A Technical Paper presented at the Canadian Mineral Processors Annual
Meeting, a division of CIMM
(Week of the 18 January 2004)
Crystallex’s Las Cristinas Gold Project
Prepared by,
J.R. Goode, P.Eng., Metallurgical Consultant1
K.G. Thomas, P.Eng., PhD, Chief Operating Officer2
1J.R. Goode and Associates
2Crystallex International Corporation
ABSTRACT
The Las Cristinas deposit contains about 10.2 million ounces of gold reserves at a grade of 1.3 g/t. The deposit comprises a layer of fully oxidized saprolite (SAPO), a layer of sulphide-enriched saprolite (SAPS), carbonate leached bedrock (CLB) and carbonate stable or un-leached bedrock (CSB). Gold occurs at about the same level in all lithological groups. The bedrock units contain minor amounts of copper (about 0.1%) as chalcopyrite, while SAPO contains virtually no copper it having been leached and deposited in the SAPS zone.
A previous owner started investigation of the Las Cristinas deposit in Venezuela in 1991. Over the next seven years, 1174 holes with a total length of 159 km were drilled, extensive metallurgical testwork, including pilot plant operations, were conducted and feasibility studies were completed. Construction was started in 1997 and again in 1999 but was suspended.
Crystallex has been producing gold in Venezuela since the early 1990’s. In 1997, Crystallex acquired Inversora Mael which had held two of the claims to the Las Cristinas concessions since 1986. In September 2002, Crystallex entered into a definitive agreement with the Corporación Venezolana de Guayana, (CVG), to develop the Las Cristinas deposit.
An extensive program of studies and testwork was started in early 2003 to determine an optimum development plan for Las Cristinas. Mine Development Associates, SNC–Lavalin Engineers and Constructors, SGS Lakefield Research Limited, J.R. Goode and Associates, and Professor A. Laplante at McGill University acted as the main contractors.
Because of the potential value of by-product copper, earlier flowsheets used carbon-in-leach for SAPO, but processed all other ore types by flotation to produce a gold-copper concentrate with cyanide leaching of cleaner tailings to maximize gold recovery. Cyanide recovery was necessitated by the high copper content of the leach feed. Initial studies of available data by Crystallex revealed that direct leaching of all ore types would provide about 11% more gold recovery than the previously selected flotation route with reduced capital and operating costs.
Bench and pilot scale studies have confirmed that a SAG-ball mill-gravity-CIL route is very effective for all ore types and will give about 89% gold recovery. This paper describes the recent testwork, discusses the proposed plant design, and presents economic data.
INTRODUCTION
Venezuela is best known as a major oil producer supplying about 12% of US imports in 2002 or roughly the same as Saudi Arabia or Mexico. Venezuela is less well known as a gold producer having produced only 9 t in 2002. Most gold production has come from Bolivar State located in the south-east of the country and in particular from the El Callao district. Gold is produced by local mining companies such as Compañia General de Mineria de Venezuela (CVG Minerven) but, recognizing the potential of the country, several foreign mining companies are also active in Venezuela including Crystallex International, Hecla Mining, Bolivar Gold, Gold Reserve, Gold Fields, and China’s Shandong Gold Group.
The Bolivar State deposits are in Archean to early Proterozoic granite-greenstone terrain of the Guayana Shield. The Shield hosts several world class deposits besides those in Venezuela including Cambior’s Omai mine in Guyana and its Gross Rosebel mine in Surinam. These deposits are geologically similar to those in Bolivar State and comprise an oxidized saprolite layer overlying primary ore.
Crystallex has been producing gold in Venezuela since the early 1990’s and the company is well versed in operating mining and milling facilities in the region. In 1997, Crystallex acquired Inversora Mael which had held the rights to two of the Las Cristinas concessions since 1986. Figure 1 indicates the location of Crystallex’s current operations and Las Cristinas.
On September 17, 2002, Crystallex entered into a definitive Mining Operation Contract (MOC) with Corporación Venezolana de Guayana, (CVG), to develop the Las Cristinas deposit. The MOC provides Crystallex with the exclusive right to explore, design and construct facilities, exploit, process and sell gold from Las Cristinas. An official translated version of the MOC is available on the Company’s website (
Following signing of the MOC, Crystallex initiated studies of the significant amount of geological and metallurgical data that was available on the deposit. These studies confirmed the viability of the project and also suggested an alternative processing route to that which had previously been considered. In early 2003, Crystallex shipped several tonnes of samples to Canada for metallurgical testing to confirm the suitability of the selected process route.
This paper describes the metallurgical work that has been recently completed and the resulting development plan for Las Cristinas.
LOCATION AND EXISTING FACILITIES
The Las Cristinas concessions are located in Bolivar state, Venezuela, about 6 km from Troncal 10, the main paved highway that runs from Puerto Ordaz to the Brazilian border. The mining concessions are in relatively flat terrain ranging from 130 to 160 m above sea level. The climate is tropical.
The property is provided with an airstrip and a 3000 man construction camp. A major 400 kV power line supplying power to Brazil parallels the highway and a 150 MVA switching station was constructed in 2001 for the Las Cristinas project at Las Claritas some 6 km from the site.
GEOLOGY, MINERALOGY, RESERVES, AND MINING
There are two main deposits at Las Cristinas: Conductora/Cuatro Muertos and Mesones/Sofia (see Figure 2). At Conductora/Cuatro Muertos, gold and copper mineralization are associated with pyrite-chalcopyrite disseminations, veinlets (2-5% sulphides) and blebs generally oriented parallel to the foliation. The occurrence of sulphide mineralization is not associated with any particular rock type, but rather with alteration assemblages that include secondary biotite and a younger carbonate-epidote assemblage. On a microscopic scale, gold can be found as free grains in quartz and as blebs and fracture fillings in pyrite and/or chalcopyrite. Silicate-carbonate-sulphide veins tend to parallel foliation. At Mesones/Sofia, gold-copper mineralization occurs within tourmaline breccia zones, which have obliterated primary tuffaceous textures. Sulphide concentrations are coarser grained and more chalcopyrite rich than those at Conductora/Cuatro Muertos.
Extensive weathering has led to the development of saprolite to depths of over 90 m locally. The upper part of the saprolite is oxidized. Within the oxidized saprolite, copper has been predominantly leached, but the gold remains generally in its original distribution. The sulphide saprolite, which has been enriched in copper leached from the overlying oxide saprolite, also retains the original gold distribution. The secondary copper minerals in the sulphide saprolite are soluble in cyanide solutions. Copper and gold grade distributions in the bedrock have not been affected by weathering.
Earlier exploration work generated a database including information on 1,174 drill holes covering 160,600 m of drilling, 108 trenches, 162,806 gold assays, 145,221 silver assays, 145,547 total copper assays, and 40,655 cyanide-soluble copper assays. Crystallex determined that the prior exploration and sampling procedures conformed to or exceeded industry standards. Nevertheless, Crystallex drilled an additional 2,188 m in twelve diamond drill holes, for a total of 1,087 core samples, to verify the presence and tenor of mineralization. In addition, 275 quality assurance/quality control (QA/QC) samples were analyzed. The Crystallex drill results and check samples corroborate the general tenor of gold mineralization reported by the previous operator. For additional confirmation, Crystallex re-assayed 262 pre-existing pulps, 200 pre-existing coarse rejects and 342 pre-existing quarter core samples. Mean grades are similar for both datasets.
Based on all available data, MDA generated a total resource estimate of 499,000,000 t grading 1.17 g Au/t for a total of 18,807,000 ounces of gold (Measured, Indicated and Inferred) which agrees well with earlier estimates.
MDA developed a mine plan based on a conventional truck and shovel operation. The total reserves, using the Canadian Institute of Mining, Metallurgy and Petroleum reserve definitions, are 246 Mt at a grade of 1.29 g/t for a total of 10.2 million ounces of gold.
From a processing viewpoint, the mass and analysis of the different ore types is important and especially the mass of the sulphide saprolite. This material contains cyanide soluble copper minerals which affect processing costs. The distribution of the different lithologies is summarized in Table 1.
Table 1. Distribution of ore types and grades
Lithology / TonnageMt / Percentage of total reserve
% / Gold grade
g/t / Cyanide soluble copper
%
Saprolite Oxide / 38,868 / 16 / 1.12 / ~0.005
Saprolite Sulphide / 23,532 / 10 / 1.38 / 0.093
Carbonate Leach Bedrock / 54,666 / 22 / 1.20 / ~0.01
Carbonate Stable Bedrock / 128,593 / 53 / 1.36 / ~0.005
METALLURGICAL TESTWORK
Background
Earlier investigators recognized that cyanide leaching could be economically applied to all ore types. However, it was decided to additionally recover copper from the deposit and a gravity-flotation circuit was developed that would produce a copper-gold flotation concentrate for custom processing by an off-shore smelter. To give adequate overall gold recovery, it was necessary to cyanide leach certain flotation products.
The flotation flowsheet was demonstrated in several pilot plant runs operated at solids flowrates of up to 150 kg/h. As the metallurgical development work continued, an acidification-volatilization-recovery (AVR) plant was added to the flowsheet because of excessive cyanide consumption when leaching a copper-bearing flotation intermediate product for gold recovery.
In early 2003, Crystallex and its consultants reviewed available metallurgical test data and performed various trade-off studies. These analyses indicated that the production and off-shore, smelting of a copper-gold flotation concentrate, as proposed earlier, was not the preferred alternative. Direct leaching of most or all of the ore and on-site production of bullion would give higher gold recovery, simplify the process, improve plant operability, and give lower capital and operating costs, and a higher Internal Rate of Return (IRR).
Crystallex arranged for new samples to be prepared in Venezuela and SGS Lakefield Research Limited (Lakefield) was engaged to test the direct leach process. The program ran from the time that samples arrived at Lakefield in early April until September 2003.
Samples
Nine composite samples of the four different ore types from the Conductora deposit were prepared from drill core stored at the mine site in Venezuela. Each sample was composited from individual drill core intervals with a mass of between 0.5 and about 7 kg and averaged about 2 kg across all samples. A listing of the major samples and their assays is presented in Table 2.
Table 2. Summary of main Conductora samples tested
Sample / Mine estimateAu – g/t / Major Assays – Lakefield
Au – g/t / Ag – g/t / Cu – % / CNSCu – %
SAPO 1 / 1.59 / 1.63 / 1.2 / 0.038 / 0.004
SAPS1 / 1.55 / 1.32 / 2.2 / 0.14 / 0.018
SAPS2 / 2.29 / 2.16 / 1.9 / 0.15 / 0.033
SAPS(2) / 1.29 / 1.33 / 1.4 / 0.11 / 0.037
SAPS3 / 1.64 / 2.15 / 6.1 / 0.21 / 0.12
SAPS4 / 1.53 / 1.84 / 1.7 / 0.43 / 0.31
CSB1 / 1.38 / 1.28 / 0.9 / 0.15 / 0.006
CLB/CSB2 / - / 1.38 / 1.8 / 0.14 / .016
Crystallex also shipped samples of Conductora waste and ore from the Mesones deposit.
Graphitic carbon assays were obtained as the difference between CTotal and CCO2 on all samples and found to be in the range of 0.01 to 0.08%. Preg robbing tests were done on the earlier samples and CSB, SAPS2, and SAP(2) were found to be mildly preg robbing with 11, 9, and 16% of a 10 ppm spike adsorbed after 24 h. SAPO and the other SAPS samples returned values of 4% or less. Mercury assays in the various samples were either 0.3 g/t or <0.3 g/t except for SAPS1 which was reported as 0.4 g/t.
The as-received screen analyses of SAPO, SAPS2 and SAPS 3 were 63, 182, and 69 micrometres m respectively. All other samples were competent rock provided as fragments of drill core.
Various composites were produced for metallurgical testwork as presented in Table 3. The composites were made to represent possible operating combinations and to investigate the impact of varying cyanide soluble copper levels on operating parameters.
Table 3. Main composites used in testwork
Composite name / CompositionSAPO-CSB / 20% SAPO1, 80% CSB
Comp S1 / 10% SAPO1, 10% SAPS2, 80% CSB1 (target 85 ppm CNSCu)
Comp S2 / 10% SAPO1, 20% SAPS2, 70% CSB1 (target 112 ppm CNSCu)
Comp S3 / 10% SAPO1, 10% SAPS3, 80% CSB1 (target 172 ppm CNSCu)
Comp S4 / 10% SAPO1, 20% SAPS3, 70% CSB1 (target 286 ppm CNSCu)
Comp S5 / 10% SAPO1, 10% SAPS4, 80% CSB1 (target 362 ppm CNSCu)
SAPS350 / 40% SAPS1, 50% SAPS2, 10% SAPS3 (target 350 ppm CNSCu)
CLB-CSB Comp. 1 / 68% CLB, 32% CSB (selected from CLB-CSB sample)
CSB2 / 100% CSB (balance of CLB-CSB sample)
CLB-CSB Comp. 2 / 27% CLB, 73% CSB
Mine blend / 15% SAPO, 5% SAPS (350ppm CNSCu), 10% CLB, 70% CSB
Grinding tests
A great deal of grinding testwork had been performed on Las Cristinas ore by earlier investigators, including A.R. MacPherson Consultants Ltd. Bond rod and ball mill work index determinations and abrasion index measurements were made in the recent Lakefield program which confirmed the earlier data.
The Bond ball mill work index data (metric) for CSB and CLB are 15.3 and 10.5 respectively. The abrasion indices are about 0.1 g for CLB, and 0.2 g for CSB.
SAPO and SAPS have not been subjected to Bond work index tests because the as-received material is too fine to test. However, an apparent work index for saprolite can be calculated from the work index measurements for blends containing this material. This calculation method yields values between 6 and 11.
Gravity recovery of gold
The feed for bottle roll leach tests were prepared by grinding 2 kg batches of ore to the desired grind then passing the sample through a 3” Knelson concentrator, then upgrading the concentrate on a Mozley table. The leach feed was then made by mixing the Knelson and Mozley tailings. Average data for the different ore types from twenty small-scale gravity recovery tests are provided in Table 4.
Table 4. Average data from gravity tests ahead of bottle roll leach tests
Sample / Grind / Gravity Conc / Tail / Head, g/t AuK80, µm / Wt % / Au, g/t / % Rec'y / Au, g/t / calc. / direct
SAPO / 35 / 0.031 / 252 / 5.3 / 1.35 / 1.47 / 1.63
SAPS / 50 / 0.060 / 900 / 18.4 / 1.39 / 1.71 / -
COMP S / 63 / 0.097 / 356 / 22.9 / 1.14 / 1.48 / 1.43
SAPO/CSB1 20/80 / 77 / 0.082 / 278 / 15.7 / 1.00 / 1.19 / 1.38
CSB / 67 / 0.091 / 328 / 22.5 / 0.96 / 1.24 / 1.24
CSB depth / 94 / 0.086 / 254 / 17.2 / 1.03 / 1.25 / 1.29
CLB/CSB2 / 99 / 0.026 / 1198 / 22.2 / 1.07 / 1.38 / 1.46
The four samples of Mesones CSB and CLB-CSB mixtures were also processed by gravity concentration and responded well. From an average feed grade of 1.1 g/t, 37% of the gold was recovered to a 711 g/t concentrate.
As is described later, a pilot plant was operated to process about 1 tonne of Las Cristinas material over a 20-day period. The first part of the pilot plant run used a feed comprising 20% SAPO and 80% CSB. The second part used a feed comprising 15% SAPO, 5% SAPS, 10% CLB, and 70% CSB – the Mine Blend. The feed for the pilot plant was prepared in 30 kg batches which were processed by the same Knelson-Mozley flowsheet as described above. Gravity recovery data from the pilot plant are provided below in Table 5.
Table 5. Gravity concentration data from pilot plant feed preparation work
Feed / Phase / Gold assays – g/t / Recovery to conc. – %Head / Tail / Conc / Mass / Gold
SAPO-CSB / PP1-1 / 1.50 / 0.95 / 775 / 0.071 / 36
SAPO-CSB / PP1-2 / 1.39 / 0.95 / 1760 / 0.025 / 32
Mine Blend / PP2 / 1.38 / 0.90 / 1920 / 0.025 / 34
In the first part of the pilot plant (Phase PP1-1), the mass of Mozley concentrate was set at 15 to 25 g per 30 kg batch grind or about 0.07% mass pull. In the later operation (PP1-2 and PP2) the mass pull was reduced to 5 to 10 g of concentrate or about 0.025% mass. The tabulated concentrate assays are based on the assay head and the gravity tail assay estimated from the cyanidation data.
Gravity recovery in the pilot plant was far higher than in the small-scale tests as indicated in Figure 3. This is as expected and reinforces the importance of processing large samples to determine the potential for the gravity recovery of gold.
About 40 kg of SAPO and 100 kg of CSB were studied by Professor André Laplante at McGill University using the Laplante Gravity Recoverable Gold (GRG) test procedure. This work established that SAPO contained 39% GRG while CSB contained 46%. It was noted that about 10% of the total gold in each sample was –20 m in size and would be difficult to recover. Based on an analysis of the data, Laplante concluded that about 25% gold recovery would be obtained by gravity processing.
Figure 3. Gravity concentration data illustrating sample size effect
Intensive cyanidation of gravity concentrate
Samples of the concentrates produced during the three different segments of the gravity recovery portion of the pilot plant were subjected to intensive cyanidation using 2% NaCN solution, H2O2 as an oxidant, and a leach time of 48 h. Results are summarized in Table 6.
Table 6. Intensive cyanidation of gravity concentrate
Feed / PP / NaCNkg/t / Metal extraction - % / Tail / Calc head
Add / Cons / 2 h / 6 h / 12 h / 24 h / 48 h / g/t / g/t
SAPO-CSB / 1-1 / 233 / 74 / Au / 90 / 84 / 90 / 95 / 98.6 / 6.6 / 484
Ag / 95 / 90 / 96 / 96 / 95.7 / 2.3 / 54
1-2 / 240 / 78 / Au / 91 / 93 / 101 / 99 / 99.5 / 6.3 / 1,378
Ag / 102 / 95 / 103 / 100 / 98.3 / 2.3 / 138
Mine blend / 2 / 260 / 100 / Au / 101 / 96 / 108 / 99 / 99.3 / 8.5 / 1,246
Ag / 104 / 95 / 101 / 98 / 98.3 / 2.3 / 136
The intensive cyanidation testwork gave very encouraging results and this method will be used to process the gravity concentrates at Las Cristinas.
In summary, the data show that gravity recovery should be very effective at Las Cristinas and give well over 20% recovery of the gold in the feed. The concentrates are very amenable to intensive cyanidation.
Bottle roll leach tests
All bottle roll tests were preceded by the removal of coarse gold by gravity concentration as detailed earlier. The results discussed in this section are overall gold recovery, i.e., gravity recovery plus leach extraction.