Unraveling the Geologic History of the Hemlo Archean Gold Deposit, Superior Province, Canada: A U-Pb Geochronological Study
Unraveling the Geologic History of the Hemlo Archean Gold Deposit, Superior Province, Canada: A U-Pb Geochronological Study
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DOI：
10.2113/gsecongeo.98.1.51
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发表时间：
2003
期刊：
影响因子：
5.8
通讯作者：
D. Davis;Shoufa Lin
中科院分区：
文献类型：
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作者：
D. Davis;Shoufa Lin
赫姆洛存款是加拿大最大的黄金营地之一。金受构造和岩性控制，但与许多其他太古代矿脉矿床相比，显示出不寻常的关系。现场证据表明，金被安置在机械和化学陷阱沿着一个主要的左行剪切带的慢跑，最有可能在早期形成的第二代（G2）的结构，这是最激烈的阶段，区域变形。成矿作用后，伴随角闪岩相区域变质作用。对与该存款有关的岩石进行了锆石U-Pb年代学研究，以确定地质事件的精确年代学及其与成矿的关系。锆石种群通常受到遗传和热液/变质晶体生长的影响。只有在严格的构造和地层背景下，通过对岩石进行精确的单锆石定年，才能确定有意义和精确的年龄信息。金主要发现在穆斯湖石英斑岩火山杂岩和地层下伏的变质沉积岩之间的变形接触。这些岩石被一个营地规模的非圆柱形F2褶皱所褶皱。穆斯湖石英斑岩中的锆石以继承性为主，年龄为2800 ~2690 Ma。位于穆斯湖斑岩之下和之上的变质沉积岩显示出更窄的源区年龄范围，主要由26.9亿至26.93亿年的锆石组成。两个来自斑岩正下方的变质沉积岩的碎屑锆石确定的年龄为2685 ± 4 Ma。这应该是火山作用、变形作用和金矿化作用的一个较老的年龄界限。Cedar Lake岩体中4个最年轻的近整合锆石的岩浆年龄为2680 ± 1 Ma。该岩体侵位在G2变形之前或早期，而金的侵位很可能受G2构造控制。因此，2680 ± 1 Ma可能也代表了金成矿作用的一个更老的年龄界限。切割金矿石的长石斑岩脉的年龄为2677 ± 1 Ma。锆石定年结果表明，本区在2693 ~ 2685 Ma期间形成了一个火山-沉积杂岩。这是在早期的地壳上发展起来的。2720 Ma的年龄，但有证据表明，从碎屑和捕虏晶更古老的岩石可追溯到2800 Ma以上。表壳岩沉积之后，在2680 ± 1 Ma发生花岗闪长岩深成岩作用，然后发生大变形（G2）和金矿化，叠加角闪岩相变质作用（之前根据钛铁矿年龄确定为2676 Ma）。晚构造期“sanukitoid suite”岩体侵位时间为2677 ± 1 Ma。因此，最初的金形成事件发生在一个持续最多几百万年的深成岩作用和地壳改造时期的开始附近。这一时间组合支持了花岗岩类岩浆是含金流体来源的观点。侵入活动变形地壳的深圳湾岩也可能是区域变质作用的热源。最有可能的环境是一个构造活跃的蒂米斯卡明型沉积盆地。同样古老的盆地被发现在Timmins地区东部和Shebandowan绿岩带西部。
The Hemlo deposit is one of Canada’s largest gold camps. Gold is both structurally and lithologically controlled, but shows unusual relationships relative to many other Archean lode deposits. Field evidence suggests that gold was emplaced within mechanical and chemical traps along a jog in a major sinistral shear zone, most likely during early formation of second-generation (G2) structures, which represent the most intense phase of regional deformation. Mineralization was followed by amphibolite-facies regional metamorphism. U-Pb geochronology has been carried out on zircons from rocks associated with the deposit, in order to determine the precise chronology of geologic events and their relationship to mineralization. Zircon populations are commonly affected by both inheritance and hydrothermal/metamorphic crystal growth. Meaningful and precise age information can only be determined by precise single zircon dating on rocks from a well constrained, structural, and stratigraphic context. Gold is mostly found near the deformed contact between the Moose Lake quartz porphyry volcanic complex and stratigraphically underlying metasedimentary rocks. These rocks have been folded by a camp-scale, noncylindrical F2 fold. Zircon from the Moose Lake quartz porphyry is dominated by inheritance and gives ages that extend from ~2800 to ~2690 Ma. Metasedimentary rocks that underlie and overlie the Moose Lake porphyry show a much narrower range of provenance ages, and are dominated by 2,690- to 2,693-million-year-old zircon. Two detrital zircons from a metasedimentary rock directly beneath the porphyry define an age of 2685 ± 4 Ma. This should be an older age limit on volcanism, deformation, and gold mineralization. The four youngest near-concordant zircons from the Cedar Lake pluton define a magmatic age of 2680 ± 1 Ma. This pluton was emplaced prior to or early during G2 deformation, while emplacement of gold was most likely controlled by G2 structures. Therefore, 2680 ± 1 Ma probably also represents an older age limit on gold mineralization. A feldspar porphyry dike that cuts gold ore gives a younger age bracket of 2677 ± 1 Ma. The results of zircon dating suggest that a volcanosedimentary complex was constructed in the area over the period of 2693 to 2685 Ma. This developed on earlier crust, ca. 2720 Ma in age, but there is evidence from detritus and xenocrysts for older rocks dating back to over 2800 Ma. Deposition of supracrustal rocks was followed by granodiorite plutonism at 2680 ± 1 Ma, then major deformation (G2) and gold mineralization, overprinted by amphibolite-facies metamorphism (previously dated at 2676 Ma from titanite ages). Late-tectonic "sanukitoid suite" plutons were emplaced at 2677 ± 1 Ma. Thus, the primary gold-forming event occurred near the beginning of a period of plutonism and crustal reworking that lasted, at most, a few million years. This time association supports the view that granitoid magmas were the source of auriferous fluids. Plutons intruded into an actively deforming crust and were probably also the heat source for regional metamorphism. The most likely environment was a tectonically active, Timiskaming-type sedimentary basin. Similarly aged basins are found in the Timmins area to the east and the Shebandowan greenstone belt to the west.