The high-grade 100%-owned Cascavel Gold Mine (Cascavel) lies within Orinoco’s Faina Goldfields Project (Faina Project) in the State of Goiás in central Brazil.
Cascavel was discovered by the Portuguese 'Bandeirantes' in the 1740's during Brazil's initial gold rush, however Orinoco was the first company to conduct extensive exploration at the site in modern times.
In 2013, Orinoco confirmed the presence of a large gold system at Cascavel, and in 2016 the Company commenced development aimed at establishing a 50,000 – 100,000tpa underground mine feeding an on-site gravity processing plant.
Cascavel hosts high-grade, structurally-controlled coarse gold shoots within a broad mineralised quartz vein system, where underground sampling has returned bonanza grades including 15 metres grading 88 g/t Au. Bulk samples from the Cuca winze (350m north of Cascavel portal) and the nearby Mestre winze (90m south of Cascavel portal) have recorded grades of 27g/t Au (from a 2.8 tonne sample) and 39g/t Au (from a 500 kg sample) respectively.
While the coarse non-refractory nature of the gold offers many advantages in terms of low-cost processing, it does make the task of resource estimation (within the JORC code) incredibly difficult due to the very coarse nature of the gold (80% > 100microns) and the small volume of sample that can be collected through diamond drilling (approximately 8kg/m HQ core).
Whilst drilling provides a highly effective measure of the geological continuity of the Cascavel quartz vein system, it is not particularly useful for grade estimation in such high-grade, coarse gold systems.
However, the Company has undertaken a significant amount of work at Cascavel since 2012, including over 8,000m of diamond drilling to delineate the gold-bearing quartz vein structure. This has been coupled with underground development, bulk sampling and specific vein and alteration sampling to provide an understanding of the distribution of gold grades within this extensive mineralised system.
Based on the nuggetty nature of the gold mineralisation at Cascavel, the Company considered that the commencement of low-cost trial mining would be the most effective way to take the project forward.
An initial phase of mine development and stoping trials was undertaken at Cascavel between August and October 2016. This initial mining campaign provided important information regarding the optimal strategy for mining and processing at Cascavel and, following a thorough operational review, the Company is now implementing extensive operational and strategic improvements. Additional review of mining dilution and grades was undertaken in December 2017 to improve the overall gold outcome.
The initial improvements were focused on increasing mechanisation, reducing dilution and boosting productivity. Following this, the mining method has been taken “Back to Basics” to selectively mine the high grade veins and then remove the waste material with mechanised means.
A crushing and gravity recovery circuit has been installed onsite only 300m from the Cascavel mine portal, with a processing capacity of 90-100,000 tonnes per annum.
Metallurgical test work undertaken following the initial phase of mining confirmed the amenability of the ore to gravity processing. These results confirm both the Company’s previous metallurgical test work programmes.
The optimal process plant configuration is currently being reviewed, but a “Back to Basics” approach of using traditional hammer mills to fine grind the ore to liberate the gold particles is now being undertaken in order to evaluate results compared with the Gecko gravity circuit.
Drilling and exploration along strike from Cascavel indicates that the gold-bearing structures have considerable continuity, with gold mineralisation shown to be occurring at multiple points over more than 3km immediately along strike from Cascavel, and on the same generation of mineralised shear zones 18km to the south at the Sertão Gold Project (Sertão). Similarly, the down-plunge continuity of the gold-bearing structures has been demonstrated in drilling at both Cascavel and Sertão to over 600m down-dip – and remains open.
Regionally, this high level of continuity is a common feature of the shear zone hosted gold deposits in the greenstone belts, with both AngloGold and Yamana having multi-million ounce resources in similar structures.
The evidence for mineralisation in and around the Cascavel area gives the Company great confidence that it can rapidly grow the potential of the Faina Project.