The uncommon Mg-rich and Ti-poor Zhaoanzhuang serpentine-magnetite ores within Taihua Group of the North China Craton （NCC） remain unclear whether the protolith was sourced from ultramafic rocks or chemical sediment...The uncommon Mg-rich and Ti-poor Zhaoanzhuang serpentine-magnetite ores within Taihua Group of the North China Craton （NCC） remain unclear whether the protolith was sourced from ultramafic rocks or chemical sedimentary sequences. Here we present integrated petrographic and geochemical studies to characterize the protoliths and to gain insights on the ore-forming processes. Iron ores mainly contain low-Ti magnetite （TiO2 -0.1wt%） and serpentine （Mg#=92.42-96.55）, as well as residual olivine （Fo=89-90）, orthopyroxene （En=89-90） and hornblende. Magnetite in the iron ores shows lower AI, Sc, Ti, Cr, Zn relative to that from ultramafic Fe-Ti-V iron ores, but similar to that from metamorphic chemical sedimentary iron deposit. In addition, interstitial minerals of dolomite, calcite, apatite and anhydrite are intergrown with magnetite and serpentine, revealing they were metamorphic, but not magmatic or late hydrothermal minerals. Wall rocks principally contain magnesian silicates of olivine （Fo=83-87）, orthopyroxene （En=82-86）, humite （Mg#=82-84） and hornblende [XMg=0.87--0.96]. Dolomite, apatite and anhydrite together with minor magnetite, thorianite （Th-rich oxide） and monazite （LREE-rich phosphate） are often seen as relicts or inclusions within magnesian silicates in the wall rocks, revealing that they were primary or earlier metamorphic minerals than magnesian silicates. And olivine exists as subhedral interstitial texture between hornblende, which shows later formation of olivine than hornblende and does not conform with sequence of magmatic crystallization. All these mineralogical features thus bias towards their metamorphic, rather than magmatic origin. The dominant chemical components of the iron ores are SiO2 （4.77-25.23wt%）, Fe203T （32.9-80.39wt%） and MgO （5.72- 27.17wt%） and uniformly, those of the wall rocks are also SiO2 （16.34-48.72wt%）, MgO （16.71- 33.97wt%） and Fe203T （6.98-30.92wt%）. The striking high Fe-Mg-Si contents reveal that protolith 展开更多
基金funded by the National Natural Science Foundation of China (Grant No. 41672078)
文摘The uncommon Mg-rich and Ti-poor Zhaoanzhuang serpentine-magnetite ores within Taihua Group of the North China Craton （NCC） remain unclear whether the protolith was sourced from ultramafic rocks or chemical sedimentary sequences. Here we present integrated petrographic and geochemical studies to characterize the protoliths and to gain insights on the ore-forming processes. Iron ores mainly contain low-Ti magnetite （TiO2 -0.1wt%） and serpentine （Mg#=92.42-96.55）, as well as residual olivine （Fo=89-90）, orthopyroxene （En=89-90） and hornblende. Magnetite in the iron ores shows lower AI, Sc, Ti, Cr, Zn relative to that from ultramafic Fe-Ti-V iron ores, but similar to that from metamorphic chemical sedimentary iron deposit. In addition, interstitial minerals of dolomite, calcite, apatite and anhydrite are intergrown with magnetite and serpentine, revealing they were metamorphic, but not magmatic or late hydrothermal minerals. Wall rocks principally contain magnesian silicates of olivine （Fo=83-87）, orthopyroxene （En=82-86）, humite （Mg#=82-84） and hornblende [XMg=0.87--0.96]. Dolomite, apatite and anhydrite together with minor magnetite, thorianite （Th-rich oxide） and monazite （LREE-rich phosphate） are often seen as relicts or inclusions within magnesian silicates in the wall rocks, revealing that they were primary or earlier metamorphic minerals than magnesian silicates. And olivine exists as subhedral interstitial texture between hornblende, which shows later formation of olivine than hornblende and does not conform with sequence of magmatic crystallization. All these mineralogical features thus bias towards their metamorphic, rather than magmatic origin. The dominant chemical components of the iron ores are SiO2 （4.77-25.23wt%）, Fe203T （32.9-80.39wt%） and MgO （5.72- 27.17wt%） and uniformly, those of the wall rocks are also SiO2 （16.34-48.72wt%）, MgO （16.71- 33.97wt%） and Fe203T （6.98-30.92wt%）. The striking high Fe-Mg-Si contents reveal that protolith
