The Konongo Gold Project, Ghana: An Example of How Geology Makes All the Difference to a Resource Estimate

Transcription

1 The Konongo Gold Project, Ghana: An Example of How Geology Makes All the Difference to a Resource Estimate 1. Introduction 2. Geology Regional 3. Geology Project scale 4. Geology Mine scale 5. Approach 6. Results 7. Summary & Conclusions René Sterk, MSc MAusIMM MAIG

2 1. Introduction - Konongo Gold Project Situated on the western margin of the 125 Moz Ashanti Gold Belt in Ghana Managed/funded by Liongold Corporation, operated subsidiary, Signature Metals Historic underground operations produced 1.6 Moz Au at a head grade of 11.8 g/t Au Mining was focused almost exclusively on free-milling gold in quartz Obenemase operation is the first of eight main operations to be assessed Obenemase: Signature drilled a total of 20,000 m of DD, RC and AC holes Obenemase: Existing 111,000 m of RAB, DD, and RC holes drilled between 1972 and 2009 by previous operators 5,250 m of underground channel sampling and drilling was completed mainly in the 1950s and 1970s. Last Resource by Ball (2010): 6,875, g/t for 552,693 oz Au

3 1. Introduction - Konongo Gold Project Situated on the western margin of the 125 Moz Ashanti Gold Belt in Ghana Managed/funded by Liongold Corporation, operated subsidiary, Signature Metals Historic underground operations produced 1.6 Moz Au at a head grade of 11.8 g/t Au Mining was focused almost exclusively on free-milling gold in quartz Obenemase operation is the first of eight main operations to be assessed Obenemase: Signature drilled a total of 20,000 m of DD, RC and AC holes Obenemase: Existing 111,000 m of RAB, DD, and RC holes drilled between 1972 and 2009 by previous operators 5,250 m of underground channel sampling and drilling was completed mainly in the 1950s and 1970s. Last Resource by Ball (2010): 6,875, g/t for 552,693 oz Au

4 2. Geology - Regional Ashanti Belt: well-studied orogenic belt, the most prominent gold-bearing structure of the Paleoprot. in W-Africa Marks the boundary between a continental domain (Archaean craton) and an oceanic domain (Birimian crust) Consists of the volcano-sedimentary rocks of the Birimian Supergroup and Tarkwaian Group Birimian consists of the Sefwi Group (composed of alternating micaschists, metavolcanics and intrusive rocks) and Kumasi Group (metasediments, volcaniclastics and phyllites that are locally rich in graphite) Tarkwaian Group consists of a variety of sandstones (quartzites), conglomerates, argillites and phyllites Timing of gold mineralisation is somewhat contentious.

5 3. Geology Project Scale Birimian Supergroup (Kumasi and Sefwi Groups), Tarkwaian Group, Voltaian Group and basin granitoids Sefwi Group are the oldest rocks in the area and include basalt flows, andesites, tuffs, and metasediments Sefwi Group includes "Mine Sequence : a ~150 m package of tuffaceous and volcaniclastic metasediments Sefwi Group appears to be stratigraphically overlain by volcaniclastics of the Kumasi Group Tarkwaian rocks consist of a series of conglomerates, coarse sandstones, wellsorted sandstones. Within the area they have been interpreted to occur as an isoclinal syncline Contacts with the stratigraphically lower Birimian metavolcanics and metasediments corridors are structural. Mineralisation predominantly occurs within the western limb of the Sefwi Group metavolcanics and metasediments, with parasitic folds controlling the wider and higher grade mineralisation

6 4. Geology Obenemase Mine Scale Obenemase ore zones are hosted within Sefwi Group rocks Stratigraphy consists of a lower turbidite member (UB-3), overlain by a volcanicvolcaniclastic "Mine Series" member (UB- 2), in turn overlain by another turbidite member containing graphitic argillites and siltstones (UB-1) The stratigraphic sequence is commonly overturned and steeply NW-dipping Sequence is wedged in between the main Ashanti shear to the NW and the Obenemase shear to the SE (the latter interpreted to be a lower-order backthrust to the Ashanti shear zone) Macroscopic isoclinal folding and shear zones are related to NW SE thrusting at the onset of the Eburnean orogeny Most economic mineralisation is controlled by parasitic and fault-drag folds

7 4. Geology Obenemase Mine Scale Folds are isoclinal with fold plunging ~40 to the NE. Geometries controlled by the Obenemase shear zone as well as a pervasive set of N S (350 ) faults with oblique (sinistral-reverse) movement Folds and faults were likely formed at same time as the regional change in stress regime to NNW SSE shortening and accompanying sinistral shearing on the main shear zones This change in stress regime, in combination with a preceding period of stress quiescence and uplift, led to increased crustal permeability and the main phase of circulation of gold-bearing fluids at Obenemase, similar to the nearby 60 Moz Obuasi mine. Two styles of mineralisation are present at Obenemase: an early disseminated sulphide phase and a later quartz vein phase

8 5. Approach Data Validation? Team of 4 RSC geologists going through existing data 12-month programme of data reconciliation focused on validating and correcting these data All available core was re-logged and resampled where required field checking and correcting location data verifying assay and quality control data verifying mine production data + = New data were recorded for recovery and density of all core Three-dimensional geological and grade shell models

9 6. Results Location Data Error (m) OGM drilling - Horz Diff N E Northing Sort out local grids and projections Re-surveying old holes where possible Collect enough survey data from each drill campaign, across a good geographic spread of the drill area, and from each drill type, to assess the relative errors and their drift for each subset Corrections could easily be made using second and third order conformal polynomial functions ( rubber band correction) Downhole surveys: previous corrections made? Uncertainty of the location of underground workings Geo-referencing: using two surveyed shafts. All maps re-defined, now stope intercepts lining up with actual voids in drill holes. Knowing previous stopes allows for proper depletion of resource.

10 6. Results Assay Data Grade validation by: Orig Au-2013 Au Scatter Plot Priority <= 1 50 QQ Plot Orig Au [Priority <= 1]-2013 Au [Prior Data Set 1: Orig Au Priority <= 1Data Set 2: 2013 Au Priority visually checking sections checking database vs logs and lab sheets, 2013 Au (534 values) Au Priority <= 1 {534 values} check-sampling a representative part of the available core and pulps to investigate bias Results show bias on lower grade range 1,800 samples from remaining core sent for re-analyses A re-run of the resource estimate with lower grades shows no effect on resource: mostly below (cut-off grade) Orig Au (534 values) Orig Au Priority <= 1 {534 values}

11 6. Results Geology Data Previous resource estimations restricted by lack of understanding of geology Geological continuity could not be demonstrated and grade continuity was also difficult to show due to the lack of knowledge on structure, lithology and mineralisation controls biggest problem with large projects that span many years and various drilling campaigns is that there have been many geologists involved with the logging and interpretation of them Attempting to manipulate the data to fit (e.g. bulk changing logging codes) usually only makes matters worse Re-logging: QA and QC Data in the correct location, data of good quality geology becomes clear without effort

12 6. Results Density Data Density always poorly treated and previous was lost or poorly applied. 13,500 new density data points collected Great tool to verify lith_1 logging Representative for mineralisation as well as for lithologies Density modelled into blocks

13 6. Results Estimation New domains constructed to constrain estimation 3D geological model as a main constraint and guidance on interpretation & extrapolation High-resolution grade control data from historic pit and underground operations Inferred Indicated Measured Total Tonnes Grade Ounces Tonnes Grade Ounces Tonnes Grade Ounces Tonnes Grade Ounces Oxide 22, , , , , ,000 Transitional 70, , , , , ,500 Sulphide 435, ,000 2,183, ,500 2,374, ,150 4,991, ,300 Total 435, ,000 2,275, ,000 2,885, ,047 5,600, ,000 Resources are reported at 0.5, 0.7 and 1.0 g/t cut-off for oxide, transitional and sulphide zones respectively. Inferred Resources exclude Indicated and Measured Resources. Estimation: Structure is complex, with multiple orientation changes, faulting, folding and anastomosing unfolding not possible Wireframe divided into several estimation domains depending on orientation Estimated using 3D ordinary kriging (low nugget, low CV%), with soft boundaries between orientation domains Results show increase in grade and contained ounces, significantly higher confidence Estimated >100% Project value increase

14 7. Summary Diligent data collection Solid geological interpretations Good Exploration targets Higher confidence resource estimates Signature had clear strategy for this Project: adequate resources provided to complete the job with little compromise Mothballed projects based on old legacy data: common situation lots of value to be added simply by reworking existing data.