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

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).

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