HYDROTHERMAL METAMORPHISM AND TRANSIENT DEPTH FLUCTUATIONS OF THE BRITTLE-DUCTILE TRANSITION DURING SHEAR-ZONE HOSTED IOCG MINERALIZATION

In the Carajás province of northern Brazil Fe-oxide-Au-Cu (IOCG) style ore deposits (e.g., Sossego-Sequeirinho, Salobo, Igarapé Bahia, Alemão) are hosted in shear zones with both brittle and ductile deformation and hydrothermal metamorphic assemblages different from the surrounding wallrocks (greenschist grade Archean meta-sedimentary and meta-volcanic rocks with local gabbroic to granitic intrusions). In contrast, shear zones contain hydrothermal metamorphic assemblages including early sodic (albite-scapolite) or calcic-sodic (actinolite-apatite) assemblages and later potassic and calcic assemblages including K-feldspar, biotite, grunerite, magnetite, and in higher temperature zones fayalite, garnet, and sillimanite. Structural features in the shear zones include mylonitic foliation in the hydrothermal metamorphic assemblage, open-space fill veins and breccia with foliated clasts and a chalcopyrite-rich hydrothermal matrix associated with potassic and calcic assemblages, and mylonitic foliation with strongly deformed sulfides.

A model to explain these features suggests that the shear zones developed in the crustal brittle-ductile transition zone defined thermally by greenschist metamorphic grade in the wallrocks (~10-15 km depth). However, in the shear zones the brittle-ductile transition was elevated, possibly due to shear heating and microscopic advective fluid flow. Shear zone strain features suggest that inter-seismic displacement by ductile flow oscillated with seismic displacement by brittle fracturing and hydrothermal fluid flow. Pre-mineralization calcic-potassic hydrothermal metamorphism occurred during inter-seismic phases with fluid-pressure build-up. Sulfide ore-mineral precipitation occurred during high-strain rate seismic phases with fluid pressure release brecciation. Post-seismic ductile flow displacement then mylonitized ore-phase sulfide minerals. The brittle-ductile transition during the seismic ore precipitation phase is defined by high-fluid pressure (lower mean stress) and high strain rate and was transiently elevated only within the shear zone; the brittle-ductile transition as defined by thermally-activated crystal plasticity remained regionally sub-horizontal outside the shear zone.