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Mechanized cut-and-fill mining as applied at the Homestake Mine Abstract-TheHomestake Mine,located in Lead,SD,has been in business for 113 years.Until 10 years ago,conventional mining methods were used to extract one. Due to complexities in the ore bodies and integral part of this technology was mechanized cut-and fill mining .This mining method currently produces about 50% of the annual production and has proven to be safe and economical.AS a result, the Homestake Mine remains competitive in todays market. IntroductionThe Homestake Mine, located in the Black Hills of South Dakota,has a long history dating back to 1876, In the 113 years of its existence ,a variety of mining methods have been applied to the ore bodies. Some of the more traditional methods included open cut-and-fill, shrinkage, aquare-set,block caving and blasthole sublevel mining.Due to increasing costs and ore body complexities ,new and innovative mining methods had to be developed. As a result, mechanized cut-and-fill was introduced to mine erratic ore zones and ore areas with dips too shallow effective recovery using vertical crater retreat mining techniques.Mechanized cut-and-fill mining consists of interconnecting a series of small ore zones accessible with a common ramp. This affords the flexibility of multiple ore headings to ensure consistent production and high equipment use .GeologyThe Homestake ore body is comprised of metasediments alterd through progressive regional metamorphism.Three main Precambrian rock formations consisting of the oldest Poorman formation, the intermediate Homestake formation, and the youngest Ellison formation, are intricately folded into a series of synclines and anticlines plunging 10 to 80 southeast and dipping 65 to 70 northeast on the average. Additionally, Tertiary rhyolites and phonolites resulting from intrusive activity cut across all three formations.The Homestake formatiom,which hosts all ore-grade mineralization,is a sideroplesite-grunerite-quartz schist. Ore bodies are replacement pipes oriented parallel to fold plunges and the gold is erratically associated with quartz , arsenopyrite and pyrrhotite. These ore zones, which strike 35 northwest ,can range from 3 to 30 m in width and 9 to 152 m in length .Mineralization may commonly extend for 91 m down 50 to 80 dips . Compressive strengths vary from over 206.8 Mpa to 131 Mpa in lower levels where highly fractured formation is characteristic. Ore grades are determined by extensive diamond core drilling and secondary wet-sludge sampling for each stoping block.Mechanized cut-and-fill stops design Ore zones are initially delineated by diamond drill holes on 30 m intervals . Once ore is intercepted,drill hole spacing is reduced to 15 m to define the limits of the mineralized zone. A computer program processes the information realized from the diamond drilling and produces a cross-section depiction of the hole date.Geologists then interpret the information to quantity the size and value of the ore body .This information is converted from cross section to plan view and lift maps are constructed on 3-m intervals. The lifts are digitized into separate layers on a computer-aided design system and can be displayed as needed for planning purpose.Next, the cross sections and lift maps are reviewed by the planning engineer. Due to safty and productivity advantages , geologic information is initially evaluated for the feasibility of vertical crater retreat mining . If the ore is not amenable to VCR mining , MCF is incorporated.The mechanized cut-and-fill method requires evaluation of : safety, logistics, ore body configurations , geotechnical considerstions , quality of geologic information and economic viability.Main mine levels are established on 46-m intervals . From these levels, a footwall ramp ranging from 0% to +18% is designed to effectively access the geologic reserves as mining progresses up or down dip . An extraction boreholes is designed along the vertical extent of the ore zone to allow access to various mining horizons. Ventilation considerations to minimize air recirculation are incorporated and may include boreholes that allow dumping of waste rock into previously mined areas .Development costs required to bring the ore body into production are calculated and applied to a cost comparision graph to determine the economic viability of the ore zone . The cost comparison graph is constructed using general equations derived from the revenue generated by a specific geologic inventory and the mining this inventory . The equations are as follows :Money available for development =(Revenue generated)-(Cost)-(Profit) and Feet of development =(Money available)+(Cost per foot)Where :A) Revenue generated considers:1) Variables a) Tons b) Grade c) Price per ounce of gold 2) Constants a) RecoveryB) Costs :1) Site specific constants a) Mining type-location cost per ton of ore b) Location cost per foot of development 2) Mine-wide constants a) Dilution factor b) Contingency factor to account for unkowns 3) Variable tons C) Margin of profit where:1) 0% indicates additional ounces are required to cover overhead 2) 10% is a minimum return necessary to continue on a long-term , mine-wide basis.3) 20% is the optimal return on most mining ventures because of risk .Summary of mechanized cut-and-fill mining cycles Mechanized cut-and-fill mining follows a standard drill, blast and muck cycle and the availability of several mining headings is integral to the success of this system . The mining sequence follows .DrillingDrilling in MCF stopes is accomplished with rubber-tired, two boom pneumatic jumbos .Currently ,Jarvis Clark MJM 20-B and Garder Denver Mini-Bore jumbos mounted with Tamrock E-400 drills are used in production drilling and rock bolting applications.After the initial cut is made , the preferred method of breaking the rock is breast mining . The breast slices range from 4 m to 5.5 m in vertical height and 3.7 m in length . The width varies depending on the ore body .Most development heading are 2.7-x 2.7-m arched .Blasting Upon completion of drilling ,a pneumatic loader is used to clean the holes and place an Ammonium Nitrate Fuel Oil mixture into the holes . Non-electric caps with 8 g boosters and detonating cord are used to initiate the ANFO. Ignition of the rounds occurs at the end of each shift to allow time for smoke to clear. MuckingMucking the broken rock is accomplished using 2.7 m3 or 1.5 m3 diesel-powered load-haul-dump units . The smaller machines are used in narrow ore blocks . Development rock is moved to mined out areas for backfill and ore is trammed to extraction raises .Ground controlThe Homestake ore body is characterized by a variety of ground conditions resulting in various support techniques throughout the mine . These techniques include :Driving development heading with an arch ;Miniing overbreak with perimeter blasting methods ;Application of rock bolts to support exposed ground . These bolts include split set, expansion shell , swellex and cablebolts;Pressure grouting in water problem areas .The raveled ground encountered at the Homestake Mine lends itself to the application of friction stabilizers of which split set bolts account for 90% of total bolt consumption . Cablebolting is used to supplement rock bolting in situations where ground conditions require extended support beyond the influence of the standard friction bolts .A standardized bolting pattern is applied throughout the mine and consists of 1.5 m bolts on a 1.8-x 1.8-m pattern with a 2.4- bolt installed in the center . The miners and supervisors decide if this pattern is sufficient. For permanent openings , galvanized bolts and 1.2-x 2.4-x expanded metal mats are applid .The majority of bolting is accomplished with pneumatic hand-held jackleg drills. Jumbos are used for bolt installation as a second priority to production drilling.Ore and waste handing LHDs tram ore to 2 m-diam bored raises that are intercepted at progressive elevations to minimize haulage distances as mining horizons advance upward. All-steel chutes are constructed at the base of the raises to facilitate loading of ore into 2.7-or 4.5-t rail granbys . The ore is then hauled to the shaft and hoisted in 9-t skips to the surface for treatment.As in any mining method,not all rock is breaken to the proper size for the extraction system . To minimize the affects of these boulders , secondary blasting , slanted grizzlies and hydrautic rockpicks are used . Secondary blasting consists of redrilling and blasting the boulders . Slanted grizzly bars are constructed at dump points to allow the proper sized material to pass through . The slant provides a surface to drag the oversized rocks off the grizzly and allow secondary blasting . In high tonnage areas , hydraulic rockpicks and grizzlies are used to produce propertly sized material.To maximize ore tonnage skipped , development rock is absorbed in the mining cycle and not skipped . Waste material, supplied through ventilation boreholes during the backfilling cycle ,is used to fill previously mined areas in MCF stops .The voids are capped off with sand backfill and filled to within 0.3 m of the back to allow expansion for blasting in the next breast cut . The sand backfill provides a smooth mucking surface and aids in minimizing dilution during the subsequent lift of the mining cycle.VentilationAdequate ventilation is critical for cooling work ares and maintaining fresh air for diesel equipment operation. A minimum quantity of 0.073 m3 /sec per rated engine KW is provided in the mining areas .MCF stops are normally ventilated by a combination of flowthrough and auxiliary practices .Boreholes,reamed from a mine level above the stope, provide an upcast exhaust air passageway . Regulators may be necessary in main ramp systems to ensure airflow passes through mining areas .Auxiliary fans or coolers are necessary for dead-end headings .One 30 KW centaxial fan is capable of ventilating three headings by wying duct into each heading . This is effective when the duct going to the inactive working areas is regulated.Mining has progressed to deeper , hotter areas of the mine. Consequently, ventilation requirements have increased. To address this situation , three air conditioning methods are employed:Chilling service water supplied to working areas to lower rock temperatures and provide cooling to swamp coolers ;Underground bulk air cooling ; and Portable air conditioning units .Application of these cooling methods enables maintaining temperatures between 27 and 29 wet bult in areas with rock temperatures up to 56.Services To minimize maintenance down-time , mobile equipment is serviced in underground shops located within easy access to mining areas and supply routes . These service areas used for major repairs , equipment servicing and parts .Fual and oil storage areas are conveniently located and allow storage of 3785 L diesel fuel tanks and several 190 L drums of oil . These areas are designed to contain the total volume of fluid assuming all vessels rupture simultaneously.Both types of the aforementioned areas are constructed with fire doors and fire suppression sprinkler systems . Further precautions include Ansul fire protection systems mounted on the major pieces of mobile equipment .Grade control Currently , MCF stopes are mined at a 4.5-g/t cutoff . control of mining areas is achieved with applixcation of the following :Pick and grab samples-This involves collecting face chip samples or representive grab samples from a muck pile :Test holes These samples are obtained by collecting cutting every 1 m from a 2.7 m hole drilled in the wall or face ;Longholes Holes ranging from 6 to 18 m are drilled with cuttings collected every 1.2 m ;Back samples 35 holes 3.6 m deep are drilled using a standard pattern in the back and cuttings are collected every 1 m ;and Visual face inspections The active mining face is inspected on a regular basis by supervisors and geologists to aid in defining mining lines .ProductionFor comparison purposes , a production history of MCF and VCR mining due to the gradual decline in size and continuity of individual o