CRC LEME
Open File Report 129
ABSTRACT
Alluvial landscapes of the Maronan area, Cloncurry - McKinlay
District, Queensland
M. R. Jones
Location
The Maronan area extends approximately 70 km northwest of McKinlay
and includes the slopes of the Selwyn Range and the adjacent plains
to the east. In the north, drainage is provided by the Williams
and Fullarton Rivers, which have upper catchments incised in the
Selwyn Range. Most of the numerous minor streams in the south only
drain the foothills along the range's eastern margin. The McKinlay
River and its tributary, Boorama Creek, are the other major streams
in the south. Braided channels are common on the plains.
Known minerals in the area include copper, gold, lead, and zinc,
and most are found in outcrops of steeply dipping metamorphics along
the eastern flank of the Selwyn Range. The area contains the Eloise
copper-gold mine, and the Maronan lead-zinc prospect. The Proterozoic
rocks are considered the most likely hosts for mineralisation in
the region.
The Proterozoic rocks form most of the Selwyn Range, but, except
for a few small outcrops, are concealed beneath the plains east
of the foothills. There is very little in-situ regolith over Proterozoic
bedrock. The largely unconsolidated sediments on the plains are
inferred to be of Tertiary and Quaternary age. This regolith cover
is primarily thin alluvium which occurs as a blanket mainly less
than 10 m thick over Cretaceous rocks. The Cretaceous rocks are
as much as 100 m thick, and cover the Proterozoic basement.
Landscape Studies
Field investigations involved inspections at 73 sites across the
plains and foothills. Many of the sites were in stream channels
where good sections of the regolith were exposed in the banks. The
field observations were given a regional context by relating them
to geology maps and to a Landsat TM image of the area.
Geomorphology and Landscape Evolution
The main geomorphic units are:
- Isa Highlands - the Selwyn Range.
- Cloncurry Plain - the narrow foothills zone along the base of
the Selwyn Range.
- Wondoola Plain - the extensive plain east of the ranges and
north of the McKinlay River.
- Julia Plain - the plain southeast of the McKinlay River; this
plain is slightly higher than the Wondoola Plain.
The Isa Highlands have been a long-term source of sediments for
the eastward flowing streams. Weathering products have been eroded
from the bedrock and transported onto and across the plains to the
east and northeast. Fluctuations in sediment supply from the upper
catchments have caused cycles of accretion and erosion on the plains.
Here the surficial deposits comprise interfluve sediments and channel
floor deposits. The interfluves contain fluvial deposits which accumulated
during earlier cycles of catchment accretion. The channel floor
deposits are very coarse sands and gravels washed from the upper
catchments and are now in transit to downstream areas.
Former accretionary cycles built up the plains to form the once
extensive Julia Plain. However with more recent diminishing supply
from the upper catchments, the plains have entered an erosional
phase. Erosion has developed the Cloncurry Plain as a partially
denuded pediment along the front of the Selwyn Range. The sediments
from the Cloncurry Plain now extend downstream in the braided channels
that have incised the Wondoola Plain. A large but thin store of
sediment exists on the plains, mainly as alluvial interfluves. Erosional
remnants from the southward retreat of the Julia Plain are found
in the McKinlay River flood corridor.
Age and distribution of regolith
In this study, no laboratory determinations of the age of the regolith
were made. The ages referred to are estimations, based on the degree
of consolidation and on soil profile development.
Implications for Mineral Exploration
Hydromorphic dispersion of trace elements.
Hydromorphic dispersion of trace elements into the regolith is
likely to produce a detectable geochemical halo if the regolith
is undisturbed for a long time. However, in the Maronan area, the
in -situ regolith on the Proterozoic rocks is generally quite thin.
Most has been eroded from the Selwyn Range and transported on to
the plains where it has been reworked intermittently. The oldest,
and likely to be the least disturbed of the interfluves are on the
Wondoola Plain and on the Julia Plain. Iron staining in some of
these deposits may have continued, following deposition, due to
the breakdown of minerals such as mica. The chemical activity provides
further opportunities for hydromorphic dispersion in these transported
deposits. However, the thick sequence of Cretaceous sediments beneath
much of the alluvial plains probably forms a barrier to upward hydromorphic
dispersion from the Proterozoic rocks. Such dispersion may produce
geochemical haloes in the Cretaceous rocks more readily than in
the younger surficial deposits.
In the upper catchment, the in situ regolith on Proterozoic bedrock
is thin and is mostly alluvium, subject to frequent overturning.
Overall, the prospects for finding geochemical haloes in the Quaternary
regolith appear to be poor.
Mechanical dispersion of trace elements
Diffusion of geochemical tracers by mechanical processes should
be discernible by stream sediment geochemistry. Most of the sand
and gravel supplied from the upper catchments is confined to the
flood corridors crossing the plains, rather than being dispersed
widely. Hence, sources in the upper catchment may be detectable
by sampling in the channels on the plains. Further downstream, trace
elements from sources in the Proterozoic rocks would become too
diffuse for detection. Windows of Proterozoic bedrock such as at
Kevin Downs could be investigated for trace elements and compared
with Selwyn Range samples to determine regional variability between
Proterozoic rocks in the ranges and beneath the plains.
Interpretations
The regional structure of the bedrock in the Selwyn Range is discernible
on TM images. For the most part, the bedrock consists of steeply
dipping metamorphics having a north-south strike, approximately
parallel to the boundary between the plains and the ranges. The
known mineral localities enable along -strike extrapolation of potentially
prospective horizons between the Fullarton and Williams Rivers.
The bedrock strike curves to the north-east, and it can be inferred
that this general trend continues beneath the plains. Amalg Resources
NL have achieved some encouraging results by prospecting along strike
from known mineral occurrences such as Fairmile.
In the area to the south of Fullarton River, it is difficult to
infer the type of Proterozoic bedrock as there is no along-strike
exposure. Mineral exploration is made more difficult by the limited
knowledge of the Proterozoic bedrock concealed beneath the plains.
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