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1.
Gold nuggets (centimetre scale) have formed in a supergene alteration zone on hydrothermal gold deposits, and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals, and along fractures in quartz and host rocks. The supergene alteration was driven by groundwater-driven water-rock interaction near to a regional unconformity beneath fluvial sediments, and involved clay alteration and oxidation that extended up to 50 m below the unconformity. Oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals. The nuggets have some crystalline form, and internally they consist of anhedral grains, elongated gold plates, and intimate intergrowths of gold and iron oxyhydroxide. Nugget surfaces have further micron scale overgrowths of microparticulate gold, gold plates, and gold crystals. Nuggets were eroded and recycled into nearby proximal Miocene quartz pebble conglomerates, where they concentrated in placers near the basal unconformity. Later recycling transferred gold into Pleistocene fluvial channels. Gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles, with little change in mass. All groundwater maintained high pH throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation. Hence, gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets, albeit with lower dissolved sulphur concentrations. Despite aridification of the climate in the late Cenozoic, with resulting localised high dissolved chloride concentrations, chloride complexation did not contribute to gold mobility. 相似文献
2.
A. R. Cabral G. Beaudoin M. Choquette B. Lehmann J. C. Polônia 《Mineralogy and Petrology》2007,90(1-2):141-150
Summary The Córrego Bom Sucesso alluvial deposit near Serro, Minas Gerais, probably provided the specimens from which the element
palladium was first discovered. Its Pt–Pd nuggets are characteristically botryoidal, arborescent and coralloidal, and exhibit
an external halo with the composition of palladiferous platinum to virtually pure platinum. X-ray mapping of an arborescent
Pt–Pd nugget from the historical occurrence documents selective palladium depletion, similar to the high-fineness gold haloes
developed on detrital Au–Ag grains under supergene conditions. The Pd-depleted alteration zone truncates inclusions of crystals
stoichiometrically close to PdPt within the Pt–Pd aggregate. Selective metal leaching in the weathering environment can account
for the frequently observed Pt-enriched rims on alluvial platiniferous alloys which may lead to the formation of platinum
nuggets at an advanced degree of weathering.
Present address: Department of Geology: Exploration Geology Rhodes University PO Box 94, Grahamstown, 6140 South Africa 相似文献
3.
Contamination of gold ring and natural gold grains into plastic capsules used in INAA analysis and the fine fraction of till (0.064 mm) have been studied. An artificial contamination of till samples with a gold ring caused Au contents of 100–600 ppb in the fine fraction. A few rubbings of plastic capsules with a gold ring gave Au amounts of 15–170 ng per capsule. Natural gold grains of sizes 0.1–1.0 mm added into “gold-free” till samples before drying and sieving caused Au contents of 1–27 ppb into the fine fraction. In a regional geochemical survey an anomaly of a few hundreds of square kilometers with concentrations of 100–600 ppb Au in the fine fraction of till was observed. Later studies showed that this area was very low in Au, concentrations being generally below one ppb. The anomaly was interpreted as a contamination caused by the gold rings of the samplers. 相似文献
4.
室温条件下微生物环境中块金生长机理的实验研究 总被引：6，自引：1，他引：5
室温条件下,用真菌、硅藻、固氮菌及芽孢杆菌等不同微生物,经３个月至１８个月的时间,可将砖红色的海绵金培育成片状、丝状及粒状的Ａｕ颗粒,这表明在表生条件及微生物环境中,Ａｕ可以被活化、迁移、再生长、测定结果表明,培养液中的Ａｕ质量浓度与其中的Ｈ２Ｏ２质量浓度有正相关关系;而且,Ａｕ和Ｈ２Ｏ２质量浓度高的样品,其中的金粒生长速率也快,说明表生条件下块金的生长与溶液中的Ｈ２Ｏ２质量浓度密切相关。 相似文献
5.
A rare collection of centimetre-scale proximal gold nuggets has been examined in the context of the Late Pleistocene to Holocene eluvial and colluvial sediments in which they occur, in a range of environments from arid temperate to periglacial. Liberation of supergene nuggets from their basement source in fault zones occurred progressively by physical erosion as the host basement rocks were uplifted between middle Miocene and Holocene. The host sediments are made up of poorly sorted angular greenschist facies schist debris. Some nuggets were recycled through several generations of these colluvial deposits, and additional nuggets were liberated to younger sediments during this time. The nuggets did not migrate laterally more than a few tens of metres during this repeated recycling, and were passively elevated vertically with the rising basement on to the crest and slopes of a 1600 m high mountain range. The most recently liberated nuggets retain almost all the morphological features of their supergene origin, including coarse (cm scale) crystal shapes, delicate crystalline internal structure, and imprints of oxidised pyrite crystals. Minor transport in colluvium has caused some abrasion and rounding of gold crystals. Repeated recycling progressively obscured the crystal shapes, although relict crystals are still recognisable on parts of most nuggets. Differential timing of liberation from basement has resulted in a wide range of rounding effects in groups of closely-coexisting nuggets. Variably crystalline gold overgrowths (micron scale) coat abraded surfaces of all or part of most nuggets, commonly intergrown with authigenic smectite clay minerals. These overgrowths developed from alkaline groundwater (pH 7–9) that had undergone extensive chemical interaction with labile minerals in the host colluvium. Abrasion during recycling removed some of the overgrowths, but this was replaced in the new sedimentary hosts. Physical and chemical processes affecting nuggets in these sediments have similarly affected eluvial and colluvial nuggets in a wide range of settings around the world, including Yukon (Canada), California (USA) and arid parts of Australia. 相似文献
6.
安徽铜陵狮子山矿田夕卡岩型金矿床的矿物组合、化学成分及成因意义 总被引：19，自引：1，他引：19
包村和朝山金矿床位于安徽铜陵狮子山矿田，属于夕卡型岩金矿床，侵入岩体分别为包村石英闪长岩体和白芒山辉石闪长岩体。通过矿相鉴定和电子探针分析，对矿床中的自然金进行了详细的观察和研究。结果表明，在两个矿床中，金矿物均以独立的自然金颗粒形式产出，且常具有“银边结构”，但在自然金的嵌布方式、载体矿物、形态、大小、金银含量变化和成色等方面表现出显著差别。根据自然金的不同特征，对金矿床的成因进行分类，包村金矿床属于Cu-Au共生，而朝山矿床为Cu-Pb-Zn-Au共生型金矿床。不同矿床不同的金产状主要是由于金的运移、络合物种类、温度、pH和金银络合物活度比值等因素赞成的。此外，包村矿床和朝山矿床在地表均遭受氧化作用形成表生矿床，其表生金成色均明显高于原生金，且成分均一。 相似文献
7.
8.
江西九瑞矿集区硅质断裂磨砾岩带的厘定及其成岩成矿意义 总被引：4，自引：4，他引：0
在九瑞矿集区研究叠合断裂和叠加成矿作用的基础上,我们进一步详细研究了出露在洋鸡山-丁家山-望夫山一线的硅质角砾岩,指出它们不是原先认为的石炭系沉积硅质岩,而应属于一种断裂磨砾岩,并深入探究其形成过程及与成矿之关系.断裂磨砾岩是断裂分带结构成熟的标志之一,多在剪切作用和热液作用下,断裂岩石经硅化-破裂-碎裂-粉碎-研磨,形成具有一定圆度和球度,大小差异较大的磨砾或磨粒,且又会反复的集结-破碎,不断拓宽断裂构造形成磨砾-角砾岩带.本文研究的断裂磨砾岩,呈北东向展布,延长达十几千米.成分上以硅化角砾岩为主,SiO2含量一般大于90％,石英颗粒由隐晶到显晶.一些角砾岩中含Fe2O3较高,有可能是原先的硫化物经氧化形成的褐铁矿.本区洋鸡山-丁家山-望夫山一线产出的断裂磨砾-角砾岩带,很可能是燕山期构造-岩浆-成矿事件的产物.在城门山和武山铜矿,我们之前的工作己发现存在产于泥盆系五通组和石炭系黄龙组层滑构造体系中的黄铁矿角砾岩,则有可能属于海西期同生断裂活动的产物.因此,这些不同的角砾岩具有多阶段活动和叠加成矿的特征.本文还进一步指出,九瑞地区其他层位(如泥盆系与志留系之间、志留系与奥陶系之间)发育的层滑构造体系和断裂角砾岩及热液蚀变岩,也很可能是成矿有利部位,值得今后找矿工作的关注. 相似文献
9.
安徽铜陵某铁帽型金-铁矿床金的赋存状态研究 总被引：3，自引：0，他引：3
对某铁帽型金-铁矿床金的赋存状态进行了研究,分析了矿石的化学组成和矿物组成,通过溶解度和物相分析,区分矿物金和分散金,测定了金矿物的成分、粒度和嵌布关系,进行了金的平衡配分,指出了金的回收方向。 相似文献
10.
E.K. Yanful P.H. Simms R.K. Rowe G. Stratford 《Geotechnical and Geological Engineering》1999,17(2):65-84
An experimental soil cover was constructed near London, Ontario and monitored for more than a year for percolation and water content data. The cover was a multi-layer system consisting of compacted till barrier soil placed between gravel layers, and a final topsoil cap in one half and a coarse stone cap in the other half. The lower gravel layer was intended to provide a capillary break that would minimize gravity driven drainage in the till, while the capillary barrier created at the upper gravel-till interface would reduce evaporative losses in the till during dry periods. The results showed that while the compacted till maintained a relatively high degree of saturation and low hydraulic conductivity under the coarse stone, it desiccated under the topsoil and resulted in high water percolation rates. The textural or grain size contrast between the relatively fine topsoil and the underlying gravel layer created a capillary break which, together with the relatively low hydraulic conductivity of the topsoil, prevented infiltrating waters from recharging the underlying till. Thus the till was not able to rebound to its pre-desiccation water content. The saturated hydraulic conductivity of the till under the topsoil was about 1 × 10-8 m/s after one year, compared to 2.0 × 10-10 m/s at construction. 相似文献
11.
12.
Alain Plouffe 《Journal of Geochemical Exploration》1997,59(3):219-232
The physical partitioning of Hg into different grain size fractions of till is predominantly controlled by the primary bedrock mineralogy, the distance of glacial transport, and the relative stability of cinnabar (HgS) in the soil weathering environment. At sites located short distances down-ice from bedrock cinnabar mineralization, the highest Hg concentrations in unoxidized till were measured in the sand- and granule-sized fractions reflecting the abundance of cinnabar in those size ranges. Similar partitioning was measured in oxidized till as cinnabar was found to be relatively resistant to postglacial weathering. Discrete clay-sized cinnabar grains obtained from the unoxidized till were viewed under the scanning electron microscope and suggest that the terminal grade of cinnabar is in the clay-sized range. In till collected from areas barren of cinnabar mineralization, the highest Hg levels were found in the clay-sized fraction which is attributed to the high adsorption of Hg by clay minerals. 相似文献
13.
砂金富集规律可归纳为五条:(1)砂金比重大,体积小,一般下沉于砂砾层底部基岩之上,形成富矿带。(2)河流流速减缓处是砂金富集之场所,如河流内湾处;两河会流处;河谷由宽突变窄处;河谷由窄突变宽处。(3)河床起伏不平是砂金富集之主要条件。最理想的河床是软硬相间之岩层,如砂页岩互层,且走向横穿河谷,倾斜较陡,这样,便形成具有无数隔梁与隔槽的起伏不平的河床,砂金受阻,易于停积。(4)位于谷旁之古老河床沉积阶地是无水患的易采砂金矿床,不容忽视。(5)红黄杂色铁砂及石英碎屑常为砂金富集标志。 相似文献
14.
A small Cu---Co---Au occurrence was found by means of till geochemistry. The metal anomalies in the till are highly local and the fineness of the gold grains and their association with goethite suggest that Au may first have been dissolved and then reprecipitated. The till very probably contains remnants of an older preglacial weathering crust. There are also high concentrations of Cu, Co, As, Mo and Zn in the humus horizon. Only one humus sample had a Au content above the detection limit, 0.02 ppm. The heavy-mineral samples from the near-surface part of the till proved useful in the regional prospecting phase. 相似文献
15.
D. V. Petrovsky V. I. Silaev V. A. Zharkov V. A. Petrovsky 《Geology of Ore Deposits》2012,54(6):474-486
Clastic gold in the Cenozoic sediments of the Ural Foredeep is referred to the apron and aureole dynamic types of alluvial placers, which are formed at a distance of 10–30 km from their sources. The gold grains vary appreciably in size and chemical composition of individuals and are characterized by good round-ness and high degree of alluvial refining. It is suggested that endogenic gold occurrences in mountainous regions of the Polar and Subpolar Urals were the main sources of clastic gold grains. 相似文献
16.
Yu.A. Kalinin K.R. Kovalev E.A. Naumov M.V. Kirillov 《Russian Geology and Geophysics》2009,50(3):174-187
A comparative analysis of morphology and geochemistry was made for gold from the primary ores and weathering crust of the Suzdal' gold deposit, Eastern Kazakhstan. The deposit is localized in Carboniferous carbonaceous-terrigenous strata and is of gold-sulfide type. Study of gold from primary ores showed that it occurs mainly in two species: free and so-called invisible. Free gold is crystallomorphic segregations and irregular-shaped grains up to tens of microns in size; it occurs in intergrowths with sulfides, quartz, carbonate, and mica-chlorite aggregate. Most of gold particles have a fineness of 930–980‰, with some grains showing wide variations in composition. Invisible gold (probably chemically combined) is present in fine-acicular arsenopyrite and, less frequently, pyrite.Being transported to the weathering crust, all this gold served as a source for “neogenic” gold of diverse morphologic forms. We recognized crystalline (isometric, prismatic, acicular, and tabular) particles and drusoid gold aggregates in the form of exotic intergrowths of crystallomorphic and sinter-shelly grains. The grains tend to coarsen from bottom to top of the weathering crust. Several generations of gold of different granulometric classes are observed. We have revealed seed and layer growth and dissolution structures in crystals of early generations overgrown with fine grains. All these gold varieties are associated with hypergene minerals. Most of this gold is of high fineness (on the average, 995‰). The hypergene gold particles are chemically homogeneous high-grade, without rims.The results of studies suggest that the high-grade hypergene gold formed in the weathering crust as a result of the dissolution of invisible gold of sulfides and its local redistribution and deposition in oxidizing media. This is also evidenced from the tendency of gold to coarsen from bottom to top of the weathering crust. A distinctive feature of secondary gold is well-expressed crystals and their great diversity. 相似文献
17.
Separated gold grains from 94 samples of the Vaal Placer, Klerksdorp gold field, South Africa have been analyzed for Au, Ag and Hg. Average gold grain compositions in these samples range from 80 to 95 weight percent Au, 4 to 18 weight percent Ag and 0.5 to 4 weight percent Hg with an average composition around Au 90, Au 8, Hg 2. Individual grains are homogeneous but significant differences exist between gold grains from single small samples and also between average compositions in separate samples. The data do not fit any simple model of gold compositional control by provenance or by metamorphic homogenization. 相似文献
18.
K. von Gehlen 《Mineralium Deposita》1983,18(3):529-534
The contents of silver and mercury in 323 spots on gold grains from seven localities of the Witwatersrand palaeo-placer and Archaean vein deposits from Barberton were measured using an electron microprobe. The objective was to obtain information on the extent of gold alteration during fluvial transport and post-depositional geological processes. The results, however, show that Ag and Hg are distributed homogeneously in the gold grains studied. No indications were found that the gold was transported in solution nor that leaching took place in an oxidizing fluvial environment. This strongly suggests that the Ag and Hg contents in Witwatersrand gold grains represent geochemical ‘fingerprints’ inherited from their eroded primary sources. Combined analysis for Ag and Hg in Witwatersrand gold grains by electron microprobe can therefore be a valuable tool in establishing the types of primary sources for the gold. 相似文献
19.
黔东内生金矿自然金成色及其研究 总被引：5，自引：0，他引：5
研究区13个金矿床／点，24个自然金样品，161个点的电子探针测试表明，区内自然金成色平均值几乎均大于900％0，属高成色；自形晶比它形晶的金银含量均匀；位于交代前峰的金成色比尾部低，包体金随主矿物的种类不同成色不同。银是金中最主要的杂质元素；铋在该区金中具有很高的富集程度，是影响成色的主要因素。本区与湖南相比，具有地层越老、成色越高、银含量越低的共同规律，而成色高低与成矿时间和温度的关系较前人总结的规律出入甚大；比值关系表明，前者变化范围很小，后者很大，碱金属含量变化对前者影响不大。 相似文献
20.
Panning for placer gold in the river beds of Punna Puzha and Chaliyar Puzha in the Nilambur Valley, southwest India, has been in vogue among the local people for the last few centuries. The auriferous quartz veins and reefs associated with the Precambrian migmatitic gneisses of the region had been mined for several years. Of late, Au is also being recovered from the laterite matrix of gravels in the old river terraces and from the residual laterites overlying the gneisses. The size of Au nuggets associated with the laterites is much higher (0.5 mm) than that of the Au in vein quartz (0.2 mm) and the associated heavy minerals in the placers (0.2 mm). The Au nuggets show surface and internal textures characteristic of redeposition under low-temperature conditions. Our field and laboratory investigations suggest that dissolution, transport and redeposition of Au occurred in Nilambur concomitant with the lateritisation process. 相似文献