A preliminary stable isotope study on Mogok Ruby, Myanmar

The primary occurrence of ruby in the Mogok area, northern Myanmar is exclusively found in marble along with spinel–forsterite-bearing marble and phlogopite–graphite marble. These marble units are enclosed within banded biotite–garnet–sillimanite–oligoclase gneisses. Samples of these marbles collected for C–O stable isotope analysis show two trends of δ¹³C–δ¹⁸O variation resulting most likely from fluid–rock interactions. Ruby-bearing marble and phlogopite–graphite marble follow a trend with coupled C–O depletion, whereas spinel–forsterite-bearing marble follows a δ¹⁸O depletion trend with relatively constant δ¹³C values. Ruby formation might have resulted from CO₂-rich fluid–rock interaction, while spinel–forsterite-bearing marble was genetically related to CO₂-poor fluid–rock interaction. Both fluids may have arisen from external sources. Based on graphite Raman spectral thermometry, the estimated temperature for phlogopite–graphite marble, and probably ruby-bearing marble, was lower than 607 °C, and for spinel–forsterite-bearing marble, lower than 710 °C. Contrasting C/O diffusion between graphite/ruby/spinel/forsterite and calcite, local variations of isotopic compositions of newly formed minerals as a result of non-pervasive fluid infiltration, and open-system isotopic disturbance during cooling may have affected C-/O-isotopic fractionations between minerals. The estimated high formation temperatures for ruby and spinel/forsterite imply that the parental fluids may have been related to nearby igneous intrusions and/or metamorphic processes. Whether these two types of fluid were genetically related is unclear based on the present data.     

The use of WorldView-2 satellite imagery to model urban drainage system with low impact development (LID) Techniques

Within a wide range of best management practices for stormwater management in urban areas, there has been an increasing interest in source control measures. Source controls such as low-impact development (LID) techniques are potentially attractive as retrofit options for older developed areas that lack available land to implement conventional measures such as stormwater management ponds. Hence, distributed urban drainage models requiring detailed representation of developed drainage areas should be developed to accurately estimate the benefits that LIDs may provide. This study (1) presents a two-stage classification process on a high-resolution WorldView-2 image, and (2) demonstrates how to use the extracted land cover information in the subsequent hydrologic modelling and assessment of different LIDs’ performance. The proposed two-stage classification method achieved an overall accuracy of 80.6%, whereas a traditional pixel-based achieved 68.4% in classifying the same urban area into six land cover classes. From the classification results, the hydrologic properties of micro-subcatchments were imported in the United States Environmental Protection Agency Storm Water Management Model to assess the performance of LIDs. A reduction of run-off volume 18.2% and 37.1% was found with the implementation of porous pavement and bioretention, respectively, in a typical low-rise residential area located in the city of San Clemente, California, US. The study demonstrates the use of high-resolution remote sensing image to aid in evaluating LID retrofit options, and thus benefits in situations where detailed drainage area information is not available.     

Advances in our understanding of the gem corundum deposits of the West Pacific continental margins intraplate basaltic fields

Recent discoveries over the last decade of new gemfields, exploitation of new and existing deposits, and application of relatively new techniques have greatly increased our knowledge of the basalt-derived gem sapphire–ruby–zircon deposits. In this paper we focus on the Late Mesozoic to Cenozoic intraplate basaltic fields of the West Pacific continental margins. We review advances made in understanding the genesis of these deposits, based on the application of newer techniques. We also critically review existing data on the gem corundum deposits, in order to further refine a model for their origin.In some of the intraplate basaltic fields, corundum-bearing xenoliths have been found showing a range of PT formation conditions from 790 °C at 0.85 GPa to as much as 1100 to 1200 °C at 1.0 to 2.5 GPa. Although most magmatic sapphires contain syngenetic inclusions of columbite-group phases, zircon, spinel and rutile, some contain additional nepheline and K-feldspar, suggesting crystallization from more undersaturated alkaline magma while the Weldborough field of NE Tasmania also contains molybdenite and beryl, suggesting at least some interaction with more fractionated ‘granitic-type’ magmas. There is a large range in PT conditions calculated for the metamorphic rubies (from 780 to 940 °C, through 800 to 1150 °C up to 1000 to 1300 °C). Fluid/melt inclusion studies on magmatic corundums generally suggest that they formed in a CO₂-rich environment from a ‘syenitic’ melt under a range of T conditions from 720 to 880 °C up to 1000 to 1200 °C. Oxygen isotope studies reveal that typical magmatic corundums have values of + 4.4 to 6.9‰, whereas metamorphic corundums from the same basaltic host have lower values of + 1.3 to 4.2‰.Geochronological studies have shown that some fields produced a simple eruptive and zircon/corundum crystallization event while others had multiple eruptive events but only one or two zircon crystallization events. For a few fields, some corundums/zircons crystallized in storage regions and then remained relatively inert for periods of 200 to 400 Ma without significant change before transport to the surface in the Cenozoic. Tectonic studies of the Australian region suggest that many of the corundums crystallized from magmas related to episodic basaltic volcanism in a tectonic regime of extension, associated with the opening of the Tasman and Coral Seas. For the Asian region, the magmatic–polygenetic corundums within the basaltic fields largely crystallized in a tectonic regime of distributed E–W extension, whereas the metamorphic-metasomatic corundums crystallised in a transpressional regime associated with the collision of the Indian Plate with the Eurasian Plate (e.g., [Garnier, V., Giuliani, G., Maluski, H., Ohnenstetter, D., Deloule, E., 2003. Ar–Ar and U–Pb ages of marble-hosted ruby deposits from Central and South-east Asia. Geophysical Research Abstracts 5, 03751; Garnier, V., Giuliani, G., Ohnenstetter, D., and Schwarz, D., 2004. Les gisements de corindon: classification et genese. Les placers a corindon gemme. Le Regne Mineral 55, 7-47; Garnier, V., Ohnenstetter, D., Giuliani, G., Maluski, H., Deloule, E., Phan Trong, T., Pham Van, L., Hoang Quang, V., 2005a. Age and significance of ruby-bearing marble from the Red River Shear Zone, Northern Vietnam. Canadian Mineralogist 43, 1315–1329]).     

Effect of rising atmospheric CO2 on sediment and water 15N interactions in experimental riparian wetland

The experiment was conducted to ascertain net production and consumption rates of ¹⁵NH₄ ⁺ and ¹⁵NO₃ ^? for water and sediment in a wetland. This was done using ¹⁵N isotope pool dilution methodology under ambient and elevated atmospheric CO₂ concentrations in experimental riparian wetlands to obtain the gross N transformation rates. The ¹⁵N budget for sediment was also estimated. The results suggested that the differences in high proportion of ¹⁵N concentration in the overlying water body under elevated CO₂ could be attributed to the low production and high consumption rates of ¹⁵NH₄ ⁺ in sediment. The elevated CO₂ effect on production and consumption of NH₄ ⁺ decreased by 144 % (P = 0.014) and increased by 153 % (P = 0.009), respectively. Thereby, ¹⁵NH₄ ⁺ production rates are negatively related with ¹⁵NO₃ ^? consumption rates and this accounted for the decreases in net ¹⁵NO₃ ^? consumption under CO₂ enrichment in the wetland sediment by 11 % (P = 0.528). Therefore, ¹⁵NO₃ ^? production and consumption rates may strongly depend on NH₄ ⁺ production. Inorganic ¹⁵N and total ¹⁵N exported from sediment to overlying water body by the effect of CO₂ were 41 % (P = 0.071) and 18 % (P = 0.000), respectively. Therefore, low net ¹⁵NH₄ ⁺ production and high ¹⁵NH₄ ⁺ consumption rates under elevated CO₂ may partly explain the significant reduction of N from the sediment.     

澳大利亚造山型金矿和侵入岩有关金矿系统流体包裹体资料和矿化过程的比较

We have examined the fluid inclusion data and fluid chemistry of Australian orogenic and intrusion-related gold deposits to determine if similar mineralization processes apply to both styles of deposits.The fluid inclusion data from the Yilgarn craton,the western subprovince of the Lachlan orogen,the Tanami,Tennant Creek and Pine Creek regions,and the Telfer gold mine show that mineralization involved fluids with broadly similar major chemical components(i.e.H_2O NaCl CO_2±CH_4±N_2).These deposits formed over a wide range of temperature-pressure conditions(<200 to>500℃,<100～400MPa).Low salinity, CO_2-bearing inclusions and low salinity aqueous inclusions occur in both systems but the main difference between these two types of deposits is that most intrusion-related gold deposits also contain at least one population of high-salinity aqueous brine.Oxygen and hydrogen isotope data for both styles of deposit usually cannot distinguish between a magmatic or metamorphic source for the ore-bearing fluids.However,sulfur and lead isotope data for the intrusion-related gold deposits generally indicate either a magmatic source or mixing between magmatic and sedimentary sources of fluid.The metamorphic geothermal gradients associated with intrusion-related gold deposits are characterized by low pressure,high temperature metamorphism and high crustal geothermal gradients of>30/km.Where amphibole breakdown occurs in a granite source region,the spatially related deposits are more commonly associated with Cu-Au deposits rather than Au-only deposits that are associated with lower temperature granites.The dominant processes thought to cause gold precipitation in both types of deposits are fluid-rock interaction(e.g.desulfidation)or phase separation.Consideration of the physical and chemical properties of the H_2O-NaCl-CO_2 system on the nature of gold precipitation mechanisms at different crustal levels infers different roles of chemical(fluid-rock interaction)versus rheological(phase separation and/or fluid mixing)host-rock controls on gold deposition.This also implies that at the site of deposition,similar precipitation mechanisms operate at similar crustal levels for both orogenic and intrusion-related gold deposits.     

U–Pb Zircon Age Constraining the Source and Provenance of Gem‐Bearing Late Cenozoic Detrital Deposits,Mamfe Basin,SW Cameroon

Zircons and other heavy minerals (corundum, rutile, ilmenite, magnetite, sillimanite) are identified in the Nsanaragati gem corundum placer deposit, in the western part of the Mamfe sedimentary basin, SW Cameroon. These alluvial minerals have different morphological characteristics and zircons, in particular, vary mostly in colour and shape. They are reddish, brownish, yellowish, pink or colourless. These minerals form rounded and sub‐rounded alluvial grains, prismatic, pyramidal or dipyramidal crystals. Reddish zircons retain their original crystallographic shape. Trace element and U–Pb isotopic geochemical analyses of these reddish zircons, using the LA‐ICP‐MS method give significant Hf (4576–6334 ppm), Th (46–1565 ppm) and U (66–687 ppm) contents, with Th/U ratio ranging from 0.6 to 3.0. The ²⁰⁶Pb/²³⁸U corrected mean age gave 12.39 ± 0.55 Ma, which characterizes an Upper Cenozoic (Serravallian) magmatic event. The zircons are probably sourced from a magmatic field in the South eastern boundary of the Cross River Formation. The Cameroon Volcanic Line of basaltic and alkaline lavas and intrusions which lie east of the Mamfe Basin mostly range in age from 37 Ma to <1 Ma. The zircons may also relate to the Mount Bambouto plateau lavas which lie northeast of the Mamfe sedimentary basin and have an eruptive age range of 21–14 Ma. The oldest Nsanaragati reddish zircon ages overlap within error with the end stages of the Bambouto eruptions. This eruptive or a related episode provides a potential source for megacrystic reddish zircons within the Nsanagarati placer deposit.     

Structural Mapping of the Bentong‐Raub Suture Zone Using PALSAR Remote Sensing Data,Peninsular Malaysia: Implications for Sediment‐hosted/Orogenic Gold Mineral Systems Exploration

The Bentong‐Raub Suture Zone (BRSZ) of Peninsular Malaysia is one of the major structural zones in Sundaland, Southeast Asia. It forms the boundary between the Gondwana‐derived Sibumasu terrane in the west and Sukhothai Arc in the east. The BRSZ is genetically related to the sediment‐hosted/orogenic gold deposits associated with the major lineaments in the Central Gold Belt of Peninsular Malaysia. In this investigation, the Phased Array type L‐band Synthetic Aperture Radar (PALSAR) satellite remote sensing data were used to map major geological structures in Peninsular Malaysia and provide detailed characterization of lineaments and curvilinear structures in the BRSZ, as well as their implication for sediment‐hosted/orogenic gold exploration in tropical environments. Major structural lineaments such as the Bentong‐Raub Suture Zone (BRSZ) and Lebir Fault Zone, ductile deformation related to crustal shortening, brittle disjunctive structures (faults and fractures) and collisional mountain range (Main Range granites) were detected and mapped at regional scale using PALSAR ScanSAR data. The major geological structure directions of the BRSZ were N–S, NNE–SSW, NE–SW and NW–SE, which derived from directional filtering analysis to PALSAR fine and polarimetric data. The pervasive array of N–S faults in the Central Gold Belt and surrounding terrain is mainly linked to the N–S trending of the Suture Zone. N–S striking lineaments are often cut by younger NE–SW and NW–SE‐trending lineaments. Gold mineralized trend lineaments are associated with the intersection of N–S, NE–SW, NNW–SSE and ESE–WNW faults and curvilinear features in shearing and alteration zones. Compressional tectonic structures such as the NW–SE trending thrust, ENE–WSW oriented faults in mylonite and phyllite, recumbent folds and asymmetric anticlines in argillite are high potential zones for gold prospecting in the Central Gold Belt. Three generations of folding events in Peninsular Malaysia have been recognized from remote sensing structural interpretation. Consequently, PALSAR satellite remote sensing data is a useful tool for mapping major geological structural features and detailed structural analysis of fault systems and deformation areas with high potential for sediment‐hosted/orogenic gold deposits and polymetallic vein‐type mineralization along margins of Precambrian blocks, especially for inaccessible regions in tropical environments.     

Complex relationships between salt type and rock properties in a durability experiment of multiple salt–rock treatments

A laboratory salt weathering experiment was performed using five salts to attack eight types of rocks to determine the relative significance of rock durability and salt aggressivity to salt crystallization damage. The influence of individual rock properties on the salt susceptibility of the rocks was also evaluated. To study the relation between pore characteristics, salt uptake, and damage, the pre‐ and post‐experiment pore size distributions of the rocks were also examined. It is observed that both salt type and rock properties influenced the damage pattern. The durability ranking of the rocks became significantly altered with the salt type while the variation in salt efficacy ranking with rock type was less pronounced. Of the five salts used, sodium chloride and aluminium sulfate were invariably ineffective with all rock types while sodium carbonate, sodium sulfate, and magnesium sulfate, were markedly more effective in damaging most types of rock used. Of the rock properties investigated, the microporosity (of pores smaller than 0·05 or 0·1 µm) showed the most significant influence on deterioration of the rocks associated with salt crystallization, whereas microporosity of pores smaller than 5 µm played a more important role in salt uptake. Pore size distribution was thus the key factor controlling salt uptake and damage. Rocks with a large number of pores (<5 µm) and a high proportion of pores (<0·05 or 0·1 µm) were particularly susceptible to salt crystallization damage. However, anomalies arose that could not be explained in terms of rock properties or salt efficacy alone. Overall, the relative influences of salt type/efficacy and rock type/properties on salt damage propensity were not clear enough to draw a reasonable conclusion. Salt crystallization damage appears to be influenced by the individual interactions between salts and rocks, which could explain the anomalous results. Copyright © 2009 John Wiley & Sons, Ltd.     

Zn–Pb–Cu volcanic-hosted massive sulphide deposits: criteria for distinguishing brine pool-type from black smoker-type sulphide deposition

Eight Zn–Pb–Cu massive sulphide deposits that appear to have formed on the sea floor (seven in Spain, one in Tasmania) are believed to have been precipitated in brine pools, based on the salinities and temperatures of fluid inclusions in underlying stockworks. Comparing the geological features of these deposits with those of the Zn–Pb–Cu massive sulphide ores of the Hokuroku Basin, Japan, which have formed as mounds from buoyant fluids of low salinity, shows that brine pool deposits have: (1) potentially very large size and tonnage, and high aspect ratio, (2) higher Zn/Cu and Fe/Cu values, (3) no evidence of chimneys, (4) relatively abundant framboidal pyrite and primary mineral banding, (5) reduced mineral assemblages (pyrite-arsenopyrite/pyrrhotite), and minor or rare barite in the massive sulphide, (6) associated stratiform and/or vein carbonates, (7) relatively unimportant zone refining, (8) lack of vertical variation in sphalerite and sulphur isotopic compositions, and (9) evidence of local bacterial sulphate reduction. Application of these criteria to the Rosebery deposit in Tasmania, for which there are no fluid inclusion data, leads to the conclusion that the southern section was deposited as separate lenses in a brine-filled basin or basins. Other potential candidates include Brunswick no. 12 and Heath Steele (Canada), Woodlawn and Captains Flat (New South Wales), Hercules and Que River (Tasmania), and Tharsis and the orebodies at Aljustrel (Spain and Portugal). Recently published fluid inclusion data for Gacun (China) and Mount Chalmers (Queensland) suggest that not all ores deposited from highly saline fluids have reduced mineral assemblages.