Abstract:The Mount Morgan Mine is a historic mine site located in Central Queensland, Australia. Between 1882 and 1981, a total of 7.6 million ounces of gold and 360,000 tonnes of copper were extracted using underground and later open mining methods. The mine closed in 1990 after the re-treatment of 28 Mt of tailings, which were placed into the open cut. Most of the mine waste is acidgenerating and acidic runoff and seepage has heavily impacted portions of the adjacent Dee River.A groundwater flow model was developed for the Mount Morgan mine site to evaluate current seepage conditions and assess closure options. The calibrated groundwater flow model indicates that the backfilled (and flooded) Open Cut/Sandstone Gully represents the largest single source of ARD seepage (8.0 L/s) on the site with tailings impoundments representing important secondary sources of seepage. An estimated 80% of all seepage is collected in a seepage interception system (SIS). The remaining 20% (or ~3 L/s) of ARD impacted seepage by-passes the SIS and enters the Dee River and underlying aquifer.The model predicts that seepage from the open cut would increase exponentially with a further increase in the water level in the open cut. The model suggests that a grout curtain or sealing the upstream side of Sandstone Gully Dam using a "blanket" of low permeability tailings would reduce seepage out of the open cut by about 40%. The amount of seepage reduction in response to placing a dry cover system onto mine waste (tailings and mine rock) is predicted to vary significantly across the site. The modeling results suggest that a combination of rehabilitation measures (including the placement of dry cover system and measures to control seepage out of the flooded Open Cut/Sandstone Gully) will be required to effectively control seepage at Mount Morgan. The calibrated groundwater flow model is currently being used to assess the effects of different closure scenarios (e.g. cover placement versus full relocation) on seepage rates and loading to the Dee River.
The International Council on Mining and Metals (ICMM) is an international organisation that brings together 27 mining and metal companies and over 30 regional and commodities associations to strengthen environmental and social performance and enhance mining's contribution to society. The ICMM Closure Working Group, made up of closure specialists from ICMM company members, developed an information document on the financial concepts for mine closure for use by ICMM members and the wider industry to communicate and enhance the understanding of key financial concepts as they relate to mine closure. Several types of cost estimates are used to characterise the financial aspects of closure. As understanding of these approaches vary, consistent communication and engagement within companies and with external stakeholders is challenging and limits the industry's ability to benchmark performance. It is important for mining companies to make clear distinctions between the different types of cost estimates as they serve different purposes. While much of the underlying source information for each of these estimates should be based on the most recent version of the mine closure plan, the numerical values are typically different as each responds to different user needs and will be prepared under different sets of frameworks. Understanding the differences and points in common for these types of estimates is key to the efficient use of information and to providing useful estimates. The document defines and contextualises the key concepts related to closure costing, accounting and reporting requirements, and the purpose of each. The document is intended to provide general conceptual guidance across a wide range of factual circumstances.
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