Angostura -- Metallurgy of Ore and Occurrence. A work in progress.
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(106 microns is relatively coarse as gold milling goes, it interpolates very closely to a standard US seive size of 140 mesh openings to the inch. (Wire size counts in mesh, so it is not an exact division. There are 25,400 microns to the inch)
A mass pull (indicated below in bulk flotation test work) of 12% indicates that the ratio of concentrate to feed bulk is 8.5 to one. This is rather low grade concentrate, but it does not defeat post-treatment in-house by intensive cyanidation. (20 to 30 to one is preferable)
All in all I would say Angostura is not the easiest ore to handle but it is by no means a show stopper, especially given its generous grade and volume and anticipated high nugget effect. You want to have a reliable gravity circuit in the mill.)
The have a bit of a war between oxidized ore which does not float and unoxidized ore which does. I don't see how they get 90% recovery into a sulphide con by flotation, unless they are referring to only the unoxidized ore in this case. I think this is what they mean. "Fresh ore" below where they got their 90% recovery, would seem to mean the unoxidized part and in tables in the report this is how they distinguish it. Fresh ore is separate from oxidized. So if that is the case they must come up with a way of separating the two ores. I think they mean that the two components may be intermixed but I cannot tell from their description. Since they have two resource calcs for each category it would seem that they are not closely mixed, so can be separately mined.
Would it be flotation, special treatment on that component, and then leaching the waste of the flotation circuit? Or would it mean oxidizing the whole schlemiel like Barrick did at Poste Betze with an autoclave? Full tilt boogie. I don' think they can do that with the grade being what it is. they have to concentrate to get a grade good enough to oxidize, and I am not sure that it would come up that well with only 8.5 to one mass pull. In the sulphide zones that is only as far as I can see 11.9 grams per ton Au in the Con.
I would also consider sink-float or flotation after a relatively coarse grind, (as the fresh sulfides are the coarser ones, to get most of the sulphides, oxidize that concentrated portion in an autoclave, or use roast or other process, and then grind finer and cyanide leach the remainder. Of course you will get some fine sulfides that are refractory. There will always be losses.
This assumes that there is a separation necessary. If this is not the case, then we simply need two differing processes for different areas and ores and the problem is much simpler. I lean towards the latter interpretation, although Strathcona did not make this perfectly clear.
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Strathcona Mineral Services Limited
greystarresources.com
Continuing metallurgical testwork currently being undertaken by two laboratories has concentrated on the treatment of the ore by heap-leaching and by flotation. The results indicate that the oxidized mineralization is amenable to heap leaching, and that the unoxidized mineralization contains a refractory component that makes conventional heap leaching unattractive.
The response to the flotation testwork has been the reverse, with oxidized ore being unresponsive.
The recovery of gold from fresh mineralization into a bulk sulphide flotation concentrate was superior to the recovery by direct cyanide leaching at a grind of 106 microns, and appears to be a promising approach.
Additional flotation testwork including locked-cycle tests are planned to confirm initial batch results that indicate gold and silver recoveries of 90% into a bulk sulphide concentrate with a mass pull of 12%.
The subsequent recovery process of gold and silver from the concentrate has yet to be identified. Leach and flotation test work is continuing and the results will be incorporated into the preliminary economic assessment by Hatch.
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Angostura is part of the Angostura-California gold province, a belt of epithermal gold occurrences of the high sulphidation type and characterized by the association of gold with silver, copper, arsenic, bismuth, molybdenum and tellurium. The existence of a breccia body hosting copper and molybdenum mineralization four kilometres from the site points to a connection with a (buried) porphyry system. Most of the gold is contained within several sets of anastomosing veins and tabular breccia zones. Alteration within the vein-like structures is dominated by silica, both in the form of free quartz and as silicification, and sericite, while the host rocks are strongly argillized. Several hydrothermal pulses are discernable and show a decrease of temperature with time, from >300° C to about 250° C, according to fluid inclusion studies.
The Angostura gold-silver mineralization occurs in a swarm of veins and mineralized structures that strike east-west, northeast-southwest and northwest-southeast and dip steeply to the north. Recent structural studies have identified five stages of mostly brittle deformation, four of which are mineralized. It is the continued history of deformation and intrusive activity that provided the thermal impetus and structural pathways for a major precious-metal deposit.
The mineralized structures may be single veins, but are more often made up of several, closely spaced composite veins. Almost two hundred individual veins and composite structures have been correlated to date on the basis of surface mapping, mapping of underground workings, and interpretation of drill hole data that vary in width from less than two metres for individual veins to over 40 metres for composite structures.
The gold mineralization has two grade populations. The majority of values (92%) are less than two grams per tonne (g/t) and show good continuity within the veins. A small high-grade population represents high-grade shoots and vein segments located in structurally favourable locations such as at the junction of two veins of different direction. Mapping and sampling of underground openings has confirmed the general grade character of the veins and of a few high-grade shoots.
The current drill hole density, however, is generally not capable of providing reliable data on the size and shape of the shoots, which have an influence on the overall resource grade larger in proportion than their small volume. The realistic modelling of the volume and fo the grade of the shoots is thus a prerequisite for any reliable resource estimate.
There are two areas in the northern part of the deposit that were modelled as relatively large bulk zones. This concept was introduced to account for continuous mineralization found in these areas that is difficult to resolve into individual veins.
In addition to veins, disseminated mineralization above cut-off grade occurs in several areas between the veins, most notably in the south of the Angostura property. Part of this mineralization constitutes obvious veins that have not been found in adjacent holes, while the remainder is more properly called disseminated.
Surface oxidation has affected the rocks at Angostura to depths of up to 170 metres along specific structures. It is more generally in the range of 10 to 30 metres at the edge of the deposit, and attains depths that vary from 40 to 100 metres in its central parts. The oxidation is irregular in shape and is of partial character, i.e., fine-grained sulphides in dense quartz often remain fresh even near surface, while the coarser sulphides are often completely oxidized.
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A critical review has shown that the previous concept of attempting to physically model the highgrade mineralization by designing shoots around high-grade intersections was arbitrary since the detailed information required is not yet available for much of the deposit. The current estimate takes a different approach by estimating the probability of each block in the block model to contain high-grade mineralization. A number of trial runs were undertaken until a set of probability
interpolation parameters was found that produced realistic results. This is a critical issue since it is estimated that the high-grade population of the deposit, which occupies less than 10% of the deposit volume, contains more than 50% of the contained gold. The interpolated probabilities were used to calculate high-grade and low-grade tonnages for each block.
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All of the above was from an early study of the ore. The same things exists today as then, but more alternatives are under review including bio-ox and full scale heap leaching. This is what the new tech reports indicate.
They speak of oxide, transitional, low grade sulphide and higher grade sulphide ore groups. They all have their amenability to different treatment processes. However usually dual circuit approaches seem to have some efficacy in test work.
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The current testing involves detailed evaluation of the following:
• Heap Leach Cyanidation – oxide, transitional and intermediate sulfide ore types: including optimization of processing parameters (feed size, cyanide concentration, etc.), and further evaluation of ore variability, as well as required geotechnical and environmental testing.
• Milling/Cyanidation – intermediate and high grade sulfide ore types: including optimization of processing parameters (grind size) and further evaluation of ore variability.
• Flotation and Flotation Product Processing – high grade sulfide ore type: including optimization of processing parameters (grind size, reagents, pull weight, etc.), evaluation of concentrate processing (biooxidation, shipping to smelter, roasting, etc.) and tailings processing (agitated leaching or heap leaching).
• Heap Bio-oxidation/Cyanidation – intermediate sulfide ore type
• Comminution Characteristics – rock type composites
• Environmental Characterization – process residue samples generated from select
metallurgical testing.
Available testing results from the current testing program have generally been consistent with those generated during earlier testing programs. The oxide, transitional and low grade ore types have been confirmed to be amenable to simulated heap leaching treatment at 19mm feed size. Preliminary data indicate that the oxide ore types have shown comparable gold recoveries when the crush size was coarsened to 38mm. Follow-up testing currently being initiated will include evaluation of run-of-mine (ROM) heap leaching.
Other ore types have not yet been evaluated at coarser (than 19mm) feed sizes. The intermediate sulfide ore types were varied in their response to heap leach cyanidation treatment at the 19mm feed size, and work is in progress to obtain a better understanding of the parameters controlling response to heap leaching of these ore types.
Milling/cyanidation treatment generally resulted in incrementally higher gold and silver recoveries than obtained by heap leach cyanidation. Results from the GRD-Minproc trade off study probably indicate that this processing option will not merit further consideration.
The sulfide ore types evaluated responded well to conventional flotation processing. The flotation rougher concentrate generated from a master composite responded well to roasting, followed by cyanidation of the roasted calcine. Combined cyanidation gold recovery (including cyanidation of the calcine and flotation tailings) was approximately 85%. Detailed work is currently in progress to optimize roasting conditions, and determine the possibility of improving this recovery. Stirred tank biooxidation testing is in progress on flotation concentrate generated from the same master composite to confirm amenability to this processing alternative. Testing to evaluate processing of lower grade refractory sulphide ore types using heap biooxidation pretreatment followed by heap cyanidation treatment is in progress.
Preliminary results indicate that heap bio-oxidation pretreatment has been effective for improving gold recovery from the intermediate sulfide ore types by approximately 20%, after 105 days of pretreatment. These tests are ongoing, so the ultimate improvement in gold recovery has yet to be determined.
16.4 PROCESS SELECTION
Based on results from the Hatch Scoping study and GRD Minproc Trade Off studies, both incorporating available metallurgical results at the time of the study, a two circuit process flowsheet is currently being considered:
1. Oxide, transitional and low grade sulphide ore types would be processed using conventional valley-fill heap leach operation with tertiary crushing (nominal 19mm feed size) of ore. ROM leaching was also considered as an alternative in the Minproc study. Metallurgical testing is planned to evaluate ROM leaching, but data are not currently available to support that alternative.
2. Higher grade sulphide ore would be treated by milling (nominal 106um feed size)/flotation treatment. In the Hatch study, stirred tank biooxidation treatment of the flotation concentrate, followed by cyanidation of the biooxidized
concentrate was considered. In the Minproc study, a number of
processing options, including POX, stirred tank biooxidation and roasting of the flotation concentrate were considered, with cyanidation of the oxidized residue.
The same two general processing circuits, along with the possible shipping and treatment of flotation concentrate at a smelter are also currently being considered. Detailed
metallurgical testing currently being conducted is designed to generate data sufficient for final selection of the optimum processing option for the Angostura flotation concentrate.
Ongoing economic evaluation of the possible processing options requires estimation of commercial processing parameters for the various alternatives, which are described in the following sections.
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