Mineralogy, Geochemistry, and Age Constraints on the Zn-Pb Skarn Deposit of Maria Cristina, Quebrada Galena, Northern Chile
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The Maria Cristina Zn-Pb skarn deposit, hosted by carbonate rocks of the Lower Cretaceous Chanarcillo Group in northern Chile, is described to provide a basis of comparison to adjacent (~2 km) barite deposits and regional Pb-Zn and Ba deposits with Mississippi Valley-type affinities. Strongly retrograded garnet (andradite to Ad30Gr70), diopsidic pyroxene, and epidote skarn, and barite-bearing massive sulfides (sphalerite-pyrite-galena-marcasite-magnetite) occur at the contact of potassically altered diorite porphyry of mid-Cretaceous age. The skarn mineral compositions, similar to those of Cu skarns, may reflect emplacement in a high f O2 context. Sulfur isotope values of sulfides (–8.8 to +7.1‰) and barite (14.0–26.1‰) indicate disequilibrium conditions and different sulfur sources, including magmatic sulfur. Lead isotope ratios of galena and strontium isotope ratios of barite indicate a similar metal reservoir for the different ore deposit types, constituted by the intercalated Lower Cretaceous volcanic rocks or mid-Cretaceous intrusive rocks. 40Ar/39Ar dating of hydrothermal K feldspar suggests an age between 94 and 90 Ma for the mineralization.Andradite
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Abstract Galena, also known as PbS, was widely used in the production of lead glazes from the beginning of the 18th century to the second half of the 20th century. Although the PbO‐SiO 2 system has been studied for years, the PbS–SiO 2 phase diagram, involved in the formation of a glaze with galena, has not yet been investigated. Temperature transformations for the system 75 wt% PbS‐25 wt% SiO 2 are investigated in a high‐temperature resolved X‐ray diffraction experiment with synchrotron radiation and compared to those of the equivalent system 70 wt% PbO‐30 wt% SiO 2 . Lanarkite, PbO·PbSO 4 , is the phase predominantly formed as soon as galena decomposes during the heating. The results show that the system melts at a temperature higher than the PbO–SiO 2 system, but far lower than those expected for the PbO–PbSO 4 –PbS system. A historical misfired lead glaze produced with galena is also studied. The presence of galena, lanarkite, and mattheddleite, Pb 10 (SiO 4 ) 3.5 (SO 4 ) 2 Cl 2, is determined and discussed in terms of the composition of the galena mineral used and the firing conditions in light of the high‐temperature transformations previously obtained.
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Abstract Cu and Fe skarns are the world's most abundant and largest skarn type deposits, especially in China, and Au‐rich skarn deposits have received much attention in the past two decades and yet there are few papers focused on schematic mineral deposit models of Cu–Fe–Au skarn systems. Three types of Au‐rich deposits are recognized in the Edongnan region, Middle–Lower Yangtze River metallogenic belt: ∼140 Ma Cu–Au and Au–Cu skarn deposits and distal Au–Tl deposits. 137–148 Ma Cu–Fe and 130–133 Ma Fe skarn deposits are recognized in the Edongnan region. The Cu–Fe skarn deposits have a greater contribution of mantle components than the Fe skarn deposits, and the hydrothermal fluids responsible for formation of the Fe skarn deposits involved a greater contribution from evaporitic sedimentary rocks compared to Cu–Fe skarn deposits. The carbonate‐hosted Au–Tl deposits in the Edongnan region are interpreted as distal products of Cu–Au skarn mineralization. A new schematic mineral deposit model of the Cu–Fe–Au skarn system is proposed to illustrate the relationship between the Cu–Fe–Au skarn mineralization, the evaporitic sedimentary rocks, and distal Au–Tl deposits. This model has important implications for the exploration for carbonate–hosted Au–Tl deposits in the more distal parts of Cu–Au skarn systems, and Fe skarn deposits with the occurrence of gypsum‐bearing host sedimentary rocks in the MLYRB, and possibly elsewhere.
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Skarn deposits occur throughout the world and have been mined for a variety of elements. 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, with a plea for field mapping as a fundamental basis for future studies.
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