Characteristics and Evolution of the Hydrothermal Fluid in the North Zone High-Grade Area, Porgera Gold Deposit, Papua New Guinea
Characteristics and Evolution of the Hydrothermal Fluid in the North Zone High-Grade Area, Porgera Gold Deposit, Papua New Guinea
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DOI：
10.2113/gsecongeo.99.5.843
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发表时间：
2004-08
期刊：
影响因子：
5.8
通讯作者：
E. Ronacher;J. Richards;M. Reed;C. J. Bray;E. Spooner;P. Adams
中科院分区：
文献类型：
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作者：
E. Ronacher;J. Richards;M. Reed;C. J. Bray;E. Spooner;P. Adams
2000万盎司（Moz）的Porgera金矿位于巴布亚新几内亚。古碱性侵入体和侵入体所在的白垩系沉积岩。含金矿脉分为三个阶段：(1)磁铁矿-硫化物-碳酸盐±含少量金的石英矿脉（前阶段I），(2)贱金属-硫化物-碳酸盐±石英±金矿脉（阶段I），(3)石英-辉长石-黄铁矿-金矿脉及角砾岩（阶段II）。II期矿脉在经济上最为重要。石英-辉长石-黄铁矿-金矿脉与Roamane断裂（一条横切侵入杂岩体的晚期正断层）和断层下盘（北带）形成高品位带。北带成矿作用是本次研究的重点。石英-辉长石-黄铁矿-金组合以三种不同的结构形式赋生：(1)细细（1 ～ 5 mm）脉状结构，其中辉长石-黄铁矿-金比石英丰富，且辉长石和金也赋生在围岩中；(2)脉体（5 ～ 10 cm），脉体边缘呈菱铁矿-黄铁矿-金带状，含少量石英，其次为粗粒石英，脉体中心常充填硬石膏和碳酸盐；(3)角砾岩脉和角砾岩中围岩碎块以辉长石—黄铁矿—金和少量石英镶边，其次为溶洞型石英充填。这些岩脉和角砾岩中的石英流体包裹体大多富含液体，单个样品的平均盐度在7.5±1.0 ～ 9.6±0.2 wt ％ NaCl当量之间，27个样品中有5个样品的盐度在4.4 ～ 6.2 wt ％ NaCl当量之间。这些相对低盐度的包裹体出现在脉中心，比出现在脉边缘的高盐度包裹体少。三个样品呈现出4.5％至10.2％ NaCl当量的连续盐度趋势。对于可进行CO 2分析的样品，平均校正盐度范围为5.1％至8.0 wt ％ NaCl当量。单个样品的平均均质温度（T h）范围为127°±12°至167°±25°C。低盐度包裹体（145°±9°C）的平均温度略低，但与所有高盐度包裹体（152°±17°C）的平均温度重叠。气相色谱分析表明，高盐度流体含有高达2mol ％的co2, 0.11 mol ％的ch4, 0.065 mol ％的n2，以及痕量的c2h4， c2h6和COS。离子色谱法测定了Cl - (310 ～ 609 mM/l)、Br - (0.28 ～ 0.75 mM/l)、Li + (1.25 ～ 8.80 mM/l)、Na + (462 ～ 1126 mM/l)、K + (0 ～ 81 mM/l)、Mg 2+ (0 ～ 7.0 mM/l)、ca2 + (0 ～ 185 mM/l)的浓度。石英的δ 18o值在13.9 ～ 18.3 / mil之间，δ 18o ～ h2o值在-1.2 ～ 4.1 / mil之间，δ dh2o值在-77 ～ -52 / mil之间，热液的同位素组成介于岩浆和大气水之间。碳酸盐岩δ 18o值为15.2 ～ 16.6‰/ ml，碳酸盐岩δ 13c值为-3.3 ～ -2.4‰，蚀变和未蚀变沉积岩δ 13c值分别为-5.4 ～ -4.0‰和-1.5 ～ 0.0‰，页岩有机碳值为-23.6 ～ -16.8‰。δ 34 S黄铁矿值为-9.4 ～ 6.1 / mil， δ 34 S硬石膏值为12.4 ～ 17.5 / mil。岩石学和分析结果表明，一种上升流体与沉积岩和/或沉积岩所承载的流体在深部（而不是在矿床所在地）相互作用。这种相互作用是由有机衍生挥发物的存在和包裹体流体中\(NH_{4}^{+}\)的高浓度所提示的。在一些样品中，两种液体参与了II期静脉的形成。富气流体包裹体表明热液局部沸腾，连续的盐度趋势表明部分II期矿脉中发生了流体混合。分析数据和共生信息用于估算Porgera热液中的元素浓度，并使用CHILLER软件进行热力学反应路径建模。这种流体经历了沸腾、与沉积地层水混合、冷却和流体-岩石反应。在沸腾过程中，形成石英、黄铁矿、金和钾长石；混合导致黄铁矿、金和云母的沉积；与闪长岩反应形成了观察到的围岩蚀变组合；冷却形成石英加上少量云母、黄铁矿、金和高岭石。观察、分析数据和建模结果表明，在不同的位置和不同的时间，II期静脉形成涉及多个过程。沸腾、混合和流体-岩石反应，所有这些都伴随着冷却，间歇性地发生在北部带的整个深度范围内，Porgera的大小可能是所有这些过程在矿床中发生的结果。
The ~20-million-ounce (Moz) Porgera gold deposit, Papua New Guinea, is hosted by 6-m.y.-old alkalic intrusions and Cretaceous sedimentary rocks in which the intrusions were emplaced. Gold-bearing veins occur in three stages: (1) magnetite-sulfide-carbonate ± quartz veins with minor gold (prestage I), (2) base metal-sulfide-carbonate ± quartz ± Au veins (stage I), and (3) quartz-roscoelite-pyrite-gold veins and breccias (stage II). Stage II veins are economically the most significant. Quartz-roscoelite-pyrite-gold veins form high-grade zones associated with the Roamane fault (a late normal fault that crosscuts the intrusive complex) and in the footwall to the fault (the North zone). The North zone mineralization is the main focus of this study. The quartz-roscoelite-pyrite-gold assemblage occurs in three texturally distinct styles: (1) thin (1–5 mm) veinlets in which roscoelite-pyrite-gold are more abundant than quartz and in which roscoelite and gold also occur in the wall rock; (2) veins (5 mm to 10 cm) in which roscoelite-pyrite-gold with minor quartz form a band at the vein edges, followed by coarse-grained quartz and the vein centers commonly filled with anhydrite and carbonate; and (3) breccia veins and breccias in which wall-rock fragments are rimmed by roscoelite-pyrite-gold and minor quartz, followed by vuggy quartz infilling. Fluid inclusions from quartz in these veins and breccias are mostly liquid rich, and average salinities in individual samples range from 7.5 ± 1.0 to 9.6 ± 0.2 wt percent NaCl equiv. In five of 27 samples, an additional cluster of salinities between 4.4 and 6.2 wt percent NaCl equiv was observed. These relatively low salinity inclusions occur toward the vein center and are less abundant than high-salinity inclusions that occur toward the vein margins. Three samples exhibit a continuous salinity trend from 4.5 to 10.2 wt percent NaCl equiv. For samples where CO 2 analyses were available average corrected salinities range from 5.1 to 8.0 wt percent NaCl equiv. Average homogenization temperatures (T h ) of individual samples range from 127° ± 12° to 167° ± 25°C. The average T h of the low-salinity inclusions (145° ± 9°C) is marginally lower but overlaps with that of all high-salinity inclusions (152° ± 17°C). Gas chromatographic analyses showed that the high-salinity fluid contains up to 2 mol percent CO 2 , 0.11 mol percent CH 4 , 0.065 mol percent N 2 , and traces of C 2 H 4 , C 2 H 6 , and COS. Concentrations of Cl – (310–609 mM/l), Br – (0.28–0.75 mM/l), Li + (1.25–8.80 mM/l), Na + (462–1126 mM/l), K + (0–81 mM/l), Mg 2+ (0–7.0 mM/l), and Ca 2+ (0–185 mM/l) were determined by ion chromatography. The δ 18 O values of quartz range from 13.9 to 18.3 per mil, and calculated δ 18 O H 2 O values range from –1.2 to 4.1 per mil. The δ D H 2 O values lie between –77 and –52 per mil. The calculated isotopic composition of the hydrothermal fluid lies between that of magmatic and that of meteoric water. The δ 18 O values of carbonates range from 15.2 to 16.6 per mil. Carbon isotopes were analyzed on carbonates ( δ 13 C = –3.3 to –2.4‰), altered and unaltered sedimentary rocks (–5.4 to –4.0 and –1.5 to 0.0‰, respectively), and organic carbon in shales (–23.6 to –16.8‰). The δ 34 S pyrite values range from –9.4 to 6.1 per mil, and δ 34 S anhydrite values range from 12.4 to 17.5 permil. The petrographic and analytical results suggest that an ascending fluid interacted with the sedimentary rocks and/or fluid hosted by the sedimentary rocks at depth (rather than at the site of ore deposition). This interaction is suggested by the presence of organic-derived volatiles and high \(NH_{4}^{+}\) concentrations in inclusion fluid. In some samples, two fluids were involved in stage II vein formation. Vapor-rich fluid inclusions indicate that the hydrothermal fluid boiled locally, and continuous salinity trends suggest that fluid mixing occurred in some stage II veins. The analytical data and paragenetic information were used to estimate element concentrations in the Porgera hydrothermal fluid that were used for thermodynamic reaction path modeling using the software CHILLER. This fluid was subjected to boiling, mixing with sedimentary formation water, cooling, and fluid-rock reaction. During boiling, quartz, pyrite, gold, and K-feldspar formed; mixing resulted in the deposition of pyrite, gold, and mica; reaction with diorite produced the observed wall-rock alteration assemblage; and cooling formed quartz plus minor mica, pyrite, gold, and kaolinite. Observations, analytical data, and modeling results suggest that more than one process was involved in stage II vein formation in different locations and at different times. Boiling, mixing, and fluid-rock reaction, all accompanied by cooling, occurred intermittently over the entire depth extent of the North zone, and the size of Porgera may be a result of all of these processes occurring at the deposit.