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Mineralogical characteristics and geological significance of skarn in the Saishitang copper deposit, Xinghai county, Qinghai Province

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Abstract

The Saishitang copper deposit is an important skarn deposit located in the Southeast of Ngola Mountain Area, Eastern margin of East Kunlun orogenic belt. The skarn occurs along the contact between Indosinian quartz diorite and middle-lower Triassic T1-2b2 lithologic section. The ore bodies which are stratoid or lenticular mainly occur in exoskarn. The T1-2b2 lithologic section consists of intermediate volcanic rocks, marble and metamorphic siltstone. Cu mineralization is adjacent to the metamorphic andesitic tuff and andesite spatially. Petrography studies of Cu-bearing skarn show four stages of skarn formation and ore development, including skarn stage, retrograde alteration stage, quartz-sulfide stage and quartz-carbonate stage. Skarn stage consists of garnet, pyroxene and wollastonite and retrograde alteration stage dominates by epidote, amphibole and magnetite. In quartz-sulfide stage a large amount of sulfides were precipitated. Electron microprobe analysis shows that components of garnet and pyroxene are Gro0.00~91.00And7.02~100.00(Pyr+Alm+Spe)0.00~4.27 and Di12.80~91.75Hd2.41~79.80(Jo+Jd+Opx)0.00~13.47, respectively. The result indicates that the skarn in Saishitang deposit belongs to typical calcic skarn. High concentrations of grossular and epidote are found in skarns located closer to the contact between quartz diorite and andesite. Andradite is richer near the marble with wollastonite and Mn-bearing hedenbergite. Mineralogical characteristics and compositional variations suggest: from skarn stage to quartz-sulfide stage, the property of fluid changed episodically with two redox fluctuations at least, probably resulting from the influx of meteoric waters in ore-forming fluid. Saishitang copper deposit is a skarn deposit. The quartz diorite-dominated intrusions carrying plenty of heat and fluids intrude into middle-lower Triassic strata. The materials exchange between intrusions and wall rocks may be the mechanism of the development of skarn and ore bodies in Saishitang deposit, accompanying the bimetasomatism between marble and metamorphic siltstone and intermediate volcanic rocks.

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... The paragenetic sequence of the Saishitang deposit is shown in Figure 4. The skarn period is characterized by the precipitation of anhydrous skarn minerals such as grossular-andradite garnet [41], diopside, and wollastonite (Stage I), which are replaced by the hydrous skarn minerals, such as amphibole and epidote (Stage II). ...
... The hydrothermal period can be divided into four stages. Stage III is characterized by the precipitation of the magnetite, replacing the garnet ( The skarn period is characterized by the precipitation of anhydrous skarn minerals such as grossular-andradite garnet [41], diopside, and wollastonite (Stage I), which are replaced by the hydrous skarn minerals, such as amphibole and epidote (Stage II). ...
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Zn-Cd-S series minerals not only comprise industrial resources for Zn and Cd, but are also significant mineralogical indicators for hydrothermal ore-forming processes. Due to its unique formation conditions and rare occurrence, our understanding of the formation of zincian greenockite in natural systems is limited. Zincian greenockite was discovered during mineralogical studies in the Saishitang Cu skarn deposit, Qinghai Province, Northwest China. This provided an ideal opportunity to assess the occurrence and formation of zincian greenockite in skarn-type deposits. Ore minerals were observed using reflected-light microscopy, and the zincian greenockite was further analyzed using electron-probe microanalysis (EPMA) and X-ray diffraction (XRD). The zincian greenockite occurs in the bornite-chalcopyrite ores and is composed of subhedral to anhedral grains approximately 50 × 150 µm² to 200 × 300 µm² in size, replaces the bornite, and is replaced by native silver. Two phases (I and II) were identified based on back-scattered electron images, X-ray element-distributions maps, and EPMA data. The textural relationship indicated that Phase I was replaced by Phase II. Phase I contained high Zn (14.6 to 21.7 mol % ZnS) and low Cd (72.4 to 82.2 mol % CdS), while Phase II contained low Zn (5.6 to 9.1 mol % ZnS) and high Cd (85.4 to 89.9 mol % CdS). The zincian greenockite was formed at temperature of 300~270 °C during the transformation from a reducing environment to an oxidizing one in the late stage of the mineralization process in the Saishitang deposit.
... Due to the diversity of ore deposit types in the area, there is no consensus for the genesis of the Saishitang Cu deposit. Three genetic models have been proposed, including skarn (Cheng, Zhu, Ding, He, & Wang, 2014;Wang et al., 2015;Xin et al., 2013), exhalative sedimentary (Li et al., 1993;Song, Zhang, Li, Zhang, & Wang, 1995) and porphyry (Li et al., 2009;Wei, Zhang, Li, & Zhang, 2012;Wu, 2010). The argument for the ore deposit genesis obliges further mineral exploration. ...
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This paper describes the basic stages of skarn formation and the main causes of variation from the general evolutionary model. Seven major classes of skarn deposits (Fe, W, Au, Cu, Zn, Mo and Sn) are briefly described, and relevant geological and geochemical features of important examples are summarized in a comprehensive table. The important geochemical and geophysical parameters of skarn deposits are discussed, followed by a summary of important petrologic and tectonic constraints on skarn formation. Finally, exploration models are presented for several major skarn types. -from Author
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This is an introductory article to a special issue of Economic Geology devoted to skarn deposits. -K.A.R.
Chapter
Skarn deposits are one of the more abundant ore types in the earth's crust and form in rocks of almost all ages. Skarn is a relatively simple rock type defined by a mineralogy usually dominated by calc-silicate minerals such as garnet and pyroxene. Although the majority of skarns are found in lithologies containing at least some limestone, they can form in almost any rock type during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of magmatic, metamorphic, meteoric, and/or marine origin. Although most are found adjacent to plutons, they also can occur along faults and major shear zones, in shallow geot-hermal systems, on the sea floor, and at lower crustal depths in deeply buried metamorphic terranes. Thus, nei-ther a pluton nor limestone is necessarily required to form skarn. Most skarn deposits are zoned, and the gen-eral pattern is proximal garnet, distal pyroxene, and minerals like wollastonite, vesuvianite, or massive sulfides and/or oxides near the marble front. Recognition of distal alteration features such as bleaching, fluid escape structures, and isotopic halos can be critically important in exploration. Because most economic skarn deposits are related to magmatism, details of igneous petrogenesis and tectonic setting form a framework for exploration and classification. For the seven major skarn types (Fe, Au, Cu, Zn, W, Mo, and Sn) a general correlation exists among igneous major and trace element composition and skarn type. Plutons associated with Fe and Au skarns contain signif-icantly more MgO and less K2O or SiO2, Au and Sn skarn plutons are more reduced, and Cu, Zn, and Mo skarn plutons are more oxidized than average skarn plutons. In terms of geochemical evolution, there is a fairly lin-ear array from relatively primitive calcic Fe skarn plutons through Au, Cu, Zn, to W, Mo, to relatively evolved Sn skarn plutons. Calcic Fe skarn plutons are metaluminous, high in compatible elements such as Ni, V, and Sc, and have Rb/Sr <1. Relative to Fe skarn plutons, Cu skarn plutons have higher Si, K, Ba, Sr, La, and Fe 3+ /Fe 2+ , and contain significantly less Mg, Sc, Ni, Cr, and V. Au skarn plutons are similar to Fe skarn plutons in their metaluminous nature and their Si, Mg, Cr, and Sc contents and are similar to Cu skarn plutons in their Ni, V, and Y content. In general, plutons associated with Sn, Mo, and W skarns have a much stronger crustal signature than do plutons associated with other skarn types. The seven major skarn classes have some distinctive differences. Iron skarns are mined for their magnetite content although minor amounts of Cu, Co, Ni, and Au may be present. Endoskarn may exceed exoskarn, in-cluding widespread albite, orthoclase, and/or scapolite that sometimes are of regional extent. Gold skarns span a range of geologic environments but most deposits are relatively reduced, are mined solely for their gold con-tent, and have a distinctive Au-Bi-Te-As ± Co geochemical association. Most are associated with reduced, mafic plutons and dike/sill complexes with distal/early biotite ± K-feldspar hornfels aureoles; Fe-rich pyroxene is a characteristic mineral. Copper skarns are associated with I-type, magnetite series, calc-alkaline, porphyritic plutons, many of which have cogenetic volcanic rocks, stockwork veining, brittle fracturing and brecciation, and intense hydrothermal alteration; features collectively indicative of a relatively shallow environment of forma-tion. Copper skarn mineralogy is dominated by andraditic garnet. Most Zn skarns occur distal to associated ig-neous rocks and are mined predominantly for Zn, although Cu, Pb, and Ag can be economically important. They can be distinguished from other skarn types by their Mn-and Fe-rich mineralogy. Most Zn skarn districts grade outward from skarn-rich mineralization to skarn-poor ores, veins, and massive sulfide bodies, which may contain few if any skarn minerals. Tungsten skarns are associated with coarse-grained, generally unaltered, equigranular batholiths (with pegmatite and aplite dikes) surrounded by large, high-temperature, metamorphic aureoles; subcalcic garnet is a characteristic mineral. Molybdenum skarns are associated with leucocratic gran-ites and commonly contain accessory W, Cu, Zn, Pb, Bi, Sn, and U; some are truly polymetallic in that several metals need to be recovered together in order for the deposits to be economic. Hedenbergitic pyroxene is the most common calc-silicate mineral in Mo skarns and wollastonite, amphibole, and fluorite may be unusually abundant. Tin skarns are almost exclusively associated with high silica granites, typically generated by partial melting of continental crust, and have a characteristic suite of trace elements (Sn, F, B, Be, Li, W, Mo, and Rb). Many Sn skarn deposits develop a greisen alteration stage, characterized by high fluorine activities and the Corresponding author:
Article
Oscillatory zoning in low δ18O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre-scale oscillatory zoning of varying grossular–andradite composition (XAdr = 0.13–0.36). The δ18O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with XAdr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ18O indicate that a high Fe3+/Al, high δ18O fluid mixed with a lower Fe3+/Al and δ18O fluid. The high δ18O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ18O fluids were meteoric in origin.These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.
Article
The solubility of the albite-paragonite-quartz mineral assemblage was measured as a function of NaCl and fluorine concentration at 400°C, 500 bars and at 450°C, 500 and 1000 bars. Decreasing Al concentrations with increasing NaCl molality in F-free fluids of low salinity (mNaCl < 0.01) demonstrates that Al(OH)4− dominates Al speciation and is formed according to the reaction 0.5 NaAl3Si3O12H2(cr)+2 H2O = 0.5 NaAlSi3O8(cr)+Al(OH)4−+H+. Log K results for this reaction are −11.28 ± 0.10 and −10.59 ± 0.10 at 400°C, 500 bars and 450°C, 1000 bars, respectively. Upon further salinity increase, Al concentration becomes constant (at 400°C, 500 bars) or even rises (at 450°C, 1000 bars). The observed Al behavior can be explained by the formation of NaAl(OH)40(aq) or NaAl(OH)3Cl(aq)0. The calculated constant for the reaction Al(OH)4−+Na+=NaAl(OH)40(aq) expressed in log units is equal to 2.46 and 2.04 at 400°C, 500 bars and 450°C, 1000 bars, respectively. These values are in good agreement with the predictions given in Diakonov et al. (1996). Addition of fluoride at m(NaCl) = const = 0.5 caused a sharp increase in Al concentration in equilibrium with the albite-paragonite-quartz mineral assemblage. As fluid pH was also constant, this solubility increase indicates strong aluminum-fluoride complexation with the formation of NaAl(OH)3F(aq)0 and NaAl(OH)2F20(aq), according to 0.5 NaAl3Si3O12H2(cr)+Na++HF(aq)0+H2O = 0.5 NaAlSi3O8(cr)+ NaAl(OH)3F(aq)0+H+, log K = −5.17 and −5.23 at 400°C and 450°C, 500 bars, respectively, and 0.5 NaAl3Si3O12H2(cr)+Na++2 HF(aq)0 = 0.5 NaAlSi3O8(cr)+NaAl(OH)2F20(aq)+H+, log K = −2.19 and −1.64 at the same P-T conditions. It was found that temperature increase and pressure decrease promote the formation of Na-Al-OH-F species. Stability of NaAl(OH)2F20(aq) in low-density fluids also increases relative to NaAl(OH)3F(aq)0. These complexes, together with Al(OH)2F(aq)0 and AlOHF20(aq), whose stability constants were calculated from the corundum solubility measured by Soboleva and Zaraisky (1990) and Zaraisky (1994), are likely to dominate Al speciation in metamorphic fluids containing several ppm of fluorine.
Article
CALCMIN, an open source Visual Basic program, was implemented in EXCEL™. The program was primarily developed to support geoscientists in their routine task of calculating structural formulae of minerals on the basis of chemical analysis mainly obtained by electron microprobe (EMP) techniques. Calculation programs for various minerals are already included in the form of sub-routines. These routines are arranged in separate modules containing a minimum of code. The architecture of CALCMIN allows the user to easily develop new calculation routines or modify existing routines with little knowledge of programming techniques. By means of a simple mouse-click, the program automatically generates a rudimentary framework of code using the object model of the Visual Basic Editor (VBE). Within this framework simple commands and functions, which are provided by the program, can be used, for example, to perform various normalization procedures or to output the results of the computations. For the clarity of the code, element symbols are used as variables initialized by the program automatically. CALCMIN does not set any boundaries in complexity of the code used, resulting in a wide range of possible applications. Thus, matrix and optimization methods can be included, for instance, to determine end member contents for subsequent thermodynamic calculations.Diverse input procedures are provided, such as the automated read-in of output files created by the EMP. Furthermore, a subsequent filter routine enables the user to extract specific analyses in order to use them for a corresponding calculation routine. An event-driven, interactive operating mode was selected for easy application of the program. CALCMIN leads the user from the beginning to the end of the calculation process.
Article
Ketoprofen-loaded microspheres made with a polymeric blend were prepared by a spray-drying technique. Organic solutions of two polymers, cellulose acetate butyrate (CAB) and poly(epsilon-caprolactone) (PCL), in different weight ratios, and of ketoprofen (Ket) were prepared and sprayed, in different experimental conditions, achieving drug-loaded microspheres. The obtained spray-dried microspheres were characterized in terms of yield of production, shape, size, surface properties and drug content, and their in vitro drug release behaviours were determined at different pH values.