Reef Adjacent To Structures at TauTona Mine, AngloGold Ashanti South African Operations DE DAVIES Section Manager TauTona Mine, AngloGold Ashanti South African Operations SYNOPSIS The paper describes the extraction of reef adjacent to geological structures in the Carbon Leader Reef Section at TauTona Mine. Traditionally long wall mining has left feasible and economical blocks of ground adjacent to structures when negotiating major geological features. This meant that mining through an up- throw fault, rolling to the reef elevation on the displaced side of the fault left reef in the long wall. High grade areas were abandoned and gold was sterilized. In these tight economic times and with the need to continuously improve safety standards the need arose to develop a technique to extract these blocks economically and safely. It was believed that the structures in these abandoned areas were de-stressed and could now be mined in small volumes at a high grade. The term RATS is an acronym derived from “reef adjacent to structures” and aptly describes the process of identifying and extracting these blocks. The viability of this method was addressed in terms of the mine design, underground investigations and financial risks. The paper concludes with an analysis of the successes achieved to date. 1 INTRODUCTION TauTona Mine is one of the AngloGold Ashanti Southern Africa operations. It is close to the town of Carletonville in the province of Gauteng and about 70km south-west of Johannesburg. TauTona is 46 years old and employs ± 4 000 people. Mining operations are conducted at depths ranging from 1,800m to 3,500m at which the world’s deepest stoping sections are found. TauTona Savuka Ergo Mponeng Western Ultra Deep Levels Tau Lekoa Moab Khotsong Great Noligwa Kopanang Figure 1: The geographical map of TauTona Mine 2 TauTona’s values are: People are our business… Our business is people. Safety is our first value: We place people first and correspondingly put the highest priority on safe and healthy practices and systems of work. We are responsible for seeking out new and innovative ways to ensure that our workplaces are free of occupational injury and illness. We live each day for each other and use our collective commitment, talents, resources and systems to deliver on our most important commitment… to care. We treat each other with dignity and respect: We believe that individuals who are treated with respect and who are entrusted to take responsibility respond by giving their best. We seek to preserve people’s dignity, their sense of self-worth in all our interactions, respecting them for who they are and valuing the unique contribution that they can make to our business success. We are honest with ourselves and others, and we deal ethically with all of our business and social partners. We value diversity: We aim to be a global leader with the right people for the right jobs. We promote inclusion and team work, deriving benefit from the rich diversity of the cultures, ideas, experiences and skills that each employee brings to the business. We are accountable for our actions and undertake to deliver on our commitments: We are focused on delivering results and we do what we say we will do. We accept responsibility and hold ourselves accountable for our work, our behaviour, our ethics and our actions. We aim to deliver high performance outcomes and undertake to deliver on our commitments to our colleagues, business and social partners, and our investors. The communities and societies in which we operate will be better off for TauTona Mine having been there: We uphold and promote fundamental human rights where we do business. We contribute to building productive, respectful and mutually beneficial partnerships in the community in which we operate. We aim to leave host communities with a sustainable future. We respect the environment: We are committed to continually improving our processes in order to prevent pollution, minimise waste, increase our carbon efficiency and make efficient use of natural resources. We will develop innovative solutions to mitigate environmental and climate risks. (Mark Cutifani, Chief Executive Officer – AngloGold Ashanti) 3 Our objective is to mine gold safely at the correct profit margin. We are a team that produces gold by believing that the workplace can be injury free. We are achieving our targets through people, we want to maximise TauTona’s contribution to AGA shareholders by exploring and managing life of mine extension opportunities and striving for continual improvement. As depicted in the following graph, safety statistics for the “RATS” operations from 2005 to 2008 proofs that mining reef adjacent to structures at depth was done safely. Injury Statistics for “RATS” 2005 - 2008 5 4 3 Dressing Cases Loss Time Injuries 2 Serious Injuries Fatalities 1 0 2005 2006 2007 2008 •26 June 2007 Laceration Finger Tools and equipment Serious injury •07 Aug 2007 Contusion Thumb Tools and equipment Serious injury Figure 2: Injury Statistics for RATS 2005 – 2008 At TauTona Mine gold production declined from 2005 to 2008 by 27% to 9 332kg, owing to a greater-than-scheduled decrease in volumes of ore mined. This was a result of increased seismic activity in the vicinity of the CLR shaft pillar which is being mined. Both face length and face advance were negatively affected by seismicity during 2008. (AngloGold Ashanti’s Report to Society, 2008, South Africa: Johannesburg.) At TauTona Mine we believe in continuous improvement and therefore it is of utmost importance to review all our current processes, to find ways to make it even more effective and value creating. The concern we have is that TauTona mine traditionally used the longwall mining method. Our gold production has decreased and a need to create additional face length arose in order to create flexibility. Because of increased seismicity, we had to change our mining strategy. To remain sustainable we decided to develop a method of extracting gold from the reef adjacent to structures without interfering with the current operations, thus enhancing our gold profile. 4 REEF ADJACENT TO STRUCTURES (RATS): The long wall mined through an up-throw fault, rolling to the reef elevation on the displaced side of the fault and leaving reef in the hanging wall. Depicted in the following figures are the original abandoned blocks left by longwall mining and a sketch indicating reef displaced between 5 and 10 leaving reef adjacent to structures. Figure 3: Geological complexity of TauTona Figure 4: Abandoned blocks adjacent to Mine structures Reef left in hanging OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO RATS Reef band Fault OOOOOOOOOOOOOOOOOOOOOOOOOOOOOO Reef band Figure 5: Sketch indicating reef displaced between 5 and 10 metres leaving reef adjacent to structures 5 METHODOLOGY Support design: The support design was based on the Fall of Ground Management Process (FOGM) with specific reference to mine wide design guidelines – FOGM 1 and the Local area design guidelines – FOGM 2. FOGM 1 is aimed at the prevention of rock burst and FOGM 2 is aimed at the prevention of rock bursts and fall of ground. FOGM 1: Mine wide design guidelines used Both modelling and seismic trends must indicate that the planned RATS mining are sufficiently removed from current working places not to have any interaction. No mining into the stressed abutments at the top or bottom of a long wall including pillars. All access ways must be positioned under mined out ground where no stress changes will occur due to the new mining activity. FOGM 2: Local area design guidelines used In order not to mine into existing excavations all stoping and development must be clearly indicated and avoided. To leave crush pillars against major faults. To consider preconditioning of prominent dykes. Design considerations We assumed that shallow mining conditions would prevail as the area had been de-stressed with the original mining. Due to the regeneration of stress in the back areas, the stress would now be zero. Careful modelling shed some light on the expected stress conditions. The initial elastic modelling indicated stresses of approximately 7 MPa. It was thus assumed that the back area stress would be between 0 MPa and 10 MPa. Stope closure is a combination of elastic convergence and in-elastic bed separation – especially in the hanging wall. This bed separation is at maximum close to the stope face and decreases in magnitude further away (into the hanging wall). It was unknown how much of this bed separation would be visible and what affect it would have on the planned mining. 6 Ideally, the support installed must prevent the first bed from falling, thereby supporting the additional layers in the hanging wall. The first layer is however unknown, but drilling support into the hanging wall will improve the integrity of the initial support system design. If drilling is impossible, the support design may have to be adjusted as soon as the stoping has commenced and new information becomes available. Low closure rates were expected, thus stiff active support had to be designed. Because the expected closure rates could not be determined, yielding support was considered. Low seismic rates were probable, but close to abutments the seismic risks increased. The possibility of back break conditions on large spans could not be disregarded. As proven by the platinum mines breaker lines of packs were often used to prevent back break from forming up to the face area. With packs on the gullies and limited panel spans, these breaker lines would not be required. Support design The standard approach is to use the accident statistics for the applicable geotechnical settings at the mine for the past 5 years to determine the fallout thickness to be supported. From this the support resistance and energy absorption criteria can be calculated.