All posts by John C. Menzies

The Geophysical Signature of Mt Isa Cu, Zn-Pb-Ag Ore Bodies

At a regional scale the Mt Isa Cu and Zn-Pb-Ag deposits do not have a noticeable gravity or magnetic response. In addition there is little recent literature on the geophysical signature of the deposits and the data which is available is dated.  Fallon and Busuttil 1992 and Valenta 2020 provide summaries of the available geophysical data. Given that pyrite extends up to 10 km north of economic grade mineralization within the Urquhart Shale and across a width of >1km, pristine mineralization would have  strong IP, EM and MT responses however this data is not readily available.

Continue reading The Geophysical Signature of Mt Isa Cu, Zn-Pb-Ag Ore Bodies

The Eastern Creek Volcanics: Source for the 8 Mt Copper Deposits at Mt Isa

Discovery of the Mt Isa Copper Deposits

Silver-lead ore was discovered at Mt Isa in 1923 but it was not until 1927 that a surface drill hole aimed to test silver-lead bodies at depth intersected 15 m of oxide copper and chalcocite grading 17% copper in the Black Rock area. This secondary mineralization was explored underground in 1937, mined intermittently for flux between 1941 and 1962, and by open cut from 1957.  Combined oxide and chalcocite ore mined from the Black Rock open cut between 1963 and 1967 totaled 2.26 Mt at an average grade of 3.9% copper.  It was not until 1930  while drilling lead-silver mineralization at depth that primary chalcopyrite mineralization was encountered reporting a best result of 8.8m at 8.5% Cu. Follow-up drilling in 1953 reported 17m at 2.0% Cu and in 1954 a most respectable 202m at 2.2% Cu (Perkins, 1999). 

Deposit Geology

The copper mineralization is hosted entirely within a broad zone of intense silica-dolomite alteration developed above the Paroo Fault within the Urquhart Shale (~1650-1955MA).   Economic copper ore bodies extend across a combined width of more than 1,000 metres and along a strike of 4,200 metres and entirely within the Urquhart Shale.

Continue reading The Eastern Creek Volcanics: Source for the 8 Mt Copper Deposits at Mt Isa

A Giant Cu-Pb-Zn Deposit Geochemical and Geophysical Signature

The Mount Isa Province in northern Australia is one of the world’s most prospective regions for minerals. It hosts three of the ten largest Zn–Pb deposits in the world, the world-class sediment- hosted Mount Isa copper deposit, and the Ernest Henry IOCG. 

The Mount Isa copper (225 Mt at 3.3% Cu) and zinc-lead-silver ore (150 Mt at 7% Zn and 6% Pb) deposits are hosted within the Mesoproterozoic (1653 Ma) Urquhart Shale, an around 1000 m thick succession of carbonaceous, pyritic, dolomitic siltstone that belong to the Mt Isa Group, which lies within the Leichhardt River Fault Trough, and belongs to Calvert Superbasin in the Western Fold Belt of the Mt Isa Inlier.

Urquhart Shale outcrop along Downs Road, Mt Isa (Courtesy Ian Withnall)
Continue reading A Giant Cu-Pb-Zn Deposit Geochemical and Geophysical Signature

A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland

This post is a review and summary of an excellent paper: Bradley Cave, Richard Lilly, Alexander Simpson, Lucy McGee, A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes, Ore Geology Reviews 154 (2023) 105311. https://doi.org/10.1016/j.oregeorev.2023.105311

SUMMARY

  1. The George Fisher and Hilton Zn-Pb-Ag deposits are located approximately 20 km north of Mount Isa.
  2. A dolerite dyke has been discovered at the George Fisher Zn-Pb-Ag deposit and the dolerite dyke have a large spike in TiO2 and V values.
  3. In drill core, the dolerite dykes occur as a light grey to grey-brown coloured rock that is commonly overprinted along its margins by Zn-Pb-Ag mineralisation.
  4. In-situ U-Pb geochronology performed on igneous apatite produce a lower intercept age of 1611 +/- 21 Ma and 1619 +/- 22 Ma for the dolerite dykes at the George Fisher and Hilton deposits, respectively.
  5. The dolerite dykes have experienced multiple stages of post-emplacement hydrothermal alteration/veining.
  6. Monazite from a quartz-albite-K-feldspar vein in the Hilton dyke produces a lower intercept age of 1513 +/- 16 Ma.
  7. To assess the timing of alteration in the adjacent George Fisher Zn-Pb-Ag deposit, in-situ Lu-Hf geochronology was performed on pre-mineralisation calcite from a section of stratabound Zn-Pb-Ag mineralisation, and a paragenetically late cross-cutting sphalerite-calcite vein.
  8. Calcite from the pre-mineralization alteration assemblage produces a Lu-Hf age of 1501 +/- 32 Ma.
  9. Calcite from a cross-cutting vein that post-dates Zn-Pb-Ag deposition produced a Lu-Hf age of 1289 +/- 26 Ma. The 1289 Ma age is associated with late faulting and movement along the adjacent Mount Isa Fault (B. Cave pers comms 2023).
  10. The paragenetic equivalents of the hydrothermal alteration/veining in the dolerite dykes are found in the adjacent Zn-Pb-Ag orebodies.
  11. The maximum age of alteration within the dolerite dykes is constrained by the monazite age of 1513 +/- 16 Ma, and the maximum age of stratabound Zn-Pb-Ag mineralisation is constrained by the Lu-Hf age of 1501 +/- 32 Ma.
  12. The dolerite dykes intruded during the early Isan Orogeny at ca. 1620 Ma, and experienced subsequent hydrothermal alteration during D3 of the Isan Orogeny coeval with Zn-Pb-Ag mineralization. Post mineralization faulting occurred during D2 of the Isan Orogeny, at ca. 1290 Ma.
Continue reading A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland

“The Most Important Mining Discovery of the Last Decade”

Below is an exceptional article from the Queenslander of 1924.  A remarkable record so soon after the discovery of Mt Isa and replete with the excitement of such a significant discovery.  At the time of writing a mere 145 tons of ore had been shipped. The original has been transcribed from the Queenslander newspaper, where the ink has been absorbed by the newsprint making transcription locally difficult. Some editorial licence has been taken and minor alterations, including conversion into g/t, have been made and images added.

Prospector John Campbell Miles (left) in 1924 with Walter John Davidson (Current Minister Scott Stewart’s great-grandfather), Will Purdy, S Boyce, and EC Saint-Smith, State Geologist.

MOUNT ISA.
A GREAT SILVER-LEAD FIELD.

By RANDOLPH BEDFORD.
Saturday, January 26, 1924

In 1918 I travelled by Argylla and West Leichhardt to Lagoon Creek – 18 miles west of West Leichardt Telegraph Station returning to Cloncurry by a little east of the Sulieman Creek-Camooweal railway survey, via Bushy Park and Duchess. My journey was made to locate Mica deposits on Mica Creek, and returning from these I passed the Mount Isa field at a distance of a few miles, and on the wrong side of the range, as a hundred other men have done. For, as John Forrest said in explanation of failing to see gold although hw was on the Coolgardie field 23 years before their discovery: “A man gets what he’s looking for, and Forrest was looking for pastoral country.

Continue reading “The Most Important Mining Discovery of the Last Decade”

A comparison of the evolution of arc complexes in Paleozoic interior and peripheral orogens

This post is a summary of the interesting paper on the geodynamic relationships  between Paleozoic arc development along the flanks of the interior (e.g. the Iapetus and Rheic) oceans and the exterior Paleopacific Ocean. Murphy, B., van Staal, C and Collins, W, A comparison of the evolution of arc complexes in Paleozoic interior and peripheral orogens: Speculations on geodynamic correlations, Gondwana Research, 2011. doi:10.1016/j.gr.2010.11.019.

Continue reading A comparison of the evolution of arc complexes in Paleozoic interior and peripheral orogens

K-Alteration in Porphyry Cu-Au Deposits formed Isochemically in a Gas Reactor!

Here is an excellent paper from Richard Henley et al that provides support for the argument that potassic alteration is largely isochemcial and and not an introduced component. Henley has elequently defined the porphyry copper environment as that of a dynamic, internally and externally stressed, gas phase reactor where repetitive fracturing generates high permeability flow paths for expansion of the magmatic gas phase from source to surface.

SUMMARY of the Abstract

Distribution of K-Feldspar and Copper in cross-section, Grassberg
Continue reading K-Alteration in Porphyry Cu-Au Deposits formed Isochemically in a Gas Reactor!

Where does you cell phone come from? a tantalum mine in eastern congo.