Hydrothermal Quartz Vein Formation, Revealed by Coupled SEM-CL Imaging and Fluid Inclusion Microthermometry: Shuteen Complex, South Gobi, Mongolia

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1 RESOURCE GEOLOGY, vol. 55, no. 1, 1 8, 2005 Hydrothermal Quartz Vein Formation, Revealed by Coupled SEM-CL Imaging and Fluid Inclusion Microthermometry: Shuteen Complex, South Gobi, Mongolia Bayaraa BATKHISHIG, Greg BIGNALL 1 and Noriyoshi TSUCHIYA Geomaterial and Energy Laboratory, Graduate School of Environmental Studies, Tohoku University, Aramaki, Aoba-ku, Sendai , Japan [ bkhishig@geo.kankyo.tohoku.ac.jp] 1 Present address: Institute of Geological and Nuclear Sciences, Wairakei Research Institute, Private Bag 2000, 144 Karetoto Road, Taupo, New Zealand Received on February 20, 2004; accepted on September 2, 2004 Abstract: Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of vein quartz in the Cu-mineralised, Shuteen Complex (South Gobi, Mongolia) has revealed a complex history of crystal growth, dissolution and microfracture healing, associated with several hydrothermal events that could not be detected using other observational techniques (e.g. transmitted/reflected light microscopy, back-scattered electron imaging, or secondary electron imaging). The quartz initially grew as CL-bright/grey crystals in a 345±30 C liquid reservoir, as inferred by the analysis of primary liquid fluid inclusions (average Th of 343 C; 6.6~7.7 wt% NaCl eq ). Quartz precipitation occurred at the edge of the crystals as reservoir fluids cooled to 260±25 C, as indicated by micron-scale CL-dark/CL-bright quartz growth bands containing abundant fluid inclusions (with an average T h values of 261 C). Pressure fluctuations were the likely cause of dissolution, as SEM-CL imaging reveals the quartz have corroded or rounded crystal edges, and precipitation of later quartz into open space. SEM-CL imaging shows the quartz contains healed microfractures that trapped low salinity fluids (3.9 wt% NaCl eq ) with Th values of 173±15 C. SEM-CL imaging provides a means of deciphering the thermal and chemical evolution of the fossil Shuteen hydrothermal system, and the nature of hydrothermal quartz vein-forming processes, by facilitating the correlation of distinct fluid inclusion populations and their relative chronology, with specific hydrothermal events. Keywords: cathodoluminescence, SEM-CL imaging, fluid inclusions, hydrothermal quartz vein, fossil hydrothermal system, Shuteen Complex, Mongolia 1. Introduction The process of quartz vein formation in hydrothermal ore deposits is poorly understood. There is little knowledge of how such veins are really formed: from multiple pulses of fluids?, associated with one hydrothermal event?; by continuous re-fracturing along a plane(s) of weakness?; and what effect does dissolution processes, temperature/pressure changes and/or fluid composition have on the nature of quartz deposition? Much of what is known about hydrothermal quartz vein formation comes from fluid inclusion case studies, which highlight crosscutting veins that trapped discrete fluid inclusion populations, with chemically distinct fluids inferred to relate to specific hydrothermal events. Examination of quartz vein textures by transmitted light microscopy, however, provides limited information about the history of multigenerational vein formation. In contrast, cathodoluminescence (CL) imaging reveals microtextural features in quartz and other minerals (such as cryptic alteration, dissolution and fracturing) that are not recognized by other methods of observation (Bignall et al., 2004). Indeed, Wilkinson et al. (1999) showed how fluid inclusion populations could be related to unique mineralisation events, due to the CL properties of the quartz that hosted the fluid inclusions, whilst Pagel et al. (2000) and Rusk and Reed (2002) demonstrated how SEM-CL analysis of vein quartz could reveal the complex growth history of hydrothermal vein formation. In this paper, we highlight the application of a coupled SEM-CL imaging/fluid inclusion study, to resolve the nature of vein formation in a high-temperature, hydrothermal quartz vein system on the margin of the mineralised Shuteen intrusive complex (~450 km SSE of Ulaanbaatar City, in the South Gobi District, Mongolia; Fig. 1). The focus of the work is to use new SEM-CL techniques (providing information on crystal growth textures and mineral-fluid relationships) to interpret the formation history of hydrothermal quartz veins in the Shuteen Complex, detail temperature-pressure changes and character (e.g. chemistry) of the fluid(s) that produced the veins, and consequently better understand the nature of aqueous fluid flow in the Earth s crust. 1

2 2 B. BATKHISHIG, G. BIGNALL and N. TSUCHIYA RESOURCE GEOLOGY : Russia Mongolia Ulaanbaatar 0 200km Shuteen Legend Quaternary sediments China Khanbogd Lithocap Area Tsogt Ovoo Formation Shuteen Complex Granite porphyry, aplite dykes Diorite/syenite porphyry stocks Shuteen Pluton Epithermal quartz veins Dusiin Ovoo Formation Ikh-Shankai Formation 2 km Normal or circular faults Fig. 1 Simplified geological map of the Shuteen Complex (after Hovan et al., 1983), showing the location of the main alteration zone (Khanbogd lithocap area) and hydrothermal quartz vein system that is the focus of this study. Inset (top right) shows the location of the Shuteen area in southern Mongolia. 2. Geological Setting The Middle Carboniferous Shuteen volcano-plutonic complex consists of monzodiorite, granodiorite and granite of the Shuteen Pluton (Batkhishig and Iizumi, 2001); andesite, porphyry and related pyroclastics of the Dusiin Ovoo Volcanic Formation; as well as granodiorite, monzonite and granite stocks, and andesite and aplite dykes (Hovan et al., 1982, 1983, 1984). The Shuteen Complex likely represents the petrochemical characteristics of an island arc, associated with subduction of Ordovician-Silurian oceanic rocks beneath the South Mongolian microcontinent (Ruzhentsev and Pospelov, 1992; Lamb and Badarch, 1997; Sengör et al., 1993). The Complex has a ring structure (Fig. 1), characterized by circular normal and radial faults (Delgertsogt and Daramsenge, 1997). Volcanic and plutonic rocks of the Shuteen Complex are likely to have a common magma source (Bignall et al., 2003); they are high Al 2 O 3, high-sr series adakitic, calc alkaline-type, with Rb-Sr whole rock ages of 321±10 Ma (Iizumi and Batkhishig, 2000). The Shuteen Complex is inferred to derive from partial melting of oceanic slab, with variations in rock chemistry best explained by fractional crystallisation (Batkhishig et al., 2003). Cretaceous sediments cover all older formations. Russian-Mongolian mineral prospecting surveys were undertaken at Shuteen in the 1950 s, and drilling/trench investigations were conducted by Czech-Mongolian geologists in the 1970 s, but only recently has its Au and Cuporphyry mineral potential been investigated in detail. A combined Japan International Co-operation Agency (JICA) and Metal Mining Agency of Japan (MMAJ) programme explored the prospect (JICA-MMAJ, 1995, 1996; Oyunchimeg et al., 1998), as did Quincunx Ltd., with the latter subsequently entering into a joint venture relationship with Ivanhoe Mines Ltd. Recent investigations have recognised that the Shuteen prospect comprises an extensive (20 km 2 ) silicified hydrothermal breccia zone (the main alteration zone, or MAZ ), in the Shuteen Khanbogd lithocap area, with anomalous copper (30 to 1180 ppm), molybdenum ( ppm), zinc ( ppm) and bismuth (20 30 ppm) (Atkinson, 1999; Delgertsogt, 2003). The north-south trending silicified lithocap and hydrothermal breccia zone is associated with intense leaching, silicification and argillic/propylitic alteration (Atkinson, 1999). It is yet to be conclusively demonstrated, however, if the Shuteen alteration zone and ring structure is related to an intensely leached, sub-volcanic, high-sulphidation breccia-pipe complex, or intrusiverelated (As, Sb and Mo) mineralisation with associated breccia pipes.

3 Detailed SEM-CL imaging of the Shuteen vein quartz revealed a complex history of mineral deposition, dissovol. 55, no. 1, 2005 Quartz Vein Formation Revealed by SEM-CL Imaging A 100 µm B 10 cm C Fig. 2 (A) Hydrothermal quartz veins exposed in the southern part of the Shuteen Complex; (B) detail of quartz vein system, showing numerous quartz veinlets (up to 5 cm wide) that bifurcate from the main vein (knife is 9 cm long); and (C) SEM- CL image of quartz vein revealing: euhedral quartz crystals, areas of growth zoning, late stage quartz precipitation and microfracturing (described in text). Dashed line marks contact between wall rock and the quartz vein. 3. Quartz Veins Numerous quartz and brecciated quartz-tourmaline veins occur towards the southern periphery of the MAZ (Fig. 1), within the granodioritic intrusive rocks (Fig. 2A). In general, the steeply dipping (almost vertical) veins are up to 20 cm wide, strike at 100 to 115, and are associated with bifurcating, 1 2 cm quartz veinlets extending metres from the main vein (Fig. 2B). The veins contain drusy and vuggy quartz, with pyrite, chalcopyrite and lesser galena mineralisation, in places masked by strong limonitisation. Individual quartz crystals in the veins are irregularly oriented, and up to 3 cm in length. The quartz veins are also Au-bearing, with one quartz vein sample assayed at 57 g Au/t (average 1 4 g/t Au), whilst smaller bifurcating quartz and quartz-tourmaline veinlets have Au contents in the range of 250 ppb to 1.2 g/t (Delgertsogt, 2003). 4. Methodology We examined several 100 µm-thick, double polished sections of quartz from a hydrothermal vein system about 2 km SSE of the Shuteen Khanbogd lithocap area (shown in Fig. 1). The thick sections were initially examined using optical microscopy, to identify zones of primary (and/or secondary) fluid inclusions, then carbon coated for SEM-CL observation using a SEM-EDX Hitachi- S2460N scanning electron microscope, equipped with an Oxford Mini-CL detector and photomultiplier. The SEM-CL technique has high spatial resolution, and a range of beam currents and acceleration voltages, which makes it useful for observing minerals with weak luminescence (e.g. quartz). The thick sections were analysed at 15 to 25 kv, with a beam current set at 10 to 20 na to obtain optimal contrast in observed luminosity. In this method, grey-scale ( CL-dark, -grey or -bright ) images are produced, with apparent intensity of observed luminescence dependent on machine operating conditions, i.e. beam current, acceleration voltage, photomultiplier contrast and brightness. Homogenization temperature (T h ) and salinity (T m ) measurements were determined for fluid inclusions in the vein quartz, using a Linkam THMS 600 heating-freezing stage (with Olympus 40 long focus lens), with the examined fluid inclusions occurring in growth zones, interstitial quartz and along healed microfractures. The thermocouple for the heating-freezing stage was calibrated using metal melting (tin, C; lead, C; zinc, C) and the ice melting temperature of pure water. The T h microthermometry has a calibration error of <2 C, whilst T m values for ice melting are accurate to ±0.1 C. A 1 C/min heating rate was applied to all heating and cooling experiments. 5. SEM-CL Imaging

4 4 B. BATKHISHIG, G. BIGNALL and N. TSUCHIYA RESOURCE GEOLOGY : lution and microfracturing, associated with several hydrothermal events (Fig. 2C). The veins are composed of an interconnected mosaic of sub-euhedral quartz crystals, with predominantly low intensity (CL-grey) luminescence. SEM-CL imaging has shown that most of the crystals are characterized by a somewhat brighter (CLpale grey) core, compared to micron-sized concentric growth bands at the edge of the quartz. SEM-CL imaging reveals microtextural evidence of multi-stage vein development, with; (i) many quartz crystals having been partly dissolved; (ii) evidence of late-stage CL-bright quartz having precipitated to open space; and (iii) randomly oriented, healed microfractures. As some quartz crystals grew, they impinged on other crystals and produced impediment textures that promoted vein closure. Backscattered electron and transmitted light images of the vein quartz do not reveal the generations of crystal growth, dissolution and fracturing evident observed in SEM-CL, instead they appear optically continuous Growth zones SEM-CL imaging reveals concentric growth zoning, up to 20 µm in width at the edge of most quartz crystals, which consist of alternating bands of micron-scale CLdark and CL-bright quartz layers. The growth zones are 3 10 µm thick, although individual CL-bright bands tend to be narrower than the CL-dark bands (Fig. 3). In places, SEM-CL imaging reveals a pattern of concentric, euhedral-shaped crystal rhombs that may be indicative of growth to open space (Fig. 4A). Elsewhere, SEM-CL imaging shows a fusion of two or more growth zone regions and/or crystals, with apparent overlapping of growth zone regions, which may vary in CL intensity Quartz dissolution and growth impediment Textural relationships revealed by SEM-CL indicate the quartz crystals initially precipitated into open space, and at some later time they were fractured and partially dissolved. Subsequently, open fractures and space between the crystals were partly filled with late-stage quartz. The most obvious evidence of quartz dissolution is the occurrence of rounded CL-bright edges (Fig. 4B), and shallow indented or corroded faces on some quartz crystals, which is not clear by optical microscopy. In places, SEM-CL imaging shows how two or more quartz crystals impinge, and cause growth impediments, with indentations and truncated crystal form highlighted by an abrupt cessation of growth zone textures Replacement and late-stage precipitation Replacement and late-stage quartz precipitation is most obvious where CL-bright quartz overprints CL-grey quartz, and where quartz with CL-bright luminescence has filled angular spaces between euhedral, CL-dark Growth impediment CL-dark polygons at microcrack intersection CL-dark and bright quartz in fractures 20 µm Crystal core Continuous CL-pale grey quartz 20 µm Growth zone CL-dark oscillation band CL-grey bright growth zone Fig. 3 SEM-CL images of microtextures in vein quartz crystals (top). The euhedral quartz crystals have a continuous CL-pale grey core, but typically contain alternating CL-grey/bright growth bands at the crystal edge. In places crystal growth impediments are evident, as well as anomalous bands of CL-dark quartz. Healed microfractures, filled with CL-bright or -dark quartz are revealed (marked in sketch), as well as CLdark polygons at microcrack intersections. crystals (Fig. 4C). Typically, late stage replacement is indicated where the boundaries of original CL-dark quartz is preserved, with late, CL-bright quartz within the crystal cutting across previously-formed CL-dark/CLpale grey growth zones. In parts of the vein(s), CL-grey quartz occurs at the edge of CL-bright quartz, which is also indicative of recrystallisation having occurred along some grain boundaries Microfracturing Throughout the veins, quartz-filled microfractures irregularly cut CL-dark or CL-bright quartz crystals (Fig. 4D), with variation in CL luminescence indicating at least three phases of fracturing. The microfractures are sometimes braided, irregularly oriented, and elsewhere planar, with some containing rare secondary fluid inclusions. SEM-CL imaging indicates that the fracture contact with the host quartz is typically sharp, even though many fractures are optically continuous with the host crystal quartz. Fractures filled with CL-bright quartz crosscut and offset

5 vol. 55, no. 1, 2005 Quartz Vein Formation Revealed by SEM-CL Imaging 5 Euhedral quartz Open space Late stage CL-bright quartz, with growth zones infilling open space Initial quartz crystal growth Euhedral quartz A 30 µm C 5 µm Late-stage quartz filling open space Initial quartz with rounded (dissolved) crystal edges Euhdral quartz (growth zones) Open space Cross-cutting microfractures B 30µm 5 µm Fig. 4 Textures observed by SEM-CL imaging. A. Euhedral, CL-grey quartz crystals, with growth zoning at crystal edges, inferred to have grown into open space; B. Quartz crystal with rounded surface, indicative of dissolution, with subsequent late-stage quartz precipitation infilling the resultant open space; C. Late-stage CL-bright quartz growth, infilling space between CL grey quartz; and D. Quartz-filled, cross-cutting healed microfractures, indicative of at least three phases of microfracturing and quartz precipitation, filled with CL-bright or CL-dark quartz. D one another, and cut earlier CL-grey quartz-filled fractures. A second phase of CL-dark, quartz-filled microfractures, cutting the CL-bright quartz-filled fractures, is the last quartz generation phase (Fig. 4D). 6. Fluid Inclusion Microthermometry Our fluid inclusion study uses the same 100 µm-thick, doubly polished vein quartz plates initially used for SEM- CL imaging, with fluid inclusion microthermometry undertaken using a Linkam THMS-600 heating-freezing stage. The quartz plates were examined to test if there was any variation in the temperature and/or salinity of fluids trapped in the vein quartz either in the CL-pale grey crystal core, growth zone, in CL-bright interstitial quartz, or in healed microfractures revealed by SEM-CL imaging. Fluid inclusions in the core of the quartz crystals and in growth zones at crystal edges are mostly liquid-rich and primary (some have negative crystal form). Vapour inclusions were identified, co-existing with liquid inclusions, but these vapour inclusions are necked and were not analysed as they would produce erroneous T h data. No daughter minerals or other phases (e.g. clathrate) were observed. The fluid inclusions are subrounded and up to 10 µm long, although most are <3 µm in length (no thermometric data was obtained on fluid inclusions <2 µm long). Microthermometry was undertaken on fluid inclusions in several crystals, from different parts of the vein (summarized in Table 1, and plotted in Figs. 5A and B). T h values for fluid inclusions in the core of vein quartz range from 269 to 391 C (average 343 C), with a calculated salinity of 6.6 to 7.7 wt% NaCl eq. In

6 6 B. BATKHISHIG, G. BIGNALL and N. TSUCHIYA RESOURCE GEOLOGY : Table 1 Summary of fluid inclusion microthermometric data for hydrothermal vein quartz from the Shuteen Complex. Microtexture No. Size Incl. type Th values Tm values Salinity Other Incl. (µm) (L:V ratios) ( C) ( C) wt% NaCl eq (a) shape; (b) daughter minerals; range (mean) range (mean) range (mean) (c) SEM-CL character Crystal core Primary (L:V 80:20) 269 to 361 (343) -4.1 to -4.9 (-4.5) 6.7 to 7.7 (a) elongate/subrounded; (b) none; (c) continuous, (7.2) CL-pale grey/bright Growth zone Primary (L:V 80:20) 210 to 293 (261) -1.7 to -2.2 (-1.9) 2.7 to 3.7 (3.2) (a) elongate; (b) none; (c) alternating CL-bright/ dark bands, of variable width (up to 20 µm wide) Interstitial quartz Primary (L:V 80:20) 170 to 275 (233) -2.5 to -2.9 (-2.7) 4.1 to 4.8 (4.6) (a) typically elongate; (b) none; (c) CL-bright (compared to adjacent quartz) (a) irregular elongate; (b) none; (c) CL-bright/ Fracture (L:V Secondary 85:15) 156 (173) to dark. Mean Th is 160 C for fluid inclusions in CL-bright fractures, and 185 C for fluid inclusions in CL-dark fractures (A) Crystal cores 261 Growth zones 233 Interstitial quartz 173 Micro fractures 343 Salinity (wt% NaCl equiv ) (B) Homogenisation temperature ( o 50 C) Homogenisation temperature ( o C) Fig. 5 (A) Homogenisation temperature (T h ) values for fluid inclusions in crystal core, growth zones, late-stage interstitial quartz, and trapped in healed (quartz-filled) microfractures revealed by SEM-CL; showing the range ( ) of T h values; average T h (x); and ±1 s.d. from the average T h value ( ); and (B) Plot of T h values and salinity (wt% NaCl eq ) data for selected fluid inclusion populations in different microtextural zones ( crystal cores; growth zones; interstitial quartz; and microfractures). contrast, fluid inclusions in well-defined growth zones have T h values that range from 210 to 293 C (average 261 C), with a calculated salinity of 2.7 to 3.7 wt% NaCl eq. The fluid inclusion data was plotted on a boiling point for depth curve (Haas, 1971), for brine with constant composition (i.e. ~7 wt% NaCl). From that plot, it is inferred that initial quartz crystal growth occurred at shallow depth of at least 600 m below its palaeo-watertable, in a 345±30 C water reservoir (based on the average T h value of 343 C±1 s.d., for fluid inclusions from the crystal cores). Rare, isolated fluid inclusions were examined from interstitial, CL-bright quartz between the early-formed vein quartz crystals. These fluid inclusions are small (<3µm long), liquid-rich, with T h values of 170 (consistent with T h values of fluid inclusions in microfractures) to 275 C (similar to the conditions when growth zone quartz was precipitated), with inferred salinities that range from 4.1 to 4.8 wt% NaCl eq. Secondary fluid inclusions in microfractures are elongate, with T h values of 156 to 213 C (average 173 C), and have a calculated salinity of about 3.9 wt% NaCl eq. 7. Discussion SEM-CL imaging indicates vein quartz in the Shuteen Khanbogd lithocap area initially grew as individual CL-grey crystals into fluid-filled space, under stable hydrostatic and thermal conditions, in an inferred 345±30 C liquid-reservoir (average T h value for fluid inclusions in the crystal cores is 343 C). Furthermore, SEM-CL imaging shows the edges of the sub-euhedral quartz crystals contain micron-scale bands of varying CL intensity, reflecting quartz precipitation at some later time, when the southern part of the Shuteen hydrothermal system had cooled (i.e. when reservoir conditions were about 260±25 C; based on the average T h value of 261 C). The zoned CL growth pattern may result from variations in trace element abundances (e.g. Al, Ti, Fe, Ge or P) in the quartz (Götze et al., 2001),

7 vol. 55, no. 1, 2005 Quartz Vein Formation Revealed by SEM-CL Imaging 7 Growth zone Growth impediments CL-grey Growth growth zone zone Crystal core Continuous CL-bright grey crystal quartz core CL-dark microfracture 20 µ 20 µm CL-bright fracture 20 µm Cross cutting CL-dark CL-bright/dark band (micro microfractures fracture) Fig. 6 Quartz microtextures revealed by SEM-CL Imaging (left), showing growth impediment and CL-grey growth zoning at the edge of euhedral quartz crystal with continuous CL-pale grey/bright core, and cut by truncated CL-dark band. Sketch detail of the SEM-CL image is also shown (right). corresponding to a change in the chemical composition of the hydrothermal fluid, different rates of quartz precipitation (as band widths vary), or changing physical (temperature/pressure) conditions in the Shuteen hydrothermal system. Fluid inclusion evidence points to an initial fluid chemistry of wt% NaCl eq being responsible for precipitation of the quartz crystal cores, whereas later formed fluid inclusions in growth zones at the crystal edges have salinities of wt% NaCl eq. As well as crystal growth, there is SEM-CL evidence of the vein quartz having been partly dissolved. Numerous quartz crystals exhibit euhedral growth zones, but also rounded crystal edges indicative of dissolution by a quartz-undersaturated fluid, even though open spaces created by quartz dissolution were filled by later CL-bright quartz. Pressure change has a large effect on quartz solubility and may have been responsible for quartz dissolution and precipitation textures in the cooling (fossil) hydrothermal system. Pressure fluctuations almost certainly occurred in the extensively faulted Shuteen area, and/or accompanied microfracturing of the vein quartz. In places, overprinting by CL-bright quartz obscures textural relations in early-formed CL-dark quartz, although the original shape of the quartz crystal is preserved. A change in CL luminescence intensity may also result from diffusion of elements in response to chemical gradients between quartz and hydrothermal fluid. SEM-CL evidence of abundant crosscutting microfractures, filled with CL-bright or -dark quartz, underlines a history of repeated fracturing and resealing (by quartz deposition) during vein development, with microfracturing likely to be linked to regional faulting. Whatever the cause of the microfracturing, however, quartz precipitation plugged most of the fractures, and this may have produced a slight pressure increase, accounting for an increase in quartz solubility and resultant dissolution. Clearly, SEM-CL imaging reveals that quartz vein formation at Shuteen did not occur at one time, and neither was it related to a single hydrothermal event. Rather, vein development occurred as a progressive process, overlapping microfracturing episodes, with quartz deposition related to distinct hydrothermal events. The present study shows SEM-CL is an effective tool to understand fluidmineral interactions in hydrothermal systems, and reveals quartz microtextures (e.g. growth zoning, dissolution, replacement and fracturing) that cannot be observed by optical microscopy, back-scattered- or secondary-electron imaging. SEM-CL images, coupled with fluid inclusion microthermometry can help to decipher the evolution of a hydrothermal system; correlate fluid inclusion populations and their relative chronology with specific hydrothermal events (or mineralisation); and resolve fluid characteristics and the nature of vein-forming processes. 8. Conclusions Back-scattered electron and transmitted light examination shows quartz crystals in hydrothermal veins in the southern part of the Cu-mineralised Shuteen Complex (Mongolia) are fractured, but these methods reveal no other distinguishing features, whilst SEM-CL imaging highlights a complex history of vein formation (e.g. Fig. 6). SEM-CL imaging of vein quartz crystals has provided

8 8 B. BATKHISHIG, G. BIGNALL and N. TSUCHIYA RESOURCE GEOLOGY : evidence of multi-stage crystal growth, dissolution, recrystallisation and microfracturing. The crystal cores are composed of CL-grey/bright quartz that are inferred to have grown within the fossil Shuteen hydrothermal systems, >600 m below the palaeo-watertable under stable physico-chemical conditions, in a 345±30 C liquid reservoir (of wt% NaCl eq salinity). Quartz edges contain micron-scale bands of CL-dark/bright quartz, containing fluid inclusions that are inferred to have been trapped during a later period of crystal growth, from somewhat cooler, less saline fluids (260±25 C; wt% NaCl eq ). SEM-CL examination of the quartz underlines a complex history of repeated microfracturing and sealing (through quartz deposition) during crystal growth. As well as growth textures, we find CL-dark quartz is partly dissolved, with corroded crystal edges, as a result of pressure fluctuations related to faulting affecting on quartz solubility, with subsequent overprinting by CL-bright quartz. Acknowledgements: The authors thank Professor Ochir Gerel (Mongolian University of Science and Technology) for logistic support during our 2002 and 2003 field surveys, and Dr. Kotaro Sekine (Institute for Fluid Science, Tohoku University) for his advice in measuring and interpreting our SEM-CL images. We thank Professor Hiroaki Kaneda for his review and suggestions that improved the paper. This study was supported financially by Grant-in- Aid (B ) to GB from Japan Society for the Promotion of Science (JSPS). References Atkinson, J. (1999) Report of Exploration Works in the Shuteen Area. Unpubl. Rept., Quinqunx Limited, Canada, 20p. Batkhishig, B. and Iizumi, S. 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