U-Be-bearing and Mn-ore associated posphate mineralization of the rhyolite (quartzporphyry)-tuff, at Bükkszentkereszt (NE Hungary)

Abstract

This paper concerns the U-Be (REE)-bearing phosphate bodies — associated with the Mn-oxide ore known in the Ladinian rhyolite (quartzporphyry) tuffs, near the village of Bükkszentkereszt — which are relevant for the results of
prospecting performed by the Mecsek Ore Enterprise (1967–1973). New geological, petrographical and tectonic details have also been dealt with. In order to achieve this, the manuscripts of numerous reports are summarized briefly.
The phosphate–Mn-oxide bodies can be found along the fractured, cataclastic tectonic zones, which occur in a NNW–SSE direction. They occur in a mildly pressed, anchimetamorphic, mafic/acidic volcanic series, built from lavaflows and tuffs. Underlying, overlying and intercalated limestone beds belong to the series.
The main rocks present in series are microfoliated shales, and slates with a porphyroblastic structure. Originally their main components could have been acidic, partly aleuropelitic pyroclastite and pelitic sediments. The outcrops of the phosphate–Mn-oxide bodies have been investigated in the locality of a great exposure (No 40), near the „Heroes' Spring”. The area surrounding this spring has been penetrated by two shafts and numerous shallow drillings (Figures 3–6). As a result of the project, it can be declared to represent a complex U-Be (REE)-P-Mn accumulation, with some industrial concentrations (although the respective sizes of these depend on the exact location).
The phosphatic and Mn–oxide-bearing tuffaceous rock-formed bodies have irregular thicknesses, as well as different configurations, bands, and lenses. The bodies are limited in their relation to the wall rock by their sharp contact and (micro) brecciated zones. Their respective structures represent symmetric or assymetrical (incomplete) alterations of the phosphatic and Mn-oxide microfolded bands (Figures 3–6, Plates II–III).
The acidic tuffaceous series has suffered from repeated weak regional tectonic, and significant hydrothermal metasomatic effects (e.g. silicification, K-Na-migrations, phosphatic and Mn-oxide mineralization — see Figures 3–7). The inner parts and the neighbouring rocks of the P-Mn metasomatic bodies can be characterized by secondary quartz, feldspar (e.g. albitization), and hidromica-illite-chlorite mineralizations (Plate I).
The main rock-forming minerals of the phosphatized rock-bodies are the cryptocrystallic-microcrystallic, and spherulitic collophane aggregates. The phosphatization underwent polymineralic, amorphous phases, and these were completed by semicrystalline ones: e.g. carbonate - F-apatite (francolite, dahlite), hydroxyl-apatite, and Mn-apatite.
The following opaque Mn-minerals are present: psilomelane (manganomelane), cryptomelane, ramesdellite, manganite, todorokite, vernadite, pirolyte, and gelic-like, dendritic, needle-radial spherulitic precipitations may occur on the phosphatic field and the ore bands (Plates II–IV). As accessory minerals, Fe-hydroxyl phosphate and, Fe-oxidehydroxide minerals may also be present.
The geochemical evaluation of the analyses carried out clearly indicated a close correlation between P-V-Be-REE contents (Tables 1–2, Figures 7–9).
The respective correlations between these elements with MnO2 proved to be negative and very loose. The alpharadiographs of the U-contents indicated dense and unequally dispersed alpha-tracks inside the phosphatic fields, but the
Mn-oxide ones were shown to be barren (Plates III–IV). These facts suggest that U, Be- and REE, are built into the lattice of the apatite. On the bases of radiographs (Plate III, 1; IV, 5–6) and optical microanalyses the presence of the following
minerals can only be presumed: U-oxide (“U-black”), uranophane, bertrandite and/or berillonite. The Pb-isotope ages of the U-accumulations (47±13 Ma) conform to the “fission track” age of apatite (47,7 Ma). The mildly
anomalous U and U-barren background ages (90–135 and 165–220 Ma, respectively) conform to the age of metamorphic and early metasomatic actions, and acidic volcanism. The latter could be determined by the Rb/Sr (98,7 Ma) K/Ar (79 and 121,6 Ma) “fission track” ages of zircon (89, 5 and 122,9 Ma). (See Table 3, Figures 3, 10–12 and the bibliography.) The supposed genetic processes occurred inside the volcanogenic series. This suffered regional metamorphism, and
metasomatic-fault tectonics-belts (with fractured zones) were raised due to the tectonic movements. Rejuvenation of the volcanism (owing to the tectonics) brought submarine, hydrothermal exhalations. The latter comprised solutions