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Testing Silver Mobility: An investigation into supergene silver enrichment at the Rochester Mine in Pershing County, Nevada.

机译：测试银的流动性：在内华达州潘兴县罗切斯特矿对超基因银的富集进行了调查。

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摘要

Supergene silver enrichment, comparable to chalcocite supergene enrichment blankets in porphyry copper deposits, has long been called upon to explain high grades in silver deposits that diminish in grade with depth. Rochester is a large-tonnage, low-grade disseminated silver-(gold) deposit hosted in Early Triassic rhyolites located in northwestern Nevada, where the role of supergene processes in controlling the distribution of ore and silver grade remained unclear prior to this study. The effect of supergene processes at Rochester was investigated by observing patterns of silver occurrences in structures, weathering zones, and ore and gangue mineral paragenesis.;The overall tabular geometry of the ore body straddling the Weaver-Rochester lithologic contact is defined by high grade zones in the lower Weaver oxide and upper Rochester oxide and mixed oxide-sulfide zones. Grades drop off significantly in the underlying protore sulfide zone. Protore hydrothermal alteration is dominated by quartz-sericite-pyrite with minor relict K-feldspar. Hypogene sulfides include pyrite, sphalerite, galena, tetrahedrite-tennantite, chalcopyrite, arsenopyrite, and rare molybdenite. Silver phases in the protore include silver-bearing tetrahedrite-tennantite; acanthite; and rare stromeyerite, pearceite, and polybasite. Geologic characteristics most closely associated with higher silver grades in oxide and mixed oxide-sulfide zones include faults, acanthite, and goethite-rich rocks. High grade zones (≥2 opt Ag) are controlled by high- and low-angle faults cutting the Weaver-Rochester contact. Acanthite is the dominant silver phase. Much of the acanthite formed by supergene enrichment based on its common occurrence with covellite as rims on hypogene sulfides in mixed zones. Chlorargyrite and native silver are commonly found in the oxide and mixed zone with acanthite. Other silver-halides, such as iodargyrite, are found in the oxide zone.;Based on these observations, coupled with published phase equilibria, early oxidation of protore caused the formation of silver-halides resulting in a decrease in silver mobility, particularly when iodargyrite was stable. Upon oxidation, a small amount of silver is able to migrate to lower portions of the supergene profile forming acanthite. However, if iodide (I-) was not continually replenished the I/Cl ratio would have decreased and the chlorargyrite field would have expanded at the expense of iodargyrite, which would have allowed more downward transport of Ag +, as chlorargyrite is more soluble than iodargyrite. A dropping water table caused by uplift, erosion, and climatic changes, controlling silver deportment resulted in continued oxidation. As the water table dropped due to uplift related to Basin and Range extension, as well as decreasing precipitation from a cooling and drying climate, enriched zones would have oxidized. When groundwater was not saline chlorargyrite released Ag+ and upon oxidation acanthite released silver as AgHSo for downward transport. New acanthite could have then formed at the deeper water table.;The amount of silver that became fixed as silver-halides and native silver in the oxide zone controlled the amount of aqueous silver in downward moving supergene fluids, which in turn controlled the extent and grade of supergene enrichment expressed as zones of abundant supergene acanthite. This was dependent on environmental conditions, including the salinity of groundwater, duration and periodicity of weathering, tectonic activity resulting in uplift and dropping of the water table, and composition of gangue minerals. The possibility that the overall tabular geometry of the Rochester ore body that sits just above the sulfide zone, resulted from supergene enrichment processes cannot be ruled out at this time without further work. Nevertheless, supergene silver enrichment certainly occurred at Rochester and resulted in high grade zones (?2 opt Ag up to 70 opt) along high- and low-angle faults. Identifying the controls on such high grades is important for future exploration and mine planning.
机译：与斑岩铜矿床中的辉绿岩超基因富集层相比，表层超生银富集长期以来一直被要求解释银矿的高品位，其品位随深度而逐渐降低。罗切斯特（Rochester）是位于内华达州西北部的三叠纪早期流纹岩中的一种大吨位，低品位弥散性银（金）矿床，在此研究之前，尚不清楚超基因过程在控制矿石和银品位分布中的作用。通过观察银在结构，风化带，矿石和脉石矿物共生中的发生模式，研究了罗切斯特表生过程的影响。跨越Weaver-Rochester岩性接触的矿体的整体表格几何形状由高品位区域定义在下部的韦弗氧化物和上部的罗切斯特氧化物以及混合的氧化物-硫化物区。在下面的原始硫化物带中，品位显着下降。原型热液蚀变以石英绢云母-黄铁矿为主，遗迹为钾长石。次生硫化物包括黄铁矿，闪锌矿，方铅矿，四面体-辉石，黄铜矿，毒砂和稀有辉钼矿。原型中的银相包括含银的四面体-钙云母。 can石以及稀有的辉锰矿，白云母和多钛铁矿。与氧化物和氧化物-硫化物混合区中较高的银品位最密切相关的地质特征包括断层，钙长石和富针铁矿岩石。高品位区域（≥2 opt Ag）由切割Weaver-Rochester触头的高角度和低角度故障控制。 can石是主要的银相。通过超基因富集形成的许多棘皮石，是由于它在混合区中次生硫化物上普遍以角铁石为边缘而出现。绿菱铁矿和本机银通常存在于氧化层和与a石的混合区中。在氧化物区中发现了其他卤化银，如碘长晶石；基于这些观察结果，再加上已公布的相平衡，原型的早期氧化导致卤化银的形成，导致银迁移率下降，特别是当碘长晶石时很稳定在氧化时，少量的银能够迁移到表生石轮廓的下部，形成a石。但是，如果不持续补充碘化物（I-），则I / Cl比率会降低，而绿泥石域会扩大，而以碘酒黄铁矿为代价，这将允许更多的Ag +向下传输，因为绿泥石比iodargyrite。由隆起，侵蚀和气候变化引起的地下水位下降，控制了银的迁移，导致继续氧化。由于地下水位由于与盆地和范围扩展有关的隆升而下降，以及由于凉爽和干燥的气候而导致的降水减少，因此富集区将被氧化。当地下水中不含盐时，绿藻石释放出Ag +，而氧化钙长石释放出银作为AgHSo，用于向下运输。然后可能在较深的地下水位上形成新的a石。；固定为卤化银和氧化带中的天然银的银量控制着向下运动的超基生流体中含水银的量，从而控制了范围和大小。以丰富的表皮长寿石区域表示的表皮富集等级。这取决于环境条件，包括地下水的盐度，风化的持续时间和周期性，导致地下水位升高和下降的构造活动以及脉石矿物的组成。如果不做进一步的工作，此时就不能排除由于超基因富集过程而导致的位于硫化物区上方的罗切斯特矿体总体呈板状几何形状的可能性。然而，超基因银的富集确实发生在罗切斯特，并导致了高角度和低角度断层的高品位区（？2 opt Ag，最高70 opt）。确定此类高品位的控件对于将来的勘探和矿山规划很重要。

著录项

  • 作者

    Anderson, Tracy L.;

  • 作者单位

    University of Nevada, Reno.;

  • 授予单位 University of Nevada, Reno.;
  • 学科 Geology.;Geochemistry.
  • 学位 M.S.
  • 年度 2016
  • 页码 113 p.
  • 总页数 113
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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