Stratabound tungsten mineralization in regional metamorphic calc-silicate rocks from the Austroalpine Crystalline Complex,Austria

Stratabound tungsten mineralization in regional metamorphic calc-silicate rocks of probably Lower Paleozoic age is described from the polymetamorphic Austroalpine Crystalline Complex (ACC) of the Eastern Alps. Scheelite-bearing calc-silicate rocks which are often associated with marbles and tourmalinites are intercalated conformably with metaclastic rocks. Alkalipoor calc-silicate rocks with high amounts of clinozoisite/ zoisite, grossular, quartz, plagioclase, etc. are the most important host rocks for tungsten mineralization. These unusual calc-silicate rocks are products of regional metaorphism and are interpreted as reaction skarns. They have formed in the presence of a water-dominated fluid phase with very low X_CO2.In the Koralpe estimated P-T conditions are 650–700 °C at 5–7 kb. The mineralogical composition and the mineral zoning of the calc-silicate rocks is controlled by the degree of the Hercynian and Eoalpine metamorphism. There are no signs of graniteelated skarn formation. Tungsten preconcentration is thought to be syngenetic/syndiagenetic. It is genetically linked to exhalative hydrothermal processes in other Lower Paleozoic terrains of the Eastern Alps.     

Fluid metasomatic genesis of stratiform skarn in the Suoerkuduke Cu-Mo deposit,East Junggar,NW China

Stratiform skarns associated with ore deposits are widespread in the north of East Junggar, particularly in the Suoerkuduke Cu-Mo deposit. The Suoerkuduke stratiform and stratoid skarns are hosted by Devonian intermediate-mafic volcanic and pyroclastic rocks, mainly andesite, andesitic porphyry and tuffaceous sandstone, without carbonate or calcareous rocks. The skarns consist of dominant andradite-grossular, epidote, diopside-hedenbergite and minor actinolite, quartz, magnetite and metallic sulfides. The garnet and epidote composition, especially Fe^3 + and Al contents, is largely a function of the bulk composition and physicochemical environment (particularly fO₂) during crystallization. Such mineralogy indicates a relatively oxidizing environment and medium acidity of solution during skarnization.The Suoerkuduke skarns are distinct from typical contact metasomatic skarn in wall rock, as no carbonate or calcareous rocks were found, and differ in the distribution patterns of skarn zonation in that gradually weakened skarn zones are not quite symmetrically distributed on both sides of the alteration center (a garnet skarn). Abundant remnants of andesite, andesitic porphyry and tuffaceous sandstone in the weakened skarn zone indicate that the protolith of the skarn is andesite, andesitic porphyry and tuffaceous sandstone. Magmatic water, meteoric and seawater are involved in skarn alteration. Moyite and granitic porphyry are not coeval with skarn, and their emplacement resulted in the hornfelization of wall rock instead of skarnization, and themselves keep away from skarn alteration. Therefore, there was probably a huge batholith supplying magmatic fluid for skarn formation. Mass balance estimates suggest that hydrothermal fluid must contribute a portion of Ca and Fe to ensure sufficient supply for skarn formation in the absence of local carbonate and calcareous rocks. In conclusion, the stratiform skarns in the Suoerkuduke are products of intermediate-mafic volcanic and pyroclastic rocks metasomatised by hydrothermal fluid that probably leached calcareous wall rock during ascent.     

Tectonic setting and petrogenesis of the Çelebi granitoid, (Kırıkkale-Turkey) and comparison with world skarn granitoids

Many studies have shown systematic correlations between the composition of plutons worldwide and the metal content of associated skarns. This is the first report of similar correlations between the composition of Çelebi granitoid and skarns of the Çelebi district in Central Anatolia, Turkey. The Çelebi district is well known for its polymetallic Fe–W and Cu vein ores. These are hosted by calcic skarn zones. Both exoskarns (pyroxene–garnet) and endoskarns (epidote–pyroxene) occur in the district formed mainly along the granitoid contacts and along the fractures within the marble. Based on mineralogy, petrology and geochemistry, two different igneous rocks were recognized in the Çelebi granitoid, referred to as leucocratic (felsic) and mesocratic (intermediate) Çelebi granitoid. The leucocratic Çelebi occurs as dominant rock type, and is classified as granite. The mesocratic Çelebi is not widespread and is classified as adamellite, tonalite, quartz monzonite and quartz monzodiorite. The mesocratic Çelebi has I-type characteristics, and have subalkaline, calc-alkaline and metaluminous characteristics like most worldwide skarn granitoids.A post-collisional tectonic setting is proposed on the basis of field evidence, the relative timing of intrusions with respect to metamorphic and obducted ophiolitic rocks and trace element geochemistry. The high abundance of La and Ce and the enrichment of V in mafic components suggest that Çelebi granitoids are formed by partial melting of mantle rocks, but have been contaminated by interaction with continental crust involving possible magma mixing processes (i.e. mixing of coexisting felsic and mafic magmas). In the district, the mesocratic type and mafic microgranular enclaves (MME) mainly within leucocratic type represent a mafic underplating magma that was mixed with and/or injected into felsic magma of the leucocratic type.The present study shows that Fe mineralization is associated with mesocratic Çelebi type, whereas W mineralization is associated with leucocratic type. Mesocratic Çelebi granitoid is significantly different from the worldwide average of plutons associated with Fe skarns. In particular, MgO vs. SiO₂, FeOt+CaO+Na₂O/K₂O vs. SiO₂, Fe₂O₃/Fe₂O₃+FeO vs. SiO₂ and V vs. Ni vary from typical values (are lower than values typical for plutons associated with Fe skarns) for plutons associated with Fe skarns. Instead, it resembles the geochemical characteristics of plutons associated with worldwide Cu and possibly Au skarns. This suggests new exploration possibilities for copper and gold in the Çelebi district.     

Origin and evolution of the calcic and magnesian skarns hosting the El Valle-Boinás copper–gold deposit, Asturias (Spain)

The El Valle-Boinás copper–gold deposit is located in the southern part of the Rio Narcea Gold Belt 65 km west of Oviedo (NW Spain), within the Cantabrian Zone (Iberian Hercynian Massif). The deposit is related to the Boinás stock, which ranges from quartz-monzonite to monzogranite and intruded (303 Ma) the carbonated Láncara Formation (early Cambrian) and the siliciclastic Oville Formation (middle-late Cambrian).A copper–gold skarn was developed along the contact between the igneous rock and the carbonated sedimentary rocks. The skarn distribution and mineralogy reflects both structural and lithologic controls. Two types of skarn exists: a calcic skarn mainly developed in the upper calcic member of the Láncara Formation, and a magnesian skarn developed in the lower dolomitic and organic-rich member. The former mainly consists of garnet, pyroxene and wollastonite. Retrograde alteration consists of K-feldspar, epidote, quartz, calcite, magnetite, ferroactinolite, titanite, apatite, chlorite and sulfides. Magnesian skarn mainly consists of diopside with interbedded forsterite zones. Pyroxene skarn is mainly altered to tremolite, with minor phlogopite and serpentine. Olivine skarn is pervasively altered to serpentine and magnetite, and is commonly accompanied by high sulfide and gold concentrations. This altered skarn results in a very dark rock, referred to as “black skarn”, which has great importance in gold reserves. Sulfide mineralization mainly consists of chalcopyrite, bornite, arsenopyrite, pyrrhotite and pyrite, while wittichenite, sphalerite, digenite, bismuthinite, native bismuth and electrum occur as accessory minerals.After extensive erosion, reactivation of the northeast-trending fracture zone provided conduits for the subsequent emplacement of porphyritic dikes (285±4 Ma) and diabasic dikes (255±5 Ma). Alteration, characterized by sericitization, silicification, carbonatization and hypogene oxidation took place, as did sulfide mineralization (pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, bournonite, and Fe–Pb–Sb sulfosalts). Veins with quartz, carbonate, adularia and sulfide minerals crosscut all previous lithologies. Jasper and jasperoid breccias developed at the upper parts of the deposits.The fluid inclusion and stable isotope studies suggest a predominantly magmatic prograde-skarn fluid characterized by high-salinity (26–28 wt.% KCl and 32–36 wt.% NaCl) and high temperature, above 580°C. This fluid evolved into two immiscible fluids: a CO₂- and/or CH₄-rich, high-salinity aqueous fluid. Temperatures for the first retrograde-stage are between 350 and 425°C. A second stage is related to a more diluted aqueous fluid (3–6.2 wt.% NaCl eq.) and temperatures from 280 to 325°C. The fluid inclusion study performed on quartz from low-temperature mineralization indicates a very low salinity (0.2–6.2 wt.% NaCl eq.), low-temperature aqueous fluid (from 150 to 250°C), and trapping pressure conditions less than 0.2 kbar. In addition, the stable isotope study suggests that an influx of metamorphic waters derived from the country rocks produced these lower temperature fluids. The last control for the Au mineralization is the Alpine tectonism, which developed fault breccias (cataclasites to, locally, protomylonites) and gold remobilization from previous mineralization.     

A new occurrence of garnetiferous skarn rocks in Saudi Arabia: a case study from Bahrah area, Jeddah–Makkah Al Mukaramah highway

Metacarbonate rocks (including marble and skarn deposits) at Bahrah area are confined to a Precambrian island-arc suite made up mostly of massive basalts and volcaniclastics aligned in a NE-trending belt. The marbles are either pure (almost made up of calcite) or contain considerable amounts of tremolite, actinolite, epidote, and diopside. Garnet-bearing rocks at Bahrah area are classified into garnetiferous marble and skarn calc-silicate assemblages that are described here for the first time. The calc-silicates become more abundant when the marble becomes interbedded with foliated metabasalt. Such contact is delineated by an epidote zone of variable thickness. Microscopically, the skarns are enriched in Ca-bearing minerals such as grossular garnet, epidote, titanite, diopside, and augitic salite. There are evidence that calc-silicate skarns were formed due to a thermal effect of a concealed underground shallow granitic intrusion. The basaltic rocks furnished Mg²⁺, Fe²⁺, Ti⁴⁺, and Al³⁺ that were first concentrated in the epidote zone. This was followed by pervasive replacement of epidote by large idiomorphic garnet (grossularite) that attains up to ～1.5 cm wide. It is evident that diopside is earlier than garnet with no replacement fabrics between the two minerals. Two types of titanite (sphene) can be distinguished: The first is secondary in the metabasalt host where titanite develops after titanomagnetite during regional metamorphism (i.e., metamorphic). On the other hand, the second type of titanite is found in the garnet-bearing calc-silicate skarn where it is typically euhedral with no link to any opaque phase and it is believed to be formed due to the event of superimposed thermal metamorphism (i.e., metasomatic). There are several evidence of the thermal metamorphic effect such as distinct granoblastic and annealing textures and K-metasomatism and formation of phlogopite at the expense of tremolite in the marble, in addition to poikiloblastic hornblende in the metabasalt host with distinct recrystallization. Also, there are some evidence of shearing such as brecciation along microshear planes, microfolding, introduction of fine euhedral pyrite, and presence of injected silica postdating crystallization of garnet in the calc-silicates.     

Use of intrusive and calc-silicate compositional data to distinguish contrasting skarn types in the Darwin polymetallic skarn district,California, USA

The polymetallic Pb-Zn-Ag-W-Cu-Au skarn and vein ores of the Darwin area, California, represent a metal association not common in skarn deposits. Detailed and regional geologic mapping combined with modal and chemical analyses of igneous rocks and electron microprobe and assay studies of calc-silicate rocks shows that there are three separate mineralizing systems which have been superimposed by structural and intrusive events. Scheelite-bearing skarns are associated with the alkaline Darwin stock; small Cu±Au skarns are adjacent to sheared (thrust-emplaced) granite-granodiorite masses; and Pb-Zn-Ag vein skarns are zoned around small granite porphyry dikes and plugs. Calc-silicate mineral compositions are also indicative of three different mineralization systems: W-skarns have high Fe pyroxenes and moderate Fe garnets; Cu-skarns have low Fe-pyroxenes and high Fe-garnets; and Pb-Zn skarns have high-Mn pyroxenes and garnets (patterns seen in other skarn deposits). Vein ores contain metals compatible with the subjacent skarns and apparently represent lower-temperature remobilization of skarn ores.     

西藏墨竹工卡地区甲玛铜多金属矿床矽卡岩矿物学特征及其地质意义

为研究西藏甲玛铜多金属矿床中矽卡岩的矿物学特征,进一步确定矿床成因类型,利用电子探针测试和镜下鉴定手段对矽卡岩矿物中的石榴子石、辉石、硅灰石等矿物成分进行了分析。测试结果表明,矽卡岩中石榴子石以钙铁榴石-钙铝榴石为主,辉石以透辉石为主,甲玛矿床矽卡岩属于交代矽卡岩中典型的钙矽卡岩。结合前人对矿区矽卡岩、围岩和花岗岩类的岩石地球化学、矿床成矿年代学等的研究,进一步证实甲玛铜多金属矿床系矿区花岗岩类岩浆交代大理岩形成的典型矽卡岩型矿床。     

Die Stellung des Amphibols in der Mineralparagenese westmährischer Skarngesteine

Summary The iron-ore skarns of the core of the Bohemian Massif is an old petrographic unit which is already doeply metamorphosed under conditions of an abyssal metamorphism. The paragenetical analysis as well as the chemical study of their mineral assemblages makes it clear that the amphibole occuring in subordinate quantities in these skarns is not a relic mineral (having not originated during the premetamorphic skarnization period). It appeared only in the course of the regional metamorphism in connection with the migmatization processes which took place in the rocks adjoining the skarn bodies and in connection with the injections of pegmatites traversing the skarn rocks.

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Phosphorus in regionally metamorphosed skarns of the Bohemian Massif and its genetic meaning

Concentrations of phosphorus have been determined in 130 specimens o regionally metamorphosed skarn rocks, including their enclosing rocks Average P₂O₅-values found were 0.11% in the pyroxene skarns from the Core of the Bohemian Massif and 0.07% in those of the Kruné hory Mountains, which correspond to those commonly present in primary, i.e. contact metamorphic skarns, but are quite different from the high P₂O₅-values usually found in sedimentary iron ores. The P-contents of the rocks from the cores of the skarn bodies studied equal those determined in relic carbonate rocks, from which they probably originated, whereas the P-contents of the rocks from the outer parts of the skarn bodies correspond to those found in the enclosing rocks. The distribution of P in migmatites surrounding the skarn bodies suggests that P did not migrate during the regional metamorphism, so that the P-contents in the skarns may be considered as primary (premetamorphic). Thus, a study of P-percentages in regionally metamorphosed skarns may be useful in the investigation of skarn genesis.

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Zusammenfassung Die Phosphorgehalte von 130 Proben regionalmetamorpher Skarne, sowie ihrer Hüll- und Begleitgesteine wurden ermittelt. Der P-Durchschnittsgehalt der Pyroxenskarne beträgt im Kern der Böhmischen Masse 0,11%, im Erzgebirge 0,07%. Diese Gehalte entsprechen denjenigen kontaktmetamorpher Skarne, sind aber von den großen gewöhnlich in sedimentogenen Eisenerzen vorkommenden P-Gehalten verschieden. Die Phosphor-Gehalte der Skarngesteine aus den Kernpartien der Skarnkörper entsprechen denjenigen der reliktischen Carbonatgesteine, die die Skarne begleiten und auf deren Kosten sich die Pyroxenskarne wahrscheinlich entwickelt haben. Demgegenüber entsprechen die P-Gehalte der Gesteine aus den äußeren Partien der Skarnkörper denen der Hüllgesteine. Die Verteilung des Phosphors zwischen Metatekt und Paläosom in Migmatiten, die die Skarnkörper umhüllen, zeigt, daß Phosphor während der Regionalmetamorphose kaum migrierte. Seine Gehalte in Gesteinen lassen sich also für primär, d.h. prämetamorph halten. Wie ersichtlich, können Kenntnisse über die P-Verteilung zur Lösung der Fragen der Skarngenese beitragen.

     

湖南杏枫山钨矿床热液榍石的地球化学特征、U-Pb定年及其地质意义

杏枫山钨矿床是湘中地区近年新发现的、产出于浅变质岩系中的矽卡岩型钨矿。本文通过电子探针与LA-ICP-MS等技术手段,对杏枫山矿区矽卡岩中的热液榍石进行了主、微量元素组成以及U-Pb年代学研究。研究表明,榍石具有以下特征:(1)较低的∑REE、Ti O2含量,极低的Th/U,以及较高的Al_2O_3和F含量,这些均为典型热液榍石的地球化学特征;(2)富W而亏损Mo,表明其形成的热液流体具有富W贫Mo的特征;(3)稀土配分模式为左倾的、轻稀土显著亏损的重稀土富集型,正Ce、Eu异常表明其形成环境较为还原。此外,榍石原位U-Pb定年结果显示该矿形成于215.2±2.7Ma,与邻近白马山钛铁矿花岗岩形成的时间一致,基于地质及年代学证据,我们认为该矿的形成与白马山印支期的岩浆热液活动有关。结合前人的研究结果,我们发现湘中地区钨成矿与印支期还原性花岗质岩浆岩密切相关,为湘中印支晚期区域性成矿作用的产物。     

Geology and Metallogeny of the Shizhuyuan Skarn-Greisen Deposit,Hunan Province,China

The Shizhuyuan deposit is the largest among the economically important polymetallic tungsten deposits in China. The deposit occurs within the thermal aureole of Yanshanian felsic intrusions that were emplaced into Devonian carbonates and marls. The mineralization can be divided into three phases that are genetically associated with three episodes of granitic emplacement-pseudoporphyritic biotite granite, equigranular biotite granite, and granite porphyry. During the emplacement of pseudoporphyritic biotite granite, thermal metamorphism and subsequent skarnization developed around the stock. The pure limestone was transformed to marble, whereas marls and argillite interlayers were changed to a series of metamorphic rocks such as grossular-diopside hornfels, wollastonite hornfels, diopside hornfels, wollastonite-vesuvianite hornfels, muscovite-K-feldspar-anorthite hornfels, and prehnitevermiculite hornfels. Because of the subsequent strong skarn development, most hornfelses later were transformed into skarns. The skarns distributed around the granite stock are mainly calcic. They are massive in structure, and are composed mainly of garnet, pyroxene, vesuvianite, and wollastonite, with interstitial fluorite, scheelite, and bismuthinite. Although there is no cassiterite in the early skarns, their tin contents average 0.1%. The distribution and compositional and mineralogical relationships of skarn minerals suggest that they formed as a result of progressive reactions of a hydrothermal solution with a limestone of generally constant composition, and that the dominant process was progressive removal of Ca and addition of other constituents to the rocks.

Following the primary skarn formation, some of the assemblages were retrograded to new assemblages such as fluorite-magnetite-salite rock, magnetite-fluorite-amphibole rock, and magnetite-fluorite-chlorite rock. The retrograde alteration of the skarns is characterized by a progressive addition of fluorine, alkali components, silica, tin, tungsten, and bismuth. A zonation from garnet-pyroxene skarn or garnet skarn, through fluorite-magnetite-salite rock, to magnetite-fluorite-chlorite rock frequently can be recognized in the deposit. All retrograde-altered rocks contain scheelite, cassiterite, molybdenite, and bismuthinite.

During the emplacement of equigranular biotite granite, skarn veins several tens of centimeters wide were developed; they contain large crystals of garnet and vesuvianite, and interstitial scheelite, wolframite, cassiterite, and molybdenite. This second stage of mineralization occurs predominantly as coarse and fine stockwork greisens, which were superimposed on the massive skarns and surrounding marble. Such W-Sn-Mo-Bi-bearing greisens can be divided into topaz greisen, protolithionite greisen, muscovite greisen, and margarite greisen. Besides calcic skarn veins and greisens, manganese skarn veinlets also were developed; they consist of rhodonite, spessartine-almandine solid solution, spessartine, and helvite. The distribution of greisens is responsible for a metal zonation—i.e., W-Sn-Mo-Bi and Sn-Be-Cu-F zones from the contact boundary between the granite stock and skarns outward in the deposit. A third stage of mineralization is represented by lead-zinc veins, which also are accompanied by manganese skarns consisting of spessartine, rhodonite, manganese-rich pyroxene, helvite, tephroite, fluorite, tourmaline, and manganese-rich phlogopite.     

Composition, mineral assemblages, and genesis of titanite and malayaite in skarns

The composition of minerals of the titanite-malayaite series and their mineral assemblages and genesis were examined at the Bol’shoi Kan’on deposit in Magadan oblast and at other deposits. These minerals were demonstrated to be typomorphic Sn-bearing silicates in postmagmatic bimetasomatic hypabyssal calc skarns and skarnoids in tin-bearing provinces. The series of these minerals with similar crystal structures has a miscibility gap, and the minerals are characterized by notably different Sn concentrations. Moreover, titanite may contain Al, Fe, F, and OH, whose concentrations decrease in the Sn-bearing members of the series (malayaite). These silicates were formed at many deposits after the successive transforms of skarn mineral assemblages. The early assemblages include wollastonite in calcic carbonate rocks and diopside and salite in skarnoids. The latter minerals are replaced first by hedenbergite with subordinate amounts of vesuvianite and garnet first of grossular and then andradite composition. This process was syngenetic with the formation of borosilicates (danburite, axinite, and tourmaline). Ti thereby may be accommodated in grossular and Sn in Fe-bearing silicates, mostly, in andradite. Skarns often contain both titanite and malayaite, which were produced in these rocks earlier than cassiterite. The isomorphic series of these minerals has a miscibility gap. The oreforming processes ended with the crystallization of quartz, fluorite, and rare sulfides, including stannite. The late Sn-bearing minerals at some deposits are stokesite and Mg, Fe, and Ca stannates, which crystallized during malayaite replacement by newly formed calcite-quartz aggregates. The Sn-bearing sulfides are replaced by varlamoffite during supergene processes.     

In-situ LA–ICP-MS trace elemental analyses of magnetite: The Mesozoic Tengtie skarn Fe deposit in the Nanling Range,South China

The Nanling Range in South China hosts numerous world-class W–Sn deposits and some Fe deposits. The Mesozoic Tengtie Fe skarn deposit in the southern Nanling Range is contemporaneous with the regional Sn mineralization. The deposit is composed of numerous ore bodies along the contacts between the late Paleozoic or Mesozoic carbonate rocks and the Yanshanian Lianyang granitic complex. Interaction of the magma with hosting dolomitic limestone and limestone formed calcic (Ca-rich) and magnesian (Mg-rich) skarns, respectively. The Tengtie deposit has a paragenetic sequence of the prograde stage of anhydrous skarn minerals, followed by the retrograde stage of hydrous skarn minerals, and the final sulfide stage. Magnetite in the prograde and retrograde skarn stages is associated with diopside, garnet, chlorite, epidote, and phlogopite, whereas magnetite of the final stage is associated with chalcopyrite and pyrite. Massive magnetite ores crosscut by quartz and calcite veins are present mainly in the retrograde skarn stage. Laser ablation ICP-MS was used to determine trace elements of magnetite from different stages. Some magnetite grains have unusually high Ca, Na, K, and Si, possibly due to the presence of silicate mineral inclusions. Magnetite of the prograde stage has the highest Co contents, but that of the sulfide stage is extremely poor in Co which partitions in sulfides. Magnetite of magnesian skarns contains more Mg, Mn, and Al than that of calcic skarns, attributed to the interaction of the magma with compositionally different host rocks. Magnetite from calcic and magnesian skarns contains 6–185 ppm Sn and 61–1246 ppm Sn, respectively. The high Sn contents are not due to the presence of cassiterite inclusions which are not identified in magnetite. Instead, we believe that Sn resides in the magnetite structure. Regionally, intensive Mesozoic Sn mineralization in South China indicates that concurrent magmatic–hydrothermal fluids may be rich in Sn and contribute to the formation of high-Sn magnetite. Our study demonstrates that trace elements of magnetite can be a sensitive indicator for the skarn stages and wall-rock compositions, and as such, trace elemental chemistry of magnetite can be a potentially powerful fingerprint for sediment provenance and regional mineralization.     

Fractal distribution of particle size in carbonate cataclastic rocks from the core of a regional strike-slip fault zone

We present particle size data from 31 samples of carbonate cataclastic rocks collected across the 26 m thick fault core of the Mattinata Fault in the foreland of the Southern Apennines, Italy. Particle size distributions of incoherent samples were determined by a sieving-and-weighting technique. The number of weight-equivalent spherical particles by size is well fitted by a power-law function on a log–log space. Fractal dimensions (D) of particle size distributions are in the 2.091–2.932 range and cluster around the value of 2.5. High D-values pertain to gouge in shear bands reworking the bulk cataclastic rocks of the fault core. Low D-values characterise immature cataclastic breccias. Intermediate D-values are typical of the bulk fault core. Analysis of the ratio between corresponding equivalent particle numbers from differently evolved cataclastic rocks indicates that the development of particle size distributions with D>2.6–2.7 occurred by a preferential relative increase of fine particles rather than a selective decrement of coarse particles. This preferentially occurred in shear bands where intense comminution enhanced by slip localisation progressed by rolling of coarse particles whose consequent smoothing produced a large number of fine particles. Our data suggest that during the progression of cataclasis, the fragmentation mode changed from the Allègre et al.'s [Nature 297 (1982) 47] “pillar of strength” mechanism in the early evolutionary stages, to the Sammis et al.'s [Pure and Applied Geophysics 125 (1987) 777] “constrained comminution” mechanism in the subsequent stages of cataclasis. Eventually, localised shear bands developed mainly by abrasion of coarse particles.     

云南个旧塘子凹锡多金属矿床夕卡岩地质地球化学特征

从地质产状、矿物组合和岩石化学成分等方面探讨了个旧塘子凹接触带不同类型夕卡岩的特征。该夕卡岩带从内侧到外侧常具有辉石夕卡岩带和石榴子石夕卡岩带交替出现的现象，其岩石化学成分也相应地发生韵律变化，表现为在辉石夕卡岩带中SiO2和MgO含量较高，而在石榴子石夕卡岩带中CaO、TFe和Al2O3含量较高。认为夕卡岩带中的韵律变化一方面与被交代围岩中存在灰质白云岩和大理岩的互层带有关，另一方面与岩浆期后热液的渗滤交代作用有关。围岩中的灰质白云岩层被交代后形成辉石带，大理岩层被交代后形成石榴子石带。     

Composition, mineral assemblages, and genesis of serendibite-bearing magnesian skarns

Variations in the composition and mineral assemblages of boronaluminosilciates (serendibite, grandidierite, kornerupine, and tourmaline) were studied in the abyssal and hypabyssal skarns of New York and California, United States, the Taezhnyi deposit of southern Yakutia, and deposits of the Pamirs, and compared to their occurrences around the world. The genesis of the boronaluminosilicates depends on the facies of the replaced skarns and the calcareous-skarn alteration of the primary composition of the host rocks. The substitution between Mg and Fe, as well as between Al, Si, and B, was studied in complex boronaluminosilciates and associated minerals. It was shown that f of serendibite is determined by that in the replaced skarn minerals (pyroxenes, spinel, sapphirine, and grandidierite) and is inherited in the replacing tourmaline and late silicates. Unlike serendibite, kornerupine is a typomorphic mineral of only bimetasomatic skarns of the abyssal facies. Serendibite, grandidierite, kornerupine, and tourmaline crystallized during the postmagmatic stage of the evolution of boron mineralization at skarn deposits of both the abyssal and the hypabyssal facies, at contact with magnesian carbonate sequences and desilicified aluminosilicate rocks.     

Evolution of Ore-Forming Metasomatic Processes at Large Skarn Iron Deposits Related to the Traps of the Siberian Platform

The paper presents systematized and synthesized data on the parameters and evolutionary sequence of metasomatic processes that accompanied interaction between Permian–Triassic trap complex and rocks of the sedimentary cover of the Siberian Platform at the large skarn iron deposits. Relations of the textural–compositional, morphological, and genetic diversity of the skarns and ores with the phases and stages of the origin of ore-bearing volcano-tectonic edifices are demonstrated with reference to the Korshunovskoe and Rudnogorskoe deposits. The genetic reconstructions are based on survey materials and data on the mineralogy of the rocks and ores (obtained by optical and scanning electron microscopy, microprobe analysis, EPR, Raman and IR spectroscopy, and by studying inclusions in minerals). A principally important feature of the volcano-tectonic edifices of the large mineral deposits is their multistage evolution and combinations of fluid-conducting zones, which are related to (1) volcanic apparatuses, (2) shallow-depth magmatic chambers (laccoliths) hosted in carbonate–salt rocks, and (3) multistage fracture structures produced by the collapse of the leached space. The major ore-bearing structures were formed simultaneously with the development of an intermediate magmatic chamber hosted in Cambrian carbonate–salt rocks beneath a seal of terrigenous sedimentary rocks. Magmatic-stage magnesian skarns with disseminated ores in them and in the calciphyres were produced during the prograde stage in the apical parts of the laccoliths, at contacts between the dolerites and dolomites. During the early prograde stage, skarn–ore bodies developed around injection bodies of globulated dolerites, laccoliths, and sills; stockworks and steep bodies of fragmentary magnesian and calcic skarns and ores were formed within the diatremes; and conformable bodies and veins were produced in the splay fracture zones. The later reactivation of faults and fractures and the involvement of connate brines and solutions from the evaporite complex triggered the redeposition of the ore masses, crystallization of the mineral assemblages of hydrated skarns, development of large domains of serpentine–chlorite–epidote–amphibole rocks, calcic skarns, and ores. Data on multiphase fluid inclusions in the forsterite, apatite, and halite indicate that the mineral-forming fluid initially was a highly concentrated solution–melt (total salinity of 60%) with high-density reduced gases. The magnesian skarns were formed during the following stages: (1) forsterite + fassaite + spinel + first-population magnetite (820–740°C); (2) phlogopite + titanite + pargasite + second-population magnetite (600–500°C), and (3) clinochlore + serpentine + tremolite + pyrrhotite + chalcopyrite (≥450°C).     

Granite- and gabbrodiorite-associated skarn deposits of NW Iran

Field and laboratory studies show that there are two types of skarn deposits in NW Iran: granite-associated (type I) and gabbrodiorite-associated (type II). Granite-associated deposits are accompanied by Cu and Fe mineralisation, whereas Mn and Fe are the main ore metals in gabbrodiorite-associated skarn deposits. There are some differences in the mineralogy of these skarn deposits. Bixbyite, piemontite and Cr-bearing garnet are found only in gabbrodiorite-associated skarns, whereas idocrase occurs only in granite-associated deposits. Type II skarns show exoskarn features, whereas some type I skarns have developed endoskarn as well. Evidence of boiling of hydrothermal fluid can be seen in both types and seems to be a common mechanism of mineral deposition. Gabbrodiorite-associated skarns show higher fO₂ than granite-associated deposits. Based on mineralogical and textural evidence, mineralisation in both groups has started from about 550 °C. Early formed anhydrous minerals have begun to be replaced by hydrous minerals from about 400 °C.It seems that due to low fluid content in the gabbrodioritic magma, heated meteoritic water in the surrounding volcanoclastic and tuffaceous rocks was the main source of hydrothermal solution in the gabbrodiorite-associated skarn system.     

Mary Kathleen metamorphic‐hydrothermal uranium — rare‐earth element deposit: Ore genesis and numerical model of coupled deformation and fluid flow

The Mary Kathleen U‐REE orebody of the Proterozoic Mt Isa Block was the product of chemical and physical interaction between regional metamorphic/hydrothermal fluids and preexisting calcic skarns. The deposit provides excellent examples of mechanical control on ore localisation and of the complexity of ores in rocks with protracted thermal histories. Host skarns were produced by contact metasomatism around the 1740 Ma Burstall Granite, whereas the allanite‐uraninite ore formed under amphibolite‐facies conditions, late during the D₂ phase of the ca 1550–1500 Ma Isan orogeny. Observations of ore geometry are consistent with previous geochronologic data demonstrating a large time gap between skarn formation and ore genesis. Numerical modelling of coupled deformation and fluid flow suggests that veins at the core of ore shoots may have formed as tensile or shear fractures during coupling of the competent skarn host with the late‐D₂ Mary Kathleen Shear Zone, allowing a change of orientation of ore shoots with distance from the shear zone. Mineral chemistry and petrographic observations suggest the possible role of a redox control on chemical localisation of ore by conversion of Fe²⁺‐rich clinopyroxene‐rich skarn host to Fe³⁺‐rich secondary garnet ‘skarn’ and uraninite‐allanite ore. Alternately, fluid pressure drops as a consequence of fracturing of the host skarn may have triggered fluid unmixing, or fluid mixing, leading to ore precipitation. Available data do not allow clear definition of the ultimate source of the U and REE, nor of the specific chemical ore‐forming mechanism. However, regional constraints, previous Sm–Nd modelling, and our numerical models suggest a combination from proximal skarn hosts and from distal sources accessed by flow of metamorphic and/or late tectonic igneous‐derived fluids. The deposit has some similarities with ironstone‐hosted Cu–Au ± U deposits found in the nearby Cloncurry Belt.     

Genesis of wollastonite- and grandite-rich skarns in a suite of marble-calc-silicate rocks from Sittampundi, Tamil Nadu: constraints on the P–T–fluid regime in parts of the Pan-African mobile belt of South India

The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite?+?clinopyroxene (mg#?=?71–73)?+?grandite (16–21 mol% Adr)?+?quartz?±?calcite, Type II: grandite (25–29 mol% Adr )?+?clinopyroxene (mg#?=?70)?+?calcite?+?quartz, and Type III: grandite (36–38 mol% Adr)?+?clinopyroxene (mg#?=?55–65)?+?epidote?+?scapolite?+?calcite?+?quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn₁ related to the Fn₁ folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn₂ folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite?+?SiO₂?→?wollastonite?+?V, 2. calcite?+?clinopyroxene?+?O₂?→?grandite?+?SiO₂?+?V, 3. scapolite?+?calcite?+?quartz?+?clinopyroxene?+?O₂?→?grandite?+?V and 4. epidote?+?calcite?+?quartz?+?clinopyroxene?+?O₂?→?grandite?+?V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (X_CO2?<?0.15) led to production of large volumes of wollastonite in the wollastonite-rich skarn whereas mobility of FeO, SiO₂ and CaO across the interface of marble and calc-silicate and infiltration of aqueous fluids (X_CO2?<?0.35) were instrumental for the formation of grandite skarns. Composition of minerals in type II skarn indicates that Al₂O₃ was introduced in the host marble by the infiltrating fluid. Interpretation of mineral assemblages observed in the interlayered metapelites and the calcareous rocks in pseudosections, isothermal P-X_CO2 and isobaric T-X_CO2 diagrams tightly bracket the “peak” metamorphic conditions at c.9?±?1 kbar and 750°?±?30°C. Subsequent to ‘peak’ metamorphic conditions, the rocks were exhumed on a steeply decompressive P–T path. The estimated ‘peak’ P–T estimates are inconsistent with the “extreme” metamorphic conditions (>11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn₁ folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.