Stope Stability at the Big Gossan Skarn Deposit, Papua, Indonesia

Transcription

1 Stope Stability at the Big Gossan Skarn Deposit, Papua, Indonesia Katerina Sari, Geoffrey de Jong, Pormando Silalahi, Wahyu Sunyoto Underground Geology Operation, PT Freeport Indonesia PACRIM 2015 Hong Kong, China March 18-21,

2 Background The tonnage (also grade) difference between production forecast and production reconciliation at some stopes at the Big Gossan mine area is caused by overbreak. The Matthew s stability method, to estimate the stability of a stope, can give a false impression of stability as it does not take in consideration potential overbreak (due to weakening of the stope face by mineralogy). 2

3 Introduction Ertsberg Mining District, Contract of Work A (COW A) PTFI INDONESIA PAPUA NEW GUINEA LEGEND AUSTRALIA 1000 m N 3

4 Geology of BG Mine Plan View the Big Gossan 2540mL Mine Layout and the Geology Ekmai shale as marker bed boundary (weak mineralisation) Two copper-gold mineralisation type: Prograde skarn : grossularite, andradite, diopside, hedenbergite Retrograde skarn : tremolite, actinolite, chlorite GEOLOGY HISTORY 1. In ± 2.7 MA, Ertsberg diorite intruded sedimentary rocks (become silicified, hornfelsic, and marbleized) through NW-SE Faults trends. 2. In ± 2.5 MA, Ertsberg diorite (dyke / unaltered) intruded and altered the sedimentary rock (become skarn) and diorite (become potassic-propylitic) through NW-SE Faults trends. 3. Mineralisation inside skarn (prograde and retrograde skarn).

5 Reserves of BG Mine Overview the Big Gossan Deposit, Looking to North Proved and probable reserves Tons (milions) Cu (%) Au (g/t) Ag (g/t) Grasberg Open Pit + Block Cave Kucing Liar DOZ + DMLZ (EESS) Big Gossan Total Reserves Source : Independent Mining Consultant, Inc, 2013 BIG GOSSAN INFRASTRUCTURE BIG GOSSAN ORE BODY BIG GOSSAN STOPES 5

6 Cross Section Along A-B Line, Looking to NE Showing EqCu Model and Stope Design STOPE h = 40m A PLAN VIEW 2540mL STOPE h = 20m A B B 6

7 Cross Section Along A-B Line, Looking to NE Showing RQD Model and Stope Design STOPE h = 40m A PLAN VIEW 2540mL STOPE h = 20m A B B 7

8 BG 2004 Mine Sequence Mine Design and Ground Basis Stability Ground Stability Mathew s stability method 8

9 Matthew s Stability Method Stope Stability Graph Diederichs and Kaiser (1996) 9

10 The Overbreak Forming N = Q x A x B x C Q = modified rock mass rating (Q) A = induced stress = RQD Jn x Jr Ja x A x B x C B C = discontinuity of joint orientation = the orientation of the excavation surface Low N High S unstable zone not supported OVERBREAK 10

11 BG Mine Stope Data 2540 ml 2560 ml 2600 ml LEGEND 1% EqCu Boundary Ekmai Shale Stope The 14 stope hanging wall faces studied in the Big Gossan mine are located at the 2540 level (nine stopes), 2560 level (four stopes) and 2600 level (one stope). 11

12 Stability Graph of BG Stopes STOPE NAME N' S Unsupported Transition Zone Stable Zone Overbreak at transition zone > at stable zone??? 2540_29_ _27_ _23_ _30_ _14_ _45_ _43_ _22_ _33_ _25_ _11_ _34_ _12_ _29_

13 Overbreak Data at BG Stopes STOPE NAME STOPE TONS Overbreak Tons % 2540_29_ % 2540_27_ % 2540_23_ % 2600_30_ % 2540_14_ % 2540_45_ % 2560_43_ % 2540_22_ % 2560_33_ % 2540_25_ % 2560_11_ % 2540_34_ % 2540_12_ % 2560_29_ % Unsupported Transition Zone Stable Zone The correlation between overbreak and stope stability is not clear cut Other factor?? 13

14 Mineralogy Control Hardness Texture Minerals Content Factor A N Number Type Connectivity Presence OH-bearing minerals well-interlocked mineral fabric Low Mohr s scale High rock strength Point load UCS Retrograde Skarn < Prograde Skarn 14

15 Role of Mineral and Water The percentage of overbreak increases with increasing retrograde skarn alteration the overbreak percentage is higher in stopes containing higher amounts of pyrite and anhydrite Meteoric water percolating and recharging in less than 24 hours through shear zones and faults also reducing the rock strength causing cavities 15

16 SUMMARY The Mathew stability method does not take into account that groundwater effect. At the Big Gossan skarn, groundwater infiltration reduces the rock strength and support. Relying only on rock mechanical parameters and not considering the mineralogical factors at Big Gossan would be a mistake. The stability number, N, cannot alone explain the overbreak behavior in the Big Gossan stopes. The percentage and type of skarn, prograde or retrograde, that is dominant in the stope and the minerals like pyrite and anhydrite are important in understanding the potential overbreak behavior in the stopes of the Big Gossan skarn deposit. 16

17 Thank You Corresponding author : katerina_sari@fmi.com 17