All posts by John C. Menzies

The Taylor Zn-Pb-Ag Deposit – South32 Writes off Acquisition Cost

South32 has undertaken a review of the carrying value of the Hermosa Project consisting of the Taylor Zn-Pb-Ag deposit and the Clark Mn deposit.  South 32 acquired the Hermosa Project through the all-cash offer for TSX listed Arizona Mining Inc. in August 2018, paying C$6.20 per share.  This was a 50% premium to market which implied a total equity value of Arizona Mining of C$2.1 billion. 

As a result of the review of carrying value, South32’s FY23 financial statements will include a non-cash impairment expense of US$1.3 billion in relation to the Taylor deposit. Following this impairment expense, the carrying value of the Hermosa project will be US$1 billion, with US$482M for the Taylor deposit. The carrying value of the Clark deposit and regional exploration land package is unchanged at US$519M.

The Arizona Mining, 2018 PEA, estimated CAPEX of US$1.2 billion, a post-tax NPV(7) of US$2 billion and an IRR of 48%.

Since concluding the acquisition in 2018, South32 have posted US$900 million in accruals to the Hermosa Project, which would appear singularly excessive for a discrete project which has yet to complete a Feasibility Study or report ore reserves. The very significant delay in progressing the project since 2018, has negatively impacted project economics and shareholder value.


The Property is located approximately 50 miles (81 km) southeast of Tucson, Arizona; 15 miles (24 km) northeast of Nogales in Santa Cruz County, Arizona, and eight miles (13 km) north of the international border with Mexico.

Geology and Mineralization

The Taylor deposit is predominantly hosted in Permian carbonates of the Pennsylvanian Naco Group of south­eastern Arizona. It is a CRD (Carbonate Replacement Deposit) style Zn-Pb-Ag massive sulphide deposit. The deposit comprises upper Taylor Sulphide and lower Taylor Deeps domains that have a general northerly dip of 30° and are separated by a low angle thrust fault. Mineralisation within the stacked profile of the thrusted host stratigraphy extends 1,200m from near-surface and is open at depth. Mineralisation is modelled for multiple litho-structural domains for an approximate strike of 2,500m and width of 1,900m.

Solid geological map of the Patagonia Mountains region of southern Arizona Source: USGS
Solid geological map of the Patagonia Mountains region of southern Arizona Source: USGS
Continue reading The Taylor Zn-Pb-Ag Deposit – South32 Writes off Acquisition Cost

Mining News and Developments – Cloncurry and Gympie, Queensland, 1923

The Brisbane Courier
June 30th 1923

The Cloncurry Field

There is no question that the future of the Cloncurry copper field is of vital moment, not only to Queensland, but to Australia.  Here we have some of the richest copper deposits in the British empire, but, possibly, through lack of co-operation and proper co-ordination of their owners, practically nothing is being produced.

Apart altogether from the proven areas there is an enormous amount of country to the northwest of proven ground which is known to carry copper, and its examination and development will merely be a matter of time once operations are resumed on the proven orebodies. An encouraging sign that better times are on the way was the recent visit to the north of prominent mining men connected with the Mt Elliot Company, Mr. W. L. Baillieu, of Melbourne, who was with the party, stated on his return from Cloncurry that he strongly favoured co-operation in the development of the field.  Along that line prosperity may lie.

Great Australia Mine, Cloncurry 1920. (Fryer Library UQ)
Continue reading Mining News and Developments – Cloncurry and Gympie, Queensland, 1923

Samples from the 20km2 Hills Intrusive Centre

Samples taken on a regular 500m by 250 m grid at the 20km2 Hills Intrusive Centre in the last three days. A High-Sulphidation alteration ecosystem. It is one of the 6 or so intrusive canters within the ~200km2 Khvav intrusive cluster all of which shows strong to advanced argillic alteration and vuggy silica.

This large alteration system is coincident with a large gravity anomaly which lies with a large 3,000 km2 gravity low which we interpret to be a large batholith in the mid crust. These gravity features are evident in a 48,000km2 airborne gravity data set and less evident in the 2.5 million km2 satellite gravity data collation we have acquired and reprocessed.

SWIR has detected alunite, pyrophyllite, diaspore, white mica, dickite and minor topaz in rocks from this area, indicative of hydrothermal fluids with temperature locally >300 °C. Alunite in this area is sodic rich.

The Jungle in the Summer

It hardly gets more challenging than SE Asian jungles at the height of summer. What’s important:

  • Water and electrolytes are very important and at the height of summer needs to have added electrolytes
  • Take more water than you need and drink early
  • We always keep ice and abundant water in the vehicle – ready to be dropped into a back-pack
  • Everyone is responsible for their own water
  • Coconuts are a fantastic source of potassium (1,200 mg /coconut!)
  • Sports drinks are NOT electrolyte replacements as they contain a poor electrolyte balance and way too much glucose.
  • Hats, long sleeve shirts, trousers, boots (and gaiters) are essential.
  • Gloves are great, they protect from the sun and against the inevitable ants and vines and trees with spikes and spines.
  • Do not push yourself, take it easy as it is all too easy to have little capacity left for the unexpected (bushfire, snake bite, injury etc.)
  • Wildlife is for observing (see the caption below)
Only another 50 metres to the target outcrop! Cambodian jungle. Slow going but its the guy are the front that does all the hard yards! There was cobra seen in this area earlier in the year. When striking it stood >1.5 metres high. The gent who was holding its’ tail was dead within 20 minutes!

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.


  • 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

Is this an Analogue for the Massive Mt Isa Cu-Pb-Ag Deposit

The Mt Isa bedrock copper anomaly (>65ppm) plotted over the 20km long magnetotelluric anomaly at 1,000 metres (Simpson, J. and Brown, D., 2023). Note the coincident large SQUITEM-EM Anomalies and hydrogeochemistry copper anomalies

The purpose of this post is to delineate the key characteristics of the Mt Isa deposit from an explorer’s pragmatic perspective and then discuss a possible analogue in the Eastern Succession, north of Cloncurry. Beside the fascinating tectonic, geological and structural history of the Mt Isa region,  it represents the largest repository of Pb-Zn metal.

Continue reading Is this an Analogue for the Massive Mt Isa Cu-Pb-Ag Deposit