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Abstract
The Archean Hollandaire volcanogenic massive sulfide deposit is a felsic–siliciclastic VMS deposit located in the Murchison Domain of the Youanmi Terrane, Yilgarn Craton, Western Australia. It is hosted in a succession of turbidites, mudstones and coherent rhyodacite sills, has been metamorphosed to upper greenschist/lower amphibolite facies and includes a pervasive S1 deformational fabric. The coherent rhyodacitic sills are interpreted as syndepositional based on geochemical similarities with well-known VMS-associated felsic rocks and similar foliations to the metasediments. We offer several explanations for the absence of textural evidence (e.g. breccias) for syn-depositional origins: (1) the subaqueous sediments were dehydrated by long-lived magmatism such that no pore-water remained to drive quench fragmentation, (2) pore-space occlusion by burial, and/or (3) alteration overprinting and obscuring of primary breccias at contact margins. Mineralisation occurs by sub-seafloor replacement of original host rocks in two ore bodies, Hollandaire Main (∼125 × >500 m and ∼8 m thick) and Hollandaire West (∼100 × 470 m and ∼5 m thick), and occurs in three main textural styles, massive sulfides, which are exclusively hosted in turbidites and mudstones, and stringer and disseminated sulfides, which are also hosted in coherent rhyodacite. Most sulfides have textures consistent with remobilisation and recrystallisation. Hydrothermal metamorphism has altered the hangingwall and footwall to similar degrees, with significant gains in Mg, Mn and K and losses in Na, Ca and Sr. Garnet and staurolite porphyryoblasts also exhibit a footprint around mineralisation, extending up to 30 m both above and below the ore zone. High precision thermal ionisation mass spectrometry of zircons extracted from the coherent rhyodacite yield an age of 2759.5 ± 0.9 Ma, which along with geochemical comparisons, places the succession within the 2760–2735 Ma Greensleeves Formation of the Polelle Group of the Murchison Supergroup. Geochemical and geochronological evidence link the coherent rhyodacite sills to the Peter Well Granodiorite pluton ∼2 km to the W, which acted as the heat engine driving hydrothermal circulation during VMS mineralisation. This study highlights the importance of both: detailed physical volcanological studies from which an accurate assessment of timing relationships, particularly the possibility of intrusions dismembering ore horizons, can be made; and identifying synvolcanic plutons and other similar suites, for VMS exploration targets in the Youanmi Terrane and worldwide.
太古代Hollandaire火山成因块状硫化物矿床是一个长英质-硅质VMS矿床，位于西澳大利亚尔加恩克拉克拉通的Youanmi地体的默奇森域。它居于浊积岩、泥岩和连续流纹英安岩岩床序列之中，已经变质为上绿/下角闪岩相，包括一个普遍的S1变形结构。根据与知名VMS相关的长英质岩石在地球化学上的相似性和变质沉积岩类似的叶理，该连续流纹英安岩岩床被解释为同时沉积。对于同沉积起源缺乏结构证据（如角砾），我们提供多种解释：（1）水下沉积物由于长期岩浆活动而脱水，以致没有孔隙水来导致淬火碎裂；（2）孔隙空间由于埋藏而闭塞；以及/或者（3）在接触边缘,蚀变叠印并模糊了初级角砾岩。海底上两个矿体中的原主岩替换导致矿化作用的发生：Hollandaire Main（约125米×500多米，约8米厚）和Hollandaire West（约100米×470米，约5米厚）。矿化作用以三个主要的结构方式出现：完全存在于浊积岩和泥岩中的大量硫化物，以及存在于连续流纹英安岩中的细脉和侵染硫化物。大多数硫化物具有与再转移和再结晶一致的结构。热液变质已经相似程度地改变了上盘和下盘，获取大量的镁、锰和钾，失去大量的钠、钙和Sr。石榴石和十字斑状变晶也表现出矿化作用的影响，在矿石带之上及以下延伸至30米。从连续流纹英安岩提取得到的锆石的高精度热电离质谱法得出2759.5±0.9Ma的年龄。和地球化学比较一起，这样的年龄则将这个地层置入2760至2735Ma的Murchison超群Polelle群的Greensleeves组之内。地球化学和地质年代学证据链接连续流纹英安岩岩床和往西约2公里的彼得嘛花岗闪长岩岩体，这充当VMS矿化作用过程中驱动热液循环的热引擎。这项研究突出以下两者的重要性：详细的物理火山学研究，准确评估时间关系，特别是肢解矿石层的侵入作用的可能性；为了在Youanmi地体和世界各地进行VMS勘探而识别同火山岩体和其它类似岩套。
ACKNOWLEDGEMENTS
This research was supported by Silver Lake Resources through an Honours project by S. E. Hull and through ongoing research as part of ARC Linkage Project LP110200747. TJI publishes with permission from the Executive Director of the Geological Survey of Western Australia. Careful reviews by Steve Hollis and Chris Yeats greatly improved the manuscript.