Hugh de Souza - From King Croesus to Chrysos, the Evolution of Gold Assaying

Hugh de Souza, Ph.D., P.Geo

Geochemical Consultant

Talk Abstract:

The fire assay method for gold determination is the most widely used geoanalytical method in commercial laboratories with millions of samples assayed annually. It is also one of the earliest analytical methods still in use, with references to the method as far back as 2600 B.C.

The developments around 550 B.C. in gold metallurgy and assaying at Sardis in the Kingdom of Lydia that allowed the introduction of the first gold and silver coins in the reign of King Croesus will be discussed. The art of assaying was revived in the Middle Ages and the fire assay method described by Agricola in De Re Metallica published in 1556 more closely resembles the modern method.

Although new methods of gold analysis using XRF and Neutron Activation Analysis were introduced in the last 50 years they were never able to displace fire assay methods as the dominant method. Cyanide leach and acid dissolution methods are alternatives to fire assay but in general Au recoveries are lower than fire assay.

In 2018 Chrysos Corporation commercialized the first PhotonAssay system developed by CSIRO using gamma activation analysis allowing Au determination in 2 minutes. With over 27 units deployed in 2024 this technology appears to be rapidly displacing fire assay methods of gold analysis.

Speaker Bio:

Hugh de Souza is a consulting geochemist who recently retired as Director of Geological Services, Business Development with SGS Minerals in Mar 2022. In this role he provided technical advice on geochemical, mineralogical and other SGS services applied in exploration and mining. In addition, he was involved in implementation of new technologies such as XRF and Hyperspectral Core Scanning and laser ablation ICP-MS. He has managed SGS geochemical, mineralogical and diamond labs over the last 27 years and has worked on the development of new mineralogical and geochemical methods for mineral exploration. Prior to that he was with the Ontario Geological Survey and carried out post-doctoral work at the Universities of New Brunswick and Geneva. He holds a BA Honours in Environmental Sciences (Lancaster), MSc in Geochemistry (Leeds), PhD in Isotope Geology (Edinburgh).

Hugh de Souza | LinkedIn

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