Thallium geochemistry in the metamorphic Lengenbach sulfide deposit, Switzerland: Thallium-isotope fractionation in a sulfide melt
Thallium geochemistry in the metamorphic Lengenbach sulfide deposit, Switzerland: Thallium-isotope fractionation in a sulfide melt
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DOI：
10.2138/am.2014.4591
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发表时间：
2014-04-01
影响因子：
3.1
通讯作者：
Markl, Gregor
中科院分区：
文献类型：
--
作者：
Hettmann, Kai;Kreissig, Katharina;Markl, Gregor
Lengenbach（瑞士）Pb-As-Tl-Zn存款是在阿尔卑斯变质作用期间由大约500摄氏度的硫化物熔体形成的，但关于其形成的细节，特别是金属的来源仍然存在争议。在这项研究中，我们提出了两个样本集来解决这些问题：（1）MC-ICP-MS分析的硫化物，硫盐，熔融包裹体中的铊同位素从阿尔卑斯变质Lengenbach存款在瑞士的宾谷，非变质维斯洛赫密西西比河谷型存款在德国南部，并在Pizzo Cervandone约2公里西南Lengenbach存款，这已被讨论为潜在的Lengenbach金属来源。(2)对Lengenbach存款和存款与Pizzo Cervandone之间的围岩中的云母进行LA-ICP-MS分析，以追踪潜在的含金属流体路径。我们发现，所有调查样品中的Tl同位素组成（表示为Tl-205值）范围为-4.1 +/- 0.5至+ 1.9 +/- 0.5。整个变化仅在Lengenbach存款中就可以看到，因此即使在高温过程中，硫化物熔体也记录了相当大的分馏。这一大范围的PT 1 -205被认为是由核体积依赖性分馏引起的。有趣的是，常见的fahlores在Lengenbach的行为不同，从所有其他调查的硫盐：基于他们的重同位素组成与低的As/S比，他们似乎不从硫化物熔体结晶，但被解释为已形成的热液流体中富含重Tl同位素。尽管基于次生亚砷酸盐，Lengenbach存款周围的片麻岩和辉长岩中的As活化是明显的。在伦根巴赫的裂隙云母中，没有发现这种原生岩石流体的痕迹。因此，考虑到硫化物和云母中的K/Rb、Rb/Tl、As/S和Pb/Tl比值，Lengenbach存款中的元素富集要么是阿尔卑斯山前的，要么与高峰变质作用有关，但肯定发生在Lengenbach云母生长之前。
The Lengenbach (Switzerland) Pb-As-Tl-Zn deposit was formed from a sulfide melt at about 500 degrees C during Alpine metamorphism, but details on its formation and especially the source of the metals are still under debate. In this study we present two sample sets to address these questions:(1) MC-ICP-MS analyses of thallium isotopes in sulfides, sulfosalts, and melt inclusions from the Alpine metamorphic Lengenbach deposit in the Binn Valley of Switzerland, the non-metamorphic Wiesloch Mississippi Valley-type deposit in Southern Germany, and the Cu- and As-rich mineralization at Pizzo Cervandone about 2 km SW of the Lengenbach deposit, which has been discussed as potential source of the Lengenbach metals.(2) LA-ICP-MS analyses of micas from the Lengenbach deposit and surrounding country rocks between the deposit and the Pizzo Cervandone to trace potential metal-bearing fluid pathways.We found that Tl isotope compositions expressed as epsilon Tl-205 values in all investigated samples range from -4.1 +/- 0.5 to + 1.9 +/- 0.5. The whole variation can be seen in the Lengenbach deposit alone, which hence records considerable fractionation even during high-temperature processes involving a sulfide melt. This large range of epsilon Tl-205 is thought to be caused by nuclear volume-dependent fractionation. Interestingly, the common fahlores at Lengenbach behave differently, from all other investigated sulfosalts: based on their heavy isotopic composition together with a low As/S-ratio, they do not seem to be crystallized from the sulfide melt, but are interpreted to have formed from hydrothermal fluids enriched in the heavy Tl isotopes. Although As mobilization in the gneisses and dolomites surrounding the Lengenbach deposit is evident based on secondary arsenites. no traces of such a country rock fluid could be found in fissure micas at Lengenbach. Hence, considerations involving K/Rb, Rb/Tl, As/S, and Pb/Tl ratios in the sulfides and micas imply that the element enrichment in the Lengenbach deposit is either pre-Alpine or related to peak metamorphism, but occurred definitely before mica growth at Lengenbach.