Chemical Fingerprinting Of Natural Gold

Alluvial gold contains other elements that can help identify the type of deposit in which the gold ore may be found.

• Sample gold from the Leadhills area in southern Scotland (optional)
• Characterisation using scanning electron microscopy (SEM) 
• Develop method for ultra-trace element analysis of individual gold grains (Ag, Cu, Ni, Hg, Pt, Se) by ICP-AES / ICP-MS / AAS.

• Range of analytical techniques for a challenging sample type
• Properties and natural behaviour of some economically and industrially important metals
• Field methods used by mining explorationists and environmental protection agencies (optional). 

The Process of Gold Fingerprinting

Gold fingerprinting is a technique for categorising a specific component created of gold, grounded on the impurities it encloses (Liu et al. 2008). The process adopts a qualitative approach from the unusual patterns created by the small and trace element impurities present in gold. The production of the Golden Bullion Bank offers an efficient means of determining the source of gold.  It is usually used to characterise gold by its trace elements by a mineralising event and to a particular mine or bullion source. Components that measure above the detection limit include Ag, Pd, Ti, Ni, Cu, Se, Hg, Pt, Te, Mn and Cr which are used for gold fingerprinting and geochemical characterisation (Jarošová, MilDe and Kuba 2014).

Gold panning is usually a manual technique which has been used since the ancient times to separate gold from other trace elements. It involves the use of large sifter pans filled with both sand and the rock that contains gold grains. Here the pan is immersed in water while shaking back and forth and sorting the gold grains from sand and other substances. Gold settles down on the pan since it is more dense while the other elements settles at the top making it possible for miners to separate it in what is referred as placer deposits of gold. However, this technique has been associated with environmental degradation mostly downstream of rivers, thus affecting the ecosystem. Moreover, miners have been faced with the challenges on matters health due to high levels of mercury during mining (De Lacerda and Salomons 2012). 

There is a substantial distinction in the structure of natural gold and the nature of surrounding trace elements attached to it, and this reveals variations in the landscape ecology and the interaction of mineral forming processes. In gold mining sites, where erosion is the main active subject, more especially in mild climatic establishments, any distinct particles of natural gold go through deposited sediment with little alteration (Liu et al. 2008). The chemical features of sedimentary particles and the nature of preserved trace elements provide a monogram which reveals the type of the ore. The monogram may be established using scanning electron microscopy and can be used to provide information about the source of the mineral. Identification of the type of ore using the technique will be more economical at the onset of exploration of the mining site of interest (Vysetti et al. 2014).

The above has been made possible by use of scanning electron microscopy (SEM) which has been useful by providing a detailed study on the specimen’s surface. It works by use of an electron beam that interacts with sample surface producing electrons of different types near or on the surface of the specimen (Leese, Morton, Gardiner and Carolan 2016). The signals that come out as a result of the interactions reveal information about the sample like appearance, chemical formation, structural lattice and alignment of materials that make up the sample. The data collected in a given area like Leadhills Scotland, on surface distribution of the sample and the two-dimensional images exhibit spatial variations in properties.

Analyzing Trace Elements in Gold

For analysis of trace elements in gold,  ICP-MS is the most popular method used as it covers a wide range of elemental analysis levels even with those having low detection limit up to parts per trillion. The trace elements commonly analysed include Ag, Pd, Ni, Cu, Se, Hg and Pt. In ICP-MS, elements with known composition but with isotopes that are outside the range of multiplier detector output can be analysed (Zhang et al. 2014). It specifically applies the method of external calibration for analysis of trace elements in gold. Here, trace elements can be investigated even of the lowest detection limit with a lot of accuracy.  However, external standardisation for quantitative analysis of gold can lead to erroneous results even to samples that are identically the same with the calibration standards. Also, it is time-consuming in sample preparation and the difficulty in resolving spatial variations within individual samples.

The above mentioned problem can be overcome for natural gold samples by using an external calibration sample only for the significant elements gold and silver, by eliminating the ion pathway and using isotopic silver as an internal calibration standard for each sample for trace elements determination (Scalbert et al. 2009). From the results obtained, it will probably show how they are aligned in the ore where gold is found. The earlier mentioned kind of analysis can be used where there are an insufficient number of calibration samples for determination of trace elements.  However, with the use of LA-ICP-MS, the above drawbacks are minimised since the method reduces sample preparation time, no sample size requirement, there is reduced spectral interference and increased sample throughput (Zhang et al. 2014). The method can uncover more on sample information on the manufacturing process and relationship to each other. With these drawbacks, the effective way to avoid matrix effect is the separation of the matrix element, but it is time-consuming and will require ultra-pure chemicals to prevent contamination. Direct analysis is much easier since it reduces sample preparation time and there is no contamination (Marcus 2013). 

There are stages in the mining process that start with an exploration of the mining site, production of the mineral of interest and closure of the mining field. But, still, there is a need for reclamation of the land for other uses which has become a big challenge. The above has necessitated for more cooperation between stakeholders in the mining sector to work closely with the government to find a sustainable solution that is beneficial to both the consumer and the mining industrialists (Liu et al. 2008).

With this regard, geochemical and geophysical methods have been used in the exploration with researchers focusing on models of extraction that pose minimal impact to the environment, with more emphasis on in-situ extraction processes. Since land reclamation and post-mining land use is expensive, long-term environmental monitoring should be incorporated during mining feasibility studies, health assessment and environment aspects of the mining ore should be well understood during the exploration tenure.

References

Bressy, F.C., Brito, G.B., Barbosa, I.S., Teixeira, L.S. and Korn, M.G.A., 2013. Determination of trace element concentrations in tomato samples at different stages of maturation by ICP OES and ICP-MS following microwave-assisted digestion. Microchemical Journal, 109, pp.145-149. [Online]. Retrieved at: https://www.sciencedirect.com/science/article/pii/S0026265X12000562, [Accessed on 25 December 2018].

Chu, H.W., Unnikrishnan, B., Anand, A., Mao, J.Y. and Huang, C.C., 2018. Nanoparticle-based laser desorption/ionization mass spectrometric analysis of drugs and metabolites. Journal of food and drug analysis. [Online]. Retrieved at:  https://www.sciencedirect.com/science/article/pii/S1021949818301170, [Accessed on 25 December 2018].

De Lacerda, L.D. and Salomons, W., 2012. Mercury from gold and silver mining: a chemical time bomb?. Springer Science & Business Media. [Online]. Retrieved at:   https://books.google.com/books?hl=en&lr=&id=aFd-BgAAQBAJ&oi=fnd&pg=PA1&dq=De+Lacerda,+L.D.+and+Salomons,+W.,+2012.+Mercury+from+gold+and+silver+mining:+a+chemical+time+bomb%3F.+Springer+Science+%26+Business+Media.&ots=RFe2zixjZg&sig=qqgESsBqLvrtc9gLyYlvh7glLbU, [Accessed on 25 December 2018].

Jarošová, M., MilDe, D. and Kuba, M., 2014. Elemental analysis of coffee: a comparison of ICP-MS and AAS methods. Czech J. Food Sci, 32, pp.354-359. [Onlnie]. Retrieved at: https://www.academia.edu/download/34694272/399_2013_Milde.pdf, [Accessed on 25 December 2018].

Kurfürst, U. ed., 2013. Solid sample analysis: direct and slurry sampling using GF-AAS and ETV-ICP. Springer Science & Business Media. [Online]. Retrieved at: https://books.google.com/books?hl=en&lr=&id=urToCAAAQBAJ&oi=fnd&pg=PA1&dq=AES,ICP-MS+and+AAS&ots=FJPtr_iPjG&sig=gZrONxaOVAewUki-wB42b0cf01Q, [Accessed on 25 December 2018].

Leese, E., Morton, J., Gardiner, P.H. and Carolan, V.A., 2016. Development of a method for the simultaneous detection of Cr (iii) and Cr (vi) in exhaled breath condensate samples using μLC-ICP-MS. Journal of analytical atomic spectrometry, 31(4), pp.924-933. [Online]. Retrieved at: https://pubs.rsc.org/en/content/articlehtml/2016/ja/c5ja00436e, [Accessed on 25 December 2018].

Liu, Y., Hu, Z., Gao, S., Günther, D., Xu, J., Gao, C. and Chen, H., 2008. In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS without applying an internal standard. Chemical Geology, 257(1-2), pp.34-43. [Online]. Retrieved at: https://www.sciencedirect.com/science/article/pii/S0009254108003501, [Accessed on 25 December 2018].

Marcus, R.K. ed., 2013. Glow discharge spectroscopies. Springer Science & Business Media. [Online]. Retrieved at: https://books.google.com/books?hl=en&lr=&id=KiUDCAAAQBAJ&oi=fnd&pg=PA1&dq=AES,ICP-MS+and+AAS&ots=j6nAzThIPk&sig=-dmmGeC0XZJld33_6WqhXcIBapM, [Accessed on 25 December 2018].

Scalbert, A., Brennan, L., Fiehn, O., Hankemeier, T., Kristal, B.S., van Ommen, B., Pujos-Guillot, E., Verheij, E., Wishart, D. and Wopereis, S., 2009. Mass-spectrometry-based metabolomics: limitations and recommendations for future progress with particular focus on nutrition research. Metabolomics, 5(4), p.435. [Online]. Retrieved at:  https://link.springer.com/article/10.1007/s11306-009-0168-0, [Accessed on 25 December 2018].

Vysetti, B., Vummiti, D., Roy, P., Taylor, C., Kamala, C.T., Satyanarayanan, M., Kar, P., Subramanyam, K.S.V., Raju, A.K. and Abburi, K., 2014. Analysis of geochemical samples by microwave plasma-AES. Atomic Spectroscopy, 35(2), pp.65-78. [Online]. Retrieved at:  https://www.researchgate.net/profile/Manavalan_Satyanarayanan/publication/287253399_Analysis_of_Geochemical_Samples_by_Microwave_Plasma-AES/links/5742f1bd08ae9f741b379289/Analysis-of-Geochemical-Samples-by-Microwave-Plasma-AES.pdf, [Accessed on 25 December 2018].

Zhang, J., Deng, J., Chen, H.Y., Yang, L.Q., Cooke, D., Danyushevsky, L. and Gong, Q.J., 2014. LA-ICP-MS trace element analysis of pyrite from the Chang'an gold deposit, Sanjiang region, China: Implication for ore-forming process. Gondwana Research, 26(2), pp.557-575. [Online]. Retrieved at: https://www.sciencedirect.com/science/article/pii/S1342937X1300364X, [Accessed on 25 December 2018].