Germanium-silicon fractionation in the weathering environment

We present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the Ge/Si system as a tracer of silicate weathering processes in both modern and ancient environments. Intensely weathered soils developed on Hawaiian basalts have bulk soil Ge/Si ratios 2 to 10 times higher than fresh basalt (e.g., 10 to 25 μmol/mol vs. 2.5 μmol/mol). Soil Ge concentrations increase with Si, and decrease with Fe, suggesting that Ge sequestration is related to accumulation of secondary soil silicates, rather than retention in soil Fe oxy-hydroxides. Sequential extractions of these soils suggest that Ge/Si fractionation takes place by Ge sequestration during the initial precipitation of secondary soil aluminosilicates (principally allophane). Further Si loss and changes in mineralogy as these soils age result in little additional Ge/Si fractionation. Ge/Si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals. Kaolinite has a Ge/Si ratio (5.9 μmol/mol) higher than the plagioclase from which it forms (3.1 μmol/mol), whereas accumulation of primary quartz (Ge/Si 0.5 μmol/mol) prevents granitic soils from attaining high Ge/Si ratios. Laboratory synthesis of allophane confirms that Ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution.     

Germanium-silicon fractionation in a tropical, granitic weathering environment

Germanium-silicon (Ge/Si) ratios were determined on quartz diorite bedrock, saprolite, soil, primary and secondary minerals, phytolith, soil and saprolite pore waters, and spring water and stream waters in an effort to understand Ge/Si fractionation during weathering of quartz diorite in the Rio Icacos watershed, Puerto Rico. The Ge/Si ratio of the bedrock is 2 μmol/mol, with individual primary mineral phases ranging between 0.5 and 7 μmol/mol. The ratios in the bulk saprolite are higher (∼3 μmol/mol) than values measured in the bedrock. The major saprolite secondary mineral, kaolinite, has Ge/Si ratios ranging between 4.8 and 6.1 μmol/mol. The high Ge/Si ratios in the saprolite are consistent with preferential incorporation of Ge during the precipitation of kaolinite. Bulk shallow soils have lower ratios (1.1-1.6 μmol/mol) primarily due to the residual accumulation of Ge-poor quartz.Ge/Si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths. Spring water and baseflow stream waters have the lowest Ge/Si ratios (0.27-0.47 μmol/mol), reflecting deep initial weathering reactions resulting in the precipitation of Ge-enriched kaolinite at the saprolite-bedrock interface. Mass-balance calculations on saprolite require significant loss of Si and Al even within 1 m above the saprolite-bedrock interface. Higher pore water Ge/Si ratios (∼1.2 μmol/mol) are consistent with partial dissolution of this Ge-enriched kaolinite. Pore water Ge/Si ratios increase up through the saprolite and into the overlying soil, but never reach the high values predicted by mass balance, perhaps reflecting the influence of phytolith recycling in the shallow soil.     

Marble weathering in an industrial environment,eastern Australia

Qualitative evidence from monuments and buildings in industrialized countries indicates that rates of stone deterioration rise in the presence of urban and industrial pollutants. Measurements presented here on surface reduction of marble tombstones show that mean weathering rates have increased over the period 1885 to 1955. Weathering rates were lower before the establishment of sulfur dioxide-emitting plants. Marble weathering involved solution etching along grain boundaries and within individual grains, and proceeded by surface roughening and dislodgement of grains with little apparent accumulation of gypsum. Absolute surface reduction of marble was less than that recorded in industrial environments in Europe and North America.     

Genetic environment of germanium-bearing gold-silver vein ores from the Wolyu mine,Republic of Korea

The Wolyu mine is one of the largest vein-type gold-silver-bearing epithermal systems in the Youngdong district and is the first gold-silver deposit in Korea found to contain significant germanium, in the form of argyrodite (Ag₈GeS₆). Mineralized veins (78.9 ± 1.2 Ma) crosscutting Late Cretaceous hostrock tuff and quartz porphyry (81.5 ± 1.8 Ma) consist of three stages of quartz and carbonates, the first of which contains pyrite, basemetal sulfides and Au-Ag-minerals. Stage I Au-Ag-Ge-mineralized veins show a systematic variation of mineral assemblage with time: (1) quartz + pyrite; (2) quartz + pyrite + sphalerite + electrum + argentite; (3) carbonate + quartz + sphalerite + electrum + argentite; (4) carbonate + native silver + argentite + Ag-sulfosalts + argyrodite + sphalerite. Calculated values of temperature and sulfur activity are: assemblage (1), 360-280°C and 10^–7-10^–10; (2), 280-210°C and 10^–10-10^–14; (3), 210-180°C and 10^–14-10^–16; (4), 180-155°C and 10^–17-10^–18. These data, the frequent association of gold with sulfides, and the abundance of pyrite in alteration zones indicate that decreasing sulfur activity and cooling were important in triggering gold deposition. Hydrogen and oxygen isotope compositions of ore fluids display a systematic variation with increasing time. Within the main Ag-Au-Ge mineralization, D and ¹⁸O values decrease with the transition from quartz to carbonate deposition (from -78 and –2.8% to –90 and –8.7%., respectively), indicating increasing involvement (mixing) of less evolved meteoric water which resulted in progressive cooling and dilution of ore fluids in the shallow ( 370–600 m) Wolyu epithermal system.     

A study on the behavior of mercury in weathering environment in Wanshan district

Studies on the mechanism of supergenic dispersion, migration and enrichment of mercury and other elements in strata-bound ore deposits are of great importance both in theory and in practice. In special reference to the Wanshan mercury ore deposits, this paper discusses the mechanism of supergenic migration of mercury and the associated elements lead, zinc, antimony, etc. In this paper the focus of the discussion is placed on the existing forms of mercury, the rules governing the migration and enrichment of mercury and associated elements in the process of weathering and some aspects of the simulating experiments on weathering of carbonate rocks. Three existing forms of mercury are recognized in carbonate rocks. The migration of mercury together with the associated elements in the process of weathering follows the sequence: SiO₂→Pb→Hg→Sb→Zn. The results of simulating experiments on weathering of carbonate rocks show that weathered materials contain more mercury and associated elements than the original rocks.     

广东长坑-富湾是金矿床金银矿化规律统计研究

对矿区７０个见矿钻孔的金银矿化进行了统计研究,包括矿化的厚度分布,见矿次数,化整为零见矿厚度和平均矿化厚度。矿化主要出现于硅质岩类中,但非硅质央类的矿化也不可忽视。各种岩性有其自己的成矿趋势和特点。金矿化较厚,高品级矿石比例大,矿化品级与厚度基本上呈正向消长,成矿与厚层角砾状硅质岩的专属性关系密切。金矿化的规律呈现得较清晰。银矿化在这些方面不如金矿化,缺乏清晰的规律。出现上述情况的原因与矿床发育两     

Chemical alteration in the micro weathering environment within a spheroidally-weathered anorthosite boulder

A chemical and mineralogical study of the innermost rindlets of a spheroidally weathered boulder from the Wichita Mountains, Oklahoma details incipient alteration of minerals along solution channels. Alteration products in plagioclase areas are highly aluminous, whereas ferruginous residuum is commonly found in olivine alteration zones. Alteration of plagioclase along solution channels results in sharp weathering contacts as contrasted with wider zones of “limonite” bordering solution channels in olivine. In certain instances, cores of fresh olivine are surrounded by limonite indicating that diffusion may play a role in this type of alteration.Mobility of sesquioxides is evidenced by aluminum-rich debris adjacent to mafic materials in solution channels and iron-bearing material in channels cutting through plagioclase. Not resolved is whether the movement of sesquioxides in solution channels is by transport of colloidal particulates or by diffusion of dissolved species.Calcic plagioclase alters to calcite along the innermost solution channel. It is postulated that solution, exterior to the boulder and undersaturated with respect to calcite, enters the rind system via solution channels. Cut off from the CO₂ reservoir of the atmosphere by the narrow confines of this solution channel, the solution reacts with the bytownite to precipitate calcite.Apart from the occurrence of calcite, no crystalline weathering products were found in the innermost rindlet samples. A poorly crystallized illite was identified in the outermost rindlet. These findings suggest that initial alteration products on weathering rinds are amorphous and that, with time, these products attain a degree of identifiable crystallinity.     

Chemical weathering of granite under acid rainfall environment, Korea

Chemical weathering was investigated by collecting samples from five selected weathering profiles in a high elevation granitic environment located in Seoul, Korea. The overall changes of chemistry and mineralogical textures were examined reflecting weathering degrees of the samples, using polarization microscopy, X-ray diffraction (XRD), electron probe micro analysis (EPMA), X-ray fluorescence spectroscopy (XRF), and inductively coupled plasma–mass spectroscopy (ICP–MS). The chemical distribution in the weathering profiles shows that few trace elements are slightly immobile, whereas most major (particularly Ca and Na) and trace elements are mobile from the beginning of the granite weathering. On the other hand, there were mineralogical changes initiated from a plagioclase breakdown, which shows a characteristic circular dissolved pattern caused by a preferential leaching of Ca cation along grain boundaries and zoning. The biotite in that region is also supposed to be sensitive to exterior environmental condition and may be easily dissolved by acidic percolated water. As a result, it seems that some rock-forming minerals in the granitic rock located in Seoul are significantly unstable due to the environmental condition of acidic rainfall and steep slopes, where they are susceptible to be dissolved incongruently leading some elements to be highly depleted.     

Calcium isotopes in a proglacial weathering environment: Damma glacier, Switzerland

The biogeochemical cycling and isotopic fractionation of calcium during the initial stages of weathering were investigated in an alpine soil chronosequence (Damma glacier, Switzerland). This site has a homogeneous silicate lithology and minimal biological impacts due to sparse vegetation cover. Calcium isotopic compositions, obtained by TIMS using a ⁴³Ca-⁴⁶Ca double spike, were measured in the main Ca pools. During this very early stage of weathering, the young soils which have formed (δ^44/42Ca=+0.44‰) were indistinguishable to the rocks from which they were derived (δ^44/42Ca=+0.44‰) and stream water (δ^44/42Ca=+0.48‰) was also within error of the average rock. This lack of variation indicates that the dissolution of the bulk silicate rock does not strongly fractionate Ca isotopes. The only Ca pool which was strongly fractionated from bulk rock was vegetation, which exhibited an enrichment of light Ca isotopes. Significant Ca isotope fractionation between bulk rock and the dissolved flux of Ca is likely to only occur where the Ca biogeochemical cycle is dominated by secondary processes such as biological cycling, adsorption and secondary mineral precipitation.     

Observations on the behavior of gold during supergene oxidation at Summitville,Colorado, U.S.A., and implications for electrum stability in the weathering environment

The textural relations, morphology, and composition of gold grains in both oxidized and unoxidized ore from the Summitville, Colorado AuCuAg deposit have been investigated in this study. Gold from oxidized Summitville ore is finer grained than that from unoxidized ore, and occurs exclusively in intimate intergrowths with goethite of presumed supergene origin. These oxide-gold grains show no evidence of silver depletion and are enriched in silver relative to primary gold in a few cases. This trend can be explained using the stability relations of silver in electrum at 25°C. At this temperature, electrum with up to 20 wt % Ag may be relatively stable in the weathering environment, provided that chloride contents are sufficiently low in the coexisting aqueous phase. This is in accord with the maximum silver values observed in gold grains in the oxidized zone of the Summitville deposit, and with the negligible chloride content of present mine drainage. The absence of chloride that is implied by these electrum compositions effectively rules out gold-chloride complexes as an explanation for the local remobilization of gold in the oxidized zone at Summitville. It strongly suggests that gold-chloride complexes are not necessary to transport gold, at least on a local scale, in the acidic environment that characterizes many weathering sulfide ore bodies. Local gold mobilization in this environment is believed to reflect the formation of transient gold-thiosulfate complexes.     

河南省钙红土风化壳型铝土矿沉积规律及找矿远景概论

河南省是我国第二产铝大省,采用目前已查明的75个大中型铝土矿床勘查资料和样品测试结果,通过铝土矿成矿物质来源;矿床沉积环境及形成方式;矿床成矿规律等三方面对河南省铝土矿沉积规律进行全面阐述.然后应用所得成果总结了河南省铝土矿的分布规律,运用六个影响铝土矿找矿的因素:①成矿时代找矿;②沉积环境找矿③古地理找矿;④古岛边缘的分布规律;⑤沉积盆地内的喀斯特地貌;⑥构造控矿条件.划出了进一步找矿中的三个大的重点找矿远景区.     

浙江治岭头金银矿体的控矿构造

治岭头金银矿基底发育八都群变质系,盖层为晚侏罗世火山岩系,金银矿体赋存于变质岩系中,金银矿体受控于变质基底的韧性共轭剪切带。研究认为,NW方向的韧性剪切带具有右行平移性质,NE方向的韧性剪切带具有左行平移性质,EW方向剪切带未见剪切动向,主要为含变质岩角砾的石英脉。NE和NW两个方向的韧性剪切带具有共轭性质,EW方向剪切带位于共轭剪切带交汇部位,受到相向挤压造成的尖端钝化,SN向的脆性断裂对矿体起到破坏作用。     

京九铁路粤北段风化花岗岩、残积土的斜坡稳定性研究

文章总结了京九铁路广东段风化花岗岩、残积土路堑边坡病害的勘察、设计及治理经验，分析了沿线花岗岩风化壳综合分带特征，并对相关的滑坡形成机理、变形特征和治理措施作了简要阐述。     

The effect of weathering on the distribution of strontium isotopes in weathering profiles

The strontium isotope ratio in sea water has varied through geologic time owing to the input of strontium from rock weathering. To evaluate the possibility that $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratios might be altered during weathering, seven weathering profiles developed on Mesozoic arkoses located along the length of New Zealand were investigated. The rubidium-strontium-strontium isotope relations in these profiles give ‘isochron’ ages less than the ages of deposition of the arkoses. These ages appear to result from the weathering of a homogeneous source rock. The age calculated from the rubidium-strontium system (t′) is related to the original age (t) by the equation t′/t = (n ? 1)/n, where n is the ratio of the amounts of common and radiogenic strontium leached from the rock. Shales formed by the accumulation of these residual solids may inherit misleading isochron relationships which may not be erased during deposition or early diagenesis. The strontium which goes into solution and is transported to the sea is slightly less radiogenic than the strontium in the unweathered rock, while the residual clays may be much more radiogenic.     

Quantitative evaluation of stability changes in historical stone bridges in Kagoshima,Japan, by weathering

This study deals with a weathering problem of historic arch stone bridges built of welded tuff about 150 years ago in Kagoshima, Japan. Based on a detailed comprehensive weathering evaluation we previously carried out, the way in which the weathering affects the structural stability of the bridges is quantitatively evaluated herein by using a distinct element method (DEM).The suitability of this method of analysis was confirmed through a comparison of the numerical analysis results with the in situ load testing data. The analysis results show that when considering the stability of a stone bridge, the slide safety factor and eccentric safety factor are more important than the compressive safety factor. That is, the types of possible failure include sliding and openings between arch blocks. In 150 years of weathering, the shear stress in the arch ring generally increased about 20%. The eccentric safety factor decreased about 9%, from 4.35 to 3.97, and the slide safety factor decreased about 20%, from 8.02 to 6.46. Deflections at the arch crown increased about two times, from 0.60 to 1.28 mm. Despite these changes, the stone bridges are considered to be still in a good condition after 150 years of weathering.     

Chemical alteration of tephra in the depositional environment: theoretical stability modelling

The study of the chemical stability of vitreous material in aqueous media is well‐established. There has to date been little consideration of the implications of variations in the chemical durability of tephra in Quaternary tephrochronology. Chemical alteration can take the form of cationic leaching from the matrix, or complete destruction of the silica network, either of which could constrain the ability to chemically identify distal tephra. Here we apply established models of vitreous durability to the published chemical analyses of a large number of Icelandic tephras in order to predict their relative durabilities under equivalent conditions. This suggests that some important tephras have relatively poor chemical stability, and that rhyolitic tephras are, in general, more stable than basaltic. We conclude that tephras should be expected to show predictable differential chemical stability in the post‐depositional environment. Copyright © 2003 John Wiley & Sons, Ltd.