Magnetite and garnet trace element characteristics from the Chagangnuoer iron deposit in the western Tianshan Mountains,Xinjiang,NW China:Constrain for ore genesis
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Located in the eastern Awulale metallogenic belt of western Tianshan Mountains,the large-scale Chagangnuoer iron deposit is hosted in the andesite and andesitic volcaniclastics of the Lower Carboniferous Dahalajunshan Formation,with one lentoid marble as footwall rock beneath the main ore bodies which exhibit as lamellar,stratoid and lenticular.The alteration zonation is similar with typical hydrothermal deposits.According to ore fabric and mineral paragenesis,this deposit can be divided into two ore-forming stages,which are magmatic stage and hydrothermal stage(included prograde sub-stage and quarts-sulfide sub-stage).In the magmatic stage,REE in magnetite is very low,rich in LREE and HREE but depleted in MREE with a U type pattern.In addition,this kind of magnetites has a higher Ti,V,Cr,indicating that Fe might come from the crystallization differentiation of andesitic magma.On the other hand,in the prograde sub-stage,magnetites have a lower REE content,a bit rich in LREE but other REE strongly depleted.Compared with the magnetites in magmatic stage,these magnetites are poor in Ti,V but a bit abundant in Ni,Co and Cu content.Garnets in barren and ore-bearing skarn distribute the same REE patterns,having a relatively high REE content,enriched in HRRE but depleted in LREE,and with a not pronounced positive Eu anomaly,which displays the feature of garnet with metasomatic origin in the calcic skarn.And this hints that the magnetites,which have a paragenesis relationship with ore-bearing garnets,should be also a product of hydrothermal fluid replacement with wall rocks,and most of the mineralizing materials(Fe) probably are derivate from andesitic strata.In combination geological characteristics with trace element geochemistry,we hold that the Chagangnuoer iron ore is probably one polygenetic deposit with the skarn type(predominated) superposition upon the magmatic type.Keywords:
Metasomatism
Paragenesis
Ore genesis
Igneous differentiation
Trace element
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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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Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type ( eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.
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Abstract Magnetite-apatite iron ores of the Kiruna type, unaffected by deformation, have structures and textures similar to those of igneous rocks. The best examples are the El Laco deposits in northern Chile which resemble lava flows, pyroclastic deposits and dikes. El Laco magnetites have δ18O values between 2.3 and 4.2‰ (V-SMOW). Magnetite from ore with a magmatic texture has a mean of 3.7‰, and the mean for magnetite intergrown with pyroxene in veins is 2.4‰. Oxygen isotope data given here, fluid inclusion results and geological evidence indicate that ore formation took place in a cooling magmatic system. Major orebodies resembling lava flows and near-vent pyroclastic deposits crystallized from magma at ca. 1000°C. Fluids from cooling magma deposited magnetite and pyroxene (±apatite) at ca. 800°C in fissures and open spaces, now present as veins cutting major orebodies. There is little evidence for significant magnetite precipitation during hydrothermal conditions. A large province of magnetite-apatite iron ore in central Chile (the Cretaceous iron belt) and the Kiruna district in northern Sweden also contain primary ore of magmatic appearance. Major deposits in the Chilean iron belt and Kiruna contain magmatic-textured magnetites with the following δ18O means: Algarrobo = 2.2‰, Romeral = 1.2‰, Cerro Imán = 1.6‰, and Kiirunavaara = 1.5‰. We consider all oxygen isotope data for unoxidized, magmatic-textured magnetite as representative of the Fe-rich magmas. Magnetites affected by hydrothermal alteration, recrystallization and subaerial oxidation have modified isotope signatures. Key Words: Oxygen isotopemagnetiteiron oreEl LacoKirunaChileSweden
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