辽宁红透山块状硫化物矿床蚀变带元素迁移特征及定量计算

Element mobilization and mass-change quantification of highly metamorphosed footwall alteration zones in Hongtoushan volcanogenic massive sulfide deposit,Liaoning Province

辽宁红透山块状硫化物矿床(MSD)位于华北地台东北部的浑北花岗岩-绿岩地体内,矿区岩石在30～28亿年期间受到了600～650℃的高级角闪岩相变质。研究结果表明,广泛分布于红透山层状矿体下盘数百米处的和直接产于矿体下盘的堇青-直闪片麻岩,分别代表了变质后的MSD成矿系统以绿泥石化为特征的半整合和筒状不整合海底热液蚀变带。微量元素特征显示,层状堇青-直闪片麻岩的原岩并非同一种岩石,而是由5种不同岩性的岩石组成,筒状堇青-直闪片麻岩的原岩主要由流纹质岩石组成,而在堇青-直闪片麻岩走向上与之过渡的角闪片麻岩和黑云片麻岩则代表了不同岩性蚀变岩的未蚀变原岩。质量变化计算表明,相对于未蚀变原岩而言,两种蚀变岩的成分发生了显著变化,其中层状堇青-直闪片麻岩的Fe、Mg发生了富集,Na、K、Ca、Cu、Pb和Zn等元素被迁出,而筒状堇青-直闪片麻岩的Fe、Mg、Si、Na、Pb、Cu和Zn等元素则发生了富集,K被迁出。重稀土元素(HREE)和高场强元素(Zr、Ti、Nb、Hf和Ta)在海底热液蚀变过程中保持惰性,而Rb、Sr、Ba和轻稀土元素(LREE,尤其是Eu)则被强烈的迁出。这些元素变化特征表明海底热液蚀变以绿泥石化和硅化为特征,同时海底水-岩反应体系具有高水/岩比值。层状堇青-直闪片麻岩可作为红透山矿区成矿潜力评价的重要依据,而含硫化物石英脉的筒状堇青-直闪片麻岩不但本身可成为工业矿体,还可作为上覆层状矿体的近矿找矿标志。

The Hongtoushan Archean Cu-Zn Massive Sulfide Deposit （MSD）,which was metamorphosed （3.0～2.8 Ga） to upper amphibolite facies at temperatures between 600℃ and 650℃,occurs in the Hunbei granite-greenstone terrane,Liaoning Province. Cordierite-anthophyllite gneiss （CAG） is widespread in Hongtoushan. The CAG that occurs hundreds of meters below the ore horizon corresponds to the simiconformable alteration zone of the MSD hydrothermal system,whereas the CAG immediately below the main ore bed represents the metamorphosed pipe-like alteration zone. Whole-rock geochemical analyses indicate that the semiconformable CAG was derived ultimately from five kinds of lithology （basalts,basaltic andesites,andesites,dacites,and rhyolites）,whereas the pipe-like CAG from a single lithology （rhyolites）. Amphibolite and biotite gneiss are identified as metamorphosed unaltered parent rocks. As for the comparison with unaltered rocks,mass change calculation indicates that,compared with least-altered rocks,stratiform CAG is enriched in Fe and Mg and depleted in Na,K,Ca,Cu,Pb and Zn,while discordant CAG is enriched in Fe,Mg,Si,Na,Pb,Cu and Zn and depleted in K. HREE and HFSE （Zr,Ti,Nb,Hf and Ta） behaved inertly during submarine alteration,whereas Rb,Sr,Ba and LREE,especially Eu,were leached off. Mass change calculations of the CAG indicate that the water-rock interaction within both types of alteration zones is characterized by water dominated processes. Stratiform CAG could be used to evaluate mineralization potential of VMS in metamorphic terranes,while discordant CAG containing sulfide-bearing quartz veins could serve as a good indicor for overlying stratiform massive sulfide ores as well as an exploration target itself.