summary assessment of biooxidation testwork-historical to present 5 28 10

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BRIERLEY CONSULTANCY LLC

Prepared by Corale L. Brierley May 29, 2010

ASSESSMENT OF BIOOXIDATION TESTWORKON ANGOSTURA ORE AND CONCENTRATE:

HISTORICAL TO PRESENT

Executive Summary

Biooxidation test-work on materials from the Angostura Project began in 2006 withbatch reactor testing of ground whole-ore. Whole ore studies continued with columntesting of two sulfidic-refractory ores at three different particle sizes (19 mm, 13 mmand 1.7 mm) to evaluate a heap biooxidation process with heap leach cyanidation of thebiooxidized residues. Both whole ore tests confirmed the amenability of the ore tobiooxidation and the correlation between pyrite oxidation and gold recovery. Thegreater the amount of sulfide oxidized the greater the gold recovery. About 70% goldrecovery was achieved with 30% sulfide oxidation for whole ores in column biooxidation

tests at a 13 mm particle size. More test-work is needed to confirm whether a smallerparticle size would improve the kinetics. Silver recovery was erratic as would beexpected, since argentojarosite formation increases with time and cyanidation does notrecover silver from this by-product. Whole ore testing suggested that cyanideconsumption increases with decreasing particle size. Lime consumption is higher forbiooxidized residues than for unoxidized ore, as a result of the presence of oxidizedsulfur species requiring neutralization.

Batch reactor testing of Angostura concentrates at two different laboratories showthe concentrates to be readily biooxidized by mesophilic bacteria and confirm the

relationship observed for Angostura whole ores that gold recovery is directly related tothe percent of pyrite oxidized. Approximately 90% gold recovery was demonstrated with85% pyrite oxidation; 60% pyrite oxidation yielded 90% gold recovery with oneconcentrate that was tested. Copper extraction is also reported at 90% at 85% sulfide-sulfur oxidation. Mineralogy indicated copper is mainly present as chalcopyrite andcovellite. The high copper extraction is somewhat unusual because mesophilic bacteriatypically do not leach chalcopyrite effectively. Lime/limestone requirements to maintainan optimal biooxidation pH of 1.2 are anticipated to be high as the concentrate has littleneutralization capacity; the sulfide content is high (~36%) and much of this will beoxidized. Batch reactor testing at SGS South Africa, under the direction of BiominTechnologies/Gold Fields Limited, is in progress. This test-work is preparatory for acontinuous pilot plant campaign using the Biomin/GFL BIOX ™ technology.





Introduction

Biooxidation test-work on ores and concentrates from the Angostura Project datesfrom 2006 and extensive testing is on-going. Test-work has included batch biooxidationamenability tests on finely ground whole ore and on concentrates, and columnbiooxidation tests on whole ores. Testing will soon commence on continuous, stirred-tank biooxidation of flotation concentrates. Biooxidation test-work has been completedor is ongoing at three separate laboratories: Little Bear Laboratories, Inc. (formerly ofGolden, Colorado, USA); McClelland Laboratories, Inc., Reno, Nevada, USA; and SGSSouth Africa Ltd., Johannesburg, South Africa. The test-work at SGS-South Africa is underthe direction of Biomin Technologies SA, a subsidiary of Gold Fields Limited (GFL) and isdirectly related to GFL’s BIOX ™ process.

This report:

Explains the objectives of the various biooxidation tests that have beencompleted or are underway,

Summarizes the results and conclusions of the biooxidation test-work, Examines the biooxidation data in view of mineralogical information on the

Angostura ore and concentrate, and Compares and assesses biooxidation test-work results among the

laboratories.

Angostura Whole Ore Biooxidation Test-Work

The first reported whole-ore biooxidation test-work was carried out by Little BearLaboratories in 2006 to determine the amenability of Angostura ore to biooxidationpretreatment. Extensive whole-ore, biooxidation, column testing has been underway atMcClelland Laboratories. These test programs are summarized.

Little Bear Laboratories: Whole-Ore Biooxidation Batch Amenability Testing

Objective:

The purpose of the test-work at Little Bear Laboratories (Little Bear Laboratories, 29Dec 2006) was to evaluate the amenability of Angostura ore to biooxidation. Suchtesting provides information on whether the leaching bacteria can grow on the ore,confirms there are no toxic constituents present in the samples that will slow or preventbacterial growth, and correlates the biologically-mediated oxidation of sulfide-sulfurwith gold and silver extraction.





Procedures:

Two whole-ore samples (composites from drill core interval 380) were each groundto a nominal -106 µm and assayed for C TOT, CO3

2-, Au, Fe TOT, STOT, S°, SO42- , and S 2- (by

difference). Sample preparation was done by Phillips Enterprises (26 Dec 2006) and

head assays were done by Hazen Research, both Golden, Colorado based companies.Sample #1 at 1.9% S 2- -S could be classified as a “low -grade” sulfide ore, while Sample #2at 4.71% S 2- -S could be classified as a “medium -grade” sulfide ore. A total of eight batchtests were conducted – four batch tests per sample. These tests entailed loading each ofeight 4-liter glass reactors with 200 g of -106 µm ore – four reactors each with 200 g ofSample #1 and four reactors each with 200 g of Sample #2. Two liters of nutrientsolution 1 adjusted to pH 1.75 with H 2SO4 were added to each reactor yielding 10%solids. Ferrous iron (2 g/L Fe 2+) was added to each of the four reactors containingSample #1; the objective was to accelerate the growth and activity of the bacterialculture, since Fe 2+ is easily oxidized by the organisms. Each reactor was inoculated with

a mixed culture of mesophilic2

, iron- and sulfur-oxidizing bacteria, adapted to Angosturaore. The reactor contents were stirred with mixers, aerated with humidified air andincubated at 36-37°C. Eh, pH, Fe TOT and Fe2+ of the reactor solutions were measuredrecurrently. Based on solution iron assays, reactors for each sample were terminatedafter various time periods, the solids recovered, dried, weighed and analyzed for S TOT and S species. The percent oxidation was determined by comparing the percent S 2- -S inthe head sample versus percent S 2- -S in the tails of the terminated reactors.

Results:

The added Fe 2+ was rapidly oxidized in the four Sample #1 reactors with some ironprecipitation noted as the pH increased to 2; pH later declined as pyrite oxidationinitiated. The pH in the four reactors containing Sample #2 was maintained at a lowerpH of 1.5 to 1.6 to minimize iron precipitation.

The Eh in the batch reactors increased to over 900 mV (Standard HydrogenElectrode) in one to three days, indicating good bacterial growth and activity. The goldand silver extraction based on the percent of S 2- -S oxidized is shown in Figures 1 and 2,respectively. Cyanide consumption for the unoxidized head samples was 4.7 kg/t forSample #1 and 5.4 kg/t for Sample #2. Cyanide consumption was approximately twice as

high for the biooxidized ores as for the unoxidized ores.

1 Nutrient solution composition: (NH 4)2SO4, 0.4 g; MgSO 4.7H2O , 0.4 g; KH 2PO4, 0.04 g; deionized (ordistilled) H 2O, 1.0 L

2 Mesophilic bacteria are microorganisms that grow at an ambient temperature range.





Figure 1 Gold extraction versus biooxidation in Angostura ore samples #1 and #2

Figure 2 Silver extraction versus biooxidation in Angostura ore samples

Conclusions:

Conclusions reached from batch testing of -106 µm Angostura whole ore are:

Bacterial oxidation initiated quickly with no indication of any toxicity of the oreto the organisms, Gold recovery increased with increasing sulfide oxidation, 86-92% gold recovery was achieved with 90-92% oxidation of the sulfide-sulfur,

and Silver extraction was erratic.

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Solution pH affects silver recovery. If the solution pH increases resulting in ironprecipitation, silver precipitates as a silver jarosite; as a jarosite compound, silver is notextracted with cyanide. During biooxidation silver-containing sulfide minerals areoxidized and low concentrations of silver go into solution. However, the solubility ofsilver is very low and will precipitate as a jarosite compound, as the pH approaches 2.

Whole-Ore Column Leach Test-Work at McClelland Laboratories, Inc. (MLI)

Objectives:

The purpose of the whole-ore column leach tests at MLI was to determine therecoveries of gold and silver from intermediate-grade, refractory-sulfidic ore using aheap biooxidation process with heap leach cyanidation of the biooxidized residue. Thetest-work also evaluated precious metal recoveries at three different particles sizes

providing information on the kinetics of biooxidation and on reagent consumption.

Procedures:

Two intermediate-sulfide grade drill core composites, LG (“Low -Gra de” sulfide ; alsodesignated as B-01 ) and MG (“Medium -Grade” sulfide ; also designated as B-02), wereevaluated at three different particles sizes -- 19 mm, 13 mm and 1.7 mm. For eachcomposite and for each particle sizes four columns (1.8 m high X 15 cm diametercontaining approximately 70 kg of ore) were set up. One column for each composite andfor each particle size was a “baseline” column test ; for these “baseline” column testslime (1.5 kg/t) was added to the ore and the ore was leached with 1 g/L NaCN. Thepurpose of these “ baseline ” columns was two-fold: (1) to determine precious metalrecoveries and reagent (lime and cyanide) consumptions with no oxidation of the sulfideminerals locking the gold and silver, and (2) to compare the “baseline” data with thatobtained from ore columns subjected to biooxidation pretreatment.

The purpose of the remaining three columns for each ore composite and for eachparticle size was to assess biooxidation on precious metal recovery. The ore in thesecolumns was agglomerated with a mixed culture of mesophilic bacteria adapted to theAngostura ore. This agglomeration accomplished three things: it inoculated the oreevenly throughout the column with the microbial culture; it acid conditioned the ore so

that biooxidation initiated quickly; and it caused the adherence of fine particles to largerore particles minimizing permeability problems. The columns were operated at roomtemperature. Two of the columns for each ore composite and for each particle size weredesignated as “continuous” columns, which mean s that the ore in the columns wasundisturbed during the oxidation period. The third column was designated as a“sacrificial” column, which means that periodically the ore was dumped from thecolumn, grab-sampled and the ore put back in the column for continuing biooxidation.





The timing for sampling was typically based on solution iron assays as a guide to theextent of biooxidation. Solution iron assays are not precise as determinants for sulfideoxidation, because iron precipitation can take place within the column; however, thistechnique was normally used, because there are no other reliable ways of estimatingthe extent of sulfide oxidation. The grab-samples were assayed for S TOT, sulfur species,

FeTOT and the un-ground residues subjected to bottle-roll cyanidation to determine theextent of sulfide oxidation with time, precious metal recovery in relation to both timeand percent sulfide oxidation, and reagent (CN and lime) consumption.

During the biooxidation period, the columns of ore were irrigated with an ore-adapted mixed culture of bacteria 3 in a nutrient-containing leach solution (0K medium 4 adjusted, as needed with either H 2SO4 or lime, to maintain a pH range of about 1.2 – 1.5. Additions were measured to later determine acid consumption or acid production.The biooxidation columns were aerated from the bottom to supply the microorganismswith O 2 and CO 2. Effluent solutions from the columns were measured for volume and

assayed for ORP (oxidation-reduction potential; Ag/AgCl electrode), pH, Fe TOT, ironspecies, and dissolved oxygen (DO). The collected effluent from each column wasrecycled to the same column with evaporative losses made up with water. The solutiontotal Fe values were converted to percent iron extraction, based on the Fe TOT in the headassay.

At the completion of biooxidation, the biooxidized residue was removed from thecolumns. Triplicate splits of the entire residue were collected and analyzed. Sampleswere metallurgically and chemically analyzed and subjected to bottle-roll cyanidationafter liming to determine precious metal recovery and reagent consumption.Biooxidized residues from some columns were, after sampling, agglomerated with limeand subjected to cyanidation in another column.

Complete metallurgical and chemical analyses were performed on each compositeat each particle size to obtain a head assay and particle size distribution. When thebiooxidation and leach tests were concluded, complete metallurgical and chemicalanalyses of the tails were carried out. A metallurgical balance was conducted using thehead and tails assays with consideration of the effluent solution data.

3 A mixed culture of mesophilic (ambient temperature) bacteria typically contains a number of differentgenera and species of iron- and sulfur-oxidizing bacteria.

4 0K (zero K) medium composition: (NH 4)2SO4, 3.0 g; KCl, 0.1 g; K 2HPO4, 0.5 g; MgSO 4. 7H2O, 0.5 g;Ca(NO3)2, 0.01 g; distilled or deionized H 2O, 1 L; H2SO4 for pH adjustment.





Results:

The results from whole ore column leach tests are extensive and only salient resultsare graphically summarized (McPartland, J. Personal Communication, 25-26 May 2010).Figure 3 summarizes gold recovery versus percent sulfide oxidation for both the LG (B-

01) composite and MG (B-02) composite for the three particle sizes tested. The baselinedata -- that is, no biooxidation (represented at 0% oxidation) -- are also included on thisgraph. Gold recoveries presented in Figure 3 are from column leach cyanidation testingon column biooxidized residue.

Figure 4 summarizes the gold recovery over time for the LG (B-01) composite) in thebiooxidation columns. Gold recoveries were derived from bottle-roll cyanidation of grabsamples (un-ground) taken from the sacrificial columns.

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Figure 3 Gold recovery vs. sulfide oxidation, Angostura sulfide ore composites,baseline cyanidation and biooxidation/cyanidation (heap/heap )

B-01 (19mm) B-01 (13mm) B-01 (1.7mm)B-02 (19mm) B-02 (13mm) B-02 (1.7mm)





Figure 5 illustrates gold recovery over time for the MG (B-02) composite ore in thebiooxidation columns. These gold recoveries were also derived from bottle-rollcyanidation of un-ground grab samples taken from the sacrificial columns.

Lime consumption for bottle-roll cyanidation tests on unoxidized ore (baseline)ranged from 1.5 to 2.6 kg/t ore and cyanide consumption ranged from 2.62 to 3.52 kg/tore. Lime consumption was somewhat higher for bottle-roll tests of biooxidized residuesfrom sacrificial columns with consumption increasing with higher percent sulfideoxidation. Cyanide consumption was one-third to one-half less for biooxidized samplesthan for the baseline samples in bottle-roll tests. Reagent consumptions are notavailable yet for those biooxidized ores being cyanide leached in columns.

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Figure 4 Gold recovery vs. oxidation time,column biooxidation/bottle roll cyanidation,

Angostura LG (B-01 composite) sulfidic heap biooxidationcomposite

19mm 12.5mm 1.7mm

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Figure 5 Gold recovery vs. oxidation time,column booxidation/bottle roll cyanidation,

Angostura MG sulfide heap biooxidation composite

19mm 12.5mm 1.7mm





Conclusions:

Some test-work on the whole ore is still underway. However, several conclusions canbe derived from the raw data that are available:

The ore is readily biooxidizable as evidenced by a rapid increase in ORP (data notshown in this report) indicating excellent ferrous iron oxidation by the bacteriaand by rapidly increasing Fe TOT in solution (data not shown in this report). Steadyoxidation of pyrite by the bacterially-generated ferric iron was noted.

There is no evidence of any lag times associated with the start-up ofbiooxidation. This indicates there are no constituents being solubilized thatnegatively affect microbial growth or activity.

Regardless of feed size, gold recovery is strongly correlated with sulfideoxidation (Figure 3).

About 70% gold recovery with 30% oxidation was achievable at a 13 mm particle

size, There may be some kinetic advantage to a finer crush size prior to biooxidation;

however, more data are needed to validate this. Silver recoveries are respectable (data not shown) given that recoveries can be

quite variable depending on iron precipitation in the columns. Lime consumption is higher for biooxidized residues in bottle-roll cyanidation

tests than for unoxidized ore, because there is acid from sulfide oxidation toneutralize in biooxidized ore. Comparable cyanide consumption in bottle-rolltests for the unoxidized and oxidized ores is somewhat unexpected, as oxidizedores often exhibit higher cyanide consumption.

There is variability of bottle-roll cyanidation data from grab samples collected fromthe sacrificial columns. This variability may be due in part to these being grab samples,but the variability may also be due to the characteristics of the gold. Mineralogicalinformation (Brown, 2006) indicated that some of the pyrite-associated gold is quitecoarse with some fragments in the range of 40 to 80 µm; grab sampling of materialswith such large fragments can bias assays.

Assessment of Whole-Ore Biooxidation Tests on Angostura Ore

The response of the -106 µm batch biooxidation testing of whole ore and the coarse

ore column biooxidation tests is comparable in that both testing regimes showed theore to be amenable to biooxidation. There was no evidence of any toxic constituentspresent in the ore that may result in unduly long lag times before biooxidationcommences or that would inhibit microbial activity. Both whole ore test programsconfirmed the correlation between pyrite oxidation and gold recovery. The greater theamount of sulfide oxidized the greater the gold recovery. Cyanide consumption foroxidized samples of finer ground ore appeared to increase; whereas this was not the





case for the biooxidized coarse ore. It may be that greater exposure of surfaces in thefiner grind increased the cyanide consumption or it may simply be that the ore samplesused for the -106 µm batch tests differed mineralogically from those used in the coarseore column tests.

Angostura Concentrate Biooxidation Test-Work

Test-work on concentrates produced from Angostura ores was performed atMcClelland Laboratories, Inc. (MLI) and is currently underway at SGS South Africa. Thelatter test-work is under the direction of Gold Fiel ds’ BIOX™ program. The objectives,procedures, results and conclusions of the concentrate testing to date are summarizedand assessed in view of mineralogical characterization of the material.

Angostura Concentrate Batch Biooxidation Test-Work at MLI

Objectives:

The objectives of the biooxidation testing on a flotation concentrate composite atMLI were to confirm the amenability of the concentrate to biooxidation and to quantifythe relationship between sulfide-sulfur oxidation and precious metal recoveries.

Procedures:

Testing, which is completed, was undertaken on a bulk sulfide flotation concentrateproduced from a master composite. The composite was subjected to analyses to obtaina head assay.

The batch test program consisted of 19 tank reactors, each containing approximately1.2 L of nutrient solution (see footnote #4) and approximately 300 g of concentrate toyield a pulp density of 20%; the exception was test AM-11, which had 10% solids.Concentrate samples were reground to 80% -37µm in size before biooxidationtreatment. Ferrous iron in the form of FeSO 4.7H2O was added to selected reactors.Ferrous iron promotes rapid initial growth of the bacteria, because Fe 2+ is a readilyoxidizable source of energy for the organisms. Exact solution volumes, concentrate

weights, Fe2+

additions and nutrient supplementation are fully documented on MLI rawdata sheets. The tank reactors (designated AM-1 through AM-19) were placed in waterbaths to maintain the temperature of the reactors at 30°C; reactors were aerated andstirred. pH was maintained in the range of 1.5 - 2 in the reactors by addition of H 2SO4 orCaCO3, as needed.





All reactors were monitored daily for temperature, oxidation-reduction potential(mV Ag/AgCl electrode), Fe TOT, Fe species, dissolved oxygen and pH. All reagentadditions were recorded as were solution removal volumes. Percent oxidation wasestimated from solution Fe TOT values, based on the Fe assay of the head sample. InitialFe2+ additions were accounted for in determining estimated percent oxidation. Each

reactor underwent various treatments during the course of biooxidation; these aresummarized in Table 1. Tank reactors were terminated after varying time periods ofbiooxidation and the biooxidized solids were weighed, water-washed and subjected tobottle-roll cyanidation or in some cases bottle-roll CIL (see Table 1). The biooxidizedsolids were also subjected to a tails analysis.





TABLE 1Summary of metallurgical results of batch reactor biooxidation of Angostura concentrate

AmenabilityTestNo.

BiooxidationTime,days

Est.Oxidation,

%Biooxidation

TestConditions

TestType

AuRecovery,

%

AgRecovery,

%

Reagent Requirementskg/mt Conc.

NaCN Cons.Lime

AddedBase-line 0 0.0% N/A Direct CN 51.0 19.2 6.24 2.6

AM-6 5 6.8% Initial Bio-ox time series-no lime Direct CN 57.9 45.7 7.21 2.9

AM-16 7 6.6%New Bio-ox time series: Daily bleeds 100 ml, Lime daily pH

<2.0 CIL 60.3 64.2 12.38 8.1



<2.0 CIL 68.6 61.1 8.57 5.9



<2.0 CIL 72.6 69.2 10.70 23.7


<2.0 CIL 78.0 70.6 9.16 36.7

AM-11 30 51.2%10% solids, Daily bleed: 100 ml/day from day 1; maintain

pH 1.7 w/lime: Day 0 Direct CN 81.9 87.6 12.05 43.2


AM-1 45 62.2%Initial Bio-ox time series-Lime addition starting day 28,

pH=1.7 Direct CN 89.5 80.7 9.34 32.8

AM-3 60 68.1%Initial Bio-ox time series-Lime addition starting day 34,

pH=1.7 Direct CN 89.3 57.5 7.37 8.7

AM-8 45Bleed: 600 ml day4, 100 ml daily thereafter, lime addition

starting day 18 pH=1.7 Direct CN 85.9 63.0 7.81 19.1

AM-7 10Started significant bleed: 400 ml day 4, 100 ml daily

thereafter Direct CN 53.4 47.1 6.69 2.5

AM-19 28 Additional confirmatory test, same conditions as AM-13 CIL 63.7 52.0 8.13 7.3

AM-17 28 Ground/washed concentrate (before biooxidation) CIL 65.8 48.3 8.49 6.8

AM-18 28 Un-ground/washed concentrate (before biooxidation) CIL 58.0 57.8 8.04 5.2





Results:

Table 1 summarizes salient results of the batch reactor testing of the flotationconcentrate: the number of days of biooxidation; the estimated percent oxidation;conditions maintained in each reactor; the type of bottle-roll test carried out on the

biooxidized solids from each reactor, including the baseline sample (unoxidized headsample) cyanidation; precious metal recoveries; and reagent consumption. Estimatedpercent oxidation data for tests AM-7, AM-8, AM-179, AM-18 and AM-19 are not yetavailable

Figure 6 and Figure 7 illustrate the gold and silver recoveries, respectively, with thelength of the biooxidation period. Figure 8 illustrates gold recovery with respect to thepercent of sulfide oxidized, based on sulfide-sulfur analysis of feeds and final residueafter accounting for weight lost during biooxidation.

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Figure 6 Gold recovery vs. biooxidation time,Angostura flotation concentrate

Au





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Figure 7 Silver recovery vs. biooxidation time,Angostura flotation concentrate

Ag

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90100

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Figure 8 Gold recovery vs. biooxidation time,Angostura flotation concentrate (percent oxidation based on S 2- -S assays infeed and final residues after accounting for weight loss during biooxidation)

Au





Conclusions:

The main conclusions derived from MLI’s batch testing of Angostura bulk flotationconcentrate are:

90% gold recovery was achieved with biooxidation of the flotation concentrate;gold recovery was 50% without biooxidation (Figure 6).

Approximately 65-70% of the sulfide in the concentrate required oxidation toachieve 90% gold recovery (Figure 8).

Silver recovery was improved with biooxidation. Approximately 19% silverrecovery was attained from unoxidized concentrate, whereas with biooxidation60 to 90% recoveries were realized (Figure 7). Silver recoveries were highlyvariable, as is expected in batch reactors. The longer the oxidized productsremain in the batch reactors the greater the likelihood of argentojarositeformation. Argentojarosite effectively locks the silver, preventing recovery with

cyanide. The highest silver recoveries were noted about 30 days into thebioleach test (Figure 7) with silver recoveries declining thereafter, probably dueto increasing formation of argentojarosite.

Lime and cyanide consumptions are higher with biooxidized concentrate thanwith unoxidized concentrate. The longer the biooxidation time, the greater thereagent consumption.

Angostura Concentrate Batch Biooxidation Test-Work at SGS South Africa

The test-work currently underway at SGS South Africa is under the direction ofBiomin Technologies, a subsidiary of Gold Fields Limited, and is in support of Biomin’sBIOX™ technology. The test-work was initiated in late February 2010 with samplepreparation, adaptation of the BIOX ™ culture to an Angostura concentrate sample, andan Inoculum Build-up stage. Once sufficient quantities of the Angostura concentrate-adapted culture were prepared, batch testing proceeded and these tests are currentlyunderway. This initial test-work is in preparation for a continuous stirred-tankbiooxidation campaign using the temperature-controlled, 120-L, mini-pilot at SGS SouthAfrica.

Objectives of SGS South Africa/Biomin BIOX™

Test-work to Date:

The objective of the “adaptation test ” is to allow the bacterial culture that is used inthe BIOX™ technology to become accustomed to the Angostura concentrate.

The objective of the “Inoculum Build -Up” tests is to increase the vol ume ofAngostura concentrate-adapted bacteria so that these adapted bacteria can be used for





inoculating the “Bulk Batch Tests” and for inoculating the mini -pilot plant for thecontinuous, stirred-tank campaign.

The objectives of “Bulk Batch Tests” are: to determine the amenability of the BIOX ™ culture to biooxidize Angostura concentrate; to ascertain precious metal recovery

versus the percent sulfide-sulfur oxidized; to measure dissolution of other importantconstituents, such as copper, that are present in the concentrate; and to authenticatethe behavior of the concentrate by measuring parameters such as Fe TOT, iron species,pH, redox, acid production and acid consumption over time, correlating theseparameters with percent oxidation of the sulfide minerals. This last objective isparticularly important, because Biomin/GFL use these data to compare the behavior ofbulk flotation concentrates that are produced at different times, produced fromdifferent drill core samples or produced from different ore zones. Each new Angosturaconcentrate will be evaluated in “Bulk Batch Tests” and the results of the test scompared with “Bulk Batch Tests” from the Angostura concentrate (or concentrates)

that will be used for the pilot plant campaign. If future concentrate samples behavedifferently in the “Bulk Batch Test”, this information will be used to alert Biomin/GFL ofchanges in mineralogy or chemical composition of the concentrate and to adjustconditions in the pilot plant, if warranted.

Procedures:

Adaptation of the BIOX ™ culture was undertaken by first growing the culture in a 9Kmedium, which is 0K medium (see footnote #4) with 33.3 g/L Fe 2SO4.7H2O added, and~8% solids of the Angostura concentrate in an aerated batch reactor. The added Fe 2+ enables the bacterial culture to grow rapidly. The batch reactor contents weremonitored almost daily for iron species, redox, pH, and dissolved oxygen. Lime or H 2SO4 were added to maintain the culture to approximately 1.2, which is the optimum pH forthe BIOX™ culture. The adaptation reactor was maintained at 40°C, which is theoptimum pH for the BIOX ™ culture. As the redox increased and the Fe 3+-to-Fe 2+ ratioincreased, additional concentrate was added. This protocol of adding increasingamounts of concentrate was repeated several times, which increases the solids contentand exposes the bacteria to higher and higher concentrations of constituents leachedfrom the concentrate. Periodically, solution or slurry is removed and replaced with 0Kmedium (no added Fe 2+); the purpose is to refresh the solution with nutrients(ammonium, potassium and phosphate ions) to insure the organisms have sufficient

nutrients to continue to grow and adapt to the concentrate. S TOT and S species in theresidues, metal dissolved in solution and cyanidation of oxidized residues are not doneon adaptation tests; the addition of concentrate over time to adapt the organismsinvalidates these results.

Inoculum build-up is accomplished by growing the Angostura concentrate-adaptedBIOX™ culture on 0K medium (no added Fe 2+) at 25% solids density in aerated batch





reactors at 40°C. Typically several 3- L “Inoculum Build -Up” batch es are prepared,because the solutions/slurries are sacrificed at various times as inoculum for the “BulkBatch Tests” and for the continuous pilot plant run. The “Inoculum Build -Up” reactorsare monitored for Fe TOT, iron species, pH, temperature, redox potential and dissolved O 2 and the pH is maintained close to pH 1.2 with lime or H 2SO4 addition. No additional

concentrate is added to the “Inoculum Build -Up” batch reactors over time, because 25%solids density is about the maximum that can be accommodated in batch reactors,particularly at the S 2-- -S content (approximately 36%) of the Angostura concentrate. Theoxygen demand for high-sulfide concentrates is high and it can be difficult in batchreactors to satisfy the O 2 demand, when oxidation rates are high. Sulfide-sulfuroxidation, metal dissolution and cyanidation of oxidized residues are not done onInoculum Build-Up tests, because some of the culture may be used at various times forinoculating other tests.

Bulk Batch Tests are conducted similarly to the batch Inoculum Build-Up reactors

using 25% solids at 40°C, but are larger in volume (16.15 L). The larger volume allows forinterim sampling of solutions and solids. Solids are cyanide leached to quantify preciousmetal recoveries and solids will be assayed for S TOT, sulfur species, Fe TOT and othermetals to correlate gold recovery and metal dissolution with percent sulfide oxidized.Variants of Bulk Batch Tests are also conducted to evaluate different grind sizes of theconcentrate and to evaluate sources of water that will be used in the commercial BIOX ™ plant. Bulk Batch Tests may be used to test certain component parts in contact with thebacteria or even nutrient supplies for the commercial BIOX ™ plant to insure there is notoxicity.

Results:

Adaptation of the BIOX ™ culture occurred relatively quickly and without problems asevidenced by the rapid increase in redox potential initially and after repeated additionsof concentrate. Slurry removal and nutrient solution/concentrate replacement after aperiod of days in batch are normal procedures as fresh nutrients augment theadaptation process. Removal of solution from the batch reactor and replacement withfresh solution also dilute by- products of bacterial growth that can slow the organism’sgrowth.

Inoculum Build-Up in all four batch reactors also proceeded smoothly as

demonstrated by rapid increases in Fe3+

and high redox potential. Figure 9, which isfrom Inoculum Build-Up batch reactor #1, is characteristic of the response of theremaining three Inoculum Build-Up reactors. The leveling off of the redox potential isnormal. Some decline in the redox potential in batch reactors is normal, as theorganisms can become nutrient deficient or simply slow down because of accumulatingmetabolic by-products that may be slightly inhibiting. In Inoculum Build-Up reactor No.1 1.5 L of solution were removed and replaced with fresh nutrient solution at day 20.





Figure 9 Iron species and redox potential in Inoculum Build-Up reactor no. 1 (SOURCE:SGS Greystar Progress Report 24 May 2010)

Redox potential and iron speciation trends noted in the Inoculum Build-Up reactorsare also being observed in the three Bulk Batch Reactors, which are on-going tests. Goldrecovery is closely paralleling sulfide-sulfur oxidation as illustrated in Figure 10, whichare data from the three Bulk Batch reactor tests. The highest gold recovery, 90%, wasnoted in Bulk Batch Reactor No. 1 with 85% sulfur oxidation. While days of biooxidation

are shown in Figure 10, it is important to note that rates of oxidation in batch reactorsare considerably slower than will be seen in the continuous pilot plant campaign. Figure11 illustrates copper leaching into solution versus the percent of sulfur oxidized. Copperextraction is approaching 90%. The copper content of the concentrate used in the BulkBatch tests is 1.5%. There appears to be two separate extraction rate curves associatedwith the copper dissolution; these may suggest the leaching of two different coppersulfide minerals with time. Mineralogical work points to the presence of several copperminerals with chalcopyrite and covellite as the two most prevalent species (SGS, 2007;SGS, 2010). Chalcopyrite is quite refractory and typically does not leach well usingmesophilic bacteria. Therefore, more testing and additional mineralogical work would

be required to confirm which copper minerals are actually being leached.

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Figure 10 Gold recovery and percent sulfide oxidation in three Bulk Batch reactor tests(SOURCE: SGS Greystar Progress Report 24 May 2010)

Figure 11 Copper recovery versus sulfide oxidation for three Bulk Batch reactor tests.Gold recovery for BAT 1 shown (SOURCE: SGS Greystar Progress Report 24 May 2010)

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Silver recoveries have not been reported to date for the Bulk Batch reactors,although Biomin confirmed that silver is being tracked (van Niekerk, J. PersonalCommunication, 25 May 2010). The expectation is that silver recoveries will be erratic inbatch reactors, because of the long residence time of the material in these reactorsresulting in formation of argentojarosite; the silver in argentojarosite is not extractable

with cyanide. The shorter retention time of the concentrate in continuous operationmeans less opportunity for argentojarosite formation and precipitation. Silverrecoveries in continuous pilot plant operations are usually more consistent.

The Angostura concentrate has little neutralization capacity and with the high S 2- -Scontent (~36%) and the need to oxidize much of this to achieve 90% gold recovery,limestone and lime requirements are expected to be high. Bulk Batch Tests (andInoculum Build-Up reactors) at SGS South Africa are recording lime consumption tomaintain the batch reactors at pH 1.2. These final results are not available as these testsare still underway.

Conclusions:

Because the test-work at SGS South Africa is still on-going it is somewhat prematureto draw too many conclusions. However, some obvious conclusions are:

The concentrate is readily biooxidizable with no evidence of any constituentsthat may pose problems for the bacteria,

The BIOX™ culture adapted easily and quite rapidly to the concentrate, Gold recovery increases with increasing percent of sulfide-sulfur oxidized, Gold recovery of 90% is achievable and correlated with 85% sulfide-Sulfur

oxidation, 90% of the copper in this particular concentrate is leached, Lime/limestone requirements to maintain the pH at 1.2 in the commercial plant

are anticipated to be high.

Assessment of Concentrate Biooxidation Tests on Angostura Ore

The batch testing of Angostura concentrates at MLI and at SGS South Africa confirmsthe amenability of Angostura concentrate to biooxidation. Neither lab reported any lag

times in biooxidation that would signify the presence of inhibitory constituents. BothMLI and SGS South Africa test results point to 90% gold recovery with biooxidation.However, there were some differences reported in the percent sulfide-sulfurbiooxidation required to achieve 90% gold recovery. The differences are likely the resultof different concentrates being tested. Nevertheless, both laboratories confirmed thatgold recovery increases with increasing sulfide-sulfur oxidation. These leach data





support mineralogical reports (Brown, 2006; SGS South Africa, 2007; SGS South Africa,2010) confirming the locking of gold in sulfide minerals.

Silver recoveries in batch reactor testing are erratic, resulting from the formation ofargentojarosite because of the length of time of the batch tests. More consistent silver

recoveries will occur when the concentrate is biooxidized in continuous mode ofoperation in the pilot plant.

Copper extraction is in the 90% range, which is somewhat unexpected given thatmineralogical reports suggest that 50% or more of the copper is associated withchalcopyrite, which doesn ’t leach well using mesophilic bacteria. If much of the copperis leached this would be advantageous, as the presence of copper minerals increasescyanide consumption.

The concentrate biooxidation data to date are all derived from batch tests, which do

not provide data on rates of S2-

-S oxidation. The expectation is that the residence timeof the concentrate in the continuous pilot plant will be in the five-day range given theamenability of the concentrate to biooxidation as demonstrated by the MLI tests andthe SGS South Africa tests.

References

Brown, P., 2006. A Preliminary Assessment of Metallurgical Response – AngosturaProject, North Eastern Santander, Colombia, KM1836.

Little Bear Laboratories, Inc., 29 December 2006. Biooxidation of Angostura OreSamples.

Phillips Enterprises, 26 December 2006. Biooxidation Versus Extraction Data.

SGS South Africa, 23 August 2007. Mineralogical Report No: MIN 0707/117.

SGS, 6 April 2010. An Investigation of QEMSCAN ™ into the Mineralogical Characteristicsof the Concentrates Samples from the Greystar Project.

SGS South Africa, 24 May 2010. SGS Greystar Progress Report.

summary assessment of biooxidation testwork-historical to present 5 28 10

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