Mineralizacja syenitów ełckich w strefach przeobrażeń metasomatyczno-hydrotermalnych (NE Polska)

Abstract

Przedstawiono historię badań ełckiego masywu syenitowego, rozwój poglądów na jego budowę i obecny stan jej znajomości. Na tym tle omówiono kompleksową mineralizację U - Th, Nb, TR i Zr w strefach przeobrażeń metasomatyczno-hydrotermalnych, porównując ją z mineralizacją w innych masywach tego typu.MINERALIZATION IN ZONES OF METASOMATIC-HYDROTHERMAL ALTERATIONS OF THE EŁK SYENITES, NE POLANDThe Ełk syenite massif has been found by a drilling in 1954. In geophysical image, it is marked as a negative gravity anomaly almost circular in outline, coinciding with a set of magnetic anomalies of the ring type. Detailed seismic surveys and subsequent parameter drillings made possible preliminary reconstruction of its structure, petrographic-mineralogical composition, radiometric age, geochemical features, metallogenic specialization and deposit perspectives. This is a polyphase, ring-like, ply separated platform intrusion with erosionally truncated Mesozoic subcrops about 400 km2 in area and top surface situated at depths over 800 m. The intrusion was formed in result of repeated tectonic-igneous activity, presumably in 5 cycles. The cycles were connected with the following. Events: ring intrusions of alkaline granites, quartz syenites, microcline syenites, sodalite and nepheline-bearing micropertite syenites with numerous phases of microsyenites and lamprophyries. Advanced tectonics (faults and fractures) facilitated origin of zones of metasomatic-hydrothermal alterations, acting as traps for U - Th, Nb and TR mineralization. Such mineralization was found by drillings Ełk IG 3 (where rocks of the agpaitite series predominate) and Ełk IG 4 (series of miaskite rocks). The alteration zones are characterized by increased natural radiation. Anomalies are of the mixed (U - Th) or, sometimes, either uranium or thorium in character. The latter may be treated as indicative of TR and, sometimes, partly Nb mineralization. Gamma logs and results of gamma-spectrometric measurements may, therefore, give indirect characteristics of mineralization.