The rocks in the Dannemora area constitute the Dannemora Formation, which can be divided into eight different sections, A-H, on the basis of stratigraphy and geochemistry. The great thickness and alteration pattern of the deposit indicate deposition in a caldera environment, related to both intensive and waning volcanic stages and hydrothermal processes. The partly strong alteration is shown by the sericite dominated matrix and the completely sericite replaced plagioclase and volcaniclastic clasts. The northern part of the Dannemora area in general shows a lower degree of alteration than the central part in comparable stratigraphic levels, which implies that these parts were located in different positions to hydrothermal cells.
A lapilli-bearing ash layer is present between the massive pyroclastic beds in section A and C that have distinctly different alteration patterns suggesting that the strong alteration in section A occurred before deposition of section C. During the waning volcanic stage, the calmer environment allowed stromatolitic limestone interlayered with thin horizons of ash-siltstone to deposit. During subsequent dolomitisation of the limestone the interlayered ash-siltstone was affected by great enrichment of CaO and depletion of MgO. Despite this strong alteration this section is classified as least altered by alteration index, which most likely reflect a two stage alteration with initial alkali enrichment, followed by CaO enrichment that “reset” the alteration index. The great enrichment of Ba in the upper member is supported by the occurrence of hyalophane. On the basis of lithogeochemical signature of immobile element ratios together with the reoccurrence of an accretionary lapilli-bed the position of a previously inferred anticline has been established.