True Open Pits ©Summer 2003 Dr. B. C. Paul

Telling a True Open Pit Strip mines advance across the deposit backfilling pits with new overburden as they advance Strip mines advance across the deposit backfilling pits with new overburden as they advance True Open Pits True Open Pits –Develop in place They may get larger and follow the deposit but they remain open until pit reaches its economic limits They may get larger and follow the deposit but they remain open until pit reaches its economic limits –True open pits may or may not be backfilled with overburden

Tabular Limitations Note that the Strip and Open Pit Techniques so far work for large tabular deposits Note that the Strip and Open Pit Techniques so far work for large tabular deposits What happens when the deposit has a pipe- like geometry?

The Porphyry Deposit Barren Intrusive Decreasing alteration and Mineralization out from center

The Bulls-Eye Exploration Feature has been key to finding copper, moly others

Superimposed Weathering Leached Capping Secondary Enrichment Zone

True Open Pit Mining Leached Capping Secondary Enrichment Zone Developes a Conical pit

Advance of the Open Pit Start by digging off the leached capping Start by digging off the leached capping –(Pre-Stripping) Begin with a small pit near the center and richest part of the deposit Begin with a small pit near the center and richest part of the deposit Pit expands outward with “Push Backs” Pit expands outward with “Push Backs” Pit expands downward by sinking new benches Pit expands downward by sinking new benches Pit expands outward and downward to its economic limit Pit expands outward and downward to its economic limit

The Ajo Open Pit in Arizona They mined around the barren core

Driving the Economic Limit Stripping Ratio Stripping Ratio –As pit gets deeper more of the material above to be removed is low grade or barren mineralization –Will also hit that barren core at middle some time Equipment Factors Equipment Factors –Open pit benches have ramps between –Truck routes from the bottom get longer and longer (and its up hill) – costs increase

Planning Key Open Pit Features Open Pit characterized by series of benches and access roads between Open Pit characterized by series of benches and access roads between Planning the benches Planning the benches –Need to maintain as steep a slope as possible to control the SR Feature is controlled by the height and width of benches Feature is controlled by the height and width of benches

Setting Bench Height The equipment constraint The equipment constraint –Need to keep bench in tact when blasting (if it slumps over it could go over edge or force you to have very wide bench – low slope – high SR) We blast to loosen the material We blast to loosen the material Digging equipment then cuts the softened up material Digging equipment then cuts the softened up material –This means that shovels must be able to dig through the height of the bench Bench cannot be higher than digging height of your shovel Bench cannot be higher than digging height of your shovel

More Setting Bench Height Grade Control Considerations Grade Control Considerations –Benches general dug as a whole on up Can only control selection of high and low grades in two dimensions Can only control selection of high and low grades in two dimensions –If benches are too high the vertical variations may average out and make all ore scoops the same Usually have to have some selective mining to be economic Usually have to have some selective mining to be economic

Bench Width Considerations Bench must be wide enough to catch spills so they don’t cascade into the pit Bench must be wide enough to catch spills so they don’t cascade into the pit –Blasting design can avoid spreading –As a minimum must still be a reasonable catch or stop Bench must be wide enough to support equipment functions Bench must be wide enough to support equipment functions –Must be able to work loading equipment when extracting –Must be able to maintain roadways for truck travel

Determining the Pit Slope Having side of the pit slump in on your equipment will ruin your whole day Having side of the pit slump in on your equipment will ruin your whole day –Have to make the slopes stable Rock mechanics classes play with stereo nets looking at fractures and fracture density Rock mechanics classes play with stereo nets looking at fractures and fracture density –Two tiered process Have one slope between benches And another slope over-all Having a little piece of temporary road Slump is much more acceptable than Loosing the whole side of the pit

Slope vs. Working Space Often slopes will stand At an angle that does not Allow enough room on Benches for equipment to Work. Mining Sequences must allow For pits to develop at a “Working Slope” and then Push back to a final slope as The stripping ratio limit is reached.

Slope Selection Criteria Ultimately slope stability will limit pit slope Ultimately slope stability will limit pit slope When benches are being worked When benches are being worked –Need to have a work area for the shovels and trucks Often the controlling factor Often the controlling factor –Called a Working Slope When benches push back to limit When benches push back to limit –Must still act as a catch –Must support the road network to access the lower levels

Final Pit Slopes Developed when pit has reached limit on that side Developed when pit has reached limit on that side –Won’t have room for equipment to work another push-back Pit Slope may be determined by geologic stability Pit Slope may be determined by geologic stability –Might still be need for road room Some pits SieRita put a hoist on pit wall Some pits SieRita put a hoist on pit wall In pit crushing with steep conveyors In pit crushing with steep conveyors Rail-road seldom used because grade must be so gradual Rail-road seldom used because grade must be so gradual

Design Tricks The slope and bench geometry are controlled by many things The slope and bench geometry are controlled by many things –Don’t know which one will actually control till you get into it Planning the sequence of when and where to push-back and when limit is reached is part of open pit planning lectures Planning the sequence of when and where to push-back and when limit is reached is part of open pit planning lectures