In the present paper the morphological description, mineralogical and chemical composition of diagenetic carbonate formations occurring in low-grade oil shale, known as Tremadocian graptolitic argillite or Dictyonema shale, is given. The role of these formations in the redistribution of several microelements during diagenesis is discussed. The results obtained permit us to draw some conclusions about the mechanism and time of authigenic minerals formation.