Determination of open pit-underground mining limit for deeply depth deposits that have possibility of open pit and underground mining is one of the primary decisions that should be taken in mining design process. The model that has been presented by Nilsson was based on choosing the depth with maximum total net present value (NPV) both open pit and underground mining. In this Project after using some corrections in determination of open pit–underground mining limit and providing some necessary parameters to maximize the net present value, the Nilsson’s model completed and based on corrected model software was developed with visual basic programming language. This software has been used to determination of open pit-underground mining limit of Chahar-Gonbad copper mine with studying of seven alternatives.To supply the necessary input data of software, in the first, the mine deposit was estimated in ordinary kriging method by Datamine Studio of version of 2.1. Then ultimate pit limits related to alternatives that their bottoms are placed in 2378, 2346, 2330, 2314, 2298 and 2250 respectively and with cut off grade of 0.3 in percent were designed by Datamine then ore and waste tonnage related to those were calculated. Based on University of British Colombia (UBC) method and the software that developed for mining method selection based on UBC method and with due attention to in-field experiences about underground mining exploitation, cut and fill stopping was chosen as the best method. One underground mine was considered with cut off grade of 0.5 in percent for each open pit mine. The income and costs were estimated and finally the software import data were prepared.
The results of software indicated that level of 2330 has maximum both open pit (12996 million Rials) and total of open pit and underground (14775 million Rials) NPV, therefore this is either open pit or open pit - underground mining optimum limit. With considering of two concentrate prices in years of 2001 (3285034 Rials per ton) and 2005 (3353218 Rials per ton), the effect of the cut off grade and interest rate variations on this limit were studied. The results showed that depth associated to this limit goes upward when these two parameters increase, but as an increasing of the price, it dose not vary and only NPV associated to alternatives increases.
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