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Open File Report 129

Alluvial landscapes of the Maronan area, Cloncurry - McKinlay District, Queensland

M. R. Jones


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.


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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