Rio das Velhas Greenstone Belt (RVGB) in Brazil is largely located in the Iron Quadrangle region of the Minas Gerais state, which is the most important gold district in the country. Crrego do Stio (CdS) Mines, herein investigated, are orogenic gold deposits hosted by meta-volcano-sedimentary units operated in underground- and open pit mines for decades by AngloGold Ashanti (AGA) Brazil. Reverse Circulation (RC) Drilling has been re-implemented during 2018 on CdS Open Pit Mines, instead of using conventional Diamond Drilling (DD) to optimize drilling efficiency. The aim was to convert reserves, bring assurance to the mine production plan and gain ounces on a historic gold deposit characterized by multi-narrow veins with high nugget-effect. Previous RC drilling campaigns on the deposit, without the required quality control on the sampling procedures, led to unreliable assay data that was eventually excluded from the modelling and resource estimation processes. This work presents the quality controls that were implemented on the drilling campaigns to ensure sample quality and representativeness. The main controls are comprised of duplicate analysis, mass balances, recovery rates, density measurements, moisture control and water table influences, twin-holes, validation with DD-holes and comparisons with other sampling systems. Parameters of the RC drilling performance in terms of sample quality were analysed to compose Quality Assurance and Quality Control (QA/QC) and method validation reports. Meticulous use of the RC sample validation controls are expected to bring reliability to the mineral resources/reserves of the deposit, especially in terms of ore positioning, gold content, grade and thickness. The method evaluated in this work provides sufficient data and controls to be considered valid for subsequent use in the mining processes, although restrictions and related sampling errors have been identified. The clear knowledge of any drilling and sampling method restrictions, combined with the full understanding of the deposit geological characteristics, are valuable since it better facilitates proper management decisions in mining projects and operations.
CITATION:Gomes, C C O, Spangenberg, I C, Vieira, M M S, Bhering, A P and Costa, J FC L, 2019. Reverse circulation drilling for gold: geological context and sampling method validation Crrego do Stio Mines, Brazil, in Proceedings World Gold 2019, pp 3554 (The Australasian Institute of Mining and Metallurgy: Melbourne).