U-Th-He dating of native gold: First results,problems, and outlooks

In this paper, we consider the application of the U-Th-He method of isotope geochronology for native gold dating. It was shown that inclusions of uranium- and thorium-bearing minerals, including rare earth element phosphates, are one of the main form of uranium and thorium occurrence and, consequently, the main source of radiogenic helium in native gold. Since these inclusions are submicrometer-sized, the radiogenic helium formed in them is not accumulated but implanted in the structure of native gold, which suggests its good preservation over billions of years. This suggestion was experimentally supported by the investigation of the kinetics of radiogenic helium release from native gold. The first results of the U-Th-He dating of native gold from the Pedrolampi (central Karelia) and Witwatersrand (South Africa) deposits are in adequate agreement with available independent geochronological data. This allows us to consider native gold as a U-Th-He mineral geochronometer for the direct dating of ore-forming processes.     

X-ray diffraction investigation of native Si-Fe alloy minerals from Luobusha, Tibet

The origin of native Si-Fe alloy mineral is thought to be related with mantle and aerolite. The native Si-Fe alloy minerals from podiform chromites of the Luobusha ophiolite in the Yarlong Zangbo suture zone were examined by a new method for powder-like diffractograms of small single crystals, using an SMART APEX-CCD area-detector X-ray diffractometer. The powder diffraction pattern shows that the minerals are composed of FeSi, FeSi₂, β-FeSi₂ and native silicon. The association of these minerals suggests that the crystallization order of the mineral may be from early to late FeSi→FeSi₂→native silicon, accompanied by gradually increasing deoxidization. Translated from Acta Petrologica et Mineralogica, 2005, 24(5): 453–456 [译自: 岩石矿物学杂志]     

甘肃礼县-岷县成矿带西段寨上金矿床中自然金的发现及成因意义

位于西秦岭礼(县)-岷(县)成矿带西段的寨上金矿床是近年发现的一个大型微细浸染型金矿。在研究甘肃寨上金矿床物质组分的过程中,采用显微镜观察、电子探针扫描、化学分析等综合分析技术发现了显微自然金颗粒。矿石中的自然金颗粒形态多样,粒度变化较大(5～150μm)。电子探针分析结果显示,自然金中Au=88.23%～92.73%,Ag=7.41%～9.08%,为含银自然金,成色905～926。金的载体矿物有砷黄铁矿、方铅矿、碲汞矿、铁白云石等。金的嵌布类型有粒间金、裂隙金和连生金3种。赋矿围岩中含Fe碳酸盐矿物溶解释放出Fe和溶解Fe的大量硫化物化,可能是寨上金矿床中存在显微可见自然金的最重要因素。     

汶口盆地发现自然硫矿

Some typical examples illustrating the tenor contraction of such metallic ore veins or bodies as Au, Pb, Zn, W, Sn, Ta and Nb have been recorded by Chinese geologists in the course of ore exploration and mining. The similar demonstrations have also been observed in Japan, Peru and the U.S.S.R. by their respective geologists. Yet its origin as a problem in the field of ore deposits remains unsolved. In this paper, by adopting the photon-based chemical thermodynamical method proposed by the author, the gravitational field position Z related to the intensity of the diffusion force of ore fluids are proved to be the leading factor causing the tenor contraction of metallic veins or bodies. The tenor contraction of metallic veins or bodies under suitable ore-controlling structural conditions is an objective fact in the science of ore deposits, though in no case should it be regarded as a rule to deny the necessity of searching for ore at depth. The variation of the deep-seated ore-controlling structures might alter the tenor of the metallic veins or bodies. The Taipushi wolframite deposits in Nei Monggol could be taken as a convincing example. The tungsten content increases at great depth owin,g to the dilatation during ore deposition along the schistosity of the Proterozoic crystalline schists. There also have been observed similar phenomena in other deposits of China.     

Rujevac Sb-Pb-Zn-As polymetallic deposit,Boranja orefield,Western Serbia: native arsenic and arsenic mineralization

Rujevac is a low-temperature hydrothermal polymetallic Sb-Pb-Zn-As vein-type ore deposit, hosted within a volcanogenic-sedimentary zone situated in the Rujevac-Crvene Stene-Brezovica Diabase-Chert Formation (DCF) of the Podrinje Metallogenic District (PMD), Serbia. It is located several kilometers SE from the Boranja contact aureole, which is an integral part of the PMD in Western Serbia. Genetically related to the Tertiary granodioritic magma, the mineral assemblages are characterized by specific features. The mineral association of this deposit consists of sulfides, Pb-Sb(As) sulfosalts, native metals, oxides, hydroxides and gangue minerals. Chemical composition of the ore is very complex, where contents of valuable metals range as follows: Sb (0.17–24.31 wt.%), Zn (0.21–6.29 wt.%), Pb (0.15–6.33 wt.%), As (0.06–1.28 wt.%), Cd (25–747 ppm), Ag (7–408 ppm), Hg (13–473 ppm), and Tl (<1–29 ppm). Electron Probe Microanalyses (EPMA) of native arsenic from both the Rujevac and Stragari deposits showed contents of As up to 98.8 and 97.1 wt.%, with impurity contents of Sb up to 1.3 and 6.6 wt.%, and Tl up to 2 and 1.3 wt.%, respectively. Rhombohedral unit-cell parameters for native arsenic from Rujevac and Stragari deposits amount to: a?=?3.760(2), c?=?10.555(3) Å, V?=?129.23(7) ų and a?=?3.763(1), c?=?10.560(5) Å, V?=?129.48(8) ų, respectively. Mineral assemblages, deposition order and genesis of the Rujevac polymetallic deposit were also discussed in detail. Native arsenic mineralization here has been additionally compared with similar well-known global deposits.     

Surface and subsurface geochemistry of the native Bee-Jasper-Biotite copper prospects, northwest Queensland

Surface and subsurface geochemical exploration in the areas around the Native Bee, Jasper and Biotite prospects in the Mount Isa Group of northwest Queensland has shown that low-grade anomalies in surface soils can overlie significant mineralization. The three prospects discussed occur in similar geological situations, but one is strongly leached (Jasper) and largely buried under a thin veneer of non-mineralized scree, the second is highly contaminated by old workings dating back to 1921, and the third represents a primary dispersion halo around deeply buried mineralization.     

Cohenite,native iron and troilite inclusions in garnets from polycrystalline diamond aggregates

Syngenetic garnet of eclogitic/pyroxenitic composition included in a polycrystalline diamond aggregate from the Venetia kimberlite, Limpopo Belt, South Africa shows multiple inclusions of spherules consisting of 61±5 vol% Fe₃C (cohenite), 30±2 vol% Fe-Ni and 9±3 vol% FeS (troilite). Troilite forms shells around the native iron-cohenite assemblage, implying that both compositions were immiscible melts and were trapped rapidly by the silicate. It is proposed that this polycrystalline diamond-silicate-metallic spherule assemblage formed in very local pressure and fO₂ conditions in cracks at the base of the subcratonic lithosphere from a C-H-O fluid that reacted with surrounding silicate at about 1,300–1,400 °C. In a mantle fluid consisting of CH₄>H₂O>H₂ near fO₂=IW, the H₂ activity increases rapidly when carbon from the fluid is consumed by diamond precipitation, driving the oxygen fugacity of the system to lower values along the diamond saturation curve. Water from the fluid induces melting of surrounding silicate material, and hydrogen reduces metals in the silicate melt, reflected by an unusually low Ni content of the garnet. The carbon isotopic composition of ¹³C=–13.69 (PDB) and the lack of nitrogen as an impurity is consistent with formation of the diamond from non-biogenic methane, whereas ¹⁸O=7.4 (SMOW) of the garnet implies derivation of the silicate from subduction-related material. Hence, very localized and transient reducing conditions within the subcratonic lithosphere can be created by this process and do not necessarily call for involvement of fluids derived from subducted material of biogenic origin.Editorial responsibility: J. Hoefs     

Delta Chromium-53/52 isotopic composition of native and contaminated groundwater,Mojave Desert,USA

Chromium(VI) concentrations in groundwater sampled from three contaminant plumes in aquifers in the Mojave Desert near Hinkley, Topock and El Mirage, California, USA, were as high as 2600, 5800 and 330 μg/L, respectively. δ^53/52Cr compositions from more than 50 samples collected within these plumes ranged from near 0‰ to almost 4‰ near the plume margins. Assuming only reductive fractionation of Cr(VI) to Cr(III) within the plume, apparent fractionation factors for δ^53/52Cr isotopes ranged from ε_app = 0.3 to 0.4 within the Hinkley and Topock plumes, respectively, and only the El Mirage plume had a fractionation factor similar to the laboratory derived value of ε = 3.5. One possible explanation for the difference between field and laboratory fractionation factors at the Hinkley and Topock sites is localized reductive fractionation of Cr(VI) to Cr(III), with subsequent advective mixing of native and contaminated water near the plume margin. Chromium(VI) concentrations and δ^53/52Cr isotopic compositions did not uniquely define the source of Cr near the plume margin, or the extent of reductive fractionation within the plume. However, Cr(VI) and δ^53/52Cr data contribute to understanding of the interaction between reductive and mixing processes that occur within and near the margins of Cr contamination plumes. Reductive fractionation of Cr(VI) predominates in plumes having higher ε_app, these plumes may be suitable for monitored natural attenuation. In contrast, advective mixing predominates in plumes having lower ε_app, the highly dispersed margins of these plumes may be difficult to define and manage.     

Trace-element variability of native copper within a large ore body: The Kingston mine,Michigan, USA

This study attempts both single-site and regional trace-element variability assessments focusing on the Kingston Mine, Keweenaw County, Michigan, USA, and other sources of native copper in the Lake Superior region of the United States and Canada. This information is of potential significance to all studies trying to characterize and distinguish between individual mineral sources. It is of particular importance to geoarchaeologists/archaeometrists who are attempting to assign geologic/geographic origin to prehistoric copper artifacts and to those interested in the trace-element variability (vertically and horizontally) within large ore bodies.     

Non-Stoichiometric magnesian spinels in shale xenoliths from a native iron-bearing andesite at Asuk,Disko, central West Greenland

A strongly reduced native iron-bearing andesite breccia from Disko contains graphite-rich modified shale xenoliths with magnesian spinels. These spinels are free from or very poor in ferric iron and vary considerably within the MgO-FeO-Cr₂O₃-Al₂O₃ compositional space. Through a simple substitution of the type 3 (Mg, Fe)²⁺ ⇆ 2(Al, Cr)³⁺+□_vacancy, the spinels vary from stoichiometric (Mg, Fe)²⁺ (Al, Cr)³⁺ ₂O₄ towards (Al, Cr)₂O₃. The simple substitution of Cr for Al suggests that Cr was accepted into the spinel structure as Cr³⁺, despite the reduced nature of the enclosing andesite. The most magnesian spinels are cation deficient spinel_ss in the synthetic systems MgO-Al₂O₃ and MgO-Al₂O₃-Cr₂O₃. The absence of exolved (Al, Cr)₂O₃-component is probable due to rapid quenching.     

新疆东天山觉罗塔格地区自然铜矿化玄武岩的成岩年代及其地质意义

新疆觉罗塔格地区自然铜矿化玄武岩沿着阿齐库都克深大断裂北侧分布,前人对其成岩年代一直存在争议.本文测得玄武岩条带中部的黑尖山玄武岩同岩浆期锆石SHRIMP U-Pb年龄为306.2±4.0Ma;测得东部东尖峰玄武岩的同岩浆期锆石LA-ICPMS U-Pb年龄为308.9±5.8Ma,确定了玄武岩是晚石炭世的岩浆作用,且归属于晚石炭世马头滩组.与觉罗塔格地区著名的晚石炭世-早二叠世铜镍矿化基性-超基性岩成因对比表明,该晚石炭世玄武岩与之具有共同岩浆源区特征,它们都是觉罗塔格地区晚石炭世-早二叠世岩石圈地幔深部受热熔融而形成岩浆作用.马头滩组玄武岩层是自然铜矿化的重要矿源层.     

Glacial geography and native North American languages

This study tests the hypothesis that the number and distribution of some native American languages may be related to ice-margin changes of the Wisconsin glaciation. The analysis indicated that the number of languages per unit area is much greater in unglaciated areas of the last glacial maximum than in glaciated areas. The pattern of languge overlap between land areas sequentially exposed during deglaciation appears to indicate the direction of movement of populations from the periphery toward the core of the area once covered by the Wisconsin Ice Sheet. The data strongly indicate that North America was inhabited prior to the Wisconsin glacial maximum, because glacial maximum conditions apparently influenced linguistic distributions. Evidence suggests that ancestral Eskimo-Aleut and Na-Dene speakers occupied the northwestern edge of the continental ice mass, and that ancestral Algonquian speakers were south of the ice mass during the Wisconsin glacial maximum (approximately 18,000 yr ago). These three linguistic groups were the principal ones to spreas into areas exposed by the recession of the Wisconsin ice.     

A shale buchite xenolith with Al-armalcolite and native iron in a lava from Asuk,Disko, central West Greenland

A native iron bearing buchite xenolith enclosed in basalt in a native iron bearing lava from Disko is similar in chemistry, apart from volatile components, to unheated Mesozoic to early Tertiary carbonaceous shale sediments from West Greenland, believed to have been the major contaminant and reducing agent causing the formation of the native iron in the volcanic rocks on Disko. The shale buchite has preserved original sedimentary lamination and experienced only a comparatively short-lived heating which allowed only short-range equilibration. The xenolith contains native iron and graphite, while the immediately enclosing basalt does not, and demonstrates the indigenous reducing properties of the shales when heated to high (1,150–1,200 ° C) temperatures at low pressures. Sedimentary clastic iron titanium oxide grains have reacted with the immediately surrounding matrix and completely recrystallized to aggregates of rutile and Al-armalcolite with up to 24 mol% Al₂TiO₅. The Al-armalcolites contain 5 to 11 mol% of the anosovite (Ti₃O₅) component. They show a considerable variation in iron-magnesium ratio due to small-scale variations in bulk chemistry to varying f _O2 and to the effects of progressive melting of the sediment. Glassy veins in the buchite, formed along original fractures in the sediment, contain an assemblage of cordierite, low Ca-pyroxene, plagioclase, ilmenite, ferropseudobrookite-rich armalcolite, iron metal, troilite and acid glass, and crystallized under higher oxygen fugacities than the buchite interior, probably due to influx from the less reduced enclosing basalt magma.     

Lead isotopic composition and lead source of the Tongchanghe basalt-type native copper-chalcocite deposit in Ninglang, western Yunnan, China

1 Introduction The native copper-chalcocite deposits associated with the Emeishan basalt have attracted great attention of many scientists engaged in ore deposits in recent years, but their focus is more put on the deposits hos- ted in the high-Ti basalt …     

The abundance of mercury in rocks,minerals and native sulphur by a microwave-excited argon plasma

Igneous rocks, silicate minerals and native sulphur have been analysed for their mercury content by a microwave-excited argon plasma. Basic and ultrabasic igneous rocks have abundances between 3 and 31 ppb Hg. The granitic rocks show much larger variations. The content in the silicate minerals is highly variable, ranging from less than 1 ppb Hg in an igneous biotite to 55 ppb Hg in a pegmatitic biotite. The analysed clay minerals give 110 and 200 ppb Hg. Volcanic elemental S contain from 11 to 9300 ppb Hg, the values on the lower side being from oceanic volcanoes and on the higher side for continental or marginal volcanoes.     

Trace‐element analysis of native copper: The view from Northern Minnesota,USA

Native copper, both finished artifacts and worked pieces, are relatively common in pre‐Contact sites of northeastern Minnesota. Fifteen artifact samples and seven sources of native copper were submitted for trace‐element analysis as a test of the procedures developed by G. Rapp, Jr. and colleagues (2000). Although the specific sources chosen had high typicality probabilities, very low posterior probabilities preclude confidence that the artifacts were accurately sourced. Potential problems include incomplete characterization of several sources, as well as the limited number of sources in the database. Of more interest than identification of specific sources is the selection of apparent source pairs. Michipicoten (eastern Lake Superior) and Isle Royale (western Lake Superior) are often chosen together; Weyerhauser Mine (Wisconsin) and the Champion Mine (Keewenaw Peninsula) may form another pair. Future research should concentrate on identifying patterns of sources in the region rather than focusing on specific mines. © 2007 Wiley Periodicals, Inc.