Tokur gold ore–placer cluster in Amur Province: Geological–structural features and perspective of its development

The geological–structural features and gold-potential perspective of the Tokur ore–placer cluster in the Amur Province have been studied. This ore–placer cluster is a syncline composed of Paleozoic blackshale sequence, which is cut through by Late Paleozoic and Late Mesozoic intrusions. A granitoid massif is suggested at the depth on the basis of geophysical data. The cluster is distinguished by high productivity. Approximately 37 t gold from ore deposits, primarily from the Tokur deposit, and 60 t from placers have been mined out. The Tokur deposit is compared with the Natalka and Degdekan deposits from the Yana–Kolyma Province. Perspectives of the cluster area for large gold–quartz deposits and technogenic gold placers have been estimated.     

River drainage reorientation during placer gold accumulation, southern New Zealand

Many alluvial placer deposits around the world occur in river systems that have been affected by tectonic events, causing drainage reorientation and severance of links between placers and their sources. This study documents tectonic rejuvenation of topography in the Otago giant placer goldfield, New Zealand, which has resulted in numerous river capture and drainage reorientation events. These events have induced changes to gold transport directions and numerous stages of separation of detrital gold from primary sources. Goldfield-wide reconstructions of drainage patterns through time are as yet only possible for Miocene–Recent, and numerous earlier drainage changes back to Cretaceous primary orogenic mineralisation are probable. Variations in basement lithologies permit auriferous gravel provenance determinations, facilitating paleodrainage pattern reconstruction and documentation of river capture events. River capture events and timing of these events for gold-bearing paleodrainages have also been documented using genetic divergences of populations of freshwater galaxiid fish that were isolated by drainage reorientation. Gold-bearing quartz pebble conglomerates had a southeastward drainage in the Miocene. This was disrupted in the Pliocene by mountain range uplift and gold placer recycling, with deposition of lithic conglomerates containing only minor gold placers. The most dramatic changes in gold transport directions occurred through the Quaternary, as antiformal ranges grew across the pre-existing drainages. Miocene and Pliocene placers were recycled with numerous local (1–10 km scale) changes in river directions and numerous capture events. Large axial rivers were segmented into a more complex drainage pattern, and on-going river capture resulted in growth of the main Clutha River catchment at the expense of neighbouring catchments. The most productive placers developed in the Clutha River in late Quaternary when increased discharge from captured mountain catchments enhanced gold transport and concentration. Similar river drainage reorientation has occurred in other placer fields around the world, but the lack of preserved evidence inhibits documentation of most such changes.     

Delayed accumulation of placers during exhumation of orogenic gold in southern New Zealand

The giant gold placer system on the Otago Schist of southern New Zealand was derived from Mesozoic orogenic gold deposits in the underlying schist basement. The core of the schist basement was exhumed in the middle Cretaceous, coeval with the accumulation of the oldest preserved nonmarine sedimentary rocks in the area (ca 112 Ma). Those sedimentary rocks contain quartz clasts, with distinctive ductile deformation textures, that were derived from structural zones in, or adjacent to, major orogenic gold deposits. Quartz textures in these structural zones are readily distinguishable from the rest of the schist belt, and hence provide a fingerprint for erosion of gold. The earliest sedimentary rocks on the margins of the gold-bearing schist belt are immature, and were derived from unoxidised outcrops in areas of high relief. Gold was not liberated from unoxidised basement rocks during erosion, and was removed from the system without placer concentration. Placer concentration did not begin until about 20 million years later, when oxidative alteration of gold deposits had facilitated gold grain size enhancement from micron scale (primary) to millimetre scale (secondary). Subsequent erosion and recycling of gold in the early Cenozoic, and again in the late Cenozoic, caused additional concentration of gold in progressively younger deposits. The Klondike giant placer goldfield of Canada had a similar geological history to the Otago placer field, and Klondike placer accumulation occurred in the late Cenozoic, at least 70 million years after Mesozoic exhumation of orogenic gold. The giant placer deposit on the western slopes of the Sierra Nevada in California occurs in Eocene and younger sedimentary rocks, at least 40 million years younger than the timing of major exhumation of the source rocks. Circum-Pacific giant gold placers formed under entirely different tectonic regimes from the emplacement of their source orogenic deposits, and these giant placer deposits do not form in foreland basins associated with convergent orogens. Formation of giant placers requires less active erosion and more subdued topography than the collisional orogenic activity that accompanied emplacement of source gold deposits in basement rocks, as well as oxidative alteration of the primary deposits to liberate gold from sulfide minerals and enhance secondary gold grain size.     

Microinclusions of ore minerals in gold from the Miass placer zone (South Urals): Evidence for source rocks

Microinclusions of ore minerals were found and studied in grains of native gold for characterization of the mineralogy of placer gold of the South Urals. One hundred ten unrounded and poorly rounded grains with a size of 1–2 mm from eight placer zones were studied. Microinclusions of ore minerals were detected in six placers of the Miass zone. The list of minerals includes sulfides, arsenides, Cu-bearing Au, and PGEs. All microinclusions show links to certain deposits of ore gold and chromite occurrences. It is suggested that the northern flanges of the Talovsky and Nurali massifs containing gold placers with PGEs require a search for PGE mineralization.

     

内蒙古虎拉林地区砂金与岩金的关系

虎拉林地区砂、岩金矿的空间位置关系十分密切。在虎拉林河上中下游及虎拉林岩金矿分别采取样品，结果表明虎拉林河砂金粒度大，分选较差，磨圆度差；砂金形态总体上以粒状为主；溶蚀系数均值小于1。微量元素含量特征表明砂金的主要物质来源是近源，与岩金矿关系密切且具有继承性。虎拉林岩金矿床的矿石中自然金颗粒粗大，矿体的上部遭到剥蚀，虎拉林河砂金主要来源于虎拉林岩金矿床，砂金可作为岩金的找矿标志。自然金化学-结晶温度图表明砂金的原生Au源为中低温热液矿床。     

Native gold in Cu- and Ni-bearing pyroxenite-cortlandite intrusions and placers of the Dambuki ore cluster,the Amur region

The Dambuki ore cluster of the Upper Amur region is characterized by numerous and high-grade gold placers. A few small primary deposits and occurrences of gold-quartz type were formerly considered to be the main source of placers. However, the study of widespread Early Cretaceous Cu- and Ni-bearing ultramafic intrusions has shown that gold occurring in primary Cu-Ni ores and products of their weathering practically does not differ from placer gold in morphology, fineness, or geochemical features. In both cases, elements typical of Cu-Ni ore (Cu, Fe, Pd, Ni, etc.) are detected as impurities of gold particles. These data indicate that the Au-and Pt-bearing Cu-Ni ore mineralization may be the main source of placer gold. This ore mineralization is genetically related to small pyroxenite-cortlandite sills and dikes, abundant in the Dambuki ore cluster. The primary gold-quartz deposits and occurrences also contributed to the placers, but their part was rather small.     

黑龙江省东部梧桐河—都鲁河流域流水成因砂金矿特点

梧桐河-都鲁河流域是我国主要砂金富集区之一.区内以流水成因河漫滩砂金矿和阶地砂金矿为主.砂金矿的分布,明显地受矿质来源、新构造运动、水动力条件和沉积环境的控制.砂金矿体多呈带状、条带状赋存于河谷堆积物的底部,在矿体中经常出现一至几个富矿地段.砂金粒度为0.03-3.5mm.区内砂金矿的找矿前景好,主要找矿标志是旧采金迹、金矿化异常区和老变质岩分布区.根据砂金矿的分布和富集规律,可在五号山、小梧桐河北支沟、西梧桐河二支沟一带寻找岩金矿.     

从陕、甘、川三省接壤地区砂金矿矿质来源看在该区寻找岩金矿的可能性

本区6个砂金富集区与6个岩金集中区的空间分布完全相吻合;新构造运动频繁,地质环境动荡,金的次生富集作用效果较差;一部分砂金金粒与石英、蚀变岩石等脉石连生在一起;一部分砂金金粒的成色偏低;有的砂金矿体中部分砾石含金,有的高达3×10^-6~10×10^-6.据此认为研究区内砂金物质来源是以高含金地质体——岩金矿床(点)补给为主,低含金地质体补给为辅.碧口群、志留系、泥盆系作为矿源层,构造条件、岩浆活动均有利于本区岩金矿的形成,而且在矿源层的出露区已有近百处岩金矿床点分布.研究区化探异常、重砂异常30多处且分布较密集,异常峰值较高.研究认为本区确已具备形成更多更大的岩金矿床的地质条件,为此笔者提出勉县-平武找矿远景区.     

Native gold in complex Ti–Zr placers of the southern West Siberian Plain

Typomorphic features of native gold and its contents in complex Ti–Zr placers in the southern West Siberian Plain are reported. Three of the placers are of littoral-marine genesis, and two formed under conditions of an alluvial piedmont plain. Native gold from the studied Ti–Zr placers occurs mainly as flattened thin (?0.1 mm) particles which underwent mechanical action. It is marked by wide fineness variation and the abundance of a very fine (990–1000‰) variety. Most likely, this is chemically transformed clastogenic metal. The gold content of the productive bed (5–30 mg/m³ native gold and 8–140 ppb bulk gold) is consistent with the dispersion of heavy ore and accessory minerals during mechanical migration in water flows simultaneously with their concentration on geochemical barriers. The native-gold content of complex Ti–Zr placers shows a higher negative correlation with the primary source–placer distance than those of Ti and Zr minerals and a positive correlation with the degree of hydrodynamic reworking (gravity concentration) of transit terrigenous material. On the southern framing of the West Siberian Plain, some regions of northern Kazakhstan are promising for gold of complex Ti–Zr placers as well as fine- and thin-gold placers, gold-bearing weathering crusts, and primary gold deposits.     

Gold and platinum group minerals in placers of the South Urals: Composition,microinclusions of ore minerals and primary sources

Gold and platinum group minerals from the gold placers of the South Urals are studied in order to identify the metal sources. In placers from the Main Uralian fault zone (MUF), the primary gold contains Ag (up to 29 wt.%), Cu (up to 2 wt.%) and Hg (up to 4 wt.%) and its fineness ranges from 538 to 997‰. Tetra-auricupride and cupriferous gold (up to 20 wt.% Cu) are common for the Nizhny Karabash placer of the MUF zone. In the eastern part of the South Urals, the placer gold is mainly characterized by high fineness of 900–1000‰ and low Cu contents (max 1.38 wt.%). Most of the placer gold grains consist of the primary domains, which are rimmed by secondary high-fineness gold with diffuse and clear boundaries. The secondary gold also develops along the shear dislocations of primary gold. Gold contains microinclusions of geerite, balkanite, chalcopyrite, Se-bearing galena, sphalerite, pyrite, pyrrhotite, arsenopyrite and hematite.Twenty four (including five unnamed) platinum group minerals (PGMs) were found in 28 placers; those from the Kialim and Maly Iremel placers of the Miass placer zone were studied in details. In the Kialim placer, ruthenium is most abundant PGM, which hosts microinclusions of isoferroplatinum, ferroan platinum, laurite, cupriferous gold, a mineral similar in composition to tolovkite, heazlewoodite and unnamed RhSbS phase. The osmium contains microinclusions of erlichmanite and laurite. The iridium grains hosts various sulfides and arsenides of platinum group elements (PGEs). The inclusion-free PGMs form Ru compositional trend in contrast to Os–Ru trend of the Ir-depleted inclusion-hosted PGMs. The isoferroplatinum from the Maly Iremel placer hosts laurite, rhodarsenite, bowieite, a mineral similar in composition to miassite and unnamed sulfide of Pt (Pt_1.11S_2.00) and antimonide of Pd ((Pd_2.41Rh_0.43Fe_0.17)_3.01(Sb_0.91Te_0.09)_1.00). Ruthenium is a host to isoferroplatinum, PGE sulfides and arsenides, and heazlewoodite. Osmium contains microinclusions of ferroan platinum; iridium is a host to a mineral similar in composition to hongshiite. Three types of PGM intergrowths were identified in the Maly Iremel samples: (1) the intergrowths of platy grains of ruthenium with isoferroplatinum and a mineral similar in composition to tulameenite; (2) the open-latticework intergrowths of platy crystals of ruthenium with interstitial aggregates made up of gold, isoferroplatinum and a mineral similar in composition to xingzhongite and (3) the intergrowths of osmium and irarsite and iridarsenite, which are developed along cleavage of the osmium grains. Nickel sulfides associated with some PGMs contain Ru (11.32 wt.%) and Rh (2.21 wt.%) in millerite and Ir (31.00 wt.%), Ru (5.81 wt.%) and Rh (2.87 wt.%) in vaesite.The primary metal sources were determined on the basis of the mineral assemblages and composition of minerals, taking into account the nearby mineral deposits and directions of rivers. The rodingite-associated gold, gold-bearing massive sulfide and chromite deposits are major sources of gold and PGMs in placers of the Miass placer zone confined to the MUF structure of the South Urals. In the southern part of this structure, gold was mainly originated from orogenic gold–sulfide deposits associated with volcanic/volcaniclastic rocks and listvenite-associated gold deposits. The placer PGMs were derived from the adjacent ultramafic massifs of ophiolitic origin. The distance between the placers and primary deposits varies from 2 to 5 km (up to 20 km in the extended valley of the Miass River). Usage of ore microinclusions and associated PGMs in study of placer gold is far more advanced than an ordinary consideration of gold composition alone. This approach allowed us to identify the concrete sources for individual placers and to predict some mineralogical findings in already known primary occurrences.     

Yasny lode-placer cluster: Geological and structural features and gold potential

The geological and structural features and gold potential of the Yasny lode-placer cluster in Amur province have been investigated. The lode-placer cluster is an intrusive domal uplift elongated in the nearmeridional direction and surrounded by Neogene loose sediments. The cluster comprises placers that yielded 15 t gold mined from there and small occurrences of gold-quartz and gold-base-metal lodes. Association of native gold with cinnabar in the Yasny Creek placer allows us to forecast a new source of gold-mercury mineralization in the basin of this creek, which could be compared with the Kyuchyus deposit in Yakutia. Gold nuggets 79 kg in total weight were mined from Gar-2 River placer. They are comparable in weight and association with quartz to the world’s largest Holtermann Plate nugget from Australia. Gold-quartz lodes have been forecasted in the basin of the Gar-2 Creek.     

Residual-eluvial gold placers in northeastern Russia (Dal’nii placer)

We consider the formation of the Dal’nii (Dal’nyaya) eluvial gold placer (Bol’shoi Anyui ore–placer district, western Chukchi Peninsula), related to the Dal’nii (Dal’nee) gold-bearing porphyry Mo–Cu occurrence. The Dal’nii placer is located within the transition between the Kur’ya Ridge and Anyui basin, which has been relatively stable at the recent (Pliocene–Quaternary) tectonic stage. Minor recent uplift has determined the slight denudation of interfluves, the leading role of eluvial processes in the formation of a loose cover on them, and the preservation of the relict matter of pre-Pliocene chemical-weathering crusts (including the oxidized zones of orebodies) in present-day eluvium. The Dal’nii placer consists of relict weathering-crust placers altered by recent eluvial processes in different degrees. Therefore, it is relatively rich in metal, whereas the primary lode contains mainly fine-sized gold, which is almost not released from ore under periglacial lithogenesis in present-day interfluves. We suggest calling this genetic type of placers “residual-eluvial.” The primary lodes being highly eroded (during the formation of residual concentrations, which serve as an intermediate reservoir for these placers), residual-eluvial placers or their parts might not be directly related to specific orebodies at the present-day level of erosional truncation.     

玻利维亚的矿产资源及开发前景

玻利维亚是矿物原料出口国,锡和钨的生产在世界上占重要地位.砂金矿的开采主要集中在拉巴斯省,砂金矿床星罗棋布,品位较富.主要为现代河流冲积砂金矿,可分为河床、河漫滩和阶地3种砂金矿床类型.玻利维亚政府鼓励外商投资,目前矿业开发投资以砂金开采为首选.     

玻利维亚的矿产资源及开发前景

玻利维亚是矿物原料出口国，锡和钨的生产在世界上占重要地位。砂金矿的开采主要集中在拉巴斯省，砂金矿床星罗棋布，品位较富。主要为现代河流冲积砂金矿，可分为河床、河漫滩和阶地3种砂金矿床类型。玻利维亚政府鼓励外商投资，目前矿业开发投资以砂金开采为首选。     

河南省砂金矿床地质特征

河南省砂金矿床广泛分布于黄河、长江两大水系的支流中,具代表性的有淅川寺湾及嵩县高都川两矿床,其成因类型分属于河床-河漫滩型与阶地型两大类。矿体展布形态与全新统地层分布状态一致,为近水平沿现代河流边部赋存的长条带状。在地质找矿方面,在两大水系交汇处、河流地形的缓坡以及河流的内湾一侧均为寻找砂金矿床的有利区段。     

Some geological-morphological features of the formation of gold-bearing placers in the Amur region

The intensity of placer formation is controlled by many factors, mainly by the erosion level of orebearing structures. Their deep erosion and long-term denudation resulted in the formation of numerous, often large placers with fine gold, which lost their relation with bedrocks and were derived mainly by rewashing of loose deposits from the depressions adjacent to the ore-bearing orogens. These placers are developed in many gold-bearing areas: Dambuki, Sutara, Erik, and others. Rich placers with coarse gold of high fineness were formed via erosion of gold mineralization that was previously metamorphosed by post-ore granitoids (Niman and Kerbi regions, Central Sikhote Alin).     

Gold resource potential of the eastern Siberian Platform: Placers and their feeding sources

The data on typomorphism of placer gold and its localization in the eastern Siberian Platform have been integrated for the first time. The alluvial, pseudolode, and eolian morphology of gold determines the origin of placers, which is explained not only by hydrodynamic but also by eolian processes. The recent and older alluvial (Au-bearing reservoir rocks in the second case), as well as eolian (unconventional type), genetic types of placers are recognized in the given territory. Precambrian and Mesozoic stages of ore formation are distinguished by the set of typomorphic attributes of placer gold (chemical composition, microinclusions, internal structure, etc.). On the basis of distinguishing features of placer gold, Precambrian lode deposits of great depth spatially related to outcrops of the basement have been forecasted for the first time and classified into low-sulfide gold-quartz, Au-bearing banded iron formation, Au-Cu porphyry, and Au-PGE types. The inferred shallow-seated gold-silver and gold-sulfide-quartz disseminated deposits occur locally and are related to the Mesozoic tectonomagmatic reactivation. The elaborated methods and approaches to the study of placer gold typomorphism in the eastern Siberian Platform also could be helpful in providing insights into placer origin, and forecasting and prospecting gold deposits on the East European Platform, which is close to the Siberian Platform in geological evolution.