Sediment‐Hosted Gold Deposits in Southeast Asia

Sediment‐hosted gold (SHG) deposits are becoming more widely recognized in the Southeast Asian region. They are characterized by very fine‐grained gold, typically occurring in stratabound jasperoid replacement and structurally controlled orebodies in calcareous or carbonaceous sedimentary rocks, which are commonly distal to porphyry copper–gold deposits. Gold mineralization is associated with anomalous As‐Sb‐Hg‐Tl and is normally concentrated in the arsenic‐rich rims of pyrite and arsenopyrite grains (commonly refractory). The examples discussed in this paper are located in Southeast Asian countries, and all occur in island or continental arc terrains. SHG deposits have many features comparable to the Carlin deposits in Nevada; however, they differ in tectonic setting, their shallow level of emplacement, fluid composition, and probable metal source. As such, the Southeast Asian examples described in this compilation may be classified as “Carlin‐like” but not “Carlin‐type.” Furthermore, they appear to represent a subclass of low‐temperature epithermal deposits, with most occurrences showing late‐stage colloform quartz vein events. Indeed, they are better described as sediment‐hosted epithermal gold deposits. Several examples are multimillion ounce deposits, such as Sepon, Laos, with its endowment of more than 4 million ounces of gold. Mesel, Sulawesi, and Bau (Jugan and Tai Parit), Sarawak, also possessed multimillion ounce resources prior to mining. Two centuries of Chinese alluvial and eluvial mining at Buduk in northwest Kalimantan has produced a significant unrecorded amount of gold. Structurally controlled feeder zones frequently contain higher‐grade gold mineralization (e.g. Mesel), and constitute valid exploration targets. More recently discovered deposits, such as those in Myanmar (Gegalaw, Taungzaw, and Katsena) and the Philippines (Hijo and Las Cogon), remain to be thoroughly explored. Most of the deposits (Sepon, Mesel, Abong, Sihayo, Jugan, Tai Parit, Hijo, and Lascogon) can be directly linked spatially and genetically to gold–copper porphyry intrusive centers. It is postulated that localities with no obvious connection to known porphyry deposits (Buduk, Ban Bak, Kyaukpahto, Gegalaw, Taungzaw, and Katsena) may represent district‐scale exploration targets for porphyry copper–gold deposits. In terms of discovery techniques, of the 13 SHG deposits discussed here, several were sites of historical mining (Mesel, Bau, and Kyaukpahto). The majority, however, are virgin deposits discovered in the past 25 years by intuitive area selection followed by systematic regional geochemical exploration programs and routine geological inspections. At the project scale, electrical resistivity techniques, such as controlled‐source audio frequency magnetotellurics (CSAMT) and induced polarization (IP), have proven to be useful for mapping blind jasperoid bodies and associated feeder zones and silicified structures. Traditional field mapping and stream float sampling in tropical regions are also effective exploration tools. Deposits that have significant jasperoid development may have conspicuous positive topographic relief, depending on erosion level.     

Carlin-type Gold Prospects in Surigao del Norte, Mindanao Island, Philippines: Their Geology and Mineralization Potential

Abstract. Three calcareous sedimentary rock-hosted Carlin type-like gold prospects were mapped in a mineral production sharing agreement area of Philex Gold Philippines Inc. in Taganaan municipality, Surigao del Norte province in Mindanao island in the Philippines. They occur along a 20–25 km long trend of known epigenetic gold and porphyry copper deposits that lie close to several splays of the Philippine Fault Zone. The gold district forms part of the Late Cretaceous Eastern Mindanao Range that hosts early Paleogene and late Pliocene to Quaternary intrusive rocks.
Gold is invisible in the jasperoid outcrops in Lascogon, Napo, and Danao prospects. The jasperoids occur in lenses of marls belonging to the Taganaan Marl Member that is associated to a turbiditic member of the Middle Miocene Mabuhay Formation. The marl lenses include gently dipping interbedded silty limestones and calcareous shales. The "invisible gold" mineralization in silicified calcareous rocks resembles Carlin-type deposits. Based on the mapped igneous and sedimentary rocks, a possible heat source for the gold mineralization is either or both of the two main phases of intrusion, Mabuhay An-desite or Alipao Andesite Porphyry. Forty-eight rock samples, fifteen stream sediment samples, and one soil sample were critical in delineating the general features of the potential Carlin-type prospects. The gold grades of jasperoids in the three prospects range from trace amounts to 20 g/t Au. Regional studies of gold and porphyry copper mineralization in the Surigao del Norte mineral district are important in delineating ore targets for drilling in the three prospects.     

http://www.sciencedirect.com/science/article/pii/S1674987111000430

Mineral assemblages formed during hydrothermal alteration reflect the geochemical composition of ore-forming fluids. Gold is mainly transported in solution as AueCl and AueS complexes. The change of physicochemical conditions such as temperature, pressure, oxygen fugacity, and sulfur fugacity are effective mechanisms for gold precipitation. Gold tends to be concentrated in the vapor phase of fluids at high temperatures and pressures. AueAs and AueSb associations are common in gold deposit. Native antimony and/or arsenic e native gold assemblages may precipitate from hydrothermal fluids with low sulfur fugacity. Hydrothermal fluids forming epithermal gold deposits are Au-saturated in most cases, whereas fluids of Carlin-type are Au-undersaturated. Quasi-steady As-bearing pyrite extracts solid solution Au from hydrothermal fluids through absorption. The capability of As-bearing pyrite to absorb Au from under-saturated fluid is the key to the formation of large-scale Carlin-type deposits. With increasing new data, studies on the geochemistry of gold deposits can be used to trace the origin of ore-forming fluids, the source of gold, and the transporting form of Au and other ore-forming elements, such as Si, S, F, Cl, As and Ag.     

Alkali element mobility as a vector for gold mineralization: An example from Central Brazil

The Brazilian Pilar and Três Buracos Paleoproterozoic mid-tier gold deposits are located in the south western Pilar de Goiás greenstone belt, containing 61,688 kg and 7,144 kg of gold, respectively, in addition to approximately 30,000 kg of past production (1736–2018). This research aimed to assess the hypothesis that the mobility of alkali elements may be used to facilitate the discovery of new gold deposits in hydrothermal systems by creating geochemical vectors based on alkali element mobility. The hypothesis was first proposed for the mafic rocks in the West Australian Goldfields and has been adopted in other deposits worldwide (E.g., Andorinhas greenstone belt, Brazil, Pataz gold field, Peru, and Rio das Velhas gold deposit, Brazil).The procedures consisted in the analysis of metasedimentary and mafic rock types from two drill holes from the Pilar de Goiás greenstone belt, central Brazil. These drill holes have intercepted mineralized and barren rock types. The petrographic and energy dispersive spectrometric results indicated that the host rocks around the orebodies present intense sericitization, albitization, and sulfidation. The chemical results reveal that the mobile elements Cs, Rb and Ba tend to be enriched in gold-bearing rock units and depleted in barren rock units. These elements are carried by fluids and can replace K in the structure of the newly formed hydrothermal phyllosilicates forming a halo of alkali-rich elements of up to 50 m around the orebodies.The upper continental crust normalized ratio ((Cs + Rb)/Th)_UCC was adopted as an alteration index to evaluate and distinguish possible mineralized zones, with ((Cs + Rb)/Th)_UCC > 3, from barren zones, with ((Cs + Rb)/Th)_UCC < 3. The ratio ((Cs + Rb)/Th)_UCC increases towards mineralization, providing a vector to possible gold mineralization, with a threshold value > 3 indicating high prospectivity.     

The Alshar epithermal Au-As-Sb-Tl deposit, southern Macedonia

The results of tectono-metallogenic, geological-structural, mineralogical, isotopic, and thermobarogeochemical studies at the Alshar deposit and in the adjacent area are discussed. The data obtained show that the gold mineralization at the deposit is similar to that observed at the Carlin-type deposits in the western United States. Similar characteristics include the Au-As-Sb-Tl-Hg geochemical assemblage; low Pb, Zn, Cu, and Ag contents in ore; widespread jasperoid and argillic metasomatic alterations of host siliciclastic-carbonate sedimentary rocks; and the spatial relations to fault zones. At the same time, the Alshar deposit differs from the Carlin-type deposits by the following features: (1) Pliocene age of mineralization, (2) expansion of mineralization over younger volcanics, (3) a high Tl grade in ore, and (4) localization of the ore field in a long-lived central-type magmatic structure. The results obtained can be used as an exploration model.     

中、下扬子地区下寒武统黑色页岩微量元素富集特征

中国南方扬子地区下寒武统黑色页岩分布广泛,主要由黑色页岩、黑色白云质页岩,黑色粉砂质页岩和黑色硅质页岩等组成;并伴生有较厚的磷块岩矿床、钒矿床和镍、钼多元素硫化物矿床。对黑色页岩岩石学特征、元素地球化学特征进行系统研究,并详细分析微量元素的富集成因,结果表明：黑色页岩主要形成于静水还原的浅海-半深海缓坡环境,层状元素富集带为沉积成因,海底热流体提供了丰富物质来源。总体上,Co、Be、Mn、Ga等元素含量较低;Cu、V、Ni、Mo等元素的含量较高,多数超过北美页岩平均值;以Ni、Mo为主的多元素富集层通常位于磷块岩之上,以黑色页岩、黑色白云质页岩中的硫化物和硫酸盐矿物为主要载体,其中有机碳（TOC）含量可达12.2%以上,远高于其他元素富集层,随着页岩内硅质成分增加,Ni、Mo、Fe、Co等元素含量明显降低;以V为主的多元素主要赋存于黑色硅质页岩中的水云母,与页岩内硅质成分具有较好的正相关关系;黑色页岩中稀土元素含量为(76.22~290.67)×10-6,轻重稀土比值LREE/HREE介于1.21~4.22,说明轻稀土更为富集,Sr/Ba值为0.04~0.34,δEu正异常,且北美页岩标准化配分曲线呈现平缓左倾,为沉积过程中存在海底热水流体提供了进一步证据。     

我国卡林型金矿研究现状

卡林型金矿是我国重要的金矿类型之一,通过对比研究前人有关卡林型金矿的资料发现,我国卡林型金矿产出于具有拉张-沉积-碰撞造山地史演化的地区,为中低温热液矿床,主要赋矿岩石为不纯的碳酸盐岩,其余为浅变质岩、火山碎屑岩等。流体包裹体及同位素组成揭示了我国卡林型金矿成矿热液来源的多样性和复杂性,其不仅仅是前人认为的来自于赋矿地层中。     

Geochemical indicators of gold-rich zones in the La Josefina epithermal deposit,Deseado Massif,Argentina

The gold deposit at La Josefina, in the Deseado Massif of Argentina, is a low-sulfidation epithermal deposit with some features of the intermediate sulfidation style; the Au occurs in quartz veins and hydrothermal breccias hosted by acid pyroclastic rocks produced by Jurassic bimodal volcanism. Exploration for this deposit type uses geochemical data for vectoring to mineralized rocks. Although a general suite of elements with anomalous concentrations around low-sulfidation deposits is known, that suite varies amongst individual deposits, which should be studied individually. The aim of this study was to determine, in the La Josefina deposit, geochemical indicators of Au-rich rocks at different scales and to assess the effects of weathering on those indicators. To reach these objectives, a mineralized zone (hosting Au-rich veins) and a non-mineralized zone (hosting barren veins) were compared in terms of vein mineralogy, alteration minerals and geochemistry. These zones were also compared with estimated element concentrations of the protolith. Element concentrations in the mineralized zone were then plotted against distance to the Au-rich veins, separating the dataset into two subsets: one from strongly oxidized rocks and the other from weakly oxidized rocks, considering the degree of oxidation as indicative of relative degree of weathering. Based on alteration and vein mineralogy, we interpret that the mineralized zone was hydrothermally active before, during and after Au-mineralization, while the non-mineralized zone was active only before Au-mineralization. Therefore, differences in element concentrations between the estimated protolith and the non-mineralized zone represent geochemical changes produced before mineralization, whereas differences between mineralized and non-mineralized zones represent changes produced during and after mineralization. We conclude that in rhyolitic to rhyodacitic pyroclastic rocks of the study area, affected by predominantly argillic and silicic hydrothermal alterations, high concentrations of Au, Cu, Hg, V, Co and P, and low concentrations of Ba are probably the best deposit-scale geochemical indicators to distinguish Au-mineralized zones from non-mineralized zones. High concentrations of As, Mn, Fe, Pb, Cr, Ni, MgO and Zn (as well as high values of chlorite-carbonate-pyrite index), and low concentrations of Be and CaO can also distinguish mineralized from non-mineralized zones. High concentrations of Au, Cu, Pb, As, Hg, and Co indicate Au-rich zones, independently of lithology and alteration, because they were introduced into the veins and host rocks together with Au. At detailed-scale, the concentrations of 15 elements vary with distance to Au-rich veins; therefore, those elements are considered potential indicators of proximity to Au-rich veins. Of those elements, Au, P, Cu, Pb, Hg, Ba and MgO are the strongest indicators of proximity to the Au-rich veins. In strongly oxidized rocks, some indicators of mineralized zones (Co, Mn, Pb, Zn and MgO) and some indicators of proximity to Au-rich veins (P, Au, Cu, Hg and Ba) should be used with caution or be avoided because weathering modifies their quality as indicators.     

Role of basement in epithermal deposits: The Kushikino and Hishikari gold deposits,southwestern Japan

The magma–ore deposit relationship of most low-sulfidation epithermal ore deposits is still unclear, partly because many stable isotopic studies of such deposits have indicated the predominance of meteoric waters within hydrothermal fluids. However, it is certainly true that hydrothermal systems are ultimately driven by magmatic intrusions, and epithermal gold deposits might therefore be produced by magmatic activity even in deposits having has no obvious links to a magma. We re-examine the genesis of two typical low-sulfidation epithermal gold deposits, the Kushikino and Hishikari deposits, using structural simulations and isotope data.Many epithermal gold deposits including the Kushikino and Hishikari deposits have been discovered in Kyushu, southwestern Japan. The Kushikino deposit comprises fissure-filling veins within Neogene andesitic volcanics that overlie unconformably Cretaceous sedimentary basement. The veins consist of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite and sulfides. Although carbon and oxygen isotopic data for the veins indicate a meteoric origin of the ore fluid, finite element simulations suggest that the vein system might have formed in direct response to magma intrusion. In particular, geophysical data suggest that intruding magma has uplifted the basement rocks, thereby producing fractures and veins and a positive Bouguer anomaly, and providing the heat necessary to drive an ore-forming hydrothermal system.The second component of this study has been to investigate the nature and evolution of the Kushikino and Hishikari epithermal systems. Isotope data document the geochemical evolution of the hydrothermal fluids. We conclude that the existence of sedimentary basement rocks at depth might have affected the strontium and carbon isotopic ratios of the Kushikino and Hishikari ore fluids. The ⁸⁷Sr/⁸⁶Sr ratios and δ¹³C–δ¹⁸O trend reveal that major ore veins in the Hishikari deposit can be distinguished from shallow barren veins. It was suggested isotopically that fluids responsible for the barren veins in nearby shallow and barren circulation systems were only controlled by the shallow host rocks. Such multi-isotope systematics provide a powerful tool with which to determine the center of hydrothermal activity and thereby document the evolution of hydrothermal fluids.     

Geochemical characteristics and metallogenesis of Carlin-type gold deposits in the Sandu-Danzhai metallogenic zone, Guizhou Province, China

Geochemical studies of the Paiting and Miaolong Carlin-type gold deposits in the Sandu-Danzhai metal-logenic zone,Guizhou Province,have shown that the mineralized-altered rocks show LREE-enrichment patterns,generally displaying negative Eu anomalies(δEu=0.51?0.97) and unobvious negative Ce anomalies(δCe=0.86?0.99).Calcite and fluorite in relation with metallogenesis show MREE-enrichment patterns,generally displaying rather weak negative Eu anomalies(δEu=0.74?0.93) and weak negative Ce(δCe=0.70?0.98) anomalies.The δ13CPDB values of carbon in calcite are-1.61‰?-5.82‰,the δ18OSMOW values of oxygen are 13.97‰?19.24‰,and the δ34SCDT values of sulfur in stibnite are 17.72‰?21.68‰.In regard to δD and δ18O,ore-forming fluids pos-sess the characteristics of metamorphic water.The process of metallogenesis of the Carlin-type gold deposits is con-trolled by the Yanshanian tectonic activities.The Yanshanian movement promoted the migration and mobilization of metamorphic fluids in the extensively developed medium-to high-grade metamorphic rocks in this region,carrying primarily enriched gold and associated elements such as Hg,As,and Sb in the Sinian metamorphosed black shales and Lower Cambrian black shales.The ore-forming fluids found their way into a suitable metallogenic environment along the fault zone,followed by gold precipitation to form gold deposits.     

秦岭卡林型金矿床及相关问题探讨

鉴于目前对沉积岩型金矿、卡林型金矿概念和类型划分存在分岐,该文在对秦岭沉积岩区浸染状金矿研究基础上,主张将沉积岩为容矿岩石的金矿床划分为广义和狭义两类：沉积岩型金矿床是具有广义内涵的一类金矿,包容了从沉积－轻微改造,到沉积－强烈改造的一类热液金矿;可划分四种亚类：卡林型、变质细碎屑岩型、热水沉积岩型、脉状体型。卡林型金矿床是狭义内涵的金矿类型,属于沉积岩型金矿类型中的重要亚类;在区域金成矿密集区内,可构成卡林型矿床地球化学系列。建议不要将沉积岩型金矿和卡林型金矿相互代替使用或等同起来。秦岭沉积岩区卡林型金矿与美国卡林典型矿床有较好的类比性。矿床具有沉积－轻微改造的中—低温热液成因特征。     

Role of basement in epithermal deposits: The Kushikino and Hishikari gold deposits, southwestern Japan

The magma–ore deposit relationship of most low-sulfidation epithermal ore deposits is still unclear, partly because many stable isotopic studies of such deposits have indicated the predominance of meteoric waters within hydrothermal fluids. However, it is certainly true that hydrothermal systems are ultimately driven by magmatic intrusions, and epithermal gold deposits might therefore be produced by magmatic activity even in deposits having has no obvious links to a magma. We re-examine the genesis of two typical low-sulfidation epithermal gold deposits, the Kushikino and Hishikari deposits, using structural simulations and isotope data.Many epithermal gold deposits including the Kushikino and Hishikari deposits have been discovered in Kyushu, southwestern Japan. The Kushikino deposit comprises fissure-filling veins within Neogene andesitic volcanics that overlie unconformably Cretaceous sedimentary basement. The veins consist of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite and sulfides. Although carbon and oxygen isotopic data for the veins indicate a meteoric origin of the ore fluid, finite element simulations suggest that the vein system might have formed in direct response to magma intrusion. In particular, geophysical data suggest that intruding magma has uplifted the basement rocks, thereby producing fractures and veins and a positive Bouguer anomaly, and providing the heat necessary to drive an ore-forming hydrothermal system.The second component of this study has been to investigate the nature and evolution of the Kushikino and Hishikari epithermal systems. Isotope data document the geochemical evolution of the hydrothermal fluids. We conclude that the existence of sedimentary basement rocks at depth might have affected the strontium and carbon isotopic ratios of the Kushikino and Hishikari ore fluids. The ⁸⁷Sr/⁸⁶Sr ratios and δ¹³C–δ¹⁸O trend reveal that major ore veins in the Hishikari deposit can be distinguished from shallow barren veins. It was suggested isotopically that fluids responsible for the barren veins in nearby shallow and barren circulation systems were only controlled by the shallow host rocks. Such multi-isotope systematics provide a powerful tool with which to determine the center of hydrothermal activity and thereby document the evolution of hydrothermal fluids.     

Quantitative analysis of anomalous sources and geochemical signatures in the Walker Lake quadrangle of Nevada and California

A new quantitative transportation model for geochemical elements in stream sediment samples is proposed and used to estimate mineral sources of anomalous concentrations and to adjust both location and concentration for the transportation effect. Subsequent to transportation analysis, data on 14 geochemical elements in the Walker Lake quadrangle are analyzed for geochemical signatures for three types of mineralization environments: epithermal gold-silver deposits, porphyry-skarn copper deposits, and a mixed occurrence. Geochemical signatures are analyzed using a multidimensional scaling method referred to as the p_ijk model, which is a factor-like model generalized to include nonlinear associations, as well as linear, among triple variables (elements). A result of this analysis is that Pb is the best indicator for epithermal gold-silver mineralization, Mo and Zn for mixed mineralization, and Cu for porphyry-skarn mineralization.     

新类型难识别矿地球化学勘查

讨论了4种新类型难识别矿的地球化学勘查研究进展,包括使用深穿透地球化学方法在吐哈盆地砂岩型铀矿的试验和在胜利油田伴生金的地表地球化学显示,从全国铂的地球化学分布看西南黑色岩系中寻找铂族元素的前景,地球化学填图增加到70余种元素分析对稀有、分散元素的研究和寻找将起到巨大的作用。     

Cluster and R-mode factor analyses on soil geochemical data of Masjed-Daghi exploration area, northwestern Iran

Masjed-Daghi is located in Julfa sheet (1:100,000 series) in the northwest of Iran. The area consists of a very likely gold mineralization bearing epithermal mineralization which appears to be associated with a porphyry Cu–Mo system at deeper levels. Ninety-three soil samples were collected and analyzed for 13 elements (Au, Mo, Cu, Pb, Sn, Ag, Zn, Cr, Mn, Ba, Be, Ni, Co) by using emission spectrometry and atomic absorption spectrophotometry. The data were processed and interpreted using univariate and multivariate statistical analyses. The distribution of the majority of variables is slightly to moderately positively skewed which can be interpreted by log-normal model. Only Ni, Be, and Mn show normal distribution. Based on cluster analysis, the variables can be classified into two main groups. The first group consists of the main ore forming elements such as Au, Ag, and Ba which belong to epithermal system and Mo, Sn, and Cu which have more affiliation to porphyry mineralization. The principal component analysis extracted three factors. These factors calculated using varimax rotated R-mode factor loading matrix account for more than 65 % of the total variance. The first factor represents the main constituents of the epithermal system (Au, Ag, Ba) and its geochemical halo at the northeast of the study area. The second factor represents the main constituents of the porphyry system (Cu, Mo, Sn) and its geochemical halo in the western part of the study area which is overlapped with the volcanic rocks affected by local intrusions with higher alteration overprint. The third factor, however with less significance, represents Pb and Zn which are not the main ore constituents but can be considered as pathfinder elements. The results have been used to locate hidden orebodies using presented factor score mapping.     

The effect of scale on the interpretation of geochemical anomalies

The purpose of geochemical surveys changes with scale. Regional surveys identify areas where mineral deposits are most likely to occur, whereas intermediate surveys identify and prioritize specific targets. At detailed scales specific deposit models may be applied and deposits delineated.The interpretation of regional geochemical surveys must take into account scale-dependent difference in the nature and objectives of this type of survey. Overinterpretation of regional data should be resisted, as should recommendations to restrict intermediate or detailed follow-up surveys to the search for specific deposit types or to a too limited suite of elements. Regional surveys identify metallogenic provinces within which a variety of deposit types and metals are most likely to be found. At intermediate scale, these regional provinces often dissipate into discrete clusters of anomalous areas. At detailed scale, individual anomalous areas reflect local conditions of mineralization and may seem unrelated to each other. Four examples from arid environments illustrate the dramatic change in patterns of anomalies between regional and more detailed surveys.On the Arabian Shield, a broad regional anomaly reflects the distribution of highly differentiated anorogenic granites. A particularly prominent part of the regional anomaly includes, in addition to the usual elements related to the granites, the assemblage of Mo, W and Sn. Initial interpretation suggested potential for granite-related, stockwork Mo deposits. Detailed work identified three separate sources for the anomaly: a metal-rich granite, a silicified and stockwork-veined area with scheelite and molybdenite, and scheelite/powellite concentrations in skarn deposits adjacent to a ring-dike complex.Regional geochemical, geophysical and remote-sensing data in the Sonoran Desert, Mexico, define a series of linear features interpreted to reflect fundamental, northeast-trending fractures in the crust that served as the prime conduits for mineralizing fluids. At a larger scale, the linear, northeast-trending anomalies can be shown to result from a series of discrete mineralized systems with different ages and mineral assemblages. The linear pattern of anomalies disintegrates.A regional geochemical survey in the Sonoran Desert in southwestern Arizona displays a cluster of samples anomalous in Pb, Mo, Bi and W. In detail, the original regional anomaly separates into four discrete anomalous areas, each with its own distinctive suite of elements, geographic distribution and age of mineralization.A prominent regional gold anomaly in the Gobi Desert, Xinjiang, Peoples Republic of China, extends southeastward for 30 km from known lode gold deposits. Because the anomaly cuts both lithologic units and the structural grain, and because it parallels the prevailing direction of high-velocity winds, it was originally attributed to eolian dispersion. In detail, the regional anomaly consists of several east-west-trending anomalies, parallel to local lithology and structure that most likely reflect independent sources of lode gold. The regional anomaly results from smoothing of an en-echelon set of local anomalies.These examples emphasize that interpretation of regional anomalies must be tempered to consider regional-sized geologic features. Attempts to overinterpret anomalies by assigning deposit-scale attributes to regional anomalies can lead to confusion and incorrect interpretations. Potential targets that can be readily resolved only at intermediate or detailed scales of study may be overlooked.     

大巴山地区水系沉积物地球化学特征及找矿方向

大巴山地区跨秦岭、扬子两个一级构造单元,基于1:5万水系沉积物测量成果,统计计算了成矿元素地球化学参数,分析研究了元素分布特征、元素相关性及异常元素组合特征,认为研究区Pb、Zn、Ba、Cu、Mn找矿潜力较大,推断研究区所在的秦岭构造单元成矿类型主要为与震旦系—下寒武统黑色岩系有关的低温热液锌铜矿和沉积型钡矿;研究区所在的扬子构造单元成矿类型主要为与寒武系碳酸盐地层有关的低温热液铅锌矿和与震旦系碳酸盐岩有关的沉积型锰矿;在此基础上,划分出杨家坪—葛洞钡锌成矿远景区、镇坪钡锌成矿远景区、黄安坝—迷魂阵锌铜成矿远景区、百羊坪铅锌成矿远景区和修齐镇—石坊乡锰成矿远景区。     

The giant Carlin gold province: a protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary

Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit “trends.” Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the trends. Interestingly, the large Cretaceous Alaska–Yukon intrusion-related gold deposits (e.g., Fort Knox) are associated with the northern extension of the same lithospheric margin in the Selwyn basin, which experienced an analogous series of geologic events.     

Letters to the Editor

The considerable gold resource potential in China has been increasingly recognized in recent years, following detailed geological investigation of the major orogenic belts since the 1980s. The major tectonic components of China include the Sino-Korean, Yangtze, Tarim, and Cathaysian Precambrian platforms surrounded by the Altaides, Tianshan-Inner Mongolia, Daxinganling, Alkin, Kunlun, Qilian, Qinling, Himalaya-Tethys, and Nanling orogenic belts. A large number of gold deposits and geochemical anomalies have been discovered in all of these lithotectonic provinces. The most important deposit types are Precambrian lode, Carlin-type, ophiolite-related, porphyry-related, volcanic epithermal, and possible Muruntau-type gold deposits, occurring mainly in seven major metallogenic districts in eastern and southwestern China. Many lithotectonic belts provide favorable locations for the formation of various large gold deposits and are comparable with other productive gold provinces of the world, such as the Canadian Shield and Circum-Pacific magmatic arcs. The geochemical anomalies and many existing gold deposits and metallogenic belts in China require additional detailed study on the basis of modern exploration technologies.     

黔西南灰家堡金矿田有机岩相学和地球化学

灰家堡金矿田是黔西南卡林型金矿床的集中产地之一,对其中的水银洞、紫木凼、太平洞和簸箕田4个代表性金矿床进行了有机岩相学和地球化学研究。灰家堡金矿田的矿床中固体沥青主要产于矿石内,与矿化关系密切,产出形态受孔隙、构造裂隙控制;沥青与热液期矿物的共生组合关系表明其形成时间与成矿时间大致一致。矿石中普遍发育有机质包裹体。有机地球化学特征表明,金矿石和围岩中的有机质成熟度均较高,经历了类似的地质作用过程;有机质形成于强还原、中低盐度的沉积环境;有机质来源以海相菌藻类等低等生物为主,有少量陆源高等植物的输入。