大兴安岭北段岔路口斑岩Mo-热液脉状Zn-Pb成矿系统硫化物微量元素的分布、起源及其勘探指示

岔路口斑岩Mo-Zn-Pb矿床位于大兴安岭北段,是近年来新发现的超大型斑岩-热液脉状Mo-Zn-Pb成矿系统,脉状Zn-Pb矿化直接叠置在斑岩Mo矿化顶部。本文挑选岔路口斑岩型矿化及热液脉型矿化的黄铁矿、闪锌矿、方铅矿,通过EMPA、ICP-MS等多种方法分析硫化物的主微量元素组成,发现岔路口各阶段硫化物均富集Mo元素,相比于斑岩型矿化各阶段中的黄铁矿,Zn、Pb、Mn、Cd、Ga、Ag、Bi等元素在铅锌矿阶段内相对富集;相比于过渡阶段,铅锌阶段闪锌矿中Mo、Co元素及方铅矿中的Bi、Cd和Ag元素含量下降。微量元素在不同阶段内的变化可能是流体降温和天水混合的结果。黄铁矿的稀土总量与成矿岩体最接近,且与成矿岩体和围岩有相似的稀土配分模式,并有较明显的Eu负异常;黄铁矿宽广的Y/Ho比值(25.0~39.0)与成矿岩体的Y/Ho比值范围(27.4~38.7)最接近,同时包括了围岩相对较窄的Y/Ho比值(25.7~31.3),这表明成矿物质主要与成矿岩体同源,可能加入了一定量的围岩物质,岔路口硫化物富Mo的特征受控于深部斑岩Mo矿化岩浆-热液系统。对比东秦岭-大别W-Mo-Pb-Zn矿集区的远源热液脉状Pb-Zn矿床,岔路口浅部近源脉状矿化中的黄铁矿具有更高含量Mo/Ag-Bi/Sb比值和Mo/Pb-Sn/Sb比值,因此浅部硫化物的高Mo含量以及黄铁矿中相关元素比值的高值,可为脉状Zn-Pb矿化附近隐伏斑岩钼矿化的勘探提供新线索。此外,与其他热液脉状和斑岩型矿床相比,岔路口矿床硫化物更富集中高温元素;且综合分析多类矿床的硫化物的微量元素后,本文还初步查明不同矿床类型硫化物富集的微量元素,这一尝试可为矿床成因的判断提供新的思路。

Trace element distribution in sulfides from the Chalukou porphyry Mo-vein-type Zn-Pb system, northern Great Xing'an Range, China: Implications for metal source and ore exploration

Located in the northern Great Xing'an Range, the giant Chalukou Mo deposit has been the largest porphyry Mo deposit so far in China, with the direct superimposed vein-type Zn-Pb ore body on the porphyry Mo mineralization. Major and trace elements compositions in pyrite, sphalerite and galena from different stages at Chalukou deposit are analyzed by EMPA and ICP-MS. Mo is enriched in all sulfides, and other elements in sulfides from different stages do not show regularity, except for the enrichment of Zn, Pb, Mn, Cd, Ga, Ag, Bi in the pyrite of Zn-Pb stage. Mo, Co content of sphalerite and Bi, Cd and Ag content of galena decrease from transitional stage to Zn-Pb stage. They are the consequence of fluid cooing and mixing with meteoric water. The REE patterns of pyrite are characterized by LREE enrichment, HREE depletion, and obvious Eu negative anomalies. These features are similar to those of ore-forming intrusions and wall-rocks. Furthermore, most of the Y/Ho values in pyrite (25.0~39.0) strap the wide range of ore-forming intrusions (27.4~38.7). It is suggested that ore-forming material is sourced predominantly from ore-forming intrusions with some contribution from wall-rocks. And the exceptional enrichment of Mo in sulfides is attributed to the deep giant porphyry Mo magmatic-hydrothermal system. Compared to the distal hydrothermal vein-type Zn-Pb deposit in east Qinling-Dabie W-Mo-Pb-Zn district, trace elements compositions of pyrite from proximal vein-type Zn-Pb ore body at Chalukou deposit show higher Mo/Ag-Bi/Sb ratio and Mo/Pb-Sn/Sb ratio. That means high Mo content in shallow sulfides and elevated ratios of some elements of pyrite may provide new clues for prospecting the underlying porphyry Mo mineralization beneath the vein-type Zn-Pb ore body. Compared to other vein-type and porphyry deposits, enrichment of moderate-high temperature elements in sulfides at Chalukou deposit is obvious. In addition, the enrichment of trace element in sulfides from different types of deposits show different features, according to the collected data (porphyry, epithermal, vein-type, skarn, VMS, MVT deposits), and this attempt could indicate a new perspective of distinguishing the ore genetic type.