Titaniferous heavy mineral aggregates as a tool in exploration for pegmatitic and aplitic rare-metal deposits (SE Germany)

Black heavy mineral (HM) aggregates of metallic luster and composed of ilmenite and rutile were named “nigrine” (amount of rutile > ilmenite) and “antinigrine” (amount of ilmenite > rutile). They contain inclusions of, e.g., columbite-(Fe), pyrochlore group minerals, wolframite solid solution series (= s.s.s.), monazite, zircon, Fe oxides and sulfides as well as alteration minerals such as pseudorutile and Fe–Ti–Nb–Ta–Al–P compounds, whose precise chemical composition and mineralogical affiliation cannot be determined. These titaniferous HM aggregates are of widespread occurrence in gneisses and shear zones cutting through them as well as alluvial, fluvial and colluvial deposits at different distances from rare-element phosphate pegmatites within the crystalline basement in SE Germany. “Nigrine” and “antinigrine” may be subdivided into three types which formed at different periods relative to the Variscan tectonic disturbances and which show different qualities as markers for the origin and presence of Nb-bearing pegmatites along the western edge of the Bohemian Massif, SE Germany.Type A developed pre- to synkinematically within or near deep-seated shear zones which formed below 730 °C as early as 321 to 329 Ma. These HM aggregates are poor in Nb and impoverished in accessory minerals. The HM aggregates having developed in shear zones mark the “kitchen” where friction and heating contributed to the formation of felsic intrusive bodies, such as pegmatites and aplites. This type of Ti-bearing HM aggregates may be held as distal proximity indicator.Type B is early postkinematic and enriched in niobian rutile, rife with lots of inclusions, especially columbite-(Fe). It precipitated in the crystalline country rocks at temperatures around 600 °C concomitant with the nearby rare-element pegmatites between 302 and 311 Ma. It is the most proximal member of this group of HM aggregates.Type C “nigrine” is enriched in W and late postkinematic relative to the shearing processes in the crystalline basement. It formed around 470 °C during the same period of time as type C. It does, however, not qualify as a marker for rare-element pegmatites, as it is unrelated to these felsic intrusives.During the late Neogene the Variscan basement was strongly uplifted and many pegmatites were stripped off their roof rocks. As a consequence of this, “nigrine” and “antinigrine” were released from their host rocks and became part of the terrigenous sediments laid down in the drainage systems which upstream cut into the crystalline basement and the pegmatites/aplites. Due to their high resistance to chemical weathering these titaniferous HM aggregates acted as “armored relics” for less resistant minerals such as columbite and pyrochlore. When these titaniferous HM aggregates got decomposed by attrition these mineral inclusions of lesser stability appear in the fluvial sediments further afield from their source than expected, considering the low stability to weathering of single crystals of columbite- and pyrochlore group minerals.“Antinigrine” and “nigrine” may be spotted in the HM suites of drainage systems around pegmatites at a distance of ≤ 10 km with correlation coefficient of RNb–Ti = + 0.42, while in the range 2–5 km RNb–Ti increases to + 0.77, in the range 1–2 km RNb–Ti is + 0.85 and around 1 km from the rare-element pegmatite RNb–Ti stands at + 0.92.“Nigrine” and “antinigrine” are no ideal mineral aggregates to form placer-type deposits of their own due to their variegated mineralogy, excluding some Ta-enriched subtypes in Colombia and Sierra Leone. The variable mineralogy detrimental to its use as a source of Ti, is the strong point as an exploration tool for rare-element pegmatites.