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首页> 外文期刊>Journal of Volcanology and Geothermal Research >From andesitic volcanism to the formation of a porphyry Cu-Au mineralizing magma chamber: the Farallon Negro Volcanic Complex, northwestern Argentina
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From andesitic volcanism to the formation of a porphyry Cu-Au mineralizing magma chamber: the Farallon Negro Volcanic Complex, northwestern Argentina

机译：从安第斯山脉火山到斑岩型铜金矿化岩浆室的形成：阿根廷西北部的法拉隆黑人火山群

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摘要

The geological relations and geochemical composition of extrusive and subvolcanic intrusive rocks of the Upper Miocene Farallon Negro Volcanic Complex (NW Argentina) were studied to reconstruct the evolution of a deeply eroded andesitic volcano over most its life history of some 3.5 my. This paper is part of an integrated study of the processes that generate magmatic-hydrothermal fluids responsible for the formation of porphyry-Cu-Au deposits. Here we present new data on bulk rock chemical and Nd, Sr and Pb isotopic compositions of unaltered volcanic and subvolcanic intrusive rocks, for which geological relations and ages are constrained by field-based stratigraphy and extensive Ar-Ar geochronology. The combined results indicate that voluminous volcanism started at ca. 9.7 Ma with the eruption of basaltic andesite to andesite lava flows and pyroclastic deposits. This basic to intermediate volcanism persisted until 7.5 Ma and was followed at 7.35 Ma by the eruption of dacites as the last recorded major extrusive event. The oldest andesitic intrusions, emplaced at 9.0 Ma, were followed by hypabyssal intrusions of basaltic andesite, andesite and dacite composition, which intruded the volcanic sequence between 8.3 and 6.1 Ma. Intrusions at 6.8 ± 0.2 Ma led to the formation of over 800 Mt of porphyry-Cu-Au ore at Bajo de la Alumbrera. It was formed by a large flux of magmatic-hydrothermal fluids channeled through several phases of fractured dacitic porphyry, which together with immediately following barren andesitic intrusions make up a composite stock. After this magmatic-hydrothermal event, only minor rhyodacitic to rhyolitic intrusions were emplaced until magmatic activity ceased at 6.1 Ma. Geochemical data for the magmatic rocks indicate that the evolution of the system was dominated by mixing of a mafic magma with a felsic magma that contained a component of crustal material, as indicated by linear compositional correlations between compatible and incompatible elements. Initial strontium isotopic ratios vary between 0.7047 and 0.7118 while ~(143)Nd/~(144)Nd range from 0.5127 to 0.5124, and both isotopic variations correlate with each other and with SiO_2 content of the rocks. After 8.5 Ma, a continuous increase in the silica content of intrusive rocks suggests the gradual formation of a magma chamber. A substantial magma reservoir ( > 7 km~3) must have been established by the time of porphyry-Cu-Au mineralization, to allow rapid release of a large volume of ore-forming magmatic brine as a result of wholesale crystallization and de-volatilization of the magma chamber.
机译：研究了上中新世法拉隆黑人火山复合体（阿根廷西北部）的挤压和次火山侵入岩的地质关系和地球化学组成，以重建深腐蚀的安第斯火山在其大约3.5 my的大部分生命历史中的演化。本文是对负责形成斑岩-铜-金矿床的岩浆-热液的过程的综合研究的一部分。在这里，我们介绍了未改变的火山岩和次火山岩侵入岩的块状岩石化学成分以及Nd，Sr和Pb同位素组成的新数据，这些数据的地质关系和年龄受到基于现场的地层学和广泛的Ar-Ar地质年代学的限制。综合结果表明，大量的火山活动始于约。 9.7 Ma，玄武质安山岩喷发至安山岩熔岩流和火山碎屑沉积物。这种基本到中级的火山活动一直持续到7.5 Ma，随后是7.35 Ma，最后一次记录的主要挤压事件是dacits的喷发。最早的安山岩侵入为9.0 Ma，其次是玄武质安山岩，安山岩和辉绿岩成分的海底侵入，侵入了8.3 Ma至6.1 Ma的火山岩层序。在6.8±0.2 Ma处侵入，导致在Bajo de la Alumbrera形成了800 Mt的斑岩-铜-金矿石。它是由大量的岩浆热液流经裂缝性的Datictic斑岩的几个阶段形成的，这些岩浆和紧随着贫瘠的安山岩侵入立即构成了一种复合储层。在这次岩浆热液事件之后，仅注入了少量的流纹岩至流纹岩侵入体，直到岩浆活动在6.1 Ma停止。岩浆岩的地球化学数据表明，该系统的演化主要由铁镁质岩浆和含地壳物质成分的长英质岩浆混合而成，这由相容和不相容元素之间的线性成分相关性表明。初始锶同位素比在0.7047至0.7118之间变化，而〜（143）Nd /〜（144）Nd在0.5127至0.5124范围内，并且两种同位素变化相互关联并且与岩石的SiO_2含量相关。 8.5 Ma之后，侵入岩中二氧化硅含量的持续增加表明岩浆室的逐渐形成。斑岩-铜-金矿化时，必须建立大量的岩浆储集层（> 7 km〜3），以允许大量的成矿岩浆盐水由于大量结晶和脱挥发分而快速释放。岩浆室。

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