Khin Zaw

Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the India‐Eurasia collision in Myanmar

Situated south of the eastern Himalayan syntaxis at the western margin of the Shan-Thai terrane the highgrade Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia. The first sensitive high-resolution ion microprobe U-Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (~170 Ma) zircons that have partly recrystallized during ~43 Ma high-grade metamorphism. A hornblende syenite from Mandalay Hill also contains Jurassic zircons with evidence of Eocene metamorphic recrystallization rimmed by thin zones of 30.9 plus or minus 0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time. Mid- Cretaceous to earliest Eocene (120 to 50 Ma) I-type granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra. Metamorphic overgrowths to zircons in the orthogneiss near Mandalay date a period of Eocene (~43 Ma) high-grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (at 65 to 55 Ma). This was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 Ma. Similar syntectonic syenites and leucogranites intruded the Ailao Shan-Red River shear belt in southern China and Vietnam during the Eocene-Oligocene to Miocene, and the Wang Chao and Three Pagodas faults in northern Thailand (that most likely link with the MMB) were also active at this time. The complex history of Eocene to early Miocene metamorphism, deformation, and magmatism in the MMB provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of India and resulting extrusion or rotation of Indochina.

Evolution and source of ore fluids in the stringer system, Hellyer VHMS deposit, Tasmania, Australia: evidence from fluid inclusion microthermometry and geochemistry

The Hellyer deposit is a classic, large tonnage, high-grade, mound style volcanic-hosted massive sulphide (VHMS) deposit in the Cambrian Mt Read Volcanic belt of western Tasmania. In the footwall directly underlying the deposit, there is an extensively altered pipe which cootains a well developed and preserved stringer zone. The vein paragenesis at Hellyer indicates that premineralization Stage I veins consist entirely of quartz, and occur throughout the alteration pipe. The synmineralization Stage 2 veins are the most abundant veins in the stringer zone and consist of three sub-stages: Stage 2A veins of crustiform quartz, pyrite, and carbonate with minor amounts of chalcopyrite, sphalerite and galena, Stage 2B veins with abundant base metal sulphides, minor quartz, carbonate and barite gangue and Stage 2C veins of coarsely crystalline barite with variable amounts of pyrite, sphalerite, galena and carbonate. Stages 3-6 veins are postmineralization veins and are related to the Devonian Tabberabberan Orogeny. Textural, petrographic and microthermometric investigations of fluid inclusions in the Hellyer stringer system indicate that Type I, primary, liquid-vapour inclusions occur along growth planes of crustiform quartz crystals or within colour banding of zoned sphalerite. These inclusions are 10-15 µm in size, and yielded homnogenisation temperatures of 170-220°C in early 2A veins, 165-322°C in main-stage 2B veins and 190-256°C io late-stage 2C veins. These data suggest a waxing and waning thermal history. However, the average salinity remained between 8-11 NaCI equiv. wt% in all Stage 2 veins. Chalcopyrite-bearing primary fluid inclusions have been recognised in the base metal-rich Stage 28 veins. No evidence for presence of CO2 (e.g. formation of clathrates) was recorded by rnicrothermometry. However, Laser Raman spectroscopic (LRS) analysis indicates the presence of CO2 (< 1 mole%) in the Stage 2B veins, and no detectable CO2 in 2A and 2C vein stages. Semi-quantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Hellyer ore fluid was enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz indicates that the Stage 2B ore fluids have a significantly higher base metal concentration compared to the Stage 2A veins. The postmineralization Stage 4 veins have a variable but lower base metal content. In this study, there was no fluid inclusion evidence of boiling. Cation composition, higher salinities relative to seawater and the presence of CO2, suggest that recycled seawater alone cannot be the sole source of the ore fluids. This interpretation is in agreement with previous isotopic studies in the Hellyer stringer system. Although direct input of bulk ore constituents from a magma chamber cannot be demonstrated from the present fluid inclusion data, such a contribution of ore fluids from a magmatic source cannot be ruled out. The possible input from the magmatic source may have occurred during the base metal-rich Stage 2B vein formation characterised by the intensifying temperature of deposition, higher base metals and CO2 contents.

Post-collisional crustal extension setting and VHMS mineralization in the Jinshajiang orogenic belt, southwestern China

Abstract The Jinshajiang orogenic belt (JOB) of southwestern China, located along the eastern margin of the Himalayan–Tibetan orogen, includes a collage of continental blocks joined by Paleozoic ophiolitic sutures and Permian volcanic arcs. Three major tectonic stages are recognized based on the volcanic–sedimentary sequence and geochemistry of volcanic rocks in the belt. Westward subduction of the Paleozoic Jinshajiang oceanic plate at the end of Permian resulted in the formation of the Chubarong–Dongzhulin intra-oceanic arc and Jamda–Weixi volcanic arc on the eastern margin of the Changdu continental block. Collision between the volcanic arcs and the Yangtze continent block during Early–Middle Triassic caused the closing of the Jinshajiang oceanic basin and the eruption of high-Si and -Al potassic rhyolitic rocks along the Permian volcanic arc. Slab breakoff or mountain-root delamination under this orogenic belt led to post-collisional crustal extension at the end of the Triassic, forming a series of rift basins on this continental margin arc. Significant potential for VHMS deposits occurs in the submarine volcanic districts of the JOB. Mesozoic VHMS deposits occur in the post-collisional extension environment and cluster in the Late Triassic rift basins.

Gem-corundum megacrysts from east Australian basalt fields : trace elements, oxygen isotopes and origins

Gem corundum, a minor but persistent megacryst in east Australian basalt fields, is mined from some placer concentrations. Laser ablation, inductively coupled plasma mass spectrometry analyses and O isotope determinations on a colour range of corundum from different fields, show that chromophore (Fe, Cr, Ti, V) and genetic indicator (Ga, Mg, δ18O) values can distinguish corundum sources (magmatic, metamorphic and metasomatic) before basalt incorporation. They also characterise corundum groups from different fields. This identified two metamorphic groups, one carrying ruby at Barrington Tops, and a magmatic group distinct from those from other gem fields (lower Fe, northeast Tasmania; higher Fe, Yarrowitch). Ruby-bearing groups show clear provincial characteristics and include lower temperature spinel-facies groups (Barrington, Yarrowitch) and higher temperature garnet-facies groups (Cudgegong–Macquarie River). High Mg/Fe and Ni values in the latter approach those for corundum in diamond, and are a possible diamond indicator. The corundum derived from diverse fold-belt and felsic sources in underlying lithosphere forms a dataset for comparing corundum from other basalt fields.

A petrological and fluid inclusion study of magnetite–scheelite skarn mineralization at Kara, Northwestern Tasmania: implications for ore genesis

Abstract The Kara magnetite–scheelite deposit is located about 40 km south of Burnie in northwestern Tasmania. The deposit consists of a number of orebodies and the total mineable reserves are 1.7 Mt of magnetite at >30% Fe and 0.3 Mt of scheelite at 0.52% WO3. The major orebodies at Kara are hosted by the Ordovician Gordon Limestone at the southern end of the Devonian Housetop Granite, adjacent to the granite or separated from it by the Ordovician Moina Sandstone. At least four paragenetic stages of skarn formation and ore deposition have been recognized: Stage I Clinopyroxene±garnet±vesuvianite±wollastonite±quartz±scheelite, Stage II Garnet–vesuvianite–magnetite±scheelite±apatite±quartz, Stage III Magnetite–amphibole–epidote–fluorite–quartz±chlorite±garnet±vesuvianite±scheelite±carbonate±pyrite±clinopyroxene, and Stage IV Hematite±fluorite±calcite±quartz. Fluid inclusions in clinopyroxene from Stage I homogenize at 460–620°C (mode 520°C). Stage II gave homogenization temperatures of 349–578°C (mode 500°C) from scheelite and 360–570°C (mode 500°C) from vesuvianite. The Stage I clinopyroxene gave a salinity range of 10.0–12.0 equiv. wt.% NaCl with only one low value of 2.0 equiv. wt.% NaCl. The Stage II mineral assemblages also showed moderate salinities of 12.0 and 17.8 equiv. wt.% NaCl from scheelite and vesuvianite. Fluid inclusions in quartz, scheelite, calcite and fluorite from the main scheelite mineralization phase, Stage III, homogenize at 230–360°C with a mode at 300°C and salinity varies from 0.2 to 19.8 equiv. wt.% NaCl. The mineral paragenetic and fluid inclusion studies indicate that magnetite–scheelite mineralization at Kara was formed as a proximal skarn assemblage in carbonate host. The skarn formation and ore deposition occurred in stages starting with prograde anhydrous clinopyroxene–garnet metasomatism at relatively high temperatures, >500°C. This early assemblage was replaced by the subsequent retrograde deposition of hydrous mineral phases (e.g. amphibole, epidote and chlorite) as temperatures decreased to 300°C or less. Deposition of scheelite predominantly occurred in association with hydrous minerals (e.g. amphibole in Stage III). The high-temperature and moderate-to-high-salinity ore fluids in early anhydrous skarn formation (Stages I and II) are consistent with fluids of magmatic origin. In comparison, the lower temperature and variable salinity from high (19.8 equiv. wt.% NaCl) to as low as 0.2 equiv. wt.% NaCl in Stage III suggests a possible involvement of low-salinity meteoric waters mixing with high-salinity fluids originating from the nearby granite in the later hydrous skarn and ore formation.

Yulong Deposit, Eastern Tibet: A High-Sulfidation Cu-Au Porphyry Copper Deposit in the Eastern Indo-Asian Collision Zone

The Yulong ore body is the largest Cu deposit (6.22 million metric tons [Mt] at 0.99% Cu) in the 300 km long Himalayan porphyry copper belt, and is controlled by major Cenozoic strike-slip faults in the eastern Indo-Asian collision zone. It is associated with a steeply dipping, pipe-like multiphase (42-35 Ma) monzogranitic stock. The host rocks are potassic calc-alkaline or shoshonitic, and show geochemical affinities with adakites. They appear to have been derived from a thickened lower crustal source in East Tibet. The Yulong deposit consists of a ring-shaped, high-grade Cu-Au zone overlying and/or surrounding a porphyry-type Cu-Mo ore body. Cu-Mo mineralization produced a steeply dipping, pipe-like, veinlet-disseminated ore body within the stock. Associated hydrothermal alteration produced K-silicate and quartz-sericite assemblages within the stock, and contemporaneous propylitic alteration in the Upper Triassic sandy-slate wall rock. Fluid inclusion and δ18O-δD data indicate that the ore-forming fluid was supercritical, and exsolved from a high-level magma chamber at >620°C; it then separated into a hypersaline aqueous liquid and a coexisting low-salinity vapor at 340°-600°C. The high-grade Cu-Au zone (3 Mt at 4.74% Cu, and 4.5 g/t Au) is dominated by a supergene chalcocite-malachite blanket resting on an underlying supergene/hypogene sulfide transition unit and a hypogene pyrite-chalcopyrite sulfide unit. The Cu-Au zone was controlled by a subhorizontal or gently outward dipping breccia horizon developed along the marginal fracture zone near the roof of the stock, produced by hydrothermal brecciation due to regional uplift and/or fluid boiling. Alteration associated with the hypogene Cu-Au mineralization was texture-destructive advanced argillic alteration, characterized by associations of quartz, kaolinite, dickite, endellite, montmorillonite, hydromica, and minor alunite. It mainly developed within the breccia horizons, and partially over-printed the early-formed K-silicate zone and the quartz-sericite zone. Associated mineralization was of the high-sulfidation epithermal-type, characterized by chalcocite, tennantite, covellite, bornite, and minor pyrite, which formed the main ore body in the high-grade Cu-Au zone. Epithermal fluids also caused the dissolution of early-formed sulfides and remobilization of Cu-Mo, the latter transported into the intense advanced argillic alteration halo within the mineralized stock. This late-stage alteration and mineralization is attributed to a CO2-rich, low-temperature (<350°C), low-salinity (<12 wt% NaCl equiv.) meteoric fluid, involving input of magmatic fluid. Based on alteration, mineralization, fluid inclusion and stable isotopic data, a two-stage genetic history has been reconstructed for the Yulong deposit. It spans (1) a magmatic hydrothermal environment reflecting the emplacement of the monzogranite stock and Cu-Mo introduction through (2) hydrothermal fluid infiltration of breccia zones to epithermal overprinting.

Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

Abstract Corundum xenocrysts from alkaline basalt fields differ in characteristics and hence lithospheric origins.Trace element, fluid/melt inclusion and oxygen isotope studies on two eastern Australian corundum deposits are compared to consider their origins. Sapphires from Weldborough, NE Tasmania, are magmatic (high-Ga, av. 200 ppm) and dominated by Fe (av. 3300 ppm) and variable Ti (av. 400 ppm) as chromophores. They contain Cl, Fe, Ga, Ti and CO2-rich fluid inclusions and give δ18O values (5.1−6.2 ‰) of mantle range. Geochronology on companion zircons suggests several sources (from 290 Ma to 47 Ma) were disrupted by basaltic melts (47 ± 0.6 Ma). Gem corundums from Barrington, New South Wales, also include magmatic sapphires (Ga av. 170 ppm; δ18O 4.6−5.8 ‰), but with more Fe (av. 9000 ppm) and less Ti (av. 300 ppm) as chromophores. Zircon dating suggests that gem formation preceded and was overlapped by Cenozoic basaltic melt generation (59−4 Ma). In contrast, a metamorphic sapphire-ruby suite (low-Ga, av. 30 ppm) here incorporates greater Cr into the chromophores (up to 2250 ppm). Fluid inclusions are CO2-poor, but melt inclusions suggest some alkaline melt interaction. The δ18O values (5.1−6.2%) overlap magmatic sapphire values. Interactions at contact zones (T = 780−940ºC) between earlier Permian ultramafic bodies and later alkaline fluid activity may explain the formation of rubies.

Different mineralization styles in a volcanic-hosted ore deposit: the fluid and isotopic signatures of the Mt Morgan Au-Cu deposit, Australia

Quantitative laser ablation (LA)-ICP-MS analyses of fluid inclusions, trace element chemistry of sulfides, stable isotope (S), and Pb isotopes have been used to discriminate the formation of two contrasting mineralization styles and to evaluate the origin of the Cu and Au at Mt Morgan. The Mt Morgan Au-Cu deposit is hosted by Devonian felsic volcanic rocks that have been intruded by multiple phases of the Mt Morgan Tonalite, a low-K, low-Al(2)O(3) tonalite-trondhjemite-dacite (TTD) complex. An early, barren massive sulfide mineralization with stringer veins is conforming to VHMS sub-seafloor replacement processes, whereas the high-grade Au-Cu. ore is associated with a later quartz-chalcopyrite-pyrite stock work mineralization that is related to intrusive phases of the Tonalite complex. LA-ICP-MS fluid inclusion analyses reveal high As (avg. 8850 ppm) and Sb (avg. 140 ppm) for the Au-Cu mineralization and 5 to 10 times higher Cu concentration than in the fluids associated with the massive pyrite mineralization. Overall, the hydrothermal system of Mt Morgan is characterized by low average fluid salinities in both mineralization styles (45-80% seawater salinity) and temperatures of 210 to 270 degreesC estimated from fluid inclusions. Laser Raman Spectroscopic analysis indicates a consistent and uniform array Of CO(2)-bearing fluids. Comparison with active submarine hydrothermal vents shows an enrichment of the Mt Morgan fluids in base metals. Therefore, a seawater-dominated fluid is assumed for the barren massive sulfide mineralization, whereas magmatic volatile contributions are implied for the intrusive related mineralization. Condensation of magmatic vapor into a seawater-dominated environment explains the CO(2) occurrence, the low salinities, and the enriched base and precious metal fluid composition that is associated with the Au-Cu. mineralization. The sulfur isotope signature of pyrite and chalcopyrite is composed of fractionated Devonian seawater and oxidized magmatic fluids or remobilized sulfur from existing sulfides. Pb isotopes indicate that Au and Cu. originated from the Mt Morgan intrusions and a particular volcanic strata that shows elevated Cu background

Formation of the Denchai gem sapphires, northern Thailand: evidence from mineral chemistry and fluid/melt inclusion characteristics

Abstract The Denchai gem sapphire deposits in Phrae Province, northern Thailand are closely associated with late Cenozoic alkaline basaltic rocks. The sapphires occur in alluvial placer deposits in palaeo-channels at shallow depths. Electron microprobe analysis of minor and trace element contents (Fe, Ti, Cr, Ga and V) of the sapphires indicate the following oxide abundances: Fe2O3 (0.32−1.98 wt.%), TiO2 (0.01−0.23 wt.%), Cr2O3 (<0.01 wt.%), Ga2O3 (0.01−0.03 wt.%) and V2O5 (<0.03 wt.%). Optical studies of sapphires revealed three types of primary fluid/melt inclusions. CO2-rich inclusions (Type I) contain three phases (LH2O + LCO2 + V) with the vapour phase comprising <10−15 vol.%. The presence of CO2 was confirmed by microthermometry and laser Raman analysis. Polyphase inclusions (Type II) (vapour + liquid + solid) contain a fluid bubble (20−30 vol.%), an aqueous phase (10−15 vol.%) and several solid phases. Silicate-melt inclusions (Type III) comprise vapour bubbles, silicate glass and solid phases. Proton-induced X-ray emission (PIXE) analysis revealed high concentrations of K (~4 wt.%) as well as Ca (~0.5 wt.%), Ti (~1 wt.%), Fe (~2 wt.%), Mn (~0.1 wt.%), V (<0.03 wt.%), Rb (~70 ppm) and Zr (~200 ppm) in the silicate glass. The Ga2O3 abundances and Cr2O3/Ga2O3 values (<1) of the sapphires favour their formation by magmatic processes. The presence of CO2-rich fluids and high K concentrations in the silicate melt inclusions link the origin of the Denchai gem sapphires to CO2-rich alkaline magmatism.

Microthermometry and chemical composition of fluid inclusions from the Mt Chalmers volcanic-hosted massive sulfide deposits, central Queensland, Australia: implications for ore genesis

Mt Chalmers is a mound-style, volcanic-hosted massive sulfide (VHMS) deposit in central Queensland, Australia. The ore lenses are hosted by rhyolitic volcanics and sedimentary rocks of Early Permian age. The two ore lenses (Main lode and West lode) consist of Cu-Zn-Pb massive sulfide underlain by Cu-rich stringer mineralization. Textural and petrographic investigations of fluid inclusions indicate that primary Type I inclusions up to 25 microns are found in quartz from the stringer mineralized zone, and microthermometric studies of these inclusions yielded homogenization temperatures of 160-285 degrees C and salinities of 5-10.5 NaCl equiv. wt.%. Laser Raman spectroscopic (LRS) analysis indicates the presence of CO2 (0.1-1 mol%)in the Mt Chalmers VHMS systems. Semiquantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates indicate that the Mt Chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater. PIXE microanalysis of fluid inclusions in quartz from the stringer zone also indicates a significant base metal concentration in these fluids. Cation composition and higher salinities relative to seawater suggest that recycled seawater cannot be the sole source of the ore fluids. High base metal content and the presence of CO2 in the fluid inclusions imply that magmatic input of ore metals, copper in particular, accompanying seawater leaching of the footwall volcanic pile is a distinct possibility. In terms of fluid composition, the K-Ca-Fe variation of the Mt Chalmers ore fluid is comparable with those of typical epithermal deposits (e.g., Thames, New Zealand) and porphyry copper deposits (Bingham, UT). The Cu-Zn-Fe/10 plot of the Mt Chalmers ore fluids, indicates that there is significant copper in the system, comparable to copper enrichment in a porphyry copper system. The Mt Chalmers ore fluids also show similar copper content with the Cu-rich end-member ore fluid composition of the mineralized Stage 2B veins of the Hellyer VHMS deposit, whereas the Stage 2A veins of Hellyer are more enriched in lead and zinc. Overall, the ore fluids have a variable chemistry with a continuum of compositional data from VHMS to epithermal-porphyry style ore fluids. Shallow-water emplacement ( < 300 m) for the VHMS mineralization has been postulated for the Mt Chalmers deposit based on the presence of trace fossils in the footwall and hanging wall sedimentary rocks and volcanic facies studies. However, fluid inclusion studies do not rule out a moderate to deeper submarine environment, as there is no fluid inclusion evidence of boiling. At Mt Chalmers, boiling of ore fluids would probably have occurred (as in most epithermal systems) if the ore fluid exhaled onto the seafloor under a shallow environment.