The origin of alluvial gold in the Leadhills area of Scotland: evidence from interpretation of internal chemical characteristics

A detailed chemical study has been carried out of 1794 gold grains from 55 sites within an area approximately 7 km by 18 km around Leadhills in the turbidite-facies, wacke-dominant Southern Uplands terrane of Scotland, formed as an accretionary thrust complex at the Laurentian continental margin during Ordovician and Silurian subduction of the Iapetus Ocean. The Leadhills area has had a long history of alluvial gold working, since at least 1510, and probably much earlier, but few bedrock sources of mineralisation have been located. Quantitative electron microprobe point analyses were obtained of sectioned and polished gold grains and of the associated microscopic inclusions, which in total comprised 35 types of opaque minerals. Microchemical X-ray maps were also obtained using a fully automated electron microprobe to show internal chemical compositional variation, to resolve the components of multiple and composite inclusions and to help in the identification of the smallest examples. Silver is present within most grains but Cu, Hg and Pd were only detected in about 3% of the grains. The shapes of cumulative frequency plots of the Ag contents of grains from different sites allow recognition of populations from different sources and enables comparison and differentiation between sites. In one of the main rivers, the Shortcleugh Water, the nature of the gold is generally similar, both in terms of Ag content and inclusion type, but a further type of gold, relatively rich in Ag and containing distinctive Ni and Sb-rich inclusions, is also present in 4 adjacent samples in one sector of the watercourse. This component of the samples reflects downstream dispersion, of up to 400 m, of gold grains from a mineralised structure cutting the river almost perpendicularly. Elsewhere abrupt changes in the nature of the alluvial gold within streams reflect a series of different sources and indicate limited glacial and alluvial dispersion. Four main types of gold which are considered to be derived from different varieties of source mineralisation can be recognised on the basis of composition and inclusion assemblage. The commonest type (ca. 70%) contains 8.4–13.1% Ag and inclusions, mostly of arsenopyrite, pyrrhotite, pyrite, chalcopyrite, galena, sphalerite and cobaltite, characteristics typical of mesothermal gold from shear-zones within a turbidite environment, suggesting sources within the local Lower Palaeozoic sedimentary rocks. The second type is richer in Ag (15.8–31.3%) with a basic igneous signature indicated from inclusions of Sb-rich gersdorffite, pyrite, Ni arsenide and other Ni-rich minerals, and shows some spatial association with discordant Tertiary basaltic dykes which cut the sedimentary rocks. The third type, mostly found in an area mantled by glacial drift, is poor in Ag (<1.2%) but frequently Cu-bearing and with inclusions of platinoids and Cu oxide. Associated with this type of gold are grains of the intermetallic compound Au₃Cu and platinoids such as isoferroplatinum and a complex Ir-rich phase. Its source is problematical and could partly reflect detrital material associated with ophiolitic debris, which is common in the northernmost turbidites, and partly a local source of lode gold. The fourth gold type is also Ag-poor (<3.2%), but frequently contains Pd, up to 6.3%, while the inclusions comprise selenides of Pb, Hg and Cu. In nearby areas this type of gold is associated with Permian red beds and associated basaltic volcanics and also structures in the Lower Palaeozoic into which oxidising solutions derived from the red bed sequence have penetrated.     

Detrital pyrite in Witwatersrand gold reefs: X-ray diffraction evidence and implications for atmospheric evolution

Pyrite is easily oxidized, and therefore unambiguous evidence of detrital pyrite grains in metasediments is a significant constraint on when an oxygenated atmosphere developed. Compact rounded pyrite in the Witwatersrand gold reefs of South Africa has a detrital habit and is texturally equivalent to and spatially associated with detrital zircon and chromite. X-ray precession photography reveals that petrographically featureless As-poor grains are untwinned single crystals of high diffraction quality. This new evidence from crystallography is consistent with mechanically abraded pyrite from primary lode gold deposits, and excludes an origin by replacement of a pre-existing detrital phase. Further evidence of a detrital origin for the compact rounded pyrite is afforded by isolated grains of arsenian pyrite displaying truncated As-rich growth bands. The geographically extensive Witwatersrand fluvial conglomerates evidently had a matrix of quartz and pyrite sand and pyritic mud in their unconsolidated state and, thus, the late Archean atmosphere of Earth was likely essentially anoxic.     

Morphology and Chemistry of Placer Gold from Attappadi Valley, Southern India

We report the morphological, textural and chemical characteristics of gold grains in stream gravels from the Siruvani River in Attappadi Valley, southern India. The placer gold deposits contain both primary grains with jagged grain contours and secondary grains with smooth grain margins. The primary and secondary gold grains are also distinguished by marked contrast in microtextures with the latter displaying a range of corrosion textures including striations, etch pits and chemical corrosion cavities that coalesce to form honey-comb patterns. Some of these cavities are filled with fine clay derived from lateritic weathering front. While the primary grains are characterized by high silver content (up to 35.77 wt.%) with marginal overgrowths of high purity gold, the secondary grains show exceedingly high fineness (1000 Au/Au+Ag) levels (up to 984) with no marked compositional variation indicating selective extraction of Ag and/or reprecipitation of Au. From morphological and chemical characteristics, we propose that the high purity gold grains were not derived directly from primary sources, but underwent chemical refinement in the weathering front before they were transferred to the fluvial systems. Our findings have important implications for gold exploration in the Attappadi Valley.     

A New Mechanism of Formation of the Silver—Rim Structure on Primary Gold Grains

Gold minerals in the Baochun skarn-type gold deposit are Au-Ag allogy with Ag contents within the range of 15-35.5wt% and the inerals vary from 5μm to 50μm in size.As viewed from the electron microprobe images,most gold grains show silver rims with a high Ag content as compared with the core,ranging generally from 2 to 3μm in width.From dynamic calculations based on the mass action and mass balance constratints on the crystallization of native gold,it is considered that the enrichment of silver in the rim of gold minerals in due to gradual enrichment of silver in the ore-forming solutions with its evolution from the early to the late metallogenic stage.     

湘西沃溪钨锑金矿床超纯自然金

湘西沃溪钨锑金矿床产有自然界中十分罕见的超纯自然金。超纯自然金的Au含量在99 %以上 ,Fisher成色接近1000。自然金中含有Ag、Pb、Zn、Cu、As、Sb、Hg、Bi等显微化学组分。与一般自然金相比 ,超纯自然金的Ag 含量显著偏低 ,而Pb、Zn、As、Sb、Hg、Bi等显微化学组分含量趋于偏高 ,且较稳定。理论分析表明 ,超纯自然金的形成与强氧化性的酸性成矿流体有关。流体中金、银主要以MeCl2-(Me=Au或Ag)的络合物形式迁移。推断矿床成矿可能与区域中酸性岩浆活动有成因联系。     

Composition of gold grains in the Vaal Placer,Klerksdorp, South Africa

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.     

西藏多龙尕尔勤——一个遭受过剥蚀的浅成低温热液金矿床

西藏具有形成巨型斑岩铜成矿系统的地质背景。浅成低温热液型金矿常发育于斑岩铜成矿系统上部,极具经济价值,但由于形成深度较浅,易于剥蚀,目前西藏地区尚未发现独立的浅成低温热液型金矿床。多龙超大型铜金矿集区位于西藏班公湖-怒江成矿带西段,产出有大型-超大型斑岩-浅成低温热液型铜金矿床,而是否存在独立的浅成低温热液型金矿,既是重大科学问题,也是勘查的重点方向。文章运用超景深高分辨数码显微镜、扫描电镜及电子探针等手段,系统研究了多龙矿集区尕尔勤矿区内的残坡积物和外围水系沉积物中746颗砂金的大小、形貌、核部元素及不透明矿物包体特征,发现尕尔勤矿区内广泛分布的砂金不是表生环境下生物化学或化学成因金,而是尕尔勤产出的金矿(化)体剥蚀的产物。砂金普遍含有较高的w(Bi)、极低的w(Cu),部分含有高的w(Hg)、w(Te);砂金w(Ag)变化较大（0.05%～40.21%）;金成色变化范围较大（588～999）;砂金富含Bi-Pb-Te-S系列矿物包体等,显示尕尔勤矿区可能存在一个遭受过剥蚀的浅成低温热液金矿床。     

Gold and sulphide minerals in Tertiary quartz pebble conglomerate gold placers, Southland, New Zealand

Auriferous quartz pebble conglomerates (QPC) formed during Tertiary sedimentary recycling in the Waimumu district, Southland, New Zealand. These sediments contain fine-grained gold of detrital origin with abundant surface textures and gold-forms associated with authigenic gold remobilisation. Most authigenic gold contains no detectable silver and occurs as overgrowths on detrital Au–Ag and Au–Ag–Hg alloys that contain up to 13 wt.% Ag, and 9 wt.% Hg. Fine-grained Au–Ag and Au–Ag–Hg alloys are compositionally heterogeneous, exhibiting both well-defined silver-depleted and silver-enriched rims. Rare coarse Au–Ag alloy is intergrown with quartz and is homogenous. Discrete grains of authigenic, porous, sheet-like gold occur in carbonaceous mudstone within a QPC sequence. Some QPC contain abundant sulphide minerals. Some of these sulphides (pyrite and arsenopyrite) are of long-distance detrital origin, presumably from the Otago Schist, whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin, derived from the erosion of QPC and underlying Tertiary sediments. There has also been authigenic deposition of sulphide minerals in the QPC themselves. These diagenetic sulphides include framboidal and anhedral marcasite, and framboidal and euhedral pyrite. Sulphur isotope data for the sulphide minerals range from − 45‰ to + 18‰ (relative to VCDT). Sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from − 9‰ to − 1‰ and are most likely derived from the Otago Schist to the north. Both framboidal and anhedral marcasite have lower values (< − 20‰) reflecting microbial sulphate reduction as a source for the precursor hydrogen sulphide. Anhedral marcasite contains elevated concentrations of Ni, Co, As and Cr, commonly with compositional banding of these metals.Both the gold and diagenetic sulphides from the Belle-Brook QPC are compositionally similar to gold and sulphides from Archaean QPC. Porous, sheet-like authigenic gold is morphologically similar to gold associated with carbonaceous material in the Witwatersrand. In addition, Southland marcasite textures resemble the rounded and banded pyrite in Witwatersrand QPC placers. There is abundant evidence from these Tertiary QPC in southern New Zealand for sedimentary transport of sulphide minerals and post-depositional sulphide mineralisation in the surficial environment despite an oxygen-rich atmosphere. These young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers. Many of these textures are similar to those commonly ascribed to metamorphic processes in Archaean auriferous QPC.     

The Witwatersrand deposit and the problem of the ore formation

The problem of the origin of the Witwatersrand gold-uranium deposit is considered. The fluvial and explosion hydrothermal origin of pyrite spheroids and Au-bearing pseudoconglomerate is denied. Evidence for the primary character of spheroids and the autochthonous crystallization of pyrite under endogenic conditions is furnished. The association of gold with carbonaceous rocks is interpreted in terms of the nuclear magnetic physicochemical phenomena. It is concluded that the Witwatersrand deposit is a hydrothermal metasomatic deposit.     

Geological Characteristics and Occurrence of Gold in the Xiling Gold Deposit,Jiaodong Peninsula北大核心CSCD

西岭金矿床是胶东金矿集区内近来新发现的超大型破碎蚀变岩型金矿床(475吨@4.56 g/t),位于著名的三山岛金矿床的东侧。本文在详实的野外地质观察的基础上,系统介绍了西岭金矿床的基础地质特征,并运用光学显微镜和扫描电镜观察,结合电子探针分析,系统研究了西岭金矿床金的赋存状态。西岭金矿床大量金矿物(银金矿和自然金)主要赋存在Ⅱ阶段石英-黄铁矿和Ⅲ阶段灰石英-多金属硫化物脉中。西岭金矿床金矿物主要有晶隙金、裂隙金和包体金三种赋存状态,以晶隙金为主。金的主要载体矿物为黄铁矿,次为石英、黄铜矿和方铅矿等。金矿物粒度大小包括粗粒金、中粒金、细粒金和微粒金,以细粒-微粒为主。金矿物形态有粒状、叶片状、线状、钩状、枝杈状和哑铃状等,以粒状为主。金矿物成分以Au和Ag为主,含微量的Cu、Cr、Fe、Ni、Te、S等元素。金成色为685~831,以银金矿为主,含少量自然金。综合地质和地球化学特征,西岭金矿床为蚀变岩型金矿床,是由早白垩世中温岩浆热液充填-交代形成。     

黔东内生金矿自然金成色及其研究

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。     

Mineral Grains of Base and Precious Metals in the Surface Layer of Bottom Sediments in the Chukchi Sea

Data on electron microprobe X-ray analysis of mineral grains of base (Cu, Zn, Sn, Pb, Ni, Bi, and Mo) and precious metals (Ag, Au, Pd, and Pt) detected for the first time in the sediments of the Chukchi Sea are presented. The peculiarities of the morphology and chemical composition of the grains are considered along with their sources and distribution in the sediments.     

Indicative features of placer gold for the prediction of the formation types of gold deposits (east of the Siberian Platform)

Prediction and search for gold deposits in the east of the Siberian Platform are problematic because the study area is overlain by a thick cover of MZ-KZ deposits. Search for gold deposits by the largest geological institutions using conventional methods have not yielded positive results, because the main attention was focused on the discovery of ancient gold-bearing conglomerates of the Witwatersrand type and on the evaluation of the gold ore potential of basic magmatism. Typomorphism of placer gold bears huge information about the genesis of native gold, both its primary endogenous origin and its exogenous transformation, which makes it possible to identify the formation type of mineralization, increases the reliability of the prediction of gold deposits, and ensures their purposeful search in the platform areas. The revealed indicative features of placer gold made it possible to substantiate the formation of the gold ore sources of Precambrian low-sulfide gold-quartz, gold-iron-quartzite, porphyry gold-copper, and gold-PGE mineralization and Mesozoic gold-silver, gold-rare-metal, and gold-sulfide-quartz mineralization in the east of the Siberian Platform. We have established that high-fineness placer gold with microinclusions of pyrite, arsenopyrite, quartz, and carbonates with recrystallized structures and lines of plastic deformation is specific to the ore sources of low-sulfide gold-quartz mineralization. A high content of Cu (up to 4%) in flaky high-fineness gold is one of indicators of porphyry gold-copper mineralization. The angular shape of gold grains, the fine fraction and high fineness of gold, its completely recrystallized and regrown internal structure, and the permanent presence of Fe, Bi, and Cu microimpurities and hematite, ilmenite, and corundum microinclusions are typical of gold-iron-quartzite mineralization. Flaky and laminated high-fineness gold particles with steady Pt, Pd, and Ni impurities and Pt-mineral phases and Au-Pt intergrowths in them testify to the ore sources of gold-PGE mineralization. Laminated and cloddy gold fractions of > 0.25-2.0 mm, the medium and low fineness of gold, its single-crystal or, sometimes, porous internal structure, the wide range of microimpurities (Pb, Zn, As, Sb, Cu, Te, etc.), and microinclusions of native Ag, adularia, Sr-barite, and calcite are indicators of gold-silver mineralization. Laminated, dendritic, and cloddy-angular gold grains, wide variation in gold fineness (307-950‰), and the presence of microinclusions of native bismuth, maldonite, arsenopyrite, and silver tellurides are indicative of gold-rare-metal mineralization. Laminated and cloddy gold grains, their size varying from dust to > 0.25 mm, their mono- and coarse-grained internal structure, wide variation in gold fineness (600-900‰), and the presence of Hg microimpurities (up to 6% and more) and microinclusions of quartz, calcite, pyrite, arsenopyrite, tellurides, selenides, and REE phosphates point to gold-sulfide-quartz mineralization. The established placer gold indicators of the particular formation types of ore sources in the east of the Siberian Platform made it possible to predict Precambrian gold deposits with low-sulfide-gold-quartz mineralization similar to the Kirkland Lake and Porcupine mines and gold deposits with Mesozoic gold-silver mineralization similar to the Cripple Creek mine. The developed criteria for determining the types of mineralization by indicative features of placer gold give an insight into the ore genesis and can be successfully applied to prediction and search for gold deposits and to evaluation of their gold resources.     

The relationship of silver and gold in the basal reef of the Witwatersrand System,South Africa

Ag/Au ratios of individual gold particles and of ore-samples obtained from the Basal Reef conglomerate horizon of the Upper Witwatersrand System in the Orange Free State Goldfield (South Africa) are discussed. The Ag/Au values of individual gold grains were found to be exceptionally constant at about 0.0812, whereas the Ag/Au ratios of the ore-samples have markedly higher values and show a large variation. These values indicate that the detrital gold has been homogenized during transportation and sedimentation and that part of the original silver content has been redistributed. It is likely that the first refining of detrital gold particles occurred in streams which eventually drained into the Witwatersrand basin. Further redistribution of the silver relative to the gold took place in the littoral zone of the basin, where the conglomerates were reworked by wave action and erosion. It is contended that supergenic enrichment of gold might also have taken place close to the original shore-line. These processes are believed to account for an increase in the Ag/Au values of ore-samples towards the northwest in the area investigated. Finally, the gold has been homogenized during the metamorphic period of the Witwatersrand System, when pseudohydrothermal transport and redeposition of certain constituents took place. It is suggested that the main portion of the silver content of the original alloy, which was redistributed during the various processes outlined above, is now present as finely dispersed silver minerals. The hypothesis of an elevation control of the Ag/Au values has been found to be improbable.

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Es werden Ag/Au Verhältnisse von Goldkörnern und von Erzproben aus dem Basal Reef-Konglomerathorizont des Oberen Witwatersrand-Systems im Oranje-Freistaat-Goldfeld mitgeteilt. Die Ag/Au-Werte der Goldkörner sind außerordentlich konstant und bewegen sich um 0,0812, während die Ag/Au-Verhältnisse der Erzproben merklich höher sind und starken Schwankungen unterliegen. Das detritische Gold muß daher während seines Transportes und während seiner Ablagerung homogenisiert worden sein und dabei einen Teil seines ursprünglichen Silbergehaltes verloren haben. Eine erste Raffinierung des Goldes geschah wahrscheinlich während des Flußtransportes der Partikel in den Sedimentationsraum. Weitere Umlagerungen von Silber relativ zu Gold fanden in der Küstenzone des Witwatersrand Beckens statt, im Bereich starker Wellenbewegung und intensiver Aufbereitung des Sedimentmaterials. Es wird zudem angenommen, daß in der Nähe der ehemaligen Küste deszendente Prozesse aktiv waren, welche Gold anreicherten. Diese Silberumlagerungen werden als Grund dafür betrachtet, daß im Untersuchungsgebiet die Ag/Au-Werte von Erzproben in der Haupttransportrichtung gegen Nordwesten hin zunehmen. Schließlich wurde das Gold im Gefolge der Metamorphose der Witwatersrand-Sedimente pseudohydrothermal über kurze Distanzen umgelagert und dabei weitgehend homogenisiert. Es wird angenommen, daß die Silbergehalte, welche bei den verschiedenen Umlagerungsprozessen freigesetzt wurden, heute größtenteils in Form fein verteilter Silbermineralien vorliegen. Eine Abnahme der Ag/Au-Werte mit zunehmender Teufe konnte nicht bestätigt werden.

     

Mercuriferous gold and amalgams in Mesozoic-Cenozoic rocks of the Vyatka-Kama depression

Mercuriferous gold and amalgams of two types have been detected in the Lower Triassic, Middle Jurassic, and Holocene alluvial deposits in the sedimentary cover of the Vyatka-Kama depression. Electron microscopic studies revealed that some of these aggregates represent intergrowths of individual segregations (globules) ranging in size from 1 to 300 μm. At high magnifications (up to 100 000 times or more), electron photomicrographs show that the smallest globules include tiny (up to nanoscale) gold particles. Microprobe analysis made it possible to detect significant chemical variations in the globules mainly due to alterations in the Hg content (from 0 to 58.25 wt %). The major part of such aggregates belongs to solid Au-Hg solid solutions. Amalgams with Hg >19.8 at % are represented by several phases: (Au,Ag)₃Hg and (Au,Ag)₂Hg prevail; (Au,Ag)₃Hg₂, (Au,Ag)₅Hg₄, and (Au,Ag)Hg are subordinate. In terms of composition, some globules correspond to argentiferous gold (without Hg), mercuriferous electrum, and Au-Pb intermetallides with Hg, Ag and Cu. Another, more widespread type is represented by lumpy massive grains (with mercuriferous gold shell) and porous amalgam. Their formation is related to natural amalgamation initiated by the Early Mesozoic tectonomagmatic activation of the platform and adjacent deep faults. The results obtained can be used in prospecting for Au-Hg mineralization in the sedimentary cover of the Vyatka-Kama depression.     

Size, form and composition of gold grains in glacial drift in Ilomantsi, eastern Finland

Gold grains were studied in 102 till and gravel samples from an area with Au anomalies in the fine fraction of till. The form and size of gold nuggets were studied under the microscope, and the composition of selected grains was determined on a microprobe. The number of visible nuggets per sample was less than 10 per 81. In size, the nuggets were between 0.07 and 1 mm. Most of them were indefinite but flat in form, although idiomorphic gains were also observed. In composition, the gold grains in bedrock differed somewhat from those in till and gravel. Some of the nuggets found in gravel and in till were probably secondary in origin. The possibility that all the grains are not primary is emphasized in the interpretation of geochemical results.     

Microfabric analyses applied to the Witwatersrand gold- and uranium-bearing conglomerates: constraints on the provenance and post-depositional modification of rock and ore components

Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle-ductile crystal-plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.     

Auriferous Precambrian conglomerates of Witwatersrand

Geological and mineralogical specificity of the world’s largest Witwatersrand gold deposit was defined by geodynamic processes in the Late Archean. The primary crust composed of felsic rocks (“gray gneisses”) was intruded 2.9–3.1 Ga ago by mafic melts that gave rise to greenstone belts. This was followed by the appearance of long-lived granulite mobile zones that promoted the formation of hydrosphere and atmosphere accompanied by an intense acidic leaching (weathering) of rocks in the greenstone belts. Numerous conglomerate interbeds were formed in the Witwatersrand Basin section due to vigorous eolian processes and floodwater washouts that produced a fan channel system mainly filled with quartz pebbles. At present, most researchers support a modified paleoplacer model of the origin of gold mineralization in Witwatersrand, suggesting a hydrothermal-metamorphic redistribution of the primary placer gold. According to various hydrothermal models, gold was introduced into sedimentary rocks in a water medium from outer deep sources or during the filtration of postsedimentary hydrothermal fluids. The present communication suggests that a significant portion of gold contained in reefs could be delivered to the sedimentation basin by the auriferous hydrothermal quartz of pebble or sand dimension that was metamorphosed at approximately 350–400°C. Metamorphism of gold was accompanied by its purification, transfer to matrix, and hydrothermal intrastratal redistribution.     

Witwatersrand gold particle chemistry matches model of metamorphosed, hydrothermally altered placer deposits

Analyses of gold particles from the Ventersdorp Contact Reef in the Klerksdorp and Carletonville goldfields of the Late Archaean Witwatersrand Basin reveal a wide range in Au, Ag and Hg concentrations of 80.9 to 92.9, 6.0 to 17.6, and 0.6 to 5.8 wt.%, respectively. However, individual gold particles are generally homogeneous. This observation is predicted by modelling of Ag and Hg diffusion through gold at the peak metamorphic temperature of about 300 °C. The only exception is gold in hydrothermal quartz veins, the formation of which can be ascribed to the mobilisation of originally detrital gold particles during a chloritisation event that was triggered by tectonically expelled fluids, as a consequence of the 2023 Ma Vredefort meteorite impact event. Inter-particle homogenisation on a hand-sample scale was generally not achieved. The mean gold composition for the same reef in different mines (kilometre scale) can vary drastically. These results confirm the model of a metamorphosed and hydrothermally altered placer deposit. Furthermore they indicate that, in spite of a significant post-depositional modification of the composition of individual gold particles, the mean gold composition at a given reef locality may be used to distinguish different source areas of the originally detrital particles. Received: 3 December 1996 / Accepted: 19 May 1997     

内蒙赤峰红花沟金矿田的金矿物及其微量元素特点

对赤峰西部的红花沟、连花山、索虎沟金矿床的矿物共生组合和金矿物的赋存状态进行了研究，同时对金矿物和寄主硫化物进行了电子探针分析测试。结果表明，从成矿早阶段至晚阶段、从包体金至裂隙金，金矿物的Au／Ag或金成色逐渐降低。总体来讲，金成色高的金矿物，其Fe和Bi含量也高，但不同温度区段形成的金矿物，其局部规律有所不同。对红花沟金矿与银金矿共生的铋硫盐矿物-针硫铋铅矿也进行了详细分析。矿床中富含铋矿物的地方往往也富含金，可作为有意义的找矿标志。