Sulphide mineralization and wall-rock alteration in ophiolites and modern oceanic spreading centres

Massive and stockwork Fe-Cu-Zn (Cyprus type) sulphide deposits in the upper parts of ophiolite complexes represent hydrothermal mineralization at ancient accretionary plate boundaries. These deposits are probable metallogenic analogues of the polymetallic sulphide deposits recently discovered along modern oceanic spreading centres. Genetic models for these deposits suggest that mineralization results from large-scale circulation of sea-water through basaltic basement along the tectonically active axis of spreading, a zone of high heat flow. The high geothermal gradient above 1 to 2 km deep magma chambers emplaced below the ridge axis drives the convective circulation cell. Cold oxidizing sea-water penetrating the crust on the ridge flanks becomes heated and evolves into a highly reduced somewhat acidic hydrothermal solvent during interaction with basaltic wall-rock. Depending on the temperature and water/rock ratio, this fluid is capable of leaching and transporting iron, manganese, and base metals; dissolved sea-water sulphate is reduced to sulphide. At the ridge axis, the buoyant hydrothermal fluid rises through permeable wall-rocks, and fluid flow may be focussed along deep-seated fractures related to extensional tectonic processes. Metal sulphides are precipitated along channelways as the ascending fluid undergoes adiabatic expansion and then further cooling during mixing with ambient sub-sea-floor water. Vigorous fluid flow results in venting of reduced fluid at the sea-floor/sea-water interface and deposition of massive sulphide. A comparison of sulphide mineralization and wall-rock alteration in ancient and modern spreading centre environments supports this genetic concept.Massive sulphide deposits in ophiolites generally occur in clusters of closely spaced (< 1–5 km) deposits. Individual deposits are a composite of syngenetic massive sulphide and underlying epigenetic stockwork-vein mineralization. The massive sulphide occurs as concordant tabular, lenticular, or saucer-shaped bodies in pillow lavas and pillow-lava breccia; massive lava flows, hyalcoclastite, tuff, and bedded radolarian chert are less commonly associated rock types. These massive sulphide zones are as much as 700 m long, 200 m wide, and 50 m thick. The pipe-, funnel-, or keel-shaped stockwork zone may extend to a dehpth of 1 km in the sheeted-dike complex. Several deposits in Cyprus are confined to grabens or the hanging wall of premineralization normal faults.Polymetallic massive sulphide deposits and active hydrothermal vents at medium- to fast-rate spreading centres (the East Pacific Rise at lat. 21°N, the Galapagos Spreading Centre at long. 86°W, the Juan de Fuca Ridge at lat. 45°N., and the Southern Trough of Guaymas Basin, Gulf of California) have interdeposit spacings on a scale of tens or hundreds of metres, and are spatially associated with structural ridges or grabens within the narrow (< 5 km) axial valleys of the rift zones. Although the most common substrate for massive sulphide accumulations is stacked sequences of pillow basalt and sheet flows, the sea-floor underlying numerous deposits in Guaymas Basin consists of diatomaceous ooze and terrigenous clastic sediment that is intruded by diabase sills. Mound-like massive sulphide deposits, as much as 30 m wide and 5m high, occur over actively discharging vents on the East Pacific Rise, and many of these deposits serve as the base for narrow chimneys and spires of equal or greater height. Sulphides on the Juan de Fuca Ridge appear to form more widespread blanket deposits in the shallow axial-valley depression. The largest deposit found to date, along the axial ridge of the Galapagos Spreading Centre, has a tabular form and a length of 1000 m, a width of 200 m, and a height of 30 m.The sulphide assemblage in both massive and vein mineralization in Cyprus type deposits is characteristically simple: abundant pyrite or, less commonly, pyrrhotite accompanied by minor marcasite, chalcopyrite, and sphalerite. With few exceptions, the composition of massive sulphide ranges from 0.3 to 5 wt. % Cu, from 0.1 to 3 wt. % Zn, from 0.5 to 30 ppm Au, and from 1 to 50 ppm Ag. The only common gangue minerals — quartz, chlorite, calcite, and gypsum generally make up less than 10 percent of the massive zone.Sulphide assemblages in massive sulphide samples recovered from the Juan de Fuca Ridge (abundant sphalerite, wurtzite, and pyrite; minor marcasite, chalcopyrite, and galena), East Pacific Rise (abundant sphalerite, pyrite, and chalcopyrite; minor wurtzite, marcasite, and pyrrhotite), and Guaymas Basin (abundant pyrrhotite and sphalerite; minor chalcopyrite) contrast with ophiolitic deposits. Bulk analyses of two zinc-rich sulphide samples from the Juan de Fuca Ridge yield the following average values: Zn, 56.6 wt. %; Cu, 0.2 wt. %; Pb, 0.15 wt. %; Fe, 4.9 wt. %; Ag, 260 ppm; and Cd, 775 ppm. Other minerals precipitated with sulphides at hydrothermal-vent sites include anhydrite, barite, gypsum, Mg-hydroxysulphate-hydrate, talc, sulphur, and amorphous silica.Massive sulphide lenses in some Cyprus-type deposits are underlain by a silica-rich zone consisting of massive quartz, opaline silica, red jasper, or chert mixed with disseminated and veinlet Fe-Cu-Zn sulphides. Some deposits are overlain by ochre, a gossanous Mn-poor Fe-rich bedded deposit composed of goethite, maghemite, quartz, and finely disseminated sulphide. In the Solomon Islands, ochre is overlain by siliceous sinter containing anhydrite, barite, and sulphide; the sinter contains anomalous Ag, Au, Cu, Zn, and Hg, and grades upward into Fe-rich chert and manganiferous wad. Amorphous Fe-Mn deposits (umber) and Mn-bearing chert enriched in Ba, Co, Cu, Ni, Cr, Pb, and Zn are common features near the top of ophiolite sequences. Although their genetic relation to sulphide mineralization is uncertian, they probably formed during off-axis hydrothermal discharge.At modern, medium-rate spreading centres, thin blankets of unconsolidated hydrothermal sediment have been observed near hydrothermal sulphide deposits. Basalt fragments recovered with massive sulphide from the Juan de Fuca Ridge have surfaces coated with smectite, magnetite, hematite, opaline silica, and Fe---Mn-oxyhydroxides. Sediment mounds composed largely of nontronitic clay and hydrated Fe and Mn oxides, and more distal metalliferous (Fe, Mn, Cu, Ni, Pb, Zn) sediment on the flanks of ceanridges, are also products of off-axis hydrothermal processes.Pillow lavas, diabase dikes, and gabbro in ophiolite sequences, and deeper, layer 2 basalt and diabase recovered from oceanic ridges, are altered to greenschist-facies assemblages (albite + chlorite + actinolite ± sphene ± quartz ± pyrite) during high-temperature sub-sea-floor hydro-thermal metamorphism near the axis of spreading. Chemical changes in the wall-rock during this large-scale sea-water/rock interactive episode depend on the water/rock ratio and temperature but generally include gains in Mg, Na and H₂O and losses of Ca. Subsequent low temperature sea-water/rock interaction away from the axis of spreading results in fracture-controlled zeolitefacies alteration, characterized by smectite, caledonite, zeolite, calcite, prehnite, hematite, marcasite, and pyrite. This retrograde alteration involves increases in total Fe, K, and H₂O and decreases in Mg and Si in the wallrock; Ca may be lost or gained.Wall-rock alteration in Cyprus type stockwork zones is more striking, in that the basalt and diabase between veins of Fe---Cu-Zn sulphides, quartz, and chlorite have undergone partial to complete conversion to fine-grained aggregates of quartz + chlorite + illite + pyrite; kaolinite and palygorskite may be present in minor amounts. Calcium and Na are strongly depleted; K, Al, Ti, Mn, and Ni are leached to a lesser extent; and Fe, S, Cu, Zn, and Co are strongly enriched in the wall-rock underlying massive sulphide. Mafic rocks at depth in the volcanic pile may be enriched in K, Rb, and Li, and depleted in Cu, Co, and Zn. Lavas lateral to and overlying massive sulphide mineralization may have low concentrations of Cu and high concentrations of Zn and Co relative to background levels.Mutual consideration of hydrothermal sulphide deposits and associated wall-rock alteration in ophiolites and at modern oceanic spreading centres can provide useful criteria for the development of regional exploration models for ophiolitic terrains.     

河南洛宁沙沟Ag-Pb-Zn矿床银的赋存状态及成矿机理

位于河南洛宁境内的沙沟热液脉型Ag-Pb-Zn矿床是熊耳山地区近年来新发现的大型矿床.野外观察和矿相学研究表明成矿过程包含4个阶段, 分别为石英-菱铁矿阶段(Ⅰ)、石英-闪锌矿阶段(Ⅱ)、石英-银矿物-方铅矿阶段(Ⅲ)和石英-碳酸盐阶段(Ⅳ), 其中Ⅱ、Ⅲ阶段为主成矿阶段.扫描电子显微镜-能谱分析(SEM-EDS)和电子显微探针微区成分分析(EMP)结果显示, 沙沟矿床中的银以不可见银和可见银两种形式存在, 但以可见银为主.不可见银主要以次显微包体(< 1 μm)的形式被包裹在黄铜矿和闪锌矿等硫化物中, 而可见银通常以各种银的独立矿物形式交代方铅矿和黄铜矿等硫化物或充填在硫化物和石英的显微裂隙内.结合本文研究和前人对沙沟矿床流体包裹体的研究认为, 银和铅、锌等金属离子在成矿早期高温阶段以氯络合物的形式搬运, 随着成矿热液温度和氧逸度的降低以及pH值的升高, 氯络合物因稳定性降低而解体, 硫氢络合物成为银、铅、锌的主要迁移形式.随着成矿热液温度的继续降低, 铅、锌等金属硫氢络合物开始分解, 方铅矿、黄铜矿和闪锌矿等硫化物得以沉淀, 此时部分银以显微和次显微包体银的形式被包裹于这些硫化物中.铅锌硫化物的大量沉淀引起成矿热液组成和性质的显著变化, 最终导致银从硫氢络合物中彻底解体, 并与Cu+、Sb3+等离子结合形成大量独立银矿物(如含银黝铜矿、硫锑铜银矿和辉铜银矿等), 而溶液中过饱和的银则以自然银的形式沉淀.      

Stromeyerite and mckinstryite from the Godejord polymetallic sulphide deposit,central Norweigan Caledonides

The Cu-Ag-S minerals, stromeyerite and mckinstryite, have been found for the first time in a stratabound polymetallic pyritic deposit in the Caledonides of central Norway. The surface specimens examined contained approximately 0.5% Ag, 1.8% Cu, 15.0% Zn and over 10 g/t Au and showed the mineral association pyrite, sphalerite, chalcopyrite, galena, tennantite, bornite, Cu-Ag sulphides, covelline, native Au, a Cu-Sn sulphide, and a new mineral of composition Ag₅CuTeS₂. The Cu-Ag sulphides appear to be replacing preexisting sulphides, with the exception of pyrite and sphalerite. The nature of this replacement is discussed. Analyses, by microprobe, of the Cu-Ag-S phases are reported and compared with published data. The stromeyerite shows an average composition Cu_1.01Ag S, the mckinstryite Cu_0.77Ag_1.19S. Values are reported of the reflectance at 542 nm for both minerals. The data indicate that stromeyerite is optically positive with Rg: 30.7%, Rm: 27.3%, Rp: 25.8% while mckinstryite is negative with Rg: 32.5%, Rm: 31.9%, Rp: 27.6%.     

The genesis of metal zonation in the Weilasituo and Bairendaba Ag–Zn–Pb–Cu–(Sn–W) deposits in the shallow part of a porphyry Sn–W–Rb system,Inner Mongolia,China

The Weilasituo and Bairendaba Zn–Pb–Ag–Cu–(Sn–W) sulphide deposits are located in the southern part of Great Xing'an Range of Inner Mongolia in China. The deposits are located at shallow depths in the newly discovered Weilasituo porphyry hosting Sn–W–Rb mineralization. The mineralization at Weilasituo and Bairendaba consist of zoned massive sulphide veins within fractures cutting the Xilinhot Metamorphic Complex and quartz diorite. The Weilasituo deposit gradually zones from the Cu-rich Zn–Cu sulphide mineralization in the west to Zn-rich Zn–Cu sulphide mineralization in the east. The Bairendaba deposit has a Cu-bearing and Zn-rich core through a transitional zone devoid of copper to an outer zone of Zn–Pb–Ag mineralization. Three main veins contain more than 50 wt.% of the contained metal in the two deposits with their metal ratios displaying a systematic and gradual increase in Zn/Cu, Pb/Zn and Ag/Zn ratios from the western part of Weilasituo to the eastern part of Bairendaba.Three stages of vein-type mineralization are recognized. Early, sub-economic mineralization consists of a variable proportion of euhedral arsenopyrite, pyrite, quartz, and rare wolframite, scheelite, cassiterite, magnetite and cobaltite. This was succeeded by main stage mineralization with economic concentration of zoned Cu, Zn, Pb and Ag sulphide minerals along strike within the veins. The zones consist of the assemblages: (1) pyrrhotite–Fe-rich sphalerite–chalcopyrite(–quartz–fluorite) at west Weilasituo; (2) pyrrhotite–Fe-rich sphalerite–chalcopyrite(–galena–tetrahedrite–quartz–fluorite) at east Weilasituo; (3) pyrrhotite–Fe-rich sphalerite–chalcopyrite(–galena–tetrahedrite–quartz–fluorite) in the centre of Bairendaba; (4) pyrrhotite–Fe-rich sphalerite–galena(–chalcopyrite–tetrahedrite–quartz–fluorite) in the transition zone of Bairendaba; and (5) pyrrhotite–Fe-rich sphalerite–galena–tetrahedrite(–chalcopyrite–falkmanite–argentite–pyrargyrite–quartz–fluorite) in the outer zone at Bairendaba. Post-main ore stage is devoid of sulphides and characterized overprinting of fluorite, sericite, chlorite, illite, kaolinite and calcite.Zircon SHRIMP U–Pb dating, Zircon LA–ICP–MS U–Pb dating, molybdenite Re–Os isochron dating, and muscovite Ar–Ar dating indicate the Beidashan granitic batholith was intruded at 140 ± 3 Ma (MSWD = 3.3), the porphyritic monzogranite from marginal facies of the Beidashan batholith was intruded at 139 ± 2 Ma (MSWD = 0.75), the mineralized quartz porphyry was intruded at 135 ± 2 Ma (MSWD = 0.91), the greisen mineralization occurred at 135 ± 11 Ma (MSWD = 7.2), and the post-main ore stage muscovite deposited at 129.5 ± 0.9 Ma. The new geochronology data show the porphyry Sn–W–Rb and vein-type sulphide mineralization are contemporaneous with granitic magmatism in the region.The metal zonation at the Weilasituo and Bairendaba deposits is a result of progressive metal deposition. This was during the evolution of a metal-bearing fluid along the strike of the veins and during the main stage of ore formation at the upper part of the deep-seated porphyry Sn–W–Rb system. This progressive zonation indicates that the deposits represent end-numbers formed from one ore-forming fluid, which moved from west to east from the porphyry. The metal zonation patterns of the major veins are consistent with metal-bearing fluid entering the system with the precipitation of chalcopyrite proximally and sphalerite, galena and Ag-bearing minerals more distally. We show that the mechanism of metal deposition is therefore controlled by thermodynamic conditions resulting in the progressive separation of sulphides from the metal-bearing fluid. The temperature gradient between the inflow zone and the outflow zone appears to be one of the key parameters controlling the formation of the metal zonation pattern. The sulphide precipitation sequence is consistent with a low fS₂ and low fO₂ state of the acidic metal-bearing fluid. The metal zonation pattern provides helpful clues from which it is possible to establish the nature of fluid migration and metal deposition models to locate a possible porphyry mineralization at depth in the Great Xing'an Range, which is consistent with the geology of the newly discovered porphyry Sn–W–Rb system.     

山西省阳高县堡子湾金矿床矿物标型特征

根据成因矿物学及找矿矿物学观点，系统研究堡子湾金矿床黄铁矿、金矿物、方铅矿、黄铜矿和闪锌矿等金属矿物，石英、碳酸盐、绢(白)云母和金红石等非金属矿物的产状、形态及化学成分标型，结果表明：①矿床中矿物组合复杂，硫化物种类多，有少量硫盐矿物出现；②矿物中微量元素成分复杂，富含As，Sb，Bi，Se，Te等Au活化、迁移有利的矿化搬运剂，Cu，Ph，Zn，Au，Ag等成矿元素和Cr，Ni，V等深源元素；元素矿物组合及其特征比值指示金矿化与深源(下地壳或上地慢)浅成岩浆热液活动(斑岩系统)有关，燕山期石英二长斑岩(角砾岩)是成矿的主导因素；③矿石中大量出现铁白云石、富铁闪锌矿，粒状、富Ti金红石的大量分布，反映矿床剥蚀深度较大，目前可能已揭露至中深部中温带，位于斑岩系统的中下部，深部金矿化不利；④矿石含丰富的铜矿物，其他硫化物矿物中含铜量大，指示深部可能存在斑岩型Cu(Au)矿化。     

The Lerokis and Kali Kuning submarine exhalative gold-silver-barite deposits, Wetar Island, Maluku, Indonesia

Wetar, an island in the southern part of the Banda Arc, is made up submarine volcanic rocks, with the oldest rocks exposed being subvolcanic intrusions and flows dated at 12 Ma. Basaltic andesite pillow lavas and intercalated volcaniclastic sedimentary rocks grade upward into more felsic volcanic lavas, tuffs and breccias, and sedimentary rocks and epiclastic mudflows cap the sequence.Gold-silver mineralization occurs at Lerokis and Kali Kuning, 3.5 km apart on the north coast of the island, in stratiform barite sand, clay or silt. The sediments are underlain by Cu-rich massive pyrite in volcanic breccias and overlain by a limestone dated at about 4 Ma. Foot wall volcanic breccias and lavas show intense clay-pyrite alteration indicating temperatures higher than 230°C.In situ geological resources, prior to the inception of mining in 1990, totalled 2.9 Mt at 3.5 g/t Au, 114 g/t Ag and 40% barite at Lerokis and 2.2 Mt at 5.5 g/t Au, 146 g/t Ag and 60% barite at Kali Kuning. Most of the Au occurs as electrum, associated with limonite, jarosite and goethite, and most of the Ag is in tetrahedrite and sulphosalts. High Pb contents, between 0.5% and 1.4% Pb on average, derive from plumbojarosite, sulphosalts and cerrussite, and there is, on average, between 200 ppm to 900 ppm Cu, 0.2% Sb, 0.1% As, 100 ppm to 200 ppm Zn, and 18 ppm Hg. Underlying massive sulphide mineralization is mainly pyrite-marcasite with zones of Cu enrichment. A significant part of the Cu is contained in enargite.Formation likely took place at less than 600 m water depth in a sea floor caldera setting similar to the Kuroko district in Japan. The ferruginous sediments hosting the Au-barite deposits may have originated through erosion like the ochres of Cyprus. These stratiform Au-barite deposits highlight a new style of sea floor mineralization not clearly recognized before. Modern-day analogues have been described from a variety of sea floor settings.     

二郎坪群海相火山岩中块状硫化物矿床地质特征及其找矿方向

以阴沟矿床为例,通过岩相学、岩石学、地球化学研究,指出该类矿床指示矿物(岩石)为重晶石、硅质岩、黄铁矿、黄铜矿、方铅矿、闪锌矿等,指示元素为Cu、Pb、Zn、Ag、Au、As、Sb、Mo、Ba等;并分析这些矿物和元素垂直分带,水平分带规律;确定了找矿标志,为重晶石脉或帽和黄铁矿化带.然后分析了二郎坪群海相火山岩形成海底火山喷流块状硫化物矿床的地质环境和条件,并指出在该区进一步扩大找矿的远景和方向.     

Sensitive High-Resolution Ion Microprobe U-Pb Zircon and 40Ar-39Ar Muscovite Ages of the Yinshan Deposit in the Northeast Jiangxi Province, South China

The Yinshan deposit in the Jiangnan tectonic belt in South China consists of Pb‐Zn‐Ag and Cu‐Au ore bodies. This deposit contains approximately 83 Mt of the Cu‐Au ores at 0.52% Cu and 0.8 g/t Au, and 84 Mt of the Pb‐Zn‐Ag ores at 1.25% Pb, 1.02% Zn and 33.3 g/t Ag. It is hosted by low‐grade metamorphosed sedimentary rocks and mafic volcanic rocks of the lower Mesoproterozoic Shuangqiaoshan Group, and continental volcanic rocks of the Jurassic Erhuling Group and dacitic subvolcanic rocks. The ore bodies mainly consist of veinlets of sulfide minerals and sulfide‐disseminated rocks, which are divided into Cu‐Au and Pb‐Zn‐Ag ore bodies. The Cu‐Au ore bodies occur in the area close to a dacite porphyry stock (No. 3 stock), whereas Pb‐Zn‐Ag bodies occur in areas distal from the No. 3 stock. Muscovite is the main alteration mineral associated with the Cu‐Au ore bodies, and muscovite and chlorite are associated with the Pb‐Zn‐Ag ores. A zircon sensitive high‐resolution ion microprobe U‐Pb age from the No. 3 dacite stock suggests it was emplaced in Early Jurassic. Three ⁴⁰Ar‐³⁹Ar incremental‐heating mineral ages from muscovite, which are related to Cu‐Au and Pb‐Zn‐Ag mineralization, yielded 179–175 Ma. These muscovite ages indicate that Cu‐Au mineralization occurred at 178.2±1.4 Ma (2σ), and Pb‐Zn‐Ag mineralization at 175.4±1.2 Ma (2σ) and 175.3±1.1 Ma (2σ), which supports a restricted period for the mineralization. The Early Jurassic ages for the mineralization at Yinshan are similar to that of the porphyry Cu mineralization at Dexing in Jiangnan tectonic belt, and suggest that the polymetallic mineralization occurred in a regional transcompressional tectonic regime.     

Gold potential of massive sulfide ores in the Kamennoozero structural unit,Eastern Karelia

Two types of massive sulfide ores have been identified in the Kamennoozero segment of the green-stone belt: (1) hydrothermal volcanic-sedimentary strata-bound ores with massive, banded, and disseminated structures and (2) massive, brecciated, and stringer-disseminated Au-bearing base-metal ores, crosscutting the rocks of the Vozhmozero Group. The strata-bound, slightly metamorphosed orebodies are located at several levels along the contact between the Kamennoozero and Kumbuksa groups in the deep fault zones of the same names. These ores are composed of pyrite and pyrrhotite, small amounts of chalcopyrite and sphalerite, and distinguished by low grades of base metals and not higher than 0.06 g/t Au. In the Lebyazhino and Svetloozero areas, close to the sulfide Cu-Ni ore hosted in ultramafic rocks, the strata-bound bodies contain pentlandite and are enriched in Co, Ni, Cu, Zn, and up to 2.0–9.2 g/t Au. Brecciated and recrystallized pyrite ores contain up to 0.08–0.4% Sb and As, and up to 0.6–1 g/t Au in the Kumbuksa Fault Zone near Zolotye Porogi. The North Vozhma and Upper Vozhma base-metal massive sulfide occurrences, composed of pyrite, chalcopyrite, sphalerite, pyrrhotite, galena, bornite, and chalcocite, are considered to be promising Au-bearing prospects. Some samples from the North Vozhma occurrence contain up to 1.2–2.8 g/t Au and up to 167 g/t Ag. A gold grade of up to 20 g/t has been detected in the Upper Vozhma occurrence. The potential gold resources of the North Vozhma occurrence are estimated at about 600 kg.     

The mineralogy and chemical composition of the Woodlawn massive sulphide orebody

The main Woodlawn ore lens is a polymetallic, massive sulphide deposit’ with pyrite the major constituent, variable sphalerite, galena and chalcopyrite, and minor arsenopyrite, tetrahedrite‐tennantite, pyrrhotite and electrum. The silicate gangue minerals are chlorite, quartz, talc and sericitic mica. Other mineralization in the vicinity consists of footwall copper ore in chlorite schist and several smaller massive sulphide lenses. The predominant country rocks are felsic volcanics and shales, with abundant quartz, chlorite and mica, and talc in mineralized zones.

An important textural feature of the massive ore is the fine compositional banding. Bands, which vary in thickness from a few tens of micrometres to several millimetres, are produced by variations in the sulphide content. Post‐depositional metomorphism and minor fracturing have only slightly modified this banding.

Apart from the major element constituents—Pb, Zn, Fe, Cu and S—the ore is characterized by significant (100–1000 ppm) values for Ag, As, Cd, Mn, Sb and Sn, and lower (1–100 ppm) values of Au, Bi, Co, Ga, Hg, Mo, Ni, Tl. In and Ge. Variations in the base‐metal sulphide content, the gangue mineralogy, and trace elements, are used to separate the orebody into hanging‐wall and footwall zones. The hanging‐wall zone shows a more variable trace element content, with higher Tl, Sn, Ni, Mn, Ge and Sb, but lower Ag, Cd, and Mo, than the footwall zone.

In general style of mineralization, mineralogy, and chemistry, the Woodlawn deposit resembles other volcanogenic massive sulphide deposits in eastern Australia, in New Brunswick in Canada, and the Kuroko deposits of Japan.     

Genesis of fahlore in the Tianbaoshan lead–zinc deposit,Sichuan Province,China: a scanning electron microscopy–energy dispersive spectroscopy study

The Tianbaoshan deposit, located in the southwestern part of the Yangtze Block, is a representative Pb–Zn deposit in the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province. The Pb–Zn orebodies are hosted in the upper Sinian Dengying Formation dolostone. The predominant minerals are sphalerite, galena, pyrite, chalcopyrite, quartz, and calcite with minor arsenopyrite, fahlore, and dolomite. The deposit is characterized by relatively strong Cu mineralization. However, the relationship between Pb–Zn and Cu mineralization is unknown. We analyzed the mineralogy and composition of fahlore, chalcopyrite, arsenopyrite, sphalerite, and galena using scanning electron microscopy–energy dispersive spectroscopy, with the aim of providing new evidence for the genesis of the Pb–Zn–(Cu) ore. The results show that the Cu ore in the deposit is dominated by chalcopyrite and fahlore, both of which formed before or during the Pb–Zn ore-forming stage. The fahlore showed dramatic compositional variation and was characterized by negative correlations between Ag and Cu, and between As and Sb, suggesting substitution of Ag for Cu, and that As and Sb substitute in the same site in the fahlore lattice. Based on backscattered electron images and composition, the fahlore was divided into two types. Type I fahlore crystallized early and is characterized by enrichment of Cu and depletion in Ag and Sb. Type II fahlore formed after Type I, and is rich in Ag and poor in Cu and As. Moreover, galena and fahlore are the host minerals of Ag. The variation of valence state with As host mineral—from fahlore to arsenopyrite—indicates the metallogenic environment changed from relatively oxidizing to reducing with a high pH. In the light of Gibbs energies of reciprocal reactions and isotherms for cation exchange, the composition of the fahlore implies its ore-forming temperature was lower than 220 °C, corresponding with typical Mississippi Valley-type (MVT) deposits. Based on the geologic character and geochemical data of this deposit, we suggest that the Tianbaoshan deposit belongs to the MVT deposit category.     

新疆哈密卡拉塔格块状硫化物矿床金银赋存状态研究

新疆哈密红海黄土坡VMS矿床位于东天山卡拉塔格隆起带,是卡拉塔格矿集区内新发现的块状硫化物矿床。矿体产于卡拉塔格隆起带核部火山沉积岩建造中,具有典型的VMS型矿床“上层下脉”二元结构特征。该矿床中含金硫化物矿石主要有块状黄铁矿黄铜矿、块状黄铁矿黄铜矿闪锌矿、块状黄铁矿闪锌矿黄铜矿和块状闪锌矿。文中在对各类含金硫化物矿石进行详细的矿相学研究基础上,结合扫描电子显微镜与能谱仪联用技术(SEM/EDS),对硫化物样品中金、银的赋存状态进行研究。结果表明,4种块状硫化物中的主要矿物形成于多个期次,主要包括VMS成矿期(黄铁矿阶段、闪锌矿黄铜矿黝铜矿方铅矿阶段、石英重晶石阶段)、热液叠加期(石英黄铁矿黄铜矿闪锌矿方铅矿阶段)和表生期(铜蓝纤铁矿阶段)。矿区首次发现4颗金银金属互化物(银金矿、碲银矿),其较大的化学成分差异指示了热液环境由中酸性中性转变为更有利于Au、Ag迁移沉淀的偏碱性。后期的偏碱性热液对VMS成矿期形成矿物产生了交代作用,使得Au、Ag活化再富集。由于后期热液叠加改造,红海VMS型矿床中Au、Ag不仅赋存于VMS成矿期后期中低温闪锌矿黄铜矿阶段,也赋存于VMS成矿期早期中高温黄铁矿阶段,并贯穿整个热液叠加期。各含金矿物组合中除4颗金银金属互化物外Au多呈显微不可见状态,推测Au、Ag主要以原子或离子形式赋存于矿物晶格中或矿物空位处。     

Geological and isotopic evidence for magmatic-hydrothermal origin of the Ag–Pb–Zn deposits in the Lengshuikeng District,east-central China

The Lengshuikeng ore district in east-central China has an ore reserve of ～43 Mt with an average grade of 204.53 g/t Ag and 4.63 % Pb?+?Zn. Based on contrasting geological characteristics, the mineralization in the Lengshuikeng ore district can be divided into porphyry-hosted and stratabound types. The porphyry-hosted mineralization is distributed in and around the Lengshuikeng granite porphyry and shows a distinct alteration zoning including minor chloritization and sericitization in the proximal zone; sericitization, silicification, and carbonatization in the peripheral zone; and sericitization and carbonatization in the distal zone. The stratabound mineralization occurs in volcano-sedimentary rocks at ～100–400 m depth without obvious zoning of alterations and ore minerals. Porphyry-hosted and stratabound mineralization are both characterized by early-stage pyrite–chalcopyrite–sphalerite, middle-stage acanthite–native silver–galena–sphalerite, and late-stage pyrite–quartz–calcite. The δ³⁴S values of pyrite, sphalerite, and galena in the ores range from ?3.8 to +6.9‰ with an average of +2.0‰. The C–O isotope values of siderite, calcite, and dolomite range from ?7.2 to ?1.5‰ with an average of ?4.4‰ (V-PDB) and from +10.9 to +19.5‰ with an average of +14.8‰ (V-SMOW), respectively. Hydrogen, oxygen, and carbon isotopes indicate that the hydrothermal fluids were derived mainly from meteoric water, with addition of minor amounts of magmatic water. Geochronology employing LA–ICP–MS analyses of zircons from a quartz syenite porphyry yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 136.3?±?0.8 Ma considered as the emplacement age of the porphyry. Rb–Sr dating of sphalerite from the main ore stage yielded an age of 126.9?±?7.1 Ma, marking the time of mineralization. The Lengshuikeng mineralization classifies as an epithermal Ag–Pb–Zn deposit.     

Typomorpic Characteristics of the Major Minerals in the Puziwan Gold Deposit, Datong, Shanxi, China

Utilizing theories of minerageny and prospecting mineralogy, the authors studied the attitude, morphotype and chemical composition of metallic minerals of pyrite, gold, chalcopyrite, galena and sphalerite, non-metallic minerals of quartz, carbonate, dolomite and rutile in the Puziwan gold deposit. The study shows the following results. （1） The mineral assemblage is complex and the species of sulfide are abundant with occurrences of sulfosalt minerals. （2） The composition in the minerals is complex and there rich micro elements, including As, Sb, Bi, Se, Te, Au, Ag, Cu, Pb, Zn, and Cr, Ni, V. The typomorphic characteristics of the association of the elements and their specific value suggest that gold mineralization is associated with shallow magmatic hydrothermal activity, the oreforming fluid is the mixture of abundant rising alkali magmatic water originating from the mantle or the lower crust and the descending acid atmospheric water. （3） Ankerite, Fe-rich sphalerite, granular Ti-rich rutile are widely distributed, which indicate great denudation depths, high mineralization temperature. The deposit is found in the middle and shallow positions of the porphyry series. The deep layers are not favorable for gold mineralization. （4） Copper minerals are rich in the ores and sulfides have high content of copper, suggesting possible porphyry-type Cu （Au） mineralization in deep positions and the surrounding areas.     

Mineralogy of gold in the Elshitsa massive sulphide deposit, Sredna Gora zone, Bulgaria

The Elshitsa volcanic hosted massive sulphide deposit occurs in the central part of the Srena Gora metallogenic zone in Bulgaria. The gold-bearing massive sulphide mineralization is considered to be the product of an island arc volcano-plutonic process and hydrothermal activity that took place during the Late Cretaceous. In addition to the major gold-hosted opaque minerals such as pyrite, chalcopyrite, sphalerite and galena there are minor phases of tennantite, goldfieldite, Se-bearing aikinite, native silver and bornite in the massive sulphide lenses and stringer zones. Most of the sulphide minerals are Se-bearing. All of the six mineral assemblages that were deposited during the pyrite and copper-pyrite stages of mineralization are gold-bearing. The gold tenor as a rule is less than 1 g/t. Native gold and electrum occur as blebs or intergranular particles in the sulphide minerals. Gold in the early massive pyrite is of submicroscopic type (< 0,1 μm) and of colloidal ori-gin. Pyrite deformation and recrystallization in the temperature range 250°–160 °C has led to Au and Ag migration to cracks and grain boundaries of the sulphide minerals. As a result of these process the native gold and electrum grain size increases from submicroscopic (< 0,1 μm) in the early colloform pyrite to microscopic (0,1–100 μm) and macroscopic (> 100 μm) in the late gold-sulphide assemblages. The electrum fineness in 41 individually studied grains varies between 780 and 992‰ with a mean of 895‰. Native silver was found in association with bornite. Cu, Te, Sb and Bi are the most common trace-elements in gold and electrum. The Cu-Zn-Pb association is most important as a Au-Ag-carrier. A model for gold behaviour during sulphide deformation is proposed involving coarsening of gold grain size from the earlier to the later sulphide mineral assemblages. Received: 4 December 1995 / Accepted: 23 September 1996     

Exploration rock geochemistry — detection of trace element halos at heath steele mines (N.B., Canada) by discriminant analysis

The Heath Steele massive sulphide deposit in northern New Brunswick lies conformably within a sedimentary-volcanic sequence of probable Ordovician age which has been metamorphosed to the greenschist stage. The dominant sulphide mineral is pyrite, and the main economic minerals are sphalerite, galena, and chalcopyrite; the general grade of the ore is 5% Zn, 2% Pb, and 1% Cu.The distribution of Pb and Zn in acid volcanic rocks stratigraphically above the massive sulphides is compared with the distribution in similar rocks stratigraphically below the sulphides. Whereas there are discernable differences in the populations, there is also considerable overlap between them. To enable individual samples to be classified, linear discriminant functions were calculated for the two groups; Pb and Zn were found to be the most useful variables to separate the two populations. The functions were then tested on hanging wall and footwall samples not used in computing the functions. A halo region, extending about 1,200 ft above the sulphides and 4,000 ft along the same stratigraphic horizon as the sulphides was outlined by samples classified as “hanging wall”. Beyond the halo zone there is no significant difference in the distribution of Pb and Zn between the hanging wall and footwall acid volcanic rocks.The results demonstrate that rocks at Heath Steele, which show no evidence of mineralogical alteration attributable to mineralization, have a trace element halo of considerable extent spatially associated with the sulphides. If similar halos can be shown to be a general feature of massive sulphide deposits, the technique described should have wide application for exploration for deeply buried deposits of this type.     

Intermediate sulfidation epithermal Pb-Zn-Cu (±Ag-Au) mineralization at Cheshmeh Hafez deposit, Semnan province, Iran

The Cheshmeh Hafez epithermal base metal deposit is located in Troud-Chah Shirin mountain range in the Alborz magmatic belt of northern Iran. In this area, the Eocene volcanism and associated mineralization are controlled by NW-SE trending Anjilo and Troud major faults. Geological units are composed of porphyritic andesite, andesitic basalt, dacite, rhyodacite, trachyandesite and basalt, which are typically high-K igneous rocks transitional to shoshonites. Alteration in Cheshmeh Hafez area comprise of propylilitization, sericitization, argillization and silicification. Mineralization consists of three stages. Stage 1, quartz, carbonate with early pyrite I and chalcopyrite assemblages. Stage 2, the main stage of sulfide deposition, comprises early euhedral galena I followed by galena II and sphalerite, then galena III, chalcopyrite, tetrahedrite, pyrite II, bornite and digenite. Stage 3 involves the deposition of quartz and calcite barren veins with minor pyrite. The average assays from 12 channel samples of Cheshmeh Hafez veins are 0.15 g/t Au, 3.23 g/t Ag, 4.47 wt % Pb, 2.64 wt % Cu, and 1.73 wt % Zn. Fluid inclusion homogenization temperatures (Th) in quartz fall within the range of 140°-280°C with salinities ranging from 4.7 to 18 wt. % NaCl equivalent. Comparison of Th versus ice melting (Tm_ice) values indicates fluid dilution.     

Mineralogy and trace element geochemistry of sulfide minerals from the Wocan Hydrothermal Field on the slow-spreading Carlsberg Ridge,Indian Ocean

The basalt-hosted Wocan Hydrothermal Field (WHF), located on the NW slope of an axial volcanic ridge at a depth of ∼3000 m at 6°22′N on the slow-spreading Carlsberg Ridge, northwest Indian Ocean, was discovered in 2013 during Chinese DY28th cruise. Preliminary investigations show that the field consists of two hydrothermal sites: Wocan-1, which shows indications for recent high-temperature hydrothermal activity, is located near the peak of the axial volcanic ridge at a water depth of 2970–2990 m, and Wocan-2 site, located at a water depth of 3100 m, ∼1.7 km to the northwest of Wocan-1. The recovered hydrothermal precipitates can be classified into four groups: (i) Cu-rich chimneys; (ii) Cu-rich massive sulfides; (iii) Fe-rich massive sulfides; and (iv) silicified massive sulfides. We conducted mineral texture and assemblage observation and Laser-ablation ICP-MS analyses of the hydrothermal precipitates to study the mineralization processes. Our results show that there are distinct systematic trace element distributions throughout the different minerals in the four sample groups. In general, chalcopyrite from the group (i) is enriched in Pb, As, Mo, Ga, Ge, V, and Sb, metals that are commonly referred to as medium- to low-temperature elements. In contrast these elements are present in low contents in the chalcopyrite grains from other sample groups. Selenium, a typical high-temperature metal, is enriched in chalcopyrite from groups (ii) and (iv), whereas Ag and Sn are enriched only in some silicified massive sulfides. As with chalcopyrite, pyrite also shows distinct trace element associations in grains with different habitus. The low-temperature association of elements (Pb, Mo, Mn, U, Mg, Ag, and Tl) is typically present in colloform/framboidal pyrite, whereas the high-temperature association (Se, Co, and Bi) is enriched in euhedral pyrite. Sphalerite in the groups (i) and (iii) at Wocan-1 is characterized by high concentrations of Ga, Ge, Pb, Cd, As, and Sb, indicating that sphalerite in these sample groups likely precipitated at intermediate temperatures. Early bornite, which mainly occurs in the central part of the Cu-rich chimney, is typically enriched in Sn and In compared to the other minerals. In contrast, late bornite that likely formed during increasing interaction of hydrothermal fluids with cold, oxygenated seawater has low Sn and In, but significantly higher concentrations of Ag, Au, Mo and U. Digenite, also forming in the exterior parts of the samples during the late stages of hydrothermal fluid venting, is poor in most trace elements, except Ag and U. The notable Ag enrichment in the late-stage mineral assemblages at both Wocan-1 and Wocan-2 may therefore be related to lower temperatures and elevated pH. Our results indicate that Wocan-1 has experienced a cycle of heating with Cu-rich chimney growth and subsequent cooling, followed by late seafloor weathering, while Wocan-2 has seen intermediate- to high-temperature mineralization followed by intense silicification of sulfides. Seafloor weathering processes or mixing of hydrothermal fluids with seawater during the waning stages of hydrothermal fluid flow result in significant redistributions of trace elements in sulfide minerals.     

安徽铜陵新桥Cu-Au-S矿床黄铁矿微量元素LA-ICP-MS原位测定及其对矿床成因的制约

激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)是一种固体微区分析新技术。用该技术来分析矿床中硫化物的微量元素组成可以为研究成矿流体特征、矿床成因及找矿勘探提供有关的科学信息。文中以安徽铜陵矿集区内新桥Cu-Au-S矿床中的黄铁矿为研究对象,在详细的野外观察和室内鉴定的基础上,将矿床中的黄铁矿分为具有沉积特征的胶状黄铁矿(PyⅠ)、具有变形重结晶和热液叠加作用特征的细粒他形黄铁矿(PyⅡ)和具热液成因特征的中—粗粒自形黄铁矿(PyⅢ)3种类型。LA-ICP-MS原位微量元素测定结果显示,PyⅠ中相对富含Ti、Co、Ni、As、Se、Te;PyⅡ继承了PyⅠ中富含Ti、Co、Ni、As、Se、Te、Bi的特征,同时还含有不均匀分布的少量成矿元素(Cu、Pb、Zn、Au、Ag);PyⅢ中成矿元素Cu、Pb、Zn、Ag、Au以及Bi元素的含量较高,Co、Ni、As的含量较低。在元素赋存状态方面,Co、Ni、As、Se和Te均以类质同象的形式进入到了黄铁矿的晶格中;Bi在PyⅡ中主要以含Bi矿物的微细包裹体形式存在,而在PyⅢ中的Bi还部分取代了Fe而占据了晶格;Cu、Pb、Zn、Au、Ag这些成矿元素中,Cu和Zn分别以黄铜矿和闪锌矿的矿物包裹体存在于黄铁矿中;PyⅡ中所含的少量Au、Ag,可能分别以自然金和自然银的形式存在,而在PyⅢ中Au可能主要以银金矿的形式存在,Ag除了以银金矿的形式存在以外还可能赋存于黄铁矿中含铋的矿物包裹体内;Pb主要赋存于黄铁矿中的方铅矿或含铋矿物的包裹体中。在综合分析黄铁矿的结构形态和微量元素组成特征的基础上认为,PyⅠ型黄铁矿可能形成于前人提出的晚古生代海底沉积或喷流沉积环境,PyⅡ和PyⅢ型黄铁矿分别形成于中生代区域构造变形-热液叠加改造的过渡环境和热液环境,PyⅡ和PyⅢ的形成时间相近。新桥矿床的形成可能经历了晚古生代海底沉积或喷流沉积期和燕山期热液期,胶黄铁矿主要形成于沉积成矿期,而矿床中成矿物质Cu、Pb、Zn、Au、Ag等主要来自燕山期岩浆侵入作用形成的热液成矿系统。     

The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden, part I: geology

The Kristineberg volcanic-hosted massive sulphide (VMS) deposit, located in the westernmost part of the Palaeoproterozoic Skellefte district, northern Sweden, has yielded 22.4 Mt of ore, grading 1.0% Cu, 3.64% Zn, 0.24% Pb, 1.24 g/t Au, 36 g/t Ag and 25.9% S, since the mine opened in 1941, and is the largest past and present VMS mine in the district. The deposit is hosted in a thick pile of felsic to intermediate and minor mafic metavolcanic rocks of the Skellefte Group, which forms the lowest stratigraphic unit in the district and hosts more than 85 known massive sulphide deposits. The Kristineberg deposit is situated lower in the Skellefte Group than most other deposits. It comprises three main ore zones: (1) massive sulphide lenses of the A-ore (historically the main ore), having a strike length of about 1,400 m, and extending from surface to about 1,200 m depth, (2) massive sulphide lenses of the B-ore, situated 100–150 m structurally above the A-ore, and extending from surface to about 1,000 m depth, (3) the recently discovered Einarsson zone, which occurs in the vicinity of the B-ore at about 1,000 m depth, and consists mainly of Au–Cu-rich veins and heavily disseminated sulphides, together with massive sulphide lenses. On a regional scale the Kristineberg deposit is flanked by two major felsic rock units: massive rhyolite A to the south and the mine porphyry to the north. The three main ore zones lie within a schistose, deformed and metamorphosed package of hydrothermally altered, dominantly felsic volcanic rocks, which contain varying proportions of quartz, muscovite, chlorite, phlogopite, pyrite, cordierite and andalusite. The strongest alteration occurs within 5–10 m of the ore lenses. Stratigraphic younging within the mine area is uncertain as primary bedding and volcanic textures are absent due to strong alteration, and tectonic folding and shearing. In the vicinity of the ore lenses, hydrothermal alteration has produced both Mg-rich assemblages (Mg-chlorite, cordierite, phlogopite and locally talc) and quartz–muscovite–andalusite assemblages. Both types of assemblages commonly contain disseminated pyrite. The sequence of volcanic and ore-forming events at Kristineberg is poorly constrained, as the ages of the massive rhyolite and mine porphyry are unknown, and younging indicators are absent apart from local metal zoning in the A-ores. Regional structural trends, however, suggest that the sequence youngs to the south. The A- and B-ores are interpreted to have formed as synvolcanic sulphide sheets that were originally separated by some 100–150 m of volcanic rocks. The Einarsson zone, which is developed close to the 1,000 m level, is interpreted to have resulted in part from folding and dislocation of the B-ore sulphide sheet, and in part from remobilisation of sulphides into small Zn-rich massive sulphide lenses and late Au–Cu-rich veins. However, the abundance of strongly altered, andalusite-bearing rocks in the Einarsson zone, coupled with the occurrence of Au–Cu-rich disseminated sulphides in these rocks, suggests that some of the mineralisation was synvolcanic and formed from strongly acidic hydrothermal fluids. Editorial handling: P. Weihed