Metal transport and deposition in hydrothermal veins revealed by 213nm UV laser ablation microanailysis of single fluid inclusions
Metal transport and deposition in hydrothermal veins revealed by 213nm UV laser ablation microanailysis of single fluid inclusions
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DOI：
10.2475/ajs.304.6.533
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发表时间：
2004-06-01
影响因子：
2.9
通讯作者：
Jeffries, TE
中科院分区：
文献类型：
--
作者：
Stoffell, B;Wilkinson, JJ;Jeffries, TE
213nm 紫外激光烧蚀为单个流体包裹体的化学微量分析提供了一种强大且经济高效的新方法。与 266 nm 紫外激光器相比，该仪器在许多地质材料中的能量吸收能力显着提高，从而改善了烧蚀特性。激光不会被空气或常用光学材料强烈吸收，从而可以相对容易地传输和控制光束。人们发现，烧蚀纯石英玻璃毛细管中的多元素标准溶液是仪器灵敏度外部校准的有效方法。使用根据显微测温数据估计的氯化物浓度进行内部校准，以计算包裹体流体中的绝对元素浓度。使用这种方法，使用 30 种元素分析物菜单对同生初级和次级夹杂物组进行分析，得出碱金属和碱土元素绝对元素浓度的典型相对标准偏差约为 20%，金属为 20% 至 50%，与通过 193nm 准分子激光烧蚀获得的结果相当。 Ca 的结果与显微测温测量的独立估计一致，一系列其他元素与之前发表的大量化学分析结果相当一致。总体而言，数据给出了良好的电荷平衡，在 5% 以内。该技术用于检验以下假设：热液矿床的金属含量主要由矿化溶液的金属含量控制，而不是选择性沉淀机制。我们研究了来自英格兰西南部的铅（-Zn）矿化“交叉”矿脉，已知这些矿脉是由靠近科努比亚半岛的二叠纪-三叠纪红层盆地排出的低温盐水形成的，可能是在三叠纪时期。这些流体与形成密西西比河谷型铅锌矿床的溶液相当。对这些矿脉之一的生长带石英晶体的原生流体包裹体的分析表明，矿脉生长早期阶段的矿石流体相对富含铅（高达 295 ppm Pb，高达 86 ppm Zn；Pb/Zn = 1.3-16.1）并显示化学中明显的时间变化。在石英生长的一个显着（类似于 10 nun）间隔内观察到 K、Ca、Sr、Ba 和 Zn 逐渐增加。这些增加被解释为反映了从盆地源区排出后流体所利用的裂缝系统内发生的矿物溶解-沉淀反应。叠加在这一趋势上的是相对较短的时间尺度（石英生长-1毫米）的化学偏移，其特征是Cl、K、Ca、Ba、Sr、Pb和Zn的浓度降低高达两个数量级。这些偏移被认为是由于局部的、体积上显着的矿物沉淀事件造成的，并表明导致多个相同时过饱和的过程的运行。有人认为，最初的方铅矿沉淀（溶液中铅损失> 98%）是由于氧化流体在流出盆地源区的流动路径上遇到第一个强还原性岩性时的还原所致。后来的几次稀释事件是由流体混合引起的，并推断是导致硫化物（方铅矿、闪锌矿；溶液中金属损失高达 98%）与硫酸盐（硬石膏、重晶石；溶液中损失高达 91% Ca 和 97% Ba）和/或硅酸盐（钾云母；溶液中 K 损失高达 96%）的共沉淀。然而，在研究的样品中仅在一个阶段观察到方铅矿（+/-硬石膏、钾云母），这表明流体包裹体通常可能在上游一定距离的矿物沉淀发生后捕获贫化流体，而不是反映其附近的沉淀事件。本案例研究中的主要成矿流体含有升高的 Pb 和 Zn，并且至少在最初表现出较高的 Pb/Zn 比闪锌矿和方铅矿饱和流体的预测值要高。因此，我们认为脉状矿床的富铅品位主要受矿石流体的金属含量控制，这最终是盆地物源地球化学的函数。在本例中，源区主要是氧化陆生碎屑岩。含水层内的反应和矿石形成地点的选择性沉淀机制似乎也改变了流体地球化学，但在控制形成的矿床的金属预算方面发挥了二级作用。
213nm UV laser ablation provides a powerful and cost-effective new method for the chemical microanalysis of individual fluid inclusions. The instrument offers significantly improved energy absorption in many geological materials compared to 266nm UV lasers resulting in improved ablation characteristics. The laser is not strongly absorbed by either air or commonly used optical material, permitting relatively easy delivery and control of the beam.Ablation of multielement standard solutions in pure quartz glass capillaries was found to be an effective method for external calibration of instrument sensitivity. Internal calibration for calculation of absolute element concentrations in inclusion fluids was carried out using chloride concentration estimated from microthermometric data. Using this approach, analyses of groups of cogenetic primary and secondary inclusions using a 30 element analyte menu gave typical relative standard deviations for absolute element concentrations of around 20 percent for alkali and alkali earth elements and 20 to 50 percent for metals, comparable with those obtained by 193nm Excimer laser ablation. Results for Ca are consistent with independent estimates made from microthermometric measurements and a range of other elements are in reasonable agreement with previously published bulk chemical analyses. Overall, the data give good charge balance, within 5 percent.The technique was used to test the hypothesis that the metal content of hydrothermal ore deposits is primarily controlled by the metal content of the mineralizing solutions rather than selective precipitation mechanisms. We studied Pb(-Zn) mineralized 'crosscourse' veins from Southwest England that are known to have formed from low temperature brines expelled from Permo-Triassic red-bed basins adjacent to the Cornubian peninsula, probably during the Triassic. These fluids are comparable with the solutions that formed Mississippi Valley-type Pb-Zn deposits.Analyses of primary fluid inclusions across growth-zoned quartz crystals from one of these veins indicate the orefluids at an early stage of vein growth are relatively Pb-rich (up to 295 ppm Pb, up to 86 ppm Zn; Pb/Zn = 1.3-16.1) and display marked temporal variations in chemistry. Gradual increases in K, Ca, Sr, Ba and Zn are observed across a significant (similar to10 nun) interval of quartz growth. These increases are interpreted to reflect mineral dissolution-precipitation reactions occurring within the fracture system exploited by the fluids after being expelled from the basin source area. Superimposed on this trend are comparatively short time-scale (-1 mm of quartz growth) chemical excursions characterized by decreases in the concentrations of Cl, K, Ca, Ba, Sr, Pb and Zn of up to two orders of magnitude. These excursions are believed to be due to local, volumetrically significant, mineral precipitation events and indicate the operation of processes that result in the simultaneous supersaturation of a number of phases.It is suggested that initial galena precipitation (>98% Pb loss from solution) was due to reduction as oxidized fluids encountered the first strongly reducing lithology on their flow path out of the basin source region. Several later dilution events were caused by fluid mixing and are inferred to have caused the co-precipitation of sulfides (galena, sphalerite; up to 98% loss of metals from solution) with sulfates (anhydrite, barite; up to 91% Ca and 97% Ba loss from solution) and/or silicates (K-mica; up to 96% loss of K from solution). However, at only one stage is galena (+/-anhydrite, K-mica) observed in the sample studied suggesting that fluid inclusions may commonly trap depleted fluids after mineral precipitation has already occurred some distance upstream, rather than reflecting precipitation events in their immediate vicinity.The primary ore-forming fluid in this case study contained elevated Pb and Zn and, at least initially, displayed a higher Pb/Zn ratio than predicted for fluids saturated with respect to both sphalerite and galena. Therefore, we suggest that the Pb-rich tenor of the vein deposit is primarily controlled by the metal content of the ore fluids, which is ultimately a function of the geochemistry of the basin source. In this example, the source area is dominated by oxidized terrestrial clastic rocks. Reactions within the aquifer and selective precipitation mechanisms at the site of ore formation also appear to have modified fluid geochemistry but played a second order role in controlling the metal budget of the mineral deposit formed.