A. Kontny et al., FORMATION OF ORE MINERALS IN METAMORPHIC ROCKS OF THE GERMAN CONTINENTAL DEEP DRILLING SITE (KTB), J GEO R-SOL, 102(B8), 1997, pp. 18323-18336
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.