Economic Geology News
Dolomite – Magnesite – Metasomatism: New Insights! (15 March 2017)
Dolomite (also called dolostone) and magnesite rock of high purity and whiteness are highly valued in industrial applications. Usually, they are extracted from hard rock quarries, less often from underground mines. Typically, both form limited masses of 1-1000 Mt within marine limestones suites. Clearly, these bodies originated by chemical replacement of calcite (CaCO3) by dolomite [(CaMg(CO3)2 ] or magnesite (MgCO3), or in other terms, of partial or total replacement of Ca by Mg. The geological term for this replacement process is metasomatism. Sedimentary and diagenetic models of sparry magnesite metasomatism are closely related to concepts of dolomite formation.
Most commonly, the details and variable possibilities of ore body scale metasomatism in geological nature are discussed in geochemical terms, employing various isotopic systems or trace element characteristics. I will not deny that geochemical methods allow surprising new approaches to genetic modelling. On the contrary, concerning dolomite and magnesite, I’ll just mention the novel use of Mg isotopes (Dong et al. 2016) or of clumped isotopes (Garcia del Real et al. 2016).
Generally, pre-Cenozoic metasomatic bodies have experienced repeated passage of fluids and heat events, fudging genetic signals. Geologically young and well-preserved metasomatic deposits of dolomite, magnesite or siderite are rare, however. Therefore I found it diverting to read a case study of Miocene dolomite formation in the Gulf of Suez (Hollis et al. 2017). For gas and oil geology, dolomite often is a profuse reservoir, but in this case, we only learn a lot about fault-bound hydrothermal dolomite formation by deep convection of ordinary seawater. The authors stress that the characteristic pattern of (1) non-stratabound massive dolomite along faults and of (2) stratabound lateral tongues into host rock is not synchronous but that (2) clearly precedes (1). Formation temperatures were estimated at ∼40–70 °C (stratabound) and at ∼40–100 °C for the massive dolomite; certainly hot enough for the term “hydrothermal”. For students of “sparry” Veitsch-type magnesite, of hydrothermal dolomite, of metasomatic siderite, and of, among others, Irish-type base metals, this should be compulsory reading.
By the way, a recent comprehensive paper on Hohentauern/Sunk, one of the typical Austrian sparry magnesite deposits, provides many modern data and confirms diagenetic replacement (Azim Zadeh et al. 2015).
Fortunately, the Hollis et al. (2017) paper is Open Source classified. You may download and read it at your own leisure.
- Pinolite-textured magnesite from Hohentauern-Sunk Mine, Austria. Note the crystal rosettes and general displacement of formerly dispersed impurities by crystals growing during diagenesis. Typical for stratiform/stratabound metasomatites, the fabric is weakly sedimentary features preserving and crystals display cloudy core–clear rimmed fabrics.
Azim Zadeh, A.M., Ebner, F. & S.-Y. Jiang (2015) Mineralogical, geochemical, fluid inclusion and isotope study of Hohentauern/Sunk sparry magnesite deposit (Eastern Alps/Austria): implications for a metasomatic genetic model. Miner. Petrol. 109, 555-575.
Hollis, C., Bastesen, E., Boyce, A. et al. (2017) Fault-controlled dolomitization in a rift basin. Geology 45, 219-222.
Gold Open Access: This paper is published under the terms of the CC-BY license. http://geology.gsapubs.org/cgi/content/abstract/45/3/219?etoc
Garcia del Real, P., Maher, K., Kluge, T. et al. (2016) Clumped-isotope thermometry of magnesium carbonates in ultramafic rocks. Geochim. Cosmochim. Acta 193, 222–250.
Dong, A., Zhu, X.-K., Li, Sh.Zh., Kendall, B., Wang, Y., Gao, Zh. (2016) Genesis of a giant Paleoproterozoic magnesite deposit: Constraints from Mg isotopes. Precambrian Research 281, 673–683.
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Arsenic: Well-drillers, be wary of water from reduced, grey sediments! (14 February 2017)
Unrelated to mining, natural arsenic concentrations in aquifers and/or groundwater of lowland valleys throughout the world are a hidden hazard. In the Ganges delta, this caused humanity’s biggest mass poisoning. Earlier, millions of people in the region had relied on surface water. From 1980 to 1990 thousands of drinking water wells were drilled to provide “safe”, that is microbially clean water. Of these wells, nearly 50% had As of >10 micrograms/litre (WHO’s limit). Sadly, this was only recognized after numerous people had developed skin and internal disorders, including cancer. With 2-20 ppm, the As-concentrations in Ganges river sediments are quite ordinary. Most As is adsorbed in Fe-oxide particles. Spots of elevated dissolved arsenic occur in low-sulphate groundwater in the presence of reactive organic matter, which supports microbial reduction of iron oxides and oxy-hydroxides; this releases adsorbed arsenic (Bowell et al. 2014). Pumping increases recharge, which may trigger aquifer flushing and release of Fe(II) and As(III). Well-drillers should keep samples oft grey sediments and water, and ask for advice. Various removal technologies are available; if nothing else is possible, a cheap and efficient precaution is stirring the contaminated well water with iron oxy-hydroxide fines.
Anthropogenic arsenic contamination is unfortunately not rare; mining, ore dressing, metallurgical processing (roasting plants and smelters) and general industry may be hotspots. I wonder how many of my readers work in mines that deal with elevated traces of arsenic, such as many of those extracting copper, gold, platinum and tin.
If you are seeking an example for how to professionally investigate a potentially dangerous situation, I recommend a recent paper that describes work done for an ecological risk assessment (ERA). The site of the study is the Neoarchaean (2.61 Ga) Siilinjärvi alkaline complex (16 x 1.5 km) in Eastern Finland that comprises carbonatite and syenite. Its core hosts an important deposit of apatite (an ore of phosphorus that is here the main mine product), glimmerite and calcite.
At the Yara Siilinjärvi industrial site, the apatite is treated with sulfuric acid to produce phosphorus fertilizers. The sulfuric acid in turn is made by roasting pyrite, of which some 860,000 tonnes per year are produced as a by-product of Cu and Zn at the nearby underground Pyhäsalmi mine.
- Photograph showing Yara Siilinjärvi phosphate mine in Finland. Fertilizer plant in the foreground, calcined pyrite heaps in the background on the right. Courtesy Yara Suomi Oy.
The apatite mine, industrial plant as well as the calcined pyrite tailings storage heaps lie near a lake. Turunen et al. (2016) present an illuminating study of arsenic dispersion from the calcined pyrite tailings. The calcined pyrite consists of hematite; the tailings have a a high As content (500–654 mg kg-1) that is mobilized by seepage water, which is collected and reused as process water in the plant.
The natural (geogenic) background As concentrations are very low so that hydraulically and geochemically, the gradient from the tailings to the environs and the lake is considerable. As expected, in spite of retaining measures, the migration of As by dust and water reaches the lake and is recorded in its sediments. Analytical results of many samples near the tailings exceeded the European Chemical Agency’s (ECHA) Predicted No Effect Concentrations (PNECs). For example, the As concentration in 64 % of the water samples exceeded the PNEC value for fresh water (0.5 l μg L-1). Dissolved arsenic and other potentially harmful metals brought in with the pyrite (Co, Cd, Cu and Ni) also pose a risk but the ubiquitous iron oxides retain and fix much of the metal load.
The paper by Turunen et al. (2016) is a rich source on geochemical methods and technologies used. It is highly recommended. And it is freely downloadable under creative commons licence!
Bowell, R.J., Alpers, Ch.N., Jamieson, H.E., Nordstrom, D.K. & Majzlan, J. (2014) The environmental geochemistry of arsenic -- an overview. Rev. Mineralogy & Geochemistry 79, 1-16. DOI: 10.2138/rmg.2014.79.1
European Chemicals Bureau (2003) Technical Guidance Document on Risk Assessment. European Communities. Available online at: https://echa.europa.eu/documents/
Turunen, K., Backnäs, S., Neitola, R. & Pasanen, A. (2016) Factors controlling the migration of tailings-derived arsenic: A case study at the Yara Siilinjärvi site. Mine Water Environ 35, 407-420. Keywords include Arsenic fractionation; Arsenic mobility; Soil chemistry; Water chemistry; Risk assessment. Free download under creative commons licence. http://link.springer.com/article/10.1007/s10230-016-0393-5
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Diamond crystals forming in a metallic Fe-Ni-C-S melt – did humans re-invent nature’s secret? (17 January 2017)
In my last blog (8 December 2016) I referred to the role of subduction in the process system of gem diamond formation (Stern et al. 2016). Soon after I had this published, another brilliant paper appeared, that reports on the crystallisation of natural diamond from a metallic Fe-Ni-C-S melt (Smith et al. 2016).
Smith et al. (2016) investigated microinclusions in some of the famous great diamonds, such as the Cullinan from the Premier mine in South Africa, in rough weighing 3,106 carats, found in 1905. Based on common properties such as attribution to the low-N type II diamonds, large size and few but distinct microinclusions, the authors collected 53 similar diamonds. The microinclusions turned out to be magnetic solid phases crystallised from a metallic Fe-Ni-C-S liquid that was trapped during diamond growth. Like fluid or gas inclusions in minerals, silicate or metallic melt inclusions provide a wealth of genetic information. Other inclusions formed of silicates suggest that these diamonds formed in the mantle transition zone between 410 and 660 km depth. This is much deeper than the common gem diamonds that crystallised at the asthenosphere-lithosphere boundary (ALB) at 135-200 km. Several arguments support formation of this metallic melt within subducted eclogite. Light carbon isotopic compositions point to a crustal biogenic origin. The authors propose to call this specific group of type II diamonds CLIPPIR (Cullinan-like, inclusion-poor, pure, irregularly shaped, and resorbed).
Synthesis of diamond has been first attempted >100 years ago. Industrial production started some 60 years later. Today, most synthetic diamond is made from graphite in a metallic melt by the High Pressure/High Temperature (HPHT) technology at T>1400oC and P>59 kbar.
Industrial diamond is foremost an abrasive that is used for drilling, grinding, sawing and polishing. Useful properties apart from its hardness include toughness, resilience against aggressive chemicals and high-temperature stability. Applications are numerous, ranging from microsurgery to computer chip production, deep drilling for petroleum and cutting large monolithic dimension stones. Most of this market is served by synthetic monocrystalline and polycrystalline diamonds.
Only about 1% of the steadily growing demand for industrial diamonds is satisfied by mining natural diamonds. In 2015, world mine production of industrial grades reached 54 Mcts. China alone produces >4000 Mct/yr (or 800 tonnes) of synthetic diamonds (USGS 2017). Even the production of synthetic gem diamonds is possible, at a fraction of the price of natural diamonds. Is branding the “true” stones a long-term solution?
- The Cullinan was the largest diamond ever discovered. It was cut by Assher’s in Amsterdam into 9 major gems and 96 small brilliants. The largest gem from the Cullinan is known as the Great Star of Africa, the largest cut diamond in the world. This and no. II are part of the British crown jewels. Here we see the stones no. IV and III. Source: Wikipedia public domain
Smith, E.M., Shirey, St.B., Nestola, F. et al. (2016) Large gem diamonds from metallic liquid in Earth's deep mantle. Science Vol. 354, Issue 6318, pp. 1403-1405. DOI: 10.1126/science.aal1303
Stern, R.J., Leybourne, M.I. & Tatsuki Tsujimori (2016) Kimberlites and the start of plate tectonics. Geology 44, 799-802. http://geology.gsapubs.org/cgi/content/abstract/44/10/799?etoc
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