new uplifted sources, the Benxi Formation is composed of two sequences corresponding to long-term glacial-interglacial cycles during the Moscovian to lower Gzhelian stages which drove global changes of eustatic sea level and weathering. The spatio-temporal distribution of sediment isopachs and facies indicate there were two sediment-infilling pulses, during which the southern and the northern Ordos Basin developed tidal-reworked deltas. The age spectra from detrital zircons, trace element patterns and εHf(t) values reveal that the siliciclastics forming the southern delta was sourced in the Qinling Orogenic Belt, whereas the northern delta was derived from the Xing’an-Mongolia Orogenic Belt. The source-to-sink evolution of this Upper Paleozoic system records the progressive development of orogenic belts and uplifts forming on the southern and northern margins of the NCB prior to its collisions with the South China and the Siberian plates, respectively.
An analysis of vehicular exhaust derived nanoparticles and historical Belgium fortress building interfaces
Luis F.O. Silva, Diana Pinto, Alcindo Neckel, Marcos L.S. Oliveira
2020, (6): 2053-2060. doi: 10.1016/j.gsf.2020.07.003
Abstract:
Air pollution monitoring is one of the most important features in contamination risk management. This is because many of the compounds contained within air pollution present a serious risk both for the preservation of open air cultural heritage and for human health. New particle formation is a major contributor to urban pollution, but how it occurs in cities is often puzzling. As more and more people enjoy an increased quality of life through outdoor activity, managing outdoor air quality is vital. This study presents the application of a low-cost system for monitoring the current level of road traffic passengers’ exposure to particulate air contamination. The global rise in tourism also leads to apprehension about its probable destructive influence on various aspects of global preservation. One of the major risks encountered by tourists, stemming from modes of transport, are nanoparticles (NPs) (100 nm) and ultra-fine particles (UFPs) (100–1000 nm) consisting of potentially hazardous elements (PHEs). This study examines Steen Castle, a medieval fortress located in Antwerp, Belgium. Significant NPs with PHEs, were found in the air sampled in this area. The self-made passive sampler (LSPS) described in this study, consisting of retainers specially designed for advanced microscopic analysis, is used for the first time as a simple way to characterize the surrounding atmospheric contamination caused by NPs and UFPs, without the need of other commonly employed more expensive particulate focused active samplers such as cascade impactors. This study aims to assess the result of the utilization of a low-cost, LSPS, to determine outdoor NPs and UFPs in a Belgian urban (Steen Castle) and rural area (Fort van Schoten). This work is the first to detail the usefulness of LSPS for the evaluation of Belgium’s outdoor air for NPs and UFPs, which contain PHEs.
A 32-million year cycle detected in sea-level fluctuations over the last 545 Myr
Michael R. Rampino, Ken Caldeira
2020, (6): 2061-2065. doi: 10.1016/j.gsf.2020.06.005
Abstract(176) HTML PDF(8)
Abstract:
Spectral analyses of past relative sea-level oscillations as represented by the ages of 57 Phanerozoic (the last 545 Myr) stratigraphic sequence boundaries from the Canadian Arctic show a strong spectral peak at 32 Myr (>99.9% confidence). These findings concur with previous reports of significant cycles with periods of around 30 Myr in various records of fluctuations of sea level, and in potentially related episodes of tectonism, volcanism, climate, and biotic extinctions. Sequence boundaries commonly coincide with stage boundaries based on biostratigraphy, and are correlated with episodes of extinction and times of flood-basalt volcanism. The connection between tectonics and sea-level variations may come from changes in rates of ocean-floor spreading and subduction, intraplate stresses from plate-reorganizations, and pulsations of hotspot volcanism. These coordinated periodic fluctuations in tectonics, sea level and climate may be modulated by cyclical activity in the Earth’s mantle, although some pacing by astronomical cycles is suspected.
A machine learning approach to tungsten prospectivity modelling using knowledge-driven feature extraction and model confidence
Christopher M. Yeomans, Robin K. Shail, Stephen Grebby, Vesa Nyk€anen, Maarit Middleton, Paul A.J. Lusty
2020, (6): 2067-2081. doi: 10.1016/j.gsf.2020.05.016
Abstract(182) HTML PDF(13)
Abstract:
Novel mineral prospectivity modelling presented here applies knowledge-driven feature extraction to a datadriven machine learning approach for tungsten mineralisation. The method emphasises the importance of appropriate model evaluation and develops a new Confidence Metric to generate spatially refined and robust exploration targets. The data-driven Random Forest™ algorithm is employed to model tungsten mineralisation in SW England using a range of geological, geochemical and geophysical evidence layers which include a depth to granite evidence layer. Two models are presented, one using standardised input variables and a second that implements fuzzy set theory as part of an augmented feature extraction step. The use of fuzzy data transformations mean feature extraction can incorporate some user-knowledge about the mineralisation into the model. The typically subjective approach is guided using the Receiver Operating Characteristics (ROC) curve tool where transformed data are compared to known training samples. The modelling is conducted using 34 known true positive samples with 10 sets of randomly generated true negative samples to test the random effect on the model. The two models have similar accuracy but show different spatial distributions when identifying highly prospective targets. Areal analysis shows that the fuzzy-transformed model is a better discriminator and highlights three areas of high prospectivity that were not previously known. The Confidence Metric, derived from model variance, is employed to further evaluate the models. The new metric is useful for refining exploration targets and highlighting the most robust areas for follow-up investigation. The fuzzy-transformed model is shown to contain larger areas of high model confidence compared to the model using standardised variables. Finally, legacy mining data, from drilling reports and mine descriptions, is used to further validate the fuzzy-transformed model and gauge the depth of potential deposits. Descriptions of mineralisation corroborate that the targets generated in these models could be undercover at depths of less than 300 m. In summary, the modelling workflow presented herein provides a novel integration of knowledge-driven feature extraction with data-driven machine learning modelling, while the newly derived Confidence Metric generates reliable mineral exploration targets.
Ophiolite hosted chromitite formed by supra-subduction zone peridotite –plume interaction
Júlia Farr e-de-Pablo, Joaquín A. Proenza, Jos e María Gonz alez-Jim enez, Thomas Aiglsperger, Antonio Garcia-Casco, Javier Escuder-Viruete, Vanessa Col as, Francisco Longo
2020, (6): 2083-2102. doi: 10.1016/j.gsf.2020.05.005
Abstract(150) HTML PDF(13)
Abstract:
Chromitite bodies hosted in peridotites typical of suboceanic mantle (s.l. ophiolitic) are found in the northern and central part of the Loma Caribe peridotite, in the Cordillera Central of the Dominican Republic. These chromitites are massive pods of small size (less than a few meters across) and veins that intrude both dunite and harzburgite. Compositionally, they are high-Cr chromitites [Cr# ¼ Cr/(Cr þ Al) atomic ratio ¼ 0.71–0.83] singularly enriched in TiO2 (up to 1.25 wt.%), Fe2O3 (2.77–9.16 wt.%) as well as some trace elements (Ga, V, Co, Mn, and Zn) and PGE (up to 4548 ppb in whole-rock). This geochemical signature is unknown for chromitites hosted in oceanic upper mantle but akin to those chromites crystallized from mantle plume derived melts. Noteworthy, the melt estimated to be in equilibrium with such chromite from the Loma Caribe chromitites is similar to basalts derived from different source regions of a heterogeneous Caribbean mantle plume. This mantle plume is responsible for the formation of the Caribbean Large Igneous Province (CLIP). Dolerite dykes with back-arc basin basalt (BABB) and enriched mid-ocean ridge basalt (E-MORB) affinities commonly intrude the Loma Caribe peridotite, and are interpreted as evidence of the impact that the Caribbean plume had in the off-axis magmatism of the back-arc basin, developed after the Caribbean island-arc extension in the Late Cretaceous. We propose a model in which chromitites were formed in the shallow portion of the back-arc mantle as a result of the metasomatic reaction between the supra-subduction zone (SSZ) peridotites and upwelling plume-related melts.
Multi-stage rodingitization of ophiolitic bodies from Northern Apennines (Italy): Constraints from petrography, geochemistry and thermodynamic modelling
Emma Salvioli-Mariani, Tiziano Boschetti, Lorenzo Toscani, Alessandra Montanini, Jasmine Rita Petriglieri, Danilo Bersani
2020, (6): 2103-2125. doi: 10.1016/j.gsf.2020.04.017
Abstract:
The investigated mantle bodies from the External Ligurians (Groppo di Gorro and Mt. Rocchetta) show evidences of a complex evolution determined by an early high temperature metasomatism, due to percolating melts of asthenospheric origin, and a later metasomatism at relatively high temperature by hydrothermal fluids, with formation of rodingites. At Groppo di Gorro, the serpentinization and chloritization processes obliterated totally the pyroxenite protolith, whereas at Mt. Rocchetta relics of peridotite and pyroxenite protoliths were preserved from serpentinization. The rodingite parageneses consist of diopside þ vesuvianite þ garnet þ calcite þ chlorite at Groppo di Gorro and garnet þ diopside þ serpentine  vesuvianite  prehnite  chlorite  pumpellyite at Mt. Rocchetta. Fluid inclusion measurements show that rodingitization occurred at relatively high temperatures (264–334 C at 500 bar and 300–380 C at 1 kbar). Garnet, the first phase of rodingite to form, consists of abundant hydrogarnet component at Groppo di Gorro, whereas it is mainly composed of grossular and andradite at Mt. Rocchetta. The last stage of rodingitization is characterized by the vesuvianite formation. Hydrogarnet nucleation requires high Ca and low silica fluids, whereas the formation of vesuvianite does not need CO2-poor fluids. The formation of calcite at Groppo di Gorro points to mildly oxidizing conditions compatible with hydrothermal fluids; the presence of andradite associated with serpentine and magnetite at Mt. Rocchetta suggests Fe3þ-bearing fluids with fO2 slightly higher than iron-magnetite buffer. We propose that the formation of the studied rodingite could be related to different pulses of hydrothermal fluids mainly occurring in an oceancontinent transitional setting and, locally, in an accretionary prism associated with intra-oceanic subduction.
Pb–Pb baddeleyite ages of mafic dyke swarms from the Dharwar Craton: Implications for Paleoproterozoic LIPs and diamond potential of mantle keel
D. Srinivasa Sarma, V. Parashuramulu, M. Santosh, E. Nagaraju, N. Ramesh Babu
2020, (6): 2127-2139. doi: 10.1016/j.gsf.2020.05.014
Abstract(156) HTML PDF(1)
Abstract:
The Dharwar Craton in Peninsular India was intruded by a series of mafic dykes during the Paleoproterozoic and these mafic magmatic events have important implications on continental rifting and LIPs. Here we report ten precise Pb–Pb TE-TIMS age determinations on baddeleyite grains separated from seven mafic dykes and three sills, intruding into Archean basement rocks and Proterozoic sedimentary formations of the Eastern Dharwar Craton respectively. The crystallization age of the baddeleyite shows 2366.3  1.1 Ma, and 2369.2  0.8 Ma for the NE–SW trending dykes, 2368.1  0.6 Ma, 2366.4  0.8 Ma, 2207.2  0.7 Ma and 1887.3  1.0 Ma for the ENE–WNW to E–W striking dykes, 1880.6  1.0 Ma, 1864.3  0.6 Ma and 1863.6  0.9 Ma for Cuddapah sills, and 1861.8  1.4 Ma for the N–S trending dyke. Our results in conjunction with those from previous studies identify eight distinct stages of widespread Paleoproterozoic magmatism in the Dharwar craton. The mantle plume centres of the four radiating dyke swarms with ages of ~2367 Ma, ~2210 Ma, ~2082 Ma, and ~1886 Ma were traced to establish their proximity to the EDC kimberlite province. Though the ~2367 Ma and ~1886 Ma plume centres are inferred to be located to the west and east of the present day Dharwar craton respectively away from the kimberlite province, location of plume heads of the other two swarms with ages of ~2207 Ma and ~2082 Ma are in close proximity. In spite of the ubiquitous occurrence of dyke intrusions of all the seven generations in the kimberlite province, only few of these kimberlites are diamondiferous. Kimberlite occurrences elsewhere in the vicinity of older Large Igneous Provinces (LIPs) like the Mackenzie, Karoo, Parana-Etendeka and Yakutsk-Vilui are also non-diamondiferous. This has been attributed to the destruction of the lithospheric mantle keel (that hosts the diamonds) by the respective mantle plumes. The diamondiferous nature of the EDC kimberlites therefore suggests that plume activity does not always result in the destruction of the mantle keel.
A method to evaluate REE-HFSE mineralised provinces by value creation potential, and an example of application: Gardar REE-HFSE province, Greenland
Graham J. Banks, Símun D. Olsen, Antonio Gusak
2020, (6): 2141-2156. doi: 10.1016/j.gsf.2020.05.019
Abstract(201) HTML PDF(2)
Abstract:
The rare earth elements and high-field-strength elements (REE-HFSE) exploration sector conducts most evaluations at deposit and smaller scales. It is not evident how the sector performs a preceding exploration stage—rating and prioritising REE-HFSE mineralised provinces—to determine which provinces are prospective enough to warrant investment. Here we present an objective, repeatable, low-cost method to screen any REE-HFSE province, as a foundation for district-scale investigations or asset evaluations. It is original for REE-HFSE screening, and adapted from regional scale copper, cobalt and petroleum exploration, and CO2 storage, screening methods. It is centred upon a mineralised province’s favourability for potential value creation, and to identify: (a) its main information gaps; (b) its weakest links; (c) its exploration maturity and remaining potential category; (d) how it compares against other REE-HFSE provinces; and (e) if further investigation is justified. This method incorporates geoscience, strategy, economic and socio-environmental factors in a way that is understandable and directly usable across stakeholder groups. The workflow is systematic, yet flexible enough to accommodate organisationspecific criteria, and usable for other commodities. It provides the platform to build a global REE-HFSE province map and database consistently across national boundaries and organisations. Categories for the extent of province exploration maturity and remaining mineral potential are proposed. We illustrate the applicability of these methods using the Gardar REE-HFSE Province (GRHP) of south Greenland. We conclude that it is a moderate size, frontier province that is currently of questionable favourability for value creation. To move GRHP into a positive favourability class, its current weak links need strengthening by research, government policy and industry stakeholders: evaluate the mineral system; integrate all information geospatially and place it in the public domain; help the region improve some community health and safety issues; convert some mineral resources into an Ore Reserves category; commence mining and sales production.