Greystar Resources Announces Updated Metallurgical Recovery Model for the Angostura Gold-Silver Deposit, Colombia
VANCOUVER, BRITISH COLUMBIA -- (Marketwire) -- 07/15/10 -- Greystar Resources Ltd. (the 'Company') (TSX: GSL)(AIM: GSL) is pleased to announce an updated metallurgical recovery model (FS Recovery Model) and process flow for the Company's 100% owned Angostura gold-silver deposit located in Colombia's California mining district. The updated model, which will be incorporated into the Feasibility study (FS) scheduled for publication in the second half of 2010, replaces the metallurgical model used in the May, 2009 Preliminary Feasibility study (PFS Recovery Model).
The results of the metallurgical testing have the following average gold recoveries by ore type.
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Average Average Metallurgical Test
Metallurgical Results - FS(6)
Test Results ---------------------------
Process Ore Type -PFS(1) 19 mm(2) 38 mm(3)
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Oxide 90% 91% 91%
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Heap Leach Transitional 73% 74% 70%
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Low Grade
Sulphide 39% 33% 30%
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Flotation/
BIOX/CIP/Heap Leach High Grade
Flotation Tails Sulphide 94%(4) 86%(5)
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1) Heap Leach average metallurgical results in the PFS based on 18 Column
Leach Test (CLT) at 19 mm.
2) Heap Leach average metallurgical results in the FS based on 77 CLT at 19
mm (includes all the samples tested at 38 mm).
3) Heap Leach average metallurgical results in the FS based on 11 CLT at 38
mm.
4) High grade ore circuit average metallurgical results in the PFS based on
90% flotation gold recovery, 98.5% Smelter Recovery, 54% heap leaching
recovery of flotation tails agglomerates.
5) High grade ore circuit average metallurgical results in the FS based on
91% flotation gold recovery, 90% BIOX® /CIP recovery, and 50% heap
leaching recovery of flotation tails agglomerates.
6) Heap leach feed size sensitivity (38mm vs. 19mm) employed for the FS
recovery model was determined considering only samples tested at both
feed sizes, rather than average results as presented in the table shown
above.
Updates to the recovery model include:
-- A coarsening of the planned heap leach feed size to 38mm.
-- A new geo-metallurgical model to project heap leach recoveries.
-- A revision to the high grade recovery circuit to include stirred tank
bio-oxidation and carbon-in-pulp (CIP) cyanidation of the flotation
concentrate.
The PFS Recovery Model included heap leaching at a 19mm feed size, a simpler empirical interpretation of heap leach recoveries based on analyzed sulphur content and shipping of flotation concentrate to a smelter, rather than on-site processing.
The FS Recovery Model is based on a new geo-metallurgical model that uses a re-interpreted oxidation profile of the Angostura deposit to project heap-leach recoveries. Partial or complete oxidation has enhanced the ability of cyanide to dissolve the fine-grained Angostura gold mineralization. The geology team of Greystar, assisted and guided by Strathcona Mineral Services in Toronto, developed the updated oxidation profile based on the characterization of approximately 290,000 meter of drill core. A core interval was characterized as fresh rock in the total absence of iron oxides affecting the ubiquitous sulphide minerals. A core interval was characterized as oxide rock where pervasive oxidation has reduced the total sulphur content to less than one percent (typically less than 0.5%), and the log indicated at most a trace of sulphides being present. Transition rocks are those with a presence of both limonite and sulphides, and as a result have sulphur values above 1% sulphur. The oxidation code assigned to the core intervals was used to model in three dimensions the lower limit of oxidation that separate the sulphide or fresh rocks from oxide and transition rocks above. This lower limit of oxidation is irregular and its position depends on the permeability of the rock mass as a result of alteration and fracturing. The lower limit of oxidation can be found locally just a few metres below surface where modern creeks have provided erosion but reaches to depths of more than 250 metres elsewhere; in general, oxidation extends to an average depth of 95 metres.
The oxidation profile at Angostura is immature and has resulted in a complex arrangement of oxide, transition and remnant fresh rock above the lower limit of oxidation. This was modelled in three dimensions using indicator kriging which estimates the probability of a block to belong to one or more of the three different oxidation rock types based on the surrounding drill-hole information. As a result, each block in the block model now has an estimate of the proportion of oxide, transition and fresh rock. An oxide block contains more than 60% oxide rock, less than 10% fresh rock and has a total sulphur content of 1% or less. A fresh block contains 45% or more of fresh rock and 8% or less of oxide rock. Any block not identified as oxide or fresh is designated as transition. The choice of these parameters was derived from the gold recovery characterization of groups of samples used for the heap-leach test work. This classification reflects the often 'mixed' nature (with respect to oxidation composition) of the Angostura ores above the lower limit of oxidation. This geo-metallurgical model was adopted in resource modeling and mine planning by NCL Ingenieria y Construccion of Santiago, Chile.
The FS metallurgical processing routes for the Angostura ore will be driven by the FS Recovery Model with;
-- Oxide, transitional and low-grade sulphide ore processed by conventional
cyanide heap leach and agglomerated flotation tailings heap leach.
-- High grade sulphide mineralization will be treated via milling,
flotation, stirred tank bio-oxidation, carbon in pulp cyanidation of
bio-oxidized residue and pulp agglomeration heap leaching of flotation
tailings.
Heap Leach FS Recovery Model
Testing was carried out by Metcon Research and McClelland Laboratories (MLI). The Heap Leach Recovery model was developed by McClelland Laboratories (MLI) after extensive testing of the oxide, transitional and low-grade sulphide materials at both MLI and Metcon Research. A full copy of the MLI report will soon be posted on Greystar's website at www.greystarresources.com.
Response of the Angostura ore to simulated heap leach cyanidation treatment was determined by standard column percolation leach tests conducted on 77 metallurgical samples, at an 80%-19mm feed size. Results from those tests were extensively analyzed with consideration of ore zone, extent of oxidation, lithology, alteration composition, depth and results from detailed head analyses. Results from this data analysis were used to develop a model for estimating commercial heap leach response of the Angostura ore at a tertiary crush (19mm) feed size.
The 19mm heap leach estimates were then factored, based on actual results from comparative column leach tests on a smaller number of samples (11) conducted at a 38mm feed size. These factored estimates were used for estimating commercial heap leach response of the Angostura ore at a secondary crush (38mm) feed size. Recovery model results for heap leaching of secondary crush (38mm) Angostura ore are shown below.
Heap Leach Recovery Model, 38mm Feed Size
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Oxidation Total S Content,
Ore Types Composition(4) % S Au Rec., %
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Au Rec. = 0.246
(i) (% Oxides) +
Oxides(1) All All 67.297
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Au Rec. = 0.246
greater than (i) (% Oxides) +
Transitionals(2) 20% Oxides All 62.297
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Au Rec. = 0.246
less than 20% (i) (% Oxides) +
Transitionals(2) Oxides less than 1.0% 62.297
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less than 20%
Transitionals(2) Oxides greater than 1.0% 56
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Au Rec. = -3.600
(i)(STotal (%)) +
Sulfides(3) All All 37.469
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Reagent Consumption
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kgNaCN/ kgCaO/
Ore Types Ag Rec., % mt ore mt ore
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Oxides(1) 48 0.3 1.3
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Transitionals(2) 56 0.5 1.4
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Transitionals(2) 56 0.5 1.4
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Transitionals(2) 56 0.5 1.4
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0.8 for Cu less than 0.03%
1.6 for Cu 0.03% - 0.10%
Sulfides(3) 34 2.4 for Cu greater than 0.10% 1.6
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1) Oxides ore type is defined as all ore containing greater than 60%
oxides, less than 10% sulfides and less than 1% Total S.
2) Transitionals ore type is defined as all ore not classified Oxides or
Sulfides ore types.
3) Sulfides ore type is defined as all ore containing greater than 45%
sulfides and less than 8% oxides.
4) Oxidation composition, on a gold weighted basis.
High Grade Sulphide (Flotation/BIOX®/CIL/Heap Leach Flotation Tails) FS Recovery Model
The high grade processing circuit considers whole ore milling (106 micrometer grind), conventional sulphide flotation treatment of the milled ore, BIOX®/CIP processing of the flotation concentrate, and agglomeration heap leaching of the flotation tailings. The Flotation/BIOX®/CIL/Heap Leach Flotation Tails Recovery model was developed by MLI. A full copy of the MLI report will soon be posted on Greystar's website at www.greystarresources.com.
Extensive milling/flotation testing was conducted on multiple drill core composites from the Angostura Project over the last 4 years. Early testing was conducted at Metcon Research, SGS (South Africa) and G&T Metallurgical. More recent testing was conducted at MLI, SGS (Santiago) and SGS (South Africa).
Recovery model flotation recovery estimates are based on results from flotation variability testing, conducted on 36 ore variability drill core composites, 13 ore zone master composites and three overall master composites, as well as locked-cycle flotation tests conducted on the same ore zone and overall master composites.
Preliminary flotation concentrate stirred tank bio-oxidation testing, with agitated cyanidation of the resulting bio-oxidized concentrate has been performed at MLI. Recovery model concentrate BIOX®/CIP recovery estimates are based on results from that testing, and on Goldfields operational experience (discussed below) with other commercial concentrate BIOX® processing circuits. BIOX® bulk batch testing is ongoing at Goldfields that supports these recovery estimates.
Agitated cyanidation tests have been conducted on flotation tailings from the locked cycle flotation testing described above. The recovery model estimate of recoveries from heap leach cyanidation of the flotation tailings are based on results from these short terms (3-4 day) cyanidation tests. These tailings recovery estimates are supported by results from a 'pulp agglomeration' column leach test conducted on a crushed (38mm feed size) oxide/transitional ore composite agglomerated using flotation rougher tailings (106 micrometer feed size) produced from a high grade sulphide master composite.
A summary of the recovery model recovery estimates is shown below.
Recovery Model, Angostura HG Sulfide Circuit, Whole Ore Flotation,
with Stirred Tank Biooxidation of Flotation Concentrate, and Heap Leach
('Pulp Agglomeration') Cyanidation of Flotation Tailings, 80%-106 micrometer
Feed Size
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Tailings
Flotation Conc. Bioox./CN Cyanidation Combined
Au Ag Au Ag Au Ag Au Ag
Ore Rec., Rec., Rec., Rec., Rec., Rec., Rec., Rec.,
Zone % % % % % % % %
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Central 90 72 90 75 46 47 86 67
Los
Laches 85 78 90 75 50 56 84 71
Peresoza 90 90 90 75 39 64 85 74
El
Silencio 90 87 90 75 50 50 86 72
Veta de
Barro 86 84 90 75 65 62 87 73
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Sulphide Concentrate FS Treatment
Stirred-tank bio-oxidation of the Angostura flotation concentrate followed by cyanidation of the oxidized residue was found to be more solid and economically attractive than direct shipment of the flotation concentrate to a smelter (May 2009 PFS), roasting of the flotation concentrate with agitated cyanidation of the roasted calcine, or pressure oxidation (POX) treatment with agitated cyanidation of the POX residue. The comparative process routes were analyzed in GRD Minproc/AMEC/NCL/Greystar trade-off studies. Ultra-fine grinding and leaching of flotation concentrate was also evaluated during metallurgical testing at G&T Metallurgical, but the resulting gold and silver recoveries were too low for consideration in the trade-off study. A preliminary stirred-tank test at McClelland Laboratories, confirmed the amenability of the Angostura flotation concentrate to bio-oxidation with subsequent carbon-in-leach cyanidation of the bio-oxidation residue. Based on these positive results Greystar Resources contracted Gold Fields Limited, the world leader in tank bio-oxidation processing, for a continuous BIOX® mini-pilot plant run using bulk flotation concentrate from the Angostura deposit. Flotation concentrate preparation and testing associated with the BIOX® pilot plant is being done at SGS Lakefield Research Africa in Johannesburg under the direction of Gold Fields Technical Division, BIOX® Department. This test program, to be completed by October 2010, will provide the necessary data for designing the commercial bio-oxidation treatment plant, establishing optimum conditions for gold recovery, recycling of toxin-free solution from the BIOX® plant and generating an environmentally stable solid waste product.
Gold Fields BIOX® process has been commercially available for over 20 years. Eleven BIOX® plants have been commissioned since 1986; eight of these are in operation today. The largest BIOX® plant to date is the 1,069 tonne per day flotation concentrate facility commissioned in mid-2008 at Kolpatas, Uzbekistan. Phase 2 development of this BIOX® plant is now under way and will increase capacity to 2,137 tonne per day of flotation concentrate. This is possible because of the modular design of BIOX® plants.
The BIOX® process uses naturally-occurring microorganisms, which break-down the sulphide mineral matrix that encapsulate precious metals. This exposes the precious metals for subsequent cyanidation, substantially increasing recoveries. The BIOX® process involves the continuous feeding of flotation concentrate into a series of aerated, stirred reactors. Certain constituents, such as iron and sulphur, are dissolved during the process; the precious metals remain associated with the solids. After bio-oxidation, which requires 4 to 6 days, the solids are separated from the liquids. The BIOX® product is washed in a counter-current decantation circuit and then treated in a conventional carbon in pulp cyanide plant to extract the precious metals. Gold recoveries are typically 90% or higher. The solution is treated and recycled.
The BIOX® process has many advantages over other refractory processes such as roasting, pressure oxidation and nitric acid leaching. These include:
-- High precious metal recovery
-- Significantly lower capital and operating costs
-- Robust technology suitable for use in remote areas
-- Lower level of skills required for operation
-- Environmentally non-polluting yielding products that meet the US EPA
standards and allow recycling
-- Ongoing process development and improvement
About Greystar Resources Ltd.
Greystar Resources Ltd. is a precious metals exploration and development company that is currently completing a feasibility study on its wholly owned, multi-million ounce Angostura gold-silver deposit in northeastern Colombia. A positive prefeasibility study announced on March 25, 2009 envisions average annual production at Angostura of 511,000 ounces of gold and 2.3 million ounces of silver over a 15 year mine life.
Forward-Looking Statements
Certain statements in this news release are 'forward-looking' within the meaning of Canadian securities legislation. They include statements about future management and the Company's anticipated transition from a gold exploration company to a gold production company.. Forward-looking statements are necessarily based upon a number of estimates and assumptions that, while considered reasonable by the Company, are inherently subject to significant business, economic, competitive, political and social uncertainties and other contingencies. Many factors could cause the Company's actual results to differ materially from those expressed or implied in the forward-looking statements. These factors include, among others, conclusions or realization of mineral resources, the actual results of exploration activities, possible variations in ore grade or recovery rates, fluctuations in the price of gold and silver, risks relating to additional funding requirements, political and foreign risks, production risks, environmental regulation and liability, government regulation as well as other risk factors set out under the heading 'Risk Factors' in the Annual Information Form dated March 26, 2010 which is available on SEDAR at www.sedar.com. Investors are cautioned not to put undue reliance on forward-looking statements due to the inherent uncertainty therein.
Neither the Toronto Stock Exchange nor the AIM Market of the London Stock Exchange has reviewed and neither accepts responsibility for the adequacy or accuracy of this news release.
Contacts:
Greystar Resources Ltd.
Geoff Chater
(604) 614-7830
info@greystarresources.com
www.greystarresources.com
Renmark Financial Communications Inc.
John Boidman
Investor Relations Contact
(514) 939-3989 or (416) 644-2020
jboidman@renmarkfinancial.com
Renmark Financial Communications Inc.
Dan Symons
Investor Relations Contact
(514) 939-3989 or (416) 644-2020
dsymons@renmarkfinancial.com
www.renmarkfinancial.com
NCB Stockbrokers Limited
Christopher Caldwell
London NOMAD Contact
+44 (0) 20 7071 5200
christopher.caldwell@ncb.ie
