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.     

Lithogeochemical localisation of disseminated gold in the White River area, Yukon, Canada

Gold mineralisation in the White River area, 80 km south of the highly productive Klondike alluvial goldfield, is hosted in amphibolite facies gneisses in the same Permian metamorphic pile as the basement for the Klondike goldfield. Hydrothermal fluid which introduced the gold was controlled by fracture systems associated with middle Cretaceous to early Tertiary extensional faults. Gold deposition occurred where highly fractured and chemically reactive rocks allowed intense water–rock interaction and hydrothermal alteration, with only minor development of quartz veins. Felsic gneisses were sericitised with recrystallisation of hematite and minor arsenic mobility, and extensively pyritised zones contain gold and minor arsenic (ca 10 ppm). Graphitic quartzites (up to 5 wt.% carbon) caused chemical reduction of mineralising fluids, with associated recrystallisation of metamorphic minerals (graphite, pyrrhotite, pyrite, chalcopyrite) in host rocks and veins, and introduction of arsenic (up to 1 wt.%) to form arsenopyrite in veins and disseminated through host rock. Veins have little or no hydrothermal quartz, and up to 19 wt.% carbon as graphite. Late-stage oxidation of arsenopyrite in some graphitic veins has formed pharmacosiderite. Gold is closely associated with disseminated and vein sulphides in these two rock types, with grades of up to 3 ppm on the metre scale. Other rock types in the White River basement rocks, including biotite gneiss, hornblende gneiss, pyroxenite, and serpentinite, have not developed through-going fracture systems because of their individual mineralogical and rheological characteristics, and hence have been little hydrothermally altered themselves, have little hydrothermal gold, and have restricted flow of fluids through the rock mass. Some small post-metamorphic quartz veins (metre scale) have been intensely fractured and contain abundant gold on fractures (up to 40 ppm), but these are volumetrically minor. The style of gold mineralisation in the White River area is younger than, and distinctly different from, that of the Klondike area. Some of the mineralised zones in the White River area resemble, mineralogically and geochemically, nearby coeval igneous-hosted gold deposits, but this resemblance is superficial only. The White River mineralisation is an entirely new style of Yukon gold deposit, in which host rocks control the mineralogy and geochemistry of disseminated gold, without quartz veins.     

The origin and evolution of the Klondike goldfields, Yukon, Canada

The world famous Klondike goldfields are located in the unglaciated part of west-central Yukon, Canada. Since their discovery over 100 years ago, they have produced an estimated 311 tonnes of gold, primarily from bench and creek placers that are fluvial in origin and range from Pliocene to Holocene in age. Historically, the placers are classified into three levels of gravel with four main units. These include the high-level White Channel Gravel (Pliocene), presently the most important gold-bearing unit, which sits nonconformably on an erosional bedrock surface (i.e., the ‘White Channel strath’) and is overlain and interbedded with the glaciofluvial Klondike Gravel (Pliocene); the intermediate-level gravel (Pleistocene), the least important economically; and the low-level gravel (Pleistocene–Holocene), historically the most important gold-bearing unit, but it has been mined three or four times now. The goldfields originated from the weathering and erosion of early Cretaceous, discordant mesothermal quartz veins, and the light grey color of the matrix of the White Channel Gravel is due mainly to weathering and diagenetic alteration by groundwater flow. The concentration of placer gold is related to a hierarchy of physical scales: at the lithofacies scale (metres), bed roughness determined sites of gold deposition; at the element scale (tens of metres), gravel bars were preferentially enriched in gold; at the reach scale (hundreds of metres), stream gradient was an important factor; at the system scale (hundreds of km), braided river environments transported large amounts of gold; and at the sequence scale (thousands of km), economic placers formed initially in the high-level White Channel Gravel and later in the intermediate-level and low-level gravel. The White Channel strath is interpreted as an erosional ‘tectonic’ terrace that formed during isostatic uplift and under conditions of dynamic equilibrium. The high-level White Channel Gravel and Klondike Gravel are interpreted as a depositional ‘climatic’ terrace that formed during a reversal in the tectonically induced downcutting, which is attributed to the initial and most extensive of the pre-Reid glaciations (3 Ma) in the Yukon. The intermediate-level gravel is interpreted as minor erosional ‘complex response’ terraces that formed during static equilibrium when there were pauses in valley-floor degradation, which are attributed to the subsequent and less extensive pre-Reid glaciations. The low-level gravel formed also during valley-floor degradation and may represent a return to dynamic equilibrium conditions. Hence, the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the Yukon. In addition, the lowering of baselevel from high-level, to intermediate-level and finally to low-level gravel was accompanied by a decrease in accommodation space (as indicated by a decrease in gravel thickness), which resulted in an increase in the concentration of the placer gold.     

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.     

Structural controls on orogenic gold mineralisation in the Klondike goldfield,Canada

The Klondike Schist that forms the basement rocks for the famous Klondike placer goldfield was emplaced as km-scale thrust slices in Early Jurassic time, along with some thin (10 to 30 m-scale) slices of greenstone and ultramafic rocks. Permian metamorphic fabrics in the schists were deformed during thrust emplacement by structures formed as the rocks passed through the brittle–ductile transition. Early-formed thrust-related structures were almost-pervasive recumbent folds that affected both the schist and greenstone/ultramafic slices and imposed a spaced cleavage with minor recrystallisation of micas. These structures gave way to shallow-dipping phacoidal cleavage near (within <100 m of) thrust structures. Thrust-related structures have been overprinted locally by well-defined steeply dipping reverse fault-fold zones, and associated upright folding on regional (km) to mesoscopic (m) scales. The fold-fault zones occur as two orthogonal sets of structures oriented NW to N and NE to E. Some of these steeply dipping fault zones have been reactivated by Late Cretaceous normal faulting. Orogenic (mesothermal) gold-bearing veins were emplaced in local sites of extension during or after formation of the compressional fault-fold zones and before normal fault reactivation. Over 400 veins (m to cm-scale) observed in this study imply a general NW strike for mineralised structures (W to N), but with a broad scatter of orientations. Vein emplacement was controlled principally by fold axial surfaces of kink folds of the fault-fold generation. However, some other local extension sites have opened along preexisting structures to host veins locally, including metamorphic foliation and spaced cleavage planes. In addition, irregular extensional fractures with no obvious structural control host some veins. The Klondike mineralised veins formed as swarms with broad regional structural control, but represent relatively diffuse mineralised zones, with numerous scattered small veins, compared to most orogenic vein systems. These diffuse vein swarms appear to be sufficient sources for the rich and geographically localised placer gold deposits that formed in overlying gravels during erosion of the Klondike Schist basement.     

Studies of water quality in the Ouro Preto region,Minas Gerais,Brazil: the release of arsenic to the hydrological system

Ouro Preto county is south-east of Quadrilatero Ferrifero in the state of Minas Gerais, one of the most known Brazilian mining regions, producing gold, topaz gems and several kinds of iron and manganese ores. However, these activities are the main forms of environmental degradation, affecting the surface and groundwater which is used for public supply and are the source of heavy metal contamination, including mercury and arsenic. The mining and predatory prospecting activity (garimpos) has been carried out in Minas Gerais since the seventeenth century. At that time, hundreds of placer gold deposits were discovered on the streams in the region of Ouro Preto and Mariana, causing the first big gold rush in Brazilian history. This work presents public supply water quality data from a seasonal cycle, from September 1999 (winter) to February 2000 (summer). Chemical and physical-chemical parameters were assessed with the content of heavy metals. Leaching tests with rock samples to assess contribution to the metal content of water were carried out. Elements in the rocks have been determined and water analysis allowed the hydrogeochemical classification of water. Leaching tests demonstrate the oxidizing conditions under which the As (and the other elements) is being mobilised to the hydrological system. As and Fe are elements dispersed in the drainage of the regions. This is due to the high amounts of sulfide and to the iron formations of the region.     

Financialization,totality and planetary urbanization in the Chilean Andes

In a context of planetary urbanization, where vast swathes of the countryside are being enclosed on an ongoing basis in order to support a sprawling urban system, the relationship between finance and land-use change needs to be brought to the forefront. By engaging with Henri Lefebvre’s ideas of levels and totality, this paper draws analytical connections between the financialization of the transnational mining industry and the production of the financialized urban everyday in geographies of extraction. The paper does this by looking at the case of Pascua Lama, a multibillion open-cast mine to be developed in Chile by a major mining company in the context of the current global gold rush. Through this case, I show how a set of strategies pursued by financiers and corporate managers thousands of kilometers away from the extraction site, resulted in fractured spaces of urbanization shaped by socioecological plunder, dispossession and geographically uneven financial landscapes.     

地下工程洞体突然涌水量的理论计算

本文通过对基岩水运动的平均流速分析，提出了不同流条件突水的涌水量理论计算方法，也为现场突然涌水量计算提供了新的理论依据。     

我国金矿床中金矿物的产出特征

我国目前已发现的独立金矿物约有19种,加上亚种、变种和未定名矿物,共计约26种。主要工业矿物是自然金和银金矿,其次是金银矿和金的碲化物。我国金矿物的产出与矿床类型有着密切的联系,即不同赋矿岩石中的金矿床,其金矿物组合和性质有着明显的差异。本文着重介绍我国金矿床中金矿物的产出地质特征。     

输水管道系统气体特性与水流冲击截留气团研究

通过试验与理论相结合的方法,对输水管道系统气体特性与水流冲击截留气团进行了研究.研究表明,决定气团主要性质的气体多方指数n在输水管道系统发生水力过渡过程时存在较为显著的变化,并非单一常数,从而提出了相应的四过程:即压缩子过程1、膨胀子过程2、膨胀子过程3和压缩子过程4.在压缩子过程4与膨胀子过程2中,气体多方指数n取值为1.4~1.6;而在压缩子过程1与膨胀子过程3中,气体多方指数n取1.15.气体多方指数n的取值对压力水流冲击截留气团的结果有一定影响,n值越小,水流冲击截留气团的压力越大;反之越小.     

撒哈拉地台霍嘎地区某金矿地质特征

太古宙绿岩带是世界主要金矿赋存区,在古老的非洲地台分布很多绿岩带,其上叠加剪切带即伴随有含金建造的出现。本文根据安徽地矿"走出去"实施的某绿岩区金矿勘查资料、西霍嘎地块地质背景、区域上金矿赋存规律及邻区正在开采的金矿考察,初步分析了该区金矿地质特征、控矿要素、矿床成因及找矿标志,供后续勘查投资和周边中方相同地质背景矿权区找金参考。     

安徽铜陵地区铜金矿化关系及其成因初探

铜陵地区是长江中下游成矿带中重要的铜金成矿区之一,区内已发现数十处铜、金矿床,二者间的关系主要表现为:矿床类型有铜矿床、金矿床和铜金(或金铜)矿床,空间上有内铜外金、下铜上金、西铜东金(左铜右金)分布规律,时间上金矿化一般晚于铜矿化.本文从金、铜地球化学性质及成矿过程中物理化学条件的变化对成矿作用的影响等方面初步探讨了二者间存在上述关系的成因,以期对该区及相似地区找矿有所启迪.     

双山子太古代绿岩带金矿的基本特征

文章简要介绍了双山子太古代绿岩带的基本特征,指出双山子绿岩带同加拿大阿比提比绿岩带有许多相似之处;论述了绿岩带中三期不同类型金矿的各自主要成矿地质特征,指出早期含金石英脉是太古代的产物,为冀东地区典型的太古代绿岩带金矿;毒砂型金矿可能是太古代最晚期的产物;晚期石英脉型金矿形成于燕山期.在该区东部南北长约30km的矿带中...     

有关卡林型金矿的几点认识

本文讨论了三个问题：①中、美两国的卡林型金矿床除共同点外，在金的存在形式等方面有较明显的差异，这对金的回收率有影响；②卡林型金矿不能与微细粒浸染型金矿等同；③我国卡林型金矿之所以仅分布在扬子地块的西线是由印度洋板块和欧亚大陆板块相碰撞而导致地幔上隆（玄武岩喷发）和水平拉伸（差异性下陷）的结果，从而形成具巨厚沉积物的三叠纪海盆，为卡林型金矿分布在该区打下了基础。     

苏丹东北部金矿成矿特征及成矿远景浅析

文章在对苏丹东北部金矿区域构造背景、含金建造及金地球化学特征研究成果的基础上,分析了哈塞、洁比特、瑞达和哈马迪等4个研究程度较高的典型金矿床的矿体赋存规律、矿床类型和金矿石的结构构造等特征,认为该地区至少存在石英脉型、火山成因块状硫化物和韧性剪切带等3种以上金矿床类型.金矿床的形成与岩石建造、构造作用关系密切,总结了苏...     

甘肃省礼县赵沟金矿床地质特征及找矿远景浅析

赵沟金矿床位于礼—岷金矿带东部 ,是李坝大型金矿床外围的中型金矿床。随着找矿工作的深入 ,发现本矿床地质特征及控矿因素与李坝大型金矿床有相同性 ,矿 (化 )体受构造及岩脉的控制。据控矿因素分析 ,已知矿体深部及西部找矿潜力很大 ,应进一步开展找矿工作。本矿床有望成为礼—岷金矿带继李坝、金山大型金矿床之后的又一大型金矿床     

贵州黔东南坑头蚀变岩型金矿的发现及意义

坑头金矿位于贵州黔东南金矿带,前人对该金矿带石英脉型金矿进行了广泛研究,认为深部可能有蚀变岩型金矿存在,但都未证实。近期经综合研究,在坑头金矿施工钻孔发现坑头金矿的深部存在蚀变岩型矿体,含金量最高达6. 65 g/t。蚀变岩型金矿矿石中金的赋存状态以裂隙金为主,主要载金矿物为毒砂。硅化蚀变及断层与金矿化关系密切,金矿体位于硅化蚀变发育的构造破碎带中。目前,黔东南金矿的开采目标多为300 m以浅的石英脉型金矿体,因此该发现对黔东南地区金矿深部勘查工作具有重要意义。     

斑岩型金矿分类探讨

斑岩型金矿是重要的金矿类型,为我国金矿第三重要类型,但对其在认识远不如其它金矿 。通过研究,提出斑岩型金矿可分为三类：伴生／共生一斑岩型金矿、角砾／网脉－斑岩型金矿和单脉／网脉－斑岩型金矿。     

粤西金、银矿床成矿规律探讨

粤西是我国华南的主要金、银矿床集中医，医内金矿床与银（金）多金属矿床具有相间排列的分布规律，即从南到北分为以下5个矿带：廉江地区银（金）多金属矿带、高州-信宜金矿带、罗宁-云浮银（金）多金属矿带、德庆-清远金矿带、连山地区银（金）多金属矿带。本文从区域地层、岩浆岩及成矿作用等方面对粤西金、银矿床区域分布规律进行了探讨，认为影响金、银矿床分布的主要因素是不同区域地层、岩浆岩Au、Ag含量的差异及金矿床与银（金）多金属矿床成矿作用的差异。     

中国卡林型金矿床金的赋存状态研究

卡林型金矿的载金矿物主要为黄铁矿和粘土矿物，其次是毒砂和石英。金主要以包裹金形式存在，秦岭地区存在有单体金。金主要赋存在含砷的铁硫化物相中。氧化矿石以游离金大量出现为特征。