Open File Report 120
Geochemical exploration in regolith dominated terrain, North
Queensland (CRC LEME - AMIRA P417 Final Report)
R.R. Anand, S.J. Fraser, M.R. Jones, Li Shu, T.J. Munday, C.
Phang, I.D.M. Robertson, K.M. Scott, P. Vasconcelos, J.E. Wildman,
Extensive, variable, and generally thick regolith is a major impediment
to mineral exploration in parts of northern Queensland, as well
as in many other parts of Australia. The Mount Isa and Charters
Towers - north Drummond Basin regions host numerous base-metal and
gold deposits and have a long history of mineral exploration. The
well-exposed parts of these regions have been effectively explored
by traditional methods. Exploration is now concentrating on areas
obscured by deep weathering or by transported cover.
The main aim of this project was to develop suitable geochemical
exploration methods for regolith covered areas based on an improved
knowledge of the nature and evolution of the regolith and landscape.
To achieve this, this project undertook regolith-geochemical dispersion
studies in a variety of geomorphological and sedimentary environments
with particular emphasis on distribution, characteristics and origin
of the regolith. The activities of the project ranged from regional
to district-scale investigations, with more detailed studies at
specific sites or prospects. Sites were selected to address specific
problems and, in many cases, mapping was extended to place them
in their regolith-landform context. The outcomes of district-scale
and specific studies are available as investigation reports. The
purpose of this report is to summarise the results and to develop
models, conclusions and recommendations. Regolith maps and geochemical
data are available on a compact disc (Appendix III).
MT ISA REGION
As an aid to geochemical dispersion studies, a regolith-landform
framework was established for the Mt Isa region. There has been
complex erosion, deposition and weathering during the Mesozoic and
Cainozoic, forming complex landscapes and regoliths. Mesozoic sediments
were deposited on a land surface of broad river valleys with low
hills and interfluves. By the early Cainozoic most of the Mesozoic
sediments had been eroded except in the south-east and north of
the region studied. Field relationships and dating of Mn oxides
strongly suggest that evolution of the weathering profiles spans
the Tertiary, possibly extending into the Cretaceous. Weathering
of Cambrian, Mesozoic and Proterozoic bedrocks left lateritic profiles
capped, in places, by ferruginous or siliceous duricrust. The depth
of weathering varies and is controlled largely by landscape position,
bedrock, structural features and any overlying sediments at the
time of weathering. Palaeoplains and topographic lows are more deeply
weathered than the erosional plains and hill belts. At many locations,
Proterozoic bedrocks are weathered to greater depths where overlying
Cambrian or Mesozoic sediments have been removed or were never deposited.
In places, remnant river channel and sheet wash deposits have been
silicified and ferruginised.
Massive, fragmental and nodular duricrusts have formed in situ
on Fe-rich weathered rocks by accumulation of ferruginous materials
from mottled saprolite and were left by down-wasting of the profile
as clays and soluble elements were removed. Slabby duricrust formed
on lower slopes by induration of locally derived colluvium and saprolite
with lateral accumulation of Fe. Slabby duricust can be distinguished
by its landscape position on plateau edges, micromorphology (platy),
geochemistry (Mn and P-rich) and its goethite-rich mineralogy.
Silcretes have formed on a variety of bedrocks but are most common
on siliceous materials. In places, silica has cemented alluvial
sands or sheet wash sands and gravels. Silicified alluvial sands
and gravels now occupy topographically higher areas, because of
relief inversion since induration.
The plains feature variable thicknesses of Cainozoic and Mesozoic
sediments, underlain by weathered or fresh Proterozoic bedrock.
Soils have formed on fresh and weathered Mesozoic and Cainozoic
sediments and Proterozoic bedrock. Lithosols are associated with
resistant rocks, areas of high relief and steeper slopes. In depositional
areas, the soils vary from black through brown to grey sandy clay,
sands or clays and generally contain polymictic gravels. Some soils
have been weathered (mottling, silicification) since their deposition.
Black and brown soils have developed extensively on Cainozoic and
Mesozoic sediments. Black soils were developed progressively from
brown soils where the alluvium was fine, water was retained and
kaolinite was transformed to smectite.
Recommendations for exploration practice
Several geochemical sample media were demonstrated to have specific
application in exploration in the Mt Isa region. Appraisal of geomorphology
and regolith at a district scale is an important pre-requisite for
efficient exploration of a regolith-dominated terrain. Regolith-landform
maps and regolith stratigraphy should guide the selection of sampling
media, sample interval, sampling procedure, analytical method, element
suite and data interpretation. A regolith 'fact map' is produced
to describe regolith materials in a landform framework and to divide
these broadly into duricrusts, saprolites and colluvium-alluvium.
Each, with the exception of colluvium and alluvium, are subdivided
according to their bedrock (Proterozoic and post-Proterozoic). The
bedrocks have different prospectivities and require different interpretation.
Residual ferruginous materials (massive, fragmental or nodular
duricrusts), where they occur, should be collected for district-
to prospect-scale surveys. Data from partly transported slabby duricrust
should be interpreted with care as their Fe and trace element (Cu.
As) content may have been derived laterally.
Soil sampling is effective in areas of shallow overburden (1-5
m). The best materials are mottles or the soil matrix rather than
clastic grains. Where Fe and/or Mn oxides have adsorbed significant
quantities of indicator elements (e.g., Cu. Zn) multiple regression,
followed by a residual treatment of these indicator elements would
remove the effects of adsorption and draw attention to anomalies
that would otherwise remain hidden.
Areas dominated by thick (>5 m) Cambrian, Mesozoic and Tertiary
sediments present significant exploration problems. Coarse sediments
should be collected at and just above the unconformity (interface
sample) in areas of unweathered or slightly weathered Mesozoic cover
to detect a near-miss when drilling a geophysical target. When sediments
have been weathered, buried ferruginous bands at palaeosurfaces
or at watertables may provide a continuous sampling medium. Horizontal
ferruginous bands formed within sediments should be preferentially
collected. These are more useful than structurally controlled sub-vertical
ferruginous veins within the Mesozoic sediments.
CHARTERS TOWERS - NORTH DRUMMOND BASIN
The landscape of the region is a product of several sedimentation
and weathering episodes. The dominance of a southerly flowing river
system in the early Tertiary, the formation of a large lake system
in the middle Tertiary, and the reversal of the river system in
the late Tertiary are the main episodes of landscape evolution in
the north Drummond Basin. Deposition and erosion in the north Drummond
Basin has been dictated by drainage changes. When the southerly
drainage was choked during the Tertiary, rapid sedimentation formed
the Suttor Formation in the south and the Southern Cross Formation
in the north. In view of their extent and fluvial nature, these
were deposited over a considerable time span and were not restricted
to a single event.
Deep weathering of both the fluvial sediments and the basement
formed the duricrust, red earths and, to a lesser extent, yellow
earths. Campaspe Formation sediments were deposited on the Southern
Cross Formation. In areas where intense erosion of the Southern
Cross Formation occurred, Campaspe Formation sediments were deposited
in lower levels in the landscape. Yellow and grey earths with ferruginous
pisoliths are developed on the Campaspe Formation.
In drill spoil, the Southern Cross Formation is more clay-rich
than the Campaspe Formation and contains clasts of the basement
rocks. The Campaspe Formation tends to be sand-rich and more sorted
than the Southern Cross Formation. The Campaspe Formation may contain
detrital nodules and pisoliths throughout the sediment, whereas
most of the pisoliths and nodules in the Southern Cross Formation
are concentrated near its top. There are no consistent mineralogical
and geochemical criteria but hiatuses in feldspar and kaolinite
abundances, rounding of quartz grains and geochemical parameters
such as hiatuses in Ti/Zr ratios can be used to distinguish the
Campaspe Formation from basement volcanics.
Recommendations for exploration practice
The focus of most of the geochemical studies in the Charters Towers
- north Drummond Basin was on investigating dispersions in sedimentary
cover. Regolith-landform procedures are similar to those described
for the Mt Isa region with the exception that regolith units should
be divided into Palaeozoic or post--Palaeozoic. The geochemical
dispersions appear to be similar to those of the Mt Isa region,
with geochemical responses where the cover is shallow (1-5 m). Here,
soil sampling (including specific sampling of mottles) would be
effective. The probability of hydromorphic dispersion is better
in sediments that have been weathered since deposition.
In areas dominated by a thick (>5 m) regolith on Campaspe, Southern
Cross and Suttor Formations, dispersion is predominantly mechanical
near the base. In places, elevated indicator elements and Au are
hydromorphically dispersed with Fe oxides and dolomite-rich bands
at least 10 m above the unconformity. Thus, basal sediments and
ferruginous bands (redox products) should be sampled preferentially.
Extensive sheets of ferruginous pisoliths, developed in the Campaspe
Formation, also appear to be a promising sampling medium in the