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1.
The King Island Scheelite Mine lies in the contact aureole of a granodiorite stock. Its open cut and numerous drill cores expose a contact metamorphosed and metasomatized series of finely interbedded argillaceous and calcareous sediments, with interleaved flows of picrite‐basalt and basic pyroclastics, the scheelite ore being limited to two limestone horizons. The range and gradation in composition of the original rocks has resulted in an unusual variety of metamorphic rocks, including forsterite‐phlogopite‐spinel‐tremolite hornfels, antho‐phyllite‐cordierite hornfels, biotite hornfels, actinolite hornfels, a variety of calc‐flinta, and marbles. The original sedimentation gave rise to a rapid alternation of limestone and shale, many times repeated, and during metamorphism these rocks reacted with each other to produce narrow bands of calc‐flinta.

Subsequent pyrometasomatism selectively converted the greater part of the marble beds to scheelite‐bearing andradite skarn, leaving the various hornfels and calc‐flinta very little affected. The replacement of the marble was a volume for volume process, and the conversion of 1,000,000 tons of marble to average grade ore involved the introduction of about 350,000 tons of SiO2, 250,000 tons of Fe2O3, 55,000 tons A12O3, 30,000 tons of H2O and 82,500 tons of CaO.

The temperature of the contact metamorphism attained over 500° C., and the rocks cooled to about 400° C. before the pyrometasomatism occurred. The rocks giving rise to the various hornfels underwent varying degrees of contraction during metamorphism, whereas the limestones probably expanded during metamorphism, and became more permeable to solutions.  相似文献   

2.
Devonian evaporites and associated sedimentary rocks in the Norilsk region were contact metamorphosed during emplacement of mafic sills that form part of the end-Permian (~252 Ma) Siberian Traps. We present mineralogical, geochemical and Sr–Nd isotopic data on sedimentary rocks unaffected by metamorphism, and meta-sedimentary rocks from selected contact aureoles at Norilsk, to examine the mechanisms responsible for magma-evaporite interaction and its relation to the end-Permian environmental crisis. The sedimentary rocks include massive anhydrite, rock salt, dolostone, calcareous siltstones and shale, and the meta-sedimentary rocks comprise calcareous hornfels, siliceous hornfels and minor meta-anhydrite and meta-sandstone. Contact metamorphism took place at low pressure and at maximum temperatures corresponding to the phlogopite-diopside stability field. Calcareous hornfels have high CaO, MgO, CΟ2, SΟ3, low SiO2 and initial Sr isotopic ratios of 0.7079–0.7092, features indicative of calcareous siltstone protoliths. Siliceous hornfels, in contrast, have high SiO2, Al2O3, Na2O, low in other major element oxides and initial Sr isotopic ratios of 0.7083–0.7152, consistent with pelitic or shaley protoliths. Loss of CO2 in a subset of calcareous hornfels can be explained by decarbonation reactions during metamorphism, but release of SO2 from evaporites cannot be accounted for by a similar mechanism. Occurrences of wollastonite and a variety of hydrous minerals in the calcareous hornfels are consistent with equilibration with hydrous fluid, which was capable of leaching large quantities of anhydrite in the presence of dissolved NaCl. In this way, substantial sediment-derived sulfur could have been mobilized, incorporated into the magmatic system and released to the atmosphere. The release of CO2 and SO2 from Siberian evaporites added to the variety of toxic gases generated during metamorphism of organic matter, coal and rock salt, contributing to the end-Permian environmental crisis.  相似文献   

3.
Analysis of magmatic and sedimentary rocks of several large igneous provinces has demonstrated that the release of gas during plutonic-metamorphic processes may be linked to global climate change and mass extinctions. Aguablanca, one of the largest Cu–Ni–PGE deposits in Europe, formed during the Variscan orogeny when a mafic magma intruded limestones and shales, creating a contact aureole composed of marble, skarn and hornfels. Our petrological and geochemical investigation of the aureole provides evidence that a combination of the two processes led to the formation of the ore deposit: The assimilation of terrigenous sediments supplied S to the magma while the assimilation of carbonates changed the oxygen fugacity and decreased the solubility of sulfur in the magma. The metamorphic assemblages in the contact aureole are directly related to heterogeneity of the protolith and particularly to the original proportions of calcite and clay. We modeled carbon dioxide degassing during contact metamorphism and showed that pure limestone is relatively unproductive because of its high reaction temperature. The presence of clay, however, leads to the formation of calc-silicates and significantly enhances CO2 degassing. Our estimations suggest that degassing of the Aguablanca contact aureole released about 74.8 Mt of CO2, a relatively low volume that we attribute to the composition of the host rock, mainly a pure limestone. A far larger volume of carbon dioxide was emitted by the contact metamorphism of dolostones in the contact aureole of Panzhihua (part of Emeishan large igneous province, SW China). We propose that the level of emission of carbon dioxide depends strongly on the nature of the protolith and has to be considered when predicting environmental impact during the emplacement of large igneous provinces.  相似文献   

4.
The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic,mid-crustal, layered, mafic intrusions, emplaced into Proterozoicmetaclastic rocks of NE Scotland soon after the thermal peakof static, high-T, low-P regional metamorphism. This gabbroand its associated contact metamorphic rocks illustrate a varietyof processes operating during contact anatexis and subsequentmelt segregation and extraction. These processes may closelymirror those occurring at much larger scales in the deep crustduring high-grade regional metamorphism and the generation ofgranitic magmas. The emplacement of the Huntly mafic magma resultedin high-grade contact metamorphism and, locally, anatexis ofmetapelites, leading to the formation of migmatites. The migmatitesand country-rock schists were studied to establish the physicalconditions of metamorphism and anatexis, the nature of the meltingreactions, the compositions of the melts produced, and the extentto which melting was a closed- or open-system process. The country-rockschists immediately to the south of the Huntly Complex containmineral assemblages characteristic of the regional andalusitezone. Thermobarometry of an andalusite schist yields regionalmetamorphic conditions of 537 ± 42°C and 0·27± 0·12 GPa, consistent with previously publishedP–T estimates. The contact metamorphic rocks include sillimanitehornfelses, metatexites and diatexites. The metatexites consistof cordierite–K-feldspar hornfels melanosomes and K-feldspar-richgarnetiferous leucosomes. The diatexites consist of schollenof fine-grained granoblastic hornfels and metatexite suspendedin igneous-textured matrix rocks composed of abundant sub/euhedralgarnet, cordierite, plagioclase and, locally, orthopyroxene,with minor interstitial biotite, K-feldspar and quartz. Thehornfels melanosomes and schollen retained their structuralintegrity during partial melting, but the matrix rocks did not.In the highest-grade diatexites, the assemblage Grt + Opx +Crd + Hc + Pl characterizes both the hornfels schollen and thesub/euhedral minerals of the matrix rocks. Application of phaseequilibria to Opx-bearing rocks yields estimated peak-metamorphicconditions of 900 ± 50°C, 0·45 ± 0·1GPa and aH2O < 0·3. The pressure estimate impliesan emplacement depth of  相似文献   

5.
《International Geology Review》2012,54(11):1020-1039
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.  相似文献   

6.
J.G. Liou  Pei-Yuan Chen 《Lithos》1978,11(3):175-187
Chloritoid-rich rocks occur as thin pods or lenses in the well-foliated marble and greenschist of the pre-Tertiary metamorphic complex of eastern central Taiwan. They contain unusually abundant chloritoid: 60–90% in the massive rocks and 30–50% in the schistose rocks. Compared to the adjacent basaltic greenschist, the chloritoid rocks contain extremely low SiO2, CaO and Na2O and very high Al2O3, total Fe as Fe2O3, and TiO2. Both monoclinic and triclinic chloritoids were identified. These rocks may have been derived originally from fossil lateritic soils formed from a basaltic layer in a limestone terrain. These soils and the bedded rocks were then metamorphosed in the chlorite-biotite zone of the greenschist facies.  相似文献   

7.
Abstract In the Twin Lakes area, central Sierra Nevada, California, most contact metamorphosed marbles contain calcite + dolomite + forsterite ± diopside ± phlogopite ± tremolite, and most calc-silicate hornfelses contain calcite + diopside + wollastonite + quartz ± anorthite ± K-feldspar ± grossular ± titanite. Mineral-fluid equilibria involving calcite + dolomite + tremolite + diopside + forsterite in two marble samples and wollastonite + anorthite + quartz + grossular in three hornfels samples record P± 3 kbar and T± 630° C. Various isobaric univariant assemblages record CO2-H2O fluid compositions of χCO2= 0.61–0.74 in the marbles and χCO2= 0.11 in the hornfelses. Assuming a siliceous dolomitic limestone protolith consisting of dolomite + quartz ° Calcite ± K-feldspar ± muscovite ± rutile, all plausible prograde reaction pathways were deduced for marble and hornfels on isobaric T-XCO2 diagrams in the model system K2O-CaO-MgO-Al2O3-SiO2-H2O-CO2. Progress of the prograde reactions was estimated from measured modes and mass-balance calculations. Time-integrated fluxes of reactive fluid which infiltrated samples were computed for a temperature gradient of 150 °C/km along the fluid flow path, calculated fluid compositions, and estimated reaction progress using the mass-continuity equation. Marbles and hornfelses record values in the range 0.1–3.6 × 104 cm3/cm2 and 4.8–12.9 × 104 cm3/cm2, respectively. For an estimated duration of metamorphism of 105 years, average in situ metamorphic rock permeabilities, calculated from Darcy's Law, are 0.1–8 × 10?6 D in the marbles and 10–27 × 10?6 D in the hornfelses. Reactive metamorphic fluids flowed up-temperature, and were preferentially channellized in hornfelses relative to the marbles. These results appear to give a general characterization of hydrothermal activity during contact metamorphism of small pendants and screens (dimensions ± 1 km or less) associated with emplacement of the Sierra Nevada batholith.  相似文献   

8.
The genesis of Liangguo corundum deposit in the southern Gangdese magmatic arc, east-central Himalaya, remains unknown. The present study shows that the corundum-bearing rocks occur as lenses with variable sizes in the Eocene gabbro that intruded into marble. These corundum-bearing rocks have highly variable mineral assemblage and mode. The corundum-rich rocks are characterized by containing abundant corundum, and minor spinel, ilmenite and magnetite, whereas the corundum-poor and corundum-free rocks have variable contents of spinel, plagioclase, sillimanite, cordierite, ilmenite and magnetite. The host gabbro shows variable degrees of hydration and carbonization. The corundum grains are mostly black, and rarely blue, and have minor Fe O and TiO_2. The spinel is hercynite, with high Fe O and low Mg O contents. The corundum-bearing rocks have variable but high Al_2O_3, FeO and TiO_2, and low SiO_2 contents. Inherited magmatic and altered zircons of the corundum-bearing rocks have similar U e Pb ages(~47 Ma) to the magmatic zircons of the host gabbro, indicating corundum-bearing rock formation immediately after the gabbro intrusion. We considered that emplacement of gabbro induced the contact metamorphism of the country-rock marble and the formation of silica-poor fluid. The channeled infiltration of generated fluid in turn resulted in the hydrothermal metasomatism of the gabbro, which characterized by considerable loss of Si from the gabbro and strong residual enrichment of Al. The metasomatic alteration probably formed under Pe T conditions of ~2.2 -2.8 kbar and ~650 -700℃. We speculate that SiO_2, CaO and Na_2O were mobile, and Al_2O_3, FeO, TiO_2 and high field strength elements remained immobile during the metasomatic process of the gabbro. The Liangguo corundum deposit, together with metamorphic corundum deposits in Central and Southeast Asia, were related to the Cenozoic Himalayan orogeny, and therefore are plate tectonic indicators.  相似文献   

9.
Detailed geochemical and mineralogical investigation of four talc deposits in the Eastern Desert of Egypt (Atshan, Abu Gurdi, Darhib and Kashira) suggests that the deposits form a distinct lithological unit within the Shadli metavolcanic rocks. The talc crystallized from the replacement of siliceous carbonate beds locally intercalated with clastic sediments. Th/Yb vs. Ta/Yb ratios of the rocks suggest that the sediments and the host volcanic rocks formed in an active continental margin (ACM) environment. Thus, the talc deposits may represent relict fragments of an ancient, regionally extensive carbonate horizon within the arc-related metavolcanics. The talc-rich rocks, which contain relict carbonate, serpentinized olivine and tremolite, have low (<3 wt%) Al2O3, Cr, Ni (<20 ppm), Co and Sc (<15 ppm) concentrations, precluding mafic or felsic igneous protoliths. The deposits were locally affected by contact metamorphism, giving rise to pyroxene-hornfels and granulite facies assemblages, and by regional metamorphism which produced greenschist-amphibolite grade assemblages. Disseminated sulfides commonly occur in the talc-tremolite-rich rocks (having low Al2O3 concentrations), suggesting that the metals were probably present in the original carbonate beds, but were remobilized and reconcentrated during the various metamorphic events.  相似文献   

10.
安庆铜铁矿床是产于长江中下游铜(金)、铁成矿带内安庆-贵池矿集区中的一典型矽卡岩矿床,矿体赋存于下三叠统南陵湖组大理岩与月山闪长岩体之间的接触带上。典型剖面系统取样分析显示,矿体与围岩在空间上具有显著的矿物组合分带与岩石化学分带特征,即靠近大理岩带发育致密块状磁铁矿矿石与团块状矽卡岩型矿石,远离大理岩带发育浸染状矽卡岩型矿石,靠近闪长岩带发育透辉石矽卡岩。从大理岩到磁铁矿体, Fe2O3T含量显著增加,之后随着靠近闪长岩体,其含量呈逐渐降低趋势;而CaO显示了与Fe2O3T相反的成分变异特征。矿物与全岩微量元素研究表明,致密块状磁铁矿矿石及团块状矽卡岩型矿石均具有岩浆成因的稀土元素配分模式;而浸染状矽卡岩型矿石显示了交代成因的稀土元素配分特征。矿体地质、矿相学与元素地球化学综合研究表明,矽卡岩成矿经历了矿浆贯入期与热液成矿期,前者包括氧化物阶段和硫化物-碳酸盐阶段,后者包括进化交代阶段、早退化蚀变阶段、石英-硫化物阶段和晚退化蚀变阶段。结合已有的区域岩浆岩成岩机制研究成果,认为安庆铜铁矿床应是矿浆贯入与接触交代复合成因的矽卡岩型矿床,由于高位岩浆房中岩浆不混溶作用形成的富Fe熔浆是该矿床中成矿元素的主要来源。  相似文献   

11.
Talc mineralization of ultramafic affinity in the Eastern Desert of Egypt   总被引：1，自引：0，他引：1  
Petrographical and petrochemical studies of the talc host rocks of Rod Umm El-Farag and Wadi Thamil in the Eastern Desert of Egypt reveal that they consist mainly of metavolcanic rocks, whilst the geology, petrography, mineralogy, chemistry and quality of the enclosed talc lenses reveal that the ore has ultramafic affinity. The setting of the talc ore is similar to that hosted by metavolcanic rocks in terms of the type of host rocks, but it differs in its ultramafic affinity, resembling the talc ore hosted by ultramafic rocks. The parent ultramafic rocks occur in the form of small bodies obducted later along a tectonized fault plane within metavolcanic host rocks (Precambrian) and their tuffaceous equivalents. The metavolcanic host rocks consist mainly of metabasalts, meta-andesites and metatuffs with a smaller amount of dacite, rhyolite and tuffaceous lava. The metamorphic grade is low corresponding to greenschist facies. The calc-alkaline and tholeiitic characters of the volcanic rocks are determined by the behaviour of trace elements on some chemical discrimination diagrams. After the emplacement of the ultramafic bodies, they underwent regional metamorphism which was accompanied by further serpentinization. Metasomatic changes, related to regional metamorphism (corresponding to the emplacement of granitic plutons at a distance) include talc, carbonate, tremolite and chlorite formation. SiO2, H2O and CO2 have been supplied from hydrothermal solutions but all other constituents are considered indigenous to the ultramafic bodies, and none of the metavolcanic components have been added during talc formation. Mineralogically, the talc ore is relatively simple, including talc, tremolite, actinolite, chlorite and chromite. On the basis of mineral abundances, pure talc (>90% talc), chlorite-rich and tremolite-actinolite-rich (50–70% talc) ore types have been recognized. Chromite is largely zoned and occurs as disseminated grains within the talc matrix. Cr, Al and Mg were released during the formation of ferrite chromite and accommodated in the talc and chlorite structures. The chemical data show that there is very little variation in the contents of MgO, Fe2O3, FeO, NiO, Cr2O3, and Co between the parent ultramafic rocks and talc ore. Al2O3, CaO, Fe2O3 and FeO are the main impurity oxides in the talc ore. They decrease the whiteness of the ore and consequently limit the use of talc. Received: 26 March 1999 / Accepted: 10 October 1999  相似文献   

12.
This paper records, for the first time, the mineralization of gold (0.98–2.76 ppm) and uranium (133–640 ppm) in marbles from the Arabian-Nubian Shield of the Eastern Desert of Egypt. These auriferous and uraniferous marbles are hosted by sheared and altered ophiolitic serpentinized ultramafic rocks of Gebel El-Rukham (ER), Wadi Daghbag (DG), and Wadi Al Barramiyah (BM). They occur as massive or banded in pod-like or bedded shapes. The ER and BM-mineralized marbles are impure calcitic, whereas the DG marble is impure calcitic to impure dolomitic. Their protolith are pure limestones and dolomitic limestones with probable argillaceous components (BM marble), and their metamorphism (Pan-African) was retrograde. Peaks of metamorphism were at granulite-amphibolite facies for the ER and BM marbles, forming diopside (Al2O3?=?0.17–1.07 wt.%) at 600–900°C and augite (Al2O3?=?2.45–9.40 wt.%) at 825–975°C, and at the amphibolite facies for DG marble, recrystallising the carbonate minerals and forming tremolite. The lowest temperatures of metamorphism were at the upper subgreenschist facies as chlorite (ER and BM marbles) and kaolinite (DG marble) were formed. Metamorphic fluids were, most probably, essentially binary H2O–CO2 mixtures with low NaCl and HF concentrations. Gold in the studied mineralized marbles occurs as native nuggets (10–35 μm) having globule, rod, crescent, and streak shapes, in pores, vugs, and fissures. The source of gold in all marbles is mostly the country ultramafic rocks. Timing of gold mineralization relative to the marblization and metamorphism of the country source ultramafic rocks was both syn- and post-metamorphic. Concerning the ER and DG marbles, it was syn-metamorphic, where Au liberation and transportation were mostly by the metamorphic fluids. The composition and temperature of these fluids were most probably inappropriate for formation of the sulfide complexes of gold. The gold mineralization of BM marble, on the other hand, was mostly post-metamorphic. The mineralising fluid was of surficial origin under oxidizing conditions. The encountered uranium minerals are of secondary origin such as autunite, uranophane, and carnotite. These minerals occur as fine oval aggregates and irregular grains (10–50 μm) usually filling fissures and vugs. The uranium mineralization can be classified as surficial of ages <1.5 Ma. It is proposed that the U was transported from its source (might be flesite and trachyte dikes for the ER and DG marbles and granite rocks for BM marble) to the marble rocks by surface and/or underground water related to the pluvial periods in Egypt. In BM marble, U and Au have mutual mineralizing fluid but different paragenesis.  相似文献   

13.
The rate of mineral transformations in rocks near magmatic intrusions may be estimated using mathematical modeling for study of the duration of metamorphism and geological and mineralogical data. At the contacts of the Anakit trappean massif on the Nizhnyaya Tunguska River, where the temperature reached 900°C, the rate of growth of a wollastonite rim at the boundary between the limestone and the siliceous nodule was ～3 × 10–10 cm/s. The zone of “spotted” hornfels with a width of 300?400 m was formed during metamorphism of chlorite–sericite–epidote–albite–quartz schist near the Kharlov gabbro massif in the foothills of the Altai Mountains. The movement rate of the metamorphic front during the formation of rock may be estimated as ～2 × 10–8 cm/s. It is suggested that the rate of metamorphism is controlled by the temperature and rock composition. As a whole, the rates of metamorphism of rocks near magmatic intrusive bodies exceed the rates of regional metamorphism. Upon accumulation of the actual data, this may be applied for diagnostics of the types of metamorphism.  相似文献   

14.
大庄子金矿受胶莱盆地边缘发育于荆山群内的顺层状低角度断裂带控制,以大理岩质碎裂岩及角砾岩等张性构造岩胶结物发生矿化为特点.容矿岩石与成矿流体具有强烈的物质交换,矿化与SiO2、Fe2O3、Na2O及As、Cd、Hg、Cu、Pb等元素关系密切.矿石具有富集轻稀土元素、铕正异常的特征,其稀土元素特征和基性脉岩具有一定的相似性,又具有原岩的一些特点,表明容矿岩石受到了深部成矿流体的影响.硫、铅、碳-氧及氢-氧等同位素地球化学及流体包裹体研究显示,成矿物质较复杂,成矿流体来源于地壳深部或上地幔的岩浆热液,在成矿作用中,萃取了部分围岩,并与围岩发生了强烈反应.  相似文献   

15.
Assimilation serves as a significant part in magma's evolution. Because of rapid cooling of ultramafic‐mafic magma, assimilation between mantle‐derived magma and crustal rocks (wall‐rocks) is often neglected. The Beidaihe diabase dikes in North China Craton suggest that assimilation of the rapidly cooled mantle‐derived magma can not be ignored. The diabase dikes are very thin (0.5 to 2 m in width) and discordantly intrude into Ordovician limestone. Center of dyke is porphyritic texture while chilled margin of it is aphanitic texture, suggesting a rapid crystallization process. Moreover, limestone close to the dyke was transformed into marble because of contact‐thermal metamorphism. From the center to margin of the dykes, contents of SiO2 (48.9→ 41.6 wt.%) and MgO (8.3 → 4.0 wt.%) are obviously decreased, but contents of CaO (6.4→8.3 wt.%) and CO2 (0.8 → 5.2 wt.%) are distinctly increased. Meanwhile, calcites in the wall rocks adjacent to and far from the dykes are also greatly different. The calcites close to the dykes have high MgO and FeO contents but low CaO and CO2 contents. All these suggest an assimilation process of the mantle‐derived magma with wall‐rock limestone. Calculating CO2 contents in the dyke and wall rock suggest that the extent of assimilation is thought to reach up to 8 ‐ 12 %. Therefore, when the wall rocks of the mantle‐derived magma are carbonate rocks, its components are obviously influenced by assimilation, although it often rapidly chilled. Simultaneously, this study provides possible explanation for some mantle‐derived rocks in carbonate areas with high LOI.  相似文献   

16.
阿西金矿地处川甘陕“金三角”成矿集中区,中三叠统为本区内最重要的金矿源层和赋矿层。阿西金矿田的形成经历了一个从沉积－成岩－富集成矿的漫长复杂的由量的积累到质的飞跃过程,是多阶段、多层次、多因素作用有机结合的产物。赋矿岩系特征表明,阿西金矿田的主要成矿环境为半深海斜坡环境。区内岩石类型极为复杂,三大岩类均有产出。主要有细砂岩、杂砂岩、沉凝灰岩、碳酸盐岩、硅质岩;变粒岩、石英岩、角岩、大理岩、夕卡岩、角砾岩;岩浆岩类主要为英安斑岩（玢岩）、闪长岩等。其中最重要的赋矿岩石为石英岩,其主要特征为具块状、不规则条带状构造,呈褐黄色／浅灰色变余层状构造。由石英（75％－80％）、高龄石（10％－15％）、绢云母（2％－3％）、钛铁质（3％－4％）、玉髓（2％－3％）及少量碳质组成。其岩石化学成分与典型硅质岩相比,SiO2偏低,Al2O3总体偏高,出现较强的高岭土化。通过研究认为,该区主要赋矿岩石石英岩的原岩应属正常碎屑沉积岩。  相似文献   

17.
西藏墨竹工卡县甲玛铜多金属矿床勘查模型   总被引：31，自引：14，他引：17  
甲玛铜多金属矿床是西藏藏中有色金属开发基地的重要矿床.文章重点研究了该矿床的成矿地质条件、找矿标志、地球化学勘查模型、地球物理勘查模型、矿体空间结构等.研究表明:赋矿地层为林布宗组(K1 l)砂板岩、角岩和多底沟组(J3d)大理岩、灰岩,角岩控制了钼(铜)矿体的分布,角岩与大理岩之间的层间构造是矽卡岩型矿体的赋存部位....  相似文献   

18.
The ~2,752-Ma Weld Range greenstone belt in the Yilgarn Craton of Western Australia hosts several Fe ore deposits that provide insights into the role of early hypogene fluids in the formation of high-grade (>55 wt% Fe) magnetite-rich ore in banded iron formation (BIF). The 1.5-km-long Beebyn orebody comprises a series of steeply dipping, discontinuous, <50-m-thick lenses of magnetite–(martite)-rich ore zones in BIF that extend from surface to vertical depths of at least 250 m. The ore zones are enveloped by a 3-km-long, 150-m-wide outer halo of hypogene siderite and ferroan dolomite in BIF and mafic igneous country rocks. Ferroan chlorite characterises 20-m-wide proximal alteration zones in mafic country rocks. The magnetite-rich Beebyn orebody is primarily the product of hypogene fluids that circulated through reverse shear zones during the formation of an Archean isoclinal fold-and-thrust belt. Two discrete stages of hypogene fluid flow caused the pseudomorphic replacement of silica-rich bands in BIF by Stage 1 siderite and magnetite and later by Stage 2 ferroan dolomite. The resulting carbonate-altered BIF is markedly depleted in SiO2 and enriched in CaO, MgO, LOI, P2O5 and Fe2O3(total) compared with the least-altered BIF. Subsequent reactivation of these shear zones and circulation of hypogene fluids resulted in the leaching of existing hypogene carbonate minerals and the concentration of residual magnetite-rich bands. These Stage 3 magnetite-rich ore zones are depleted in SiO2 and enriched in K2O, CaO, MgO, P2O5 and Fe2O3(total) relative to the least-altered BIF. Proximal wall rock hypogene alteration zones in mafic igneous country rocks (up to 20 m from the BIF contact) are depleted in SiO2, CaO, Na2O, and K2O and are enriched in Fe2O3(total), MgO and P2O5 compared with distal zones. Recent supergene alteration affects all rocks within about 100 m below the present surface, disturbing hypogene mineral and the geochemical zonation patterns associated with magnetite-rich ore zones. The key vectors for identifying hypogene magnetite-rich Fe ore in weathered outcrop include textural changes in BIF (from thickly to thinly banded), crenulated bands and collapse breccias that indicate volume reduction. Useful indicators of hypogene ore in less weathered rocks include an outer carbonate–magnetite alteration halo in BIF and ferroan chlorite in mafic country rocks.  相似文献   

19.
滇西红牛矽卡岩型铜矿床石榴子石特征   总被引：13，自引：7，他引：6  
高雪  邓军  孟健寅  闫寒  李建新  杨春海  孙诺  魏超 《岩石学报》2014,30(9):2695-2708
红牛矽卡岩型铜矿床是义敦岛弧南段格咱火山-岩浆弧新探明的铜矿床之一,目前探明铜金属资源量已达大型规模。与由侵入岩和大理岩直接接触形成的典型矽卡岩矿床不同,红牛铜矿床是隐伏岩体远程矽卡岩化的产物,其矽卡岩矿体与地层产状基本一致,通常相间排列,且距离岩体较远,大理岩中可见粗粒石榴子石和硅灰石,矽卡岩中常见大理岩捕掳体。根据矽卡岩矿物组合可将该矿床矽卡岩类型划分为石榴子石矽卡岩、石榴子石透辉石(或透辉石石榴子石)矽卡岩、透辉石矽卡岩、符山石-石榴子石矽卡岩、硅灰石-石榴子石矽卡岩、绿帘石-石榴子石矽卡岩、阳起石-绿帘石矽卡岩、硅灰石矽卡岩和绿帘石矽卡岩,其中以石榴子石矽卡岩、透辉石矽卡岩和硅灰石矽卡岩为主。石榴子石是最重要的矽卡岩矿物,分布广泛、颜色变化大,且石榴子石矽卡岩中黄铜矿、黄铁矿、磁黄铁矿化最好。本文通过对0ZK10、3ZK11和7ZK16钻孔岩芯的地质编录,查明石榴子石在红牛铜矿床的空间分布和矿化特征,采集该矿区新鲜的石榴子石矽卡岩、矽卡岩化大理岩和角岩磨制成光薄片,开展详细的显微镜下鉴定工作,观察石榴子石的颜色、粒度、结构、光性等岩相学特征,并通过电子探针分析其化学成分。红牛铜矿床石榴子石集中产出于矽卡岩中,少量产出于矽卡岩化大理岩和角岩中,具有明显的两期。早期石榴子石分布广泛,多呈褐色-红褐色,非均质性,异常干涉色,粒径一般在0.2~4mm之间,半自形-自形中细粒结构,韵律环带发育。SiO2含量变化范围为35.18%~37.69%、CaO为33.34%~36.35%、Al2O3为3.64%~13.69%、FeO为11.90%~24.18%、MgO为0.00%~0.08%,FeO和Al2O3含量变化呈负相关,SiO2和CaO含量变化整体呈正相关。石榴子石端员组分总体以钙铁榴石(36.88%~82.36%)为主,其次为钙铝榴石(16.59%~60.75%),还有少量的镁铝榴石、铁铝榴石和锰铝榴石,属于钙铁榴石-钙铝榴石系列(And37-82Gro17-61Spe+Pyr+Alm0.33-3.71)。晚期石榴子石呈浅褐色-浅红色,多发育于矽卡岩化角岩和大理岩中,少量发育于矽卡岩中,半自形-他形粒状结构,均质性,全消光,常具有溶蚀结构。SiO2含量变化范围为35.06%~36.27%、CaO为33.07%~33.77%、Al2O3为0.04%~1.05%、FeO为27.38%~28.18%、MgO为0.00%~0.04%,属于钙铁榴石(94.42%~98.46%)。早期石榴子石韵律环带发育,其主量元素含量变化显示出一定的规律性,由核部向边缘,SiO2和CaO基本保持不变,FeO含量增加,Al2O3含量减少,钙铁榴石含量增加,钙铝榴石含量减少,反映在石榴子石形成早期,成岩环境为低氧逸度、酸性还原环境;形成过程中氧逸度增加,成矿溶液由酸性向弱碱性演化。黄铜矿、磁黄铁矿、辉钼矿等金属硫化物多呈他形充填于石榴子石颗粒之间,或在石榴子石的裂隙中形成细脉,或沿石榴子石生长环带面交代,表明石榴子石形成于矽卡岩早期、早于铜矿化,并为金属硫化物的沉淀富集提供了空间。  相似文献   

20.
《Ore Geology Reviews》2003,22(1-2):17-39
Many talc deposits occur in the Hwanggangri Mineralized Zone (HMZ) in dolomitic marbles of the Cambro-Ordovician Samtaesan Formation within 1 km of the contact with the Cretaceous Muamsa Granite. Talc commonly forms fine-grained, fibrous aggregates, or pseudomorphs after tremolite; abundant tremolite is included as impurities in the talc ore. Talc generally was derived from tremolite in calc-silicate rock within the dolomitic marble. Calc-silicate rock, consisting mainly of tremolite and diopside, was generated from silicic metasomatism during the prograde stage, which promoted decarbonation reactions until dolomite was exhausted locally. Hydrothermal alteration of calc-silicate rock to talc is marked by the addition of Mg and Si, and the leaching of Ca; Cr, Co, and Ni were relatively immobile during the retrograde stage. Contact metamorphism related to the granite intrusion generated the successive appearance of tremolite, diopside, and forsterite, or wollastonite-bearing assemblages in the marble, depending on the bulk rock composition. The XCO2 content of the metamorphic fluids rose initially above XCO2=0.6, and decreased steadily toward a water-rich composition with increasing temperature above 600 °C in the calcitic marble, while buffered reaction of the dolomitic marble occurred at higher XCO2 conditions above 600 °C. Talc mineralization developed under metastable conditions with infiltration of large amounts of igneous fluids along a fault-shattered zone during the retrograde stage and is characterized by the loss of Ca2+ with the addition of Mg2+. Oxygen and carbon isotopic variations of carbonate and calc-silicate minerals are in agreement with theoretical relationships determined for decarbonation products of contact metamorphism. Talc formation temperatures obtained from oxygen isotope fractionation, T–XCO2 relationships, and activity diagrams range from 380 to 400 °C.  相似文献   

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