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 POLAND

The 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.