Fine and ultrafine gold in placers of northeastern Russia

The main features of the migration and accumulation of fine and ultrafine gold during the evolution of placer-forming processes in northeastern Russia are discussed. The results of loose sediment sampling (over 2000 samples) with a screw separator and strake facilities have been taken into consideration. The approximate grades and potential resources of gold were estimated for various genetic types of placers. The most favorable conditions for the mobilization and accumulation of fine and ultrafine gold in the placers of northeastern Russia prevailed during the Paleogene to Miocene stage of the evolution of orogenic morphostructures, when the residual (illuvial and alluvial) placers were formed. During the Pliocene to Quaternary stage, the tectonic activation caused the reworking of placers by slope and fluvial processes and resulted in the formation of new hillside and alluvial placers. At this stage, the dispersal of fine gold fractions prevailed over accumulation. Under conditions of the periglacial lithogenesis of the Pliocene to Quaternary tectonic activation, the formation of new placers with fine and ultrafine gold derived from primary sources was suppressed due to the low rate of metal release in the process of physical weathering. In terms of the economic importance of placers with fine and ultrafine gold, residual, eluvial, or hillside placers, as well as alluvial stratal placers located in low-order (and partly medium-order) stream valleys, are the most attractive in northeastern Russia. The primary sources of such placers comprise gold-bearing porphry copper, gold-sulfide-quartz, and other hydrothermal deposits with similar placer-forming capability, as well as gold-quartz stockworks. The grade of fine and ultrafine gold in such placers may be as high as a few g/m³. The placers located in the constratal alluvium of graben-type valleys and the bar and floodplain placers in the areas with stockwork-like orebodies containing fine (<0.25 mm) gold are regarded as prospective objects.     

Evolution of Cretaceous–Cenozoic quartz pebble conglomerate gold placers during basin formation and inversion, southern New Zealand

Numerous auriferous fluvial quartz pebble conglomerates (QPCs) are present within the Late Cretaceous–Recent sedimentary sequence in southern New Zealand. The QPCs formed in low-relief settings before, during, and after regional marine transgression, in alluvial fan and a variety of fluvial and near-shore depositional settings: In particular, during slow thermal subsidence associated with Late Cretaceous–early Cenozoic rifting, and during the early stages of orogenic uplift following mid Cenozoic marine regression. QPC maturity characteristics are complex and vary with sediment transport and recycling history, stratigraphic proximity to the transgressive Waipounamu Erosion Surface, and the amount of first-cycle detritus incorporated during recycling. For pre-marine QPCs, the amount of first cycle detritus varies with tectonic intensity and proximity of the depositional setting to remnant Cretaceous topography. For post-marine QPCs, it varies with tectonic intensity and proximity to Late Cenozoic uplift of basement ranges.QPCs do not form during a single bedrock erosion–sediment deposition cycle: Non-oxidised and/or oxidized groundwater alteration (kaolinisation) of labile minerals in immature sediment and the upper part of underlying basement, and repeated sedimentary recycling, are fundamental processes of QPC formation regardless of the tectonic or sedimentary settings. Altered immature rock disaggregates easily upon erosion, and alteration clays are winnowed to leave quartz-rich residues containing resistant heavy minerals such as zircon and gold. Detrital sulfide survives recycling if deposition and burial in saturated sediments are rapid. QPCs result only if sediment recycling is not accompanied by excessive erosion of fresh basement rock. Uplift of many parts of the Otago Schist belt since late Miocene has raised rocks above the water table, increased erosion rates, and inhibited groundwater alteration and QPC formation. QPC formation is still occurring in Southland, where the water table is high, sediments are saturated and undergoing alteration, and uplift and erosion rates, topography, and fluvial gradients are all low. The QPCs accumulate as residual gravel on the valley floors of low-competence streams that are slowly incising pervasively altered dominantly late Miocene–Pliocene immature conglomerates.QPCs formation essentially represents physical and chemical lagging of precursor strata. Accumulation of detrital gold and other heavy minerals is an inevitable consequence, and most QPCs contain some gold. Three types of significant gold placer have developed in the QPCs. Type 1 placers are essentially eluvial and/or colluvial in origin and form without significant fluvial transport, by residual accumulation in low-competence valleys during low-rate uplift, fluvial incision and QPC formation. Type 2 placers have formed during significant fluvial transport and subsequent fluvial incision, mainly in higher energy proximal and medial reaches of larger pre-marine (Eocene) and post-marine fluvial systems. Type 3 placers formed by wave-base and marine current winnowing in the shallow shelf setting during low-rate regional marine transgression, especially in the Eocene.     

Genesis of gold-bearing placers and their possible sources (Eastern Siberian Platform)

Typomorphic features of placer gold in the eastern Siberian Platform were studied to determine its genesis. It was established that this region contains abundant alluvial bar placers, which were formed mainly by the erosion of gold reservoirs of different ages. Discovery of gold with the points of eolian reworking in the Quaternary sediments allows us to predict the eolian gold placers in individual areas of the northeastern and central parts of the eastern Siberian Platform. Identification of two types of gold on the basis of typomorphic features in the Cenozoic deposits indicates that placer gold was mainly derived from the Precambrian sources spatially confined to the exposure of ancient basement, as well as from local ore occurrences related to the Mesozoic tectonomagmatic activation. Thus, two main stages of ore formation (Precambrian and Mesozoic), were substantiated for the first time in the eastern Siberian Platform.     

辽东地区砂金矿控矿因素的初步研究

论述了辽东地区砂金矿控矿因素,揭示了砂金矿矿规律,并指出了在具备物源,气候,水动力,地貌,等条件下,形成一个有工业意义的砂金矿床,对其含金层必须要有一定适宜的封存条件。     

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.     

Dynamic classification of alluvial gold placers in the northeast of Russia

The comprehensive study of alluvial gold placers, fluvial sediments, and river valleys allowed division of these placers into subtypes, categories, and varieties on the basis of genetic principles. The comparison of observation results with reconstructions of placer-forming fluvial processes demonstrates the distinct dependence of all basic properties of placers on the dynamics of their formation, i.e., on the hydraulic size of gold grains, the dimensions of streams, and the phase of the elementary erosion cycle (PEC). Four PECs are distinguished: erosion, abrasion, equilibrium, and aggradation. A lithodynamic alluvial complex (LDAC) of the same name corresponds to each PEC. Any fractions of free gold can be transported by streams, and the mode of transportation depends on the PEC. Placers of eight dynamic categories, brush, erosion, perluvial, apron, bar, equilibrium, aggradational, and gravitational, are syngenetic to certain lithofacies belonging to different LDACs. The first four categories are combined into the channel-line subtype, which embraces all traditional placers. The nontraditional bar and equilibrium (beach subtype) and aggradational and gravitational categories (representing two subtypes of the same name (?)) are so far poorly studied but may be regarded as highly promising. The size, morphology, thickness, structure, and localization of placers; average and modal values of size and roundness of gold grains; degree of their sorting and concentration; three-dimensional distribution of gold; composition of alluvium; relationships of placers with alluvium and the gutter, river valleys of various sizes, and types of morphostructures; and other features are distinct for placers pertaining to specific categories and make up natural groups individualized for each category. The correlation of placer properties in each category makes these properties predictable at the early stages of geological exploration and enhances the efficiency of forecasting and prospecting. In high-grade gold clusters, closely spaced placers group into composite deposits. The subdivision of these placers into categories makes it possible to apply rational methods and tools of exploration and mining of placers; to estimate the character, volume, and location of previous losses of gold; and to select targets for their remining. The dynamic analysis of recently formed and ancient placers is helpful for implementation of panning and interpretation of the results obtained in the context of prospecting for gold. The proposed classification may be used not only in the study of stream gold but also in the exploration of some other placer deposits.     

辽宁、吉林、黑龙江地区流水成因的砂金矿

该区具有重要工业意义的砂金矿床,几乎皆是流水成因的。研究其矿质来源、含金地质体的风化剥蚀作用、砂金矿在河谷中的搬运方式和距离、砂金矿的成矿作用及矿床特征,不但可以扩大砂金矿的找矿前景,还能为在砂金矿分布区寻找岩金矿提供依据。     

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。     

云南红河三家河地区第四纪地质及其与砂金成矿关系研究

热带，亚热带气候，强烈切割的三家河地区的第四纪地质作用对三家河流域内砂金矿的形成具有直接意义，其砂金矿床类型，富集规律，成矿期次及矿床的工业意义均是第四纪地质作用的体现。     

自然重砂成因矿物学特征及找矿指示作用

自然重砂是地质体经自然风化、剥蚀、搬运、沉积等地质作用而分离出的单矿物(或矿物组合)。自然重砂矿物晶体由于仍然保留有许多矿物成因信息,包括颜色、形态、化学成分、物理性质和矿物组合等特征,因此常用于追溯源区地质体或者找矿勘查。这种方法被认为是一种经济实用的找矿方法——自然重砂测量。本文基于全国自然重砂找矿的数据资料,系统梳理了自然重砂的矿物类型、出现频率及其分布等特点,分析了自然重砂的矿物组合和成因矿物学特征,研究了自然重砂矿物的源区烙印、搬运距离及标型指示矿物组合特征,探讨了自然重砂成因矿物学研究意义及其找矿应用前景。自然重砂矿物的颜色、形态及内部结构依然保留着明确的成因矿物学信息:其颜色和晶体形态具有继承性而与其源区母体联系起来,体现源区母体的特性;其矿物组成可分出残余原生重矿物(包括造岩矿物、副矿物、矿石矿物等)和新生重砂矿物两个部分,如赤铜矿、孔雀石等反映着源区岩性体的成分或者赋存状态特征,其矿物组合也在很大程度上继承原生共生矿物而体现诸如有无矿化等意义;重砂颗粒的磨圆度、边界光滑性等表面特征反映搬运距离,有利于明确响应源区母体或者物源,而具有良好的找矿指示意义。     

Possibility of the discovery of eolian gold placers in Tuva

Issue of the influence of eolian processes on the formation of gold placers in the Tuva region is considered. It has been revealed that the formation of placers is related not only to hydrodynamic processes but also to eolian processes, which are reflected in the formation of relief, redistribution of metal, and reworking of the useful component. Study of typomorphic features of the eolian gold and regularities of its redistribution revealed the possibility of discovery of a new type of placer gold (eolian and heterogeneous placers).     

Unconventional gold placers of the China tectonic depression (Vitim Plateau,East Siberia)

Holocene shallow (0.5–4.5 m, rarely more) and Pliocene–Pleistocene deep (> 25 m) placers occur within the China tectonic depression. The shallow placers are associated with the formation of the present-day drainage valleys of the China River under permafrost conditions, and the deep ones are localized within the preglacial paleovalleys of the river basin. An integrated geological and geochemical study was carried out at ten shallow commercial placers, eight of which are classified as poorly studied and “unconventional.” Placers are considered “unconventional” based on their technological characteristics (commercial gold is small (? 0.25 to + 0.1 mm), thin (? 0.1 mm), and micron-sized or “bound” (invisible)), geomorphologic conditions of formation, confinement to the oxidized zone of active permafrost, significant portion of fine hydrogenic gold, and several other minor features.The formation of shallow “unconventional” placers is controlled by the conditions of active permafrost. Under aerobic conditions, suprapermafrost waters form an oxidized zone, in which iron hydroxides impart a yellowish reddish color to water-bearing rocks. Long-lived geochemical barriers (biogenic, reduction, electrochemical, sorption, and others), including gravitational differentiation, play an important role in the concentration of small and thin gold.Alluvial deposits in Meso-Cenozoic tectonic depressions, such as the China basin, are the most promising in terms of “unconventional” placers. The main factors favoring the formation of these localities and the criteria for their assessment are large feeding sources of gold (mainly carbonaceous and sulfide) mineralization, endogenic and exogenic dispersion aureoles with thin and invisible gold; increased thickness of the suprapermafrost active layer and its temporal and spatial stability, contributing to the formation and functioning of oxidized horizons with the accumulation of ferric hydroxide and hydrogenic gold; specific morphologic varieties of hydrogenic gold, which are the fundamental criterion for primary gold mineralization with migratable metal; fine-clastic clay-rich composition of recent alluvial or alluvial-talus sediments, produced by water reworking of ancient gold-bearing weathering crusts; and development of broad floodplains filled with Holocene sediments and their junction with talus-solifluction erosional slopes.     

浅谈冲积型砂金矿床成矿模式——以陕西月河砂金矿床为例

本文通过对砂金矿床形成过程的分析认为:在地壳局部下沉过程中,砂金在水等强应力作用下,与砾石、砂一起进入沉积盆地,形成砂金的中间储存体;随着地壳的间歇性上升,在蛇曲河流的作用下由于砂金与砾石的运动规律相仿,故和砾面一起被保留在河流冲刷面上的河床滞留砾石沉积中。当地壳多次间歇性上升后,使砂金中间储存体中分散的砂金集中富集在河床滞留砾石层中,从而形成砂金矿床。     

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.     

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.     

Evolution of deep‐lead palaeodrainages and gold exploration at Ballarat,Australia

Ballarat in western Victoria hosts substantial hard‐rock and palaeoplacer gold deposits. The most famous placers are the deep leads—channel deposits of a middle Cenozoic drainage system that were buried by voluminous basalt flows over the past few million years. The basalt has also shielded large areas of the highly prospective bedrock from exploration for more of the hard‐rock gold deposits. Although difficult to explore for, such deposits could express themselves as geochemical plumes in the major aquifer system hosted by the deep leads. Groundwater sampling may provide a vector to such deposits, but around Ballarat debate has long surrounded the distribution and flow directions of the deep leads, which are critical for this exploration methodology. The present landscape around Ballarat began to develop in the Early Cenozoic when a pre‐existing Mesozoic landscape was severely dissected during Australia‐Antarctica breakup. Several cycles of erosion left several generations of fluvial placer deposits scattered across the present landscape. New data from regional mapping, boreholes and compilation of historical records elucidates the positions and flow directions of the deep leads. The distribution and flow directions of the deep leads beneath the basalt are different to, and cannot be inferred from, the present drainage upon the basalt. The deep‐lead drainage divide runs beneath the city of Ballarat with divergence of up to 30 km between the deep lead and the present drainage divides. The divide was shifted northward to its present position by the process of drainage diversion because the basalt eruptions built new topography to greater heights than along the pre‐existing deep‐lead divide.     

川西高原砂金矿床形成规律

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。