Search CRC LEME :

powered by FreeFind

Publication Policy

Open File Report Series

OFRS Index


Regolith Maps

Annual Reports

Articles & Papers


Minerals Briefs

"Focus on Salt"

Other LEME Reports

Order Form

Open File Report 139

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.


Cooperative Research Centres Australia

About Us | News & Events | Research
Publications | Education | Staff Only | Links

Contact Us | Disclaimer | Sitemap
© CRC LEME 2004

CRC LEME is established and supported under the Australian Government's Cooperative Research Centres Program. The CRC Program is an Australian Government initiative which brings together research groups with common interests.

CRC LEME Core Parties