滇西姚安金矿床正长斑岩角闪石、黑云母化学特征及其地质意义

Chemical characteristics of hornblende and biotite in barren syenite porphyry from Yaoan gold deposit in western Yunnan and geological significance

姚安金矿床位于金沙江-哀牢山富碱斑岩带中段,大地构造位置处于扬子板块西缘与三江特提斯构造域结合部位,其成矿与新生代富碱斑岩密切相关。前人研究都集中在富矿岩体的岩石学、矿物学和地球化学特征,对贫矿岩体研究甚少。文章以姚安矿区贫矿正长斑岩为研究对象,在岩相学基础上对姚安正长斑岩中的角闪石,黑云母进行了电子探针分析,计算了矿物结晶的温压条件、氧逸度和含水量等要素,并与其他地区富矿岩体与贫矿岩体进行了对比,讨论了富矿岩体与贫矿岩体在矿物化学尺度上的差异性。姚安贫矿正长斑岩角闪石富MgO(11.82%~12.93%)、CaO(11.25%~11.58%)、K_(2)O(1.09%~1.37%)、Na_(2)O(1.99%~2.21%),属于镁闪石;计算得出角闪石结晶温度为822.47~850.69℃,压力112.69~151.21 MPa;侵位深度为3.72~5.44 km。黑云母为岩浆成因且富镁,w(MgO)为14.36%~18.01%,Fe^(2+)/(Mg^(+)Fe^(2+))比值0.23~0.34,属于镁质黑云母。黑云母结晶温度为719.69~738.78℃;压力51.15~178.35 MPa;侵位深度1.69~5.89 km。通过角闪石成分计算岩浆氧逸度变化介于ΔNNO+0.20到ΔNNO+0.91,略高于Ni-NiO出溶线。根据角闪石化学成分计算出岩浆含水量介于2.54%~3.02%。大型-超大型斑岩Cu矿多金属矿与高氧逸度、富水的浅侵位中酸性岩浆密切相关,对比姚安富矿岩体和成矿带上马厂菁、北衙的富矿岩体,姚安贫矿岩体与六合贫矿岩体相似,具有低氧逸度,较深的侵位深度的特点。低氧逸度致使金属元素早期以硫化物形式沉淀,无法随岩浆运移,较深的侵位深度使得流体难以出溶成矿。以上特征是其不成矿的关键因素。

The Yaoan gold deposit is located in the middle domain of the Jinshajiang-Ailaoshan alkali-rich porphyry belt as well as at the junction of the western margin of the Yangtze Plate and the Sanjiang Tethys Orogenic Belt,the mineralization is closely related to Cenozoic alkali-rich porphyries.Previous studies have mainly focused on the petrological,mineralogical and geochemical characteristics of fertile porphyries,a little on barren porphyry.Based on the detailed petrographic observation,the authors investigated the compositions of hornblendes and biotites from barren porphyry by electron microprobe analysis.The critical factors for Au mineralization such as pressure condition,oxygen fugacity,water content of magma were estimated,compared with other alkali-rich porphyry in the metallogenic belt,the differences of mineral chemistry between fertile and barren porphyries are discussed.The results show that hornblendes from barren porphyry are rich in MgO(11.82%~12.93%),CaO(11.25%~11.58%),K_(2)O(1.09%~1.37%)and Na_(2)O(1.99%~2.21%),belongs to magnesiohornblende.The characteristics of biotite are rich in MgO(14.36%~18.01%);the Fe^(2+)/(Mg^(+)Fe^(2+))ratio is between 0.23~0.34,classified as magnesia biotite.The crystallization temperature,pressure,emplacement depth calculated by hornblende and biotite thermo-barometers are 822.47~850.69℃and 719.69~738.78℃;112.69~151.21 MPa and 51.15~178.35 MPa;3.72~5.44 km and 1.69~5.89 km.The oxygen fugacity of barren porphyry calculated by hornblende varies fromΔNNO+0.20 toΔNNO+0.91,slightly higher than NNO buffer,and water contents of the magma is between 2.54%~3.02%.Super porphyry Cu deposits are closely related to shallow emplaced intermediate-acid magma with high oxygen fugacity and water content.Compared with Yaoan,Beiya and Machangjing fertile porphyries,Yaoan barren porphyry is characterized by lower oxygen fugacity and deeper emplacement depth similar with Liuhe barren porphyry.Low oxygen fugacity resulted in the early precipitation of metallic elements in the form of sulfides,which could not move with magma,the deeper emplacement depth made it difficult for fluids to dissolve.The above characteristics are the key factors of non-mineralization for the barren porphyries at Yaoan.