Mineralogy, mineral chemistry and genesis of the Hongyuntan iron deposit in East Tianshan Mountians, Xinjiang
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The Hongyuntan iron deposit is hosted in pyroclastic rocks of the Lower Carboniferous Yamansu Formation.The ore bodies occur as layers,stratoid bodies or lenses.The principal ore mineral is magnetite,together with minor maghemite,specularite,pyrite and trace chalcopyrite.The gangue minerals include garnet,diopside,actinolite,chlorite,tremolite,epidote,biotite,albite and quartz.The ore structures are mainly of massive and disseminated forms,with occasional banded or veined forms.The ore textures are of subhedral-anhedral granular and metasomatic types.The wall rock alteration shows symmetrical zoning,and the alteration colors change from dark to light from ore bodies outwards.On the basis of observed mineral assemblages and ore fabrics,two periods of ore deposition were recognized,i.e.,skarn period and hydrothermal ore-forming period,which could be further subdivided into four metallogenic stages,namely skarn stage,retrograde alteration stage(main ore-forming stage),early hydrothermal stage and quartz-sulfide stage.Electron microprobe analyses show that the end member of garnet is mainly andradite-grossularite.The composition of pyroxene is mainly diopside-asteroite.The amphiboles is composed mainly of actinolite and tremolite with minor magnesiohornblende.The composition of these skarn minerals suggests that skarn in the Hongyuntan iron deposit is calcic skarn,belonging to metasomatic skarn.The characteristics of main and trace elements suggest that the formation of magnetite was closely related to the skarn.In combination with geological characteristics,the authors suggest that the skarn might have resulted from interaction between Ca-rich pyroclastic and Fe-rich magmatic hydrothermal fluid which was transported along the fault system.The formation of magnetite was hence related to the regressive metamorphism of the skarn.Keywords:
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A horizontal zonality has been established for the skarn (Table 1,Fig.2).Themain skarn minerals are diopside and garnet,with subordinate actinolite,epidote,plagio-clase,chlorite,calcite,quartz and sulphides,etc.There are multiple stages of constituentminerals and mineralization.The ore-minerals sulphides and scheelite are generally pre-sent as replacing the earlier formed skarn minerals. Analyses have been made both of the host rocks and the secondary silicate rocksresulting from it in order to see what changes accompanied the skarn formation process.On the basis of studying the distribution and behaviour of major and trace elements inthe skarn process,the following conclusions have been made. Under the metasomatism calcium,iron,aluminium,manganese and magnesium werecarried in and alkalis,silicon,titanium and phosphorus were simultaneously carried outand partly redeposited (Table 2,3,Fig.3).Germanium,indium,nickel and rare earthelements were carried in during the process of skarn formation,the other trace elementsenter into the skarns from the surrounding rocks (chiefly granodiorite) (Table 7).Thetrace elements follow the major elements for which they can substitute in favourablecrystal lattices (Table 9). The skarn genesis is nicely coordinated with the theory of metasomatic contact-reaction processes between a granitoid and a carbonate environment with the participationof post-magnetic solutions.The variety of metasomatic rocks and the zonal structure ofthe skarn body are the results of a reactive interaction of solutions with host rocks in theprocess of an infiltration metasomatis with a subordinate significance of bimetasomatis.All the mineralogical compositions of the rocks in the skarn zones may be comprehendedby the relative activity of Ca,Al and Si. The order of formation of the skarn body has been approximately determined asthe following:altered granodiorites → diopside-plagioclase skarns → diopside-garnetskarns → garnet skarns in the inner zone;simultaneously marmorized limestones diop-side skarns → diopside-garnet skarns and banded skarns in the outer zone.
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The polymetallic Madem Lakkos sulfide deposit in northern Greece is hosted within marble of the Mesozoic (?) Kerdylia Formation, a high-grade metamorphic complex composed of migmatitic biotite gneiss interlayered with marble, hornblende gneiss, and amphibolite. The Kerdylia Formation is invaded by a variety of foliated and nonfoliated intermediate to felsic intrusions of Tertiary age. The Madem Lakkos deposit is long-believed to have formed from a single epigenetic hydrothermal replacement event related to Tertiary magmatism, but this research has recognized the presence of three different and distinct ore types in the deposit that resulted from a much longer and more complex genetic history.Based on ore mineralogy, textures, and geochemistry, the Madem Lakkos ores can be characterized as (1) massive sulfide ore, (2) disseminated sulfide ore, and (3) skarn ore. The massive pyrite-sphalerite-galena ore exhibits abundant and well-developed metamorphic structures and textures that indicate the ore has been metamorphosed to upper amphibolite grade, at temperatures of at least 600 degrees C, together with its marble and gneissic host rocks. These textures include foliated-lineated galena and sphalerite, slip planes and deformation twinning in galena and sphalerite, and granoblastic annealing-recrystallization features with the development of 120 degrees triple-point junctions in galena, sphalerite, and pyrite. Despite its metamorphism, this ore preserves a generally stratiform nature with sharp, unaltered host-rock contacts, a regional and stratigraphic association with chemical and possible evaporitic metasedimentary rocks, compositional layering, and metal zoning that are consistent with formation as a sedimentary massive sulfide deposit.Disseminated sulfide ore, the most abundant type in the deposit, consists of complex veins and irregular manto-type impregnations in altered marble that are composed of pyrite, sphalerite, tennantite, chalcopyrite, arsenopyrite, galena, seligmannite, boulangerite, and minor amounts of a wide variety of additional sulfominerals in a quartz-sericite-manganiferous carbonate gangue. Disseminated sulfide ore transects and has reacted with the earlier massive sulfide ore and does not exhibit evidence of metamorphism. Euhedral zoned crystals with mineral and fluid inclusions, open-space fillings, and complex textural relationships are characteristic of this ore type and indicate that it formed through the replacement of marble by reaction with hydrothermal solutions. Disseminated sulfide ore is enriched in Cu, As, Mn, Sb, and Bi in comparison with the massive sulfide ore.Skarn ore contains pyrite, chalcopyrite, scheelite, and minor amounts sphalerite, galena, and Pb-Bi sulfominerals in a calc-silicate assemblage of gangue minerals that includes andradite-grossularite garnet, diopside, calcite, quartz, epidote, and minor chlorite, actinolite, and magnetite. Textures similar to those found in the disseminated sulfide ore and an absence of metamorphic features are characteristic of the skarn ore. Highly saline fluid inclusions in quartz from the skarn ore suggest that high-temperature, low-pressure porphyry copper-type magmatic fluids were involved in generation of this ore. Skarn ore does not exhibit a spatial relationship to igneous rocks in the mine but may be related to porphyritic quartz diorite stocks a few kilometers to the south that have halos of propylitic and phyllic alteration and porphyry copper-type mineralization.The different ore types are characterized by a very similar lead isotope composition ( 206 pb/ 204 pb = 18.78-18.82, 207 pb/ 204 pb = 15.67, 208 Pb/ 204 pb = 38.88-38.92), which lies within the restricted field of igneous rocks from northern Greece. Although this resemblance between ore and igneous rock lead has been used to support a magmatic origin for the Madem Lakkos and related sulfide deposits, the uniform isotopic composition of all lead in this tectonically active region weakens this argument. If, as is proposed, the massive sulfide ore was initially deposited as a synsedimentary body within the Kerdylia Formation, the modern model age of the lead strongly suggests that mineralization took place only a short time before the rocks were metamorphosed.The superposition of multiple ore types having different mineralogic and chemical compositions, textures, metamorphic grades, and apparent ages indicates a complex, multistage, polygenetic origin for the Madem Lakkos deposit. An interpretation consistent with this evidence is that synsedimentary massive sulfide ore was deposited as a stratiform body within a sequence of probably Mesozoic shallow-water platform carbonate and clastic-volcaniclastic sediments, possible evaporitic sediments, and lesser amounts of volcanic rocks. This ore and its host rocks were metamorphosed to upper amphibolite grade during Cretaceous-Tertiary regional metamorphism.Coregional, post-tectonic intrusions generated heat and magmatic fluids that produced skarn and skarn ore by replacement of marble at temperatures above 360 degrees C. A continuing but cooling convective hydrothermal system mixed magmatic fluids with meteoric water. These hydrothermal fluids permeated marble and massive sulfide ore peripheral to the skarn ore, reacting with them and extensively altering the marble to form disseminated sulfide ore. Massive sulfide ore and related chemical sedimentary rocks were partly dissolved by and incorporated into the hydrothermal solutions, thereby contributing Pb, Zn, Fe, Mn, Ag, Au, and minor amounts of other constituents to the hydrothermal system. Fe, Cu, W, As, Sb, and Bi were probably magmatic contributions.
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In the Yerington district of western Nevada, large bodies of skarn formed in a limestone unit of Triassic age adjacent to a Jurassic batholith. This batholith is dominantly granodiorite but contains quartz monzonite stocks and dike swarms that host major porphyry copper deposits. The units enclosing the limestone, an andesitc tuff and a silty limestone, were also extensively metasomatized, as were outlying intrusions of granodiorite.Skarn formation is divided into an early skarnoid stage and a late metasomatic skarn stage. The skarnoid stage formed massive fine-grained granditc garnet in the andesitc tuff, limestone, and silty limestone, with subordinate amounts of pyroxene. The iron content of garnets increases significantly away from the granodiorite, whereas pyroxenes maintain a relatively constant diopsidic composition.Granodiorite apophyses, which occur in the Triassic rocks up to 0.5 km from the main batholith contact, are extensively altered to endoskarn. The main alteration type consists of massive, anhedral granditc garnet with a significant titanium component. The distribution of endoskarn suggests that proximity to limestone was a key factor in endoskarn formation and that one hydrothermal fluid altered both sedimentary and igneous rocks simultaneously.The second stage of alteration most dramatically affected the limestone, forming coarse-grained skarn. Two skarn types are recognized: (1) a magnesium-rich type in dolomitized marble, dominated by pyroxene, and (2) an iron-rich type in calcite marble, dominated by andradite. Age relations between the two are ambiguous, but constraints inferred from T-X (sub CO 2 ) stability relations suggest that magnesium-rich skarn formed first and at high temperatures, followed by iron-rich skarn. During a late stage, intermediate granditc garnet veined both skarn types, and actinolite (+ or - salite) formed locally at the andradite-marble contact.In detail, magnesium-rich skarn nearest the granodiorite is zoned from diopside through serpentine (replacing diopside) + calcite, then from clinohumite + calcite + dolomite to calcite + dolomite marble. Additional phases include monticellite, spinel, magnetite, ludwigitc, and szaibelyite. Farther from the batholith, clinohumite is absent, tremolite separates serpentine from pyroxene, and periclase occurs locally.Pyroxene and serpentine are enriched in iron with distance from the granodiorite, the pyroxene changing from nearly pure diopside to intermediate smite a kilometer away. This compositional variation is consistent with an increase in mu Fe /mu Mg , with time and with distance from the batholith. The presence of magnetite pseudomorphs after hematite and the variation in magnetite composition indicate that the oxidation state decreased with time near the granodiorite.Iron-rich skarn consists of andradite garnet, which on the outer skarn contact directly replaced calcite. Inclusions of wollastonite noted in garnet cores from one sample suggest that initially a wollastonite zone was locally present at the skarn margin. Veins of actinolite and magnetite cut garnet skarn; late actinolite at the garnet-marble contact, locally a site of sulfide deposition, suggests a reversal in the trend toward increasing mu Fe /mu Mg . At the Douglas Hill mine, apatite, quartz, and sulfides replaced andradite.Mineral assemblages and fluid inclusion data indicate that both magnesium- and iron-rich skarn formed at low X (sub CO 2 ) , generally less than 0.1. Temperatures of formation of magnesium-rich skarn are inferred to have been in the range 650 degrees to below 400 degrees C. Temperatures of formation of iron-rich skarn are not constrained by observed assemblages. Fluid inclusion data show that late apatite replaced andradite at temperatures between 120 degrees and 200 degrees C.
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Mineralogical Characteristics and Genesis of Skarn from Lizhu Iron Deposit, Zhejiang Province, China
The Lizhu iron deposit in Zhejiang Province, China is a typical skarn-type deposit located in the east part of the Qinhang Metallogenic Belt in southern China. The ore bodies are lenticular, bedded, and irregular, and are located in skarn in a wide contact zone of Nanhua system of Sinian system, of the Cambrian and Ordovician periods along the external contact belts of the Guangshan granite complex. The main skarn minerals include diopside, garnet, Fe-rich edenite, phlogopite, chlorite and titanite. The main metallic minerals are magnetite, pyrite, galena, sphalerite and molybdenite. The skarn minerals discussed in this paper were studied using electron microprobe analyses. Results show that the Lizhu iron deposit experienced a period of skarnization and a period of hydrothermal alteration. The period of skarnization is divided into a pyroxene-garnet phase, a magnetite phase and a hornblende-phlogopite phase. As well, the hydrothermal alteration period includes a quartz-sulfide phase and a quartz-carbonate phase. The pyroxene end-member is dominated by diopside, and is altered into hedenbergite(Mg2+ to Fe2+); and garnet is altered from grossular into andradite(Al3+ to Fe3+). These features suggest a progressive concentration of Fe3+ ion and increasing oxygen fugacity f(O2) in mineralization fluids during the early period of skarnization. The Na-, K-, and F-bearing Fe-rich edenite, F-rich phlogopite and titanite, and fluorite phenomenon, indicate that ore-forming fluids are alkaline and enriched in F, which favour the migration, enrichment and mineralization of iron. The formation of the Lizhu iron deposit is related to multiple periods of magmatic hydrothermal activity in the Guangshan-Shanxi plutons.
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Skarns, mantos, and breccia pipes occur at Cananea in a 2- by 4-km horst of Paleozoic carbonate rock and minor quartzite. Surface mapping and core logging reveal a sequence of events beginning with early metamorphism that converted impure carbonate lithologies to iron-poor garnet-pyroxene + or - idocrase hornfels. Subsequent metasomatism formed garnet-py-roxene skarn along the pre-Cretaceous Elisa fault contact between carbonate rock and Mesozoic volcanic rock. Skarn is zoned from an andradite-rich center, through a zone containing both andraditic garnet and salitic pyroxene, to a mineralogic sequence near the marble front which is largely a function of the sedimentary protolith; chert nodules are rimmed by wollastonite, dolomite is converted to massive phlogopite-magnetite skarn, calc-silicate hornfels is over-printed by veins of skarn garnet, and relatively pure marble is replaced by coarse blades of iron and manganese-rich pyroxene. Multiple generations of garnet and pyroxene can be distinguished by subtle variations in color and texture. Age classification on the basis of crosscutting vein and overgrowth relationships indicates that late pyroxenes are more iron and manganese rich than the early generations; garnets show a less regular iron enrichment with time. The spatial distribution of the different garnet and pyroxene generations is irregular; garnet and pyroxene in late veins have iron-rich compositions both near the skarn center and near the marble front. Pyrite and minor chalcopyrite are the only sulfides associated with this stage of metasomatism.Following the main stages of garnet and pyroxene formation, veins and orbicular patches of amphibole + or - quartz + or - calcite occur replacing pyroxene and in some cases, garnet. Most of the amphibole is actinolitic in composition and is associated with pyrite and minor amounts of chalcopyrite. About 20 percent of the amphibole is subcalcic and is associated with or replaced by massive calcite. The alteration of prograde skarn to amphibole + or - quartz + or - calcite can be best explained by a general temperature decline.The destruction of skarn by alteration related to brecciation and breccia pipe formation and the replacement of previously unaltered carbonate rocks by stratiform blankets (mantos) of iron oxides and sulfides resulted in some of the highest grade orebodies. Breccia pipe and manto formation appears to have been largely contemporaneous with emplacement and subsequent sericitic alteration of a series of quartz monzonite porphyry stocks. Where breccia pipes crosscut skarn, garnet, pyroxene, and amphibole are converted to mixtures of calcite, quartz, chlorite, hematite, siderite, and sulfides. Where breccia pipes or porphyries crosscut previously unaltered carbonate rock, mixtures of magnetite, sulfides, chlorite, siderite, calcite, quartz, and serpentine form massive mantos. Veinlets of chlorite and magnetite + or - pyrite + or - serpentine extend tens of meters beyond the zones of massive replacement. There is a rough vertical and lateral zonation of sulfide minerals with respect to the Democrata breccia pipe from chalcopyrite + or - bornite or pyrite in the core and at depth to sphalerite-pyrite-chalcopyrite to pyrite-sphalerite-galena distal to the center of mineralization. Mineral stability relations suggest that brecciation and mineralization took place at lower temperatures (275 degrees -25 degrees C) and possibly under lower X (sub CO 2 ) conditions than the earlier skarn formation.The high Zn/Cu ratios in skarn, the zonation of most hypogene mineralization relative to the breccia pipes which crosscut skarn, and the lack of skarn spatially associated with the quartz monzonite porphyry stocks which intrude carbonate rock all suggest that, unlike skarn in most porphyry copper districts, skarn at Cananea is not related to the quartz monzonite porphyry stocks that are mined elsewhere in the district for disseminated supergene-enriched sulfide mineralization. Rather, the skarn may be related to a deeper magmatic system which has not yet been encountered in subsurface exposures.
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