冈底斯明则-程巴斑岩-夕卡岩型Mo-Cu矿床成矿时代与含矿岩石成因

Mineralization age and petrogenesis of associated intrusions in the Mingze-Chengba porphyry-skarn Mo-Cu deposit,Gangdese

冈底斯东南缘克鲁-冲木达矿带以发育渐新世斑岩-夕卡岩型Mo-W-Cu成矿作用而有别于冈底斯中新世斑岩-夕卡岩型Cu-Mo-Au矿带,但是对渐新世成矿岩石类型及成因以及渐新世与中新世斑岩成矿作用存在差异的原因尚不清楚,严重制约了该带矿产资源潜力评价。为此,本文选择矿带内程巴斑岩型Mo矿床及明则夕卡岩型Cu矿床开展了成岩与成矿年代学及含矿岩石成因研究。明则矿区与夕卡岩矿化有关的二长岩的结晶年龄为30.4±0.6Ma,表明夕卡岩矿化时代约30Ma,这与程巴斑岩型Mo矿床矿石中辉钼矿Re-Os同位素分析所揭示的约30Ma矿化时代一致。明则二长岩的SiO2含量为55%～57%,K2O含量为2.7%～5.0%,属于钾玄岩,具有较高的MgO含量(3.5%～6.9%)、Mg#值(57.6～67.2)以及相容元素含量(Cr=34×10-6～379×10-6;Ni为48×10-6～116×10-6)。而程巴花岗闪长岩的SiO2含量为65%～67%,K2O含量为3.2%～4.1%,属于高钾钙碱性岩,具有较低的MgO含量(1.7%～2.1%)、Mg#值(49.5～51.1)以及相容元素含量(Cr=20×10-6～39×10-6;Ni为16×10-6～25×10-6)。二长岩和花岗闪长岩均显示富集轻稀土元素与大离子亲石元素,亏损高场强元素,具有弱的Eu负异常,但二长岩的REE含量明显高于花岗闪长岩,且具有较低的Sr/Y比值(24～49),后者Sr/Y比值为54～68。两种岩石均具有类似的Hf同位素组成,二长岩的εHf(t)值为+2.8～+6.8,花岗闪长岩εHf(t)值为+4.2～+6.1。综合分析表明,二长岩是受大洋板片流体交代的富集岩石圈地幔部分熔融的产物,而花岗闪长岩是增厚的新生下地壳部分熔融的产物,二者在形成过程中可能发生了幔源镁铁质岩浆与壳源长英质岩浆的混合作用,这种壳幔相互作用导致区域斑岩-夕卡岩型Cu-Mo矿化,但由于该区隆升剥蚀强烈,上部的斑岩型Cu矿体基本被剥蚀而没有保存下来,因此现今表现为斑岩型Mo矿床。

The Kelu-Chongmuda metallogenic belt, different from the Gangdese Miocene porphyry-skarn Cu-Mo-Au belt, is characterized by Oligecene porphyry-skarn Mo-W-Cu mineralization. The types and petrogenesis of intrusions associated with Oligecene mineralization and the reason for the difference between the Kelu-Chongmuda and Gangdese mineralization remain topics of debate, which prevents the evaluation of resource potentiality. Therefore, we conducted the geochronological analysis and research on the petrogenesis of associated intrusions for the Chengba porphyry Mo deposit and Mingze skarn Cu deposit. The Mingze monzonlite associated with skarn mineralization has zircon SHRIMIP U-Pb age of 30.4±0.6Ma, indicating that the Mingze mineralization took place at ca. 30Ma, which is consistent with the mineralization age of the Chengba porphyry Mo deposit. The Mingze monzonlite are shoshonitie, with low SiO2 （55%～57%）, high K2O （2.7%～5.0%）, MgO （3.5%～6.9%）, Mg# value （57.6～67.2）, high abundances of compatible elements （e.g., Cr=34×10-6～379×10-6; Ni=48×10-6～116×10-6）, and low Sr/Y ratios （24～49）. The Chengba granodiorite, with the zircon SHRIMIP U-Pb age of 28.7±0.9Ma, are high-K calc-alkaline, with high SiO2 （65%～67%）, high K2O （3.2%～4.1%）, MgO （1.7%～2.1%）, Mg# value （49.5～51.1）, low abundances of compatible elements （e.g., Cr=20×10-6～39×10-6; Ni=16×10-6～25×10-6）, and low Sr/Y ratios （53～76）. Both the monzonlite and granodiorite display enriched LREE and LILE, depleted HSFE, marked negative Nb-Ta-Ti anomalies, and slightly negative Eu anomalies. However, the monzonlite have high abundances of REE than the granodiorite. They also have similar Hf isotopic compositions with εHf（t）=＋2.8～＋6.8 for the monzonlite and εHf（t）=＋4.2～＋6.1 for the granodiorite. Our new data, together with previously published work, lead us to suggest that the parental magmas of the Mingze monzonlite were most likely derived from enriched lithospheric mantle beneath southern Tibet and the Chengba granodiorites were derived from partial melting of a thickened juvenile lower crust. The magma mixing between the crust-derived and mantle-derived magmas is beneficial for the porphyry-skarn Cu-Mo mineralization in the Kelu-Chongmuda and Gangdese metallogenic belts. But due to the more intensive erosion in the Kelu-Chongmuda than in the Gangdese, the upper Cu bodies were not preserved and the lower Mo bodies were present in the Chengba porphyry Mo deposit.