Seismic signatures of large mineral systems Evidence of fluid flow and alteration at depth

Transcription

1 Seismic signatures of large mineral systems Evidence of fluid flow and alteration at depth Don Pridmore, HiSeis Greg Turner, HiSeis Milovan Urosevic, Curtin University

2 Presentation Outline Seismic method: What causes reflections? What causes texture? Preserved amplitude processing 3D seismic examples from near surface environment at an epithermal mine Qld Textures from beneath major mineralised systems. St Ives Kambalda Western Australia Olympic Dam South Australia

3 Conceptual Geological Model

4 Reflection Seismic Energy travels at the seismic wave velocity Approx 6000m/sec Reflectivity is controlled by the change in the product of density and velocity across the boundary

5 Geological Scenario: Epithermal Syatem Altered fragmentals Altered Andesite Lava AI1 AI1 AI2 Andesite Lava AI2 AI2 >> AI1 AI2 > AI1 a.) Fragmentals b.) AGC Processing Partially Preserved Amplitude Processing Seismic Processing 5

6 Overlying Volcanics Epithermal Gold Vein System. Vertical Section from 3D Seismic Cube Preserved Amplitude Processing Mineralised vein. Faulted offset, clay alteration Increasing alteration 1 km Interbedded Andesitic Lavas and Fragmentals Reflections due to acoustic impedance (density*velocity) change across contacts between fragmentals and lavas

7 Epithermal Gold Vein System. Vertical Section from 3D Seismic Cube Preserved Amplitude Processing Location of gm-m intersections ±50 m either side of section 1 km Pervasive clay alteration around faults generates white snakes, zones where no coherent reflection emerge.

8 St Ives gold mining camp Major gold mining commenced in 1980s Currently producing oz pa Produced > 11.6 Moz Recent discovery: Invincible g/t Victory-Defiance 2 Moz

9 Victory seismic line (focus on preserving relative reflection amplitudes) ~10km Antiform - Focus for fluids? Low reflectivity in core and coincident gravity low - Felsic intrusion? ~5.5km Reduced reflectivity above the anticline apex - Alteration due to fluid flow? - Intrusions disrupting the continuity of reflectors? Adapted from Harrison and Urosevic, 2012

10 Victory seismic line with interpretation Victory Defiance Gold mine ~10km Antiform - Focus for fluids? Foster Thrust Fluid flow? Intrusions? Felsic intrusion Low reflectivity in core and coincident gravity low - Felsic intrusion? Repulse Fault ~5.5km Reduced reflectivity above the anticline apex - Alteration due to fluid flow? - Intrusions disrupting the continuity of reflectors? Adapted from Harrison and Urosevic, 2012

11 Rock properties (sonic log) Change in seismic velocity associated with alteration around mineralisation Low variability within porphyry P velocity Au grade basalt ultramafic porphyry ultramafic sediment basalt

12 Olympic Dam Largest known single deposit of Uranium in the world 4th largest Copper deposit Measured Resources: Olympic Dam Seismic lines 1.08% Cu, 0.32kg/t U3O8, 0.34g/t Au 2.07g/t Ag

13 Olympic Dam (Ehrig et al., 2013) HiSeis COMPANY NAME license

14 Rock properties (lab measurements) 3400 SG (g/cm 3) 2.6 AI (m/s.g/cm 3) 8840 Granite (RDG) Granitic Breccia Fe<5% Granitic Breccia Fe5-20% Hematite rich breccia Fe>20% hematite breccia+clasts (VBx) Hem rich SS/MS/conglomerate Breccia (KHEMQ) Lithology Sandstone (cover) Vp(m/s) Altered rocks have anomalously high acoustic impedances due to their high densities associated with elevated iron contents. Contrast with surrounding rocks produces strong reflection coefficients

15 Migrated seismic data on model HiSeis 15

16 Geoscience Australia E-W seismic line just south of Oympic Dam. Preserved amplitude processing Bouger Gravity Profile Olympic Dam After HiSeis, 2013

17 Geoscience Australia E-W seismic line just south of Oympic Dam Bouger Gravity Profile Olympic Dam Sedimentary cover Enhanced reflectivity Antiform base of cover Fe-rich anomaly Fluid migration path Fractures/vertical faults/dykes Antiformal structure Thrust faullt fault After HiSeis, 2013

18 Expanded section directly below projection of Olympic Dam onto seismic line Depth 1.5Km Depth 7Km

19 Kalgoorlie 2D line

20 First pass migrated seismic data 19 Km 10 Km Destruction of reflectivity

21 Conclusions Deep seismic data provides insight into mineral forming processes Only technique capable of providing detailed continuous images of structures from the surface through the full extent of economically mineable depths and down to the base of the crust. An empirical correlation exists between antiform structures at depth in the crust, arguably with evidence of fluid migration and associated alteration, and major mineral systems. Seismic processing and interpretation strategies must include consideration of the outcomes required and limitations of 2D seismic.