Loei Fold Belt Research Articles

Geochemistry of magnetite from Fe-skarn deposits along the central Loei Fold Belt, Thailand

Abstract Fe-deposits in Thailand are mostly located along the Loei Fold Belt in Central Thailand. The study areas, Khao Lek (KL) and Khao Thab Kwai (KTK), have Fe-deposits hosted in contact metamorphized Permian limestone and Permo-Triassic volcanic rocks. Magnetite forms over a wide range of geological conditions and incorporates various trace elements into its cubic spinel structure. The KL and KTK Fe-skarn deposits are mainly composed of massive magnetite. The main discriminator elements for magnetite are characterized by the distribution of Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, and Cu. Thus, this study focused on the chemical characteristics of magnetite to identify the origins of the iron ore deposits. This study investigated the trace element chemistry of the magnetite using laser ablation inductively coupled plasma mass spectrometry. The high Ca + Al + Mn and low Ti + V and Ni/Cr + Mn contents of hydrothermal magnetite in the KL and KTK deposits support a typical skarn origin, and the textural and trace element features of the KL and KTK magnetite are analogous to those of typical calcic-Fe skarn deposits worldwide.

Early Mesozoic granitoids in southern Vietnam and Cambodia: A continuation of the Eastern Province granitoid belt of Thailand

This study examines three granitoid plutons, intruded during the latest Triassic - earliest Jurassic, that are exposed in south-western Vietnam and south-eastern Cambodia. On geochemical and petrological grounds the two plutons in Vietnam (Hon Da Bac and Hon Khoai island) are composed of slightly peraluminous I-type granitoids, with a high-K calc-alkaline affinity. The Cambodian pluton (Tamao) consists of peraluminous S-type granitoids, with high-K calc-alkaline and ferroan affinity. Based on zircon U-Pb geochronology the Hon Da Bac granitoids were intruded between 200.7 ± 2.7 and 196.2 ± 3.0 Ma, those from Hon Khoai between 199.6 ± 2.7 and 197.6 ± 2.7 Ma and the Tamao granitoids at 187.8 ± 3.2 Ma. These ages are consistent with intrusion during and soon after the Indosinian II Event of the Indosinian Orogeny, which was driven by docking of the Sibumasu and Indochina Blocks. Although these granitoids have some geochemical signatures similar to those of arc-related magmas, we conclude that they are more likely derived from syn to post-collisional magmatism. In terms of timing of their emplacement and geochemistry, all three plutons can be convincingly correlated with the Late Triassic-Early Jurassic Eastern Province granitoids in the Loei Fold Belt, which stretches along the western margin of the Indochina Block, extending through eastern Thailand and southern Cambodia. By this correlation we show that in fact, the Eastern Province granitoid belt extends further southeast than previously thought, occurring in south-eastern Cambodia and south-western Vietnam.

Permian to Cretaceous granites and felsic volcanics from SW Vietnam and S Cambodia: Implications for tectonic development of Indochina

Zircon ages and geochemistry are presented for igneous rocks from SW Vietnam and S Cambodia. Four main age groupings occur: Cretaceous (107–75 Ma), Early Jurassic (195 Ma), Late Triassic (230–222 Ma), and Permian (294–265 Ma). Cretaceous and Jurassic samples are amphibole-bearing biotite granodiorites to biotite granites and occur mainly east of the Kampot Fold Belt. Pre-Cretaceous samples occur within the Kampot Fold Belt and are dominantly felsic volcanics. The rocks are primarily high-K calc-alkaline, weakly peraluminous rhyolites or granites, with similar arc-like trace element patterns. Cretaceous granites are similar to Dalat Zone Granites and formed during Pacific subduction. Two Cretaceous granites have adakite-like signatures. The youngest Cretaceous granite has an A-type signature and may represent post-collision activity following the cessation of subduction. Jurassic granites are also linked to Pacific subduction. Two Triassic samples could be affiliated to similarly aged rocks in the Chanthaburi Terrane, or linked to the Loei Fold Belt and related to closure to the Sa Kaeo back arc. We cannot exclude that they are related to Pacific Plate subduction. Correlation of Permian volcanics to paleo-Tethys subduction and the Sukhothai-Chanthaburi arc is not clear as the Cambodian volcanics are older than Chanthaburi rocks, and there are stratigraphic contrasts between S Cambodia and Chanthaburi. Furthermore, such a correlation requires extension of the Sa Kaeo suture into S Cambodia and SW Vietnam at a high angle to regional structures. We propose that the Permian volcanics are also related to Paleo-Pacific subduction as shown in regional reconstructions of the region.

Zircon Hf‐isotope constraints on the formation of metallic mineral deposits in Thailand

AbstractThe effects of the Sibumasu–Indochina Terranes collision created several kinds of mineral deposits in Thailand, which include porphyry–skarn copper–gold, epithermal gold and antimony, orogenic gold–antimony–tungsten and tin‐tungsten mineralization among others. The deposits show a distinct spatial zonal distribution and occur in specific tectonic terranes. Combining regional geological data and ore deposit distribution data with Hf‐isotopic data of zircons in igneous rocks can be used to investigate the relationship between crustal construction processes and metallogeny. In this study, we investigated the Sukhothai Fold Belt, which is composed of quartz monzodiorite, granodiorite, syenogranite, and monzogranite of I‐ and S‐type affinities. All granitoids were analyzed for zircon U–Pb geochronology and Lu–Hf isotopic analysis. The granitoids of the Sukhothai Fold Belt yielded U–Pb zircon ages ranging from ~243 to 202 Ma, which mark the timing of subduction to the syn‐collisional stage between the Sibumasu–Indochina terranes at ~243–237 Ma and the timing of post‐collision between the Sibumasu–Indochina terranes during 230–202 Ma. In addition, an age of ~43 Ma in the south of the Sukhothai Fold Belt may indicate intrusion during the sinistral movement of the Klaeng and Mae Ping fault zones resulted from the Indian–Eurasian plate collision. The Doi Tung quartz monzodiorite provided an age of ~350 Ma as a timing of formation of the Sukhothai Fold Belt. The negative and positive initial εHf values (−8.0 to +9.2) with two‐stage depleted mantle model ages (TDMC of 2.2–0.6 Ga) of zircons from the Sukhothai Fold Belt granitoids indicate that the sources of their magma derived from partial melting of old continental crust and young oceanic crust, which probably mixed with a mantle‐derived magma. A zircon Hf‐isotope compilation including the data obtained in this study and previously reported values was used to prepare a map that allows a comparison between magmatic source and mineral deposit distribution in Thailand. The spatial distribution of Hf isotopic data reveals a distinct zonation, with initial εHf values decreasing from the east to the west, that is, from the western margin of the Indochina Terrane or the Loei Fold Belt to the Sukhothai Fold Belt, the Inthanon Zone and the Sibumasu Terrane. The magmatic source for the granitoids in the Loei Fold Belt is dominated by mantle‐derived components, as shown by positive average initial εHf values (+1.0 to +12.7), and contributed to porphyry‐related skarn copper–gold and iron and epithermal gold mineralization. In contrast, magmas in the Sibumasu Terrane and the Inthanon Zone originated from melting of old crustal materials, as indicated by mostly negative average initial εHf values (−15.1 to +0.8), and are responsible for S‐type granite‐related tin‐tungsten mineralization. The average initial εHf values (−5.0 to +11.0) from the intrusions in the Sukhothai Fold Belt suggest mixed sources, including evolved and juvenile magmatic materials, which generated the orogenic gold deposits and other vein‐type antimony, tungsten, fluorite, and base metal deposits. These results imply a close spatial correlation between the source of magmatism in each tectonic terrane of Thailand and different metal ore deposits. These isotope maps can be used as a powerful tool in the exploration for various commodities at the regional scale.

The Truong Son, Loei-Phetchabun, and Kontum Terranes in Indochina: Provenance, Rifting, and Collisions

The three main regions of Indochina are defined as the Truong Son, Loei-Phetchabun, and Kontum terranes. The aim of this review is to integrate numerous petrological studies with sedimentary, palaeontological, and provenance studies in order to construct a preliminary tectonic model which shows the terranes docked in the earliest Carboniferous (Truong Son with Loei-Phetchabun) and in the Permian (Kontum). The Kontum Terrane is characterized by Proterozoic magmatism, mid-Ordovician to Early Devonian granites, and Permian charnockites. Major carbonate platforms developed in the Givetian to earliest Tournaisian on Truong Son and from the Visean to mid-Permian across Truong Son and Loei-Phetchabun terranes. The Truong Son has Silurian granites and a Late Ordovician to Silurian magmatic arc along its southern and western borders caused by subduction of oceanic lithosphere, the remnants of which are now partially preserved in the Loei and Tamky sutures. A region to the east of the Loei Suture in the Loei Foldbelt has a similar-age volcanic arc extending northwards into Laos and is included in Truong Son. A platform-margin coral-stromatoporoid reef developed on Truong Son in the Frasnian and was coincident with a gap in magmatism until the Pennsylvanian with granite magmatism became widespread until the Late Triassic. In the Loei-Phetchabun Terrane, Devonian siliciclastics, volcanics are overlain by Givetian limestones and patch reefs and in turn overlain by Late Devonian to earliest Tournaisian radiolarian cherts and siliciclastics. Tournaisian to early Visean siliciclastic turbidites were coincidental with initial uplift and deformation and were replaced by Visean terrestrial evaporites and coal and marine limestones. Arc magmatism along the Loei-Phetchabun Terrane indicates intermittent eastward subduction from mid-Devonian to mid-Triassic. Carboniferous-Triassic volcanism extends eastward along the northern margin of Cambodia and is probably an extension of Loei-Phetchabun. Ordovician within-plate basalts in Kontum and the transition from shallow marine Ordovician siliciclastics to Silurian deep-marine sediments and a volcanic arc in Truong Son suggest rifting of both terranes from Gondwana in the Late Ordovician. Initial docking of Truong Son with South China was probably in the late Silurian to Early Devonian and docking of Loei-Phetchabun and Truong Son was in the late Tournaisian to early Visean.

Zircon U–Pb chronology on plutonic rocks from northeastern Cambodia

Zircon U–Pb geochronology was carried out on plutonic rocks from Phnom Daek, Phnom Koy Rmeas, Svay Chras, Kon Mom, Koh Nheak, Andong Meas, Oyadav South, Svay Leu, and Phnom Soporkaley. The zircon U–Pb ages from the plutonic rocks determined in this study can be roughly divided into two groups. One is the Late Permian to Triassic ages of 278–202 Ma for the Phnom Daek, Phnom Koy Rmeas, Oyadav South, Svay Leu, and Phnom Soporkaley, and the other is the early Cretaceous ages of 118–98 Ma for the Svay Chras, Kon Mom, Koh Nheak, and Andong Meas samples. The plutonic rocks from Phnom Daek, Phnom Koy Rmeas, Svay Leu, Oyadav South, and Phnom Soporkaley were likely formed by magmatic activity in the Loei Fold Belt. These plutonic rocks were likely formed in an extensional setting and/or a region where the continental crust was thin. The plutonic rocks of Svay Chras, Kon Mom, Koh Nheak, and Andong Meas were likely formed by magmatic activity in the Dalat-Kratie Fold Belt, related to the NW-directed subduction of the Paleo-Pacific Ocean plate. These plutonic rocks are thought to correspond to the Dinhquan suite in southern Vietnam. The Kon Mom and Koh Nheak plutonic rocks fall within the alkaline series, which suggests that the magma genesis was deep and far from the western Paleo-Pacific Ocean plate. Magmatic activity in the Dalat-Kratie Fold Belt migrated oceanward as a whole during the Cretaceous.

Geochronology and petrogenesis of Carboniferous and Triassic volcanic rocks in NW Laos: Implications for the tectonic evolution of the Loei Fold Belt

The Loei Fold Belt is one of the most significant tectonic and metallogenic belts in the Indochina Block. However, origin and tectonic significance of volcanic rocks in several volcanic belts of this fold belt remain ambiguous. In this paper, we present new petrochemical data, zircon U-Pb ages and Lu-Hf isotopic data for volcanic rocks from the Loei Fold Belt in northwestern Laos. The basaltic and andesitic tuffaceous rocks have Late Carboniferous zircon U-Pb ages (304 ± 2.1 Ma and 299 ± 1.8 Ma respectively), and display both MORB-like and arc-like geochemical signatures. Zircon Lu-Hf isotopic analyses give highly positive εHf(t) values of 8.99–14.24, indicative of a metasomatic mantle source which formed in a back-arc basin environment. On the other hand, the andesite and rhyolite samples have zircon U-Pb ages ranging from 236 Ma to 224 Ma, erupted during the Late Triassic, and formed in a continental margin arc setting (e.g., enrichment of LREE, depletion in Nb-Ta, Sr, and positive εHf(t) values of 9.87–12.25). With a synthesis of available geochronological, petrochemical and isotopic data, magmatic events in the Loei Fold Belt occurred in four periods: Ordovician-Silurian (450–400 Ma), Late Devonian-Middle Permian (ca. 361–265 Ma), Early-Middle Triassic (250–237 Ma) and Late Triassic (ca. 236–200 Ma), with peaks in Triassic and Carboniferous. Accordingly, we propose a tectonic-magmatic evolution model for the Loei Fold Belt. The Luang Prabang-Loei belt was a spreading back-arc basin during the Late Devonian-Middle Permian, and connected with the Nan back-arc basin. The basin expanded to its largest extent in the Permian period and started subduction in the earliest Triassic; the successive subduction did not terminate until the Late Triassic.

Middle Triassic diorites from the Loei Fold Belt, NE Thailand: Petrogenesis and tectonic implications in the context of Paleotethyan subduction

Two Early-Middle Triassic magmatic arc belts occur in the Indochina Block within Thailand, namely the Lampang Volcanic Belt and the Loei-Phetchabun Volcanic Belt, which are about 150 km apart and run parallel to each other. The Lampang Volcanic Belt has been extensively studied and suggested to be induced by eastward Paleotethyan subduction, whereas the Loei-Phetchabun Volcanic Belt has not been well constrained due to the scarcity of geochronological and geochemical data. Here we report a detailed study on zircon U–Pb dating, elemental geochemistry and Sr–Nd isotopes for the Phu Rai diorites recently identified in Loei Province, NE Thailand. The Phu Rai diorites yield Middle Triassic (ca. 239 Ma) emplacement ages. They are calc-alkaline rocks characterized by arc-like trace element patterns and depleted Sr–Nd isotopic compositions ((87Sr/86Sr)i = 0.70371–0.70392; εNd(t) = +1.65 to +2.54). These geochemical and isotopic signatures reveal that the Phu Rai diorites were most likely derived from a depleted mantle wedge metasomatized by subducted sediment-derived melts and slab-derived fluids, and modified by limited degree of fractional crystallization involving clinopyroxene, amphibole and Fe–Ti oxides combined with weak plagioclase accumulation. Based on our results and others from the literature, we suggest the existence of an Early-Middle Triassic (ca. 250–239 Ma) magmatic arc belt in NE Thailand, which is not directly linked to eastward Paleotethyan subduction, but is possibly associated with eastward subduction of the Nan back-arc basin. Specifically, the ca. 239 Ma Phu Rai diorites and ca. 245–241 Ma adakites nearby form a rock association in response to slab break-off at that time, immediately after the initial stage of the Sukhothai-Indochina collision. The occurrence of this rock association could mark not only the cessation of eastward subduction of the Nan back-arc basin but also the initiation of the Sukhothai-Indochina collision.

Magma Genesis and Arc Evolution at the Indochina Terrane Subduction: Petrological and Geochemical Constraints From the Volcanic Rocks in Wang Nam Khiao Area, Nakhon Ratchasima, Thailand

Volcanic rocks and associated dikes have been exposed in Wang Nam Khiao area, Nakhon Ratchasima Province, northeastern Thailand where complex tectonic setting was reported. These volcanic rocks are classified as rhyolite, dacite, and andesite while dikes are also characterized by andesitic composition. These dikes clearly cut into the volcanic rocks and Late Permian hornblende granite in the adjacent area. Rhyolite and dacite are composed of abundant plagioclase and quartz whereas andesite and andesitic dike contain mainly plagioclase and hornblende with minor quartz. The volcanic rocks typically show plagioclase and hornblende phenocrysts embedded in fine-grained quartz and glass groundmass whereas dike rocks contain less glass matrix with more albitic laths. P-T conditions of crystallization are estimated, on the basis of Al-in-hornblende geobarometry and hornblende geothermometry, at about 4.5 – 5.5 kbar, 861 – 927 oC and 4.8 – 5.5 kbar, 873 – 890 oC for the magma intrusions that fed volcanic rocks and andesitic dikes, respectively. Whole-rock geochemistry indicates that these rock suites are related to calc-alkaline hydrous magma. The enriched LILE (e.g. Rb, Sr) and depleted HFSE (e.g. Nb, Ce, Ti), with similar REE patterns indicate arc magmatism. The results of this study are comparable to the continental arc magmatism along the Loei Fold Belt as a consequence of the Late Permian Palaeo-Tethys subducted beneath Indochina Terrane.

The Best Article Award 2018 of the Society of Resource Geology was presented to P. Nualkhao, R. Takahashi, A. Imai, and P. Charusiri for the following paper: Petrochemistry of Granitoids along the Loei Fold Belt, Northeastern Thailand. Resource Geology, 68(4), 395–424.

Resource GeologyVolume 70, Issue 1 p. 108-108 The Best Article Award 2018 The Best Article Award 2018 of the Society of Resource Geology was presented to P. Nualkhao, R. Takahashi, A. Imai, and P. Charusiri for the following paper: Petrochemistry of Granitoids along the Loei Fold Belt, Northeastern Thailand. Resource Geology, 68(4), 395–424. Daizo Ishiyama, Corresponding Author Daizo Ishiyama ishiyama@gipc.akita-u.ac.jp Chair of Awarding Committee for 2018 Society of Resource GeologySearch for more papers by this author Daizo Ishiyama, Corresponding Author Daizo Ishiyama ishiyama@gipc.akita-u.ac.jp Chair of Awarding Committee for 2018 Society of Resource GeologySearch for more papers by this author First published: 06 December 2019 https://doi.org/10.1111/rge.12223Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume70, Issue1January 2020Pages 108-108 RelatedInformation

Age, composition and tectonic implications of late Ordovician-early Silurian igneous rocks of the Loel Volcanic Belt, NW Laos

ABSTRACTVolcanic rocks preserved in the Houay kham area, Loei Fold Belt are important components of the giant Paleo-Tethyan igneous belt. These magmatic rocks, previously mapped as Triassic-Jurassic in age, are composed of dacitic tuff, brecciated rhyolite tuff and a small number of dioritic dikes. In this study, LA-ICP-MS zircon U-Pb dating results show that brecciated rhyolite tuff, dacitic tuff, and dioritic dyke samples have ages of 419.8 ± 1.6 Ma, 449.6 ± 2.6 Ma and 417 ± 4.9 Ma. These are the first documented Late Ordovician to Late Silurian volcanic rocks in the Loei Volcanic Belt or in Southeast Asia. Geochemical analyses of these rocks show that all are calc-alkali to high K, formed in a subduction-related, back-arc basin and are intermediate-acid igneous rocks. Dacitic and brecciated rhyolite tuff samples originated from partial melting of mafic crust with medium to strong crustal contamination and weak to medium fractional crystallization. Dioritic dyke samples originated from simultaneous partial melting of gabbroic and metasedimentary rocks with weak to no crustal contamination or fractional crystallization. In the early Ordovician the eastern branch of the Rheic ocean opened and the Indochina terrane drifted away from NE Gondwana and subsequently an Ordovician–Silurian Palaeo-Tethyan Ocean opened.

Petrochemistry of Granitoids Along the Loei Fold Belt, Northeastern Thailand

AbstractPetrochemical characteristics of Permo‐Triassic granitoids from five regions (i) Mung Loei, (ii) Phu Thap Fah – Phu Thep, (iii) Phetchabun, (iv) Nakon Sawan – Lobburi, and (v) Rayong – Chantaburi along the Loei Fold Belt (LFB), northeastern Thailand were studied. The LFB is a north–south trending 800 km fold belt that hosts several gold and base‐metal deposits. The granitoids consist of monzogranite, granodiorite, monzodiorite, tonalite, quartz‐syenite, and quartz‐rich granitoids. These are composed of quartz, plagioclase, and K‐feldspar with mafic minerals such as hornblende and biotite. Accessory minerals, such as titanite, zircon, magnetite, ilmenite, apatite, garnet, rutile, and allanite are also present. Magnetic susceptibilities in the SI unit of granitoids vary from 6.5 × 10−3 to 15.2 × 10−3 in Muang Loei, from 0.1 × 10−3 to 29.4 × 10−3 in Phu Thap Fah – Phu Thep, from 2.7 × 10−3 to 34.6 × 10−3 in Petchabun, from 2.4 × 10−3 to 14.1 × 10−3 in Nakon Sawan – Lobburi, and from 0.03 × 10−3 to 2.8 × 10−3 in Rayong – Chantaburi. Concentration of major elements suggests that these intermediate to felsic plutonic rocks have calc‐alkaline affinities. Concentration of REE of the granitoids normalized to chondrite displays moderately elevated light REE (LREE) and relatively flat heavy (HREE) patterns, with distinct depletion of Eu. Rb versus Y/Nb and Nb/Y tectonic discrimination diagrams illustrate that the granitoids from Muang Loei, Phu Thap Fah – Phu Thep, Phetchabun, Nakon Sawan – Lobburi, and Rayong – Chantaburi formed in continental volcanic‐arc setting. New age data from radiometric K‐Ar dating on K‐feldspar from granodiorite in Loei and Nakhon Sawan areas yielded 171 ± 3 and 221 ± 5 Ma, respectively. K‐Ar dating on hornblende separated from diorite in Lobburi yielded 219 ± 8 Ma. These ages suggest that magmatism of Muang Loei occurred in the Middle Jurassic, and Nakon Sawan – Lobburi occurred in Late Triassic. Both Nb versus Y and Rb versus (Y + Nb) diagrams and age data indicate that Nakon Sawan – Lobburi granitoids intruded in Late Triassic at Nong Bua, Nakon Sawan province and Khao Wong Phra Jun, Lobburi province in volcanic arc setting. Muang Loei granitoids at the Loei province formed later in Middle Jurassic also in volcanic arc setting. The negative δ34SCDT values of ore minerals from the skarn deposit suggest that the I‐type magma has been influenced by light biogenic sulfur from local country rocks. The Au‐Cu‐Fe‐Sb deposits correlate with the magnetite‐series granitoids in Phetchabun, Nakon Sawan – Lobburi and Rayong – Chantaburi areas. Metallogeny of the Au and Cu‐Au skarn deposits and the epithermal Au deposit is related to adakitic rocks of magnetite‐series granitoids from Phetchabun and Nakon Sawan areas. All mineralizations along the LFB are generated in the volcanic arc related to the subduction of Paleo‐Tethys. The total Al (TAl) content of biotite of granitoids increases in the following order: granitoids associated with Fe and Au deposit < with Cu deposit < barren granitoids. XMg of biotite in granitoids in Muang Loei indicates the crystallization of biotite in magnetite‐series granitoids under high oxygen fugacity conditions. On the other hand, low XMg (<0.4) of biotite in magnetite‐series granitoids in Phu Thap Fah – Phu Thep and Rayong – Chantaburi indicates a reduced environment and low oxygen fugacity, associated with Au skarn deposit (Phu Thap Fah) and Sb‐Au deposit (Bo Thong), respectively. The magnetite‐series granitoids at Phu Thap Fah having low magnetic susceptibilities and low XMg of biotite were formed by reduction of initially oxidizing magnetite‐series granitic magma by interaction with reducing sedimentary country rocks as suggested by negative δ34SCDT values.

Palaeoenvironment and palaeogeography of Middle and Upper Devonian strata from the Loei fold belt, Indochina terrane (northeast Thailand)

Limestone and chert sections from the Loei fold belt in Loei Province, northeastern Thailand, have yielded Givetian to Famennian conodonts and Late Devonian radiolarian faunas. The mid-Devonian siliciclastic section containing an abundant benthic assemblage is interbedded with volcaniclastics and is replaced laterally, in places, by pillow basalts. This siliciclastic section is conformably overlain by argillaceous limestones, divided into two microfacies. The lime mudstone (MF1) and spiculite wackestone (MF2) which contains Polygnathus linguiformis linguiformis, Belodella resima and Icriodus arkonensis, indicating the lower Givetian, deposited in a quiet subtidal to deeper water environment. This argillaceous limestone is succeeded by upper Givetian bioherm containing a diverse stromatoporoid–coral fauna. Microfacies of bioherm consists mainly of crinoidal wackestone/packstone (MF3) deposited in a flank and stromatoporoid–coral framestone/bafflestone/rudstone (MF4) forming a core of bioherm. Continuing relative sea-level rise led to drowning of the bioherms and the deposition of condensed section of non-oceanic, continental margin oozes containing the radiolarians Trilonche chiangdaoensis, Trilonche echinata, Trilonche elegans, Trilonche hindea, Trilonche minax and others and the Famennian conodonts Palmatolepis triangularis, Palmatolepis werneri, Palmatolepis minuta minuta and Palmatolepis wolskae. The transgressive Middle to Upper Devonian strata ranging from fossiliferous sandstones through argillaceous limestone and bioherm to radiolarian cherts are broadly similar to sections in South China and in the Rhenohercynian belt of Germany. Deposition in a continental margin basin, possibly on thinned continental crust, such as a back-arc basin, is more likely than previous models that suggested deposition in and around seamounts within a major ocean basin.

Arc-like volcanic rocks in NW Laos: Geochronological and geochemical constraints and their tectonic implications

The Loei Fold Belt, located at the western margin of the Indochina Block, is considered to be tectonically linked to the subduction of the Paleotethyan Ocean and subsequent collision. In northwestern Laos, the volcanic rocks are traditionally mapped as Permian–Early Triassic sequences. Our LA-ICP-MS zircon U–Pb dating results show that andesitic, rhyolitic and tuffaceous samples from the Muang Feuang region of the Loei Fold Belt have zircon U–Pb ages of 330.4±2.2Ma, 334.9±1.7Ma and 349.6±1.7Ma, which are interpreted as their eruption ages, respectively. These ages are indicative of an Early Carboniferous origin. The basaltic and basaltic–andesitic samples from the Pak Lay region yield zircon U–Pb ages of 314.6±2.7Ma and 315.4±3.8Ma, respectively, and have a Late Carboniferous origin. These samples exhibit depletion in HFSEs (e.g., Nb, Ta, Ti) with high LILE/HFSE ratios and exhibit a geochemical affinity to continental arc volcanic rocks. These signatures suggest the presence of an Early to Late Carboniferous active continental margin along the western margin of the Indochina Block. The Loei Fold Belt may extend north into NW Laos.

Geochemistry and geochronology of the Chatree epithermal gold–silver deposit: Implications for the tectonic setting of the Loei Fold Belt, central Thailand

The Chatree deposit is the largest epithermal Au deposit in mainland SE Asia. Despite its economic significance, the stratigraphy, age, geochemistry and tectonic setting of the host volcanic sequence is poorly documented in the literature. The Chatree deposit is located between Phichit and Phetchabun provinces, central Thailand, and is hosted by Late Permian to Early Triassic volcaniclastic and volcanogenic sedimentary rocks. Detailed field mapping, U–Pb zircon dating, geochemistry and stratigraphy suggest that the volcanic sequence can be subdivided into four stratigraphic units and two volcanic suites. The Late Permian Suite 1 volcanic units are sourced from a more depleted mantle relative to the overlying Early Triassic Suite 2 volcanic units. The Late Permian Suite 1 units probably formed immediately after the beginning of subduction and at the creation of a new island arc. The less depleted Early Triassic Suite 2 units were erupted during ongoing subduction once the system had achieved a steady state. The Chatree Au mineralization appears to have occurred during the switch between the two mantle sources and this switch of magma source is marked by a mixed volcano-plutonic magmatism at the Permo-Triassic boundary (ca. 250Ma).

Adakites in the Truong Son and Loei fold belts, Thailand and Laos: Genesis and implications for geodynamics and metallogeny

LA-ICP-MS U–Pb zircon dating reveals that the Phu Kham adakites in the Truong Son Belt were emplaced during the Late Carboniferous (ca. 306 to 304Ma), whereas the Puthep 1 (PUT 1) adakites in the Loei Fold Belt were formed during the Middle Triassic (ca. 244 to 241Ma). These rock formation ages are largely coeval to Re–Os molybdenite ages (Phu Kham: ca. 304Ma; PUT 1: ca. 246Ma), suggesting close temporal links between adakite formation and copper–gold mineralization. New geochemical results show that the Phu Kham and PUT 1 adakites are characterized by low HREE and Y contents, but elevated LREE, Sr, Sr/Y, and La/Yb values. In addition, the two adakites have relatively high 143Nd/144Nd ratios (0.512648 to 0.512719), Mg# (Phu Kham: 37–68; PUT 1: 40–65), and Cr and Ni contents. These results are best interpreted as representing slab melts that have interacted with supra-subduction zone mantle wedge during ascent. Tectonically, the Phu Kham and PUT 1 adakites were most likely formed during the initiation of subduction of the Ailaoshan–Song Ma and the Main Paleo-Tethys ocean plates respectively. Close temporal relations between the adakite formation and the copper–gold mineralization suggest strong genetic links between the two in the mainland SE Asia region.

The origin and evolution of skarn-forming fluids from the Phu Lon deposit, northern Loei Fold Belt, Thailand: Evidence from fluid inclusion and sulfur isotope studies

The Phu Lon skarn Cu–Au deposit is located in the northern Loei Fold Belt (LFB), Thailand. It is hosted by Devonian volcano-sedimentary sequences intercalated with limestone and marble units, intruded by diorite and quartz monzonite porphyries. Phu Lon is a calcic skarn with both endoskarn and exoskarn facies. In both skarn facies, andradite and diopside comprise the main prograde skarn minerals, whereas epidote, chlorite, tremolite, actinolite and calcite are the principal retrograde skarn minerals. Four types of fluid inclusions in garnet were distinguished: (1) liquid-rich inclusions; (2) daughter mineral-bearing inclusions; (3) salt-saturated inclusions; and (4) vapor-rich inclusions. Epidote contains only one type of fluid inclusion: liquid-rich inclusions. Fluid inclusions associated with garnet (prograde skarn stage) display high homogenization temperatures and moderate salinities (421.6–468.5 °C; 17.4–23.1 wt% NaCl equiv.). By contrast, fluid inclusions associated with epidote (retrograde skarn stage) record lower homogenization temperatures and salinities (350.9–399.8 °C; 0.5–8 wt% NaCl equiv.). These data suggest a possible mixing of saline magmatic fluids with external, dilute fluid sources (e.g., meteoric fluids), as the system cooled. Some fluid inclusions in garnet contain hematite daughters, suggesting an oxidizing magmatic environment. Sulfur isotope determinations on sulfide minerals from both the prograde and retrograde stages show a uniform and narrow range of δ 34S values (−2.6 to −1.1 ‰ δ 34S), suggesting that the ore-forming fluid contained sulfur of orthomagmatic origin. Overall, the Phu Lon deposit is interpreted as an oxidized Cu–Au skarn based on the mineralogy and fluid inclusion characteristics.

Geology and geochemistry of the Phu Lon copper-gold skarn deposit at the northern Loei Fold Belt, Northeast Thailand

The Phu Lon deposit in northeast Thailand (Nong Khai province) is the best known skarn copper-gold deposit in the Palaeozoic-Cenozoic Loei volcano-plutonic belt (Loei Fold Belt). The research project documented herein comprises a detailed study of the deposit geology, providing the basis for calc-silicate mineral chemistry and sulphur isotope and fluid inclusion investigations. This is the first result of analysis of the fluid chemistry at Phu Lon and strongly supports a direct relationship between magmatic fluid and mineralisation.This research also summarises the salient features of the Phu Lon deposit to compare with other world-class copper-gold skarns. This comparison suggests that the Phu Lon skarn deposit may represent an oxidised end-member of gold enriched types.

“How to obtain earthquake ground motions for engineering designs”: by Ellis L. Krinitzsky; Elsevier Engineering Geology Paper, 65:1–16

LA-ICP-MS U–Pb zircon dating reveals that the Phu Kham adakites in the Truong Son Belt were emplaced during the Late Carboniferous (ca. 306 to 304 Ma), whereas the Puthep 1 (PUT 1) adakites in the Loei Fold Belt were formed during the Middle Triassic (ca. 244 to 241 Ma). These rock formation ages are largely coeval to Re–Os molybdenite ages (Phu Kham: ca. 304 Ma; PUT 1: ca. 246 Ma), suggesting close temporal links between adakite formation and copper–gold mineralization. New geochemical results show that the Phu Kham and PUT 1 adakites are characterized by low HREE and Y contents, but elevated LREE, Sr, Sr/Y, and La/Yb values. In addition, the two adakites have relatively high 143Nd/144Nd ratios (0.512648 to 0.512719), Mg# (Phu Kham: 37–68; PUT 1: 40–65), and Cr and Ni contents. These results are best interpreted as representing slab melts that have interacted with supra-subduction zone mantle wedge during ascent. Tectonically, the Phu Kham and PUT 1 adakites were most likely formed during the initiation of subduction of the Ailaoshan–Song Ma and the Main Paleo-Tethys ocean plates respectively. Close temporal relations between the adakite formation and the copper–gold mineralization suggest strong genetic links between the two in the mainland SE Asia region.

Evolution of a Permian carbonate platform to siliciclastic basin: Indochina Plate, Thailand

In Thailand, during the Permian, an extensive carbonate platform developed on the margin of the Indochina Plate and near to a coeval and deeper siliciclastic-dominated marine basin. This basin separated the Indochina Plate, to the east, from the Shan Thai Plate to the west. Sedimentation in the area of both platform (Pha Nok Khao Platform) and basin (Nam Duk Basin) was ended by a late Permian/early Triassic orogenic event that caused closure of the Palaeo-Tethys ocean. Tectonism has controlled the pattern of platform sedimentation and supplies of carbonate and clastic sediments into the basin area. The Pha Nok Khao Platform developed on an extensive distally steepened ramp-like margin along which carbonates and siliciclastics accumulated. Adjacent to this platform, to the west, the Nam Duk Basin received coeval hemipelagic siliciclastics and minor amounts of carbonates. Biostratigraphic evidence, based on foraminiferal assemblages, suggests an early to middle Permian age equivalence between platform and basin lithofacies. The platform strata are exposed along the Loei fold and thrust belt in northeast Thailand and basin strata are exposed within the Phetchabun fold and thrust belt in central Thailand, respectively. The two belts extend in a north–south direction and their rocks exhibit extensive karstification. On the basis of seismic data, platform lithologies extend further east in the subsurface, beneath the continental sediments of the Khorat Group into Kampuchea and Laos. Facies groups of the platform environments include restricted platform, platform interior, and outer platform. The facies groups of the basin environments include basin margin and basin plain. Minor layers of volcaniclastics and green coloured tuffites are interbedded with both the platform carbonate layers and basinal pelagic layers, suggesting active volcanism during sedimentation. The carbonate lithofacies of the shallow platform to the east (in the Loei Fold Belt) grade westward into turbidites (in the Phetchabun fold belt), and include shales and sandstones with minor carbonates. Farther to the west they pass into hemipelagic lithofacies of shale and carbonate. Lithologies and fauna of the limestones suggest tropical to subtropical climate during deposition.