Abstract

The Val-d’Or vein field (VVF), located in the southern Abitibi subprovince (Québec, Canada), is host to ~ 47 Moz gold and is therefore an example of a greenstone-hosted orogenic gold district. Gold is contained in quartz-tourmaline-carbonate veins that cut As-poor intermediate to mafic volcanic and intrusive rocks, including dioritic, granodioritic and gabbroic sills, dikes, stocks, and plutons. Five investigated orebodies (Goldex, Triangle, Plug #4, Pascalis Gold Trend, Beaufor) host gold in vein- and wallrock-hosted pyrite-rich sulfide aggregates (> 95 vol%) that show a porous core domain (Py1), with abundant inclusions of carbonate, silicate, and Fe-oxides up to several tens of µm in size. A homogeneous pyrite rim domain (Py2) surrounds Py1 and contains most of the gold as native gold and polymetallic (Au-Ag-Te-Bi) inclusions, primarily calaverite and petzite. The two pyrites show different Au and As contents (Py1 = Au ≤ 30 ppm; As ≤ 67 ppm; Py2 = Au ≤ 1250 ppm; As ≤ 550 ppm). Pyrite shows a ubiquitous shift in δ³⁴S values of up to + 3.0‰ from Py1 (δ³⁴S = − 0.4‰ to 5.8‰, n = 32) to Py2 (δ³⁴S = 0.0‰ to 6.3‰, n = 59) and records a small, slightly negative Δ³³S signature between – 0.20‰ and 0.01‰. The δ³⁴S shift suggests that removal of reduced sulfur species from auriferous hydrothermal fluids causes the formation of inclusion-hosted gold in Py2 by a decrease in the fluid sulfur fugacity (fS₂) through wallrock sulfidation of Fe-oxides. The shift also correlates with locally enriched Co and Ni concentrations in Py1 (< 1 wt%), compared to lower, oscillatory zoned concentrations (< 0.1 wt%) in Py2, respectively, indicating an overall decrease in fluid oxygen fugacity (fO₂). Contemporaneously, a decrease in fluid tellurium fugacity (fTe₂) drives polymetallic inclusion-hosted gold formation in Py2, initially as calaverite followed by increasingly Ag-bearing petzite and hessite. The multiple sulfur isotopes and trace element compositions recorded in pyrite in the VVF indicate that a homogeneous fluid reservoir introduced gold-sulfide complexes. Even if considered a localized process at the ore-shoot scale, fluid-wallrock sulfidation reactions can lead to a coupled decrease in fS₂, fO₂, and fTe₂ of auriferous hydrothermal fluids in a greenstone-hosted As-poor gold district.

中文翻译：

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流体岩石硫化反应控制 Val-d'Or 造山金矿脉田（加拿大阿比蒂比省）的 Au-Ag-Te-Bi 沉淀

摘要

Val-d'Or 矿脉田 (VVF) 位于阿比蒂比副省（加拿大魁北克省）南部，蕴藏着约 47 Moz 的金矿，因此是绿岩造山金矿区的一个例子。金存在于石英-电气石-碳酸盐矿脉中，这些矿脉将贫砷中间岩切割成镁铁质火山岩和侵入岩，包括闪长岩、花岗闪长岩和辉长岩基岩、岩脉、岩岩群和岩体。五个调查的矿体（Goldex、Triangle、Plug #4、Pascalis Gold Trend、Beaufor）在脉和围岩中富含黄铁矿的硫化物聚集体（> 95 vol%）中含有金，这些聚集体显示出多孔的核心区域（Py1），具有丰富的碳酸盐、硅酸盐和铁氧化物的夹杂物，尺寸可达数十微米。均质的黄铁矿边缘区域 (Py2) 围绕 Py1，并含有大部分以天然金形式存在的金和多金属 (Au-Ag-Te-Bi) 包裹体，主要是钙辉石和钾铁矿。两种黄铁矿显示出不同的 Au 和 As 含量（Py1 = Au ≤ 30 ppm；As ≤ 67 ppm；Py2 = Au ≤ 1250 ppm；As ≤ 550 ppm）。黄铁矿显示 δ ³⁴ S 值从 Py1（δ ³⁴ S = − 0.4‰ 至 5.8‰，n  = 32）到 Py2（δ ³⁴ S = 0.0‰ 至 6.3‰，n  = 59 ）普遍存在高达 + 3.0‰ 的变化）并记录了 – 0.20‰ 和 0.01‰ 之间的小而略负的 Δ ³³ S 特征。 δ ³⁴ S 位移表明，从含金热液中去除还原的硫物质，通过铁氧化物的围岩硫化降低流体硫逸度（f S ₂ ），导致在 Py2 中形成包裹体金。该变化还与 Py1 中局部富集的 Co 和 Ni 浓度 (< 1 wt%) 相关，而 Py2 中分别较低的振荡分区浓度 (< 0.1 wt%)，表明流体氧逸度 ( f O ₂）。与此同时，流体碲逸度 ( f Te ₂ ) 的降低推动了 Py2 中多金属夹杂物的金形成，最初为钙辉石，随后越来越多地含银锰矿和铁锰矿。 VVF 中黄铁矿中记录的多种硫同位素和微量元素组成表明均质流体储层引入了金硫化物复合物。即使被认为是矿石规模的局部过程，流体-围岩硫化反应也可能导致绿岩贫砷金中含金热液的f S ₂、f O ₂和f Te ₂耦合减少区。