|
|||||
|
|
Thallium isotope variations in an ore-bearing continental igneous setting: Collahuasi Formation, northern Chile |
|
| Authors: | RGA Baker M Rehkämper C Ihlenfeld R Coggon |
| Institution: | a Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK b Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK c BG Group PLC, 100 Thames Valley Park, Reading, Berkshire RG6 1PT, UK d Anglo American PLC, 20 Carlton Terrace, London SW1Y 5AN, UK |
| Abstract: | Thallium is a highly incompatible element and a large fraction of the bulk silicate Earth Tl budget is, therefore, expected to reside in the continental crust. Nonetheless, the Tl isotope systematics of continental rocks are essentially unexplored at present. Here, we present new Tl isotope composition and concentration data for a suite of 36 intrusive and extrusive igneous rocks from the vicinity of porphyry Cu deposits in the Collahuasi Formation of the Central Andes in northern Chile. The igneous lithologies of the rocks are variably affected by the hydrothermal alteration that accompanied the formation of the Cu deposits.The samples display Tl concentrations that vary by more than an order of magnitude, from 0.1 to 3.2 μg/g, whilst ε205Tl ranges between −5.1 and +0.1 (ε205Tl is the deviation of the 205Tl/203Tl isotope ratio of a sample from a standard in parts per 104). These variations are primarily thought to be a consequence of hydrothermal alteration processes, including metasomatic transport of Tl, and formation/breakdown of Tl-bearing minerals, which are associated with small but significant Tl isotope effects. The Tl abundances show excellent correlations with both K and Rb concentrations but no co-variation with Cu. This demonstrates that Tl displays only limited chalcophile affinity in the continental crust of the Collahuasi Formation, but behaves as a lithophile element with a distribution that is primarily governed by partitioning of Tl+ into K+-bearing phases. Collahuasi samples with propylitic alteration features, which are derived from the marginal parts of the hydrothermal systems, have, on average, slightly lighter Tl isotope compositions than rocks from the more central sericitic and argillic alteration zones. This small but statistically significant difference most likely reflects preferential retention of isotopically heavy Tl in alteration phases, such as white micas and clays, which formed during sericitic and argillic alteration. |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ | |