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
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.
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).
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.
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.
During Vietnam and the Thai-Cambodian war the Border regions of Cambodia were heavily mined, with mines produced exclusively in China – at least 8 million of them. In the last 30 years the Cambodian Government and private de-mining charities have been systematically demining the country. Demining which is very labour intensive now achieves 250 km2 of clearance per year in Cambodia.
A total of five provinces and the capital had been declared mine-free as of December, as Cambodia’s 2025 goal of becoming mine-free approaches. Stung Treng, Kep, Prey Veng, Preah Sihanouk and Tbong Khmum were declared mine free in December 2022 and Kampong Cham, Takeo, Kampong Chhnang, Kampot, Svay Rieng, and Kandal are expected to be mine-free in 2023.
Unexploded ordnance (UXO, sometimes abbreviated as UO), unexploded bombs (UXBs), and explosive remnants of war (ERW or ERoW) are explosive weapons (bombs, shells, grenades, land mines, naval mines, cluster munition, and other munitions) that did not explode when they were employed and still pose a risk of detonation, sometimes many decades after they were used or discarded.
The major contributor to the Cambodian UXO legacy was the aerial bombing campaign conducted by the United States Airforce during the Vietnam War. The most intense bombing was area denial along the Vietnamese border and the extent of this bombing can be see in the following images. The total ordnance dropped was 2.8 Mt (6.1 billion pounds) comprising 2.5 million individual munitions carried in 230,000 sorties.
Copper concentrates come in two “flavours” – Clean and Complex.
Clean copper concentrates have more than 20% copper and possibly gold and silver and low levels of the deleterious elements As, As, Bi, Cd, Cl, F, Pb, Hg, U and Zn. In addition, asbestos (referred to as fibre) is present in a small number of mine product streams. The Complex Concentrates have high levels of one or more deleterious elements. Each of the deleterious elements will have a threshold level in the offtake contract where the smelter will charge a penalty in addition to the treatment and refining charges. The penalty accommodates the increased costs of disposal and safe disposal.
There is frequently an upper limit for some deleterious elements,
above which the smelter may refuse to accept the concentrate.
The most common deleterious element in copper concentrates is arsenic. Globally 65% of copper concentrates have less than 0.1% As. Above 0.2% arsenic, copper concentrates are considered to be Complex Concentrates and will be charged penalties. In the last decade, as new mine copper production has slowed the quantities of Complex Concentrates entering the market increased significantly while smelter capacity for these concentrates has declined.
Complex concentrates from Marcapunta in Peru (8% As), Chelopeche in Bulgaria (6% As) and Chuquicamata in Chile (1.2% As) have been the main producers. They have been joined more recently by production form Toromocho in Peru (1% As) and Ministro Hales in Chile (4% As).
Smelting Treatment Options
Prior to the 1990s there were many smelters that would accept copper concentrates with high deleterious element concentrations. However due to environmental concerns, liability concerns, tightening regulations and smelter closures at La Oroya in Peru, San Luis de Potosi in Mexico, Tacoma in the USA, Rönnskär in Sweden, PASAR in the Philippines and Kosaka in Japan, the number of smelters that will now regularly accept Complex Concentrates has declined very significantly.
Smelters that will now accept Complex Concentrates include Tsumeb in Namibia, Altonorte in Chile, Guixi in China and Horne in Canada. For complex concentrates that contain more than 1% arsenic, the Dundee Precious Metals smelter in Namibia at Tsumeb is now the only smelting option.
Most of the smelters which would previously accept high arsenic concentrates utilised roasters to fume of the Arsenic to produce arsenic trioxide (for which there is a limited market) and calcined copper with much reduce arsenic levels. Only Tsumeb still operates such a process facility.
Codelco installed an Outotec Partial Roaster (a fluid bed roaster) at its Ministro Hales mine in 2013 to reduce the as content of the copper concentrates. It is located close to Codelco’s Radomiro Tomic and Chuquicamata operations. Initially the project comprised an open pit mine, a 50 kt/d mill to produce 163 kt/a Cu and 287 t/a silver over a 14-year mine life. The ore contains a significant amount of arsenic (around 1.6-1.9% As) that results in production of concentrate with arsenic content just above 4%. To reduce the arsenic levels, a 550 kt/a fluid-bed roaster was constructed to safely process copper concentrate and recover arsenic for further confinement. In addition to calcine and sulphuric acid, the roaster produces flue dust (around 4% volume) containing 22% Cu.
Hydrometallurgical Treatment Options
There are a number of hydro-metallurgical treatment options (few of which have achieved commercial success) which do not involve roasting where the objective is to produce a residue containing arsenic in a form which is stable within a tailings dam. These process routes include atmospheric leaching, bio-oxidation and pressure leaching. Dundee Precious Metals prior to its acquisition of the Tsumeb smelter had attempted to permit a pressure oxidation circuit at its Chelopeche mine in Bulgaria but faced opposition for the usual socialist “ecological” groups.
Teck Aurubis have trialled their proprietary high pressure
oxidation technology, CESL, on concentrates with up to 10% arsenic and report greater than 99% deportment of arsenic to leach residues. Arsenic components in the residue have been
identified as ferric arsenate and scorodite – both of which are considered the
most stable forms for arsenic fixation. Teck Aurbis have achieved >97%
copper and >90% Au and Ag recovery, LME grade copper cathode and gold and
silver Dore production.
Due to the limited capacity and high costs of the smelters capable of accepting high arsenic concentrate, blending of clean and complex concentrates to produce a product that is below the smelter deleterious element thresholds has become a significant business opportunity. Generally, this is below the 0.5% Chinese threshold and the main blended concentrate target is Chinese smelters.
In 2014 Codelco set up a strategic alliance with Ocean Partners to blend high-As copper concentrate from its Ministro Hales mine with clean third-party concentrate bought in by both companies, at Ocean Partners’ concentrate blending facility in Taiwan.
In the near term it appears likely that the percentage of concentrates subject to arsenic penalties will increase, as will the percentage of Complex Concentrates in the market. In response to this Glencore has opened a new copper concentrate blending facility in Taiwan and a number of Chinese smelters are looking at locating blending and scrap processing operations in the region.