Geological Modeling and Numerical Simulation of Cold-Water Trap in the Low-Sulfidation Epithermal Sirawai Au-Ag Deposit, Mindanao, Philippines
Geological Modeling and Numerical Simulation of Cold-Water Trap in the Low-Sulfidation Epithermal Sirawai Au-Ag Deposit, Mindanao, Philippines
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菲律宾棉兰老岛低硫化物浅成热液 Sirawai 金银矿床冷水圈地质模型和数值模拟
DOI：
10.1007/s11053-021-09978-3
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发表时间：
2022
影响因子：
5.4
通讯作者：
H.
中科院分区：
文献类型：
--
作者：
Takahashi;H.;Tomita;S. A.;Koike;K.;& Yoshiyama;H.
对于全球重要的金、银来源低硫化浅成热液矿床，我们提出了一种新的由冷地下水引发的多阶段成矿机制。利用矿石矿物、x射线衍射蚀变矿物和流体包裹体数据进行地质建模，并利用TOUGH2对流体流动进行数值模拟，在菲律宾Sirawai金银矿床验证了该冷水圈闭的存在。金的品位和Ag/Au比值因其具有不同的成矿机制而备受关注。最初，~ 270°C的中性ph热液通过矿脉上升并遇到浅层含水层。低温浅层地下水对上升流体的冷却导致了早期成矿（Au ~ 1 g/t， Ag/Au比值4-89）。在这一阶段，流体从脉体流向周围的渗透带（即浅层含水层）也产生了热液蚀变晕。这些蚀变晕的渗透性大大降低，在矿脉周围形成蘑菇状的低渗透蚀变晕。由于LPAHs的低渗透性和绝缘性，当温度升高时，流体沸腾，发生了中期矿化（Au ~ 26 g/t， Ag/Au比值≤10）。随着LPAHs渗透率的进一步降低，热液角化作用优先发生在水平脆性带（即浅层LPAHs的顶部）。随着压力的降低，流体沸腾，发生晚期成矿（Au ~ 2 g/t， Ag/Au比值~ 30）。因此，冷水圈闭机制是澄清低硫化浅成热液金矿成矿作用的一种创新方法。
For low-sulfidation epithermal deposits, which are globally significant sources of Au and Ag, we propose a new multi-stage ore-deposition mechanism initiated by cold groundwater. This cold-water trap was verified in the Philippine Sirawai Au–Ag deposit through geological modeling using the data of ore minerals, X-ray diffraction-based alteration minerals, and fluid-inclusions, and the numerical simulation of fluid flow using TOUGH2. Gold grades and Ag/Au ratios were especially focused on because they were characterized by various Au ore-deposition mechanisms. Initially, neutral-pH hydrothermal fluids at ~ 270 °C ascended through veins and encountered a shallow aquifer. The cooling of rising fluids by cold, shallow groundwater resulted in early-stage mineralization (Au ~ 1 g/t, Ag/Au ratio 4–89). Hydrothermal alteration halos were also generated at this stage by fluid flow from the veins into surrounding permeable zones (i.e., a shallow aquifer). The greatly decreased permeability of these alteration halos formed mushroom-shaped low-permeability alteration halos (LPAHs) around the veins. Middle-stage mineralization (Au ~ 26 g/t, Ag/Au ratio ≤ 10) occurred by the boiling of fluids when temperatures increased due to low permeability and insulating properties of the LPAHs. As the permeability of the LPAHs decreased further, hydrothermal brecciation occurred preferentially in horizontal brittle zones (i.e., the caps of the LPAH in the shallow aquifer). Late-stage mineralization (Au ~ 2 g/t, Ag/Au ratio ~ 30) then occurred by the boiling of fluids as the pressure decreased. Consequently, the cold-water trap mechanism is an innovative approach that clarifies mineralization in low-sulfidation epithermal Au deposits.