gold deposits of the birimian and tarkwaian in ghana...detailed geology of the wassa mine, showing...

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  • GUIDEBOOK SERIESVolume 57

    April 22 – May 1, 2018

    Field Trip LeadersRael Lipson and Raymond Kudzawu-D’Pherdd

    Guidebook EditorsRael Lipson, Borden Putnam, Simon Meadows Smith, and Ryan Taylor

    SOCIETY OF ECONOMIC GEOLOGISTS FOUNDATION, INC.SEGF STUDENT FIELD TRIP 17

    Gold Deposits of the Birimian and Tarkwaian in Ghana

  • GUIDEBOOK SERIESVolume 57

    April 22 – May 1, 2018

    Field Trip LeadersRael Lipson and Raymond Kudzawu-D’Pherdd

    Guidebook EditorsRael Lipson, Borden Putnam, Simon Meadows Smith, and Ryan Taylor

    SOCIETY OF ECONOMIC GEOLOGISTS FOUNDATION, INC.SEGF STUDENT FIELD TRIP 17

    Gold Deposits of the Birimian and Tarkwaian in Ghana

  • GUIDEBOOK SERIESOF THE

    SOCIETY OF ECONOMIC GEOLOGISTS, INC.

    Guidebook 57

    PrintISSN 1547-3090

    ISBN 978-1-629490-72-4

    OnlineISSN 2374-6955

    ISBN 978-1-629494-94-4

    Digital version available fromSociety of Economic Geologists, Inc.

    7811 Shaffer ParkwayLittleton, Colorado 80127

    Tel. 1.720.981.7882Fax: 1.720.981.7874

    Bookstore: www.segweb.org/storePublication date: April 2018

    On the cover: Generalized regional geology of southwest Ghana.Modified after BRGM 1:2M Geological Map of West Africa.

  • Contents

    List of Figures and Tables .......................................................................................................... iv

    Emergency and Other Contacts ..................................................................................................vii

    Sponsors

    West African Rescue Association ....................................................................................xi

    SEMS Exploration ........................................................................................................xiii

    WARA Clinic Maps ................................................................................................................... xv

    Special Request to Student Participants ....................................................................................xix

    The SEGF Student Field Trip Program ...................................................................................... xx

    Foreword ................................................................................................................................... xxi

    Acknowledgments ....................................................................................................................xxii

    Field Trip Leaders ...................................................................................................................xxiii

    Professionals/Mentors Accompanying Student Field Trip 17 .................................................xxiv

    Student Participants ............................................................................................................... xxvi

    Itinerary ................................................................................................................................. xxvii

    Field Trip Route Map ............................................................................................................... xxx

    Syllabus – Field Guide

    Introduction ...................................................................................................................... 1

    Southwest Ghana Regional Geology ................................................................................ 7

    Site Visits

    Iduapriem (Teberebie) .......................................................................................... 23

    Damang ................................................................................................................ 33

    Wassa .................................................................................................................... 45

    Prestea .................................................................................................................. 59

    Obuasi .................................................................................................................. 73

    Homase-Akrokerri ................................................................................................ 91

    Obotan-Esaase ................................................................................................... 105

    Chirano ............................................................................................................... 131

    Ahafo................................................................................................................... 145

    Akyem ................................................................................................................. 157

    References ............................................................................................................................... 167

    Appendices

    Appendix I: Cost Definitions ........................................................................................ 175

    Appendix II: Chronology of Key Events ....................................................................... 180

    iii

  • List of Figures and TablesFigures

    1-1. Simplified geological map of the West African craton with gold deposits .......................................................21-2. Cretaceous palinspastic reconstruction of the Guiana and south West Africa shields .....................................31-3. Simplified geological map of southwest Ghana with mineral belts .................................................................51-4. Comparison of the world’s 25 largest Precambrian orogenic Au deposits .......................................................61-5. Grade-tonnage diagram for West African orogenic Au deposit resources ........................................................61-6. Stratigraphic column of southwest Ghana ........................................................................................................81-7. Age dating from southwest Ghana ore deposits ...............................................................................................91-8. Tectonic evolution in southwestern Ghana .....................................................................................................101-9. Compilation of histograms presenting radiometric age data ..........................................................................111-10. Simplified geologic map of the Ashanti Belt ..................................................................................................141-11. Cross-section across the Ashanti Belt .............................................................................................................151-12. Total magnetic intensity reduced to pole over first vertical derivative of the Ashanti Belt ............................161-13. Bouguer gravity map over the Ashanti Belt ....................................................................................................17

    2-1. Satellite image of the Tarkwa district .............................................................................................................242-2. Geology of the Tarkwa-Damang gold district.................................................................................................252-3. Type cross-section through Tarkwa ................................................................................................................262-4. Stratigraphic column of southwest Ghana ......................................................................................................282-5. Stratigraphic subdivisions of the Banket Series at the Tarkwa gold mine ......................................................292-6. Iduapriem mining blocks ................................................................................................................................31

    3-1. Location map of Damang showing generalized geology................................................................................343-2. Stratigraphic succession of the Tarkwa Group at Damang .............................................................................353-3. Mining bench plan from the Damang deposit ................................................................................................363-4. Damang area regional geologic map and cross-section ..................................................................................373-5. Photos of drill core and outcropping shear zone at Damang ..........................................................................383-6. Styles of gold-bearing extensional quartz veins in walls of the Damang pit..................................................393-7. Diagrams of the various styles of mineralized quartz veins at Damang ........................................................403-8. Longitudinal projection of orebodies at Damang ...........................................................................................403-9. Kinematic model for gold-bearing quartz vein development at Damang.......................................................413-10. Structural model for quartz vein development at Damang .............................................................................42

    4-1. Location map of Wassa showing generalized geology ...................................................................................464-2. Soil geochemistry anomaly map over the Wassa deposit ...............................................................................484-3. Detailed geology of the Wassa mine, showing resource model gold distribution ..........................................494-4. DD core samples and thin sections of main host rocks at Wassa ...................................................................504-5. Cross-section through the Wassa Main deposit showing ore zones ...............................................................514-6. Cross-section through the Wassa Main deposit showing Eburnean folding ...................................................524-7. Detailed magnetic interpretation of the Wassa area .......................................................................................534-8. Examples of Wassa mineralization .................................................................................................................544-9. Synthesis of events at Wassa and the regional evolution ................................................................................554-10. Longitudinal projection of select thicker drill intersections in the B Shoot ...................................................564-11. Longitudinal projection of B Shoot orebody and potential down plunge extension ......................................574-12. Cross-section through the geotechnical stress model of the crown pillar ......................................................58

    5-1. Location map of Prestea .................................................................................................................................605-2. Satellite image of Bogoso and Prestea ...........................................................................................................615-3. Location and simplified geology of the Prestea and Bogoso deposits ............................................................625-4. The four major litho-structural units exposed at Prestea open pit ..................................................................635-5. Oblique view of 3-D resource model of the Prestea orebodies ......................................................................635-6. Mine Level 17 drill plan with vein intersections and structural contours ......................................................64

    iv

  • 5-7. Cross-section 8200N through the West Reef ..................................................................................................655-8. West Reef vertical longitudinal projection .....................................................................................................665-9. DD core photos of West Reef mineralization styles .......................................................................................675-10. Prestea general infrastructure layout ..............................................................................................................68

    6-1. Satellite image of Obuasi mining district .......................................................................................................746-2. Simplified regional geology and location of Obuasi and other major gold mines .........................................756-3. Images from a typical underground stope showing a rock bolting machine

    installing ground support at Obuasi................................................................................................................776-4. Obuasi Au production and AISC cost trends from 1q2013 to 2q2014 ...........................................................786-5. Location, geology plans, and cross-section of Obuasi district .......................................................................796-6. Map of major structures and distribution of open pits within the Obuasi district ..........................................806-7. Location of Obuasi gold mine along the Axim-Konongo shear zone ............................................................816-8. Simplified cross-section through the Obuasi deposit, showing shear structure .............................................826-9. DDH core photo of quartz vein with free gold, Obuasi ..................................................................................836-10. Vertical longitudinal projection of the Obuasi gold mine ..............................................................................846-11. Vertical longitudinal projection of the Obuasi gold mine showing

    reserve model blocks and infrastructure .........................................................................................................856-12. Longitudinal projection through the Obuasi mine showing high-grade shoots ..............................................866-13. Oblique schematic view of the old and new mine design, Obuasi .................................................................87

    7-1. Location of Homase-Akrokerri project ..........................................................................................................917-2. Outline of the old Akrokerri underground mine workings projected to surface, Akrokerri town ..................937-3. Location of > 20 ppb gold-in-soil anomalies along the Homase-Akrokerri trend .........................................947-4. Detail of the central gold-in-soil anomaly covering Akrokerri to Homase ....................................................957-5. Photo of open pit mining of the Homase oxide ..............................................................................................967-6. Photograph of the flooded Homase open pit ..................................................................................................967-7. Simplified geology of the Obuasi, Akrokerri, and Homase district with simplified structure .......................977-8. Cross-section through the Homase open pit with drill results ........................................................................987-9. Plan of areas of gold in soil anomalies at Homase and Akrokerri with locations of

    Homase pit and proposed pits .........................................................................................................................997-10. Versatile Time Dome Electromagnetic (VTEM) image with mineralized zones highlighted ......................1007-11. Detailed aeromagnetics survey showing location of Homase-Akrokerri NE-trending

    mineralized structure passing through Homase ............................................................................................1017-12. Oblique 3-D image of the Homase resource block model showing exhausted and planned pits .................1017-13. Vertical longitudinal profile of the Homase resource block model ..............................................................1027-14. Photograph of illegal artisanal mining at Akrokerri .....................................................................................103

    8-1. Location and geological map of Asanko Gold Mine assets ..........................................................................1058-2. Location map of Asanko Gold tenements.....................................................................................................1068-3. Asanko tenements showing distributions of gold in soils geochemical anomalies ......................................1098-4. Regional total field aeromagnetic image of the Ashanti Belt and Kumasi Basin .........................................1118-5. Geological map of Asanko tenements showing major lithologies, structures, and gold prospects ..............1138-6 Geological map of the Nkran pit area ...........................................................................................................1148-7. Cross-section through the Nkran pit .............................................................................................................1158-8. Esaase deposit interpreted structural geology ..............................................................................................1178-9. Nkran Mineral Resource Estimate (MRE) progression and reconciliation

    audit work programs from 2014 to 2016-178-10 .........................................................................................1188-10. Oblique view into Nkran pit geological model, comparing 2014 and 2016 interpretations.........................1198-11. Nkran MRE block models comparison ........................................................................................................1208-12 Cross-sections through Nkran MRE block models showing the new interpretations of

    the two dominant structural ore controls ......................................................................................................1218-13. Nkran and Total Obotan MREs over time ....................................................................................................1258-14. AISC (all in sustaining cost) international gold production cost curve positions of

    Asanko P5M and P10M ................................................................................................................................126

    v

  • 8-15. Contrasted basic mine plans and outputs of the P5M and P10M mining scenarios .....................................1278-16. Nkran pit NPV and cost per ounce Whittle optimization scenarios .............................................................128

    9-1. Satellite image of the Chirano mine region ..................................................................................................1319-2. Satellite image of the Chirano mine and infrastructure ................................................................................1329-3. Regional geologic setting of the Chirano deposits .......................................................................................1349-4. Location and geology of the Chirano district ...............................................................................................1359-5. Cross-section through the Obra deposit .......................................................................................................1379-6. Geologic level plan of the large Obra deposit ..............................................................................................1389-7. Plan view of typical shape and sizes of various Chirano gold deposits .......................................................1399-8. Schematic models of structure development and controls on gold mineralization ......................................1409-9. Block model cross-section through the Akwaaba deposit showing gold grades ..........................................1419-10. Survey crew in the Chirano open-pit ............................................................................................................1429-11. Typical working face at Chirano underground .............................................................................................142

    10-1. Satellite image of the Ahafo mine region .....................................................................................................14510-2. Ahafo location map with regional geology ..................................................................................................14610-3. Site map of Ahafo South and Ahafo North ...................................................................................................14710-4. Ahafo regional geologic map........................................................................................................................14910-5. Photographs of pit walls showing ductile folding and extensional shear vein .............................................15010-6. Simplified geologic plan map and cross-sections of the Awonsu and Apensu deposits ...............................15110-7. Satellite image (annotated) showing the locations of the four open pits at Ahafo .......................................15210-8. Cross-section of Subika ................................................................................................................................15310-9. Oblique view of Subika geological model superimposed on the open pit....................................................15410-10. DD cores showing increasing intensity of alteration and mineralization .....................................................155

    11-1. Location map for Akyem in southwest Ghana..............................................................................................15811-2. Simplified cross-section from early exploration stage of the Akyem discovery...........................................15911-3. Attributed simplified cross-section of Akyem ..............................................................................................16011-4. Block model cross-section of the Akyem deposit emphasizing high grade mineralization .........................16111-5. Vertical longitudinal projection showing strong surficial gold enrichment and

    strong grade continuity along the thrust .......................................................................................................16211-6. Early schematic geologic map with stream sediment anomaly Au values ...................................................16311-7. Schematic map of preliminary geochemical signature of the Akyem deposit .............................................16311-8. Akyem mining site layout .............................................................................................................................165

    Tables1-1. Resource and Reserve Data for Orogenic Gold Deposits in West Africa .........................................................42-1. Iduapriem Resources and Reserves ................................................................................................................273-1. Damang Reserves and Resources Reconciliations 2015 to 2016 ...................................................................334-1. Wassa Reserves, Resources, and Reconciliation as at 2016 ...........................................................................475-1. Bogoso-Prestea Reserves and Resources in 2016, and Reconciliation to 2015 .............................................705-2. West Reef Search Parameters for Grade Interpolation and Estimation ..........................................................716-1. Obuasi Resources and Reserves .....................................................................................................................767-1. Homase Resources and Reserves ...................................................................................................................928-1. Asanko Gold – Reserves and Resources and Reconciliations 2015 to 2016................................................1078-2. The Nine Individual Deposits Comprising Asanko’s Mining Complex .......................................................1108-3. MRE Reconciliations for Measured and Indicated Resources .....................................................................1228-4. MRE Reconciliations for Proven and Probable Reserves ............................................................................1239-1. Chirano Reserves and Resources and Reconciliations 2015 to 2016 ...........................................................13310-1. Ahafo Resources and Reserves ....................................................................................................................14811-1. Akyem Resources and Reserves ...................................................................................................................157

    vi