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781.
Zijinshan is the largest high-sulfidation epithermal Cu–Au deposit on mainland China. The primary mineralization at the deposit is characterized by pyrite, digenite and covellite. Although some RbSr isochron ages of the fluid inclusions in quartz and the apparent K-Ar age of the Cu-bearing alunite alteration zone have been previously interpreted as the ore-forming age, the Rb-Sr and K-Ar dating systems are usually readily reset due to their closure temperature, and thus document the latest thermal event. In order to precisely determine the age of the Cu mineralization, eight-pyrite separates were Re-Os dated, and seven yielded an isochron age of 103 ± 4 Ma with an initial 187Os/188Os ratio of 0.45 ± 0.14. This date is interpreted as the age of Cu mineralization, which is in agreement with geochronological data from molybdenite and the porphyritic granodiorite that hosts Cu-Mo mineralization at Luoboling (located 2 km NE of Zijinshan), suggesting that these two deposits were formed during the same metallogenic event. The relatively low initial 187Os/188Os ratio indicates that the source of the Cu at Zijinshan is predominantly the crust with input from the mantle. 相似文献
782.
The Sangan iron skarn deposit is located on the eastern edge of the Sabzevar-Doruneh Magmatic Belt, northeastern Iran. Mineralization occurs at the contact between Eocene igneous rocks and Cretaceous carbonates. The silicate-dominant prograde skarn stage consists of garnet and clinopyroxene, whereas the retrograde stage is dominated by magnetite associated with minor hematite, phlogopite, pyrite, and chalcopyrite. Phase equilibria and mineral chemistry studies reveal that the skarn formed within a temperature range of ∼375° to 580 °C and that the mineralizing fluid evolved from a hot, low oxygen fugacity, alkaline fluid during the silicate-dominant stage to a fluid of relatively lower temperature and higher oxygen fugacity at the magnetite-dominant stage. The δ18O values of magnetite and garnet vary from +3.1 to +7.5‰ and +7.7 to +11.6‰, respectively. The calculated δ18OH2O values of fluid in equilibrium with magnetite and garnet range from +9.8 to +11.1‰ and +10.1 to +14.8‰, respectively. These elevated δ18OH2O values suggest interaction of magmatic water with 18O-enriched carbonates. The high δ34S values (+10.6 to +17.0‰) of pyrite separates from the Sangan iron ore indicate that evaporites had an important role in the evolution of the hydrothermal fluid. Phlogopite separates from the massive ores yield 40Ar/39Ar plateau ages of 41.97 ± 0.2 and 42.47 ± 0.2 Ma, indicating that the skarn formation and associated iron mineralization was related to the oldest episode of magmatism in Sangan at ∼42 Ma. Eocene time marked a peak of magmatic activity and associated skarn in the post-collisional setting in northeastern Iran, whereas Oligo-Miocene magmatic activity and associated skarn in the Urumieh-Dokhtar Magmatic Belt are related to subduction. In addition, skarn mineralization in northeastern and eastern Iran is iron type, but skarn mineralization in the Urumieh-Dokhtar magmatic belt is copper – iron and copper type. 相似文献
783.
One of the most puzzling features of the UG1 chromitite layers in the famous exposures at Dwars River, Eastern Bushveld Complex, is the bifurcation, i.e. convergence and divergence of layers along strike that isolate lenses of anorthosite. The bifurcations have been variously interpreted as resulting from: (1) the intermittent accumulation of plagioclase on the chamber floor as lenses, terminated by crystallization of continuous chromitite layers (the depositional model); (2) late-stage injections of chromite mush or chromite-saturated melt along anastomosing fractures that dismembered semi-consolidated plagioclase cumulates (the intrusive model); (3) post-depositional deformation of alternating plagioclase and chromite cumulates, resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally (the deformational model). None of these hypotheses account satisfactorily for the following field observations: (a) wavy and scalloped contacts between anorthosite and chromitite layers; (b) abrupt lateral terminations of thin anorthosite layers within chromitite; (c) in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite; many of these inclusions are contiguous with footwall and hanging wall cumulates; (d) transported anorthosite fragments enclosed by chromitite; (e) disrupted anorthosite and chromitite layers overlain by planar chromitite; (f) protrusions of chromitite into underlying anorthosite; (g) merging of chromitite layers around anorthosite domes. We propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo-chemical erosion of the temporary floor of the chamber. The melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers, resulting in lens-like remnants of anorthosite resting on continuous layers of chromitite. On cooling, the magma crystallized chromite on the irregular chamber floor, draping the remnants of anorthosite and merging with pre-existing chromitite layers excavated by erosion. With further cooling, the magma crystallized chromite-bearing anorthosite. Emplacement of multiple pulses of magma led to repetition of this sequence of events, resulting in a complex package of anorthosite lenses and bifurcating chromitite layers. This hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the Bushveld Complex. 相似文献
784.
The large Vorontsovskoe Au-Hg-As deposit in the Urals is located in the exocontact of the Early Devonian Auerbah gabbro-diorite-granodiorite massif, which intrudes volcano-sedimentary rocks. The orebodies are confined to a tectonic contact of calcareous and tuffaceous rocks. They are composed of 6 types of disseminated ores, but the main reserves of gold are associated with the following ore types: gold-pyrite-arsenopyrite in altered tuffaceous rocks, pyrite-realgar ores in limestone breccia with a carbonate-volcanogenic cement, and gold-oxide-clay from regolith with residual gold. Early ore associations have been formed at 450–300 °C, whereas the late ores have been formed at lower temperature of 260–110 °C. We propose a model for the genesis of the Vorontsovskoe deposit based on synchronicity of mineralization with the formation of the Auerbah volcano-plutonic complex. The Ar-Ar age of hydromica from the gold-arsenopyrite association is 391.1 ± 4.9 Ma, which coincides with the age of igneous rocks of the Auerbah complex. The main sources of water and carbon dioxide were composed of the fluid derived from the magma chamber and the metamorphic water equilibrated with carbonate sedimentary rocks. Magmatic fluid dominated during the development of skarns, jasperoids and quartz veins, whereas metamorphic water was dominant during quartz-sericite alteration of volcano-sedimentary rocks and calcareous breccias. The bulk of the sulfur was supplied by a deep magma reservoir, however this source prevailed only during skarn ore formation. The mixing between deep-sourced sulfur and sedimentary or biogenic sulfur was established for other ore types. Gold and other ore components were possibly introduced during the volcanic and intrusive activity and also extracted from host sedimentary rocks. 相似文献
785.
786.
The Beiya gold–polymetallic deposit is one of the largest gold deposits in China and is considered to be a typical porphyry-skarn system located in the middle of the Jinshajiang–Ailaoshan alkaline porphyry metallogenic belt. Massive magnetite is widespread in the Beiya ore district but its genesis is still the subject of debate. Five representative magnetite types are present in the Beiya deposit, namely magmatic magnetite (M1) from the ore-related porphyry, disseminated magnetite (M2) from the early retrograde alteration, massive magnetite (M3) from the early quartz-magnetite stage, massive magnetite (M4) from the middle quartz-magnetite stage and magnetite (M5) from the late quartz-magnetite stage. Compared with the M1 magnetite, the magnetites from stages M2 to M5 are depleted in Ti, Al and high field strength elements, implying a hydrothermal origin, distinct from the magmatic accessory magnetite in the ore-related porphyry (M1). The concentrations of cobalt in the hydrothermal magnetites decrease gradually from M2 to M5, and can be used to discriminate the magnetite types. The Al + Mn and Ti + V contents of the successively precipitated magnetite grains (M2–M5) suggests that the ore forming temperature decreased from M2 to M4, but increased from M4 to M5, possibly as the result of a new pulse of magma entering the chamber, which may have triggered the gold mineralization. The V content in the hydrothermal magnetite suggests that the oxygen fugacity increased from M2 to M4 but decreased as soon as the sulfides entered the system (M5). 相似文献
787.
Magnetite is a very common mineral in various types of iron deposits and some sulfide deposits. Recent studies have focused on the use of trace elements in magnetite to discriminate ore types or trace ore-forming process. Germanium is a disperse element in the crust, but sometimes is not rare in magnetite. Germanium in magnetite can be determined by laser ablation ICP-MS due to its low detection limit (0.0X ppm). In this study, we summary the Ge data of magnetite from magmatic deposits, iron formations, skarn deposits, iron oxide copper-gold deposits, and igneous derived hydrothermal deposits. Magnetite from iron formations contains relatively high Ge (up to ~250 ppm), whereas those from all other deposits mostly contains Ge less than 10 ppm, indicating that iron formations can be discriminated from other Fe deposits by Ge contents. Germanium in magmatic/hydrothermal magnetite is controlled by a few factors. Primary magma/fluid composition may be the major control of Ge in magnetite. Higher oxygen fugacity may be beneficial to Ge partition into magnetite. Sulfur fugacity and temperature may have little effect on Ge in magnetite. The enrichment mechanism of Ge in magnetite from iron formations remains unknown due to the complex ore genesis. Germanium along with other elements (Mn, Ni, Ga) and element ratios (Ge/Ga and Ge/Si raios) can distinguish different types of deposits, indicating that Ge can be used as a discriminate factor like Ti and V. Because of the availability of in situ analytical technique like laser ablation ICP-MS, in situ Ge/Si ratio of magnetite can serve as a geochemical tracer and may provide new constraints on the genesis of banded iron formations. 相似文献
788.
The ca. 1.5 Ga mafic intrusions in the Zhuqing area, predominantly composed of alkaline gabbroic rocks in the Kangdian region of SW China, occur as dykes or irregular small intrusions hosting Fe–Ti–V mineralization. All of the intrusions that intrude the dolomite or shales of the Mesoproterozoic Heishan Formation of the Huili Group are composed of three cyclic units from the base upward: a marginal cyclic unit, a lower cyclic unit and an upper cyclic unit. The Fe–Ti–V oxide ore bodies are hosted in the lower and upper cyclic units. The textural relationships between minerals in the intrusions suggest that titanomagnetite formed earlier than silicate grains because euhedral magnetite and ilmenite grains were enclosed in clinopyroxene and plagioclase. Both the magnetitess–ilmenitess intergrowths due to subsolidus oxidation–exsolutions and the relative higher V distribution coefficient between magnetite and silicate melts in the gabbros from the Zhuqing area are different from those of other typical Fe–Ti bearing mafic rocks, suggesting that the oxygen fugacity was low in the gabbric rocks from the Zhuqing area. This finding was further confirmed by calculations based on the compositions of magnetite and ilmenite pairs. The clinopyroxene, magnetite and ilmenite in the intrusions from the Zhuqing area had considerably lower Mg O than those of other typical Fe–Ti oxide-rich complexes, suggesting that the titanomagnetite from the intrusion may have crystallized at a relatively late stage of evolution from a more evolved magma. Titanomagnetite first fractionally crystallized and subsequently settled in the lower parts of the magma chamber, where it concentrated and formed Fe–Ti–V oxide ore layers at the bases of the lower and upper cycles. Moreover, the occurrence of multiple Fe-Ti oxide layers alternating with Fe-Ti oxide-bearing silicate layers suggests that multiple pulses of magma were involved in the formation of the intrusions and related Fe-Ti-V oxide deposits in the Zhuqing area. 相似文献
789.
应用ICP-MS研究黔西南地区构造蚀变体稀土元素地球化学特征 总被引：5，自引：2，他引：3
构造蚀变体(SBT)是沉积作用、构造作用和热液蚀变的综合产物。SBT作为黔西南地区金、锑、萤石等矿产的重要就位空间,金锑矿成矿与其密切相关。近年在黔西南地区发现了丰富的金资源量,关键在于SBT体系的提出及与之为核心的成矿模式的建立和应用。本文应用电感耦合等离子体质谱法(ICP-MS)分析了黔西南台地相区典型矿床的SBT样品,揭示稀土元素的地球化学特征。结果表明,不同矿区SBT的ΣREE=49.55×10~(-6)~271.72×10~(-6),含量变化较大,LREE/HREE=5.62~13.59,轻重稀土分馏明显,轻稀土富集;SBT对北美页岩和CI球粒陨石标准化配分模式图均表现为轻稀土富集的右倾型、"四分组"效应明显、强烈热液作用,为同一体系的产物;大厂至戈塘一线显示高的正铕异常,推测有两个方向含矿热液在此汇聚。本文系统对比了黔西南地区SBT稀土元素地球化学特征,反映该区成矿流体来源及演化,为本区微细浸染型金矿的成矿预测提供了依据。 相似文献
790.
湘中紫云山岩体的成因:锆石U-Pb年代学、元素地球化学及Hf-O同位素制约 总被引：5，自引：4，他引：1
位于湘中盆地周缘的紫云山岩体主要由似斑状石英二长岩(主体)和二云母花岗岩(补体)组成,前者分布于该岩体的周边,后者分布于其内部,岩体内可见有大量岩浆结构的镁铁质暗色包体。利用高精度SIMS锆石U-Pb定年方法得到主体岩体的年龄为225.2±1.7Ma和225.6±1.4Ma,补体岩体的年龄为227.0±2.2Ma,两者均形成于印支晚期,基本上是同时形成的。该区两类岩石均具有富SiO_2、Na_2O和K_2O,贫Ca O、Mg O和Al2O3的特征,A/CNK比值为0.85~1.05,固结指数较高,主体岩石为3.61~5.05,补体岩石为4.13~14.06;其微量元素均表现出富集Rb、U、La、Nd和Zr,亏损Ba、Nb、Ta、Sr、P和Ti的特征;稀土元素配分模式均呈明显的右倾"V"字型,轻重稀土分馏明显((La/Yb)N=7.35~11.7),Eu负异常较显著(δEu=0.32~0.70);该区花岗岩的主体和补体的锆石Hf-O同位素组成非常相似,εHf(t)值为-10.0~-1.6,δ18O值为7.8~11.4,两阶段模式年龄tDM2为1.22~1.79Ga;各类岩石主要氧化物之间表现出良好的线性协变关系,且明显沿I型花岗岩演化趋势线分布,反映主体和补体岩石具有密切的亲缘关系,均应归属于高分异的I型花岗岩。各类岩石样品沿岩浆混合趋势线分布、而远离结晶分异趋势线,结合Hf-O同位素分析,认为该岩体来源于扬子地块中元古代下地壳变质杂砂岩重熔,与部分幔源岩浆形成的壳幔混合岩浆源区。该岩体形成于碰撞后构造背景,暗示华南地块受太平洋板块及印支地块的双重影响,在印支晚期处于伸展-减薄的构造环境。通过与华南地块其它印支期花岗岩对比分析,提出扬子地块与华夏地块的拼合带在湖南境内应沿"攸县-双牌"一线展布。 相似文献