四川乌斯河大型锗铅锌矿床锗超常富集特征及其地质意义

锗(Ge)是我国的一种优势战略性关键矿产,闪锌矿是其主要载体和重要工业来源之一。尽管前人对闪锌矿富锗有了较为深刻的认识,但是对其中锗的替代方式和超常富集机制仍缺乏深入理解。位于扬子地块西南缘的川滇黔铅锌矿集区是我国最重要的锗和铅锌资源基地之一,区内的乌斯河大型铅锌矿床(5.4Mt@8.6%Zn,2.0%Pb)显著超常富锗(铅锌矿石中Ge平均品位为72.3×10~(-6),闪锌矿中Ge含量高达1934×10~(-6))。何种因素制约该矿床中锗的超常富集亟待解密。乌斯河矿床发育两期闪锌矿,早期闪锌矿(Sp1)单偏光下呈白色-棕黑色,发育半自形细粒结构,浸染状构造;晚期闪锌矿(Sp2)单偏光下呈白色-红色或灰色-棕黑色,发育自形-半自形细-粗粒结构,块状或脉状构造。激光剥蚀耦合等离子体质谱(LA-ICPMS)闪锌矿原位微量元素含量分析结果显示,闪锌矿中Ge含量为3.17×10~(-6)～1934×10~(-6),平均563×10~(-6),相对Ge的地壳丰度(1.5×10~(-6))具有显著超常富集特征(高达1000倍)。闪锌矿中Ge与Fe、Cu、Ag、Pb具有正相关关系,而与Cd呈负相关性。元素面扫描(Mapping)结果显示,闪锌矿中Ge主要以类质同象形式存在,且Ge与Fe、Cu、Ag、Pb、Cd等元素相关关系与含量分析结果一致。结构相似度计算结果进一步显示,Sp1中Ge与Ag相关程度最高,而Sp2中Ge与Cu相关系数最大,暗示结构相似度计算在比较元素相关关系中有较大的应用潜力。然而,乌斯河矿床闪锌矿中Cu、Ag含量常低于Ge含量,且(Ge/Cu) mol和(Ge/Ag) mol比值变化范围大,所以单一的元素相关程度分析并不能完全解决Ge替代方式问题。综合研究认为,该矿床闪锌矿中Ge替代方式与Cu、Ag等元素关系不大,而是直接替代Zn(Ge(4+)■2Zn~(2+)或Ge~(2+)■Zn~(2+))。另外,乌斯河矿床同一闪锌矿颗粒不同部位(如扇形分区和振荡环带) Ge含量变化显著,暗示Ge的超常富集很可能受闪锌矿的沉淀速率和结晶过程控制,而Sp1与Sp2中Ge含量的明显差异,则很可能与成矿流体演化过程流体成分和成矿物理化学条件改变有关。因此,乌斯河大型锗铅锌矿床锗的超常富集是流体成分、物化条件、沉淀速率和结晶过程等多要素耦合作用特殊地球化学过程的结果,并受到矿物和矿床等不同尺度苛刻成矿条件的影响。

The characteristics of the extraordinary germanium enrichment in the Wusihe large-scale Ge-Pb-Zn deposit, Sichuan Province, China and its geological significance

Germanium (Ge) is an advantageous critical metal in China, and it is mainly mined in sphalerite as a by-product. Although much progress has been made on the Ge enrichment in sphalerite, the substitution of Ge and mechanisms of abnormal Ge enrichment remain poorly understood. The Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic district, located on the southwest margin of the Yangtze Block, is one of the most important production bases of Ge in China. Wherein the Wusihe large-scale deposit (5.7Mt@8.6% Zn, 2.0% Pb) is characterized by abnormal Ge enrichment with a mean Ge ore grade of 72.3×10^-6 and Ge concentrations up to 1934×10^-6 in sphalerite. The factors controlling such abnormal enrichment warrant further investigation. The Wusihe sphalerite is mainly formed in two hydrothermal stages:the early-stage (Sp1) is white-brown-black under single polarized transmitted light, subhedral fine-grained and disseminated; whereas the late-stage (Sp2) is white-red or gray-brown-black, euhedral-subhedral fine-to coarse-grained, and massive or veined. The analytical results by the laser laser-ablation inductively coupled plasma mass spectrometry show that Ge concentrations in sphalerite are highly variable, with a range of 3.17×10^-6 to 1934×10^-6 (averaged at 563×10^-6), which is up to 1000 times of the Ge abundance of the earth''s crust (1.5×10^-6). Germanium has a positive correlation with Fe, Cu, Ag and Pb, while it is negatively correlated with Cd. Chemical maps show that Ge is incorporated into sphalerite with isomorphic substitution. Structural similarities (SSIM) of the chemical maps show that Ge has the best correlations with Ag in Sp1, and with Cu in Sp2. This implies the SSIM method has great implication potential in comparing element correlation. Nevertheless, the concentrations of Cu and Ag are often lower than those of Ge, and the mol ratios of (Ge/Cu) and (Ge/Ag) varies widely, hence a single element correlation analysis cannot directly be used to reveal the substitution. Comprehensive studies have found that the Ge substitution may have little to do with elements such as Cu and Ag, but directly replaced Zn in sphalerite at Wusihe (Ge⁴⁺↔2Zn²⁺ and/or Ge²⁺↔Zn²⁺). Furthermore, concentrations within a single sphalerite particle (sector and oscillating zones) have a significant variation, suggesting that the Ge enrichment is crystallographically controlled and related to the precipitate rate of sphalerite. The difference in Ge contents between Sp1 and Sp2 is related to the fluid composition and changes in the physicochemical conditions of the mineralization. Therefore, the abnormal enrichment of Ge in the Wusihe large-scale Ge-Pb-Zn deposit should be a coupling product of fluid composition, physicochemical conditions, precipitation rate and crystallization, and affected by special metallogenic conditions on the mineralogical and deposit scales.