Mineral Zoning and Geochemistry of Epithermal Polymetallic Zn-Pb-Ag-Cu-Bi Mineralization at Cerro de Pasco, Peru
Mineral Zoning and Geochemistry of Epithermal Polymetallic Zn-Pb-Ag-Cu-Bi Mineralization at Cerro de Pasco, Peru
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DOI：
10.2113/gsecongeo.103.3.493
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发表时间：
2008-05
期刊：
影响因子：
5.8
通讯作者：
R. Baumgartner;L. Fontboté;T. Vennemann
中科院分区：
文献类型：
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作者：
R. Baumgartner;L. Fontboté;T. Vennemann
秘鲁中部大型塞罗德帕斯科科迪勒拉贱金属矿床位于中新世diatreme-dome 杂岩体的东缘，包括两个矿化阶段。第一阶段由大型黄铁矿石英体组成，取代了中生代下生代普卡拉碳酸盐岩，并在较小程度上取代了角砾岩。该物体由带有磁黄铁矿包裹体的黄铁矿、石英以及黑色和红色玉髓（含有深生赤铁矿）组成。在与黄铁矿-石英体接触时，黄铁矿角砾岩转变为黄铁矿-石英-绢云母-黄铁矿。该矿体部分被管状磁黄铁矿矿体所取代，该矿体向外分布至含有富铁闪锌矿（高达 24 mol% FeS）的碳酸盐置换 Zn-Pb 矿石。第二个成矿阶段部分叠加在第一个阶段上，由矿床西部角砾岩中呈东西走向的铜-银-(Au-Zn-Pb)硫铜矿-黄铁矿脉和分布良好的锌-铅-(Bi-Ag-Cu)碳酸盐置换矿体组成；在这两种情况下，闪锌矿都是贫铁的，矿体内部显示出典型的高级泥质蚀变组合，包括磷酸铝硫酸盐（APS）矿物。黄铅矿-黄铁矿矿脉显示出矿物分带，从黄铅矿-黄铁矿±明矾石核心（含微量金），穿过含钾铁矿、黄铜矿和铋矿物的中间带，到含有闪锌矿-方铅矿±高岭石的发育不良的外部带。碳酸盐岩交代矿受控于 N35°E、N 90° E、N 120° E 和 N 170° E 断层。它们形成分带良好的向上展开的管状矿体，其核心为铁磷矿-黄铁矿和明矾石，中间带为四铁铜矿-黄铁矿、黄铜矿、辉铁矿、铜辉石、绿辉石和其他Bi矿物，并伴有APS矿物、高岭石和地开石，外带由贫铁闪锌矿（在0.05-3.5摩尔范围内）组成。 % FeS) 和方铅矿。最外层由赤铁矿、磁铁矿和 Fe-Mn-Zn-Ca-Mg 碳酸盐组成。第二期碳酸盐交代矿体大多向西倾角在25°~60°之间，表明热液是从深层上升的，没有发生从矿脉向碳酸盐交代矿体的侧向补给。在分别位于 Cerro de Pasco 露天矿西北 2.5 公里处和矿床南部的 Venencocha 和 Santa Rosa 地区，出现了高级泥质蚀变英安岩穹丘和带有高级泥质蚀变晕的氧化脉。后者的矿脉可能是位于矿床西部的第二阶段硫铜矿-黄铁矿矿脉的氧化等价物。与黄铁矿-石英体相关的石英-白云母-黄铁矿蚀变组合表明第一阶段在弱酸性pH下沉淀。硫化物矿物组合定义了接近黄铁矿-磁黄铁矿边界的演化路径，并且具有低硫化状态的特征；他们认为，氧化性微酸性热液受到千枚岩、页岩和碳酸盐主岩的缓冲。然而，黄铁矿-石英体中石英内赤铁矿的存在表明，在局部，流体受到母岩的缓冲较少。第二成矿阶段的矿物组合具有高硫化态到中硫化态的特征。在第二阶段矿体的核心中实现并维持了高硫化状态和氧化条件，甚至在那些替代碳酸盐岩的矿心中也是如此。观察到，在某些地方，内部和外部区域都发现了第二阶段矿物组合，这可以用热液的前进和减弱来解释。石英流体包裹体的显微测温数据表明，第一成矿阶段的不同矿石是在相似的温度和中等盐度下形成的（黄铁矿-石英体中为200°–275°C和0.2–6.8 wt% NaCl当量；磁黄铁矿体中为192°–250°C和1.1–4.3 wt% NaCl当量；以及183°–212°C Zn-Pb 矿石中 3.2–4.0 wt% NaCl 当量）。这些值与第二阶段矿石中的石英和闪锌矿中的流体包裹体获得的值类似（硫黄铁矿-黄铁矿脉中的 187°–293°C 和 0.2–5.2 wt% NaCl 当量；碳酸盐置换矿体的石英中 178°–265°C 和 0.2–7.5 wt% NaCl 当量；168°–222°C 和碳酸盐置换矿体的闪锌矿中 3–11.8 wt% 氯化钠当量；以及来自 Venencocha 的石英中 245°–261°C 和 3.2–7.7 wt% 氯化钠当量）。碳酸盐置换矿体高岭石（δ18O = 5.3–11.5‰，δD = -82 至 -114‰）以及 Venencocha 和 Santa Rosa 地区明矾石（δ18O = 1.9–6.9‰，δD = -56 至 -73‰）的氧和氢同位素组成。第一阶段和第二阶段石英的氧同位素组成的 δ18O 值为 9.1 至 17.8 每密耳。计算出与高岭石平衡的流体的 δ18O 值为 2.0 至 8.2，δD 值为 -69 至 -97 每密耳；与明矾石平衡时的值为-1.4至-6.4和-62至-79每密耳。两个阶段的硫化物的硫同位素组成的 δ34S 值范围很窄，在 -3.7 和 +4.2/mil 之间；第二阶段的硫酸盐值在每密耳 4.2 至 31.2 之间。这些结果定义了成矿流体的两种混合趋势。第一个趋势反映了已脱气（如低 δD 值所示）的中等盐度（约 10 wt% NaCl 当量）岩浆端元与大气水之间的混合。第二次混合表明岩浆蒸气与 HCl 和 SO2 凝结成大气水，形成明矾石。 Cerro de Pasco 的热液系统位于斑岩环境的浅热区和上部浅层（约 500 m）。第一阶段和第二阶段获得的相似温度和盐度，以及稳定同位素数据表明，两个阶段是相互联系的，代表斑岩系统上部浅成热液多金属成矿的连续阶段。
The large Cerro de Pasco Cordilleran base metal deposit in central Peru is located on the eastern margin of a middle Miocene diatreme-dome complex and comprises two mineralization stages. The first stage consists of a large pyrite-quartz body replacing Lower Mesozoic Pucara carbonate rocks and, to a lesser extent, diatreme breccia. This body is composed of pyrite with pyrrhotite inclusions, quartz, and black and red chalcedony (containing hypogene hematite). At the contact with the pyrite-quartz body, the diatreme breccia is altered to pyrite-quartz-sericite-pyrite. This body was, in part, replaced by pipelike pyrrhotite bodies zoned outward to carbonate-replacement Zn-Pb ores bearing Fe-rich sphalerite (up to 24 mol % FeS). The second mineralization stage is partly superimposed on the first and consists of zoned east-west–trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins hosted in the diatreme breccia in the western part of the deposit and well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement orebodies; in both cases, sphalerite is Fe poor and the inner parts of the orebodies show typically advanced argillic alteration assemblages, including aluminum phosphate sulfate (APS) minerals. The zoned enargite-pyrite veins display mineral zoning, from a core of enar-gite-pyrite ± alunite with traces of Au, through an intermediate zone of tennantite, chalcopyrite, and Bi minerals to a poorly developed outer zone bearing sphalerite-galena ± kaolinite. The carbonate-hosted replacement ores are controlled along N35°E, N 90° E, N 120° E, and N 170° E faults. They form well-zoned upward-flaring pipelike orebodies with a core of famatinite-pyrite and alunite, an intermediate zone with tetra-hedrite-pyrite, chalcopyrite, matildite, cuprobismutite, emplectite, and other Bi minerals accompanied by APS minerals, kaolinite, and dickite, and an outer zone composed of Fe-poor sphalerite (in the range of 0.05–3.5 mol % FeS) and galena. The outermost zone consists of hematite, magnetite, and Fe-Mn-Zn-Ca-Mg carbonates. Most of the second-stage carbonate-replacement orebodies plunge between 25° and 60° to the west, suggesting that the hydrothermal fluids ascended from deeper levels and that no lateral feeding from the veins to the carbonate-replacement orebodies took place. In the Venencocha and Santa Rosa areas, located 2.5 km northwest of the Cerro de Pasco open pit and in the southern part of the deposit, respectively, advanced argillic altered dacitic domes and oxidized veins with advanced argillic alteration halos occur. The latter veins are possibly the oxidized equivalent of the second-stage enargite-pyrite veins located in the western part of the deposit. The alteration assemblage quartz-muscovite-pyrite associated with the pyrite-quartz body suggests that the first stage precipitated at slightly acidic pH. The sulfide mineral assemblages define an evolutionary path close to the pyrite-pyrrhotite boundary and are characteristic of low-sulfidation states; they suggest that the oxidizing, slightly acidic hydrothermal fluid was buffered by phyllite, shale, and carbonate host rock. However, the presence in the pyrite-quartz body of hematite within quartz suggests that, locally, the fluids were less buffered by the host rock. The mineral assemblages of the second mineralization stage are characteristic of high- to intermediate-sulfidation states. High-sulfidation states and oxidizing conditions were achieved and maintained in the cores of the second-stage orebodies, even in those replacing carbonate rocks. The observation that, in places, second-stage mineral assemblages are found in the inner and outer zones is explained in terms of the hydrothermal fluid advancing and waning. Microthermometric data from fluid inclusions in quartz indicate that the different ores of the first mineralization stage formed at similar temperatures and moderate salinities (200°–275°C and 0.2–6.8 wt % NaCl equiv in the pyrite-quartz body; 192°–250°C and 1.1–4.3 wt % NaCl equiv in the pyrrhotite bodies; and 183°–212°C and 3.2–4.0 wt % NaCl equiv in the Zn-Pb ores). These values are similar to those obtained for fluid inclusions in quartz and sphalerite from the second-stage ores (187°–293°C and 0.2–5.2 wt % NaCl equiv in the enargite-pyrite veins; 178°–265°C and 0.2–7.5 wt % NaCl equiv in quartz of carbonate-replacement orebodies; 168°–222°C and 3–11.8 wt % NaCl equiv in sphalerite of carbonate-replacement orebodies; and 245°–261°C and 3.2–7.7 wt % NaCl equiv in quartz from Venencocha). Oxygen and hydrogen isotope compositions on kaolinite from carbonate-replacement orebodies (δ18O = 5.3–11.5‰, δD = −82 to −114‰) and on alunite from the Venencocha and Santa Rosa areas (δ18O = 1.9–6.9‰, δD = −56 to −73‰). Oxygen isotope compositions of quartz from the first and second stages have δ18O values from 9.1 to 17.8 per mil. Calculated fluids in equilibrium with kaolinite have δ18O values of 2.0 to 8.2 and δD values of −69 to −97 per mil; values in equilibrium with alunite are −1.4 to −6.4 and −62 to −79 per mil. Sulfur isotope compositions of sulfides from both stages have a narrow range of δ34S values, between −3.7 and +4.2 per mil; values for sulfates from the second stage are between 4.2 and 31.2 per mil. These results define two mixing trends for the ore-forming fluids. The first trend reflects mixing between a moderately saline (~10 wt % NaCl equiv) magmatic end member that had degassed (as indicated by the low δD values) and meteoric water. The second mixing indicates condensation of magmatic vapor with HCl and SO2 into meteoric water, which formed alunite. The hydrothermal system at Cerro de Pasco was emplaced at a shallow depth (~500 m) in the epithermal and upper part of a porphyry environment. The similar temperatures and salinities obtained for the first stage and second stages, together with the stable isotope data, indicate that both stages are linked and represent successive stages of epithermal polymetallic mineralization in the upper part of a porphyry system.