SRK undertook fault modelling for rock mass studies in the Deep Mill Level Zone (DMLZ) mine owned by PT Freeport Indonesia in Papua, Indonesia. The fourth phase of the DMLZ is currently being mined at a depth of approximately 1,500 m in the East Ertsberg Skarn System and porphyry deposit. The skarn formed within a carbonate-rich sedimentary sequence on the northern contact of the large Ertsberg diorite intrusion. The induced vertical stresses combined with the competent nature of the rock mass in the Ertsberg diorite intrusion present challenges for cave propagation and rock fragmentation.
Combined underground mapping efforts by SRK and PT Freeport Indonesia in 2018, helped develop a better understanding of the fault patterns and characteristics inside the Ertsberg diorite. Underground mapping identified that the faults in the diorite are typically non-continuous, segmented, and often vein-filled, tight faults (i.e. fault-veins). The faults in the sedimentary country rock are significantly more continuous, and contain weaker fault rock, including gouge and breccia. Detailed drill hole logging from core and core photographs allowed the logging quality and characterisation of faults to be reviewed.
At the depth of the DMLZ the rock mass is more competent, with low joint and weak vein densities. The lack of natural fractures presented challenges for the block cave fragmentation. Blast fracture orientations can be variable, but in the DMLZ these are commonly parallel to pre-existing rock anisotropy defined by the vein systems.
SRK developed a revised 3D wireframe fault system interpretation in 2017 that has been incorporated and continually developed by the mine geologists to keep the 3D structural understanding up-to-date and relevant for mine planning, seismic risk and damage management. Fault modelling for rock mass studies in the Deep Mill Level Zone (DMLZ) mine of PT. Freeport Indonesia SRK provides regular peer review of the fault models as they are developed by incorporation of new mapping and drilling data.
The fault systems have reacted to high excavation stresses, influencing seismicity during the mine development and initial block caving. It appears that some of the seismic events occur on the segmented faults systems at the intact relays between segments that are breached by stress-induced fracturing. These challenges have prompted the team to keep enhancing the quality of data collection and modelling. Caving is currently being assisted through use of a comprehensive hydraulic fracturing program. SRK continues to provide peer review, training courses and input to seminars for PT Freeport Indonesia.
