About possibility of isotope dating of native gold by the (U-Th)/He method

For investigation of helium in native gold, a new measuring complex was created and used: the high sensitivity mass spectrometer MSU-G (ZAO SKB “SPECTRON”). The sensitivity of measuring ⁴He was 5.3 × 10^?13 cm³/g per impulse. Experiments in stepwise heating of samples have been carried out, and the kinetics of radiogenic ⁴He emanation from native gold was investigated. Migration parameters (activation energy and frequency factor) were determined. Model calculations of stability (closure temperature) of radiogenic ⁴He in the native gold structure with a given time and temperature of thermal influences were made using the data received. The concentration of ⁴He in native gold from the original deposit Nesterovskoe is (4.7 ± 0.1) × 10^?5 cm³/g in the sample from the placer; from Chudnoe deposit, it is (3.8 ± 0.1) × 10^?5 cm³/g; from sulfide deposits of Kitoiskii knot of Eastern Sayani, it is (1.9 ± 0.1) × 10^?5 cm³/g; and from the South Muiskii ore region it is (8.7 ± 0.5) × 10^?7 cm³/g. The received curve lines of kinetics of ⁴He emanation from native gold show that radiogenic helium is well bonded in the native gold structure: in all the examined samples, most ⁴He emanates only by reaching the temperature of 950–1000°C. A specific feature of the kinetics of radiogenic ⁴He emanation in all examined samples is an outburstlike emanation in the form of a peak of large amplitude in the area of temperatures near the melting temperature point of gold. This is stipulated by the existence of helium bubbles released by metals only while they melt. The spectrum of helium thermal desorption from native gold has a complicated form and is a result of superposition of several peaks. This proves the migration of groups of atoms located in the gold structure in different energy states. Very large values of the activation energy of helium migration from native gold were received: up to 161–176 kcal/mol. Extremely large values of the frequency factor, from 2 × 10¹⁸ to 3 × 10³², correspond to such values of activation energies. This is caused probably by helium migration in the form of gas bubbles. The received data indicate the very high stability of the (U-Th)/He isotope system in native gold. Using the (U-Th)/He method of isotope geochronology seems to be very promising for isotope dating of these strategic raw materials.     

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。     

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。     

First finds of native palladium,platinum, gold,and silver in brown coals of the Erkovets field (upper Amur region)

Minerals of native elements (Pd, Pt, Au, Ag, and Au-Ag solid solutions) as well as Pb, Zn, Cu, Bi, Fe, Cr, Ni, W, Al, and their intermetallides, and a number of other ore minerals were discovered in brown coals of the Erkovets field. The structural reorganization of the noble metal grains and most of the other minerals found in the brown coals suggest their authigenic paragenesis. The input of noble metals in brown coals is possible in an ionic mode from the surface and underground waters as mineral particles transported by wind from goldfields.     

First finding of native gold from the chromitites of the Mesoarchaean Tagadur mines, Nuggihalli Schist Belt, Dharwar Craton, south India

Occurrence of native gold in ultramafic rocks is rather rare. Such occurrences serve as snapshots of the underlying processes leading to their formation and modifications thereafter. Presence of native gold as flakes were earlier reported from the chromites of rock massifs of Kraka, southern Urals, Russia (Kovalev et al. 2007). Gold spherules also occur as tiny inclusions (5–30 μm) in olivine, pyrrohotite and magnetite grains from the ultrabasic rocks of Loveld deposit, South Africa (Rozhkov, 1967). Here we report, probably for the first time, occurrence of native gold from the chromitites of the Mesoarchaean Tagadur Mines of the Nuggihalli Schist Belt (NSB), Dharwar Craton, south India. Geological Survey of India (2006) has already reported significant gold content from the bulk ultramafic rocks of Kempinakote area far south of the current study area. A short discussion on the preliminary petrographic features of the gold grains and its significance is presented.     

K-Ar dating of quaternary volcanics: Methodology and interpretation of results

The K-Ar method and its modifications play a unique role in the geochronology of young volcanic rocks, which has important fundamental and economic aspects. This method provides an opportunity for dating Quaternary rocks, from the oldest (1–2 Ma) to the most recent ones (<30 ka). This paper discusses physical and geochemical prerequisites for the use of the K-Ar method in the solution of this problem. The key factor providing favorable proportions of radiogenic and nonradiogenic ⁴⁰Ar for the K-Ar system in volcanic rocks is the low solubility of argon (0.n–0.0n ppb) in silicate melts and crystallizing rocks and minerals. The sources and controlling factors of errors in the K-Ar dating of young rocks were evaluated in detail. The main analytical problem in the K-Ar dating of young rocks is concerned with the conditions and methods of measurements of very low (0.0n–0.00n ppb) contents of radiogenic ⁴⁰Ar accompanied by much higher (by a factor of 3–10 or even more) contents of nonradiogenic ⁴⁰Ar. The main stages in the development of the K-Ar method in Russia and other countries that provided a solution to this problem are described. We describe the analytical mass spectrometer system and method designed in the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, which allowed us to carry out for the first time in Russia systematic studies on the geochronology of Quaternary volcanics up to late Pleistocene-Holocene age. The main characteristics of the method are the absolute sensitivity of measurements (5 × 10^?3 A/Torr) and background signal levels for argon isotopes (3 × 10^?3 ng ⁴⁰Ar and 1 × 10^?5 ng ³⁶Ar). The problems of excess radiogenic argon and variations in the initial ⁴⁰Ar/³⁶Ar ratio in young volcanics are discussed. The results of an investigation of the distribution of K-Ar isotopes among various constituents of young volcanics and the corresponding approach to the choice of material (geochronometer) for analysis are presented. This approach is illustrated by the example of geochronological results for three volcanic centers of the Caucasus differing in the time of occurrence and duration of the active phase: Elbrus, Samsari, and Aragats. A tentative regional time scale was proposed for Neogene-Quaternary magmatic events. This scale generalized the available data for the Greater and Lesser Caucasus and embraces the period from the late Miocene (8.5 Ma) to the late Neopleistocene-Holocene (<35 ka). An interesting feature of the young magmatism of the Caucasus is the synchronous occurrence of distinctive types of volcanic activity in particular volcanic areas. An important prognostic aspect related to the proposed time scale of the young magmatism of the Caucasus is the evidence that the most recent stage of volcanic activity, the youngest occurrences of which were dated at a few thousands to tens of thousands of years, is not yet finished.     

First results of U/Pb dating of detrital zircons from middle Riphean sandstones of the Zigalga Formation,South Urals

The results of U/Pb dating of detrital zircons from sandstones of the Zigalga Formation, which is the base level of the Middle Yurmatu Group of the Bashkir uplift, southwestern Urals, are presented. The U/Pb ages of detrital zircons from sandstones of the Lower, Middle, and Upper Riphean are compared.     

First Nordic colloquium on accelerators and dating, March llth-12th, 1983

A Nordic colloquium on accelerators and dating was held in Uppsala on March 11th to 12th, 1983, and arranged by the Tandem Accelerator Laboratory and the Institute of Physics. Its aims were: to report on the accelerator technique using ion counting instead of decay counting, to discuss the advantages and possible disadvantages of the new technique, to discuss the dating possibilities using ¹⁰Be, ²⁶A1, ³⁶CI etc. besides ¹⁴C, and to organize a Nordic committee to establish cooperation. About 80 participants representing the five Nordic countries attended the meeting. Survey lectures were given by Jürg Beer, Berne and Robert Hedges, Oxford.     

The Discovery of Native Ruthenium in Nuanquanzi and Tuling—Shihu Gold Deposits：Implications for Source of Gold

Native ruthenium and platinum-bearing hedleyite were recognized two gold deposits contained in Archaean metamorphic rocks in northern China.They are coexistent with native gold,quartz and pyrite.The high W content of native ruthenium may reflect the precipitation of ores in a W-rich hydrothermal system at moderate to high temperatures,The presence of platinum-group minerals(PGM)in the two deposits suggests that Au and PGE were both derived from mantle-source rocks.     

与侵入岩有关金矿床的研究现状、存在问题及在中国的前景

与侵入岩有关的金矿床是近年来识别出来的一种新的金矿类型。这类金矿床常在远离汇聚板块边缘的克拉通边缘环境中产出,与已知的W／Sn岩浆省叠加。它们具特征性的Au＋Bi＋W＋Te＋Sb＋As等亲石元素金属组合,在时空上和成因上与中等还原的分异Ⅰ型侵入岩密切相关,随就位深度的不同而显示不同的矿化特征。形成于地壳浅部环境（〈5km）中的金矿床常与岩株、岩墙或岩床有关,金矿化大多数以浅成低温脉、网状脉或角砾岩筒产出;形成于地壳深部环境（〉5km）中的金矿床常与深成侵入岩体有关,金矿化以席状脉、云英岩或浸染状块体产出。流体包裹体研究表明成矿流体的类型随着成矿深度的不同而显示有规律的变化,形成于较浅部环境中的金矿床一般包括高温（〉350C）、不混溶的盐水（〉30％NaCleq）和低盐度（〈5％NaCleq）含CO2液相包裹体;而形成于较深部环境中的金矿床含有大量低盐度含CO2液相（〈10％NaCleq）包裹体,有些矿床晚阶段流体含有中高盐度（10％～40％NaCleq）的盐水。含CO2流体无论在挥发份的出溶过程中还是在金属迁移过程中都起了重要的作用。与侵入岩有关的金矿床在中国有广阔的找矿前景,而中国东部金矿省和钨／锡成矿省的叠加部位是寻找该类型矿床的有利地段。     

新疆托库孜巴依和金坝矿床闪长岩与金成矿关系：锆石定年与载金矿物特征约束

托库孜巴依矿床和金坝矿床位于中国阿尔泰南缘西段,是中亚成矿域的重要组成部分。矿区内广泛分布闪长岩脉,与金矿脉具有密切的空间关系,但其与金成矿是否具有成因关系尚不清楚。本次研究采用野外调查、显微鉴定、矿物自动分析系统、LA-ICP-MS等分析方法,对金成矿作用与闪长岩脉的关系进行探讨。实验结果表明：(1)托库孜巴依矿床和金坝矿床中闪长岩脉受金矿脉穿切,其中闪长岩脉经历了明显的构造变形,而金矿脉却未见显示;(2)金主要分布在石英脉及两侧蚀变带中,托库孜巴依矿床金矿物主要为碲金矿,金坝矿床则以自然金、碲金银矿为主;(3)托库孜巴依矿床与金坝矿床闪长岩具有相近的锆石U-Pb加权平均年龄,分别为（382.3±0.9）Ma和（385.5±2.0）Ma,形成于中泥盆世古亚洲洋俯冲期。综合前人研究,金成矿于晚石炭世—早二叠世后碰撞期,闪长岩明显早于金矿形成,二者并不具备成因关系。金成矿可能与韧性剪切带和同期的岩浆活动有关,未来的勘查工作应聚焦于含金石英脉与同期岩浆岩体周围。