Geological controls on refractory ore in an orogenic gold deposit, Macraes mine, New Zealand

The Macraes mine is hosted in an orogenic (mesothermal) gold deposit in metasedimentary rocks of the Otago Schist belt. Much gold occurs within altered schist with minimal silica-addition, and this study focuses on altered schist ore types. The unmineralized host schists are chemically and mineralogically uniform in composition, but include two end-member rock types: feldspathic schist and micaceous schist. Both rock types have undergone hydrothermal alteration along a shallow-dipping foliation-parallel shear zone, but their different rheological properties have affected the style of mineralisation. Micaceous schist has been extensively recrystallized and hydrothermally altered during ductile deformation, to form ores characterized by abundant, disseminated millimetre-scale pyrite cubes (typically 1–2 wt% S) and minor silicification. The earliest pyrite contained Ni and/or As in solid solution and no gold was imaged in these pyrites or later arsenopyrite grains. The ore type is refractory and gold recovery by cyanide leaching is less than 50%, with lowest recovery in rocks that have been less affected by later brittle deformation. In contrast, hydrothermally altered feldspathic schist is characterized by mineralised black microshears and veinlets formed during shear-zone related brittle deformation. Microsheared ore has relatively low sulphur content (<0.7 wt%) and muscovite has been illitised during hydrothermal alteration. Pyrite and arsenopyrite in microshears are fractured and deformed, and contain 1–10 m blebs of gold. Later pyrite veinlets also contain micron- to submicron-scale inclusions of sphalerite, chalcopyrite, galena, and gold (10 microns). Gold in microsheared ore is more readily recoverable than in the refractory ore, although encapsulation of the fine gold grains inhibits cyanidation. Both microsheared ore and disseminated pyritic ore pass laterally into mineralised black shears, which contain hydrothermal graphite and late-stage cataclastic sulphides. This black, sheared ore releases gold readily, but the gold is then adsorbed on to gangue minerals (preg-robbed) and net cyanidation recovery can be less than 50%. Hence, low gold recovery during cyanidation results from (1) poor liberation of gold encapsulated in microcrystalline quartz and unfractured sulphide grains, and (2) preg-robbing of liberated gold during cyanidation. Introduction of pressure-oxidation of ore prior to cynidation has mitigated these issues.     

广东东坑金矿控矿条件分析

东坑金矿产于震旦系云开群中亚群,北东向吴川-四会大断裂是区内的重要控矿构造,大田顶弧形构造控制了矿床、矿点的分布,北西向韧性剪切带使金矿脉群定位;岩浆岩为成矿提供硫源外,还提供了热源和部分金质;金矿赋存于近接触带的绿片岩中,接触带为矿脉群的聚集、充填提供了空间。     

秦岭造山带柞水-山阳矿集区夏家店金矿床成矿时代及其地质意义

柞水-山阳矿集区位于秦岭造山带中部,区内矿产资源丰富,金矿找矿近年来不断取得突破,发现了多个大-中型金矿床,然而区内金矿成矿时代、成矿地质背景一直存在争议,直接制约矿集区金矿床找矿工作。夏家店金矿床是该区大型金矿床,矿体受断裂构造控制,金矿石类型有角砾岩型、碎裂岩型、糜棱岩型3种。围岩蚀变强烈,主要由硅化、方解石化、绢云母化、高岭土化、萤石化、重晶石化等,与金成矿关系最密切的是硅化、方解石化、萤石化。热液成矿期可划分为石英阶段、石英-方解石-萤石-硫化物阶段、方解石阶段。文章选取夏家店金矿床主成矿期的方解石、萤石等矿物进行了Sm-Nd同位素测试工作,获得等时线年龄为(139.64±0.98)Ma,表明成矿时代为晚侏罗世。结合区域构造-岩浆成矿作用特征,认为夏家店金矿床成矿作用与晚侏罗世—早白垩世构造-岩浆事件有关。据此将柞水山阳矿集区金矿床成矿年龄厘定在140Ma左右,为秦岭造山带伸展的地球动力背景下构造-岩浆热液演化的产物。通过与区内晚侏罗世—早白垩世构造-岩浆成矿事件对比,文章首次提出柞水-山阳矿集区存在140 Ma左右的金矿床成矿事件,为下一步在柞水-山阳矿集区开展金矿床勘查工作提供了重要依据。     

秦岭造山带金矿床类型与构造背景

独具特色的秦岭复合型大陆造山带位于中国南北大陆中央,由古生代盆－岛兼杂的小型陆块及其南北缝合构造带组成,并与华北和扬子大陆碰撞镶嵌,中一新生代陆内造山形成山盆耦合,构成独特的地质构造格局,造成了闻名于世的秦岭成矿带,形成贵金属,有色与黑色金属为特色的丰富矿产资源,根据秦岭造山带的组成与结构,阐述本区造山－成矿作用机制下在不同构造单元中形成的复杂的金矿床类型,分析了秦岭造山带金矿类型产出的地质背景,分布规律及成矿潜力。     

Paragenesis and geochemistry of ore minerals in the epizonal gold deposits of the Yangshan gold belt,West Qinling,China

Six epizonal gold deposits in the 30-km-long Yangshan gold belt, Gansu Province are estimated to contain more than 300 t of gold at an average grade of 4.76 g/t and thus define one of China's largest gold resources. Detailed paragenetic studies have recognized five stages of sulfide mineral precipitation in the deposits of the belt. Syngenetic/diagenetic pyrite (Py₀) has a framboidal or colloform texture and is disseminated in the metasedimentary host rocks. Early hydrothermal pyrite (Py₁) in quartz veins is disseminated in metasedimentary rocks and dikes and also occurs as semi-massive pyrite aggregates or bedding-parallel pyrite bands in phyllite. The main ore stage pyrite (Py₂) commonly overgrows Py₁ and is typically associated with main ore stage arsenopyrite (Apy₂). Late ore stage pyrite (Py₃), arsenopyrite (Apy₃), and stibnite occur in quartz ± calcite veins or are disseminated in country rocks. Post-ore stage pyrite (Py₄) occurs in quartz ± calcite veins that cut all earlier formed mineralization. Electron probe microanalyses and laser ablation-inductively coupled plasma mass spectrometry analyses reveal that different generations of sulfides have characteristic of major and trace element patterns, which can be used as a proxy for the distinct hydrothermal events. Syngenetic/diagenetic pyrite has high concentrations of As, Au, Bi, Co, Cu, Mn, Ni, Pb, Sb, and Zn. The Py₀ also retains a sedimentary Co/Ni ratio, which is distinct from hydrothermal ore-related pyrite. Early hydrothermal Py₁ has high contents of Ag, As, Au, Bi, Cu, Fe, Sb, and V, and it reflects elevated levels of these elements in the earliest mineralizing metamorphic fluids. The main ore stage Py₂ has a very high content of As (median value of 2.96 wt%) and Au (median value of 47.5 ppm) and slightly elevated Cu, but relatively low values for other trace elements. Arsenic in the main ore stage Py₂ occurs in solid solution. Late ore stage Py₃, formed coevally with stibnite, contains relatively high As (median value of 1.44 wt%), Au, Fe, Mn, Mo, Sb, and Zn and low Bi, Co, Ni, and Pb. The main ore stage Apy₂, compared to late ore stage arsenopyrite, is relatively enriched in As, whereas the later Apy₃ has high concentrations of S, Fe, and Sb, which is consistent with element patterns in associated main and late ore stage pyrite generations. Compared with pyrite from other stages, the post-ore stage Py₄ has relatively low concentrations of Fe and S, whereas As remains elevated (2.05～3.20 wt%), which could be interpreted by the substitution of As^? for S in the pyrite structure. These results suggest that syngenetic/diagenetic pyrite is the main metal source for the Yangshan gold deposits where such pyrite was metamorphosed at depth below presently exposed levels. The ore-forming elements were concentrated into the hydrothermal fluids during metamorphic devolatilization, and subsequently, during extensive fluid–rock interaction at shallower levels, these elements were precipitated via widespread sulfidation during the main ore stage.     

新疆伊什基里克成矿带金矿类型及其地质特征

伊什基里克成矿带系西天山成矿区带内重要的金铜成矿带.带内金矿类型主要划分有二种类型岩浆-热液型和火山-热液型,其中岩浆-热液型金矿床与浅成-超浅成侵入体(脉)有成因关系,火山-热液型金矿床与下石炭统火山热液活动有关,矿体赋存于大哈拉军山组火山地层中.两类型金矿均严格受构造、火山岩地层以及花岗岩(脉)体的控制.     

Genesis and geological and geochemical characteristics of Qixia gold deposit, Shandong, China

The Qixia gold deposit is one of the important quartz vein-type deposits hosted in metamorphic rocks in the east of Shandong, China. Compositionally the wolframite which is associated with gold mineralization in the deposit is rich in iron, but poor in manganese, showing that this mineral crystallized from hydrothermal solution at low temperature. The temperatures at the main metallogenic stage of the Qixia gold deposit are within the range of 160–270°C. The gaseous phases in fluid inclusions are dominated by H₂O and CO₂, while Na⁺, Ca²⁺ and Cl⁻ are the major species in the fluid phase with K⁺ and F⁻ present in relatively small amounts. The salinities of fluids are 4.2 wt% –8.0 wt% NaCl equiv. Meanwhile, the fluid is characterized by either Ca²⁺ < Na⁺ < K⁺ (in five samples) or Na⁺ < Ca²⁺ < K⁺ (in two samples), quite similar to the composition of ore-forming fluid derived from meteoric water. Primary data on the hydrogen and oxygen isotopic compositions of the ore-forming fluid fall within a wide range: = − 56‱ − 95‱. and = − 3. 6‱ − 4. 5‱ relative to SMOW. These isotopic values fully reflect the distribution features of meteoric water which has exchanged with the metamorphic rocks of the Jiaodong Group at different temperatures and W/ R ratios, and the metallogenic process is characterized by low W/R ratios. The Rb-Sr isochron age of the Qixia gold deposit is 125.8 ± 1.7 Ma, with (⁸⁷Sr/⁸⁶Sr)_i being 0.7168, and the time interval between the gold deposit and its country rocks (granite or metamoprhic rocks) is very large. The formation of the Qiaxia gold deposit is directly related to the evolution of geological history in eastern Shandong, and a genetic model in which the deep convective circulating meteoric water reacts with the country rocks can be used to explain the metallogenic mechanism of the deposit. This project was financially supported by the National Natural Science Foundation of China (No. 49000020).     

辽西建平烧锅营子金矿花岗岩的锆石U-Pb年代学、地球化学特征及地质意义

辽西建平烧锅营子花岗岩岩体位于华北地台北缘朝阳—赤峰金矿成矿带,岩体周围分布着烧锅营子、撰山子等金矿床或金矿点。与金矿同源关系的岩体主要为二长花岗岩。3个不同粒度的二长花岗岩LA-ICP-MS锆石UPb测年结果分别为(159.1±1.5)Ma、(160.2±4.4)Ma、(160.1±1.1)Ma,表明烧锅营子岩体形成晚侏罗世早期;其周边金矿床也形成于此时期或稍晚。结合前人资料总结认为160 Ma左右是朝阳—赤峰地区一个重要的金矿成矿时期。烧锅营子花岗岩体主量元素以富Si、低Al、中偏低碱、高钾和低Mg、低Ca为特点;微量元素表现出富集Th、Zr、Nd、Rb、K和亏损Ba、Sr、P、Ti的特点,具有强烈的负Eu(δEu=0.08~0.60)、Sr异常。岩石总体上属于经历了高度结晶分异的高钾钙碱性I型花岗岩,Sr-Yb-Y元素特征显示其源区深度为30 km。晚侏罗世整个中国东部处于构造域转折期,结合区域上构造环境分析,烧锅营子岩体是太平洋板块俯冲背景下的弧后伸展环境的产物。     

安徽铜陵冬瓜山铜金矿床流体包裹体微量元素地球化学特征及其地质意义

安徽铜陵冬瓜山铜金矿床是狮子山矿田中埋藏深度和地质储量最大的矿床,同时发育层控矽卡岩型和斑岩型矿体.该矿床流体成矿从早期到晚期经历了多个矿化阶段.本文选取了钾长石化阶段、矽卡岩化阶段、早石英硫化物阶段和晚石英硫化物阶段矿石中石英的原生流体包裹体进行了ICP-MS分析,结果表明,各成矿阶段石英流体包裹体微量元素的陆壳标准化分布曲线具有较好的一致性,显示成矿流体具有统一的来源和演化;流体包裹体的REE配分曲线与岩浆岩的稀土元素配分曲线相似,均为平缓右倾型,轻稀土明显富集,LREE/HREE比值与岩浆岩接近,显示成矿热液具有深部岩浆来源的特征.流体包裹体微量元素的直接分析显示成矿流体为矿质富集的热液流体,且各成矿阶段热液流体中的成矿元素含量高低的变化与相应阶段的矿化强度及矿石品位高低变化相一致,早石英硫化物阶段是矿质集中富集的最佳阶段,至晚石英硫化物阶段可能有浅部热液流体的混入.     

Mineralogy,geochemistry, and structural controls of a disseminated gold-bearing alteration halo around the schist-hosted Bullendale orogenic gold deposit,New Zealand

Orogenic gold-bearing quartz veins in the middle Tertiary Bullendale Fault Zone, New Zealand were mined historically for coarse gold in a narrow zone (ca. 5 m thick). However, recent drilling has revealed a broad hydrothermal alteration zone extending into the host schist, in which disseminated sulphide and gold mineralisation has occurred. The evidence of alteration is first seen over 150 m across strike from the fault zone, and the best-developed alteration halo is about 50 m wide. The extent and intensity of alteration is strongly controlled by local structures that developed during regional Tertiary kink folding of the pervasively foliated and fissile metasedimentary schist host. The earliest structures are foliation-parallel microshears (micron to millimeter scale) formed during flexural-slip folding. Later, but related, structures are predominantly normal faults and associated shear zones that have formed extensional sites during the regional folding event. All these structures facilitated hydrothermal fluid penetration and rock alteration, with localised vein formation and brecciation. Where fluid has followed structures, metamorphic chlorite, phengite, and titanite have been altered to hydrothermal ankerite, rutile, and muscovite or kaolinite. Ankerite with Fe/(Fe + Mg) < 0.4 formed in host rocks with Fe/(Fe + Mg) of 0.6, and iron released by ankerite alteration possibly formed pyrite and arsenopyrite that host disseminated gold. Fault zones were extensively silicified and veined with quartz, albite, sulphides, and gold. Host rocks have wide compositional variations because of centimeter-scale metamorphic segregation. However, the alteration halo is characterised by elevated CO₂ and S, as measured by loss-on-ignition (doubled to ca. 6 wt.%), elevated As (100–10,000 ppm), and weakly elevated Sb (up to 14 ppm). Strontium is elevated and Ba depleted in many altered rocks, so Sr/Ba ratio increases from < 1 (host rocks) to > 3 in the most altered and silicified rocks. Many altered and mineralised rocks have low Sr/Ba (< 0.5) as well. The subtle geochemical signature is not useful as a vector to ore because of the strong microstructural control on alteration. Likewise, there is no evidence for spatial mineralogical zonation across the alteration halo, although the most intense alteration is centred on the main fault zone, and intensity of alteration is controlled by microstructures at all scales. As documented in previous studies, hydrothermal alteration haloes enlarge the exploration target for some orogenic gold deposits, and may include disseminated gold, as in this Bullendale example.     

五龙沟金矿区域地球化学异常特征及找矿标志

本文以地质及区域地球化学资料为基础，通过对五龙沟金矿区域地质—地球化学异常特征和找矿标志的研究；建立了该矿床的区域地球化学异常模型及找矿标志。对已取得的大量1：20万区域化探异常的筛选和评价，以及进一步提高金矿化探普查效果，均具有一定的积极意义。     

Determination of mineralization stages using correlation between geochemical fractal modeling and geological data in Arabshah sedimentary rock-hosted epithermal gold deposit,NW Iran

This research paper aims to delineate and recognize different gold mineralization stages based on surface lithogeochemical data using factor analysis and Spectrum-Area (S-A) modeling, as well as geological data in Arabshah sedimentary rock hosted epithermal gold deposit, NW Iran. Based on the factor analysis, Au and Mn were allocated to factor 2 (F2) and then classified by the S-A fractal modeling. In addition, Au and F2 values were transformed to spectrum data, which were categorized by the S-A log-log plots. Accordingly, the main mineralization phase contains Au and F2 (Au-Mn) values greater than 800 ppb and 0.3, respectively, and is associated with the occurrence of minerals such as pyrite, arsenian pyrite, realgar, orpiment and oxidized sulfides. The first phase of gold mineralization, where Au typically ranges between 80 and 350 ppb, is associated with base metal sulfides, arsenian pyrites and F2 values between 0.1 and 0.2. The second gold mineralization phase consists of Au values from 350 to 800 ppb and F2 values between 0.2 and 0.3. Combination of the S-A modeling, factor analysis and geological data outlined three gold mineralization stages in Arabshah gold deposit. The main mineralization stage showed a strong positive correlation with the NE-SW and NW-SE trending structures, the altered intrusive rocks such as microdiorite and granodiorite, and the altered subvolcanic dacitic domes.     

西秦岭造山带天子坪金矿床成矿时代及成因

西秦岭天子坪金矿床为含金石英细脉、网脉的蚀变岩型金矿,以含金石英脉中的热液锆石作为研究成矿时代的对象,挑选的锆石部分呈局部溶蚀状,无环带或弱环带,Th、U、Pb含量较高,Th/U值较高,属成矿期热液锆石。LA-ICP-MS锆石U-Pb分析获得了110±7 Ma的谐和年龄,与矿区所在地区早白垩世晚期的推覆构造事件吻合,代表了矿床的成矿时代。分析认为,柴家庄矿集区在燕山晚期有一次强度较大的与大型构造有关的构造蚀变岩型金的成矿作用。     

北祁连车路沟斑岩型金矿床含矿岩体年代学、地球化学与岩石成因

车路沟金矿是北祁连山发现的首例与斑岩有关的金矿床, 矿体产于加里东晚期侵入的中酸性斑岩体及其与围岩接触带内。为了查明含矿斑岩体侵位时代、岩石成因和源区性质, 本文以车路沟一带含矿斑岩体作为研究对象, 对其开展了岩石地球化学、LA-ICP-MS锆石U-Pb同位素定年和Hf同位素特征研究。该岩体主要由花岗闪长斑岩、石英闪长斑岩、英云闪长斑岩组成, 对花岗闪长斑岩、英云闪长斑岩进行了LA-ICP-MS锆石U-Pb同位素定年, 分别获得锆石²⁰⁶Pb/²³⁸U加权平均年龄为438.6±3.8Ma和433.0±3.8Ma, 指示岩浆侵位时代为早志留世晚期。岩石SiO₂含量为61.4%~68.6%, Al₂O₃含量为15.0%~16.7%, MgO含量为0.36%~2.95%, σ值介于1.20~2.03之间, A/CNK值为1.0~1.1之间, 属钙碱性系列准铝质花岗岩类; 具有弱负铕异常-正铕异常(δEu=0.90~1.18), 高Sr(319×10^-6~549×10^-6)含量和Sr/Y(36.6~77.5)比值, 低Y(7.09×10^-6~10.2×10^-6)和Yb(0.69×10^-6~0.89×10^-6)含量, 具有埃达克岩地球化学特征。该岩体的锆石ε_Hf(t)值均为正值, 变化于+12.7~+15.0之间, 两阶段模式年龄t_DM2(Hf)=461~563Ma, 推测岩浆源区可能主要来自新元古代晚期至寒武纪新生洋壳物质。研究认为车路沟斑岩体形成于陆陆碰撞后期, 地壳持续加厚, 诱发中新元古代晚期至寒武纪新生洋壳发生部分熔融, 形成埃达克质岩浆, 在其不断上侵和演化过程中, 形成车路沟斑岩体与其相伴产出的金矿体。

     

河北丰宁大西沟金矿地质地球化学特征及成因探讨

河北丰宁大西沟金矿地处华北板块北缘冀北--辽西成矿区,是一个研究程度较低的矿床.文章从地质、地球化学的角度初步研究了该矿床的微量元素、稀土元素和成矿流体的特征,并对矿床成因做了初步的讨论.研究表明,大西沟金矿是与陆相次火山作用有关的中浅成中低温热液(蚀变岩)型金矿床.     

甘肃省玛曲县大水金矿原生金矿石的发现及意义

甘肃玛曲县大水金矿是西秦岭地区规模最大、类型独特的金矿床之一,多年的矿床勘探和开发过程中,在400m以上从未发现过原生金矿体或金矿石,因而对该矿床的成因认识一直存在着争论。本文利用矿山在89线竖井开拓过程中发现的原生金矿石,通过岩矿鉴定、微量元素和稀土元素分析,发现矿区存在英安质凝灰岩,是金矿石寄主岩石之一。原生金矿石具块状、条带状和角砾状构造,凝灰岩矿石或角砾具海底喷流沉积特征。新发现的原生金矿石与花岗闪长斑岩、黑云石英闪长玢岩型金矿石具有不同的微量元素、稀土元素变化趋势和硫同位素值,指示原生金矿石可能经历了喷流和后期岩浆热液改造两个阶段的成矿,这些发现对认识大水金矿床成因和今后找矿方向具有重要指导意义。     

西天山地区浅成低温热液型金矿地质特征及成矿模式

新近发现于西天山吐拉苏地区的阿希大型金矿床及其外围的伊尔曼得、恰布坎卓它等金矿床(点)的矿化类型为浅成低温热液型,并进一步划分为冰长石-绢云母型和硅化岩型。矿床赋存于伊犁晚古生代裂谷区的吐拉苏—也里莫墩火山岩带中,矿床围岩为下石炭统大哈拉军山组中基性—中酸性陆相火山岩系。冰长石-绢云母型以阿希金矿为代表,矿床为沿火山口缘环状、放射状断裂充填的石英脉型;硅化岩型以伊尔曼得金矿为代表,矿化体呈层状、似层状,受火山岩系底部沉火山碎屑岩控制。结合该地区成矿地球化学特征分析,本文讨论了区内金矿成矿特征、成矿背景和成矿条件,建立了西天山地区浅成低温热液型金矿的成矿模式。     

湖南铲子坪金矿床地质特征及找矿标志

铲子坪金矿位于雪峰弧形构造带中段,是湖南省发现最早的破碎带蚀变岩型金矿床。通过对金矿床地质特征的综合分析,总结出该矿床的找矿标志,对该区进一步找矿预测有指导意义。     

河北小寺沟铜钼矿埃达克岩: 年龄、地球化学特征及其地质意义

小寺沟斑岩型铜钼矿产于华北板块北缘,其主体岩体类型为英云闪长岩和二长花岗岩类,矿物组合为斜长石+角闪石+石英+钾长石±黑云母±辉石+不透明矿物,岩性为钙碱性Ⅰ型侵入岩.K-Ar法测得小寺沟岩体花岗斑岩年龄为(122.83±2.46)Ma.岩石主量元素具有高SiO2、Al2O3、Na2O、Mg#,低K2O、TiO2、MgO、CaO、P2O5特征.稀土元素具有负Eu异常不明显,低HREE和Y,轻、重稀土元素强烈分异特征.研究认为,小寺沟岩体与埃达克岩相似,其初始岩浆主要为加厚下地壳的部分熔融产物,即小寺沟埃达克岩为C型埃达克岩,其初始岩浆中可能还有部分地幔物质.小寺沟大型斑岩型铜钼矿的形成与埃达克岩的成矿专属性、华北板块北缘特殊构造环境密切相关.     

The Hemlo gold deposit,Ontario: A geochemical and isotopic study

The Hemlo deposit, near Marathon, Ontario, is one of the largest gold deposits in North America. It is stratiform within Archean metamorphosed volcano-sedimentary rocks. The main ore zone is composed of pyritic, sericitic schist, and massive barite. This is the first report of stratiform barite in the Archean of North America, but other occurrences have since been found west of Hemlo. The mineralization is substantially enriched in Au, Mo, Sb, Hg, Tl and V and lacks carbonate. Because of metamorphism and deformation of the body its genesis is uncertain.${}^{87}\text{Sr}{}^{86}\text{Sr}$ of .7017 for barite from the deposit is similar to that of the sedimentary barite west of Hemlo and to initial ratios of contemporaneous volcanic rocks. At the base of the main ore zone, barite with δ^34S of +8 to +12%. was deposited with ~0%. pyrite. Upward, both barite and pyrite get isotopically lighter, with minimum values for pyrite, to ?17.5%, in non-baritic schist forming the upper part of the ore zone. In drill section, Au grades correlate with the isotopic composition of pyrite. This, and the association of fractionated sulphide with sulphate, suggests that Au, pyrite and barite were deposited contemporaneously. The linked, asymmetric distributions of S minerals and isotopic distributions, which are continuous from section to section, and the isotopic similarity of the Hemlo and western barites are consistent with a syngenetic depositional model.Two sources for the S minerals are considered. In the first, exogenous sulphate from a restricted basin were partially reduced in a geothermal system to form ³⁴S-depleted sulphide. In the second, the sulphate and sulphide are of magmatic-hydrothermal origin. Sulphate and fractionated sulphide are uncommon in Archean rocks, but one or both occur with unusual frequency in major Archean gold deposits. Hydrothermal fluids of moderately high , containing sulphate and permitting isotopic fractionation between oxidized and reduced S species, may have favoured the dissolution, transport and precipitation of Au.