Epithermal precious metal deposits of Italy–an overview

Epithermal precious metal deposits have only quite recently been added to the metallogenic view of Italy. It was only in the late 1980s that two districts were recognized, in Sardinia and southern Tuscany, previously long known for a large variety of other commodities. The Sardinian epithermal precious metal district is associated with the Oligocene-Miocene Sardinian calc-alkaline magmatic cycle. The most relevant areas include the Au deposit at Furtei, already in production, and the Osilo prospect, where extensive exploration is under way. The deposit at Furtei contains at least six tonnes of gold metal. The mineralogy of the deposit and of the alteration assemblages is typical of the acid-sulfate (high-sulfidation) class of volcanic-hosted epithermal deposits. Fluids associated with alteration and mineralization have moderate temperatures (200–300 °C) and low salinities (less than 6% wt. NaCl equivalent); high-temperature, high-salinity fluids similar to porphyry-style systems also circulated at Furtei. At Osilo, a number of quartz veins containing up to several ppm Au have been identified. The alteration assemblage includes adularia and illite, i.e. is typical of low-sulfidation deposits. Fluid inclusion homogenization temperatures range from 198 to 270 °C, with salinities of less than 4% wt. NaCl equivalent. In southern Tuscany, a number of carbonate-hosted (“Carlin type”) gold showings occur at the edges of the geothermal fields of Larderello, Amiata and Latera, an area previously known for Sb mineralization. In fact, many showings coincide with former Sb mines, and stibnite, along with pyrite, is the most common sulfide mineral. Gold is typically invisible. Fluids hosted in a variety of minerals from these occurrences span a relatively large temperature range (132 to 245 °C), with constantly low salinities (less than 7% wt. NaCl equivalent). The onset of (presumably meteoric) hydrothermal fluid circulation can be ascribed to the emplacement of Neogene Tuscan magmatic rocks, but the ultimate source of gold remains speculative. Received: 13 October 1998 / Accepted: 5 February 1999     

Gangue mineral textures and fluid inclusion characteristics of the Santa Margarita Vein in the Guanajuato Mining District, Mexico

Successful exploration for mineral deposits requires tools that the explorationist can use to distinguish between targets with high potential for mineralization and those with lower economic potential. In this study, we describe a technique based on gangue mineral textures and fluid inclusion characteristics that has been applied to identify an area of high potential for gold-silver mineralization in the epithermal Ag-Au deposits at Guanajuato, Mexico. The Guanajuato mining district in Mexico is one of the largest silver producing districts in the world with continuous mining activity for nearly 500 years. Previous work conducted on the Veta Madre vein system that is located in the central part of this district identified favorable areas for further exploration in the deepest levels that have been developed and explored. The resulting exploration program discovered one of the richest gold-silver veins ever found in the district. This newly discovered vein that runs parallel to the Veta Madre was named the Santa Margarita vein. Selected mineralized samples from this vein contain up to 249 g/t of Au and up to 2,280 g/t Ag. Fluid inclusions in these samples show homogenization temperatures that range from 184 to 300°C and salinities ranging from 0 to 5 wt.% NaCl. Barren samples show the same range in homogenization temperature, but salinities range only up to 3 wt.% NaCl. Evidence of boiling was observed in most of the samples based on fluid inclusions and/or quartz and calcite textures. Liquid-rich inclusions with trapped illite are closely associated with high silver grades. The presence of assemblages of vapor-rich-only fluid inclusions, indicative of intense boiling or ??flashing??, shows the best correlation with high gold grades.     

红外显微镜红外光强度对测定不透明矿物中流体包裹体盐度的影响：以辉锑矿为例

红外显微镜是研究不透明矿物中流体包裹体的重要手段,但红外光发热可能对不透明矿物中流体包裹体的盐度和均一温度测定有影响。本文以辉锑矿中流体包裹体为例,系统评价了红外光强度对测定辉锑矿中流体包裹体冰点的影响。结果表明,红外光强度对辉锑矿中流体包裹体的冰点测定有显著影响,导致过高估计流体包裹体的盐度,并提出了解决这一问题的方法。     

The sandstone-hosted stratiform copper mineralization at Mwitapile and its relation to the mineralization at Lufukwe, Lufilian foreland, Democratic Republic of Congo

The Lufilian foreland is a triangular-shaped area located in the SE of the Democratic Republic of Congo and to the NE of the Lufilian arc, which hosts the well-known Central African Copperbelt. The Lufilian foreland recently became an interesting area with several vein-type (e.g., Dikulushi) and stratiform (e.g., Lufukwe and Mwitapile) copper occurrences. The Lufilian foreland stratiform Cu mineralization is, to date, observed in sandstone rock units belonging to the Nguba and Kundelungu Groups (Katanga Supergroup).The Mwitapile sandstone-hosted stratiform Cu prospect is located in the north eastern part of the Lufilian foreland. The host rock for the Cu mineralization is the Sonta Sandstone of the Ngule Subgroup (Kundelungu Group). A combined remote sensing, petrographic and fluid inclusion microthermometric analysis was performed at Mwitapile and compared with similar analysis previously carried out at Lufukwe to present a metallogenic model for the Mwitapile- and Lufukwe-type stratiform copper deposits. Interpretation of ETM+ satellite images for the Mwitapile prospect and the surrounding areas indicate the absence of NE–SW or ENE–WSW faults, similar to those observed controlling the mineralization at Lufukwe. Faults with these orientations are, however, present to the NW, W, SW and E of the Mwitapile prospect. At Mwitapile, the Sonta Sandstone host rock is intensely compacted, arkosic to calcareous with high silica cementation (first generation of authigenic quartz overgrowths). In the Sonta Sandstone, feldspar and calcite are present in disseminated, banded and nodular forms. Intense dissolution of these minerals caused the presence of disseminated rectangular, pipe-like and nodular dissolution cavities. Sulfide mineralization is mainly concentrated in these cavities. The hypogene sulfide minerals consist of two generations of pyrite, chalcopyrite, bornite and chalcocite, separated by a second generation of authigenic quartz overgrowth. The hypogene sulfide minerals are replaced by supergene digenite and covellite. Fluid inclusion microthermometry on the first authigenic quartz phase indicates silica precipitation from an H₂O–NaCl–CaCl₂ fluid with a minimum temperature between 111 and 182 °C and a salinity between 22.0 and 25.5 wt.% CaCl₂ equiv. Microthermometry on the second authigenic quartz overgrowths and in secondary trails related to the mineralization indicate that the mineralizing fluid is characterized by variable temperatures (Th = 120 to 280 °C) and salinities (2.4 to 19.8 wt.% NaCl equiv.) and by a general trend of increasing temperatures with increasing salinities.Comparison between Mwitapile and Lufukwe indicates that the stratiform Cu mineralization in the two deposits is controlled by similar sedimentary, diagenetic and structural factors and likely formed from a similar mineralizing fluid. A post-orogenic timing is proposed for the mineralization in both deposits. The main mineralization controlling factors are grain size, clay and pyrobitumen content, the amount and degree of feldspar and/or calcite dissolution and the presence of NE–SW to ENE–WSW faults. The data support a post-orogenic fluid-mixing model for the Mwitapile- and Lufukwe-type sandstone-hosted stratiform Cu deposits, in which the mineralization is related to the mixing between a Cu-rich hydrothermal fluid, with a temperature up to 280 °C and a maximum salinity of 19.8 wt.% NaCl equiv., with a colder low salinity reducing fluid present in the sandstone host rock. The mineralizing fluid likely migrated upwards to the sandstone source rocks along NE–SW to ENE–WSW orientated faults. At Lufukwe, the highest copper grades at surface outcrops and boreholes were found along and near to these faults. At Mwitapile, where such faults are 2 to 3 km away, the Cu grades are much lower than at Lufukwe. Copper precipitation was possibly promoted by reduction from pre-existing hydrocarbons and non-copper sulfides and by the decrease in fluid salinity and temperature during mixing. Based on this research, new Cu prospects were proposed at Lufukwe and Mwitapile and a set of recommendations for further Cu exploration in the Lufilian foreland is presented.     

Volcanic stratigraphy, structural controls, and mineralization in the san cristobal Ag–Zn–Pb deposit, southern bolivia

The Late Miocene San Cristobal Ag–Zn–Pb deposit represents syngenetic and epigenetic mineralization with low- and high-sulfidation characteristics. Rocks in the deposit are characterized by barren dacitic ring fracture domes, mineralized resurgent rhyodacite domes, strongly altered and mineralized tuffaceous lacustrine sedimentary rocks, and an extensive crystal-lithic tuff debris apron. The ore body is hosted by intracauldron sedimentary and volcanic rocks and genetically associated breccias. Fluid inclusion data suggest that silver, lead, and zinc were transported as chloride complexes and precipitated by cooling in veins from <5 wt.% NaCl eq. fluids at 170–215 °C. Silver that was spatially, and perhaps temporally, associated with an episode of rhyodacite resurgence may have been transported as a chloride complex and precipitated by increased H₂S activity or increased fluid pH. Although San Cristobal represents a major silver resource, the occurrence of stratiform wurtzite and sphalerite in cauldron-hosted sedimentary rocks represents a syngenetic component of mineralization that is very rare in continental caldera-associated epithermal deposits, which contributes to San Cristobal's significance as a zinc resource.     

Tectonically driven fluid flow and associated low-grade metamorphism during the Alpine compression in the eastern Iberian Chain (Spain)

Fluid inclusions and clay mineralogy of the Permo-Triassic rocks from the Espina and Espadà Ranges (SE Iberian Chain, Spain) have been investigated to establish their relationship with hydrothermal fluid circulation during the Alpine Orogeny. Primary fluid inclusions in quartz-filled tension gashes in Permo-Triassic sandstones reveal maximum temperatures around 230 °C and very constant salinities of 8.5% wt. eq. NaCl. Secondary fluid inclusions found in quartz from the Santonian Ba–Cu–Hg deposits show similar compositional and thermodynamic characteristics, denoting an Alpine recrystallization. Clay mineral composition of Permo-Triassic mudrocks is characterized by pyrophyillite, indicating low-grade metamorphic conditions. Field observations and experimental data suggest that the crystallization of quartz in tension gashes, the formation of secondary fluid inclusions and the development of the metamorphism are contemporaneous and related to fluid circulation during the Alpine compression. Fluid flow took place along the Hercynian fault system that was reactivated during the Mesozoic rift stage and inverted during the Alpine deformation.     

Genesis of the Ciemas Gold Deposit and Relationship with Epithermal Deposits in West Java, Indonesia: Constraints from Fluid Inclusions and Stable Isotopes

The Ciemas gold deposit is located in West Java of Indonesia,which is a Cenozoic magmatism belt resulting from the Indo-Australian plate subducting under the Eurasian plate.Two different volcanic rock belts and associated epithermal deposits are distributed in West Java:the younger late Miocene-Pliocene magmatic belt generated the Pliocene-Pleistocene epithermal deposits,while the older late Eocene-early Miocene magmatic belt generated the Miocene epithermal deposits.To constrain the physico-chemical conditions and the origin of the ore fluid in Ciemas,a detailed study of ore petrography,fluid inclusions,laser Raman spectroscopy,oxygen-hydrogen isotopes for quartz was conducted.The results show that hydrothermal pyrite and quartz are widespread,hydrothermal alteration is well developed,and that leaching structures such as vuggy rocks and extension structures such as comb quartz are common.Fluid inclusions in quartz are mainly liquid-rich two phase inclusions,with fluid compositions in the NaCl-H20 fluid system,and contain no or little CO_2.Their homogenization temperatures cluster around 240℃-320℃,the salinities lie in the range of 14-17 wt.%NaCl equiv,and the calculated fluid densities are 0.65-1.00 g/cm~3.The values of δ~(18)O_(H2O-VSMOW)for quartz range from +5.5‰ to +7.7‰,the δD_(VSMOW) of fluid inclusions in quartz ranges from-70‰ to-115‰.All of these data indicate that mixing of magmatic fluid with meteoric water resulted in the formation of the Ciemas deposit.A comparison among gold deposits of West Java suggests that Miocene epithermal ore deposits in the southernmost part of West Java were more affected by magmatic fluids and exhibit a higher degree of sulfldation than those of Pliocene-Pleistocene.     

Regional stratigraphy and distribution of epigenetic stratabound celestine, fluorite, barite and Pb–Zn deposits in the MVT province of northeastern Mexico

Northeastern Mexico hosts numerous epigenetic stratabound carbonate-hosted low-temperature hydrothermal deposits of celestine, fluorite, barite and zinc-lead, which formed by replacement of Mesozoic evaporites or carbonate rocks. Such deposits can be permissively catalogued as Mississippi Valley-type (MVT) deposits. The deposits studied in the state of Coahuila are associated with granitic and metasedimentary basement highs (horsts) marginal or central to the Mesozoic Sabinas Basin. These horsts controlled the stratigraphy of the Mesozoic basins and subsequently influenced the Laramide structural pattern. The Sabinas Basin consists of ~6,000-m-thick Jurassic to Cretaceous siliciclastic, carbonate and evaporitic series. The MVT deposits are mostly in Barremian and in Aptian-Albian to Cenomanian formations and likely formed from basinal brines that were mobilized during the Laramide orogeny, although earlier diagenetic replacement of evaporite layers (barite and celestine deposits) and lining of paleokarstic cavities in reef carbonates (Zn–Pb deposits) is observed. Fluid inclusion microthermometry and isotopic studies suggest ore formation due to mixing of basinal brines and meteoric water. Homogenization temperatures of fluid inclusions range from 45°C to 210°C; salinities range from 0 to 26 wt.% NaCl equiv., and some inclusions contain hydrocarbons or bitumen. Sulfur isotope data suggest that most of the sulfur in barite and celestine is derived from Barremian to Cenomanian evaporites. Regional geology and a compilation of metallogenic features define the new MVT province of northeastern Mexico, which comprises most of the state of Coahuila and portions of the neighboring states of Nuevo León, Durango and, perhaps extends into Zacatecas and southern Texas. This province exhibits a regional metal zonation, with celestine deposits to the south, fluorite deposits to the north and barite and Zn–Pb deposits mostly in the central part.     

Copper mineralization around the Ahar batholith, north of Ahar (NW Iran): Evidence for fluid evolution and the origin of the skarn ore deposit

This investigation presents and interprets fluid inclusion data from different lithological units of the Cu skarn deposits at Mazraeh, north of Ahar, Azarbaijan, NW Iran. The results provide an assessment of the P–T conditions and mineral–fluid evolution and suggest new exploration parameters. Five types of inclusions are recognized from quartz and garnet. The temperature of homogenization of Type I inclusions with daughter minerals halite and sylvite ranges from 312° to 470 °C with total salinity of 52 to 63 wt.% NaCl equiv.; Type II and III inclusions with halite have homogenization temperatures of 230° to 520 °C and salinity of 31 to 50 wt.% NaCl equiv. The salinity of Types IV and V biphase (liquid + vapor) inclusions, based on their final ice melting temperature, varies between 10.2 to 20.8 wt.% NaCl equiv. T_h vs. salinity plots of inclusions show that the salinity of the fluids correlates positively with temperature. The inclusions formed at low pressure. Changes in the temperature and salinity of the fluids can be reconstructed from the inclusions. Highly saline, high-temperature fluids were most abundant during the main chalcopyrite ore-forming phase in the skarn and mineralized quartz veins. Low-salinity aqueous fluids were abundant in barren veins, in which there is no evidence for early hot high-salinity brine, and might have resulted from late-stage dilution and mixing of hydrothermal fluids with meteoric water. Based on petrographic features and fluid-inclusion data, early-stage magnetite deposition is related to boiling of fluid at temperatures of about 500 °C. At a later stage, boiling at temperatures of around 320° to 400 °C favored the deposition of sulfides and Fe mobility was decreased at these lower temperatures. The following inclusion characteristics may be used as exploration parameters in the Mazraeh area. (i) Presence of high-temperature, salt-bearing inclusions, with T_h between 300 and 500 °C; (ii) High-salinity fluid inclusions; and (iii) Inclusions showing evidence of boiling of the fluid. In addition, the presence of magnetite is an important exploration parameter.     

Fluid inclusion study constraining the hydrothermal evolution of caldera-forming volcanic systems in the Sengan Area, Northern Honshu, Japan

Summary The hydrothermal evolution of the Sengan geothermal area, Northern Honshu, was studied using fluid inclusion microthermometry. Sengan is one of the most active geothermal areas in Japan, and it is dominated by andesitic to dacitic volcanic rocks of Pliocene and Quaternary age. Fluid inclusions were studied in hydrothermal minerals (quartz, calcite, anhydrite, wairakite) and in fractured igneous quartz phenocrysts from core samples retrieved from five geothermal exploration wells, which penetrated Quaternary and Tertiary formations, and intrusive rocks in or around the calderas. A caldera-hosted hydrothermal system was heated by a shallow intrusion, which produced hypersaline fluids. During the early stages some heterogeneity in fluid composition occurred, but during peak activity of the hydrothermal system, the fluid was dominantly low-salinity, and most likely of meteoric origin. We have reconstructed, by means of fluid inclusion studies, the structure of an extinct hydrothermal system in a Pliocene caldera filled with ignimbrites, and of an active hydrothermal system which is now being heated under the northern slope of Yakeyama volcano. Based on inclusion data, the aqueous fluids that circulated in the hydrothermal system ranged from very low saline (0–7 wt.% NaCl equiv.) to hypersaline (up to 60 wt.% NaCl equiv.), with temperatures ranging from 130 to 400 °C.     

Evolution of mineralizing fluids in the Zn–Pb–Cu(–Ag ± Au) skarn and epithermal deposits of the world-class San Martín district, Zacatecas, Mexico

Mineralizing fluids at the San Martín skarn show an evolution characterized by prograde and retrograde associations. The prograde mineral associations consist of (1) a massive garnet zone, (2) a tremolite ± garnet zone, and (3) a late association of quartz, sphalerite, calcite and fluorite lining the vugs in the garnet zone. The fluids of the prograde associations exhibit decreasing temperatures of homogenization (Th) and variable salinities. The fluids of the massive garnet zone have salinities of 36 wt.% NaCl equiv. and Th of 645 to 570 °C, corresponding to pressures of 1055 bar. At the tremolite ± garnet zone, Th range from 438 to 354 °C. In the late association at the endoskarn, the following evolution can be drawn: (a) salinities of 50 to 42 wt.% NaCl equiv., and Th of 455 to 346 °C in quartz, (b) salinities of 46 wt.% NaCl equiv., and Th of 415 to 410 °C in sphalerite, (c) salinities of 50 to 37 wt.% NaCl equiv., and Th of 479 to 310 °C in calcite, (d) salinities of 33 to 28 wt.% NaCl equiv. and of 24 to 22 wt.% KCl in fluorite, and (e) two types of fluids with salinities of 2 and 39 wt.% NaCl equiv. and Th 344 and 300 °C, respectively, in later saccharoidal quartz segregations. The retrograde mineral associations comprise pervasive propylitic alteration to carbonization, and mantos with sulfides. Fluids in epidote have salinities of 7.6 wt.% NaCl equiv. and Th of 287 to 252 °C, and in calcite have salinities of 9.2 to 1 wt.% NaCl equiv. and Th of 188 to 112 °C. Fluids in the sulfide assemblages in the mantos have salinities of 8 to 3 wt.% NaCl equiv. and Th 300 °C, with corresponding pressures of 94 bar. Fluids in late epithermal veins close to the intrusive body have salinities of 10 to 5 wt.% NaCl equiv. and Th of 275 to 200 °C, and distal veins show salinities of 2 to 1 wt.% NaCl equiv. and Th of 160 °C.     

西藏多龙矿集区典型矿床(点)矿化特征与成矿作用对比研究

西藏多龙矿集区是近年来中国新发现的具有世界级潜力的铜金矿集区。该矿集区现已查明多不杂、波龙、拿若和铁格隆南4个大型-超大型矿床,并新发现地堡那木岗和拿顿矿点。文章对上述矿床(点)脉体、蚀变、矿化和流体特征开展了系统研究和对比。结果表明,多不杂、波龙和拿若矿床矿化类型以斑岩型为主,同时钾硅酸盐化、绢英岩化、青磐岩化等蚀变广泛发育,而铁格隆南矿床除上述蚀变类型外,还叠加有高级泥化蚀变,并发育与之相关的浅成低温热液型矿化。根据脉体特征对比和流体包裹体温压计算推测,上述4个矿床矿化类型的差异可能由剥蚀深度的差异所引起(前三者剥蚀深度约为2～3 km,后者约为1～1.5 km)。此外,地堡那木岗矿点蚀变类型以绢英岩化、泥化为主,该矿点发育与斑岩型金矿中类似的深色条带状石英脉,指示该地区可能存在斑岩型金矿。拿顿矿点为典型的高硫型浅成低温热液型矿化,铜金矿体赋存于角砾岩筒中。野外地质调查表明,上述矿点地表蚀变岩盖(Lithocaps)发育,并且蚀变岩盖空间分布位置与下伏铜金矿体表现出良好的匹配关系,可有效地指导找矿勘查工作。流体包裹体实验进一步表明,铜金元素在斑岩型矿化中的沉淀可能与温度降低和氧逸度的变化有关,而在浅成低温热液型矿化中的沉淀则受控于温度的降低和流体的不混溶作用。最后,在前人年代学研究基础上,结合本次实验结果构建了该地区与成矿作用有关的时空演化模型。     

西天山小于赞金矿成矿流体、成矿年代学特征及其地质意义

小于赞金矿床是产于新疆西天山也列莫顿盆地的浅成低温热液型金矿床,赋存于晚古生代大哈拉军山组火山岩中。矿石类型主要为蚀变岩型和石英脉型,主要发育硅化、黄铁绢英岩化、伊利石化、青磐岩化蚀变。流体成矿过程可分为3个阶段,分别为石英黄铁矿、石英玉髓黄铁矿和石英方解石黄铁矿阶段。小于赞金矿床流体包裹体类型单一,主要为水溶液包裹体,可分为纯液相水溶液包裹体(PL类)、富液相水溶液包裹体(L类)和富气相水溶液包裹体(V类)。石英黄铁矿阶段包裹体均一温度集中于130~190 ℃,盐度w(NaCleqv.)为0.2%~8.0%;石英玉髓黄铁矿阶段均一温度介于115~161 ℃,盐度w(NaCleqv.)为0.7%~3.4%;石英方解石黄铁矿阶段均一温度介于110~138 ℃,盐度w(NaCleqv.)为0.2%~3.4%。鉴于赋矿角砾凝灰岩的锆石U-Pb年龄为(353.8±1.8) Ma,且被下石炭统阿恰勒河组不整合覆盖,故可将小于赞金矿床的成矿时代限定在(353.8±1.8) Ma至早石炭世维宪期。锆石εHf(t)变化范围为+4.1~+8.4,平均值+6.1,两阶段Hf模式年龄tDM2变化范围为822~1 095 Ma,指示该区岩浆演化过程中有少量地幔物质的加入。综合考量矿床地质特征、流体包裹体特征和成矿时代,认为小于赞矿床为早石炭世低硫型浅成低温热液型金矿。     

不透明矿物流体包裹体红外显微测温研究：进展、存在问题与解决方案

流体包裹体是古地质流体的唯一直接记录,为反演地质过程涉及的物理化学条件等提供了最为重要的证据。流体包裹体是研究热液矿床成矿流体最有利手段,常规的测试对象为透明的脉石矿物,而矿石矿物中的流体包裹体更为直接、准确地记录了成矿流体信息。大多数矿石矿物在显微镜透射光下不透明,其捕获的流体包裹体研究需要使用配备有红外成像系统的显微镜进行。近20 年来,不透明矿物流体包裹体红外显微测温研究获得了长足的进展,但其中仍有一些问题尚未解决,制约了该方法的应用;同时,国内该研究领域正处在起步阶段,多个实验室已安装了红外测试仪器,但尚未开展系统的研究工作。因此,需要对该研究领域进行综述,探讨存在的问题,以促进该研究方法的发展。本文首先介绍了红外显微镜仪器工作原理及样品制备注意事项,阐述了影响不透明矿物红外透明度的因素,讨论了红外显微测温研究中的主要问题和难点,并针对测温过程中相变观察困难、红外光可能影响测温结果等问题提出可行的解决方案,最后论述了红外显微测温系统在矿床学领域的应用前景。     

Origin of coarse-crystalline calcite cement in Early Ordovician carbonate rocks, Ordos basin, northern China: Insights from oxygen and carbon isotopes and fluid inclusion microthermometry

The calcite cement in the Lower Ordovician Majiagou Formation in the Ordos basin in northern China can be subdivided into three groups based on preliminary results of oxygen and carbon isotopes and fluid inclusion microthermometry. Group 1 has low oxygen isotopes (− 14‰ to − 18‰), low T_h values (92–103 °C), and low salinities (1.7–4.9 wt.% NaCl equivalent) and is interpreted to have precipitated during early burial from porewater influenced by meteoric water. Group 2 has much higher oxygen isotope values (− 5‰ to − 8‰), which, coupled with the higher T_h values (136–151 °C), suggest that the calcite was precipitated from fluids that were significantly enriched in ¹⁸O, possibly resulting from fluid–rock reaction during burial. Group 3 occurring along fractures is characterized by high salinities (21–28 wt.% NaCl equivalent) and is interpreted to have been precipitated from locally preserved residual evaporitic brines. The occurrence of primary hydrocarbon inclusions and its low carbon isotopes (− 11‰ to − 15‰) suggest that precipitation of group 3 calcite took place in the presence of hydrocarbons.     

Gold and copper mineralization at the El Indio deposit, Chile

A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km² which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins.Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S₂ accompanied by variations in the activity of Te₂ during the Gold stage.Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO₂ vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization.Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration.     

Fault–valve action and vein development during strike–slip faulting: An example from the Ribeira Shear Zone, Southeastern Brazil

Fluid inclusion microthermometry and structural data are presented for quartz vein systems of a major dextral transcurrent shear zone of Neoproterozoic–Cambrian age in the Ribeira River Valley area, southeastern Brazil. Geometric and microstructural constraints indicate that foliation–parallel and extensional veins were formed during dextral strike–slip faulting. Both vein systems are formed essentially by quartz and lesser contents of sulfides and carbonates, and were crystallized in the presence of CO₂–CH₄ and H₂O–CO₂–CH₄–NaCl immiscible fluids following unmixing from a homogeneous parental fluid. Contrasting fluid entrapment conditions indicate that the two vein systems were formed in different structural levels. Foliation–parallel veins were precipitated beneath the seismogenic zone under pressure fluctuating from moderately sublithostatic to moderately subhydrostatic values (319–397 °C and 47–215 MPa), which is compatible with predicted fluid pressure cycle curves derived from fault–valve action. Growth of extensional veins occurred in shallower structural levels, under pressure fluctuating from near hydrostatic to moderately subhydrostatic values (207–218 °C and 18–74 MPa), which indicate that precipitation occurred within the near surface hydrostatically pressured seismogenic zone. Fluid immiscibility and precipitation of quartz in foliation–parallel veins resulted from fluid pressure drop immediately after earthquake rupture. Fluid immiscibility following a local pressure drop during extensional veining occurred in pre-seismic stages in response to the development of fracture porosity in the dilatant zone. Late stages of fluid circulation within the fault zone are represented dominantly by low to high salinity (0.2 to 44 wt.% equivalent NaCl) H₂O–NaCl–CaCl₂ fluid inclusions trapped in healed fractures mainly in foliation–parallel veins, which also exhibit subordinate H₂O–NaCl–CaCl₂, CO₂–(CH₄) and H₂O–CO₂–(CH₄)–NaCl fluid inclusions trapped under subsolvus conditions in single healed microcracks. Recurrent circulation of aqueous–carbonic fluids and aqueous fluids of highly contrasting salinities during veining and post-veining stages suggests that fluids of different reservoirs were pumped to the ruptured fault zone during faulting episodes. A fluid evolution trending toward CH₄ depletion for CO₂–CH₄–bearing fluids and salinity depletion and dilution (approximation of the system H₂O–NaCl) for aqueous–saline fluids occurred concomitantly with decrease in temperature and pressure related to fluid entrapment in progressively shallower structural levels reflecting the shear zone exhumation history.     

Origin of epithermal Ag–Au–Cu–Pb–Zn mineralization in Guanajuato, Mexico

The Guanajuato epithermal district is one of the largest silver producers in Mexico. Mineralization occurs along three main vein systems trending dominantly northwest–southeast: the central Veta Madre, the La Luz system to the northwest, and the Sierra system to the east. Mineralization consists dominantly of silver sulfides and sulfosalts, base metal sulfides (mostly chalcopyrite, galena, sphalerite, and pyrite), and electrum. There is a broad zonation of metal distribution, with up to 10 % Cu+Pb+Zn in the deeper mines along the northern and central portions of the Veta Madre. Ore occurs in banded veins and breccias and as stockworks, with gangue composed dominantly of quartz and calcite. Host rocks are Mesozoic sedimentary and intrusive igneous rocks and Tertiary volcanic rocks. Most fluid inclusion homogenization temperatures are between 200 and 300 °C, with salinities below 4 wt.% NaCl equivalent. Fluid temperature and salinity decreased with time, from 290 to 240 °C and from 2.5 to 1.1 wt.% NaCl equivalent. Relatively constant fluid inclusion liquid-to-vapor ratios and a trend of decreasing salinity with decreasing temperature and with increasing time suggest dilution of the hydrothermal solutions. However, evidence of boiling (such as quartz and calcite textures and the presence of adularia) is noted along the Veta Madre, particularly at higher elevations. Fluid inclusion and mineralogical evidence for boiling of metal-bearing solutions is found in gold-rich portions of the eastern Sierra system; this part of the system is interpreted as the least eroded part of the district. Oxygen, carbon, and sulfur isotope analysis of host rocks, ore, and gangue minerals and fluid inclusion contents indicate a hydrothermal fluid, with an initial magmatic component that mixed over time with infiltrating meteoric water and underwent exchange with host rocks. Mineral deposition was a result of decreasing activities of sulfur and oxygen, decreasing temperature, increasing pH, and, in places, boiling.     

黑龙江省铜山斑岩铜矿床流体包裹体研究

铜山大型铜矿床位于小兴安岭西北部,是中亚-兴蒙造山带北东段最著名的斑岩型铜矿床之一,矿体产于加里东期花岗闪长岩和中奥陶世多宝山组安山岩、凝灰岩中,铜矿化与硅化-绢云母化关系密切.流体包裹体研究表明,铜山铜矿床主要发育气液两相包裹体、含CO_2包裹体和含子矿物多相包裹体.成矿流体在形成过程中经历了早、中、晚3个阶段的演化.成矿早阶段发育气液两相水溶液包裹体和少量含子矿物多相包裹体,均一温度介于420℃～＞5500C之间,流体盐度介于13.72 wt%～59.76 wt%NaCl eqv之间;中阶段为铜山矿床的主成矿阶段,发育气液两相水溶液包裹体和含CO_2包裹体,均一温度为241℃～417℃,流体盐度介于2.96 wt%～14.04 wt%NaCl eqv之间,主成矿期成矿流体总体上属H_2O-CO_2-NaCl体系;晚阶段仅发育气液两相水溶液包裹体,均一温度为122℃～218℃,盐度介于3.71 wt%～15.96 wt%NaCl eqv之间,表明晚阶段有大气降水的混入.成矿早、中阶段的流体均为不混溶流体,流体沸腾作用是金属硫化物大量沉淀的主要机制.铜山矿床形成于陆缘弧环境.     

Fluid processes in the Tesbihdere base-metal-Au deposit: Implications for epithermal mineralization in the Biga Peninsula,NW Turkey

Tesbihdere is one of a number of spatially close epithermal Cu-Pb-Zn-Ag-Au deposits hosted by andesites and rhyolites, typical of deposits in the Biga peninsula. Microthermometry of fluid inclusions shows a wide range of temperatures, ～360–170°C, and salinities, ～10-0.5 wt.% NaCl, in the different deposits studied. Dilution of a moderately saline magmatic? fluid with meteoric water occurred at constant temperature indicating, the temperature of both fluids was controlled by the geological environment. Boiling was not a major factor, but did occur in very minor amounts. The large range of temperatures within individual samples can only reasonably be explained by variations from near lithostatic to hydrostatic pressure during vein and fracture opening. That this pressure decrease did not produce extensive boiling suggests that vein opening was gradual rather than aggressive, allowing the pressure and temperature decrease to follow a path close to the L-V boiling curve. P-T reconstruction places emplacement of these ore veins at between 300–500 m beneath the surface. Similarities of LA-ICPMS of fluid inclusions from Tesbihdere, Azitepe and Basmakci, supports the conclusion that they were part of the same contemporaneous mineralizing system. The fluids are dominated by Na, with the concentrations of K>Ca>Mg combined equivalent to the concentration of Na. The range of K/Na ratios is not consistent with the fluid inclusion temperatures as the calculated temperatures are significantly higher indicating the fluids were not close to equilibrium with the enclosing rocks. Elevated K concentrations are consistent with acid-sulphate waters in shallow epithermal systems. Ore metals Cu, Zn and Pb are present in significant concentrations ～500, 300 and 200 ppm respectively and the low Fe/Mn ratios are indicative of a relatively oxidising fluid. The negative δ ³⁴S values of sulphides are consistent with boiling and oxidising redox conditions.