Nigrine - a heavy mineral aggregate indicative of pegmatites and their siliceous root zones

摘要

Nigrine is a black mineral aggregate composed of rutile (often enriched in Nb and/or W) and ilmenite (enriched in Mn). Nigrine occurs as heavy mineral grains in alluvial-fluvial drainage systems in areas underlain by aplitic and/or pegmatitic quartz-feldspar rocks. Three types of nigrine were established in the NE Bavarian Basement; they provide an insight into the unroofing story of bedrock deposits. Type A nigrine originates from the deeply eroded siliceous root zone of the pegmatites. This type formed in mylonitic quartz bearing shear zones in the course of pre- to synkinematic nigrinisation. Postkinematic type B nigrine originates from the highly differentiated parts of the pegmatite stocks abundant in rare metal concentrations. Postkinematic type C indicates the passage into the scheelite and wolframite mineralisation at the margin of a pegmatite district.