ZIRCON COMPOSITIONAL EVIDENCE FOR SULFUR-DEGASSING FROM ORE-FORMING ARC MAGMAS
ZIRCON COMPOSITIONAL EVIDENCE FOR SULFUR-DEGASSING FROM ORE-FORMING ARC MAGMAS
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DOI：
10.2113/econgeo.110.1.241
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发表时间：
2015
期刊：
影响因子：
5.8
通讯作者：
J. Dilles;A. Kent;J. Wooden;R. Tosdal;A. Koleszar;R. Lee;Lucian P. Farmer
中科院分区：
文献类型：
--
作者：
J. Dilles;A. Kent;J. Wooden;R. Tosdal;A. Koleszar;R. Lee;Lucian P. Farmer
斑岩型Cu（±Mo ±Au）矿床和浅成低温热液型金银矿床是我国主要的金属矿产来源，它们通常是由中生代会聚边缘环境中含水岩浆的岩浆热液流体形成的。与斑岩矿床有关的火成岩组合在现代会聚边缘环境中很常见，但尽管许多火成岩组合产生了酸性岩浆流体，但过去或现在很少有火成岩组合产生足够的金属、氯和硫富集，而这些富集是形成矿石存款所必需的。其原因尚不清楚。我们报告SHRIMP-RG离子探针分析铪，钛和稀土元素（REE）的锆石，几乎无处不在的和强大的微量矿物在地壳岩浆中的丰度。对比成矿岩体和非成矿岩体的锆石组成表明，成矿岩体结晶温度较低，铕负异常较小（多数Eu N /Eu N * ≥0.4）。我们解释这些小的锆石铕异常，以表明氧化岩浆条件，并假设在许多情况下，这反映了氧化由于SO2脱气岩浆与相对较低的Fe/S比。熔体中铕和铁的氧化是由岩浆硫酸盐（S6+）在脱气时还原为SO2（S4+）而产生的。这种解释加强了氧化富硫流体在斑岩和低温热液矿床存款形成中的重要作用。因此，锆石成分可用于识别释放大量富SO 2气体的古老岩浆，锆石成分的区域调查可能是矿产勘探的有价值的工具。
Porphyry Cu (±Mo ±Au) and epithermal Au-Ag deposits are major sources of mined metals and are commonly formed by magmatic-hydrothermal fluids derived from hydrous magmas in Phanerozoic convergent margin settings. The igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings, but while many have produced acidic magmatic fluids, very few, past or present, have produced sufficient metal, chlorine, and sulfur enrichments necessary to engender an ore deposit. The reasons for this remain uncertain. We report SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element (REE) abundances in zircon, a nearly ubiquitous and robust trace mineral in crustal magmas. Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (mostly Eu N /Eu N * ≥0.4). We interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to SO 2 degassing from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S 6+ ) to SO 2 (S 4+ ) upon degassing. This interpretation reinforces the important role of oxidized sulfur-rich fluids in porphyry and epithermal mineral deposit formation. Zircon compositions thus may be used to identify ancient magmas that released significant amounts of SO 2 -rich gases, and regional surveys of zircon composition are potentially a valuable tool for mineral exploration.