Direct evolution of W-rich brines from crystallizing melt within the Mariktikan granite pluton, west Transbaikalia

Uneconomic tungsten mineralization associated with the Mariktikan granite pluton of the Transbaikalian igneous province, eastern Siberia, is confined to a marginal part of the pluton referred to as the Andreyevsky body. This is composed of rocks similar to those of the main pluton, but is likely to be an autonomous, although kindred, body. On the basis of cross-cutting relationships between rock varieties, and their textural features, the crystallization history is subdivided into three stages of unequal duration. Melt and fluid inclusions related to each of them have been studied using microthermometric and micro-analytical procedures, including a technique for atomic emission spectroscopy of individual fluid inclusions opened by a laser microprobe. During the main crystallization stage (1045–1012 °C) more than 70% of the parental magma is believed to have crystallized (at the level studied), resulting in the formation of a crystalline framework rigid enough for the appearance of fractures within which some portion of the intercrystalline melt accumulated, giving rise to thin aplite veins. Almost complete crystallization of intercrystalline and fracture-hosted melt occurred during the late stage (1012–990 °C), whereas during the final stage only small bunches of schlieric, sometimes miarolitic pegmatite were formed from a few pockets of residual melt (990–917 °C). In spite of the low water content of the melt (about 1 wt.%), fluid separation took place from the onset of crystallization. During the main crystallization stage, the exsolved fluid divided into two immiscible phases, CO₂-rich gas and salt-rich liquid (brine). However, it was homogeneous thereafter. Major ore components of the brine (Mn, Fe, W) displayed different behaviour as crystallization progressed. W concentration was below 0.1 wt.% at the main stage, attained 1 wt.% during the late stage and increased to 1.8 wt.% at the final stage. Comparison of these data with calculations carried out using estimated parameters for the parental magma enables us to infer that obtained concentration values are reasonable for the special case studied here. However, they are unlikely to be attainable if the magma is H₂O enriched and lacks an anomalously high W content. In the Andreyevsky body of the Mariktikan pluton, the following features of the parental magma made possible the generation of W-rich solutions: (1) high liquidus temperature (1045 °C) and elevated Cl content (c.0.15 wt.%) that resulted in enlargement of the fluid/melt distribution coefficient (c.10), (2) low water content (c.1%) and elevated W content (c.0.001 wt.%) that provided a relatively high W/H₂O ratio within the system. At the same time, because of the low water content of the magma, the total mass of W-bearing solutions has proved to be insufficient for the production of large-scale mineralization (a reduced W content of the great bulk of the exsolved brine may have had an unfavourable effect as well). Received: 2 May 1996 / Accepted: 29 January 1997