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。     

Compositional variations and morphological evolution in platinum beach placers, southern New Zealand

Morphologies of placer platinum group minerals (PGM) are more variable and resistant to modification during transport than placer gold grains. This study documents morphological evolution of PGM placer grains during up to 120 km of transport in beach placers after river transport from inferred sources up to 200 km inland. PGM morphological changes are calibrated with changes in morphology of associated placer gold. Most of the PGM are Pt-Fe alloy and have been fed into the beach placer system from a large river at the western end of the beaches on the south coast of New Zealand. The incoming fluvial PGM suite includes Os, Ir and Ru alloys which may have been derived from distal ophiolitic sources. More proximal sources have Ural-Alaskan affinities and these contributed cooperite and braggite, or sperrylite, locally, as well as Pt-Fe alloy grains. Some PGM may have been recycled through Cretaceous-Quaternary fluvial sediments before entering the modern placer system. Recycled placer PGM grains have also been derived from elevated Quaternary beaches near the coastline. PGM grains entering beach placers have rough surfaces, with remnants of crystal faces, and these evolve to smooth flakes with progressive long-shore transport. PGM flakes have slightly thickened rims caused by impacts by saltating sand on windy beaches, and the most distal beach placers contain flakes with incipient toroidal shapes. These PGM incipient toroids are poorly developed compared to accompanying well-formed toroidal gold that has developed in nearly all beach placers, including those on elevated Quaternary beaches. Typical PGM and gold placer grain size decreases with increasing distance of transport, from fluvial grain size of 400–1,000 to ～200 microns on the most distal beaches, accompanied by eastward loss of equant PGM grains and associated increase in proportion of flakes. Although net transport distance is ～120 km in the beach placer complex, frequent aeolian transport of grains from beach to dunes and subsequent recycling by storm surges substantially increased total transport distance in a dynamic windy tectonic environment.     

Geomorphological-sedimentological studies of landform types and modern placer deposits in the savanna (Southern Malawi)

Semi-arid climatic conditions were responsible for the evolution of the vast savanna that stretches across central Africa and for the evolution of placers in this area that carry gemstones, rare earth element minerals and zircon- and titanium-bearing minerals. In combination with a polystage peneplanation, chemical weathering contributed to the emplacement of four different types of placer deposits during the Cenozoic in Malawi. Eight landform types have been defined by their geomorphological form, geometry, grain size parameters, stratification, fabric and rock contacts. Each type describes the landscape of the savanna and can be related to the emplacement of placer types within this morphoclimatic zone. Residual to eluvial placers, represented geomorphologically by landform type V, came into existence under strong chemical weathering during a period of alternating peneplanation and valley incision into soil and saprolite. Gentle valley-side slopes or elongated interfluves have a low gradient favorable for the formation and preservation of eluvial to colluvial placers (landform type Ic). Fluvial placers become economic only on outwash plains of tributaries near the confluence with their trunk rivers, beyond the point of inflection (landform types III, II). In the lower reaches of fluvial drainage systems around shallow lakes, fluvio-deltaic placers evolved during periods of rapid uplift and regression of the lake (landform type IV). Other landform types established throughout this study are cast in the role of ‘ore guides’ or, alternatively, have a negative impact on the likelihood of placer formation. The key associations of landforms discussed in this paper may be used as a geomorphological-sedimentological tool in search of modern continental placer deposits in areas where semi-arid climatic conditions occur(red) during the Cenozoic.     

Lithofacies zoning and titanium-zirconium potential of the Mansi and North Sos’va areas,Transural placer region

The lithofacies and morphostructural analyses of the Mansi and North Sos’va areas of the Transural placer region allowed us to establish factors of the spatial and lithostratigraphic control of titanium-zirconium placer potential. It was established that the placers are localized in sediments of the upper stage of the Oligocene-Miocene complex and rocks of Oligocene basin shows facies zoning. The most promising are the paleolittoral and shoal facies formed under moderate hydrodynamic activity. In tectonic aspect, the position of promising areas and their lithofacies composition are controlled by flank zones of local domes that experienced epeirogenic movements. Regressive regime in the Oligocene basin suggests the highest probability for the discovery of titanium-zirconium placers of the littoral type in inner zones of neotectonic domes.     

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.     

Inhomogeneity of the mineral field of complex polymineral placers

The mineral field structure of a complex placer is a set of features reflecting the distribution and correlation of useful and accompanying placer-forming minerals, their variation trends, the ordering degree of mineral assemblages, inhomogeneity levels, etc. The mineral field structure is one of the most important characteristics of polymineral placers. Along with other properties, it allows one to judge the regional and local formation condition of a placer and to assign it to a certain dynamic class. The informativeness of this parameter increases if the mineral field inhomogeneity of complex placers is estimated using various statistical models, in particular, the method of major components, which is one of the modifications of the factor analysis. This method allows one to restore the structure of such a multifactor system as mineral assemblages of complex heavy-mineral placers. This analysis yields particularly interesting results in the reconstruction of formation environments of fossil placers that have lost connection with the recent topography (Devonian gold-diamond-rare metal placers in the middle Timan region and Mesozoic-Cenozoic titanium-zirconium placers in northern Kazakhstan). It is demonstrated that the mineral field of a proximal polymineral placer is extremely disordered; its mineral assemblages primarily reflect the provenance rock composition and only insignificantly the formation environment of the placer itself. The mineral field of a complex coastal-marine placer of heavy minerals (CMP), in contrast, reflects practically exclusively the lithodynamic situation within the shoreline zone, including an influence of local factors. Only in exceptional cases, the inhomogeneity of the mineral field of a certain CMP reflects an additional influence of rocks of the source area.     

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。     

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。     

Problem of the Formation of Eolian Gold Placers on the East European Platform

The distribution of eolian gold in various Proterozoic–Cenozoic sediments on the East European Platform is considered. Eolian placers of the Timan Ridge are characterized by specific features: significant areal distribution and consistent strike of the thin (0–30 cm) productive bed. Prospecting criteria of eolian gold placer in the study territory can be the presence of gold flakes and other minerals with signs of eolian reworking, specific structure of productive bed, deflation structure of relief, and characteristic lithological composition of sediments. It is concluded that since gold flakes with eolian reworking make up high metal concentrations, such as placer deposits in the Timan Ridge, eolian placers of different age may be found on the East European Platform.     

The integral model of placer formation in the Voronezh anteclise area

Integral model of Ti-Zr placer formation is proposed for the Voronezh anteclise in the following system: primary source-weathering crust-intermediate collector-placer. It is shown that zircon, the most important useful component in this system, can be zirtolitized intensely during crust formation. This process is reflected ultimately in the resource potential of placers. Creation of the integral model took into consideration abundance of the major lithostructural complexes (LSC) in the crystalline basement and their productivity, hiatus and crust formation intervals, paleofacies conditions and mechanism of the formation of intermediate collectors and placers, as well as migration direction and other factors of placer localization. The most productive Cenomanian placers were formed as triple washout products according to the panning mechanism. It is suggested that the resource potential of placers should be revised with the consideration of variation in the density characteristics of the zirtolitized zircon varieties.     

Analysis of the productivity of gold deposits of Amur province

By the amount of extracted placer and primary gold (~1300 t), Amur province is one of the major gold provinces of Russia. Its numerous placers yielded almost 1132 t Au in contrast to ~180 t from primary deposits. The central part of the province is most productive for placer and primary gold in comparison with the periphery. Native gold from placers has moderately high and high fineness, and its composition corresponds to that of gold from dominant gold–quartz and gold–quartz–sulfide deposits and occurrences. The preponderance of placer gold in gold production indicates significant prospects for discovery of new gold deposits.     

Gold in the beach placer sands of Chavakkad-Ponnani,Kerala Coast,India

Although sporadic mining of placer gold from river beds is not uncommon in India, there is no documented literature on the occurrence of gold in the beach placer sand deposits of the country. While characterizing the heavy minerals of the Chavakkad-Ponnani (CP) beach placer sands along the North Kerala coast, the association of gold with the pyriboles in these sands has been observed. A native gold grain of about 25 μm was seen to occur as an inclusion within an amphibole of hornblendic composition. The pyriboles of the CP deposit are angular to sub-angular indicating a nearby provenance that may be in the upstream reaches of the Ponnani River in the districts of Malappuram and Palakkad or further north in the auriferous tracts of the Wynad-Nilambur or Attapadi regions. It is argued that the occurrence of native gold in CP deposit is not a freak occurrence and that it warrants thorough investigation of all the pyribole-ilmenite-rich placers to examine the possible presence of gold and its abundance in the beach placers of northern Kerala coast.     

Advanced geological modeling of coastal–marine cassiterite placers based on data on deposits in Russia’S Eastern Arctic Region

A mathematical model of coastal–marine tin placers formed in the alongshore drift flow is developed using the mass balance method. The physical meaning of the coefficients included in the model is considered. Coastal–marine placers are quantitatively estimated. The simulation adequacy is confirmed by the data on coastal–marine tin-bearing placer deposits at the Pevek ore cluster (Chaun Bay, East Siberian Sea). We suggest that the obtained equations be used for surveying and exploration. Identified patterns are used to estimate the tin placer potential in one of the coastal areas in the Eastern Arctic Basin.     

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.     

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。     

Styles of lode gold mineralization contributing to the placers of the Indian River and Black Hills Creek,Yukon Territory,Canada as deduced from microchemical characterization of placer gold grains

Between 1978 and 2009, approximately 430,000 oz of placer gold were obtained from the Indian River and Black Hills Creek, which equates to roughly 20% of the production for the entire Yukon Territory during that period. The area is unglaciated, exposure is poor, and there are few known lode gold occurrences present. The technique of microchemical characterization of placer gold grains has been applied to illuminate the style(s) of source mineralization and their relationship to placer gold from the Klondike gold district immediately to the north. A total of 2,613 placer gold grains from 22 localities were characterised in terms of the Au, Ag, Cu, and Hg content of their alloy and associated suite of opaque mineral inclusions. A combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the Klondike. Gold type 3b (8–25% Ag) is the main component of the placers in lower Dominion Creek but is augmented and eventually replaced by type 3a gold (10–40% Ag) in placers in the main Indian River valley, probably through erosion of gold-bearing veins in the valley floor. Type 4 gold exhibits highly variable Ag which may contain Hg to a maximum of 11 wt.%. This gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides, tellurides, sulfotellurides, and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization. Placer gold in drainages radiating from Eureka Dome exhibits various proportions of types 3 and 4 gold depending on location, but type 3 gold forms the major component in Black Hills Creek and northerly flowing tributaries of the Indian River with the exception of Eureka and Montana creeks. Type 5 gold is found only in placers in the middle and lower Indian River. It is distinguished by slightly elevated (0.05–0.17%) Cu in the gold alloy, together with low (5–9%) Ag contents. Inclusions of Bi minerals, Cr-bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion-related source. Candidates for such a source include undiscovered lode occurrences associated with Cretaceous age intrusions to the south of the Indian River, or deformed Cu-Au (−Mo) porphyry occurrences which are known to be present in the same area. This analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from Eureka Creek to the larger placers of the Indian River is minor. Consequently, the placer gold inventory of the Indian River is primarily orogenic in origin. Similarly, the characterization of placer gold in Blackhills Creek strongly suggests an orogenic source. This study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern Klondike goldfield. This information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration.     

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.     

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).