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1.
Stratiform manganese silicate rocks overlie jasper and metabasah in the ?Middle Silurian Hoskins Formation at the Hoskins manganese mine near Grenfell, NSW. Two dominant mineral assemblages occur in the Mn silicate rocks: (1) a “reduced’ assemblage, probably gradational into underlying jasper, containing abundant rhodonite and/or tephroite, plus subordinate carbonates, quartz, hausmannite, spessartine and Ba minerals, and (2) a well‐laminated ‘oxidized’ assemblage rich in red Mn‐rich alkali pyroxene and amphibole, braunite, manganoan pectolite and minor Mn‐rich mica, alkali feldspars, carbonates, quartz and barite. Several Mn silicates implicitly contain trivalent Mn. The Mn silicate rocks are rich in Mn, Ba and Sr, and also contain anomalously high Co, Cu, As and W; oxidized assemblages are alkali‐rich. Bulk compositions and geological setting suggest a submarine volcanic exhalative origin for the precursors of the Mn silicate rocks and jasper. Metamorphism has occurred at upper greenschist facies with original high oxygen fugacity conditions in the exhalative sediments being largely reflected in the resulting assemblages. Although analogues of the reduced Mn silicate rocks are widespread in metamorphosed Mn deposits, equivalents of the oxidized assemblages appear to be particularly uncommon. 相似文献
2.
The paper summarizes experimental and calculation data on the effect of oxygen fugacity on the origin of mineral assemblages in Mn-bearing rocks and demonstrates the possibility of application of these data to the reconstruction of conditions under which metalliferous deposits were metamorphosed. A new variant of the T-log \(f_{O_2 } \) diagram is proposed for the Mn-Si-O system, which differs from previous ones by the location of the lines for the formation (decomposition) of braunite and tephroite. These two minerals are the most universal indicators of oxygen fugacity during the metamorphism of Mn-bearing deposits, because these minerals are widespread in nature and can be formed in diverse environments: braunite at high \(f_{O_2 } \) values in the pore solution, and tephroite at low \(f_{O_2 } \) values. The occurrence of Mn oxides and rhodonite (pyroxmangite) in a rock makes it possible to constrain the oxygen fugacity range. An original T-log \(f_{O_2 } \) diagram is constructed for the Ca-Mn-Si-O system. As follows from this diagram, a Ca admixture expands the stability field of rhodonite toward higher oxygen fugacity values. Johannsenite can be formed in these rocks at even higher \(f_{O_2 } \). The stability of both minerals is constrained in the region of low \(f_{CO_2 } \). The paper reports data on the Fe-Si-O and Mn-Fe-Si-O systems and discusses the possibility of applying the results of experiments in the Mn-Al-Si-O system to the estimation of conditions under which andalusite, spessartine, and galaxite can be formed in Mn-bearing rocks. Data on the mineralogy of numerous Mn deposits metamorphosed under various PTX parameters indicate that the origin of Mn-bearing mineral assemblages depends not so much on the temperature and pressure as on the oxygen fugacity, which is, in turn, controlled primarily by the composition of the pristine sediments (the presence or absence of organic matter in them) and host rocks and depends on the permeability of the rocks to oxygen, the P-T conditions, and the duration of the metamorphic processes. 相似文献
3.
The first data on the mineral composition and formation conditions of manganese ore at the Chapsordag and Malosyrsky deposits
in the Askiz ore district of Khakassia are integrated and systematized. The detailed mineralogical mapping of the deposits
has been carried out. The identification of minerals and examination of the ore microstructure were performed with optical
microscopy in transmitted and reflected light and using SEM/EDS, EMPA, XRD, IRS, and other methods. It was established that
the ore mineralization is spatially and genetically related to the Early Devonian magmatism and accompanying hydrothermal
activity and metasomatism. Syngenetic braunite was detected for the first time in elevated amounts reaching an economic level
in the devitrified groundmass of volcanic rocks, in cement of lava breccia, and in fragments in pyroclastic rocks. By analogy
with iron deposits, this magmatogenic type of manganese mineralization is regarded as ore lavas and tuffs combined with metasomatic
and hydrothermal mineral assemblages into a strata-bound orebearing complex and as a source of hydrothermal metasomatic ore.
The elevated Mn content in magmatic melts of the Early Devonian trachybasalt-trachyandesite-trachydacite association is caused
by assimilation of Riphean and Lower Cambrian high-Mn carbonate sequences in crustal magma chambers. In contours of economic
orebodies, the hydrothermal economic ore is recognized as sites of massive, patchy and impregnated, brecciated, stringer-disseminated,
and disseminated varieties. High-grade massive ore occurs as stratiform and branching bodies up to 1.5 m thick and a few tens
of meters long and as smaller pocketlike bodies. Braunite and pyrolusite (polianite) are major ore minerals varying in size,
degree of crystallinity, and character of intergrowths with associating minerals. Gangue minerals include carbonates, sulfates,
albite, quartz, chlorite, actinolite, piemontite, and okhotskite, a Mn-pumpellyite identified in Russia for the first time
and studied in detail in this paper. The veined hydrothermal ore is classified as a calcite-barite-pyrolusite type. The crystallization
temperature of hydrothermal metasomatic ore is estimated at 350–180°C; oxygen fugacity is above the hematite-magnetite buffer.
The surface of high-grade ore is encrusted with supergene goethite-hydrogoethite, chalcedony-hematite, and pyrolusite-psilomelane
crusts and veinlets (less than 1% of the bulk ore mass). The data obtained facilitate prospecting for high-quality manganese
ore at walls of superimposed rifts in fold regions, including large economic manganese concentrations in the form of ore lavas
and tuffs as products of solidification of metalliferous melt. 相似文献
4.
The mineralogy of slightly metamorphosed manganese ore at the South Faizulino hydrothermalsedimentary deposit in the southern Urals has been studied; 32 minerals were identified. Quartz, hausmannite, rhodochrosite, tephroite, ribbeite, pyroxmangite, and caryopilite are major minerals; calcite, kutnahorite, alleghanyite, spessartine, rhodonite, clinochlore, and parsettensite are second in abundance. This mineralic composition was formed in the process of gradual burial of ore beneath the sequence of Middle Devonian-Lower Carboniferous rocks. The highest parameters of metamorphism are T ≈ 250°C and P ≈ 2.5 kbar. The relationships between minerals and their assemblages made it possible to reconstruct the succession of ore transformation with gradually increasing temperature and pressure. Manganese accumulated in the initial sediments as oxides and a gel-like Mn-Si phase. Rhodochrosite and neotocite were formed at the diagenetic stage. In the course of a further increase in temperature and pressure, neotocite was replaced with caryopilite; ribbeite, tephroite, pyroxmangite, and other silicates crystallized afterwards. In addition to the PT parameters, the formation of various metamorphic mineral assemblages was controlled by the Mn/(Mn + Si) ratio in ore and X CO2 in pore solution. The latter parameter was determined by the occurrence of organic matter in the ore-bearing rocks. Ore veinlets as products of local hydrothermal redistribution of Mn, Si, and CO 2 were formed during tectonic deformations in the Middle Carboniferous and Permian. 相似文献
5.
Manganese silicate rocks, interbanded with manganese oxide orebodies, constitute an important stratigraphic horizon in the Mansar formation of the Sausar Group of Precambrian age in India. The manganese silicate rocks of Gowari Wadhona occupy the westernmost flank of the manganese belt of the Sausar Group. These rocks are constituted of spessartite, calcium-rich rhodonite, quartz, manganoan diopside, blanfordite (manganese bearing member of diopside-acmite series), brown manganese pyroxene (manganese bearing aegirine-augite), winchite (manganese bearing richterite-tremolite), juddite (manganese bearing amphibole with richterite, tremolite, magnesioriebeckite and glaucophane molecules), tirodite (manganese bearing amphibole with richterite, cummingtonite and glaucophane molecules), manganophyllite, alurgite, piedmontite, braunite, hollandite (and other lower oxides of manganese) with minor apatite, plagioclase, calcite, dolomite and microcline. A complete mineralogical account of the manganese-bearing phases has been given in the text. It has been shown that the juxtaposition of manganese silicate rocks with dolomitic marble, regional metamorphism to almandine-amphibolite facies and assimilation of pegmatite veins cutting across the manganese formation, were responsible for the development of these manganese silicate rocks and the unusual chemical composition of some of the constituent minerals. It has been concluded that the manganese silicate rocks of Gowari Wadhona were originally laid down as sediments comprising manganese oxides admixed with clay, silica etc. and were later regionally metamorphosed to almandine-amphibolite facies. All evidences indicate that rhodochrosite was not present in the original sediment and the bulk composition of the sediments was rich in manganese. These rocks agree entirely to the detailed nomenclature of the gondites enunciated by Fermor (1909) and amplified by Roy and Mitra (1964) and Roy (1966). 相似文献
6.
The paper describes mineralogy of the low grade metamorphosed manganese sediments, which occur in sedimentary complexes of the Pai Khoi Ridge and the Polar Urals and volcanosedimentary complexes of the Central and South Urals. The degree of metamorphism of the rocks studied corresponds to PT conditions of the prehnite–pumpellyite (deposits of Pai Khoi and Polar and South Urals) and green schist (deposits of the Central Urals) facies. One hundred and nine minerals were identified in the manganese-bearing rocks on the basis of optical and electron microscopy, X-ray diffraction, and microprobe analysis. According to the variations in the amount of major minerals of the manganese rocks of the Urals, they are subdivided on carbonate (I), oxide–carbonate–silicate (II), and oxide–silicate (III) types. Carbonates, various Mn 2 +-bearing silicates associated with oxides and carbonates, and braunite (Mn 3 +-bearing silicate) are the major Mn hosts in types I, II, and III, respectively. Because of the different oxidation state of Mn, the rocks of types I and II are termed as “reduced” and the rocks of type III, as “oxidized”. The formation of a certain mineralogical type of metamorphic assemblage is controlled by the content of organic matter in the primary sediments. The sequence type I → type II → type III reflects the decrease in the amount of organic matter in metalliferous sediments. Mineralogical data indicate that manganese in the primary sediments accumulated in a silicate form (MnSi gel, glass, etc). During diagenesis, the Mn–Si phase was transformed to neotokite with subsequent formation of caryopilite and further crystallization of pyroxmangite, rhodonite, tephroite, and other silicates due to reactions involving caryopilite. The hydrated Mn-silicates (caryopilite and/or friedelite) and the spatially associated parsettensite, stilpnomelane, and other minerals are the index minerals of the low grade metamorphism. Under PT conditions of prehnite–pumpellyite facies, nearly 70% of silicate minerals are hydrous. The metamorphosed Mn-bearing sediments are characterized by the low-temperature caryopilite (or tephroite-caryopilite-pyroxmangite ± rhodonite) and the high-temperature caryopilite-free (or tephroite-pyroxmangite ± rhodonite) facies. Their PT conditions correspond to zeolite and prehnite-pumpellyite (the low-temperature) and green schist and higher grade (the high-temperature) facies. 相似文献
7.
The particularity of the formation of the skarn lodes of the Cretaceous-Paleogene Belogorsk deposit is the intense replacement
of the early mineral assemblages (the decomposition of garnet, pyroxene, and pyroxenoids) with decreasing temperature, the
increase in the amount of magnetite at the expense of Fe released from the decomposed minerals, and the formation of quartz
and volatile-rich compounds (calcite, fluorite, amphibole, and sulfides). The geochemical and mineralogical similarity suggests
a genetic relation between the manganese skarn lodes of the Belogorsk deposit (the Ol’ginsk ore district) and the stratabound
bodies of the manganese silicate rocks (the Triassic contact metamorphosed metalliferous sediments) of the adjacent Shirokaya
Pad area (as a source of matter). 相似文献
8.
The Engteri is a new hidden Au-Ag deposit in the Russian segment of the Pacific ore belt. The discovery of this deposit merits
special attention, because it involves repeated attempts to reappraise a lowprospective ore occurrence, which were crowned
with success as a result of fulfillment of large-scale drilling project. The average Au grade is 18.6 gpt. The deposit is
classified as the gold geochemical type of Au-Ag deposits. The major ore mineral is pyrite, which amounts to no less than
95% of the total ore minerals. The native phases comprise electrum and to a lesser extent native gold of low fineness (730).
The homogenization temperature of fluid inclusions is 125–255°C with a distinct maximum at 145–150°C. Despite blind localization
of some orebodies, the Engteri deposits bears evidence for a deep erosion level: (1) small vertical range of economic mineralization
(50–100 m); (2) predominant occurrence of massive sugarlike quartz with a low sulfide content; (3) prevalence of massive and
brecciated textures above rhythmically banded textures; and (4) lack of low-temperature propylites. The southern part of the
ore field distinguished by occurrence of rhythmically banded, framework-tabular, and brecciated texture has the best prospect
for revealing new orebodies. The Engteri deposit allowed us to outline the following prospecting guides and methods of prospecting
for hidden Au-Ag deposits: (1) these deposits are regularly arranged in ore clusters between heavy concentrate anomalies of
cinnabar and gold-silver or silver-base-metal occurrences (method of missed link); (2) findings of fragments of ore mineral
assemblages with sporadically high Au and Ag contents in barren calcite-quartz veins (method of indicators); (3) linear zones
of ankeritization in the fields of low- and mediumtemperature propylites (mapping of metasomatic rocks); and (4) pyrite-quartz
veinlets with rhythmically banded pockets (mineralogical mapping of halos of stringer-disseminated mineralization). 相似文献
9.
Manganese ores of Nishikhal occur as distinctly conformable bands in the khondalite suite of rocks belonging to the Precambrian
Eastern Ghats complex of south Orissa, India. Manganese minerals recorded are cryptomelane, romanechite, pyrolusite, with
minor amounts of jacobsite, hausmannite, braunite, lithiophorite, birnessite and pyrophanite. Goethite, graphite, hematite
and magnetite are the other opaque minerals and quartz, orthoclase, garnet, kaolinite, apatite, collophane, fibrolite, zircon,
biotite and muscovite are the gangue minerals associated with these ores. The mineral chemistry of some of the phases, as
well as the modes of association of phosphorous in these ores have been established. The occurrence of well-defined bands
of manganese ore; co-folding of manganese ore bands and associated metasedimentary country rocks; the min-eral assemblage
of spessartite-sillimanite-braunite-jacobsite-hausmannite; the geochemical association of Mn-Ba-Co-Ni-Zn together with the
Si versus Al and Na versus Mg plots of the manganese ores suggest that the Nishikhal deposit is a metamorphosed Precambrian
lacustrine deposit. Continental weathering appears to be the source for manganese and iron. After deposition and probable
diagenesis, the manganese-rich sediments were metamorphosed along with conformable psammitic and pelitic sediments under granulite
facies conditions, and subsequently underwent supergene enrichment to produce the present deposit.
Received: 14 March 1995 / Accepted: 11 April 1996 相似文献
10.
The Pogranichnoe ore occurrence of gold-bismuth type is closely related to Ordovician granitic rocks pertaining to the Sarkhoi granodiorite-granite complex of calc-alkaline series. This type of mineralization in the Eastern Sayan is described for the first time. The orebodies are represented by quartz veins and veinlets with greisenized granite as selvages. Three mineral assemblages consecutively follow one another: (1) pyrite-arsenopyrite, (2) base-metal with fahlore and Sb sulfosalts, and (3) gold-bismuth. The geological position of orebodies and character of microinclusions in accessory minerals of granites suggest that goldbismuth mineralization is related to granitoids. 相似文献
11.
Mn-rich members of the pyrosmalite-family [(Mn, Fe) 8Si 6O 15(OH, Cl) 10], friedelite and schallerite have been identified as rock-forming minerals together with caryopilite, in several metamorphosed carbonate Mn-deposits. The phase assemblages and mineral compositions are described for eight of these localities each of which represents a distinct geologic situation. Friedelite is always Cl-bearing and occurs both as a prograde phase in low-grade metamorphic rocks (Pyrenees, Haute-Maurienne) and as a secondary phase formed by retrogressive replacement of primary anhydrous phases in higher-grade rocks. Schallerite, an Asbearing relative of friedelite, occurs in the greenschist metamorphic deposit of the Ködnitztal (Austria) together with other As-minerals. In these deposits, caryopilite is typically formed during retrograde metamorphism by alteration of, generally anhydrous, Mn-silicates. Based upon these occurrences, a qualitative petrogenetic grid for the system MnO-SiO 2-CO 2-H 2O with the phases friedelite, caryopilite, pyroxmangite/rhodonite, tephroite, rhodochrosite, quartz, CO 2, and H 2O is proposed. The phase relations imply that Cl- (or As-) free friedelite is not stable in hydrous systems with respect to caryopilite. From the mineral assemblages containing hydrous Mn silicates, waterrich fluids are inferred during the retrograde metamorphic evolution of the investigated deposits. Chemical data for Mn-rich chlorites, which are basically members of the clinochlore-pennantite series which coexist with the pyrosmalite minerals, show the absence of intermediate Mn/Mg ratios. This supports the existence of a miscibility gap as previously hypothesized by other authors. 相似文献
12.
Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline
schists and gneisses from shear zones exposed in Palenyi Island in the Early Proterozoic Belomorian Mobile Belt. Qualitative
analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C
and 10–11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various
mineralogical and chemical composition. The closure temperatures of the isotopic systems obtained by methods of oxygen isotopic
thermometry are close to the values obtained with mineralogical geothermometers ( Grt-Opx and Grt-Bt) and correspond to the high-temperature granulite facies (860–900°C). Identified systematic variations in the δ 18O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically
in contact with one another, these variations in δ 18O cannot be explained by the primary isotopic heterogeneity of the protolith. The model calculations of the extent and trend
of the δ 18O variations in minerals suggest that the only mechanism able to generate the zoning was fluid-rock interaction at various
integral fluid/rock ratios in discrete zones. This demonstrates that a focused fluid flux could occur in lower crustal shear
zones. The preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at
the peak of metamorphism was very brief. 相似文献
13.
The study of the mineralogical and geochemical features of ores and their textural and structural relationships at the Nikolaevsky base-metal skarn deposit allowed us to establish the succession of mineral assemblages and to ascertain the complex distribution of elements in minerals, ores, and orebodies resulting from nonuniform development of metasomatic zones in the single ore-metasomatic process. The vertical mineralogical and geochemical zoning of the deposit is expressed in the replacement of economic Pb-Zn mineralization at the lower levels of the skarn orebodies with Ag-Pb-Zn mineralization at the upper levels of the major Vostok-1 orebody located at the contact of limestone and felsic volcanics and the block orebodies at the lower level of felsic volcanic rocks. Au-bearing vein Pb-Ag-Sb mineralization occurs in the near-surface zone of the deposit. Three mineralogical types of ores have been identified: sulfide-hedenbergite, quartz-carbonate-sulfide, and sulfide. The zonal distribution of these types within orebodies is combined with variations in mineralogy and in the distribution of major and minor elements. The statistical processing (the Geokhimiya-1 program) of 96 chemical analyses of monomineralic galena and sphalerite samples taken from skarn showed that an intimate Pb ? Bi assemblage is characteristic of the lower levels, whereas a Ag Bi assemblage is typical of the upper levels. The elemental assemblages correspond to definite mineral assemblages, determining the mineralogical and geochemical zoning of the deposit. 相似文献
14.
The results of geological-mineralogical study of stratificated manganese ores in Famennian rocks of the Lemva facies in Pai-Khoi are presented. Carbonate manganese ores make up conformable stratified and lenticular bodies (up to 0.6 m thick) in the interval between the Gromashor and Silovayakha formations that are composed of jasperoids and carbonate-siliceous rocks. Ores are characterized by fine wavy bedding and development of transverse quartz veinlets. The ores are mainly composed of kutnahorite. Secondary minerals are represented by dolomite, calcite, pyrite, ransayite, and cryptomelane (?). Mn-muscovite, micro-cline, pyrophanite, galena, barite, apatite, and monazite are accessory minerals. The average MnO content is 23.81 wt %. The interval also includes a long (up to 40 m) lens of rhodonite rocks (Silovayakha occurrence) that replace carbonate ores along the strike. The major minerals in these rocks are represented by rhodochrosite, rhodonite, pyroxmangite, and quartz. Secondary minerals are observed as tephroite, alleghanyite, friedelite, caryopilite, neotocite, sussexite, pyrite, and supergene manganese oxides. Spessartine, albandine, barite, and apatite are accessory minerals. Based on the analysis of factual material, we suggest that ore material was derived from hydrothermal paleoceanic systems associated with Devonian volcanism. Ore concentration in the sedimentation zone was related to the stagnant reduced setting of bottom water. Ore deposition was promoted by the delivery of fresh portions of ocean water. Ore matter was accumulated mainly by chemogenic mechanism and partly with the’ participation of bacteria. It is suggested that boundary between the Gromashor and Silovayakha formations should be corrected to unite rocks of the Famennian manganiferous association into a single formation. 相似文献
15.
In the metamorphosed manganese oxide ores of India, braunite is ubiquitous in all assemblages from chlorite to sillimanite grades. Chemical analyses of braunite from different prograde assemblages confirm the presence of a fixed R 2+ (=Mn 2++Mg+Ca) SiO 3 molecule in the mineral. Element partitioning between coexisting braunite and bixbyite indicates a near-ideal mixing of Fe +3/ -Mn +3 in the phases. This also indicates that braunite became relatively ferrian while equilibrating with associated phases such as bixbyite, hollandite and jacobsite during prograde reactions. Petrogenetic studies show that as a general trend, prograde lower oxide phases appeared by deoxidation of higher oxide phases. But braunite, a more reduced phase than bixbyite, appeared early from deoxidation of pyrolusite in presence of quartz. Bixbyite could appear later from the reacting pyrolusite-braunite-quartz assemblage. Inferred mineral reaction paths and the general trend of pro-grade deoxidation reactions suggest that the composition of ambient fluid phase was internally buffered during metamorphism. 相似文献
16.
马坑大型铁(钼)矿赋存于莒舟-大洋花岗岩体外接触带黄龙组(C2h)灰岩和林地组(C1l)碎屑岩层间构造破碎带中,铁矿与矽卡岩密切共生,但矿床成因尚存在争议。本文就马坑铁矿矽卡岩进行了矿物学特征研究。电子探针分析结果表明:该矿矽卡岩矿物组合主要为辉石、石榴子石和钙蔷薇辉石,退化蚀变岩矿物组合为角闪石、绿帘石、绿泥石、石英等。单斜辉石以透辉石和钙铁辉石为主,仅存在少量锰钙辉石;似辉石为钙蔷薇辉石和蔷薇辉石;石榴子石端元成分以钙铁榴石为主,钙铝榴石少量;角闪石属于钙角闪石,矿物学特征表明它们形成于相对较氧化的条件下。马坑铁矿的矽卡岩是由热流体沿灰岩与碎屑岩之间层间构造破碎带交代形成的,铁矿石大部分产于矽卡岩内,磁铁矿多稍晚于矽卡岩,不仅广泛交代矽卡岩,而且还直接交代灰岩、砂岩等围岩,呈交代结构;主矿体下盘常出现厚层石英岩,碎屑岩也出现了明显的交代,矽卡岩分带现象普遍,与典型矽卡岩矿床特征一致。结合矿床地质特征,马坑铁矿矿床类型应为层控矽卡岩型矿床。 相似文献
17.
The geological position, composition of mineral assemblages, and typomorphism of major minerals from garnet-bearing rocks at the Berezitovoe gold-base-metal deposit in the Upper Amur Region have been studied in detail. These are ore-bearing metasomatic rocks and metamorphosed porphyritic dikes. The garnet-bearing metasomatic rocks reveal zoning, which is caused by various degrees of metasomatic transformation of the Paleozoic porphyritic granodiorite that hosts the ore zone. The metasomatic replacement of granodiorite was accompanied by loss of Na, Ca, Ba, Sr and gain of K, Mn, and Rb. Garnet-bearing metamorphosed intermediate dikes occur within the metasomatic zone. The PT conditions of metamorphism and metasomatism are similar and estimated at 3.9 kbar and 500°C from various mineral equilibria. The results of physicochemical simulation of garnet-bearing mineral assemblages carried out by minimizing the Gibbs free energy and the geological data show that garnet-bearing mineral assemblages arose at the Berezitovoe deposit as a result of local high-temperature thermal metamorphism of previously formed low-temperature metasomatic rocks close in composition to classic beresite. In this connection, we propose considering garnet-bearing metasomatic rocks as high-temperature metamorphosed beresites. 相似文献
18.
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
19.
The Archaean lode-gold deposits at Norseman, Western Australia, consist of auriferous quartz veins in dextral-reverse ductile-brittle
shear zones within tholeiitic metabasalts of upper-greenschist to amphibolite facies metamorphic grade. Three types of deposits
(Northern, Central, Southern) are delineated on the basis of their spatial distribution, veining style, alteration mineraloty
and metamorphic grade of host rocks. Northern deposits, hosted in upper-greenschist to lower-amphibolite facies rocks, comprise
massive to laminated quartz veins with selvedges of quartz-chlorite-calcite-biotite-plagioclase assemblages. Central deposits,
hosted in lower-amphibolite facies rocks, consist of laminated to massive quartz veins with selvedges of quartz-actinolite-biotite-plagioclasecalcite
assemblages. Southern deposits, hosted in middleamphibolite facies metabasalts, consist of banded quartz-diopside-calcite-microcline-zoisite
veins. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, consistent with a syn-deformational
genesis at high temperatures. From Northern to Southern deposits, the alteration assemblages are indicative of higher temperatures
of formation, and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals.
These observations imply that a thermal variation of gold-related hydrothermal alteration exists within the Norseman Terrane
over a distance of 40 km, with T Northern<T Central<T Southern This thermal zonation is corroborated by T−X CO
2 phase relations between vein selvedge assemblages, which signify formation temperatures of approximately 420°–475°C, 470°–495°C
and >500°C for Northern, Central and Southern deposits, respectively. The sum of structural, petrographic and mineral chemistry
data indicates that the alteration assemblages formed in high-temperature, open hydrothermal systems and have not been subsequently
metamorphosed. The thermal differences between the deposit groups may reflect (1) a temperature gradient, at relatively constant
P, corresponding to the proximity of the deposits to regional granitoid complexes, or (2) formation of the deposits at progressively
deeper crustal levels from north to south. In either case the deposits represent a continuum of gold deposition from upper-greenschist
to amphibolite facies, now exposed in an oblique section through the Archaean crust at Norseman. 相似文献
20.
The porphyry molybdenum deposits of Climax, Colorado, are stockworks of quartz-molybdenite veinlets. They are apical to intrusive cupolas of leucocratic rhyolite-granite porphyry of Oligocene age (33 to 24 Ma). The upper parts of the orebodies are overprinted by overlying zones of phyllically altered rocks, and by stockworks of greisen-like veinlets, containing quartz, pyrite, topaz and huebnerite. The phyllic and/or greisen-like zones are surrounded by a propylitic zone, characterized by quartz-chlorite-hematite veinlets and chlorite after biotite. Late veins, radial and peripheral to the intrusive center, are common but widely scattered and discontinuous. Such veins typically contain quartz, fluorite, rhodochrosite, pyrite, dark sphalerite, galena, tetrahedrite, and traces of huebnerite, molybdenite, argentite, native gold and/or electrum. The Alma district, 4 to 10 km southeast of Climax, contains widely scattered clusters of veins and replacement deposits. The veins cut chloritized to sericitized silicate host rocks, and the replacement deposits are in carbonate host rocks. The ore- and gangue-mineral assemblages of the veins of the Alma district resemble those of veins peripheral to the Climax molybdenum deposits. Veins of the Alma district cut dikes of Late white rhyolite (35 Ma), which resemble the rhyolite porphyries of Climax. A swarm of such dikes is parallel to the long axis of an elliptical drainage pattern, which bounds a structural dome, here called the outer Alma dome. The outer Alma dome (8 × 12 km) surrounds an inner Alma dome (6 × 8 km). Veins of the Alma district generally are axial and/or marginal to the outer Alma dome, and/or radial to the inner Alma dome. Veins and alteration minerals associated with the outer Alma dome formed between 35 and 27 Ma ago. Those associated with the inner Alma dome formed about 27 Ma ago, as indicated by fission-track age determinations on thermally annealed zircon grains from hydrothermally altered rocks, within and around the inner Alma dome. Negative Bouguer gravity anomaly patterns indicate: (1) a regional, northeast-trending gravity low, associated with the composite monzonite-granodiorite-granite B batholith of the Colorado mineral belt (average density about 2.62 g/cm3); (2) a semi-regional gravity trough, within the gravity low of the Colorado mineral belt, extending northeastward across the Mosquito Range; (3) an oval pattern of gravity lows, coincident with the Alma domes; and (4) a closed gravity low associated with the Climax stock (density about 2.56 g/cm3) (Tweto and Case, 1972; Behrent and Bajwa, 1974; Corry, 1981). The semi-regional gravity trough is interpreted as the gravimetric expression of a granite batholith, here called the Climax-Alma batholith. The gravity lows of the Climax and Alma districts are interpreted as expressions of cupolas on this granite batholith. The highly differentiated rhyolite-granite intrusions of Climax plunge toward the Alma domes. The Climax intrusions are interpreted as products of apical differentiation of the inferred Climax-Alma granite batholith. Magmatic-hydrothermal fluids, associated with highly differentiated apical magmas, produced the Climax molybdenum orebodies. Most of the veins and replacement deposits of the Alma district are associated with lower, broader cupolas of the inferred granite batholith. The shape of the composite Climax-Alma magmatic-hydrothermal system resembles that of a teapot, with Climax above the spout and the Alma district above the lower, broadly domed lid. 相似文献
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