Dolomite-illitic rocks (dolillite) - the product of hydrothermal replacement of carbonate rocks in the Holy Cross Mts., Poland - a possible guide to ores

The dolomites altered to dolomite-illitic rocks were found accompanying marcasite-pyrite-hematite mineralization within the Middle Devonian carbonate rocks in the eastern part of the Holy Cross Mts. (Poland) in the fault zones and their close vicinity. The alteration consist of dolomite recrystallization and replacement by illite, accompanied by small amounts of silica, K-feldspars and disseminated pyrite or hematite. The final product of dolomite replacement by illite are massive, dolomite-illitic rocks unevenly distributed within the unaltered dolomites or in fault breccia. They form irregular metric-sized nest-like bodies most often with blurred boundaries, or in some places sharp delineated veins and lenses. Illite crystallinity, demonstrates its hydrothermal origin. Altered rocks are characterized by elevated contents of REE, Rb, and sometimes other trace metal (Zr, Ti, Zn, U, Th, Ba and al.), compared to unaltered dolomites. The enrichment in REE, Zr and Ti implies their mobility in hydrothermal solutions. The lamprophyre intrusions present in the close vicinity may be considered as the probable source of potassium rich hydrothermal fluids. The peculiar features of dolomite-illitic rocks such as: their composition and occurrence close to the sulphide and hematite mineralization, allow supposing, that they may be a guide to deeply seated unknown ore deposits.