Hi
@SoConfusedFirstly I'm not a mining expert.
Secondly I won't have a crack at your first question until I see next quarters production numbers, we might be able to do some extrapolations then.
Maybe just some general discussion on the meaning of Oxide and Sulphide might help to get things into perspective for you.
Gold ores are extremely varied but I'll try and dumb it down a bit to explain the difference and why they have the two circuits (apologies to the real experts).
OK gold normally occurs in rocks in its native form or as an alloy with silver (called electrum). It doesn't commonly for compounds (chemical complexes) in rocks, only on rare occasions.
OK so we've got that. Gold typically occurs in its native metal form in rocks.
Sometimes the metallic gold is associated with other minerals and commonly it is closely associated with sulphide minerals such as pyrite (FeS2), chalcopyrite (CuFeS2) or arsenopyrite (Fe,Co)AsS to name a few.
Sometimes the metallic gold isn't associated with sulphide minerals at all.
In cases where the gold is in a simple metallic form and not associated with sulphides the gold can generally, depending on its size and shape, be removed from the rock using gravity methods and/or through a leaching process using cyanide that takes the gold from a solid form into a water-soluble coordination complex which is later recovered through other downstream processes.
https://en.m.wikipedia.org/wiki/Gold_cyanidationhttps://en.m.wikipedia.org/wiki/Coordination_complexIn cases where the gold is closely associated with large quantities of sulphide minerals the gold is usually captured in the form of a dry concentrate with the sulphide minerals after undfoing froth floatation. This concentrate mixture typically requires roasting (as at Syama) to liberate the gold before cyanidation and further downstream processes extraction processes. AQG at Copler use pressure oxidation instead of the roasting step. At a similar pressure oxidisation centre to Copler in Russia the concentrate is re-ground prior to being put into the pressure oxidisation vessels (autoclaves).
OK you should be getting the picture of these two general types of deposits, one just comprised of metallic gold in the rock without sulphides and one comprising of metallic gold in the rock in close association with sulphide minerals. You are also probably wondering why I haven't talked about oxide ore yet. Well oxide or oxidation refers to the upper levels of the rock that have been exposed to oxygen directly through the air or through processes involving meteoric water. The upper oxide layers of the rock have had the benefit of the chemical reactions that go along with oxygen and water that tend to convert sulphide minerals to oxide minerals over time. For example pyrite (FeS2) converts to acid (H+), sulfates (SO42-), and aqueous Fe(II) in the presence of water. So ineffect nature is doing some of the work of decomposing the rock into a form where the metallic gold is dominant in the area exposed to air and water. But the water and air don't always act to oxidise the rock homogeneously as nature is rarely homogeneous. Fractures and cracks in the rock, the permeability of the rock, the height of the water table are all factors that can affect the degree and depth of oxidation. Also the downward percolation of meteoric waters can leach the upper parts of the ore body of gold meaning gold grades are lower in the upper parts of the oxidised rock and higher around the water table in the transitional zone where rocks go from being completely oxidised to partially oxidised before becoming fresh and praticaly free of all oxidation.
You can see from this description that where a gold is closely associated with sulphide minerals the characteristics of the deposit can vary according to the degree and patterns of oxidation. This complexity also flows through to the type of processing the various ore must undergo after being mined. In the case of a deposit that contains only metallic gold (no sulphides) the degree of oxidation really only comes through in the ease with which the rock can be dug (ie highly oxidised and weathered rocks are generally softer and easier to dig, often not requiring any blasting), the process route both in oxidised ore, transitional ore and fresh ore is likely in most case not to envolve more complex pre-treatments like froth floatation or roasting before cyanidation. That's the difference between the two circuits. I've read in recent quarterly reports that RSG have built and comisioned a gravity separation circuit for the ore being mined at Tabakoroni. I'd expect a high internal rate of return from the money spent there and the Dec quarterly said it was already having an impact (see statement from quarterly below).
"Commissioning of the gravity gold circuit was also successfully completed in conjunction with the commencement of treatment of the Tabakoroni material with an immediate positive impact."
I haven't read anywhere about the particular mineralogy of the ores at Syama but JW has often referred to them as double refractory which in all likelihood means that the metallic gold is in large part not only closely associated with the sulphide minerals but is also probably very fine and encapsulated by the sulphides themselves making it much harder for cyanidation fluid to make good contact with the gold grains, ie some of the metallic gold in the rock is less free and harder to release through grinding and cyanidation. The double refractory description probably refers to the carbonaceous matter in the rock that contributes to peg-robbing (see full explaination of double refractory in the link below which describes some ores found in Ghana). You will recall that most of the gold in circuit is this type of material which is awaiting the construction of the low carbon roaster to maximise the recovery of gold from these concentrate stockpiles. Another low CapEx project which should give a high internal rate of return and liberate more than $100 million dollars worth of gold that is waiting for processing.
https://www.911metallurgist.com/blo...Preg-robbing-Characteristics-of-Gold-Ores.pdfI suppose this actually answers your first question to some degree as you should be able to see from this that the recoveries will depend on a number of variables not just grade.
Once we are sourcing ore from UG in high grade fresh unoxidised rock at Syama the recoveries should improve to the targeted level which I assume have been replicated in bench testing and/or pilot tests. JW has also said on a number of occasions in announcements and in conference calls that targeted recoveries have been achieved when high grade batches of UG have been fed through the circuit. One would need to assume that this is not a lie.
Here is an extract from the Dec 2018 quarterly report that says as much.
"All elements of Resolute’s sulphide processing improvement project, Project 85, have been commissioned and are operating effectively.
Significantly, during periods when 100% underground sulphide ore has been treated, the Syama processing plant has delivered recoveries greater than 85% and provides confidence in our ability to achieve targeted recoveries from Syama."
You can either be like Colin and believe everything the company says are lies or you can read their announcements at face value, like a normal person, and wait to let results do the talking. Esh
(20min delay)
