Hyperspectral interpretation of selected drill cores from orogenic gold deposits in central Victoria,Australia

HyLogger hyperspectral data obtained from seven orogenic gold deposits in central Victoria, including Bendigo, Ballarat, Maldon, Fosterville, Costerfield, Castlemaine and Wildwood, are presented. The data demonstrate that fresh diamond drill core displays substantial mineralogical variation that can be attributed to the effects of cryptic hydrothermal alteration that might not otherwise be recognised. The most significant hyperspectral response lies in the white mica compositions, which vary in a systematic manner between high-Al muscovitic zones (Al–OH absorption around 2208 nm) that define a phyllic alteration halo around mineralised structures, and low-Al phengitic–chlorite zones (Al–OH absorption >2014 nm) inferred to represent either more distal alteration or possibly regional metamorphic background. An extensive ferroan dolomite alteration halo overlaps the phyllic and sulfidic alteration zones and extends beyond the sampled core in most instances. This ferroan dolomite halo has previously been defined petrographically, geochemically and using carbonate staining techniques, and is further characterised using thermal infrared hyperspectral data in drill core from the Ballarat goldfield. The mineralogical trends identified by the hyperspectral data are best developed in diamond drill core from the Costerfield, Fosterville and Ballarat goldfields, and are less pronounced at the other deposits. At Bendigo and Castlemaine the reasons for this are not immediately clear, but may be related to the close timing of gold mineralisation relative to peak metamorphism. The Maldon area lies within the contact aureole of the Harcourt Batholith and so has been thermally overprinted leading to the recrystallisation of earlier hydrothermal assemblages. The Wildwood deposit is similar to the Magdala deposit at Stawell and differs from the other goldfields in its geological setting, host rock lithologies and style of hydrothermal alteration, with the development of Fe-rich chlorite closely associated with gold mineralisation. The results demonstrate how hyperspectral data can be used to define large hydrothermal alteration footprints associated with orogenic gold mineralisation in central Victoria that are of direct benefit to mineral explorers, as well as independently characterising lithological variations in drill core.     

Variations in composition and abundance of white mica in the hydrothermal alteration system at Hellyer,Tasmania, as revealed by infrared reflectance spectroscopy

Short-wave infrared (SWIR) spectral reflectance of hydrothermally altered volcanic rocks in the footwall of the Hellyer massive sulfide deposit was measured with a portable PIMA-II infrared spectrometer. The Al–OH band was used to derive information on the octahedral Al content and the abundance of white mica (sericite) in the hydrothermal alteration and mineralization system. The range of the Al–OH band wavelength from 2192 nm to 2222 nm corresponds to the number of octahedral Al (Al^vi) in white mica approximately from 3.9 to 3.0 (based on 4 octahedral cations per formula). This Al^vi range represents a significant compositional variation, covering most of the compositional region between muscovite (Al^vi = 4.0) and phengite (Al^vi = 3.0). Furthermore, the spectral reflectance data show that the compositional variation of white mica is spatially related to hydrothermal alteration zoning, such that phengitic white mica tends to occur in 1) main upflow fluid channel, 2) intensely altered volcanic rocks, and 3) Pb–Zn mineralization, whereas muscovitic white mica was formed preferentially distal to massive sulfide mineralization on the margin of the footwall alteration system. The results suggest that the Al–OH band wavelength, and therefore the octahedral Al content, of white mica can be used as vectors to mineralization to map the hydrothermal system at Hellyer.     

Application of ASTER data for exploration of porphyry copper deposits: A case study of Daraloo–Sarmeshk area,southern part of the Kerman copper belt,Iran

The Cenozoic Urumieh–Dokhtar Magmatic Belt (UDMB) of Iran is a major host to porphyry Cu ± Mo ± Au deposits (PCDs). Most known PCDs in the UDMB occur in the southern section of the belt, also known as the Kerman Copper Belt (KCB). Three major clusters of PCDs are distinguished in the KCB and include the Miduk, Sarcheshmeh and Daraloo clusters. The Daraloo and Sarmeshk deposits occur in a northwest–southeast-trending fault zone that is characterized by the presence of a narrow zone of alteration–mineralization that contains a series of Oligocene granitoids and Miocene porphyritic tonalite–granodiorite plutons that cut Eocene andesitic lava flows and pyroclastic rocks. Here we use various techniques, including different ratio images, minimum noise fraction, pixel purity index, and matched filter processing to process ASTER data (14 bands) and generate maps that portray the distribution of hydrothermal minerals (e.g., sericite, kaolinite, chlorite, epidote and carbonate) related to PCD alteration zones. In order to validate the ASTER data, follow-up ground proofing and related mineralogical work was done which, in all cases, proved to be positive. The results of this work have identified the regional distribution of hypogene alteration zones (i.e., phyllic, argillic, propylitic and silicic), in addition to areas of secondary Fe-oxide formation, which are coincident with known sites of PCDs. The regional distribution and extent of the alteration zones identified also highlighted the role of regional structures in focusing the mineralizing/altering fluids. These results demonstrate very convincingly that ASTER imagery that uses the appropriate techniques is reliable and robust in mapping out the extent of hydrothermal alteration and lithological units, and can be used for targeting hydrothermal ore deposits, particularly porphyry copper deposits where the alteration footprint is sizeable.     

Mineral types of hydrothermal alteration zones in the Dukat ore field and their relationships to leucogranite and epithermal gold-silver ore,northeastern Russia

The paper considers the localization of potassic and propylitic hydrothermal alteration zones in the domal volcanic-plutonic structure controlling the position of the Dukat ore field with the eponymous unique epithermal Au-Ag deposit. Comprehensive mineralogical and geochemical data on rocks and minerals in hydrothermal alteration zones and associated intrusions have shown that quartz-jarosite-sericite, quartz-pyrite-sericite, and quartz-adularia-chlorite alterations were formed with the participation of fluid flows related to a fingerlike projection of a high-K leucogranite porphyry intrusion with large phenocrysts. These hydrothermal alterations developed in the rifted graben under conditions of divergent plate boundaries, whereas quartz-clinozoisite-calcite, epidote-chlorite, and garnet-calcite-chlorite alterations were linked to K-Na leucogranite intrusive bodies and developed under conditions of convergent plate boundaries reactivated as a result of formation of the marginal Okhotsk-Chukotka volcanic belt. Phase separation and coagulation of specific portions of ascending fluids resulted in the formation and stabilization of small-sized particles of native silver and other ore components, which enabled involvement in flows of secondary geothermal solutions and ore-forming fluids. The Sr, Nd, and Pb isotopic compositions of rocks and minerals from the hydrothermal alteration zones, associated intrusions, and economic orebodies at the Dukat deposit indicate that their components have been derived from the juvenile continental crust, which was altered in pre-Cretaceous periods of endogenic activity. The components of gangue minerals of potassic and propylitic hydrothertmal alterations and associated intrusions have been taken from deep sources differing in 87Sr/86Sr and 143Nd/144Nd at similar U/Pb and Th/Pb ratios. Chalcophile lead in products of hydrothermal activity and melanocratic inclusions in leucogranite has been taken from regions with elevated U/Pb and Th/Pb ratios.     

Ore–Waste Discrimination in Epithermal Deposits Using Near-Infrared to Short-Wavelength Infrared (NIR-SWIR) Hyperspectral Imagery

Near-infrared (NIR) and short-wavelength infrared (SWIR) hyperspectral imagery can be used to detect certain alteration minerals. At epithermal deposits, the formation of alteration minerals is, in theory, related to the mineralisation of gold and silver. In order to provide foundations for developing sensor-based sorting applications at a mine that exploits such a deposit, it was investigated if NIR-SWIR hyperspectral imagery can be used to distinguish between ore and waste particles by characterising the alteration mineralogy. Maps were produced from the NIR-SWIR hyperspectral images of 827 drill core samples that show mineral occurrences, mineral absorption feature intensities and characteristics of the iron oxide mineralogy. Partial least squares discriminant analysis (PLS-DA) was applied to the information contained in these maps to investigate if this information can be used to discriminate between ore and waste. The results showed that NIR-SWIR hyperspectral imagery could be used to segment a population of waste samples by detecting occurrences of pyrophyllite, dickite and/or illite. This result can be explained by the fact that these minerals are commonly deposited further away from the ore-bearing epithermal veins, while the absence of SWIR-active minerals or detected occurrences of alunite are more closely associated with these structures. The ability to identify waste with NIR-SWIR spectral sensors means there is potential that sensor-based sorting can be used to remove this waste from mineral processing operations. Additional research is still required to assess the economic feasibility of such a sensor-based sorting application.

     

The mineral chemistry of hydrothermally altered and metamorphosed wall-rocks at the Stollberg Fe-Pb-Zn-Mn(-Ag) deposit,Bergslagen, Sweden

The c. 1.9 Ga old Stollberg sulphide and Mnrich skarn iron ores and sulphide ores in Bergslagen, south-central Sweden are hosted by hydrothermally altered and metamorphosed felsic volcanic and volcaniclastic rocks. The ores are underlain by comformable alteration zones characterized by albite-gedrite-quartz and biotite-muscovite-plagioclase-K-feldspar-quartz +/– garnet assemblages. The present mineralogies are interpreted as medium-grade metamorphic equivalents to the original alteration mineral assemblages. PT-conditions during prograde regional metamorphism are semiquantatively determined to be 510 to 560 °C at approximately 3 kbar. With increasing modal content of gedrite and biotite in the alteration zones, the Mg/Fe ratios and X_Mg's in octahedral positions of these minerals also increase. In the gedrite-bearing strata, whole-rock Mg/Fe ratios remain constant, whereas in the biotite-rich unit the wholerock Mg/Fe trend is parallel to that of the biotites.The trends in the metamorphic mineral composition are interpreted to be a product of original changes in fluid composition during the evolution of a sub-seafloor hydrothermal system. During the initial stage of alteration, Fe-Mn-rich fluids altered the rocks, and during a later stage, the fluids became more Mg-rich, possibly due to entrainment of fresh seawater, and the alteration zones became relatively more Mg-rich. Sulphide precipitation was contemperaneous with Mg metasomatism, suggesting base metal precipitation was a function of the mixing of cool seawater with hydrothermal fluid. It is proposed that early hydrothermal alteration was associated with the deposition of areally extensive Fe-oxide formation, and that Mg metasomatism defines a second stage of hydrothermal activity during which sulphide mineralization overprinted the earlier formed Fe-oxide deposit.     

热液流体活动规律高光谱遥感分析示范研究——以新疆白杨河铀矿床为例

高光谱遥感技术可以有效地用于分析热液流体活动规律,进而指导找矿预测。利用国际先进的CASI/SASI航空高光谱遥感技术、并结合ASD便携式地面高光谱技术手段,文章从航空、地面、深部钻孔岩芯等不同尺度对白杨河铀矿区及周围地表和深部的热液蚀变类型和热液活动规律等进行了立体识别与研究。研究表明,白杨河铀矿区及周围航空高光谱遥感识别的蚀变矿物在空间上可分为北部蚀变区、矿区蚀变区、南部蚀变区等3个蚀变带。北部蚀变区发育一套以叶腊石、明矾石、高铝绢云母为主的酸性热液蚀变组合;矿区蚀变区地表发育高铝绢云母、中铝绢云母、赤铁矿、硅化等热液蚀变,矿区深部钻孔岩芯主要发育高铝绢云母、中铝绢云母、低铝绢云母、高岭石、赤铁矿、褐铁矿、绿泥石等蚀变矿物及其混合蚀变,且绢云母蚀变矿物的Al-OH吸收峰波长位置存在"单向递减"和"先减后增"等2种明显变化规律;南部蚀变区发育低铝绢云母和中铝绢云母蚀变。分析表明,北部蚀变区是相对的区域热液流体活动中心,铀矿区蚀变区是流体活动中心旁侧的一个明显的热液流体活动区,南部蚀变区是更加远离流体活动中心的,温度进一步降低的流体活动区。矿区北缘接触带的热液蚀变温度要比南缘接触带蚀变温度高,北缘接触带热液流体蚀变温度具有由中西部向东部不断降低、由接触带向接触带外围不断降低的特点。总之,白杨河铀矿区深部热液流体活动至少存在"直流型"和"分流型"2种典型的形式,前者反映热液流体活动温度自深部→接触带→浅部不断降低的特点,后者反映热液流体活动温度自接触带分别向上和向下逐渐降低的特点。此外,文章还对热液流体活动与铀矿化关系,以及矿区外围铀矿找矿方向进行了分析。     

Hydrothermal alterations in the Echassières granitic cupola (Massif central,france)

Detailed petrographic and mineralogic investigations of an albite-lepidolite granite at Echassières (Massif Central, France; scientific deep drill program) shows the existence of hydrothermal stages which are closely related to the magmatic and structural history. According to fluid inclusion data, K-Ar datations and ¹⁸O/¹⁶O-D/H compositions of secondary minerals, two successive hydrothermal periods have been recognized. The early one (273–268 million years) produced a series of aluminous phyllosilicates: muscovite, pyrophyllite, donbassite, tosudite, kaolinite which are observed as vein deposits (<10 mm wide) and alteration products of primary minerals in wall-rocks. The vein system was sealed by monomineralic assemblages during a cooling period (400–150°C). This early hydrothermal alteration stage was controlled by interactions of rock with low salinity (1–10 wt% NaCl equivalent) fluids expelled from the granitic body during the cooling processes. The chemical properties of these fluids were the following: low pH, very low Mg and Fe and high Li, Na and K contents. Thermodynamic calculations show that the sequence pyrophyllite, Li-bearing donbassite, tosudite is mostly temperature dependent. From the chemical composition of secondary minerals and isotopic data it can be deduced that these fluids, which have a meteoric origin, have been expelled from the granite body during its cooling period and after interaction with it at high temperature. The late hydrothermal stage corresponds to deposits of fluorite and Fe-Mg rich illite (151 million years) in subvertical fractures. Temperature conditions did not exceed 250° C and fluids came through the surrounding metamorphic rocks into the granitic body. IIlite/smectite mixed-layer minerals have been identified in subvertical fractures which were opened during Tertiary periods. In the host micaschists, successive hydrothermal alterations took place during the cooling of the Beauvoir granite. Early magmatic fluids interacted with these micaschists. Locally, the metamorphic assemblage is replaced by a metasomatic one. Secondary topaz and (F, Li)-rich mica crystals were formed over a range of 450 of 150°C. Later hydrothermal fluids reacted with the country rocks to form phengite-biotite, chlorite-illite and kaolinite over a range of 300 to 150°C. Illite/smectite mixed-layer minerals crystallized in the roof micaschists and within the Beauvoir granite during the Tertiary alteration period. Meteoric water invaded open fractures producing supergene alteration mineral assemblages.     

Gold Mineralization and Occurrence of Sinter in the Hoshino Area, Fukuoka Prefecture, Japan

Abstract. Several epithermal gold deposits occur in the Hoshino area, which is located in the western end of the late Cenozoic Hohi volcanic zone, north‐central Kyushu, Japan. The area is characterized by intermediate to felsic extrusive rocks of Pliocene age. In the Hoshino area, the shallow manifestation of the hydrothermal activity is exposed on the surface. Several outcrops of sinter are still preserved on the top of hydro thermally altered volcanic rocks, and high‐grade gold‐bearing quartz veins occur on the surface at lower levels. The hydrothermal alteration resulted into well‐developed alteration zones. The zonal alteration pattern, primarily of near‐neutral pH type, is characterized by the outer smectite zone of a lower temperature, and the inner mixed layer minerals zone of a higher temperature. Quartz vein‐related or fracture‐controlled alteration, is represented by the occurrence of interstratified illite/smectite and K‐feldspar, suggesting a potassium‐enriched alteration. The mineralization in the Hoshino area is recognized on the surface by the occurrence of gold‐bearing quartz veins distributed mainly in the mixed layer minerals zone. The fracture system related to the gold mineralization is mainly characterized by NW‐SE trend. The alteration pattern and the mineralogical composition of the veins suggest that the mineralizing fluids had near‐neutral pH and the mineralization is of low‐sulfidation‐type. Fluid inclusion data and textures observed in quartz veins indicate that gold precipitated during boiling. The chemical composition of quartz veins shows that high‐grade gold‐bearing quartz veins are characterized by higher content of Al₂O₃, K₂O and Rb. Several outcrops of silica‐sinters are distributed on the top of the mixed layer minerals zone. Although their structures are not very well preserved, because of later silicification, the Hoshino sinters still show characteristic textures identical to those observed in modern sinters, such as the presence of plant fossil incorporated into the sinters, the strongly developed depositional laminations and the columnar structures perpendicular to the depositional surfaces. Quartz is the only silica mineral constituting the Hoshino sinters presently. The conversion of amorphous silica into quartz was probably governed by higher temperatures resulting from later hydrothermal activity. This later hydrothermal activity is reflected by the intense silicification affecting mainly the lower parts of the sinters and also by the presence of quartz veins cutting the sinters. The distribution of sinters in the Hoshino area is very significant. The presence beneath the sinters of concealed high‐grade gold‐bearing quartz veins should be highly considered and exploration work is strongly suggested.     

Prehnite–pumpellyite to greenschist facies transition, Smartville Complex, near Auburn, California

The Smartville Complex is a late Jurassic, rifted volcanic arc in the northern Sierra Nevada, California. Near Auburn, California, it consists of a lower volcanic unit, dominated by basaltic flows, and an upper volcanic unit of andesitic volcaniclastic rocks, both of which have been intruded by dykes and irregular bodies of diabase. These rocks contain relict igneous minerals, and the metamorphic minerals albite, chlorite, quartz, pumpellyite, prehnite, epidote, amphibole, titanite, garnet, biotite, K-feldspar, white mica, calcite, and sulphide and oxide minerals.
Prehnite–pumpellyite (PrP), prehnite–actinolite (PrA), and greenschist (GS) zones have been identified. The pumpellyite-out isograd separates the PrP and PrA zones, and the prehnite-out isograd separates the PrA and GS zones. The minerals Ab + Qtz + Mt + Tn are common to most assemblages in all three zones. The MgO/(MgO + FeO) ratio of the effective bulk composition has an important and systematic effect on the observed mineral assemblages in the PrP zone. Prehnite-bearing assemblages contain the additional minerals, Pmp + Amp + Ep + Chl in MgO-rich rocks, and either Pmp + Ep + Chl or Amp + Ep + Chl in less magnesian rocks. Subcalcic to calcic amphibole is common in the PrP zone. The mineral assemblage Prh + Act + Ep + Chl, without Pmp, characterizes the PrA zone, and the mineral assemblage Act + Ep + Chl, without Prh or Pmp, characterizes the GS zone. The disappearance of pumpellyite and prehnite occurred by continuous reactions.
The sequence of mineral assemblages was produced by burial metamorphism at P–T conditions of 300° 50°C at approximately 2.5 ± 0.5 kbar. During metamorphism, the composition of the fluid phase was nearly 100% H₂O and the oxygen fugacity was between the hematite–magnetite and quartz–fayalite–magnetite buffers.     

基于GF-5高光谱数据的蚀变矿物填图及地质应用——以甘肃省瓜州县花牛山地区为例

[研究目的]"图谱合一"的GF-5 AHSI国产卫星高光谱数据可以根据光谱精细特征进行蚀变矿物的直接识别,一次过境成像即可获取宽幅大面积高光谱数据,能够为陆域自然资源调查提供重要的数据支撑,本文开展了GF-5高光谱数据蚀变矿物提取、分析与验证研究,以期推动国产卫星高光谱数据在地质领域的深化应用.[研究方法]建立了GF-...     

Borosilicate alteration associated with U-Mo-Zn and Ag-Au-Zn deposits in volcanic rocks

Borosilicate alteration is developed in many uranium-molybdenum-zinc deposits in volcanic piles such as at Novazza (Italy), Novoveska Huta (Slovakia), Ben Lomond (Queensland, Australia), the Saar-Nahe basin (Germany), the Shengyuan district (China), and at many deposits in Russia. These uranium-molybdenum-zinc deposits share similar styles of alteration with silver-gold-zinc deposits in volcanic rocks, like those of the Humboldt Range (Nevada, USA), the Basin district (Montana, USA) and Equity Mine (British Columbia, Canada). In thick volcanic piles, fluid-rock interaction commonly develops under the influence of convective hydrothermal systems. In borosilicate-bearing systems the alteration is characterized by changes in borosilicate mineralogy and chemistry under mesothermal conditions. Zonal distribution of different compositions of tourmaline can be observed including regular increases of alkali deficiency in the X(3a) site from peripheral (chlorite-epidote-prehnite) to internal (sericite and/or pyrophyllite) alteration zones. In the central part of these systems tourmaline is absent and the alteration assemblage is characterized by an andalusitedumortierite-quartz association. Such changes in mineral assemblage are related to an increase in temperature from 250 to 350–400 °C and to decreases in pH and Eh, from the outer toward the inner parts of the hydrothermal system. The zonation of the borosilicate-bearing alteration systems is similar to that of aluminosilicate-bearing hydrothermal systems (muscovite, pyrophyllite, andalusite) developed in volcanic rocks. A kaolinite-alunitejasperoid assemblage is generally associated with the borosilicate alteration, and seems related to a late hydrothermal phase mainly located in graben structures.     

Intermediate sodium–potassium mica in hydrothermally altered rocks of the Waterloo deposit,Australia: a combined SEM-EMP-XRD-TEM study

Metastable intermediate Na–K mica represents a product of hydrothermal alteration in volcanic rocks from the alteration halo of the Waterloo massive sulfide deposit, Australia. The XRD pattern of this solid solution between paragonite and muscovite is characterized by a rational series of basal reflections with d values intermediate between the end members. Transmission electron microscopy revealed that the intermediate Na–K mica forms thick stacks that belong to a two-layer polytype. Na-rich intermediate Na–K mica typically occurs together with paragonite whereas K-rich intermediate Na–K mica is intergrown with muscovite. The intermediate Na–K mica is interpreted to have formed as a result of the incomplete transformation of K-rich mica to Na-rich mica through dissolution and recrystallization processes driven by compositional changes of the hydrothermal fluids interacting with the volcanic rocks. Alteration must have proceeded under non-equilibrium conditions because the composition of the solid solution falls into the miscibility gap separating paragonite and muscovite.Editorial responsibility: T.L. Grove     

Geological Fluid Mapping in the Tongling Area： Implications for the Paleozoic Submarine Hydrothermal System in the Middle-Lower Yangtze Metallogenic Belt, East China

The Tongling area is one of the 7 ore-cluster areas in the Middle-Lower Yangtze metallogenic belt,East China,and has tectonically undergone a long-term geologic history from the late Paleozoic continental rifting,through the Middle Triassic continent-continent collision to the Jurassic-Cretaceous intracontinental tectono-magmatic activation.The Carboniferous sedimentary-exhalative processes in the area produced widespread massive sulfides with ages of 303-321 Ma,which partly formed massive pyrite-Cu deposits,but mostly provided significant sulfur and metals to the skarn Cu mineralization associated with the Yanshanian felsic intrusions. To understand the Carboniferous submarine hydrothermal system,an area of about 1046 km~2 was chosen to carry out the geological fluid mapping.Associated with massive sulfide formation,footwall sequences 948 m to 1146m thick,composed of the Lower Silurian-Upper Devonian sandstone,siltstone and thin-layered shale,were widely altered.This hydrothermal alteration is interpreted to reflect large- scale hydrothermal fluid flow associated with the late Paleozoic crustal rifting and subsidence.Three hydrothermal alteration types,i.e.,deep-level semiconformable silicification(S_1),fracture-controlled quartz-sericite-pyrite alteration(S_(2-3)),and upper-level sub-discordant quartz-sericite-chlorite alteration(D_3),were developed to form distinct zones in the mapped area.About 50-m thick semiconformable silicification zones are located at～1-km depth below massive sulfides and developed between an impermeable shale caprock(S_1)and the underlying Ordovician unaltered limestone. Comparisons with modern geothermal systems suggest that the alteration zones record a sub-seafloor aquifer with the most productive hydrothermal fluid flow.Fracture-controlled quartz-sericite-pyrite alteration formed transgressive zones,which downward crosscut the semiconformable alteration zones, and upwards grade into sub-discordant alteration zones that enveloped no economic stringer- stockwork zones beneath massive sulfides.This transgressive zone likely marks an upflow path of high- flux fluids from the hydrothermal aquifer.Lateral zonation of the sub-discordant alteration zones and their relationship to overlying massive sulfide lenses suggest lateral flows and diffusive discharging of the hydrothermal fluids in a permeable sandstone sequence.Three large-sized,14 middle-small massive sulfide deposits,and 40 massive sulfide sites have been mapped in detail.They show regional strata- bound characters and two major styles,i.e.,the layered sheet plus strata-bound stringer-style and the mound-style.Associated exhalite and chemical sedimentary rock suites include(1)anhydrite-barite,(2) jasper-chert,(3)Mg-rich mudstone-pyrite shale,(4)barite lens,(5)siderite-Fe-bearing dolomite,and (6)Mn-rich shale-mudstone,which usually comprise three sulfide-exhalite cyclic units in the area. The spatial distribution of these alteration zones(minerals)and associated massive sulfides and exhalites,and regional variation inδ~(34)S of hydrothermal pyrite and inδ~(18)O-δ~(34)C of hanging wall carbonates,suggest three WNW-extending domains of fluid flow,controlled by the basement faults and syn-depositional faults.Each fluid domain appears to have at least two upflow zones,with estimated even spacing of about 5-8 km in the mapped area.The repeated appearance of sulfide-sulfate or sulfide-carbonate rhythmic units in the area suggests episodically venting of fluids through the upflow conduits by breaking the overlying seals of the hydrothermal aquifer.     

Geochemical Aspects of Hypogene Hydrothermal Alteration Zones in the Chol-qeshlaghi Area, NW Iran: Constrains on REEs

Chol-qeshlaghi altered area lies in the northwestern part of the post-collisional Urumieh-Dokhtar magmatic arc, NW Iran. Pervasive silicic, argillic, phyllic and propylitic altered zones appears to be intimately affiliated to the fluids derivative of upper Oligocene Khankandi granodiorite. This paper is dedicated to the identification of geochemical characteristics of hydrothermal alterations, focusing on the determination of the mass gains and losses of REEs, to gain significant insights regarding the chemical exchanges prevailed between the host rocks and hydrothermal fluids. The low pH and high activity of SO_4~(-2) ligands in silicic alteration fluids, resulting in depletion of entire REEs. Decreasing of LREEs appeared in argillic zone may attributed to reduce in adsorption ability of clay minerals in low pH; whereas HREEs enrichment in phyllic zone was inclined to put it down to the abundance of sericite(± Fe oxides). A significant reduction of Eu/Eu* ratio in silicic zone can be attributed to negligible sulfides and clay minerals as some effective agents in adsorption of released Eu~(+2). Factors such as changes in pH, the abundance of absorptive neomorph mineral phases, activity of ligands play an important role in controlling the distribution and concentration of REEs in Chol-qeshlaghi alteration system.     

Mass Transfer, Oxygen Isotopic Variation and Gold Precipitation in Epithermal System: A Case Study of the Hishikari Deposit, Southern Kyushu, Japan

Abstract: Transportation of various kinds of elements occurred in wall rocks (Quaternary andesites) during the hydrothermal alteration accompanied by the Hishikari epithermal gold mineralization. For example, K₂O and MgO contents of wall rocks decrease away from the gold-quartz veins, while (CaO+Na₂O) content increases, and SiO₂ content is variable near the veins. Hydrothermal alteration zoning and bulk compositional variations in wall rocks suggest that the mixing of hydrothermal solution and acidic groundwater took place an important role as the cause for the hydrothermal alteration and bulk compositional variations. The relationship between dissolved silica concentration and temperature of hydrothermal solution mixed with groundwater is obtained based on precipitation kinetics-fluid flow–mixing model, and the computed results are compared with the distribution of SiO₂ minerals (quartz and cristobalite) in the hydrothermal alteration zones. This comparison suggests that the most reasonable flow rate of fluids migrating through hydrothermal alteration zones, and A/M (A: surface area of rocks interacting with fluid, M: mass of fluid) are estimated to be ca. 10^-4.2 m/sec, and ca. 0.10 m²/kg, respectively. The mixing of two fluids (hydrothermal solution and acidic groundwater) can also explain δ¹⁸O zoning in the altered country rocks, hydrothermal alteration zoning from K-feldspar through K-mica to kaolinite from the center (veins) to margin, and deposition of gold.     

Reflectance spectroscopic characterisation of mineral alteration footprints associated with sediment-hosted gold mineralisation at Mt Olympus (Ashburton Basin,Western Australia)

Hydrothermal ore deposits are typically characterised by footprints of zoned mineral assemblages that extend far beyond the size of the orebody. Understanding the mineral assemblages and spatial extent of these hydrothermal footprints is crucial for successful exploration, but is commonly hindered by the impact of regolith processes on the Earth's surface. Hyperspectral drill core (HyLogger?-3) data were used to characterise alteration mineralogy at the Mt Olympus gold deposit located 35 km southeast of Paraburdoo along the Nanjilgardy Fault within the northern margin of the Ashburton Basin in Western Australia. Mineralogy interpreted from hyperspectral data over the visible to shortwave (400–2500 nm) and thermal (6000–14500 nm) infrared wavelength ranges was validated with X-ray diffraction and geochemical analyses. Spaceborne multispectral (ASTER) and airborne geophysical (airborne electromagnetic, AEM) data were evaluated for mapping mineral footprints at the surface and sub-surface. At the deposit scale, mineral alteration patterns were identified by comparing the most abundant mineral groups detected in the HyLogger data against lithology logging and gold assays. Potential hydrothermal alteration phases included Na/K-alunite, kaolin phases (kaolinite, dickite), pyrophyllite, white mica, chlorite and quartz, representing low-T alteration of earlier greenschist metamorphosed sediments. The respective zoned mineral footprints varied depending on the type of sedimentary host rock. Siltstones were mainly characterised by widespread white-mica alteration with proximal kaolinite alteration or quartz veining. Sandstones showed (1) distal white mica, intermediate dickite, and proximal alunite + kaolinite or (2) widespread white-mica alteration with associated intervals of kaolinite. In both, sandstones and siltstones, chlorite was distal to gold mineralisation. Conglomerates showed distal kaolinite/dickite and proximal white-mica/dickite alteration. Three-dimensional visualisation of the gold distribution and spatially associated alteration patterns around Mt Olympus revealed three distinct categories: (1) several irregular, poddy, SE-plunging zones of >0.5 ppm gold intersected by the Zoe Fault; (2) sulfate alteration proximal to mineralisation, particularly on the northern side of the Mt Olympus open pit; and (3) varying Al^IVAl^VISi^IV_–1(Mg,Fe)^VI_–1 composition of white micas with proximity to gold mineralisation. Chlorite that developed during regional metamorphic or later hydrothermal alteration occurs distal to gold mineralisation. ASTER mineral mapping products, such as the MgOH Group Content used to map chlorite (±white mica) assemblages, showed evidence of correlation to mapped, local structural features and unknown structural or lithological contacts as indicated by inversion modelling of AEM data.     

Textural evidence for quartz and feldspar dissolution as a mechanism of formation for Maggs Pipe,Zaaiplaats tin mine,South Africa

Shallowly plunging and branching pipe systems in Lease and Bobbejaankop Granite at the Zaaiplaats mine are host to major tin mineralization. Detailed textural study of Maggs Pipe indicates that dissolution of the granite was a major process in the formation of open space which provided permeability for the passage of hydrothermal fluids, and sites for the precipitation of ore and gangue minerals. The pipe formation process initiates with the dissolution of granite quartz and subsequently extends to feldspar dissolution, particularly in the central portion of Maggs Pipe. Spaces created by mineral dissolution are filled by hydrothermal phases and the relict feldspar matrix becomes progressively more altered toward the centre of the pipe. The distribution of alteration and infill minerals defines a zoning pattern which, from the outer margin to the central core, includes calcite-quartz, chlorite (± cassiterite, albite, fluorite) and synchisite-calcite zones. It is postulated that quartz and feldspar dissolution resulted from interaction between the granite and hydrothermal fluids containing alkali-chloride, -fluoride or -carbonate complexes which had separated from the granite magma during crystallization. Preliminary observations on several other pipes at Zaaiplaats indicate that quartz and feldspar dissolution was a major procress in forming the pipe systems.     

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.     

Magnetic susceptibility of volcanic rocks in geothermal areas: application potential in geothermal exploration studies for identification of rocks and zones of hydrothermal alteration

Magnetic susceptibility and petrographic studies of drilled rock cuttings from two geothermal wells (Az-26 and Az-49) of the important electricity-generating geothermal system, Los Azufres, Mexico, were carried out to determine the relation between the magnetic susceptibility of rocks, the concentration of magnetic minerals and hydrothermal alteration. For this purpose, low-frequency magnetic susceptibility (χ _lf) was measured and compared its distribution trends with those of magnetic and Fe–Mg silicate minerals, and with the extent of hydrothermal alteration in rocks of the two geothermal wells. The study indicates a decrease in χ _lf values with depth in the two geothermal wells corresponding with: (1) an increase in the reservoir temperature and hydrothermal alteration; and (2) a decrease in the concentrations of Fe–Mg silicates and opaque minerals. The data suggest that ferromagnesian minerals and opaque minerals like ilmenite are the main contributors to the χ _lf of rocks. The decrease in χ _lf, ilmenite, and Fe–Mg mineral contents with an increase in the hydrothermal alteration degree, pyrite and haematite contents suggests the hydrothermal alteration of ilmenite and Fe–Mg minerals (characteristic of high χ _lf values) to pyrite, haematite and other opaque minerals (with low χ _lf values). The interaction of hydrothermal fluids with rocks results in the hydrothermal alteration of primary minerals. In a geothermal area, an anomaly of low magnetic susceptibility values of rocks in a homogenous litho unit characterized by high magnetic susceptibility may suggest hydrothermal alteration. Magnetic susceptibility can be a useful parameter, during the initial stages of geothermal exploration, in identifying hydrothermally altered rocks and zones of hydrothermal alteration both at the surface and from drilled wells in geothermal systems.