Cordoba Minerals Corp. (Cordoba) has announced a positive updated Mineral Resource estimate for the San Matias Project, including the Alacran replacement copper-gold deposit, in Colombia. The updated Mineral Resource estimate, prepared in accordance with the National Instrument 43-101 Standards of Disclosure for Mineral Projects, has been completed by Nordmin Engineering Ltd (Nordmin) of Thunder Bay (Ontario). Nordmin is also completing the Preliminary Economic Assessment (PEA) for the San Matias Project, which is expected later in July.
“We are very pleased with the work completed by Nordmin in updating the Mineral Resource estimate for San Matias,” stated Eric Finlayson, President and CEO of Cordoba.
“By understanding key structural controls to mineralisation, such as the high-grade vertical structures, Nordmin have been able to unlock additional value in Alacran and in the satellite deposits. We look forward to the results of the San Matias PEA, which we anticipate will demonstrate a very robust copper, gold and silver mining project.”
Nordmin update increases gold grades and adds silver resources at Alacran
Nordmin has updated the Mineral Resource estimate for the Alacran deposit with an improved geological understanding of structural controls on the high-grade gold veins within the deposit following recent re-mapping of the artisanal workings and re-logging of both historical and Cordoba drill core. This work has led to the identification of gold-rich domains controlled by vertical or near-vertical structures that are traceable along the north-south axis of the deposit. These domains were unrecognised in previous iterations of the Resource model. Definition of these structural domains has allowed for a robust geostatistical model to support the inclusion of higher gold grades into the Resource model.
Metallurgical test work completed by SGS Canada Inc. (SGS) has improved the confidence of the expected copper, gold and silver recoveries within the saprolite, transitional and fresh sulfide zones for each deposit, as previous assumptions were based on limited historical test work. SGS has confirmed that silver is recoverable, providing for the inclusion of silver in the updated Resource model. The recent test work has further demonstrated that up to 50% of the gold and silver may be recoverable by a gravity circuit, which, if verified through further metallurgical testing, would allow production of doré bars on-site.
An updated structural model, in conjunction with a geotechnical review of the Alacran and satellite deposits, has enabled an improved definition of the expected pit-wall angles within the conceptual open pit shells. All four of the pits were run using a maximum 45° pit slope in fresh and transition rock and 32.5° in the weathered saprolite rock. The application of these pit slopes at the Alacran deposit has enabled access to mineralized material up to 50 m deeper than in previous Alacran Resource estimates.
The February 2018 Alacran Mineral Resource estimate completed by Amec Foster Wheeler Americas Limited (refer to Cordoba’s news release dated 26 February 2018) included 36.1 million t of Indicated Resources grading 0.57% copper and 0.26 g/t gold (0.72% CuEq), and 31.8 million t of Inferred Resources grading 0.52% copper and 0.24 g/t gold (0.65% CuEq) at a 0.28% CuEq cut-off. The Montiel East, Montiel West and Costa Azul satellite deposits were not included in the February 2018 Mineral Resource estimate. For the 2019 Mineral Resource, a detailed geological and structural review, combined with metallurgical testing and detailed QA/QC analysis, warranted the addition of the satellite deposits to the updated Resource estimate.
The three satellite deposits are geologically distinct from the stratigraphically-controlled, carbonate-replacement copper-gold-silver and structurally-controlled high-grade gold mineralisation at Alacran. The Montiel East deposit is a porphyry copper-gold deposit hosted in a Cretaceous diorite intrusion emplaced into basaltic andesite volcanic rocks. At Montiel West, copper-gold mineralisation occurs in an intense quartz stockwork cutting basaltic andesite volcanic rocks, with no intrusive material evident. At Costa Azul, a Cretaceous diorite intrusion containing porphyry copper-gold mineralisation dips shallowly eastward and is underlain by unmineralized volcanic rocks.
Cordoba believes that the updated Mineral Resource estimate carries added confidence, due to the increases of the Indicated Resources compared to the previous estimate. When compared to the February 2018 Mineral Resource estimate, and at the reporting cut-off grade of 0.30% CuEq, the June 2019 Indicated Resource tonnage increased by 139% at the Alacran deposit, the contained copper increased by 115%, the contained gold increased by 148% and silver was added into the estimate (not previously included). Largely reflecting the conversion of Inferred Resources into the Indicated category, the June 2019 Inferred Mineral Resource tonnage decreased by 95%, the contained copper decreased by 96%, contained gold decreased by 97% and silver was added into the estimate (not previously included).
The significant increase in Indicated Resources is primarily attributed to increased confidence in the understanding of the structural controls on mineralization, re-mapping of the artisanal workings, re-logging of both historical and Cordoba drill core and the inclusion of some historical drilling. The historical drilling included drill holes completed by previous operator Dual Resources Inc. (SJ drill holes) that were later twinned by Ashmont Resources Corp. and Cordoba. A detailed QA/QC analysis and comparison between the SJ holes and twin holes warranted their inclusion. Collectively, this work recognised the key structural controls on mineralisation and improved the overall data density for the deposit, resulting in a substantial improvement in the geological and grade continuity required to support high conversion rates of Inferred to Indicated Resources.
Alacran deposit cut-off grade sensitivity
Indicated and Inferred Mineral Resources have been calculated at various copper equivalent cut-off grades to demonstrate the variability of tonnage and grades. The reporting base case of 0.3% CuEq cut-off grade compares with a 0.28% CuEq cut-off grade used in the February 2018 Mineral Resource.
Mineral Resource estimation methodology
For a detailed discussion of the Mineral Resource estimation methodology, refer to the National Instrument 43-101 technical report that will be filed on www.sedar.com within 45 days of this news release.
The updated Mineral Resource estimate for Alacran is based on geological and structural data from 178 diamond drill holes totalling 39 086 m and 30 086 samples completed by Cordoba and previous operators between 2012 and 2018. Assay data is available for 167 of the completed holes. The drill hole database for this Resource estimate has increased by 14 drill holes (8%) and 3086 samples (11%) as compared to the February 2018 Resource estimate as a result of the inclusion of the historic SJ holes. Their inclusion, in conjunction with a review of artisanal workings and trenching, resulted in an improvement of the overall data density within the deposit.
The updated structural model demonstrates significant improvements to both the geological and grade continuity within each of the mineralised domains. The mineralised domains have been explicitly modelled to constrain the higher grades within each deposit through the use of hard boundaries to separate the different styles of mineralisation/domains (vertical, sub-vertical and low-grade). These hard boundary ‘wireframes’ effectively minimise the mixing of sample data across the separate blocks and more accurately reflect the observations made in the field. This approach minimises risks compared to using implicit modelling for resource estimation. Further, these structural/mineralised domains have allowed for a more robust geostatistical model to support the inclusion of higher gold grades into the Resource model.
The domains are as follows for each deposit:
- Alacran – The models (wireframes) include five high-grade vertical structures, five sub-vertical, stratabound bodies of replacement mineralization, and an encompassing low-grade shell.
- Costa Azul – The models include two high-grade wireframes and an encompassing low-grade shell. The Mineral Resource estimate is based on geology and assay data from 118 holes totalling 4996 m completed between April 2014 and March 2017, and consisting of 112 reverse circulation and 6 diamond core holes. A total of 2275 assays were used, comprising 1193 diamond core assays and 1082 reverse circulation assays.
- Montiel East – The models include two high-grade wireframes and an encompassing low-grade shell. The Mineral Resource estimate is based on geology and assay data from 78 holes totalling 11 056 m completed between August 2013 and March 2017, and consisting of 30 diamond core holes and 48 reverse circulation holes. A total of 6946 assays were used, comprising 6406 diamond core assays
- Montiel West – The models include one high-grade wireframe and an encompassing low-grade shell. The Mineral Resource estimate is based on geology and assay data from 93 holes totalling 4056 m completed between February 2014 and May 2017, and consisting of 8 diamond drill core holes and 85 reverse circulation holes. A total of 1743 assays were used, comprising 1104 diamond core assays and 639 reverse circulation assays.
Grade outliers are assay values that are much higher than the general population of samples and that have the potential to bias (inflate) the quantity of metal estimated in a block model. Geostatistical analysis using XY scatter plots, cumulative probability plots and decile analysis was used by Nordmin to analyse the raw drill hole assay data for each domain in order to determine appropriate grade capping.
A 5 x 5 x 5 m block size was chosen for the Alacran deposit resource block model to reflect mining selectivity up to a potential 16 000 tpd mining scenario. The resource block model was sub-blocked to 2.5 x 2.5 x 2.5 m blocks to maintain geological resolution.
The satellite deposits in the San Matias Project area used a 5 x 10 x 5 m block size. The block grade model was flagged for lithological and structural domains with wireframes from the geological model. The block model was classified in accordance with CIM definition standards for Mineral Resources and Mineral Reserves (May 2014). Drill spacing for the resources in the Indicated category was approximately 50 m and in the Inferred category is between 50 - 150 m.
Read the article online at: https://www.globalminingreview.com/mining/04072019/cordoba-reports-positive-updated-mineral-resource-estimate-for-the-san-matias-project/