FORMATION OF ORE MINERALS IN METAMORPHIC ROCKS OF THE GERMAN CONTINENTAL DEEP DRILLING SITE (KTB)

The German Continental Deep Drilling Program (KTB) drilled a 9.1 km de ep profile through amphibolite facies metamorphic rocks and reached in situ temperatures of 265 degrees C. Each lithologic unit is character ized by typical ore mineral assemblages related to the regional metamo rphic conditions. Paragneisses contain pyrrhotite + rutile + ilmenite +/- graphite, metabasic units bear ilmenite + rutile + pyrrhotite + py rite, and additionally, the so-called variegated units yield pyrrhotit e + titanite assemblages. In the latter unit, magnetite + ilmenite + r utile + titanite assemblages related to the lower amphibolite facies b reakdown of ilmenite-hematite solid solution also occur locally. Retro grade hydrothermal mineralization which commenced during Upper Carboni ferous times is characterized by the following geochemical conditions: (1) low salide Na-K-Mg-CI fluids with sulfur and oxygen fugacities at the pyrite-pyrrhotite buffer and temperatures of 400-500 degrees C, ( 2) fluids with CO2, CH4 +/- N-2, and pH, Eh, sulfur, and oxygen fugaci ty in the stability field of graphite + pyrite at temperatures of 280- 350 degrees C, and (3) moderate to high saline Ca-Na-Cl fluids with CH 4 + N-2: sulfur and oxygen fugacity are in the stability field of pyrr hotite at temperatures <300 degrees C. The latter environment is confi rmed by in situ conditions found at the bottom of the deep drilling. M onoclinic, ferrimagnetic pyrrhotite is the main carrier of magnetizati on which disappears below about 8.6 km, corresponding to in situ tempe ratures of about 250 degrees C. Below this depth, hexagonal antiferrom agnetic pyrrhotite with a Curie temperature of 260 degrees C is the st able phase. Temperature-dependent transformation of pyrrhotite and the reaching of its Curie isotherm within the Earth crust are one of the striking results of the KTB deep drilling project.