Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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ORE CONTROLS AND OPEN PIT GEOLOGICAL PROCEDURES
AT STEEP ROCK IRON MINES LIMITED
DAVID C. MULDER
Steep Rock Iron Mines Limited,
Atikokan, Ontario
ABSTRACT
The ore controls of the Middle Arm orebodies of the Steep
Rock Iron Range have been well established as the result of an almost
continuous programme of mapping, sampling, and development drilling
from 1945 to the present time, during which period Steep Rock Iron
Mines Limited has shipped a total of thirty—five million tons of ore.
The remarkably uniform stratigraphic sequence of the Steep—
rock Group, which lies within a sedimentary—volcanic sequence of
Archean age, has proven to be the most useful ore control, particularly
with regard to projections on the smaller scale. A major fault system
strikes from 020 to 065 with steep dips mainly to the east; a minor
fault system strikes a fairly consistent 115 with steep dips to the
north and south. Both fault systems are of post—orezone age with the
majority of the vertical and horizontal offsets ranging from 15 feet to
60 feet. Cross—cutting and conformable altered basic dykes of post—
orezone age often occupy planes of weakness, such as faults and strati—
graphic contacts, and are erratically distributed causing considerable
dilution of high grade ore to crude ore in the mining process. Sill—
fication of the Goethite Member is quite erratic on the larger scale,
and produces a type of crude ore which is difficult to beneficiate.
The above ore controls play a very important role in pit
planning, both on the short and long term. Due to the complexity of
the total geological picture, it is continually necessary to gather
new data and reinterpret previous vertical projections. A major underground
development drilling programme, which commenced in 1967 and is
presently nearing completion, is establishing the position of the
major geological contacts at the proposed ultimate pit elevation for
pit planning purposes. Highly successful new techniques in drilling
and identifying rubbly goethitic formations were developed during the
early stages of this progranune. The drilling results provide an
invaluable control when projecting the geology on the vertical plane
below the present pit bottom. Experience has provided invaluable
guidelines in the form of approximate limits of projection with relation
to allowable limits of error. Besides estimating reserves over the
long term, the Geology Department plays a vital role in controlling the
recovery of ore during the daily mining operations.