The Earth’s lithosphere is its rigid outer shell, which rests on the more fluid asthenosphere. The thickness of the lithosphere varies, from a few kilometers at ocean spreading centers to 250-300 kilometers within continental cratons. There are two significant seismic boundaries in the crust and upper mantle: the Mohorovicic Discontinuity (Moho), indicating a change from felsic-to-mafic rocks to ultramafic peridotites, and the Lithosphere-Asthenosphere Boundary (LAB), which signifies a shift from a strong, plate-like layer to a weaker, convective asthenosphere over geological time. This transition occurs around the conductive-adiabatic geotherm intersection, where heat transfer shifts from conduction to convection.
The thickness of the continental lithosphere depends on its tectono-thermal age, increasing from 60-80 kilometers in active extensional regions to 100-160 kilometers in older terranes and up to 200-300 kilometers in ancient cratons. Some exceptions exist in cratons affected by more recent tectonic and magmatic events. The lithosphere appears as a seismic high-wavespeed layer, or “lid,” over a low-wavespeed zone or a gradual decrease in seismic wavespeed with depth. This boundary is referred to as the “8°-discontinuity” or the mid-lithosphere discontinuity (MLD). Different seismic methods may detect different depths for the LAB or MLD, depending on their sensitivity.
The cratonic LAB is subject to debate, with some proposing a broad thermal boundary zone and others suggesting a sharper transition influenced by factors such as chemical composition, melt content, or vertical anisotropy variation. The presence of an observable S-to-P (Sp) conversion in seismic data requires a thermal gradient of at least 20°C per kilometer. While such gradients are common beneath oceanic and non-cratonic areas, cratons typically exhibit much lower gradients. Multiple factors, including various scales of mantle convection, can contribute to localized high thermal gradients at the LAB.
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.
MINES AND MINING 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.
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.
Last week there were a few comments that the Quiz on Structural Geology was not difficult enough – this week we have made it more of a challenge! Your challenge is to beat the GPTAPI that got 7/10 correct. Answers at the end.
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.
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