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

摘要

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 colloidalsize chlorite.