Mesozoic In-situ and External Skarn Magmatic-Hydrothermal Mineralization in the Anhui Segment of the Lower Yangtze Metallogenic Belt
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    Abstract:

    In the Anhui segment of the Lower Yangtze metallogenic belt are distributed not only a great number of contact metasomatic and superimposed and composite skarn deposits, but also some magmatic skarn deposits. Definitions are given for different magmatic skarns and associated deposits, and discussions are held on the characteristics of Mesozoic magmatic skarns and associated deposits in the area, with a focus on an analysis of Mesozoic skarn magmatic-hydrothermal mineralization. Based on emplacement location of skarn magma, magmatic skarn can be divided into in-situ skarn and external skarn, and magmatic skarn deposit correspondingly into in-situ skarn deposit and external skarn deposit. Characteristics are evidently different between the two skarns and associated deposits. Geologically, an in-situ skarn body is closely associated with a crust-mantle syntactic magmatic intrusion and occurs in the contact belt between the intrusion and carbonate wall rock. This in-situ skarn body generally has no chilled or optalic border but contains residuals of wall rock at its margin. These xenoliths were formed by incomplete assimilation of crust-mantle syntactic magma with the wall rock and mostly metamorphosed as hornfels or marble. In contrast, an external skarn body is not associated with a crust-mantle syntactic magmatic intrusion and generally located in fault belt or detachment belt within strata. This external skarn body often has a chilled or optalic border, vesicular structure and garnet or pyroxene cumulate, but contains no residuals of wall rock at its margin. In addition, clear horizontal zoning occurs in both of the in-situ and external skarn bodies, but their style of zoning is evidently different. Granitoid, hybrid granitoid, rich-Fe skarn, rich-Ca skarn, hybrid carbonate, and carbonate occur in order from crust-mantle syntactic magmatic intrusion through in-situ skarn body to carbonate wall rock, indicating gradual weakening of assimilation and contamination. In contrast, lithologies from the centre to the margin of external skarn body vary from medium-coarse-grained skarn to medium-fine-grained skarn, showing gradual decreasing of crystallization speed with gradual increasing of temperature lowing speed. Mineralogically, in the in-situ skarn, garnet includes andradite, gralmandite and grossular, and pyroxene does diopside and hedenbergite. In contrast, in the external skarn, garnet is almost andradite, and pyroxene is all hedenbergite. Geochemically, compared with the in-situ skarn, the external skarn is evidently rich in W, F, Rb, Be and Fe, but poor in Al, Sr, Ba, Cu, Pb, Zn, Cr, Co and Ni. Petrographically, both of the in-situ and external skarns have automorphic granular texture and mostly contain melt inclusions. However, compared with those in the in-situ skarn, garnet and pyroxene in the external skarn are more automorphic and have better zonal structure, and melt inclusions indicate evidently lower homogenization temperature. In ore deposit, the in-situ skarn is generally associated with copper deposit with copper and iron sulfite ore, while the external skarn with iron deposit with iron oxide ore. A comprehensive analysis is consistent with formation of the in-situ and external skarns by cooling and crystallization of the in situ and external skarn magmas, and supports formation of deposits associated with the in-situ and external skarns by cooling and crystallization of the ore pulp formed through liquation of the skarn magmas and by alteration and replacement of the hydrothermal solution formed through crystalline differentiation of the skarn magmas.

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Du Yangsong, Cao Yi, Zhang Zhiyu, Pang Zhenshan, Li Dapeng.2011. Mesozoic In-situ and External Skarn Magmatic-Hydrothermal Mineralization in the Anhui Segment of the Lower Yangtze Metallogenic Belt[J]. Acta Geologica Sinica,85(5):699-711

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History
  • Received:January 26,2011
  • Revised:March 20,2011
  • Adopted:May 16,2011
  • Online: May 16,2011
  • Published: