Independent testing on Orinoco Gold’s (ASX: OGX) Mestre gold mineralisation and tailings has returned high-grade results, with the Mestre mineralisation within the company’s Cascavel project in Brazil averaging 28.75 grams per tonne gold.
The test involved a third-party ball mill and carbon-in-leach (CIL) processing on 8.94 tonnes of Mestre ore and 22.11t of tailings to recover gold with average grades of 28.75g/t and 6.143g/t, respectively.
During the test, 257.1 grams of gold dore was produced with gold recovery rates of 98.81%.
Orinoco compared this result by feeding 10t of Mestre ore through its gravity-only Gekko mill and only produced 85.7g of gold dore at an average grade of 8.57g/t. A similar tailings sample was also pushed through the Gekko mill and generated 50.64g of gold with an average grade of 2.11g/t.
“The extremely high-grade results go a long way to show Cascavel is a potentially rich and large gold system, which has so far not performed to its true potential,” the company stated.
Orinoco added the gold grades recovered from the tailings sample indicate the operation’s current mill has “likely been losing a lot of gold” over the past two years.
The third-party miller informed Orinoco that a high percentage of the gold it recovered was an “extremely fine” material.
Orinoco said this may explain why the mill had not been capturing the gold mineralisation.
Back to basics
Since the Orinoco commissioned its Cascavel project two years ago, the company has failed to achieve production milestones.
After initial mining was unsuccessful in generating the predicted gold ounces, Orinoco went back to the drawing board.
A review found gold ounces were lost during mining and within the gravity recovery plant.
Orinoco then implemented a “back to basics” approach in December 2017, which led to a series of hammer mill trials, involving crushing ore into smaller particles.
Multiple hammer mill trials on ore from various deposits at the project, as well as tailings generated “dramatically improved” grades.
According to the company, the “striking difference” in the third-party ball mill and CIL results and its own has prompted it to begin searching for a potential plant that can process its ore and tailings using CIL.
Additionally, Orinoco engaged Gell Street Mining to optimise the plant to improve current recovery rates.
Gell Street Mining’s optimisation involved altering the gravity mill’s inline pressure jig as well as making adjustments to enhance fine gold recoveries from mineralisation and tailings.
The company also plans to fast-track licensing to develop its own CIL plant at Rio Do Ouro (formerly Sertao), where it has achieved exploration success and a previous operator produced more than 250,000oz gold.
“Overall, the CIL tests confirm in our mind that Cascavel is extremely high-grade and the approximate 40,000t of dry stacked tails could contain a significant quantity of gold,” the company stated.
During the September quarter 2018, Orinoco produced 316oz gold and attributed the low number to “substantial losses into the tailings” and lack of reliable underground equipment.
Orinoco is preparing for a better December period, with the optimisation work on the mill completed and plans to step up processing.
As part of this plan, Orinoco has moved from one shift to two shifts per day to boost processing capacity to 3,000t of tailings a month.
“We hope the adjustments to the mill and the increased volume of feed from tailings will allow us to reach our first production milestone in the near term,” Orinoco stated.
Today’s news spurred Orinoco’s share price up more than 10% to reach $0.022 by late afternoon trade.