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首页> 外文期刊>Economic geology and the bulletin of the Society of Economic Geologists >Petrographic, Geochemical, and Fluid Inclusion Evidence for the Origin of Siliceous Cap Rocks Above Volcanic-Hosted Massive Sulfide Deposits at Myra Falls, Vancouver Island, British Columbia, Canada
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Petrographic, Geochemical, and Fluid Inclusion Evidence for the Origin of Siliceous Cap Rocks Above Volcanic-Hosted Massive Sulfide Deposits at Myra Falls, Vancouver Island, British Columbia, Canada

机译：加拿大不列颠哥伦比亚省温哥华岛迈拉瀑布的火山质块状硫化物矿床上的硅质盖层岩石学，地球化学和流体包裹体证据

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Massive sulfides at the Myra Falls volcanic-hosted massive sulfide (VHMS) camp, Vancouver Island, British Columbia, Canada, are overlain by white chert, black chert, argillite, and siltstone. White chert is best developed above the Battle orebody, where it forms a siliceous caprock (3-5 m thick) above the massive sulfides. There is a gradational lateral change from white chert above massive sulfides to black chert and unaltered argillite, 100 to 150 m south of the Battle orebody. Chert horizons are also located above the Ridge and Extension ore zones, but only minor chert lies above the HW orebody, which is instead overlain by a thick argillite sequence. The chert and argillite share similar sedimentologic and petrologic features, including abundant parallel laminations, interbedded turbidites, radiolarian-rich layers, soft-sediment deformation, scours, flame structures, and small phosphatic concretions. These features indicate that white and black chert formed as a replacement of mudstone rather than as exhalative or biogenic deposits. Silicification occurred early in the depositional history of the fine-grained sediments and was contemporaneous with some ore formation. Early syndepositional features are still visible in the chert, with primary pore spaces such as radiolarian tests filled by silica, rutile, apatite, and minor sulfides displaying open-space crystal growth. The presence of minor ore-clast breccias above the orebody indicates that at least parts of the Battle orebody were exposed on the sea floor. Metal zoning is observed in the cap-rock horizon above the Battle orebody, with higher Cu, Zn, and Cd contentrations in chert directly above massive sulfides, higher Pb, Sb, and Ag contentrations in black chert at the edge of the siliceous cap rocks and lower metal concentrations in the distal argillite. Primary fluid inclusions in spherical quartz patches in chert above the Battle orebody indicate that hy-drothermal fluids passing through sediment were between 135 to 250 deg C and had salinities ranging from 3.0 to 12.1 wt percent NaCl equiv. These data are similar to those for fluid inclusions measured in quartz interstitial to sulfides in the underlying Battle orebody, which have temperatures of homogemzation ranging from 140 deg to 250 deg C and salinities from 3.0 to 12.4 wt percent NaCl equiv. Fluid inclusions in the Battle orebody display a slight increase in salinity and homogemzation temperature with depth, which may reflect the overprinting of earlier high temperature stages by cooler fluids as the hydrothermal system waned or varying degrees of mixing between hydrothermal fluids and seawater. A minimum depth for deposition of the cap rocks (>200 m) is estimated, based on sedimentologic features such as fine parallel laminations and interbedded sandstone turbidites, which indicate deposition below storm wave base. Greater water depths (1,000-1,500 m) are suggested by the lack of evidence of boiling in fluid inclusions. Low O_2 concentrations in the bottom water of the Battle basin are suggested by the absence of bio-turbation, lack of fossils of benthic fauna, degree of pyritization values >0.90, elevated Zn, Pb, Cu, Cd, As, Sb, Ag, Ba, and V, low Fe and Mn, and V/(V + Ni) > 0.8 in the unaltered argillite. Paleosea-floor reconstructions indicate that the Battle and HW orebodies formed in small basins along a northwest-trending ridge. The finegrained sediments were deposited in depocenters within paleotopographic lows. Hydrothermal fluid densities, estimated from fluid inclusions at Myra Falls, range from 0.88 to 1.05 g/cm~3 and are higher than for many other VHMS deposits. However, they are close to the density of seawater at 2 deg C and a 2,000-m depth (1.028 g/cm~3). Replacement textures in the siliceous cap rocks above the Battle deposit, the sheetlike morphology of the siliceous cap rocks, and lateral metal zonation indicates that diffuse lateral flow of hydrothermal fluids through the porous sea-floor se
机译：在加拿大不列颠哥伦比亚省温哥华岛的迈拉瀑布火山携带的块状硫化物（VHMS）营地中，块状硫化物被白石，黑cher石，凹凸棒石和粉砂岩覆盖。白石最好在战矿体上方发育，在那里它在块状硫化物上方形成硅质盖层（3-5 m厚）。在块状矿体以南100至150 m处，从块状硫化物上方的白色石到黑色石和未改变的泥晶石有一个横向的横向变化。石层位也位于山脊和伸展矿带上方，但仅少量minor石位于硬岩矿体上方，而其上则覆盖着厚厚的泥质岩序。硅钙石和泥质沸石具有相似的沉积学和岩石学特征，包括大量平行叠层，层状浊积体，富含放射线虫的层，软沉积物变形，冲刷，火焰结构和小的磷结石。这些特征表明白色和黑色的石形成为泥岩的替代物，而不是呼出气或生物成因的沉积物。硅化作用发生在细粒沉积物的沉积历史的早期，并且与某些矿石形成同时发生。早期的共沉积特征仍在the石中可见，主要的孔隙空间例如由二氧化硅，金红石，磷灰石和次生硫化物填充的放射线虫试验显示出开放空间的晶体生长。矿体上方存在少量的矿-碎裂角砾岩，表明战斗矿体的至少一部分暴露在海底。在战斗矿体上方的盖层地层中观察到金属分区，硅质盖层边缘的块状硫化物正上方的石中的Cu，Zn和Cd含量较高，黑色石中的Pb，Sb和Ag含量较高。并降低了远端硅藻土中的金属浓度。在Battle矿体上方的石中球形石英斑块中的主要流体包裹体表明，穿过沉积物的水热流体温度在135至250℃之间，盐度范围为当量NaCl的3.0至12.1 wt％。这些数据与在下面的Battle矿体中硫化物间隙石英中测得的流体包裹体的数据相似，其均一化温度范围为140至250℃，盐度为3.0至12.4 wt％的NaCl当量。战斗矿体中的流体包裹体随深度显示盐度和均一化温度略有增加，这可能反映了随着热液系统的减弱或热液与海水之间不同程度的混合，较冷的流体对早期高温阶段的叠印。根据沉积学特征（如精细的平行叠片和互层砂岩浊度），可以估算出盖层沉积的最小深度（> 200 m），这表明沉积物位于风暴波基下。缺乏流体包裹体沸腾的证据表明，水深更大（1,000-1,500 m）。缺乏生物扰动，缺乏底栖动物化石，黄铁矿化度值> 0.90，锌，铅，铜，镉，砷，锑，银，在未改变的硅藻土中，Ba和V，低的Fe和Mn以及V /（V + Ni）> 0.8。古海底重建表明，Battle和HW矿体在西北趋势山脊的小盆地中形成。细粒沉积物沉积在古地形低点的沉积中心。根据Myra Falls的流体包裹体估算的热液密度为0.88至1.05 g / cm〜3，高于许多其他VHMS沉积物。但是，它们接近于2摄氏度和2,000米深度（1.028 g / cm〜3）时的海水密度。战役沉积物上方的硅质盖层岩石中的置换纹理，硅质盖层岩石的片状形态以及侧向金属带化表明，热液的横向流通过多孔海底扩散

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