railway east discovery, page-38

The Hamersley Basin of Western Australia holds significant tonnages of detrital iron deposits. Formed by a cyclic weathering and eroding sequence of nominally barren banded iron formation, they may form rich proximal accumulations of iron cemented gravels distal from any obvious
hard-rock iron mineralisation. While generally small, they nonetheless represent excellent economic targets as they are near surface and easy to mine. The relatively high density of iron detritals, when compared with their sedimentary hosts, makes the gravity method the primary method of exploring for these blind deposits. Identification of possible trap sites is
used as a precursor to gravity surveying, with magnetics used as an ancillary method prior to drilling.

Application of the gravity method in this terrain is not straightforward. Detrital trap sites are best developed next to the spectacular cliffs that form the range fronts to the Hamersley Ranges. Precise terrain corrections and use of first vertical derivatives are routinely applied to gravity data in an effort to map every possible gravity high that may reflect the presence of a detrital accumulation. Extensions to Hamersley Iron Pty Limited’s BS2D deposit were discovered by the routineapplication of gravity. This methodology has been used to create a model
by which exploration for detrital deposits continue to the present day.

Economic detrital iron accumulations are a valuable resource of low phosphorous, lump iron ore. One such deposit – Brockman Syncline No 2 Detritals (BS2D) was a very profitable producer
of lump iron ore for Hamersley Iron Pty Limited from 1992 - 1998.

Exploration for detrital deposits using traditional mapping is challenging. Locations of these deposits are independent of the presence of high-grade iron within the adjacent outcropping
banded iron formations. Banded iron formation (BIF) scree carpets the valley floor adjoining the range front, effectively concealing any detrital iron deposits.

Detrital deposits occur as shallow blankets of outwash scree deposited in structural depressions adjacent to iron formation escarpments (McKenna and Harmsworth, 1998). The material is
derived from the erosion of a surface enriched carapace that crusted the escarpments. Cyclic fluids resulted in ferruginisation of the matrix and lowering of phosphorous content, an
undesirable impurity in surface enriched iron ores. Cementation of the base of the detrital pile formed canga, a very hard haematite conglomerate.

The detrital sequences usually overlie the Mount McRae Shale, Mount Sylvia and Wittenoom Formations, which typically occur in the floor of valleys parallel to iron formation
escarpments. In rare cases, Dales Gorge mineralisation and BIF form the basement to the deposits.

Geological unit Average density (g/cc)
Immature Detritals (<55 % Fe) 2.2
Low-Grade Detritals (55 - 60 % Fe) 2.7
Mature Detritals (>60 % Fe) 3.0