Remobilization and adsorption of colloidal gold on nano-particulate chlorite at the Hardrock Archean orogenic gold deposits: A new tool for gold exploration

Abstract

At the Hardrock Archean orogenic gold deposit, Geraldton, Canada, a correlation was found in drill core samples between the mass of elements lost on ignition (LOI) and gold concentration, and it was demonstrated that the measured LOI was in fact controlled by chlorite. The reason why LOI and gold were correlated was investigated, analyses were made to verify if chlorite and/or other sheet silicate minerals were hosting gold, and a new geochemical exploration tool was developed based on these findings. Approximately 50 drill core samples were disaggregated by gelifraction and separated into 5 different size fractions, a process which also removed the heavy minerals out-, and preferentially enriched the sheet silicates in the (sub) microscopic fractions. The concentration of gold was measured in these size fractions and their enrichment in gold over the corresponding whole rock samples was calculated. Only for the small fractions, it was discovered that for rocks having greater amount of gold, the small fractions were preferentially depleted in gold compared to their related whole rocks, whereas for rocks having lesser amounts of gold, the small fractions were relatively enriched in gold. This was interpreted to show that gold was hosted in sheet silicates for rocks hosting lesser amounts of gold, by contrast to its usual occurrence in sulfides for gold-rich rocks. Also only for small size fractions, the gold enrichment factor was positively correlated only with chlorite, confirming that chlorite was hosting gold. X-ray diffraction analyses of the different size fractions further showed that although chlorite had resisted to meteoric recrystallization, the identification of muscovite and illite variably interstratified with montmorillonite and vermiculite, as well as palygorskite, sepiolite and halloysite highlighted some of the weathering-related recrystallization. It is thus proposed that at Hardrock, the sheet silicates were affected by groundwaters after the emplacement of these rocks and that the weathering remobilized some gold from primary pyrite/arsenopyrite, to secondary colloidal-size chlorite.The idea that gold could be transported as colloidal particles in hydrothermal solutions appeared early during the 20th century. Experiments were made showing that gold could be transported as colloids at temperatures reaching up to 350 °C, Bonanza gold veins from Nevada were interpreted to have formed from colloidal gold and silica depositing as a gel, placer deposits were interpreted to have been formed from fluids transporting colloidal gold, and colloidal gold was identified in fluids escaping from black smokers from the South Pacific Ocean. At Hardrock, the concentration of gold in colloids and that of ionic gold in the accompanying liquid was determined by dialysis and showed colloidal/ionic gold distribution coefficients ranging from 5 to 3000. This was interpreted to suggest that gold was remobilized out of sulfides and transported downstream from the hydrologic flow mostly as colloids. The gold sols must eventually have adsorbed to nano-particulate chlorite, which could also have been transported further downstream. These results demonstrate that the separation of (sub) microscopic sheet silicates using this new method and the measurement of their gold enrichment factors over the related whole rocks appears to be an efficient tool for the exploration and discovery of gold in Archean settings and could effectively lead to increased resources, although further testing of the method is required over horizontal and vertical distances from known mineralization.