Magnetite composition of Zhibo iron deposit in Western Tianshan Mountains and its genetic significance
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Chalcobamba is a Cu-skarn deposit (338 Mt @ 0.55 % Cu) located in the centre of the Middle Eocene-Early Oligocene Las Bambas district, Andahuaylas-Yauri Belt, Southern Peru. At Chalcobamba, skarns formed when limestones of the Ferrobamba Formation were intruded by a Late Eocene quartz-diorite pluton. Skarn ores were then cut by a quartz monzodiorite porphyry stock, and swarms of monzogranitic and monzodiorite dikes.
Chalcobamba has spectacular outcrops of massive magnetite, massive epidote endoskarns, and coarse grained garnet. Magnetite and epidote display a wide range of textures linked with the evolution of alteration and mineralization in the deposit.
Distinctive magnetite textures formed during magmatic-hydrothermal activity in the intrusive rocks and in the prograde and retrograde skarn assemblages. The pluton, stocks and dikes typically have mafic minerals replaced by magnetite. Early banded magnetite and diopside skarn formed in limestone horizons that have some dolomitic content. Magnetite bands are typically thin and fine grained, but locally coarsen and increase in thickness. Magnetite associated with retrograde sulfide mineralization has replaced garnet and pyroxene in a continuous progression from magnetite patches to massive magnetite skarns. Hematite-epidote-calcite is a typical late-stage assemblage. In many cases, the hematite is partially to total pseudomorphed by magnetite, producing mushketovite. Some mushketovite crystals are up to 5 cm in diameter.
The typical epidote progression in intrusive rocks at Chalcobamba is from veinlets and disseminations in plutons, stocks and dikes, through endoskarn alteration patches, to tens of meter outcrops of massive epidote endoskarns. The monzogranitic swarm dike has a mappable gradient in disseminated epidote intensity. It is abundant near the skarn mineralization centre, and progressively weaker to the periphery of the system. The disseminated epidote consists of partial or total replacements of feldspars crystals. Epidote has also locally replaced prograde garnet, pyroxene and magnetite skarns. When epidote is associated with calcite and mushketovite, it typically displays coarse radial acicular textures.
Both magnetite and epidote have the capacity to record chemical anomalies via their stable isotopic or trace element compositions, potentially allowing inferences about the evolution and fertility of hydrothermal fluids to be made, provided that interpretations are made within the framework of the textural evolution of these key gangue minerals. Work is on-going to establish if any of the textural varieties of magnetite and/or epidote can be used to aid skarn exploration in the Andahuaylas-Yauri Belt, particularly as to whether mapping characteristic textures can provides insights into ore genesis and/or proximity to the center of mineralization.
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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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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.
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