Gold Abundance and Stable Isotopes (H, O, S) in Hydrothermal Alteration Zones of Gasa Island (Gasado), Korea

Geology of the Gasa Island (Gasado), Korea, consists mainly of tuffaceous rocks, rhyolite and andesitic rocks related to Cretaceous volcanic activity. These rocks are hydrothermally altered, and are classified into the following four alteration zones based on the alteration mineral assemblages: advanced argillic alteration (alunite‐pyrophyllite‐kaolinite‐pyrite); sericitic alteration (sericite‐kaolinite‐quartz); propylitic alteration (quartz‐chlorite‐carbonate‐pyrite); and silicified zones. Alunite in the advanced argillic zone occurred in two types; a massive or disseminated type and a vein type. Most of the massive or disseminated alunites are ≥50 μm in size, whereas the size of vein alunites is <20–30 μm. Alunite grain size is greater in the central part of disseminated or massive alunite, while it is smaller toward the margins. The gold content of each alteration zone is 21–2900 ppb, 15–88 ppb, 57–1730 ppb, and 2–231 ppb, respectively. The gold content of quartz veins developed in the alteration zones is 39–715 ppb. Gold is enriched in the minerals and rocks around faults and fissures, and is strongly concentrated in the advanced argillic alteration zone around faults. Hydrothermal solutions traveling along the fracture systems might be responsible for the comparatively high gold content in the study area. δ³⁴S of alunites occurring in the advanced argillic alteration zone range from +16.5 to +3.9‰, although most are in a comparatively narrow range from +8.6 to +5.2‰. There is no difference between disseminated or massive and vein alunites. The δ³⁴S of pyrites in the advanced argillic alteration zone are from +4.8 to ?2.9‰. Oxygen and hydrogen isotope values of alunites are from +8.5 to 0‰ and from ?59.6 to ?97.3‰, respectively. With an assumed temperature of 200°C, δD and δ¹⁸O of hydrothermal solutions calculated for alunites are from ?53.6 to ?91.3‰, and from ?2.4 to ?8.1 for massive or disseminated alunites and from ?6.6 to ?10.9‰ for vein alunites, respectively. These data suggest that meteoric water dominated during the alunite formation. Isotopic data, geological setting, mineralogy, size of alunite and pure alunite composition (K end member) indicate that alunites of the study area were formed in the steam‐heated environment of acid sulfate alteration.     

Geothermometry, geochronology, and mass transfer associated with hydrothermal alteration of a rhyolitic hyaloclastite from Ponza Island, Italy

A rhyolitic hyaloclastite from Ponza Island, Italy, was hydrothermally altered, producing four distinct alteration zones based on X-ray diffraction mineralogy and field textures: (1) nonpervasive argillic zone; (2) propylitic zone; (3) silicic zone; and (4) sericitic zone. The unaltered hyaloclastite is volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli. Incomplete alteration of the hyaloclastite resulted in the nonpervasive argillic zone, characterized by smectite and disordered opal-CT. The other three zones exhibit more complete alteration of the hyaloclastite. The propylitic zone is characterized by mixed-layer illite-smectite (I-S) with 10 to 85% I, mordenite, opal-C, and authigenic K-feldspar (akspar). The silicic zone is characterized by I-S with ≥90% I, pure illite, quartz, akspar, and occasional albite. The sericitic zone consists primarily of I-S with ≥66% I, pure illite, quartz, and minor akspar and pyrite. K/Ar dates of I-S indicate hydrothermal alteration occurred at 3.38 ± 0.08 Ma.Oxygen isotope compositions of I-S systematically decrease from zones 1 to 4. In the argillic zone, smectite has δ¹⁸O values of 21.7 to 22.0‰ and I-S from the propylitic, silicic, and sericitic zones ranges from 14.5 to 16.3‰, 12.5 to 14.0‰, and 8.6 to 11.9‰, respectively. δ¹⁸O values for quartz from the silicic and sericitic zones range from 12.6 to 15.9‰. By use of isotope fractionation equations and data from authigenic quartz-hosted primary fluid inclusions, alteration temperatures ranged from 50 to 65°C for the argillic zone, 85 to 125°C for the propylitic zone, 110 to 210°C for the silicic zone, and 145 to 225°C for the sericitic zone. Fluid inclusion data and calculated δ¹⁸O_water values indicate that hydrothermal fluids were seawater dominated.Mass-transfer calculations indicate that hydrothermal alteration proceeded in a relatively open chemical system and alteration in the sericitic zone involved the most extensive loss of chemical species, especially Si. Systematic gains in Mg occur in all alteration zones as a result of I-S clay mineral formation, and systematic losses of Na, Ca, and K occur in most zones. With the exception of Ca, calculations of mass transfer associated with hydrothermal alteration on Ponza agree with chemical fluxes observed in laboratory experiments involving hydrothermal reactions of rhyolite and seawater. The anomalous Ca loss at Ponza may be due to hydrothermal formation of anhydrite and later low-temperature dissolution. On the basis of Mg enrichments derived from circulating seawater, we estimate the following minimum water/rock ratios: 9, 3, 6, and 9 for the argillic, propylitic, silicic, and sericitic zones, respectively. Hydrothermal fluid pH for the propylitic and silicic zones was neutral to slightly basic and relatively acidic for the sericitic zone as a result of condensation of carbonic and perhaps other acids.     

Geology and Hydrothermal Alteration of the Milyang Pyrophyllite Deposit, Southeast Korea

Abstract: The Milyang pyrophyllite deposit, which is embedded in the Late Cretaceous Yuchon Group of the Kyongsang Supergroup, is one of the largest hydrothermal clay deposits in the Kyongsang basin, southeast Korea. Host rocks of the deposit are porphyritic andesite lava and minor andesitic lapilli tuff. In the Milyang district, a hydrothermally altered zone is about 2 × 3 km in extent; we can recognize the concentric arrangement of advanced argillic, propylitic, and sericitic alteration zones from the central to peripheral parts of the zone. The Milyang pyrophyllite deposit forms a part of the advanced argillic alteration zone. The Milyang pyrophyllite deposit is subdivided into the following four zones based on mineral assemblages: the pyrophyllite zones 1, 2, 3, and the silicified zone. The pyrophyllite zone 1, which occupies the central part of the deposit, comprises mainly pyrophyllite, kaolinite, and diaspore without quartz. Diaspore nodules often concentrate in beds 40–50 cm thick. Andalusite, dumortierite, and tourmaline locally occur as network veins, crack‐filler, or small spherulitic spots. The Al₂O₃ content of the ore ranges from 27 to 36 wt%. The pyrophyllite zone 2, which constitutes a major part of the deposit, comprises mainly pyrophyllite, kaolinite, and quartz. The Al₂O₃ content of the ore ranges from 15 to 24 wt%. The pyro‐phyllite zone 3 is the hematite‐rich marginal facies of the deposit. The silicified zone, which occurs as beds and septa, is mostly composed of quartz with minor pyrophyllite and kaolinite; the SiO₂ contents range from 79 to 90 wt%. Comparing chemical compositions of the high‐Al ores with those of unaltered host andesite, the Fe, Ca, alkalis, HFSE, and HREE contents are significantly depleted, whereas S, B, As, Sr, and LREE are enriched. The hydrothermal alteration of the Milyang pyrophyllite deposit can be classified into the following four stages: 1) extensive sericitic and propylitic alteration, 2) medium‐temperature (200–250°C) advanced argillic alteration, 3) high‐temperature (250–350°C or more) advanced argillic alteration, and 4) retrograde low‐temperature alteration. The heat and some volatile components such as B and S would be derived from the Pulguksa Granite intruded underneath the deposit.     

Rosia Poieni copper deposit,Apuseni Mountains,Romania: advanced argillic overprint of a porphyry system

The Rosia Poieni deposit is the largest porphyry copper deposit in the Apuseni Mountains, Romania. Hydrothermal alteration and mineralization are related to the Middle Miocene emplacement of a subvolcanic body, the Fundoaia microdiorite. Zonation of the alteration associated with the porphyry copper deposit is recognized from the deep and central part of the porphyritic intrusion towards shallower and outer portions. Four alteration types have been distinguished: potassic, phyllic, advanced argillic, and propylitic. Potassic alteration affects mainly the Fundoaia subvolcanic body. The andesitic host rocks are altered only in the immediate contact zone with the Fundoaia intrusion. Mg-biotite and K-feldspar are the main alteration minerals of the potassic assemblage, accompanied by ubiquitous quartz; chlorite, and anhydrite are also present. Magnetite, pyrite, chalcopyrite and minor bornite, are associated with this alteration. Phyllic alteration has overprinted the margin of the potassic zone, and formed peripheral to it. It is characterized by the replacement of almost all early minerals by abundant quartz, phengite, illite, variable amounts of illite-smectite mixed-layer minerals, minor smectite, and kaolinite. Pyrite is abundant and represents the main sulfide in this alteration zone. Advanced argillic alteration affects the upper part of the volcanic structure. The mineral assemblage comprises alunite, kaolinite, dickite, pyrophyllite, diaspore, aluminium-phosphate-sulphate minerals (woodhouseite-svanbergite series), zunyite, minamyite, pyrite, and enargite (luzonite). Alunite forms well-developed crystals. Veins with enargite (luzonite) and pyrite in a gangue of quartz, pyrophyllite and diaspore, are present within and around the subvolcanic intrusion. This alteration type is partially controlled by fractures. A zonal distribution of alteration minerals is observed from the centre of fractures outwards with: (1) vuggy quartz; (2) quartz + alunite; (3) quartz + kaolinite ± alunite and, in the deeper part of the argillic zone, quartz + pyrophyllite + diaspore; (4) illite + illite-smectite mixed-layer minerals ± kaolinite ± alunite, and e) chlorite + albite + epidote. Propylitic alteration is present distal to all other alteration types and consists of chlorite, epidote, albite, and carbonates. Mineral parageneses, mineral stability fields, and alteration mineral geothermometers indicate that the different alteration assemblages are the result of changes in both fluid composition and temperature of the system. The alteration minerals reflect cooling of the hydrothermal system from >400 °C (biotite), to 300–200 °C (chlorite and illite in veinlets) and to lower temperatures of kaolinite, illite-smectite mixed layers, and smectite crystallization. Hydrothermal alteration started with an extensive potassic zone in the central part of the system that passed laterally to the propylitic zone. It was followed by phyllic overprint of the early-altered rocks. Nearly barren advanced argillic alteration subsequently superimposed the upper levels of the porphyry copper alteration zones. The close spatial association between porphyry mineralization and advanced argillic alteration suggests that they are genetically part of the same magmatic-hydrothermal system that includes a porphyry intrusion at depth and an epithermal environment of the advanced argillic type near the surface.Editorial handling: B. Lehmann     

Mapping hydrothermal alteration in the Comstock mining district,Nevada, using simulated satellite‐borne hyperspectral data

Hydrothermal alteration mapping with spaceborne hyperspectral data was simulated in the Comstock mining district, Nevada in order to evaluate the mineral mapping capabilities of the proposed Australian Resource Information and Environment Satellite (ARIES‐1). As a result, a suite of hydrothermal alteration minerals, including kaolinite, dickite, illite, chlorite, alunite and carbonate was identified from the simulated data in the 0.4–2.5 μm wavelength region and their areal abundance variations mapped accordingly. The recognised alteration zoning shows a major change in alteration assemblages across the Comstock and Silver City Faults, and a gradual variation from north to south along the faults. In the bleached Miocene volcanic rocks, dickite, kaolinite, illite and alunite were recognised. Coexistence of dickite of relatively high temperature, high‐crystallinity kaolinite of medium temperature and low‐crystallinity kaolinite of low temperature suggests supergene processes overprinting earlier hypogene alteration. The bleached rocks probably represent hydrothermal alteration in the fluid up‐flow zones in the central and shallower parts of the hydrothermal system. Illite in the bleached zones is characterised by relatively short AI–OH band wavelengths (2190–2200 nm), indicating no or very low Fe and/or Mg contents. Fault‐controlled propylitic alteration is mapped in the central part of the district mainly in the footwall of the Comstock Fault. The associated illite is characterised mainly by medium AI–OH band wavelengths (2200–2208 nm). This propylitic alteration may be contemporaneous with Au–Ag mineralisation. Additional and more extensive propylitic zones, containing illite with long AI–OH band wavelengths (2204–2216 nm), were mapped in the southern part of the district. These zones resulted from either a pre‐mineralisation propylitic alteration, or the peripheral hydrothermal alteration in the fluid down‐flow zones of the Miocene hydrothermal system.     

Spectral characteristics of minerals in alteration zones associated with porphyry copper deposits in the middle part of Kerman copper belt,SE Iran

Visible near infrared and shortwave infrared (VNIR-SWIR, 350 to 2500 nm) reflectance spectra obtained from an analytical spectral device (ASD) have been used to define alteration zones adjacent to porphyry copper deposits (PCDs), in the central part of Kerman magmatic arc, SE Iran. The spectral analysis identified sericite, illite, halloysite, montmorillonite, dickite, kaolinite, pyrophyllite, biotite, chlorite, epidote, calcite, jarosite, and iron oxyhydroxides (e.g. hematite, goethite) of hydrothermal and supergene origin. Identified alteration zones are classified into six principal types namely phyllic, phyllic/propylitic, propylitic, potassic, argillic and advanced argillic. The iron oxide minerals in the oxidized zone were also identified using spectral analysis. Results of spectral analyses of samples are consistent with mineralogical data obtained from X-ray diffraction (XRD) and petrographic studies. Spectroscopic studies by ASD demonstrate that this tool is very useful for semi-quantitative and cost effective identification of different types of alteration mineral assemblages. Furthermore, it can provide a valuable tool for evaluating aerial distribution of alteration minerals while coupled with remote sensing data analysis.     

Mineralogy, Lithogeochemistry and Elemental Mass Balance of the Hydrothermal Alteration Associated with the Gold-rich Batu Hijau Porphyry Copper Deposit, Sumbawa Island, Indonesia

This paper discusses the mineralogy, whole-rock geochemistry and elemental mass balance of the hydrothermal alteration zones within the Batu Hijau porphyry copper-gold deposit, Sumbawa Island, Indonesia. The hydrothermal alteration and mineralisation developed in four stages, namely (i) the early stage consisting of a central copper-gold-bearing biotite (potassic), proximal actinolite (inner propylitic) and the distal chlorite-epidote (outer propylitic) zones; (ii) the transitional stage represented by the chlorite-sericite (intermediate argillic) zone; (iii) the late stages distinguished into the sericite-paragonite (argillic) and pyrophyllite-andalusite (advanced argillic) zones; and (iv) the very late stage typified by the illite-sericite zone. In general, major elements (particularly Ca, Mg, Na and K) and some minor and rare earth elements decrease from the least altered rocks towards the late alteration zones as a consequence of the breakdown of Ca-bearing hornblende, biotite and plagioclase. Chemical discrimination by means of millicationic R₁-R₂ diagram indicates that R1 [4Si − 11(Na + K) − 2(Fe + Ti)] increases while R₂[6Ca + 2Mg + Al] decreases with increasing alteration intensity, from least-altered, through early, transitional, to late alteration zones. Rare earth elements-chondrite (C₁) normalised patterns also exhibit the depletion of the elements through the subsequent alteration zones. These results are consistent with the elemental mass balance calculation using the isocon method which shows that the degree of mass and volume depletion systematically increases during alteration. A decrease of the elements as well as mass and volume from early, through transitional to late alteration stages may imply a general decrease of the element activities in hydrothermal fluids during the formation of the alteration zones.     

黑龙江金厂金矿床18号矿体围岩蚀变及短波红外光谱特征

金厂金矿18号矿体围岩蚀变发育顺序从早到晚为:钾化、硅化、绿泥石化、绢云母化、碳酸盐化、高蛉土化,从内往外依次发育青磐岩化带、绢英岩化带和钾化带.矿化出现在泥化和绢英岩化叠加处,以及泥化和青磐岩化叠加处.通过短波红外光谱测试技术,识别出本矿区有26种蚀交矿物,其中白云母含量与金矿体呈正相关,说明绢云母化与金矿化关系密切;青磐岩化带蚀变矿物组合为绿泥石+绿帘石+伊利石±埃洛石±蒙脱石±石英;钾化带蚀变矿物组合为钾长石+高岭石+埃洛石±蒙脱石±石英;绢英岩化带蚀变矿物组合为绢云母+埃洛石±蒙脱石±高岭石±石英.     

Quaternary Volcanic-Related Acid Hydrothermal Alteration in the Ugusu Silica Stone Deposit, Western Izu Peninsula, Central Japan: Geology and Alteration

Acid alteration areas accompanying Quaternary volcanoes are widespread in the western Izu Peninsula, central Japan. The Ugusu alteration area is the largest among them and is mined for “silica stone” at the silica body in the core of the alteration area. Silica zone defined by previous studies is subdivided into highly leached brecciated silica zone and residual silica zone, which extend along a NNW‐SSE–NNE‐SSW direction of fractures/faults. Fe‐rich, alunite, advanced argillic alteration, and intermediate argillic alteration zones occur toward the outside surrounding the two silica zones. The ascent of acid hydrothermal fluid would be responsible for the silica zones and surrounding alteration zones at an earlier stage, while the hydrothermal brecciation and silica veins were caused by a limited supply of silica‐saturated fluids at later stages. Based on the available mineral stability relations and fluid inclusion thermometries, the formation temperatures are estimated to be: >300°C for the residual silica zone; >290°C for the diaspore association in the advanced argillic alteration zone; and <260°C for the kaolinite association in the intermediate argillic alteration zone. The later stage quartz druses have been formed at 200–260°C. The Ugusu–Fukata acid hydrothermal systems were active at 1.5–1.2 Ma, which were temporally related to the Tanaba Andesite volcanism. Hydrothermal system at the Seikoshi gold–silver deposit survived until 0.7 Ma after the volcanism. In the western half of the Izu Peninsula, subduction of the Philippine Sea plate underneath the Suruga Trough caused nearly N–S‐trending maximum horizontal compressive stress (σ_Hmax) and the resultant formation of similarly trending alteration areas and Au‐Ag vein‐type deposits in the Ugusu‐Toi‐Seikoshi area. From a practical viewpoint, at the Ugusu silica stone deposit, the fracture‐controlled vertical morphology of the silica body provides an important guide for exploration. Because the alteration zones occur both in the lower and upper sides of the silica bodies, it is important to make sure to which side the alteration zones correspond.     

碰撞造山型斑岩铜矿蚀变分带模式--以西藏冈底斯斑岩铜矿带为例

岛弧环境斑岩铜矿蚀变分带模式已为人们所熟知 ,但碰撞造山环境的斑岩铜矿蚀变分带特征尚不清楚。对此 ,文中以西藏冈底斯斑岩铜矿带为例 ,选择驱龙、冲江、厅宫 3个典型斑岩铜矿 ,对其蚀变系统进行了系统研究。依据蚀变矿物组合可分为 3个蚀变带 ,呈环带状分布。从中心向外依次为钾硅酸盐化带、石英绢云母化带、青磐岩化带。泥化带不太发育 ,通常叠加在其它蚀变带之上。钾硅酸盐化带主要蚀变矿物为钾长石、黑云母、石英、硬石膏 ,伴有大量的黄铜矿与辉钼矿 ,是成矿物质的主要堆积区。石英绢云母化带与钾硅酸盐化带渐变过渡或叠加其上 ,是次于钾硅酸盐化带的储矿部位。蚀变矿物组合为绢云母 +石英 +钾长石 ,金属硫化物有黄铁矿、黄铜矿、辉钼矿、斑铜矿 ,少量的方铅矿、闪锌矿。主要的辉钼矿以石英 +辉钼矿脉的形式出现于本矿带。青磐岩化在斑岩体内不发育 ,矿化极微弱。蚀变岩石组分分析表明 ,岩石蚀变及其分带是岩浆流体 /岩石反应时K ,Na ,Ca ,Mg等组分迁移的结果 ,矿化伴随着蚀变发生。钾硅酸盐化带、石英绢云母化带和青磐岩化带的蚀变岩石与未 (弱 )蚀变斑岩具有一致的稀土配分模式 ,REE含量有规律地变化 ,说明蚀变岩石均经历了源于岩浆的流体的交代 ,不同的蚀变形成于岩浆流体演化的不同阶段。蚀?     

Fluid evolution,wallrock alteration,and ore mineralization associated with the Rodalquilar epithermal gold-deposit in southeast Spain

At Rodalquilar gold mineralization is found in Late Tertiary volcanic rocks of the Sierra del Cabo de Gata and is related to a caldera collapse. Radial and concentric faults were preferred sites for gold deposition. Hydrothermal activity produced a specific alteration zoning around gold-bearing vein structures, grading from an innermost advanced argillic via an argillic into a more regionally developed propylitic zone. Advanced argillic alteration with silica, pyrophyllite, alunite, and kaolinite extends down to several hundred m indicating a hypogene origin. High-grade gold mineralization in vein structures is confined to the near-surface part of the advanced argillic alteration. Fine-grained gold is associated with hematite, jarosite, limonite, or silica. At a depth of about 120 m, the oxidic ore assemblage grades into sulfide mineralization with pyrite and minor chalcopyrite, covellite, bornite, enargite, and tennantite. Two types of fluids from different sources were involved in the hydrothermal system. Overpressured and hypersaline fluids of presumably magmatic origin initiated the hydrothermal system. Subsequent hydrothermal processes were characterized by the influx of low-salinity solutions of probable marine origin and by interactions between both fluids. Deep-reaching, advanced argillic alteration formed from high-salinity fluids with 20–30 equiv. wt% NaCl at about 225°C. Near-surface gold precipitation and silification are related to fluids with temperatures of about 175°C and 3–4 equiv. wt% NaCl. Gold was transported as Au(HS) ₂ ^– , and precipitation resulted from boiling with a concomitant decrease in temperature, pressure, and pH and an increase in fO₂. All features of the Rodalquilar gold deposit reveal a close relationship to acid-sulfate-type epithermal gold mineralization.     

西藏地堡那木岗斑岩型铜矿多光谱遥感蚀变矿物组合分带特征

文章基于ASTER和Landsat-8OLI两种多光谱遥感数据,采用高光谱遥感技术混合调谐滤波(MTMF)、多光谱遥感技术相对吸收深度(RBD)、波段比值(BR)等方法提取了西藏多龙矿集区地堡那木岗斑岩型铜金矿床地表的蚀变矿物组合。其结果表明,基于ASTER数据的MTMF技术可将Al—OH矿物划分为白云母+高岭石/蒙脱石和地开石+蒙脱石+累托石两种组合,进一步可细分出斑岩型矿床多光谱遥感地表蚀变矿物组合并呈现出良好的分带特征:地堡那木岗铜金矿床自内而外依次为白云母+高岭石/蒙脱石→地开石+蒙脱石+累托石→Mg—OH类矿物组合,分别对应于前人野外调查所勘测到的的绢英岩化带+泥化带→泥化带→青磐岩化带,Fe3+矿物叠加于绢英岩化带、泥化带及其两带的叠合部位。所提取的多光谱遥感蚀变矿物组合分带特征对该区斑岩型铜金矿床的勘查工作提供了重要的遥感线索,对定位找矿靶区具有指导意义。     

辽宁排山楼金矿围岩的岩石学特征

排山楼地区变质作用的主要因素为热量(约600℃)、化学活动流体和压力(地压应力和挤压应力).地壳震动是造成本区造山作用的原因.这种现象与板块构造理论相吻合.矿物分异作用序列为:首先,受火成侵入活动的影响,产生片麻岩、糜棱岩、花岗岩;然后,经历地压和挤压的作用;最后,发生各种类型蚀变作用,包括绢云母化、硅化、碳酸岩化、方解石化、绿泥石化、脱氧作用等.排山楼金矿成矿模式可概述如下:a)成矿作用为热液蚀变型,矿床赋存在太古宙变质岩大型韧性剪切带中;b)矿体产于裂隙中;c)矿体形态与岩脉和细脉形态一致;d)围岩经历了强烈的蚀变作用;e)黄铁矿是最重要的富金矿物,热液流体来源于侵入体,在流经破碎带和裂隙带后,在围岩中沉积黄铁矿.主要岩石类型有花岗岩、角闪岩、片岩、片麻岩和糜棱岩.区内广泛发育青盘岩化、泥化和绢云母化蚀变作用.主要蚀变矿物有石英、黄铁矿、白云母、绢云母、绿帘石、黑云母、微斜长石、方解石、角闪石、云母和锆石.矿体主要赋存在花岗岩和片麻岩中.主要蚀变作用为绢云母化、黑云母化、硅化和方解石化.蚀变过程中,铁氧化物(铁帽、云母)覆于贫硫酸盐矿石表面.蚀变类型有青盘岩化(黏土)、泥化和绢云母化.通常,氧化铁与黏土矿物的混合影响卫星影像中光谱的反射.利用遥感技术方法,适于这类矿床的进一步预测研究.     

辽宁排山楼金矿围岩的岩石学特征

排山楼地区变质作用的主要因素为热量(约600℃)、化学活动流体和压力(地压应力和挤压应力).地壳震动是造成本区造山作用的原因.这种现象与板块构造理论相吻合.矿物分异作用序列为:首先,受火成侵入活动的影响,产生片麻岩、糜棱岩、花岗岩;然后,经历地压和挤压的作用;最后,发生各种类型蚀变作用,包括绢云母化、硅化、碳酸岩化、方解石化、绿泥石化、脱氧作用等.排山楼金矿成矿模式可概述如下:a)成矿作用为热液蚀变型,矿床赋存在太古宙变质岩大型韧性剪切带中;b)矿体产于裂隙中;c)矿体形态与岩脉和细脉形态一致;d)围岩经历了强烈的蚀变作用;e)黄铁矿是最重要的富金矿物,热液流体来源于侵入体,在流经破碎带和裂隙带后,在围岩中沉积黄铁矿.主要岩石类型有花岗岩、角闪岩、片岩、片麻岩和糜棱岩.区内广泛发育青盘岩化、泥化和绢云母化蚀变作用.主要蚀变矿物有石英、黄铁矿、白云母、绢云母、绿帘石、黑云母、微斜长石、方解石、角闪石、云母和锆石.矿体主要赋存在花岗岩和片麻岩中.主要蚀变作用为绢云母化、黑云母化、硅化和方解石化.蚀变过程中,铁氧化物(铁帽、云母)覆于贫硫酸盐矿石表面.蚀变类型有青盘岩化(黏土)、泥化和绢云母化.通常,氧化铁与黏土矿物的混合影响卫星影像中光谱的反射.利用遥感技术方法,适于这类矿床的进一步预测研究.     

The Lanthanide Tetrad Effect in Argillic Alteration: An Example from the Jizvan District,Northern Iran

Intrusion of quartz‐monzodioritic igneous bodies of Oligocene age into Eocene lithic crystal tuffs and trachy‐basalts resulted in the occurrence of a widespread argillic alteration zone in the Jizvan district (northern Iran). Mineralogically, the argillic alteration zone includes minerals such as kaolinite, quartz, smectite, pyrophyllite, muscovite‐illite, alunite, rutile, calcite, feldspar, chlorite, hematite and goethite. Therefore, the non‐CHARAC behaviour for trace elements in the argillic samples is reflected in the non‐chondritic Y/Ho and Zr/Hf ratios and the irregular REE patterns, which appear related to the tetrad effect phenomenon. The chondrite‐normalized REE distribution patterns indicate both concave (W‐shaped) and convex (M‐shaped) tetrad effects in the argillic samples. Based on the field evidence and the results from geochemical studies, it can be concluded that the samples from the argillic alteration zone having high fourth tetrad effect values (>0.30) were developed in the fault and breccia zones. The results indicate that factors such as preferential scavenging by Mn‐oxides, crystallization of clay minerals, fluid‐rock interaction, overprint of hypogene mineral assemblage by supergene ones, and the structural control, have all played an important role in the occurrence of tetrad effects in samples of the argillic zone in the Jizvan district.     

Isotopic evidence for the origin of an acid sulphate alteration,Styrian basin,Austria

The Gossendorf volcanic body is the only one in the Styrian basin that shows extensive hydrothermal alteration. K‐Ar dating of primary volcanic biotite and alteration products (alunite) suggests that the emplacement of the volcanic body and hydrothermal alteration took place synchronously, 15 Ma ago. The stable isotope compositions of the alteration products such as opal, barite, pyrite and alunite combined with crystallographia investigations indicate temperatures between 150 and 200 °C for the formation of the alteration zones. The calculated stable isotopic compositions of the parent fluid, responsible for the alteration, show an exogene marine component, which interacted with the host rock. Sulphur isotopic compositions of sulphur, sulphides and sulphates indicate disequilibrium, and progressive oxidation. This fact, combined with the mineral zonation of the alteration zone, reflects not only change in the pH but also change in the fO₂ of the ascending fluids.     

Gold mineralisation and advanced argillic alteration at the Dobroyde prospect,central New South Wales

Gold mineralisation at the Dobroyde prospect in central New South Wales is hosted by a zoned alteration system characterised by peripheral propylitic alteration, grading inwards through argillic and advanced argillic alteration to a siliceous altered core. Overprinting textures indicate that propylitic, argillic, advanced argillic and siliceous assemblages were successively superimposed on each other. Au grades between 0.3–0.8 ppm are associated with siliceous alteration and cross‐cutting pyrite veinlets. Higher Au grades are associated with barite veins that cut the pyrite veinlets. Native Au, native Te, Au, Pb and Hg tellurides, Pb selenide, chalcopyrite, Zn‐sphalerite and tennantite‐tetrahedrite occur in the barite veins. Microscopic pyrophyllite shears cut the barite veins. The location of the Dobroyde prospect, the orientation of its internal alteration zonation and the orientation of auriferous barite veins in the core of the prospect are controlled by a 330°‐striking fault. Movement on this fault, synchronous with hydrothermal activity, at some time between the Late Ordovician and mid‐Devonian controlled the development of successive phases of brecciation, siliceous alteration, pyrite and later barite‐Au veining in the prospect core. The restricted distribution of auriferous barite veins within the siliceous altered core of the prospect is inferred to be controlled by the relatively brittle rheology of this assemblage during deformation, and its location on the fault that formed the main hydrothermal fluid conduit. Alteration zones distal from this fault remain unmineralised. The Dobroyde prospect may be a product of the same Early Devonian metallogenic epoch as the paragenetically similar Temora and Peak Hill deposits. All three deposits/prospects appear to be localised in splays of either the Gilmore Fault Zone or the Parkes Thrust.     

Geology and Hydrothermal Alteration of the Duobuza Gold‐Rich Porphyry Copper District in the Bangongco Metallogenetic Belt,Northwestern Tibet

The Duobuza gold‐rich porphyry copper district is located in the Bangongco metallogenetic belt in the Bangongco‐Nujiang suture zone south of the Qiangtang terrane. Two main gold‐rich porphyry copper deposits (Duobuza and Bolong) and an occurrence (135 Line) were discovered in the district. The porphyry‐type mineralization is associated with three Early Cretaceous ore‐bearing granodiorite porphyries at Duobuza, 135 Line and Bolong, and is hosted by volcanic and sedimentary rocks of the Middle Jurassic Yanshiping Formation and intermediate‐acidic volcanic rocks of the Early Cretaceous Meiriqie Group. Simultaneous emplacement and isometric distribution of three ore‐forming porphyries is explained as multi‐centered mineralization generated from the same magma chamber. Intense hydrothermal alteration occurs in the porphyries and at the contact zone with wall rocks. Four main hypogene alteration zones are distinguished at Duobuza. Early‐stage alteration is dominated by potassic alteration with extensive secondary biotite, K‐feldspar and magnetite. The alteration zone includes dense magnetite and quartz‐magnetite veinlets, in which Cu‐Fe‐bearing sulfides are present. Propylitic alteration occurs in the host basic volcanic rocks. Extensive chloritization‐silicification with quartz‐chalcopyrite or quartz‐molybdenite veinlets superimposes on the potassic alteration. Final‐stage argillic alteration overlaps on all the earlier alteration. This alteration stage is characterized by destruction of feldspar to form illite, dickite and kaolinite, with accompanying veinlets of quartz + chalcopyrite + pyrite and quartz + pyrite assemblages. Cu coexists with Au, which indicates their simultaneous precipitation. Mass balance calculations show that ore‐forming elements are strongly enriched during the above‐mentioned three alteration stages.     

Petrology and alteration geochemistry of the epithermal Balya Pb-Zn-Ag deposit,NW Turkey

The 1.5 km-large hydrothermal system of Balya is characterized by three alteration styles which from the outer halo towards the center are: (i) propylitic alteration with the hydrothermal mineral assemblage of calcite-daphnite-albite-epidote-quartz-pyrite; (ii) argillic/phyllic alteration with the hydrothermal mineral assemblage of sericite/muscovite-kaolinite-rutile-quartz ± pyrite; (iii) advanced argillic alteration with the hydrothermal mineral assemblage of alunite-jarosite-kaolinite-quartz-sericite ± pyrite. Hornblende andesite is the protolith of the hydrothermal alteration system. Enrichment in Si, Sb and Rb, and depletion in Na, Ca, Mg, Fe, Mn, P, Ba, Sr, and Zn distinguishes the argillic/phyllic and advanced alteration types from propylitic alteration and the unaltered hornblende andesite protolith. REE distribution patterns indicate an essentially immobile behaviour of REEs during the alteration cycle. K-Ar age data for unaltered and hydrothermally altered rocks define a synchronous age of 25.3 ± 1.2 Ma for both igneous and hydrothermal activity.     

REE and HFS element behaviour in the alteration facies of the Erenler Dağı Volcanics (Konya,Turkey) and kaolinite occurrence

Mainly high-K, calc-alkaline, Late Miocene to Pliocene volcanic rocks cropped out of the Konya area in Central Anatolia, Turkey. The volcanic rocks are predominantly andesitic to dacitic in composition and rarely basalt, basaltic andesite, basaltic trachyandesite and pyroclastics. Kaolinite, illite, Ca-montmorillonite, alunite, jarosite, minamiite and silica polymorphs were formed by widespread and intense hydrothermal alteration in or around the volcanic products. To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, the whole rock chemical composition (major and trace elements, including rare-earth elements (REE) was analysed. The results of the study demonstrate notable differences in the REE behaviour in the different sample groups. REE trends of fresh parent rocks to weakly-, moderately-altered, kaolinitic and alunitic rocks are characterised by strong LREE enrichment ((La/Lu)_cn = 14.57, 11,8 to 15.20, 4.54 to 13.30, 12.5 to 24.2 and 34.6 to 47.26, respectively). Most of the samples have pronounced negative and/or weakly-negative Eu anomalies ranging from 0.75 to 0.98 while three samples have weakly-positive Eu anomalies. LRE element contents are higher than those of HREE in the samples. The LRE elements were strongly enriched in the kaolinitic and alunitic alteration; in weakly- and moderately-altered rocks. LREE are nearly immobile whereas HRE elements show different behaviour in different rock groups. The HFS and TRT elements are slightly mobilised in weakly-altered rocks, but enriched in other alteration types. Elements commonly assumed to be immobile (e.g. Y, Zr, Nb, Hf, TiO₂, Al₂O₃, REE) show variation in mass calculation. LIL elements showed enrichment over LREE and MREE, and similar behaviour, in contrast with HFSE. A clear increment of trans-transition elements (TRTE) was found mainly in alunitic and partly in kaolinitic samples.