Integration of geotechnical and structural data from Savage River Mine, Tasmania

Savage River Mine consists of a number of large open-cut workings in NW Tasmania, based on a group of magnetite lenses irregularly distributed within marble, schist and metabasic rocks of the Bowry Formation, Arthur Metamorphic Complex. The ore bodies strike NNE within host rocks that have a near vertical N-S striking schistosity. Historically, the open-cuts have been subjected to small- to large-scale rock failure controlled by the geology and the groundwater regime. The majority of magnetite extraction since 2009 has been from North Pit, the largest and deepest of the open-cuts in the lease area. This research includes geotechnical engineering principles and techniques, and structural geology. The aim is to test whether any advantages can be found by correlating the results from these two fields. The project developed a structural model of three major fault bounded assemblages in North Pit and used this as a basis for predicting the geotechnical performance of future open pit designs. Historical failures have been used to inform the analysis of rock stability in North Pit. The nature of brittle and ductile movement on faults within North Pit was studied using the geotechnical data and conventional structural geology data. Unfavourably orientated brittle features controlling pit wall stability have been identified, geographically located and described. Structure infill has a major impact on pit wall failure with hematite filled structures behaving as a release surface for the majority of recent failures. The terminology of such features in the geotechnical and geological fields has been discussed. Magnetism in the area and the safety issues involved in accessing pit faces has made conventional geotechnical and geological mapping techniques difficult. Therefore, research into alternative mapping methods e.g. photogrammetry, has been carried out. There was extensive use of drill core logging. The use of the geotechnical database to retrieve geological conclusions has been attempted and vice versa. Two large-scale brittle faults bound the three rock assemblages in North Pit; the Eastern Contact Fault (ECF) and the Magnesite Fault. The highest grade ore is located immediately W of the near vertical N-S striking ECF. This causes inherent mining problems due to pit wall steepness and rock quality in the area directly affecting pit wall stability. A quartz, dolomite, talc and graphite rich damage zone is associated with the ECF, ranging from 7 m to 61 m wide along strike. The characteristics and extent of damage is discussed using structural, geological and geotechnical methods and data. Although more difficult to constrain, a zone of similar lithology is present 30 m W of the Magnesite Fault. It is vital that mine planning takes the nature and extent of the damage zone into account when planning the future expansion of North Pit. Pit mapping indicates the presence of two generations of mylonite at the Savage River Mine; one showing a dip-slip, west-block-up movement regime and the other a strike-slip, sinistral movement regime. The foliation orientation is similar for both generations of mylonite (i.e. steeply dipping, N striking), however the older mylonites (Middle Cambrian) have a subhorizontal stretching lineation. These mylonites contain biotite, and locally preserve hornblende and Na-Ca amphiboles typical of high P/T metamorphic rocks in Tasmania (e.g. ferrowinchite). Monazite dating indicates the dip slip mylonites either formed or were reactivated during the Devonian. The high P/T minerals were not found in these more widespread Devonian mylonite zones (low greenschist facies mineral assemblages). Neither of the suites of mylonitic rocks are recorded in the geotechnical database of the mine and the inability to recognise these rocks is a significant issue for the mine geology. Although the strike-slip samples show a stronger mylonitic fabric and includes the presence of ultramylonite, shear sense could also be determined within the dip-slip samples. It is likely that both generations are mylonites were active at low metamorphic grade.