新疆阿尔泰乌吐布拉克铁矿床矽卡岩矿物特征及其地质意义

摘要

The medium-sized Wutubulake iron deposit in the eastern extension of the Altay ore belt within Xinjiang is hosted in the metamorphosed volcaniclastic-sedimentary sequence of the Upper Silurian-Lower Devonian Kangbutiebao Formation.The ore bodies in the ore deposit occur as stratoids or lenses, surrounded by skars minerals.Skarn minerals include garnet, chinopyoxene, scapolite, albite, actinolite, epidote, chlorite, apatite etc.whereas ore minerals are mainly magnetite, with minor pyrite, chalcopyrite, pyrrhotite, molybdentite etc.Field evidence and petrographic observation indicate four stages of metallogenesis, i.e., prograde stage, retrograde stage, sulfide stage and supergene stage.Electron microprobe analyses show that the end member of garnet is mainly andradite, with the calculated end number dominated by andradite (And: 72.89％ ～84.09％, 78.44％ on average), followed by almandite (Alm: 8.23％ ～ 12.86％) and grossularite (Gro: 3.47％ ～ 14.97 ％ ), both of which are less than 20 ％.The composition of pyroxene is mainly diopside, with a small amount of augite.The end member of diopside is dominated by wollastonite (Wo: 43.48％～45.84％), followed by enstatite (En: 27.02％～34.14％) and ferrosilite (Fs: 18.72％～25.56％).The amphiboles show insignificant compositional variation, mostly being ferrotschermakite.The composition of these skarn minerals suggest that skarn in the Wutubulake iron deposit is calcic skarn, belonging to metasomatic skarn.The epidote is rich in Al and Ca but poor in Fe and Ti, indicating that Fe and Ti were separated out at the same time of the formation of epidote.Electron microprobe analyses show that the composition of magnetite is mainly FeOT (91.17％ ～ 93.63 ％), and most magnetite has similar chondrite-normalized rare earth element distribution patterns and is enriched in light rare earth elements, with an obvious Eu positive anomaly, which is similar to the rare earth element distribution patterns of skarns in this area.The Ni/Co values of magnetite range from 0.15 to 4.29, but most of them are lower than 1.0.All of these characteristics suggest that the formation of magnetite was closely related to the skarns.The above mineralogical characteristics are similar to things of minerals in typical metasomatic skarn iron deposits, implying that the Wutubulake iron deposit is a metasomatic skarn iron deposit.In combination with the geological characteristics, the authors hold that the skarns might have resulted form hydrothermal interaction with metamorphic volcanic and carbonate beds and lenses, and the formation of magnetite was related to the retrogressive metamorphism of the skarns.%乌吐布拉克中型铁矿床赋存于上志留统一下泥盆统康布铁堡组变质火山一沉积岩系中,矿体呈似层状、透镜状,矿体及其周围发育大量矽卡岩矿物.电子探针分析表明石榴石端员组分以钙铁榴石一钙铝榴石系列为主,辉石端员组分以透辉石为主,角闪石端员组分主要为铁镁钙闪石,这些特点表明矿区矽卡岩为交代矽卡岩中的钙矽卡岩.磁铁矿的主要组分、稀土及微量元素表明其形成与矽卡岩密切相关.结合矿床地质特征,认为矽卡岩是由岩浆热液流体交代康布铁堡组基性火山岩(熔岩和火山碎屑岩)及灰岩而形成的,磁铁矿的形成与矽卡岩的退化变质作用有关.