The tops and bottoms of porphyry copper deposits
The tops and bottoms of porphyry copper deposits
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DOI：
10.2113/gsecongeo.68.6.799
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发表时间：
1973-10
期刊：
影响因子：
5.8
通讯作者：
R. Sillitoe
中科院分区：
文献类型：
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作者：
R. Sillitoe
虽然目前人们普遍认为斑岩铜矿是由以高水平钙碱性矿床为中心的蚀变和矿化带排列的壳状矿床组成，但它们向上和向下伸展的不经济性质仍未得到证实。本文试图描述这些向上和向下延伸的特征，并将由此产生的概念整合到一个完整斑岩铜系统的假设模型中。来自智利、阿根廷和其他地方的例子被用来帮助证实模型。斑岩矿床勘探项目显然可以从这类模型的应用中受益。一个典型的斑岩含铜岩块被推断为在显岩侵入岩中向下递变为矿化和硅酸钾蚀变，而显岩侵入岩又向下过渡到一个基本未蚀变的岩体，其尺寸比该斑岩块大得多。斑岩型铜矿床通常位于岩浆火山堆下的基底，由热液蚀变柱横切，代表斑岩型铜矿体系的上部。该蚀变由丙基型和泥质型组成，并伴有局部硅化斑块和晚期泥质蚀变。火山堆被认为是一个层状火山，拥有大量的天然硫矿床和少量的贱金属，特别是铜，在其中心喷口附近的高温喷气孔的升华物中；这些表面沉积物被认为是活性斑岩铜体系的流出产物。现有的证据表明，典型斑岩铜矿的顶部位于层状火山峰顶下1.5-3公里的深度，并表明整个斑岩铜矿体系具有高达8公里的垂直延伸。在智利的Chuquicamata，一条主要的高角度断层可能切割了斑岩铜矿床，随后的侵蚀将位于凸起块体中的部分矿床移走。矿体其余部分的热液蚀变模式不完整，沿断层突兀终止。隆起块体中未蚀变的花岗闪长岩为丘基卡玛塔斑岩铜体系的根带，含小脉体和伟晶岩体。据信，在智利的洛斯洛洛斯，一个斑岩系统的下部矿化部分是暴露的。富钼贫铜钾硅酸盐蚀变带携带丰富的钾长石，在较大的显岩花岗岩岩体内部占据了一块区域。在阿根廷西北部的Farallon Negro，几个小型斑岩铜矿床穿透了一个与时间相关的安山岩层状火山的基础设施。这种不寻常的矿床位置位于次火山基底之上，可以确定斑岩铜侵位是层状火山构造的晚期事件，之后才形成了小流纹岩侵入物和“浅成热液”脉。在被侵蚀的层状火山中心，如智利北部的塞罗马尔克斯，可以看到广泛的黄铁矿蚀变带，包括广泛的硅化，在这些蚀变带中几乎没有侵入岩。这种带被解释为跨越斑岩铜矿床和未被侵蚀火山喷口区之间垂直间隔的蚀变柱。在阿根廷西北部的Cerro Queva，与超前泥质蚀变有关的铅银成矿作用位于层状火山顶下的蚀变带中。在层状火山构造的最后阶段，火山喷发和温泉活动是岩浆房中含金属岩浆流体在逆行沸腾过程中流出的表面表现，这些流体与地下水系统相互作用，并由此形成蚀变和成矿作用。该模型表明，斑岩型铜系统有效地跨越了深部和火山环境的边界。
Although it is now widely accepted that porphyry copper deposits consist of zonally arranged shells of alteration and mineralization centered on high-level, calc-alkaline stocks, the nature of their uneconomic upward and downward extensions remains undocumented. This paper attempts to characterize these upward and downward extensions and to integrate the resulting concepts into a hypothetical model for complete porphyry copper systems. Examples from Chile, Argentina, and elsewhere are used to aid in the substantiation of the model. Programs of exploration for porphyry ore deposits can clearly benefit from the application of a model of this sort.A typical porphyry copper-bearing stock is inferred to grade downward into stock-work mineralization and potassium silicate alteration in a phaneritic intrusive, which in turn is transitional downward to an essentially unaltered pluton of considerably larger dimensions than the stock. Porphyry copper deposits are normally located in the basement beneath a comagmatic volcanic pile, which is transected by a column of hydrothermal alteration representing the upper parts of the porphyry copper system. This alteration consists of propylitic and argillic types with localized patches of silicification and advanced argillic alteration. The volcanic pile is thought to constitute a strato-volcano which possesses large native sulfur deposits and small quantities of base metals, particularly copper, in sublimates at high-temperature fumaroles in the vicinity of its central vent; these surficial deposits are considered as the effluent products of active porphyry copper systems.The available evidence favors the emplacement of the tops of typical porphyry copper deposits at depths of 1.5-3 km beneath the summits of stratovolcanoes and suggests that entire porphyry copper systems possess vertical extensions as great as 8 km.At Chuquicamata, Chile, a major high-angle fault may have cut the porphyry copper deposit, and subsequent erosion has removed the portion of the deposit that was situated in the upthrown block. The hydrothermal alteration pattern in the remaining part of the ore body is incomplete and terminates abruptly against the fault. The unaltered, phaneritic granodiorite, containing minor veins and pegmatitic bodies, in the upthrown block is interpreted as the root zone of the Chuquicamata porphyry copper system.The lower most, mineralized part of a porphyry system is believed to be exposed at Los Loros, Chile. There a zone of molybdenum-rich and copper-poor potassium silicate alteration carrying abundant K-feldspar occupies an area in the interior of a relatively large pluton of phaneritic granite.At Farallon Negro, northwest Argentina, several small porphyry copper deposits pierce the infrastructure of a temporally related, andesitic stratovolcano. This unusual locus of the deposits above the subvolcanic basement enables it to be determined that porphyry copper emplacement was a late event in the construction of the stratovolcano, succeeded only by the formation of minor rhyolite intrusives and "epithermal" veins.Extensive zones of pyritic alteration including widespread silicification, in which intrusive rocks are virtually absent, are visible in the centers of eroded stratovolcanoes, as at Cerro Marquez in northern Chile. Such zones are interpreted as the columns of alteration spanning the vertical interval between porphyry copper deposits and the vent areas of uneroded volcanoes. At Cerro Queva in northwest Argentina, lead-silver mineralization associated with advanced argillic alteration is located in an alteration zone beneath the summit regions of a stratovolcano.It may be concluded that during the final stages of construction of stratovolcanoes, fumarolic and hot-spring activity are the surficial manifestations of the efflux of metal-bearing magmatic fluids from magma chambers during retrograde boiling, the interaction of these fluids with the groundwater system and the consequent formation of alteration and mineralization. The proposed model implies that porphyry copper systems effectively span the boundary between the plutonic and volcanic environments.