Środowisko sedymentacji serii węglanowej Z1 a mineralizacja osadów cechsztyńskich (rejon Zielonej Góry – Kożuchowa)

Abstract

Omówiono wykształcenie mikrofacjalne skał węglanowych pierwszego cyklotemu, podano wstępną interpretację paleośrodowisk oraz zrekonstruowano przebieg sedymentacji utworów węglanowych Z1.Wykazano wpływ mikropaleoreliefu na przestrzenne rozmieszczenie facji sedymentacyjnych i geochemicznych oraz omówiono warunki i sposoby koncentrowania się materii organicznej i metali. Wskazano na znaczenie migracji roztworów utleniających w modyfikowaniu osadów i rozszerzaniu zasięgu strefy utlenionej. SEDIMENTARY ENVIRONMENT OF THE Zl CARBONATE SERIES VERSUS MINERALIZATION OF ZECHSTEIN DEPOSITS (ZIELONA GÓRA–KOŻUCHÓW REGION) Several microfacies (Figs. 2–3) were recognized in carbonate series of the first cyclothem of the Zechstein (Z1 in the Zielona Góra - Kożuchów region (Fig. 1). Recognition of sedimentary environments of these microfacies and analysis of their spatial distribution made it possible to define facies and paleogeographic zones (Figs. 4 and 5, respectively) and to reconstruct the sedimentary environment and history. A special attention was paid to interrelations between the relief, hydrodynamic conditions and components of carbonate rocks on the one hand, and distribution of organic matter and metal compounds in basal part of the Z1 carbonate series on the other.Taking into account microfacies development of lower part of the Zl carbonate series (micrite complex) and evolution of the basin in times of sedimentation of Saxonian rocks and those of upper part of the carbonate series (supra-inicrite complex), two paleogeographic units – shallow and deep parts of shelf – were identified in the studied region. Moreover, several local elevations and depressions were found in shallow part of the shelf (Fig. 6).The elevations are characterized by condensed profiles of the Zl carbonate series, comprising deposits formed in shallow-littoral environment and displaying low geochemical variability (Figs. 2, 7).The results of studies implicate syngenetic relations between the oxidized zone and areas of subaqueous elevations (see Figs. 4 – 5).Complete profiles of the Z1 carbonate series are found in depressions in shallow part of the shelf.Micrite complex is there represented by clay micrites with bioclasts and carbonate particles of different origin and by deposits stabilized by blue-green algae (Figs. 3, 8). It seems that in the Kożuchów area the conditions turned to reducing in times of deposition of upper part of the ZI carbonate series, due to decay of organic matter accumulated in algal mats (the boundary between oxidating and reducing zones was passing somewhat below the sediment-water interface).Marls and micrites laminated with clay-bituminous matter and with rich sulfide mineralization were originating in deeper part of the shelf (Figs, 2, 6). In that area, reducing conditions began to predominate from the beginning of sedimentation of the Z1 carbonate series, due to accumulation of organic compounds in stagnant near-bottom water zone (zero level of Eh values was passing above sediment-water interface at that time). It also means that the Copper-bearing Shale horizon should be identified with reference to reconstructions of the position of the boundary between oxidating and reducing environment in relation to sediment surface (Figs. 5, 6, 7, 8).The presented interpretation implicates relatively high differentiation of paleorelief and, at the same time, small denivellations of sea floor of the Zechstein basin. The evolution of the relief, taking place due to differentiated subsidence and burial by accumulation, was bearing a decisive influence on development of sedimentary environments, paleogeochemical facies and polymetallic mineralization.The processes of oxidation of sulfides were found in several boreholes situated in the proximity of the above mentioned elevations, which suggests that external areas of occurrence of oxidated deposits are related to subsequent stages of diagenesis and/or epigenesis connected with ascension of oxidating solutions. The zone of secondary oxidation (Fig. 5) appears related to synsedimentary activity of WNW – ESE oriented faults. It is not excluded that tectonic structures were also acting as migration routes for mineralizing solutions.