Gold Enrichment in the Supergene Zone of a Southern Urals Pyrite Deposit,Russia

The Gay deposit, situated in the Orenburg region, is identified with one of Russia's principal occurrences of pyrite (pyrite deposits are an important source of Russia's gold). It belongs to the west subzone of the Magnitogorsk synclinorium and occurs in Devonian rhyolite-basaltic volcanic rocks. The deposit comprises five large pyrite-chalcopyrite, pyrite-chalcopyrite- sphalerite, and pyrite orebodies. The supergene zone extends to 120-240 m below surface and consists of the following three subhorizontal zones (from bottom to top): the secondary sulfide enrichment, the leaching, and the oxidation zone (where ores are enriched in gold).

There are two levels of secondary gold enrichment in the weathering profile. The lower level, located in the leaching zone, corresponds with the level of water table fluctuations. The rich, flat-lying horizon (1.5-10.0 m) is composed of bedded, friable native sulfur-quartz ores; it contains 19.0-52.2 ppm Au and up to 389 ppm Ag. Native gold and silver halides (chlorargyrite, iodargyrite, and embolite) are the principal precious-metal minerals. Electrum, native silver, acanthite, and uytenbogaardtite constitute the minor ones. The upper level of the enrichment is located in the lower part of gossan. This bonanza is composed of hematite-quartz ochres. Gold concentration is 13.5 to 21.2 ppm. Native gold of high fineness and silver halides apparently are associated here with poorly crystallized iron oxides. The formation of supergene gold enrichments may result partly from residual concentration and partly from mobilization and reprecipitation of the precious metal. Rich horizons form by repeated gold redeposition in accordance with weathering and a gradual erosion surface lowering. The lower bonanza forms at first in the process of oxidation involving pyrite and native sulfur. Gold may be transported by complexes with metastable sulfur oxy-anions: sulfites, thiosulfates, or polythionates. The upper enriched horizon forms in the course of further evolution of the weathering profile in the stage of hematite recrystallizaiton and its transformation into goethite.     

新疆哈密玉西银矿床特征及成因

玉西银矿床产在中元古界长城系星星峡群黑云斜长片麻岩和藓县系卡瓦布拉克群大理岩间的韧性剪切带、韧脆性破碎带中;矿体呈脉状、透镜状和似层状;矿石由辉银矿、方铅矿、闪锌矿、黄铜矿、石英和黄铁矿等４０余种矿物组成;围岩蚀变有硅化、内铁矿化和碳酸盐化;经历热液成矿期、叠加成矿期和表生成矿期。矿质源自地壳浅部和深部岩石,介质来自地壳封存水、变质水和雨水的混合水,热源主要来自地壳圈层剪切热。在同构造、中低温、中     

The geochemistry of gossans associated with Sarcheshmeh porphyry copper deposit,Rafsanjan, Kerman,Iran: Implications for exploration and the environment

The Sarcheshmeh copper deposit is one of the world's largest Oligo-Miocene porphyry copper deposits in a continental arc setting with a well developed supergene sulfide zone, covered mainly by a hematitic gossan. Supergene oxidation and leaching, have developed a chalcocite enrichment blanket averaging 1.99% Cu, more than twice that of hypogene zone (0.89% Cu). The mature gossans overlying the Sarcheshmeh porphyry copper ores contain abundant hematite with variable amounts of goethite and jarosite, whereas immature gossans consist of iron-oxides, malachite, azurite and chrysocolla. In mature gossans, Au, Mo and Ag give significant anomalies much higher than the background concentrations. However, Cu has been leached in mature gossans and gives values close or even less than the normal or crustal content (< 36.7 ppm). Immature gossans are enriched in Cu (160.3 ppm), Zn (826.7 ppm), and Pb (88.6 ppm). Jarosite- and goethite-bearing gossans may have developed over the pyritic shell of most Iranian porphyry copper deposits with pyrite–chalcopyrite ratios greater than 10 and therefore, do not necessarily indicate a promising sulfide-enriched ore (Kader and Ijo). Hematite-bearing gossans overlying nonreactive alteration halos with pyrite–chalcopyrite ratios about 1.5 and quartz stringers have significant supergene sulfide ores (Sarcheshmeh and Miduk). The copper grade in supergene sulfide zone of Sarcheshmeh copper deposit ranges from 0.78% in propylitized rocks to 3.4% in sericitized volcanic rocks, corresponding to the increasing chalcopyrite–pyrite or chalcocite–pyrite ratios from 0.3 to 3, respectively. Immature gossans with dominant malachite and chrysocolla associated with jarosite and goethite give the most weakly developed enrichment zone, as at God-e-Kolvari. The average anomalous values of Au (59.6 ppb), Mo (42.5 ppm) and Ag (2.6 ppm) in mature gossans associated with the Sarcheshmeh copper mine may be a criterion that provides a significant exploration target for regional metallogenic blind porphyry ore districts in central Iranian volcano–plutonic continental arc settings. Drilling for new porphyry ores should be targeted where hematitic gossans are well developed. The ongoing gossan formation may result in natural acidic rock drainage (ARD).     

The origin of Ag–Au–S–Se minerals in adularia-sericite epithermal deposits: constraints from the Broken Hills deposit, Hauraki Goldfield, New Zealand

The 7.1 Ma Broken Hills adularia-sericite Au–Ag deposit is currently the only producing rhyolite-hosted epithermal deposit in the Hauraki Goldfield of New Zealand. The opaque minerals include pyrite, electrum, acanthite (Ag₂S), sphalerite, and galena, which are common in other adularia-sericite epithermal deposits in the Hauraki Goldfield and elsewhere worldwide. Broken Hills ores also contain the less common minerals aguilarite (Ag₄SeS), naumannite (Ag₂Se), petrovskaite (AuAgS), uytenbogaardtite (Ag₃AuS₂), fischesserite (Ag₃AuSe₂), an unnamed silver chloride (Ag₂Cl), and unnamed Ag?±?Au minerals. Uytenbogaardtite and petrovskaite occur with high-fineness electrum. Broken Hills is the only deposit in the Hauraki Goldfield where uytenbogaardtite and petrovskaite have been identified, and these phases appear to have formed predominantly from unmixing of a precursor high-temperature phase under hypogene conditions. Supergene minerals include covellite, chalcocite, Au-rich electrum, barite, and a variety of iron oxyhydroxide minerals. Uytenbogaardtite can form under supergene and hypogene conditions, and textural relationships between uytenbogaardtite and associated high-fineness electrum may be similar in both conditions. Distinguishing the likely environment of formation rests principally on identification of other supergene minerals and documenting their relationships with uytenbogaardtite. The presence of aguilarite, naumannite, petrovskaite, and fischesserite at Broken Hills reflects a Se-rich mineral assemblage. In the Hauraki Goldfield and the western Great Basin, USA, Se-rich minerals are more abundant in provinces that are characterized by bimodal rhyolite–andesite volcanism, but in other epithermal provinces worldwide, the controls on the occurrences of Se-bearing minerals remain poorly constrained, in spite of the unusually high grades associated with many Se-rich epithermal deposits.     

Metamorphic-hydrothermal parkerite and associated minerals in the Noril’sk ore field

In the Noril’sk ore field, parkerite is a characteristic mineral of sulfide ore that metamorphosed under conditions of zeolite and prehnite-pumpellyite facies and of arsenide-calcite veins. The mineral occurs in ores containing bornite, anhydrite, magnetite, mackinawite (3–5 wt % Ni), valleriite, calcite, ankerite, native silver, native bismuth, violarite, Te-rich bismutohauchecornite, cupropentlandite enriched in Fe, Pd-rich breithauptite (1.5–2.5 wt % Pd), galena enriched in Cu (3.8 wt % Cu), and Ni arsenides and antimonides. Parkerite occurs in those place, where the primary ores have contained pockets and veins of graphic galena and chalcopyrite aggregates with associated Pt-Pd-Au-Ag minerals. Parkerite metacrysts in galena and Fe-Cu-Ni sulfides contain 6–16 and up to 5 wt % Pb, respectively. Parkerite rims replacing PGM aggregates and galena contain 1–3 wt % Pb. In calcite veins hosted in metamorphosed sulfide ores, parkerite is associated with native silver and bismuth, maucherite, cobaltite, chalcocite, and uraninite. Parkerite from these veins contains up to 0.5 wt % Pb. Thus, the Pb and Bi contents in parkerite basically depend on those of replaced minerals. Rare bismutohauchecornite is associated with parkerite.     

额济纳旗老硐沟氧化-淋滤型金矿床成矿特征

内蒙古额济纳旗老硐沟金矿原生矿化与闪长玢岩脉和似斑状花岗闪长岩株有关.金-多金属矿脉产于蓟县系平头山群白云石大理岩中,主要受近E-W向和NWW向断裂控制.氧化-淋滤型金矿床的形成,乃是原生低品位金矿化和次生淋滤富集两个过程的结果.在目前采深50m的淋滤带中,金多富集在距地表10~30m范围内.氧化矿石的矿物组合为臭葱石+砷菱铅矾+褐铁矿+针铁矿+砷钙锌石+羟砷锌石+自然金+自然银+石英+蛋白石.     

一种亟待重新认识的铜矿床——滇中土状铜矿

滇中土铜矿长期被认为是一种不可利用的铜矿床,新的湿法冶金技术应用使之成为了效益型铜资源。滇中土状铜矿产于中元古界昆阳群落雪组的土状风化壳内,分为面型和线型两种矿床类型。主要铜矿物为孔雀石、硅孔雀石、微粒状赤铜矿和黑铜矿、少量残余含铜硫化物。其矿石平均品位同于原生硫化矿。与原生矿石相比,土状氧化铜矿石ＳｉＯ２、Ａｌ２Ｏ３、Ｆｗ２Ｏ３、Ｍｎｏ明显富集,ＣａＯ与ＭｇＯ大量淋失,由碱性矿石变成了可酸浸的非     

The “Calamines” and the “Others”: The great family of supergene nonsulfide zinc ores

“Nonsulfides” is a term, which comprises a series of oxidized Zn(Pb)-ore minerals. It has also been used to define a special deposit type, mainly considered as derived from the weathering of Zn(Pb) sulfide concentrations. However, nonsulfide zinc deposits have been distinguished between supergene and hypogene, according to their mineralogy, geological characteristics and genetic setting. The supergene deposits formed by weathering and oxidation at ambient temperatures, whereas the hypogene ones are considered hydrothermal, or associated with metamorphic processes on primary sulfide ores.In this review paper, a comparison between a number of several nonsulfide deposits has been carried out: typical “Calamines”, peculiar “Calamines” and “Others”. The whole group comprises deposits of typical supergene origin, mixed supergene–hypogene mineralizations, and oxidized concentrations characterized by different metals only locally associated with zinc. The Zn–Pb nonsulfide concentrations hosted in carbonate rocks, which are mainly attributed to “wall-rock replacement” and “direct-replacement” supergene processes, are the typical “Calamines” (Liège district, Belgium; Iglesias district, Italy; Silesia–Cracow district, Poland). Peculiar “Calamine” deposits are those mineralizations that have been generally considered as supergene, but which are instead genetically related, at least partly, to hypogene processes (e.g. Angouran, Iran; Jabali, Yemen), though mineralogically and texturally similar to supergene nonsulfide deposits. The “Others” are prevailingly supergene nonsulfide zinc deposits not hosted in carbonate rocks (Skorpion, Namibia; Yanque, Peru), or characterized by other metals as main commodities, like lead (Magellan, Australia), silver (Sierra Mojada, Mexico; Wonawinta, Australia) or vanadium (Otavi Mountainland, Namibia).Minerals of current economic importance in most “Calamine” deposits are smithsonite, hydrozincite, and cerussite. This mineralogical association is generally simple but, when the “Calamines” are dolomite-hosted, one of the consequences of the “wall-rock replacement” process is the generation of a series of economically useless Zn- and Mg-bearing mixed carbonate phases. Secondary deposits hosted in silicatic (sedimentary or volcanic) rocks mainly contain hemimorphite and/or sauconite. Lead-, Ag- and V-rich nonsulfide ores are characterized by a more complex mineralogical association: mixed Pb-carbonates, Pb-sulfates, Pb-phosphates, Pb-arsenates, various Ag-sulfosalts, and Zn–Pb–Cu-vanadates.Carbon and oxygen stable isotope studies allow distinguishing between supergene and hypogene nonsulfide deposits, evaluating the effects of oxidative heating and even gaining indirect paleoclimatic information. The oxygen-isotope variation of the individual carbonate minerals within a deposit is relatively small, indicating constant formation temperatures and a single, meteoric fluid source. Carbon-isotope values are highly variable, thus suggesting several isotopically distinct carbon sources.Periods of paleoclimatic switch-overs from seasonally humid/arid to hyperarid have been considered as the most favorable conditions for the formation and preservation of supergene nonsulfide deposits. However, while several recent nonsulfide deposits throughout the world are positioned between 15° and 45° N latitude, thus pointing to a warm and humid weathering climate, others have been deposited in sub-Arctic regions.The economic value of the nonsulfide Zn(Pb–Ag–V) ores is highly variable, because more than in the case of metallic sulfide deposits, it resides not only on the geological setting, but also on their mineralogy that can directly influence processing and metallurgy.     

银和金的选择吸附实验研究及意义

吸附作用被认为是低温表生矿床的一种重要的成矿机制。文章内多种矿物对银和金的吸附实验表明,呈纳米粒级的银和金的单质对自然界矿物的吸附有明显的选择性,尤其是银对方铅矿和金对黄铁矿的强选择吸附,与自然界的实际情况符合得很好。这一实验结果为解释许多矿床中方铅矿常作为银的载体矿物及黄铁矿常作为金的载体矿物提供了科学依据。实验还证明了粘土、有机质和黑色页岩对银和金的吸附作用。     

巴布亚新几内亚奥克泰迪铜金矿床成矿特征和控制因素

巴布亚新几内亚西部Fubilan山奥克泰迪矿床是一个世界级铜金矿床,在大地构造上位于新几内亚造山带的巴布亚褶皱带。该矿床的铜金矿化赋存于Fubilan二长斑岩及其周边的磁铁矿夕卡岩和硫化物夕卡岩中。矿石类型以原生硫化物矿石为主,金属矿物包括磁铁矿、黄铁矿、磁黄铁矿、白铁矿、黄铜矿、斑铜矿等。蚀变类型包括夕卡岩化、钾化、泥化和青盘岩化。矿床氧化次生富集带发育,表生矿石矿物为蓝辉铜矿、辉铜矿、自然铜、铜蓝和银金矿。成矿作用主要受区域构造、侵入杂岩体、Darai组灰岩地层、断裂等因素的控制。根据矿床的主岩、矿石特征、蚀变特征和控矿因素,认为该矿床成因类型属于较为典型的夕卡岩一斑岩型矿床。     

Mineral and geochemical compositions,regularities of distribution,and specific formation of ore mineralization of the Rogovik gold-silver deposit (northeastern Russia)

New data on the mineral composition and the first data on the geochemical composition of ores of the Rogovik gold-silver deposit (Omsukchan ore district, northeastern Russia) have been obtained. Study of the regularities of the spatial distribution of ore mineralization shows that the deposit ores formed in two stages. Epithermal Au-Ag ores of typical poor mineral and elemental compositions were generated at the early volcanic stage. The major minerals are low-fineness native gold, electrum, acanthite, silver sulfosalts, kustelite, and pyrite. The typomorphic elemental composition of ores is as follows: Au, Ag, Sb, As, Se, and Hg. The content of S is low, mostly < 1%. Silver ores of more complex mineral and elemental compositions were produced under the impact of granitoid intrusion at the late volcanoplutonic stage. The major minerals are high-Hg kustelite and native silver, silver sulfosalts and selenides, fahlore, pyrite, chalcopyrite, galena, and sphalerite. The typomorphic elemental composition of ores is as follows: Ag, As, Sb, Se, Hg, Pb, Zn, Cu, and B. The content of S is much higher than 1%. The ores also have elevated contents of Mo, Ge, F, and LREE (La, Ce, and Nd). At the volcanoplutonic stage, polychronous Au-Ag ores formed at the sites of the coexistence of silver and epithermal gold-silver mineralization. Their specific feature is a multicomponent composition and a strong variability in chemical composition (both qualitative and quantitative). Along with the above minerals, the ores contain high-Hg gold, hessite, argyrodite, canfieldite, orthite, fluorapatite, and arsenopyrite. At the sites with strongly rejuvenated rocks, the ores are strongly enriched in Au, Ag, Hg, Cu, Pb, Zn, Ge, Se, La, Ce, Nd, S, and F and also contain Te and Bi. The hypothesis is put forward that the late silver ores belong to the Ag-complex-metal association widespread in the Omsukchan ore district. A close relationship between the ores of different types and their zonal spatial distribution have been established. In the central part of the Rogovik deposit, epithermal Au-Ag ores are widespread in the upper horizons, Ag ores are localized in the middle horizons, and rejuvenated polyassociation Au-Ag ores occur at the sites (mostly deep-seated) with ore-bearing structures of different ages.     

新探明的玻利维亚银沙(Silver Sand)超大型银矿床地质特征和找矿评价

银沙银矿(Silver Sand silver deposit)是玻利维亚锡矿带内新探明的一个超大型中硫型浅成低温热液银矿床,已探明银金属资源量约1万t,平均银品位约120 g/t,并伴有少量铅、锌、铟、镓.银矿化与中新世中酸性侵入岩和次火山岩有关.矿体赋存于白垩系蚀变褪色石英砂岩中的密集构造裂隙带内.成矿作用可以划分...     

Gold paragenesis and chemistry at Batu Hijau, Indoneisa: implications for gold-rich porphyry copper deposits

Gold is an important by-product in many porphyry-type deposits but the distribution and chemistry of gold in such systems remains poorly understood. Here we report the results of petrographic, electron microprobe, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and flotation test studies of gold and associated copper sulfides within a paragenetic framework from the world-class Batu Hijau (914 mt @ 0.53% Cu, 0.40 g/t Au) porphyry copper–gold deposit, Indonesia. Unlike many other porphyry copper–gold deposits, early copper minerals (bornite–digenite–chalcocite) are well preserved at Batu Hijau and the chalcopyrite–pyrite overprint is less developed. Hence, it provides an excellent opportunity to study the entire gold paragenesis of the porphyry system. In 105 polished thin sections, 699 native gold grains were identified. Almost all of the native gold grains occurred either within quartz veins, attached to sulfide, or as free gold along quartz or silicate grain boundaries. The native gold grains are dominantly round in shape and mostly 1–12 m in size. The majority of gold was deposited during the formation of early A veins and is dominantly associated with bornite rather than chalcopyrite. The petrographic and LA-ICP-MS study results indicate that in bornite-rich ores gold mostly occurs within copper sulfide grains as invisible gold (i.e., within the sulfide structure) or as native gold grains. In chalcopyrite-rich ores gold mostly occurs as native gold grains with lesser invisible gold. Petrographic observations also indicate a higher proportion of free gold (native gold not attached to any sulfide) in chalcopyrite-rich ores compared to bornite rich ores. The pattern of free gold distribution appears to correlate with the flotation test data, where the average gold recovery value from chalcopyrite-rich ores is consistently lower than bornite-rich ores. Our data suggest that porphyry copper-gold deposits with chalcopyrite-rich ores are more likely to have a higher proportion of free gold and may require different ore processing strategies.Editorial handling: R. P. Richards     

大地构造环境对世界主要类型银矿的控制作用

不同大地构造环境控制了不同类型的银矿床。火山弧和岛弧控制了浅成低温热液型和黑矿型银矿床。裂谷带的地垒之上产出有浅成低温热液银矿床、中深中温次火山热液银矿床、斑岩型银矿床和矽卡岩型银矿床。裂谷带的地堑之中产出有五元素矿床、碱性火山岩中的块状硫化物型银矿床、沉积岩中的块状硫化物型银矿床、砂岩型铜银矿床。     

浙西北银山银多金属矿床地质特征及成因

浙西北银山银多金属矿赋存于震旦纪蓝田组之中;矿体呈层状、似层状产出,受褶皱和断层控制明显,在褶皱的转折段和倒转翼矿明显加厚。Fe/Ti-Al/(Al+Fe+Mn)、Co/Ni比值都显示银山银多金属矿具有热水沉积特征,结合野外矿脉穿插关系和矿石的结构构造特征,可将成矿期划分为:热水沉积期和后期低温热液改造期两个成矿期;成矿流体整体属于低盐度流体,显示出从低盐度往高盐度演化的趋势。本文研究初步认为,银山银多金属矿属于浅成低温热液矿床,前寒武纪热水沉积阶段是金属元素的富集沉积阶段,但并未形成有经济价值的矿体,后来的构造运动和岩体侵入使其富集成矿。     

含金硫化物矿床的表生富集型金矿

虎圩金矿位于湿热多雨的江西省东部，赋存在东乡火山岩盆地北部，江南地体与赣中地体的拼接带上，因此，矿区断裂构造发育，岩浆活动频繁。金矿化产于燕山中期侵入的花岗闪长玢岩的内外接触带上。矿化以充填交代张性断裂构造带的脉状形式产出。矿体为不规则脉状和透镜状，具明显的垂直分带现象。上部为氧化矿石、具黑色土状、蜂窝状构造，Ａｕ含量达工业品位以上，形成金矿体、此外，氧化矿石还富Ｃｏ，相对贫Ｃｕ、Ｚｎ、Ａｇ和Ｐｂ     

Yerranderie a Late Devonian Silver–Gold–Lead Intermediate Sulfidation Epithermal District, Eastern Lachlan Orogen, New South Wales, Australia

Felsic volcanic units of the Early Devonian Bindook Volcanic Complex host the Yerranderie epithermal silver–gold–lead district 94 km west–southwest of Sydney. Mineralization in the district forms part of a fault‐controlled, intermediate sulfidation, epithermal silver–gold–base metal vein system that has significant mineral and alteration zonation. Stage 1 of the mineral paragenesis in the veins developed quartz and carbonate with early pyrite, whereas stage 2 is a crustiform banded quartz–pyrite–arsenopyrite assemblage. Stage 3, the main stage of sulfide deposition, comprises early sphalerite, followed by a tetrahedrite–tennantite–gold assemblage, then a galena–chalcopyrite–native silver–pyrite assemblage, and finally a pyrargyrite–polybasite–pearceite assemblage. Stage 4 involves the deposition of quartz veins with minor (late) pyrite and stage 5 is characterized by siderite that infilled remaining voids. Mineral zonation occurs along the Yerranderie Fault, with bornite being restricted to the Colon Peaks–Silver Peak mine area, whereas arsenopyrite, which is present in both the Colon Peaks–Silver Peak and Wollondilly mine areas, is absent in other lodes along the Yerranderie Fault. The Yerranderie Fault, which hosts the major lodes, is surrounded by a zoned alteration system. With increasing proximity to the fault the intensity of alteration increases and the alteration assemblage changes from an outer quartz–muscovite–illite–(ankerite) assemblage to a quartz–illite–(pyrite–carbonate) assemblage within meters of the fault. ⁴⁰Ar/³⁹Ar dating of muscovite from the alteration zone gave a 372.1 ± 1.9 Ma (Late Devonian) age, which is interpreted to be the timing of the quartz–sulfide vein formation. Sulfur isotope values for sulfides range from 0.1 to 6.2‰ with one outlier of ?5.6 δ³⁴S‰. The results indicate that the initial ore‐forming fluids were reduced, and that sulfur was probably sourced from a magmatic reservoir, either as a direct magmatic contribution or indirectly through dissolution and recycling of sulfur from the host volcanic sequence. The sulfur isotope data suggest the system is isotopically zoned.     

Genesis of Middle Miocene Yellowstone hotspot-related bonanza epithermal Au–Ag deposits,Northern Great Basin,USA

Epithermal deposits with bonanza Au–Ag veins in the northern Great Basin (NGB) are spatially and temporally associated with Middle Miocene bimodal volcanism that was related to a mantle plume that has now migrated to the Yellowstone National Park area. The Au–Ag deposits formed between 16.5 and 14 Ma, but exhibit different mineralogical compositions, the latter due to the nature of the country rocks hosting the deposits. Where host rocks were primarily of meta-sedimentary or granitic origin, adularia-rich gold mineralization formed. Where glassy rhyolitic country rocks host veins, colloidal silica textures and precious metal–colloid aggregation textures resulted. Where basalts are the country rocks, clay-rich mineralization (with silica minerals, adularia, and carbonate) developed. Oxygen isotope data from quartz (originally amorphous silica and gels) from super-high-grade banded ores from the Sleeper deposit show that ore-forming solutions had δ ¹⁸O values up to 10‰ heavier than mid-Miocene meteoric water. The geochemical signature of the ores (including their Se-rich nature) is interpreted here to reflect a mantle source for the “epithermal suite” elements (Au, Ag, Se, Te, As, Sb, Hg) and that signature is preserved to shallow crustal levels because of the similar volatility and aqueous geochemical behavior of the “epithermal suite” elements. A mantle source for the gold in the deposits is further supported by the Pb isotopic signature of the gold ores. Apparently the host rocks control the mineralization style and gangue mineralogy of ores. However, all deposits are considered to have derived precious metals and metalloids from mafic magmas related to the initial emergence of the Yellowstone hotspot. Basalt-derived volatiles and metal(loid)s are inferred to have been absorbed by meteoric-water-dominated geothermal systems heated by shallow rhyolitic magma chambers. Episodic discharge of volatiles and metal(loid)s from deep basaltic magmas mixed with heated meteoric water to create precious metal ore-forming fluids. Colloidal nanoparticles of Au–Ag alloy (electrum), naumannite (Ag₂Se), silica, and adularia, likely nucleated at depth, traveled upward, and deposited where they grew large enough to aggregate along vein walls. Silica and gold colloids have been reported in hot springs from Yellowstone National Park, suggesting that such processes may continue to some extent to the present. However, it is possible that the initial development of the mantle plume led to a major but short-lived “distillation” process which led to the mid-Miocene bonanza ore-forming event.

摩洛哥伊米泰尔(Imiter)浅成低温热液型银矿床地质特征与成因

伊米泰尔(Imiter)银矿床位于摩洛哥小阿特拉斯山地区,是世界级超大型浅成低温热液银矿床。矿区内矿化主要受伊米泰尔断裂带控制,矿化期可以分为以下两期:(1)贱金属矿化期(BME),与Taouzzakt花岗闪长岩(572±5Ma)有关,(2)浅成低温热液矿化期(ESE),与Takhatert流纹岩(550±3Ma)有关。通过He、S、Os同位素研究,伊米泰尔矿床的成矿物质和成矿流体都具有幔源的特征,其成矿模式表现出地幔来源的浅成低温热液成矿模式。     

A review of sulfur to selenium ratios in magmatic nickel–copper and platinum-group element deposits

Deviations in the sulfur to selenium ratios (S/Se) from mantle values in magmatic Ni–Cu–Platinum Group Elements (PGE) sulfide deposits have been widely used to constrain the ore forming processes. Basically, S/Se ratios greater than mantle values are interpreted to be the result of contamination of the mantle derived magma by S-rich sedimentary rocks, whereas S/Se ratios lower than mantle values are thought to be the result of S loss during post-crystallization. However, there are many other processes involved in producing a deposit and it is possible that these may be also important in controlling S/Se ratios. In order to investigate the relative importance of these processes, we have compiled a data base of S, Se, δ³⁴S and metal values from Ni–Cu–PGE sulfide deposits. This compilation shows that processes affecting S/Se ratios can be divided into two main classes: the magmatic processes and the late- to post-magmatic processes.

• 1)Magmatic processes include the well-known addition of S from sedimentary rocks, variations in the sulfide to silicate liquid ratio (R-factor), depletion of the silicate magma in Se by early segregation of the sulfide liquid, and the moderate incompatibility of Se into the first sulfide minerals to crystallize from a sulfide liquid, the monosulfide-solid-solution (MSS). This incompatibility results in a change in S/Se ratio between the Fe-rich and Cu-rich zones of magmatic sulfide ores. The fractionation of Se during crystallization of sulfide liquids has not previously been appreciated.
• 2)Late- to post-magmatic processes include: hydrothermal alteration, high-grade metamorphism, serpentinization and supergene weathering. Some metamorphosed Cu-deposits have low S/Se ratios suggesting S-loss by breakdown of sulfide minerals during a high-grade metamorphic event. However, the effectiveness of this process remains unclear and alternative models exist. The preferential remobilization of S relative to Se during hydrothermal alteration, serpentinization and supergene weathering leads to a moderate decrease of S/Se ratios values and can mask the initial S/Se ratio.