Item talk:Q155231

Element mobility studies of two drill-cores from the Götemar Granite (Kråkemåla test site), southeast Sweden

A pilot study was carried out on two relatively deep drill-cores (∼ 600 m) from the Götemar Granite massif in S.E. Sweden. This granite is typical of the 1400-Ma anorogenic granites of the northern hemisphere. Samples from representative, unfractured parts of the cores, together with four samples taken along a profile tangential to a fracture plane at ∼ 280-m depth, were investigated chemically, mineralogically and isotopically. The results show that after crystallisation, subtle and pervasive open-system modifications of the trace-element chemistry of the granite took place. Whereas the major-element chemistry and minera-logical data emphasised the relative homogeneity of the Götemar Granite samples investigated, trace elements such as U, Rb, and Pb revealed irregular distributions which are probably the result of large-scale hydrothermal alteration processes. This conclusion is supported by isotopic studies which indicate that whole-rock samples were open to a gain or loss of Pb and possibly U at ∼ 420 ± 171 Ma ago. In addition, isotopic data for U-Pb and U-Ra are consistent with a recent minor loss of U.

The pervasive alteration and the more recent mobilisation of U are evident to a depth of at least 600 m. The effects are most prevalent along major fracture zones and within the upper 250–300 m of one drill-hole where a high frequency of crush zones has been noted. Higher Fe oxidation ratios, higher Rb contents, lower U contents and correspondingly higher Th/U ratios, all characterise this zone.