滇西大理地区岩体黑云母化学特征及其对成矿的指示意义

Biotites chemical characteristics from Dali district, Western Yunnan and geological significance

中文关键词: 金沙江—哀牢山富碱斑岩带  黑云母  岩浆流体性质  氧逸度  岩石成因

英文关键词: Jinshajiang-Ailaoshan alkali-rich porphyry belt  Biotite  properties of magmatic fluids  oxygen fu-gacity  rock genesis

中文摘要:

      金沙江—哀牢山富碱斑岩带内发育众多成岩时代集中于 37~33 Ma的富碱斑岩，其中部分岩体(富矿岩体)与带内 Cu-Au-Mo矿化关系密切，部分岩体(贫矿岩体)尚未发现矿化，前人研究多集中于二者在岩石地球化学上的差异，缺乏矿物地球化学特征上的对比。本文以北衙富矿石英正长斑岩、马厂箐富矿花岗斑岩、白莲村贫矿正长斑岩和松桂贫矿正长斑岩为研究对象，对富矿与贫矿岩体中的黑云母进行了电子探针分析，计算了富矿与贫矿岩体岩浆流体性质、岩浆氧逸度，揭示了富矿与贫矿岩体岩石成因与源区性质，讨论了富矿岩体与贫矿岩体在矿物化学尺度上的差异。黑云母主量元素分析结果显示，富矿与贫矿岩体中的黑云母均为岩浆成因，且富镁 w(MgO)范围 8.86%~15.54%，Fe2+(/ Mg+Fe2+)比值范围 0.29~0.53，属于镁质黑云母。富矿与贫矿岩体中的黑云母具有显著不同于 S型和 A型花岗岩的 Mg/(Fe+Mg)、log(XF/XOH)和 AlVI值，指示母岩浆具有 I型花岗岩的特征，均来自新生下地壳角闪岩的部分熔融。富矿岩体中的黑云母 IV(F)(1.21～1.71，平均 1.43)、IV(Cl)(-4.25～-3.03，平均-3.84)低于贫矿岩体中的黑云母 IV(F)(1.61～1.91，平均 1.76)、IV(Cl)(-4.61～-3.29，平均-3.74)表明富矿岩体中的云母更加富集 F和 Cl。富矿岩浆流体相比于贫矿岩浆流体具有更低的 log(fH2O/fHF)fluid值(富矿:3.80~4.54，平均 4.1；贫矿:3.99~4.60，平均4.28)和略高的 log(fH2O/fHCl)fluid值(富矿:2.88～4.20，平均 3.64；贫矿 3.02~4.18，平均 3.63)，表明贫矿岩浆流体较富矿岩浆流体显示出更为富集的 HCl，但富矿岩体早期岩浆氧逸度远高于贫矿岩体，由此看出，岩浆流体 Cl含量高低并不是控制岩体成矿的第一决定性因素，岩体含矿与否的因素并不是单一的因素控制，而是多种因素综合作用的结果，氧逸度的高低对岩体的成矿与否更为重要。综上，黑云母指示地质过程较为有效，但是部分结算结果仍需谨慎使用。

英文摘要:

      A series of 37-33 Ma alkali-rich porphyries developed along the Jinshajiang-Ailaoshan alkali-richporphyry belt during the Indo-Asian continental collision period, some of these porphyries (fertile) are closely relat-edto regional Cu-Au-Mo mineralization, and the others are barren, Previous studies have mostly focused on thewhole rock geochemistry of both porphyries, and the differences in mineral geochemistry between fertile and barrenporphyry are still unclear. This paper focuses on the Beiya fertile quartz syenite porphyry, Machangqing fertile gran-ite porphyry, Bailiancun and Songgui barren syenite porphyry. EMPA analysis was conducted on biotite, and theproperties of magma fluid and magma oxygen fugacity were calculated. The rock genesis and source properties of fer-tile and barren were revealed, finally discussed the differences in mineral chemical between fertile and barren por-phyries. The analysis of major elements in biotites shows that the biotites in both porphyries are magmatic originand rich in magnesium, with MgO (8.86%-15.54%)，Fe2+(/ Mg+Fe2+)(0.29-0.53) belonging to magnesian biotite.The biotite in fertile and barren porphyries has significantly different Mg/(Fe+Mg), log (XF/XOH), and AlVI values from S-type and A-type granites, indicating that the parent magma has the characteristics of I-type granites, both ofwhich are derived from partial melting of lower crustal amphibolites. The biotite IV (F) (1.21-1.71, average 1.43)and IV (Cl) (-4.25 to 3.03, average -3.84) in the fertile porphyry are lower than those in the barren porphyry (1.61-1.91, average 1.76) and IV (Cl) (-4.61 to 3.29, average -3.74), indicating that the mica in the fertile porphyry is more enriched in F and Cl. Lower log(fH2O/fHF)fluid values and higher log(fH2O/fHCl)fluid values in fertile porphyry(fertile: 3.80-4.54, average 4.1, 2.88-4.20, average 3.64; barren: 3.99-4.60, average 4.28, 3.02-4.18, average3.63), indicating that magma fluids of barren porphyry show more enriched HCl than fertile porphyry. However, theoxygen fugacity of early magma in fertile porphyry is much higher than that in barren porphyry. Therefore, it can beseen that the Cl content of magma fluids is not the first decisive factor controlling the mineralization of rock bodies.The factor of whether the rock body contains ore or not is not controlled by a single factor, but by the comprehensiveeffect of multiple factors. As a result, the level of oxygen fugacity is more important for the mineralization of the rockmass. Overall, biotite is effective in indicating geological processes, but still need to be used with caution.