Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J₀, J₁, J₂, and J₃, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J₀ formed during an early event. Petrographic observations, geochemistry, and δ¹⁸O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J₁, J₂, and J₃ jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J₀ jasperoids caused them to have higher δ¹⁸O values than J_1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ¹⁸O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ¹⁸O values are associated with the largest deposits. The 28‰ range of jasperoid δ¹⁸O values is best explained by mixing between an ¹⁸O-enriched fluid and an ¹⁸O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ¹⁸O composition of jasperoids indicates that gold was introduced by the ¹⁸O-enriched fluid. The lowest calculated δ¹⁸O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the ¹⁸O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the ¹⁸O-enriched ore fluid is not constrained. The δ³⁴S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H₂S and sulfate were derived from sedimentary sources. Likewise, the δ¹³C and δ¹⁸O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO₂ was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.     

Evidence for a magmatic origin for Carlin-type gold deposits: isotopic composition of sulfur in the Betze-Post-Screamer Deposit, Nevada, USA

We report here new sulfur isotope analyses from the Betze-Post-Screamer deposit, the largest Carlin-type gold deposit in the world. Carlin-type deposits contain high concentrations of arsenic, antimony, mercury, tellurium and other elements of environmental interest, and are surrounded by large volumes of crust in which these elements are also enriched. Uncertainty about the source of sulfur and metals in and around Carlin-type deposits has hampered formulation of models for their origin, which are needed for improved mineral exploration and environmental assessment. Previous studies have concluded that most Carlin-type deposits formed from sulfide sulfur that is largely of sedimentary origin. Most of these studies are based on analyses of mineral separates consisting of pre-ore diagenetic pyrite with thin overgrowths of ore-related arsenian pyrite rather than pure, ore-related pyrite. Our SIMS spot analyses of ore-related pyrite overgrowths in the Screamer zone of the Betze-Post-Screamer deposit yield δ³⁴S values of about −1 to 4‰ with one value of about 7‰. Conventional analyses of realgar and orpiment separates from throughout the deposit yield δ³⁴S values of about 5–7‰ with one value of 10‰ in the Screamer zone. These results, along with results from an earlier SIMS study in the Post zone of the deposit and phase equilibrium constraints, indicate that early arsenian pyrite were formed from fluids of magmatic origin with variable contamination from sulfur in Paleozoic sedimentary rocks. Later arsenic sulfides were formed from solutions to which sulfur of sedimentary origin had been added. The presence of Paleozoic sedimentary sulfur in Carlin-type deposits does not require direct involvement of hydrothermal solutions of sedimentary origin. Instead, it could have been added by magmatic assimilation of Paleozoic sedimentary rocks or by hydrothermal leaching of sulfur from wall rocks to the deposit. Thus, the dominant process delivering sulfur, arsenic, gold and mineralizing fluids to Carlin-type systems and their surrounding country rocks was probably separation of fluids from a magmatic source. Editorial handling: G. Beaudoin     

Water quality impacts from waste rock at a Carlin-type gold mine,Elko County,Nevada

Changes in water quality in the North Fork of the Humboldt River, Nevada are caused by weathering of waste rock from an inactive Carlin-type gold mine. Review of historical water-quality data, monthly water sampling, and continuous monitoring of water-quality parameters were used to quantify these impacts. River water pH, which ranged between 7 and 8, did not show statistically significant variation from upstream of the mine to downstream. Several constituents, most notably sulfate, calcium, and magnesium, showed statistically significant increases in dissolved-ion concentrations. These data, along with geochemical modeling, suggest that oxidation of sulfide minerals and in situ acid neutralization by carbonate host rocks are occurring. Large increases in dissolved-ion concentrations were observed twice a year—during spring snow melt and the onset of the winter precipitation season. These spikes are likely caused by flushing of pore waters that have reacted with waste rock during months-long periods when shallow groundwater recharge is not occurring.     

Carlin-type gold deposits in Qinling and some related problems

The sediment-hosted disseminated gold deposits in the Oinling region are of sedimentation-slight-metamorphic origin superimposed by hydrothermal reworking at moderate-low temperatures and are well comparable with the typical Carlin gold deposits in the United States.In view of the confusing concept concerning the “sediment-hosted”and “Carlin-type” gold deposits,the authors propose that the term“sediment-hosted gold deposit”should be used in a broad sense which encompasses at least the four subtypes,i.e.,the Carlin type,the metamorphic fine clastic type,the hydrothermal sedimentary type and the vein type.In oter words,the “Carlin-type”should not be used as a synonym for “sediment-hosted”but is recommended as a subtype under the general category of “sediment-hosted gold deposits”     

Possible Carlin-type disseminated gold mineralization in the Mahakoshal fold belt, central India

The paper records the first occurrence of dolomite-hosted, disseminated gold mineralization at Barhi and Jhal, in a Late Archean-Early Proterozoic metavolcano-sedimentary belt (Mahakoshal fold belt) in central India. Gold mineralization is hosted by dolomite that occurs as discontinuous bands interbedded with phyllite. Hydrothermal alteration styles of the host rock include decalcification, silicification, and argillization. Pyrite is the most common sulfide, whereas stibnite and realgar are rare. Mineralization is characterized by persistent gold from 0.20 to 0.62 ppm and a consistent association of anomalous arsenic, antimony, and mercury with gold.     

Carlin-type gold mineralization at Saint-André-de-Ristigouche,Gaspé Peninsula (Québec), Canadian Appalachians

The Gaspé Peninsula, in the Canadian Appalachians, hosts a variety of mineral occurrences that are spatially associated with the Grand Pabos and Restigouche faults. Among these occurrences, the Saint-André-de-Ristigouche gold (SAR-Au) showing has peculiar features comparable with Carlin-type gold deposits at the regional, district, and prospect scales. The most striking ones are: (1) the calcareous nature of the host rocks; (2) the Au-As-Sb-Hg metallic signature with absence of base metals and Ag; (3) the association with a crustal scale fault zone; (4) the hydrothermal alteration characterized by silicification, calcite, and quartz veinlets at the margin of the Au-bearing zones and the occurrence of kaolinite in large halos of argillic alteration; (5) numerous felsic dikes; (6) the occurrence of Au in As-rich pyrite forming overgrowths on premineralization As-free pyrite, and in arsenopyrite; (7) hydrothermal fluids equilibrated with host rocks at high temperature (>200°), which cooled to precipitate arsenopyrite and then mixed with evolved meteoric water to precipitate stibnite in veins. Although the SAR-Au showing is relatively small, analogies with Carlin-type mineralization encourage exploration in the Gaspé Peninsula and in similar settings elsewhere in the Appalachians.     

赣西卡林型金矿的成矿特征及找矿标志

根据长期的工作和研究成果,本文归纳总结了赣西卡林型金矿的成矿特征和找矿标志.赣西卡林型金矿产于扬子板块东南缘萍乡-高安坳陷带内,赋矿围岩为二叠系下统茅口组和三叠系下统大冶组等层位的细碎屑岩和不纯碳酸盐岩,金矿体产于断裂破碎带、层间破碎带、不整合面、密集裂隙带等构造部位,呈似层状、透镜状产出.金成矿受地层岩性、构造等因素的控制,在金矿床上均有化探金异常.找矿标志:1/5万化探分散流Au、AS、Sb、Hg组合异常;地层标志为大冶组和长兴组地层的细屑岩和不纯碳酸盐岩;构造上应在不整合面、断裂破碎带、密集裂隙带中.重点找矿远景区是坳陷带内两大推(滑)覆构造对冲带南侧的化探金异常区.     

中国贵州与美国内华达卡林型金矿对比及对找矿勘查的指示作用

文章从区域地质及矿床地质等方面对内华达和贵州卡林型金矿进行详细的总结和对比,结果表明两地金矿有很多相似,也有很多差异。主要的相似性包括:两地构造背景演化均经历了早期拉张裂陷、沉积一套被动大陆边缘海相沉积地层、之后造山挤压和晚期拉张;矿体主要受富铁地层和构造控制;与金矿化相关的蚀变作用均有硫化、去碳酸盐岩化、硅化和泥化;金主要以不可见金赋存于含砷黄铁矿中;晚期雄黄、雌黄、辉锑矿、方解石等矿物以脉状充填在开放空间。重要差别体现在:矿床区域分布控制因素;绝大部分贵州金矿矿床范围内未见岩浆岩;贵州金矿特有的白云石化以及相对较弱的去碳酸盐化、硅化(似碧玉岩)和泥化;载金黄铁矿的形貌和微量元素含量;部分贵州金矿含成矿期-成矿晚期毒砂;贵州有大量晚期石英-(方解石)脉;贵州成矿流体更富集CO_2,且具有更高的温度、压力和pH值。相似性和差异性的鉴定对于卡林型金矿(尤其是贵州卡林型金矿)找矿勘查具有重要指示作用。斜坡相地层是卡林型金矿最好的赋矿层位,卡林型金矿勘查中需要多关注斜坡相地层,尤其是富铁斜坡相地层。近期在贵州玄武岩分布区的区域不整合面(又称"构造蚀变体",即SBT)中,金矿勘查取得重大突破,玄武岩富含铁,是一种潜在的卡林型金矿赋矿岩性。在贵州地区,识别茅口组和龙潭组(或峨眉山玄武岩)之间的SBT及SBT上部的构造高点对于台地相区的区域金矿勘查至关重要。识别切割泥盆纪碳酸盐岩台地的区域同沉积断裂,对于调查矿床的区域分布(尤其是盆地相中金矿的区域分布)非常关键。大量似碧玉岩露头或者石英-方解石脉表明该区发生了大量的水-岩反应,预示周围可能有大规模成矿作用发生。辉锑矿、雄黄、雌黄等成矿晚期矿物多以脉状充填在矿体周围的开放空间,比矿体的分布范围更广,对矿体有很好的指示意义。     

卡林型金矿研究的若干问题探讨

卡林型金矿作为一种重要类型的金矿床,其众多成矿特征在地学界已达成共识,但在定名、成矿物质来源、有无矿源层、矿床成因诸问题上则争议颇大.通过分析前人资料,结合在滇黔桂"金三角"地区的找矿实践和思考认为:卡林型金矿本身不具有成因意义,不是成因类型,可定义为区带上集中分布的(超)微细、浸染、中低温热液矿床;卡林型金矿不存在严...     

西秦岭岷-礼成矿带岩浆活动与金矿成矿模式探讨

岷-礼成矿带发现较多大型金矿,同时在岩体内发现斑岩型钼矿。金矿床矿石中除有低温矿物外,还有黄铜矿、白钨矿等中-高温矿物,白钨矿在寨上金矿中是载金矿物之一;钼矿中也有黄铜矿、白钨矿矿物;金、钼、钨矿化分带有一定空间关系;金矿床地球化学、稳定同位素、放射性同位素和流体包裹体的研究表明成矿物质,成矿流体有深源特征;中川岩体群的岩石学、年代学、岩石化学、微量元素、稀土元素特征表明岩体为复式岩体,具有壳幔混源特征;据此建立成矿带内受印支-燕山期岩浆活动控制的金矿成矿模式。     

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。     

卡林型金矿床中自然砷的特征与成矿物理化学条件示踪

在"滇黔桂"、"川甘陕"两个金三角密集区内的某些卡林型金矿床中,自然砷矿物的存在并非个别现象。自然砷呈致密块状、致密凝胶体状(肾状)、脉状或微细脉状产出。自然砷呈他形粒状,颗粒大小变化较大,一般为0.05～0.50mm,最大可达2mm。反光显微镜下为白色,显微硬度为114.21～150.60kg/mm2,相当于摩氏硬度3.27～3.59。矿物主要化学成分As的质量分数为92.74%～99.74%,并含有S0.18%～5.25%,Sb0.04%～3.65%。矿物为三方晶系,晶胞参数值a=0.3759nm,c=1.0527nm。利用矿床中含砷矿物的共生组合特点及热力学资料,探讨了卡林型金矿床形成的温度、成矿流体的f(O2)、f(S2)变化范围及金以金砷络合物形式迁移的可能性。     

胶东玲珑金矿田金的赋存状态及其对成矿过程的指示意义

胶东地区是我国最大的黄金基地, 探明黄金资源储量超过5000t。胶东地区金矿床主要发育两种矿化样式, 即浸染状细脉-网脉型矿化和石英-硫化物脉型矿化, 金主要以可见金形式赋存于黄铁矿和石英中。玲珑金矿田位于招平断裂带北端, 同时发育两种矿化样式的矿体, 是研究金赋存状态的理想选区, 理清金的赋存状态对于选择合适的选冶工艺以及揭示金的成矿作用和富集机制具有重要的研究意义。在详实的野外地质调查基础上, 通过显微岩相学观察、电子探针分析与矿物自动定量分析, 对胶东玲珑金矿田两种不同矿化样式的样品中可见金的赋存状态进行了对比研究, 探讨了金成矿过程。玲珑金矿田两种矿化样式矿石中金矿物的赋存状态并无显著差异, 金矿物主要为银金矿和自然金。嵌布状态为裂隙金、包体金、粒间金, 以粒间金和包体金为主。金矿物粒度以微粒-细粒为主, 石英脉中偶见中粒-粗粒金矿物。金矿物主要以独立矿物形式以及与黄铜矿、方铅矿等硫化物共生的形式赋存于黄铁矿中。石英-黄铁矿阶段金矿物成色高于石英-多金属硫化物阶段金矿物成色, 主要是受到成矿温度的影响。浸染状细脉-网脉型样品金矿物比石英-硫化物脉型样品金矿物具有更高的金成色, 是由于两种矿化样式沉淀机制差异导致。

     

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.     

南秦岭夏家店卡林型金矿床构造特征与成矿关系

从地质背景、矿床地质、控矿构造分析了夏家店金矿床特征及其形成过程及成矿机理;矿床产于南秦岭陡岭古隆起边缘寒武系-震旦系碳-泥-硅质板岩及白云岩中;控矿构造具有二次韧性剪切变形、二次构造角砾岩化及二次脉体矿化的特征,显示出式样多种、性质多样、形式多变、活动期次与阶段多次叠加的特点;具体阐述了各构造作用阶段与矿化蚀变之间的关系,建立了地层建造-剪切与张裂构造-热液渗流的成矿系统.属于沉积岩系、构造、热液活动3种地质作用叠加改造的卡林型金矿床.     

胶东-朝鲜半岛中生代金成矿作用

华北克拉通东部与朝鲜半岛相接,是中朝克拉通的重要组成部分。在华北克拉通东部的胶东半岛和朝鲜半岛内皆产出有不同规模的金矿床,并具有显著的地域特色。胶东半岛已发现金矿床(点)近200处,其中三山岛、焦家、新城、玲珑等为世界级金矿,它们为石英脉型和受构造控制的蚀变岩型,成矿时间主要集中在~120Ma,说明金成矿作用是在相当短的时间内,在同一成矿背景下和同一构造-岩浆-流体成矿系统下完成的。成矿流体主要来自幔源岩浆以及幔源岩浆与地壳相互作用产生的地质流体,就位环境与地壳/岩石圈在早白垩世强烈伸展构造变形有关,为克拉通破坏型金矿;与我国辽东相邻的朝鲜半岛北部平安北道等地金矿储量较大,成矿类型与特征辽东五龙金矿类似,为石英脉型矿化,也可能为早白垩世与华北克拉通岩石圈减薄、破坏相关的金矿床;朝鲜半岛南部的韩国金(银)矿床分成侏罗纪中温热液型和白垩纪浅成低温热液型两类,其中侏罗纪热液脉状金矿成矿特征虽与胶东金矿类似,但成矿时代(峰期~160Ma)有显著差异。而白垩纪浅成低温热液型金-银矿化主要发生在100~70Ma,与太平洋板块俯冲作用相关,为典型的环太平洋斑岩-次火山活动有关的浅成低温贱金属成矿系列。胶东和朝鲜半岛金矿床类型、特征及成矿时间的异同,与区域中生代地质演化及地球动力学背景密切相关。     

我国右江盆地与美国“大盆地”卡林型金矿对比研究

系统的成矿背景、矿床地质特征、成矿流体和成矿物质示踪、成矿时代和成矿模式的对比研究,表明美国"大盆地"与我国右江盆地(滇黔桂"金三角")卡林型金矿存在许多相似之处.包括:(1)矿集区均位于某一盆地范围内,大地构造演化史类似,均经历了大陆裂解→被动大陆边缘沉积→挤压造山→伸展变形等过程.成矿发生在造山后的伸展阶段;(2)矿床成带分布和聚集,容矿岩石以含钙质沉积岩为特征,与岩浆岩没有成因联系,同生正断层和构造高点是重要的控矿构造;(3)成矿作用相似,热液蚀变具特征的去碳酸盐化,含砷黄铁矿是主要的载金矿物,金以显微-次显微状赋存于黄铁矿的富砷环带中;(4)成矿流体具中低温,偏还原,弱酸性的特点,同位素示踪表明流体和物质来源比较复杂,没有单一的来源;(5)矿集区内矿床形成于一个很短的时间段内,表明矿床的形成受控于统一的动力学背景;(6)均缺乏一个被广泛认同的成矿模式.同时,两地的卡林型矿床也存在若干差异:①成矿前的构造背景复杂程度不一样;②赋矿地层层位不一样;③成矿时代不同;④成矿模式中关注的重点不同.根据两国卡林型金矿对比的结果,认为我国滇黔桂"金三角"仍有很大的找矿潜力.成矿时代、成矿流体来源、成矿地球动力学背景以及成矿模式仍将是今后研究的主要方向.     

中国金矿地质构造背景和成矿环境特征简介

我国地处欧亚板块、太平洋板块和印度板块的交汇地带，其区域成矿构造背景可分为古亚洲成矿域、滨太平洋成矿域和特提斯－喜马拉雅成矿域。它们有着独具的演化历史与成矿特征。因此，我国金矿成矿环境比较多样，矿床类型亦较繁多，并多具活动性陆壳的成矿特征。金矿时空分布具有一定的地域差异，热液矿床多具后生特征。矿床类型以交代－重熔花岗质杂岩中的玲珑－焦家式金矿为最重要，前寒武纪变质岩中石英脉型和糜棱岩带型金矿次之，微细浸染型和火山热液型金矿亦有一定远景，金矿资源比为：岩金占５１．７％，伴生金占３１．４８％，砂金占１６．７６％。     

Physicochemical Conditions of Differential Mineralization of Au and As in Gold Deposits,Southwest Guizhou Province,China

The conditions under which gold and arsenic are enriched separately during mineralization in gold deposits in southwestern Guizhou Province were described and the thermodynamic calculations gave: 200–150°C at 400 × 10⁻⁶ -300 × 10⁶ Pa (corresponding to a depth between 1.6 km and 1.2 km); lgf o₂,−40 to -35 Pa; lgf s₂, -20 to−16 Pa; pH 5.0 -4.2 and Eh -0.53 V. This project was jointly supported by the National Natural Science Foundation of China and the Open Lab. of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences.