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Near-surface low-grade ore deposits can be mined by using
large mechanized equipment to remove ore that
typically has been fragmented as a result of drilling and
blasting to a size to allow loading onto ore hauling trucks
or a conveyer belt system for transport to an area where
additional crushing takes place.
An open pit mine has stepped or benched side slopes that
will spiral (ramp) down into the ore deposit to allow
downward excavation. Typically a planned thickness of ore
will be mined as a bench in a horizontal extent until waste
rock is encountered. The bench thickness is based on
equipment selected to meet the economic objectives of the
mine plan. Before the economic extent of the bench is
reached, a new bench will be developed by continuing the
downward development of access roads. A large open pit may
have several benches active at the same time. The size of an
open pit will be limited by several factors which commonly
include the grade of the ore, the distribution of the ore,
the ratio of waste rock to ore that has to be removed to get
to the ore, the stability or strength of an excavated slope
to stand without collapsing, and the amount of groundwater
that may have to be pumped to keep the pit from flooding.
Another factor that now has to be
considered for new operations is the extent of reclamation
that will be required. This can vary significantly based on
regional environmental regulations.
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