Textural, Compositional, and Sulfur Isotope Variations of Sulfide Minerals in the Red Dog Zn-Pb-Ag Deposits, Brooks Range, Alaska: Implications for Ore Formation
Textural, Compositional, and Sulfur Isotope Variations of Sulfide Minerals in the Red Dog Zn-Pb-Ag Deposits, Brooks Range, Alaska: Implications for Ore Formation
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DOI：
10.2113/gsecongeo.99.7.1509
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发表时间：
2004-11
期刊：
影响因子：
5.8
通讯作者：
K. Kelley;D. Leach;C. Johnson;J. L. Clark;M. Fayek;J. Slack;V. Anderson;R. Ayuso;W. Ridley
中科院分区：
文献类型：
--
作者：
K. Kelley;D. Leach;C. Johnson;J. L. Clark;M. Fayek;J. Slack;V. Anderson;R. Ayuso;W. Ridley
红狗锌铅矿床赋存于密西西比州库纳组富含有机质的泥岩和页岩中。复杂成矿史由四种闪锌矿类型或阶段定义：(1)早期棕色闪锌矿，(2)黄褐色闪锌矿，(3)红棕色闪锌矿，(4)晚期棕闪锌矿。阶段2和阶段3构成了主要的成矿事件，在体积上是最重要的。阶段1和阶段2的硫化物以重晶石为主，阶段3主要以重晶石为主。闪锌矿不同阶段之间存在明显的化学差异。从早期的棕色闪锌矿到后来的黄褐色闪锌矿和红棕色闪锌矿，Fe、Co含量普遍增加，Mn、Tl含量普遍降低。早期棕色闪锌矿的Fe含量不超过1.9wt%，Co含量不超过63ppm，具有高的Mn(高达37ppm)和Tl(126ppm)，而黄褐色闪锌矿和红棕色闪锌矿的Fe(高达7.3wt%)和Co(高达382ppm)，低的Mn(<；27ppm)和Tl(<；37ppm)。与早期闪锌矿相比，晚黄闪锌矿的Fe(-lt；0.9wt%)明显偏低，而Tl(高达355ppm)、Mn(高达177ppm)和Ge(426ppm)明显偏高。所有闪锌矿类型，特别是黄褐色闪锌矿和红棕色闪锌矿，银、铜、铅和锑的含量范围很广，很可能反映了闪锌矿中方铅矿、黄铜矿和/或方铜矿的亚微观包裹体。原位离子探针硫同位素分析表明，从第一阶段极低的δ34S值(低至-37.2‰)到高得多的黄褐色闪锌矿(平均3.3‰；n=30)和红棕色闪锌矿(平均3.4‰；n=20)。晚期棕黄色闪锌矿的同位素很轻(-16.4~-27.2‰)。结构、化学和同位素数据表明：(1)早期棕色闪锌矿与丰富的重晶石、少量黄铁矿和少量方铅矿一起沉积在松散的海底之下；(2)在海底热液重结晶和先前存在的重晶石粗化过程中沉积了黄褐色闪锌矿；(3)开放空间沉积了重晶石、红棕色闪锌矿和其他硫化物，并对重晶石进行了同时代的交代；(4)后期硫化物沉积，包括形成晚期黄褐色闪锌矿角砾岩。阶段1成矿作用发生在低温环境中，富含Ba的流体与孔隙水或水柱硫酸盐混合形成重晶石，金属与细菌硫酸盐还原生成的硫化氢结合形成硫化物。与阶段1相比，较高的温度和盐度以及相对氧化的成矿阶段流体(阶段2和阶段3)可能是流体中金属丰度和相对数量以及由此产生的硫化物化学的重要控制因素。结构观察和同位素数据表明，先前存在的重晶石被还原溶解，为硫化物矿物的形成提供了硫化氢的来源。在第三阶段，热液的持续流动导致富含有机质的泥岩的热蚀变和甲烷的积聚，导致流体超压、水力压裂和脉体形成。矿脉中沉积了重晶石、红棕色闪锌矿等硫化物，原存在的重晶石被红棕色闪锌矿普遍取代。热液活动停止，直到侏罗纪，与布鲁克造山有关的逆冲和大规模流体流动在构造角砾岩中重新动员并形成晚期黄闪锌矿。
The Red Dog Zn-Pb deposits are hosted in organic-rich mudstone and shale of the Mississippian Kuna Formation. A complex mineralization history is defined by four sphalerite types or stages: (1) early brown sphalerite, (2) yellow-brown sphalerite, (3) red-brown sphalerite, and (4) late tan sphalerite. Stages 2 and 3 constitute the main ore-forming event and are volumetrically the most important. Sulfides in stages 1 and 2 were deposited with barite, whereas stage 3 largely replaces barite. Distinct chemical differences exist among the different stages of sphalerite. From early brown sphalerite to later yellow-brown sphalerite and red-brown sphalerite, Fe and Co content generally increase and Mn and Tl content generally decrease. Early brown sphalerite contains no more than 1.9 wt percent Fe and 63 ppm Co, with high Mn (up to 37 ppm) and Tl (126 ppm), whereas yellow-brown sphalerite and red-brown sphalerite contain high Fe (up to 7.3 wt %) and Co (up to 382 ppm), and low Mn (<27 ppm) and Tl (<37 ppm). Late tan sphalerite has distinctly lower Fe (< 0.9 wt %) and higher Tl (up to 355 ppm), Mn (up to 177 ppm), and Ge (426 ppm), relative to earlier sphalerite. Wide ranges in concentrations of Ag, Cu, Pb, and Sb characterize all sphalerite types, particularly yellow-brown sphalerite and red-brown sphalerite, and most likely reflect submicroscopic inclusions of galena, chalcopyrite and/or tetrahedrite in the sphalerite. In situ ion microprobe sulfur isotope analyses show a progression from extremely low δ 34S values for stage 1 (as low as –37.2‰) to much higher values for yellow-brown sphalerite (mean of 3.3‰; n = 30) and red-brown sphalerite (mean of 3.4; n = 20). Late tan sphalerite is isotopically light (–16.4 to –27.2‰). The textural, chemical, and isotopic data indicate the following paragenesis: (1) deposition of early brown sphalerite with abundant barite, minor pyrite, and trace galena immediately beneath the sea floor in unconsolidated mud; (2) deposition of yellow-brown sphalerite during subsea-floor hydrothermal recrystallization and coarsening of preexisting barite; (3) open-space deposition of barite, red-brown sphalerite and other sulfides in veins and coeval replacement of barite; and (4) postore sulfide deposition, including the formation of late tan sphalerite breccias. Stage 1 mineralization took place in a low-temperature environment where fluids rich in Ba mixed with pore water or water-column sulfate to form barite, and metals combined with H2S derived from bacterial sulfate reduction to form sulfides. Higher temperatures and salinities and relatively oxidized ore-stage fluids (stages 2 and 3) compared with stage 1 were probably important controls on the abundances and relative amounts of metals in the fluids and the resulting sulfide chemistry. Textural observations and isotopic data show that preexisting barite was reductively dissolved, providing a source of H2S for sulfide mineral formation. In stage 3, the continued flow of hydrothermal fluids caused thermal alteration of organic-rich mudstones and a build-up of methane that led to fluid overpressuring, hydrofracturing, and vein formation. Barite, red-brown sphalerite, and other sulfides were deposited in the veins, and preexisting barite was pervasively replaced by red-brown sphalerite. Hydrothermal activity ceased until Jurassic time when thrusting and large-scale fluid flow related to the Brookian orogeny remobilized and formed late tan sphalerite in tectonic breccias.