A Miarolitic Cavity in Section

Siem Reap has 98km of new roads  and new sidewalks much of which has been paved with interesting felsic intrusive rocks from quarries in Shandong Province in China.  The composition ranges from granodiorite to tonalite and is locally granophyric and pegmatitic.  The intrusive consists dominantly of plagioclase, quartz, pyroxene and hornblende. Ovoid structures known as Miarolitic Cavities are evident in the sidewalks to the observant and indicates that the parental magma was hydrous.

Here we have attempted to reassemble a miarolitic cavity over ~1 metre of its length. Just bear in mind that this is dimension stone and its now the pavement. There are many places where there are a large number of slabs that are clearly related. Reassembling them is quite instructive and reveals much about the evolution of these structures.

We have previously published libraries of miarolitic cavities.

Unravelling the Mystery of Graphite-Rich Magmatic-Hydrothermal Mineral Systems: MT Imaging Results from Australia and the US

MT Section through Olympic Dam. Note the very large sub-horizontal conductor in the mid crust with near vertical conductors beneath known deposits. After Selway (2015)

This post is a summary and review of Murphy, B., Hjuizenga, J. and Bedrosian, P., 2022. Graphite as an electrically conductive indicator of ancient crustal-scale fluid flow within mineral systems. Earth and Planetary Science Letters. https://doi.org/10.1016/j.epsl.2022.117700


  • Magnetotelluric (MT) imaging has shown an apparent connection between crustal-scale electrical conductivity anomalies and major magmatic-hydrothermal iron oxide-apatite/iron oxide-copper-gold (IOA-IOCG) deposits in Australia and the United States
  • The exact cause of these anomalies has been unclear
  • Murphy et al (2022), interpret the conductors to be the result of graphite precipitation from CO2-rich magmatic fluids during cooling
  • These fluids exsolved from mafic magmas at mid- to lower-crustal depths
  • Saline magmatic fluids that could drive mineralization were likely derived from more evolved intrusions at shallower crustal levels
  • The conductivity anomalies mark zones that once were the deep roots of ancient magmatic-hydrothermal mineral systems
Continue reading Unravelling the Mystery of Graphite-Rich Magmatic-Hydrothermal Mineral Systems: MT Imaging Results from Australia and the US

“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.


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”