CRC LEME
Open File Report 139
ABSTRACT
Geochronology of weathering in the Mount Isa and Charters Towers
Regions, Northern Queensland
P. Vasconcelos
Weathering geochronology of the Mount Isa region
This project is the most comprehensive weathering geochronology
study ever undertaken in a single region. More than 103 40Ar/39Ar
laser step-heating and 11 K-Ar analyses provide a substantial geochronology
database which permits answering some surficial geochemistry and
landscape evolution questions for the region.
The Mn oxide results indicate that the evolution of weathering
profiles in the Mount Isa Region spans the whole of the Tertiary,
possibly extending back into the Cretaceous. The 40Ar/39Ar results
obtained for the jarosite samples also indicate that weathering-related
mineral precipitation continued until the Quaternary. However, these
weathering reactions are driven by groundwater with little recrystallization
of Mn oxides at the surface.
A remarkable feature is the concordance in the ages from similar
geomorphic provinces, even where samples were from chemically different
and spatially separated profiles. The ages of the Mount Isa Mines
gossans (14.5-21 Ma, with 20.9 Ma as the most probable) and gossans
exposed at Lake Moondarra (17-24 Ma, with 19.5 or 20.7 Ma as the
most probable) indicate that the Mn oxide minerals in the dissected
part of the landscape were precipitated in the early Miocene. The
results for the Kennedy Gap area are also very consistent. Manganese
oxides from the Mesa I and the Gunpowder Creek Road site yield ages
ranging from 30-40 Ma, with best estimates at 32, 35, and 37 Ma.
Finally, samples from the Selwyn, Pegmont, and Tringadee prospects
are concordant with the weathering ages (12-13 Ma) for the Cannington
Region. Geomorphological provinces with more complete and stratified
weathering profiles (Overhang, Selwyn, and Kennedy Gap) are older;
the most dissected parts of the landscape yield the youngest weathering
ages (Century).
The weathering geochronology implies that the Mount Isa region
has seen some very wet periods, when dissolution, redistribution
and reprecipitation of elements within weathering profiles was facilitated
by an abundance of meteoric water. These periods were probably the
late Cretaceousearly Palaeocene, the early to middle Oligocene,
and the early to middle Miocene.
The geochronology is significant because it may be combined with
mineral chemistry to suggest mechanisms, pathways and rates of element
migration in the past that may, in turn, help explain patterns of
surficial geochemical anomalies. A complete explanation of this
is given by Vasconcelos (1997). Some important issues are summarised
below.
The Mn oxides overlying the Mount Isa deposit are rich in Pb, Zn,
and Ba. In addition to cryptomelane (KMn8O16), coronadite (PbMn8O16),
chalcophanite (ZnMn3O7.3H2O),
hollandite (BaMn8O16) and barite are present, suggesting large-scale
migration of Pb, Zn and
Ba in solution between 14-21 Ma.
The Mn oxides of the Century profiles host coronadite, chalcophanite
and plumbogummite (PbAI3(PO4)2OH5.H2O). The high concentrations
of Pb and Zn suggest that these elements were derived from a nearby
source, and did not precipitate at the Cambrian-Proterozoic unconformity
by long-range transport in the groundwater system. The most likely
source for Mn, Zn, Pb, Ba and K in the Mn oxide outcrops is the
weathering Century mineralization.
Since solubility and oxidation-reduction constraints would prevent
large-scale migration of Pb, Zn and Mn in solution by oxidising
surface waters, the precipitation of the Mn-rich "false gossans"
at Century most likely occurred in a shallow subsurface environment.
Lead, Zn and Mn, derived from weathering of the Century mineralization,
migrated in solution and were precipitated at the chemical barrier
imposed by the basal Cambrian limestone. Erosion of 5-50 m of overburden
in the past 5 Ma is implied.
The Mn oxides (coronadite and chalcophanite) from Pegmont and Cowie
are also significantly enriched in Pb and Zn, respectively, indicating
association with nearby Pb and Zn mineralization.
The Mn oxides at Tick Hill and Selwyn are different from those
associated with Pb-Zn deposits in that they are enriched in Co and
Zn (~0.5 wt%) with negligible Pb contents.
Manganese oxides from the Tringadee prospect are devoid of Pb,
contain only moderate Zn contents (~0.5 wt%), and are enriched in
Co which is consistent with association with Cu-Au or Au mineralization.
Manganese oxides on the Pilgrim Fault in the Tick Hill area replace
silcretes. Given the extreme cation-depleted (except for Si and
Ti) nature of the silcretes it is unlikely that the cations in the
Mn oxides (Mn, K, Ca, Ba, Na, Co) were precipitated from descending
groundwaters. The most likely source of these oxides is mineralised
groundwater ascending along the Pilgrim Fault. The geochemical anomalies
associated with these manganese "breccias" may reflect
leaching of elements from distal sources (100's to l000's of m)
and are unlikely to be from nearby or underlying mineralization.
Weathering geochronology of the Drummond Basin
The deep weathering profiles exposed at the Scott Lode gold deposit
were sampled in detail (Vasconcelos, 1997 (452R)). Manganese oxides
and K-bearing sulphates (alunite and jarosite) occur throughout
but only selected benches could be sampled due to safety reasons.
No mineral suitable to 40Ar/39Ar geochronology occurred in the ferruginous
channel deposits (Southern Cross Formation) overlying the mineralised
volcanic rocks so only weathering of the underlying volcanic bedrock
could be investigated. Manganese oxide samples were collected along
the mine access road bench and at approximately 20 m and 60 m below
the mine access road level. Thirty-eight grains from 13 Mn oxide
samples were analysed by the 40Ar/39Ar laser-heating method and
four grains from 2 jarosite hand specimens from approximately 20
m below the mine access road level.
Remarkably reproducible results were produced from the 13 Mn oxide
samples analysed (in excess of 500 individual analyses). Plateau
ages ranged from 3.9 + 0.1 Ma at the bottom of the pit to 16.2 +
0.2 Ma at the uppermost part of the weathering profile at the western
edge of the pit. The ages from jarosite were also remarkably reproducible.
Only one jarosite grain yielded a plateau age (4.5 + 0.1 Ma); the
other three grains displayed a plateau-like series of steps at low
temperatures, but the ages were unreasonably great at high temperature.
The climbing spectra suggest that the jarosite grains are intermixed
with unweathered or partially weathered hypogene silicates.
One Mn oxide sample from the Scott Lode was also dated by the K-Ar
bulk method. The age (39.5 + 2 Ma) is drastically different from
the 40Ar/39Ar ages. The only explanation for the discrepancy is
the presence of contaminants.
Weathering profiles overlying the Mount Leyshon and Kidston ore
deposits were dated by K-Ar analysis of alunite (Bird et al., 1990).
The results for Kidston (1.85 + 0.04, 1.61 + 0.04, 1.52 + 0.03,
3.91 + 0.07, 4.1 + 0.2 Ma) and Mount Leyshon (3.1 + 0.2 and 4.1
+ 0.1 Ma) indicate PlioPleistocene ages, surprising given the long
history of weathering in the region (Bird et al., 1990). The 40Ar/39Ar
ages for jarosite samples of this study (plateau age of 4.5 + 0.1
Ma and plateau-like steps at 3.2 + 0.1 and 2.7 + 0.1 Ma) are consistent
with these young ages.
However, Mn oxide 40Ar/39Ar ages indicate a longer history of weathering
than suggested by 40Ar/39Ar jarosite or alunite ages. The Mn oxides
dated in this study fill cavities and desiccation cracks in kaolinitised
volcanic rocks, indicating that the host sequence was already strongly
weathered at the onset of Mn oxide precipitation, at approximately
17 Ma. It is significant that Mn oxide ages are greater (10-17 Ma)
at shallower horizons and younger precipitation ages are recorded
at the bottom of the profile (4-6 Ma), indicating a downward propagation
of the weathering front.
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