THE ORIGIN AND ZONING OF HYPOGENE AND SUPERGENE Fe-Mn-Mg-Sc-U-REE PHOSPHATE MINERALIZATION FROM THE NEWLY DISCOVERED TRUTZHOFMUHLE APLITE, HAGENDORF PEGMATITE PROVINCE, GERMANY

Abstract

An aplite containing Fe–Mn–Mg–Sc–U–REE phosphates, some Cu–Pb–Zn sulfides, barite, and U–Nb–Ta–Ti–Fe–Mn oxides was recently discovered near Trutzhofmuhle (THM) at the western border of the Hagendorf Pegmatite Province, Germany. We describe the sequence of phosphate crystallization in six stages of mineralization (I to VI) covering the time span from the Late Carboniferous through the Recent supergene alteration, and six sequences (1a/1b to 5) reflecting the reaction of phosphate-bearing solutions with the gneissic country-rocks (exo-aplitic) and with intra-aplitic rock-forming minerals that formed during crystallization. Age dating was carried out on columbite-(Fe) and torbernite using laser-ablation techniques. Precipitation of columbite-(Fe) and early magmatic phosphates (Mn-rich apatite, monazite) in the THM aplite is correlated with a thermal event around 302 Ma postdating the intrusion of the post-kinematic Flossenburg granite. The sequences 1a and 1b, containing the lazulite solid-solution series, gordonite and childrenite–eosphorite series, reflect late magmatic and early hydrothermal exo-aplitic processes. The late magmatic and early hydrothermal stages of the intra-aplitic sequences 2 to 5 are characterized by triplite, wolfeite, triploidite, an unnamed K–Ba–Sc–Zr phosphate, an unnamed Zr–Sc phosphate–silicate, phosphoferrite, Mn-rich vivianite, and lermontovite – vyacheslavite (?). Complexing agents such as fluorine and phosphate control the formation of Sc phosphates and silicates. In contrast with the neighboring Hagendorf pegmatite, the magmatic and hydrothermal phosphate mineralization of the THM aplite does not contain any Li-bearing phosphates and is very low in F. Rockbridgeite, whitmoreite, “ferrolaueite”, Al-bearing rockbridgeite, mitridatite, “metamitridatite”, kolbeckite and strunzite appear during late hydrothermal processes and weathering. Kolbeckite formed at the transition from hypogene to supergene processes. Its morphology varies from a rather simple combination of faces (platy kolbeckite I) under hydrothermal conditions to complex mineral aggregates (stubby kolbeckite II) produced under weathering conditions. The latest supergene alteration consists of wavellite, beraunite, cacoxenite, strengite, P- and Mn-bearing “limonite”, autunite, Sc-bearing vochtenite, Sc-bearing churchite-(Y) and diadochite. The latter phosphates with predominantly Fe, Al and U in close association with kaolinite are the representatives of supergene alteration, which is related in time and space to the Miocene peneplanation between 4.8 and 6.9 Ma. The boron- and phosphate-bearing THM aplite is not directly linked to any of the granitic plutons nearby, and is not easily classified within the scheme of rare-element pegmatites.