稳定和放射性同位素在Cu-Ni-PGE岩浆矿床研究中的应用:实例及需要注意的问题(英文)

硫、氧同位素和放射性同位素比值在示踪岩浆与围岩的反应及在岩浆镍-铜-铂族元素矿床成因中同化过程的重要性方面有很大作用。如1.1Ga的Duluth杂岩、Norilsk地区二叠-三叠纪侵入岩和1.4Ga的Kabanga侵入岩体中矿床的硫同位素测定结果证明硫来自含硫化物和硫酸盐的围岩。1.3Ga的VoisbeysBay矿床矿石的硫同位素δ34S值通常落入公认的地幔硫同位素组成范围(0±2‰)内,而对其元古宙变质沉积围岩进行的详细研究结果显示δ34S的平均值也在此范围。全面了解同位素组成的潜在混染对合理评价提供硫来源的围岩在矿石形成中的作用是非常必要的。用氧同位素和放射性同位素示踪岩浆和围岩相互作用时,必须在开放体系没有扰动同位素系统情况下进行。在氧同位素体系和放射性同位素体系(如Re/Os、Pb同位素体系)中,元素在热液条件下活动性强或主相没有发生同位素交换与吸收,都会导致低温热液过程掩盖高温过程。只有在进行详细的岩相观察和单矿物分析之后,再应用围岩混染同位素模式才是有意义的。

APPLICATIONS OF STABLE AND RADIOGENIC ISOTOPES TO MAGMATIC Cu-Ni-PGE DEPOSITS: EXAMPLES AND CAUTIONS

Measurements of S, O, and radiogenic isotope ratios are all potentially powerful tracers of magma interaction with country rocks and the importance of assimilation processes in the genesis of magmatic Ni-CuPGE deposits. Sulfur isotope measurements of deposits such as those in the 1.1 Ga Duluth Complex, the Permo-Triassic intrusions of the Noril'sk area, and the 1.4 Ga Kabanga intrusions provide evidence for the derivation of S from both sulfide-and sulfate-bearing country rocks. The 1.3 Ga Voisey's Bay deposit provides an example where δ34S values of the ores commonly fall within the accepted mantle range of 0 ± 2‰, but detailed studies of Proterozoic metasedimentary country rocks show that their weighted average δ34S value is also within this range. A thorough knowledge of the isotopic compositions of potential contaminants is essential for a proper evaluation of the role of country rock derived S in ore formation. When O and radiogenic isotopic measurements are employed as tracers to evaluate magma-country rock interaction it is essential to provide evidence that open system processes have not perturbed the isotopic systematics. Low-temperature hydrothermal processes can mask evidence of high-temperature processes in the oxygen isotope system, and in radiogenic systems such as Re/Os and Pb where involved elements may be mobile under hydrothermal conditions, or host phases may close to exchange and uptake at different temperatures. Careful petrographic observation and analyses of individual minerals may be required before the models involving the contamination of magmas by country rocks can be meaningfully applied.