Evolution of a volcanogenic hydrothermal system recorded by the behavior of LREE and Eu: Case study of the Key Tuffite at Bracemac–McLeod deposits, Matagami, Canada
Evolution of a volcanogenic hydrothermal system recorded by the behavior of LREE and Eu: Case study of the Key Tuffite at Bracemac–McLeod deposits, Matagami, Canada
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DOI：
10.1016/j.oregeorev.2014.04.019
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发表时间：
2014-12
影响因子：
3.3
通讯作者：
Dominique Genna;D. Gaboury;G. Roy
中科院分区：
文献类型：
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作者：
Dominique Genna;D. Gaboury;G. Roy
现代和古代火山成因块状硫化物（VMS）矿床周围的蚀变带中稀土元素（REE）的分布常表现为轻稀土（LREE：La ~ Sm）和铕的增减变化。矿物组合及其化学成分记录了热液系统的演化。为了解决稀土元素在这种情况下的行为，Bracemac-McLeod太古宙VMS矿床的Matagami区使用。在那里，矿化主要是通过海底置换形成的，沿着一个具有均匀安山质成分的凝灰质标志单元（关键凝灰岩），因此提供了一个单一的原岩来解释与矿石位置有关的矿物和化学变化。我们结合联合收割机全岩和矿物稀土元素数据在远绢云母和近绢云母蚀变周围的矿化。综合观察到的结构关系，地球化学数据和目前的矿物稳定性的理解，提出了一个模型来解释的稀土元素的再分配过程中的VMS热液系统的演化。远端绢云母带代表早期低温蚀变。其特征是火山玻璃和斜长石的破坏导致全岩Eu负异常和轻稀土亏损。近变质带是在一个富水的VMS热液系统的热演化过程中形成的。绿泥石化样品中LREE和Eu普遍富集，但部分样品LREE亏损。岩相学观察表明，褐帘石与碳酸盐的共沉淀解释了LREE的增加，而质量平衡计算表明，磷灰石（±碳酸盐）的沉淀基本上控制了Eu。局部硅化，发生在热液系统生命的脉冲，也是感兴趣的，因为它可以保存一个中间的蚀变状态，从而提供更多的信息，热液系统的演变。稀土元素的活动性指示特定的热液条件，可以反映成矿热液系统的效率。在Matagami，轻稀土元素和铕的流动性可用于矢量化勘探，特别是Eu，它具有异常行为，远远超出了硫化物区的限制，进入远绢云母低温蚀变（距离矿区400米）。
The distribution of rare earth elements (REE) in the alteration zones surrounding modern and ancient volcanogenic massive sulfide (VMS) deposits is often characterized by a wide range of variations comprising both gains and losses of light REE (LREE: La to Sm) and europium. The evolution of the hydrothermal system is recorded by mineral assemblages and their chemical composition. To address the behavior of the REE in such a context, the Bracemac–McLeod Archean VMS deposits of the Matagami district are used. There, mineralization was formed mainly by subsea-floor replacement along a tuffaceous marker unit (Key Tuffite) of homogenous andesitic composition, hence providing a single protolith to account for mineral and chemical variations in relation to ore position. We combine whole-rock and mineral REE data in both distal sericite and proximal chlorite alteration surrounding the mineralization. A model, integrating observed textural relationships, geochemical data and current understanding of mineral stabilities, is proposed to explain the redistribution of the REE during the evolution of the VMS hydrothermal system. The distal sericite zone represents the early, low temperature alteration. It is characterized by the destruction of volcanic glass and plagioclase leading to a negative Eu anomaly in the whole-rock analyses and the depletion of LREE. The proximal chlorite zone was formed during the thermal evolution of a fertile VMS hydrothermal system. It is generally characterized by gains of LREE and Eu, although some chloritized samples are depleted in LREE. Petrographic observations suggest that the coprecipitation of allanite with chlorite explains the increase of LREE, whereas a mass balance calculation indicates that the precipitation of apatite (± carbonates) essentially controls the Eu. Local silicification, occurring as pulses during the hydrothermal system life, is also of interest because it could preserve an intermediate alteration state and thus provide additional information on the evolution of the hydrothermal system. The mobility of REE is indicative of specific hydrothermal conditions, which may reflect the efficiency of a mineralizing hydrothermal system. At Matagami, the mobility of LREE and Eu could be used for vectoring in exploration, especially Eu which has an anomalous behavior that extends far beyond the limit of the sulfide zone, into the distal sericitic low-temperature alteration (up to 400 m from the ore zones).