Active and fossil geothermal systems in Iceland

Transcription

1 Active and fossil geothermal systems in Iceland Exploration for epithermal deposits Hjalti Franzson Iceland GeoSurvey

2 Main points Geological framework Geological structures Geothermal systems Exploration for epithermal deposits Projects What can we contribute What do we need Hjalti Franzson Iceland GeoSurvey 2

3 40 M y Reykjanes Ridge Spreading rate ~2 cm/yr 30 M y 20 M y Today Kolbeinseyj Ridge UK Mantle plume position through time North Atlandic Large Ingneous Province, Saunders et al., km Hjalti Franzson Iceland GeoSurvey 3

4 16my my Hjalti Franzson Iceland GeoSurvey 4

5 Fissure swarm, formation of a lenticular lava series Hjalti Franzson Iceland GeoSurvey 5

6 Lenticular lava formation cross-section Hjalti Franzson Iceland GeoSurvey 6

7 Central volcano Evolved rocks Caldera Shallow intrusives Magmatic volatiles Heatsource High-T systems Hjalti Franzson Iceland GeoSurvey 7

8 Fossil eroded high-temperature areas Active high-temperature systems Hjalti Franzson Iceland GeoSurvey 8

9 General model of a high-t reservoir Hjalti Franzson Iceland GeoSurvey 9

10 Geothermal systems Dominantly freshwater Temperatures C Magmatic volatiles present but less than porphyry systems Extensive studies of structures, alteration and fluid chemistry of drilled systems Scalings in pipes (controlled by fluid compositon, pressure and temperature) Hjalti Franzson Iceland GeoSurvey 10

11 40 M y Reykjanes Ridge Spreading rate ~2 cm/yr 30 M y 20 M y Today Kolbeinseyj Ridge UK 400 km Hjalti Franzson Iceland GeoSurvey 11

12 Borehole RN-22 RN-17 First, downhole temperature and pressure measurements: Samples collected at C, below depth of first boiling Hjalti Franzson Iceland GeoSurvey

13 Borehole pressure logging clogging Black smoke seawater 260 C Chalcopyrite (CuFeS 2 ) Remnant Anhydrite Fe-Zn-sulfides SO 4 2- Ca C SO C Anhydrite (CaSO 4 ) Outer marcasite crust Lateral vent (horizontal fracturing) Chalcopyrite (CuFeS 2 ) Sph, cpy, gn, (bn, cv, py, Am SiO 2 ) Zn: wt% Cu: % Fe: 3-4 % Pb: 0.3-1% SiO2: 3-2% S: 30-28% Ag: ppm Au: ppm 330 C High-temperature fluid Poster No Hjalti Franzson Iceland GeoSurvey 13

14 Black smokers and Reykjanes Hjalti Franzson Iceland GeoSurvey 14

15 Exploration for epithermal gold in Iceland Early exploration in early twentieth century Exploration started again in about 1990 and has been ongoing intermittently to present Hjalti Franzson Iceland GeoSurvey 15

16 Present prospect areas Older prospect areas (now discarded) Málmis- Melmi prospect areas. Hjalti Franzson Iceland GeoSurvey 16

17 Hjalti Franzson Iceland GeoSurvey 17

18 Flateyjarskagi Hjalti Franzson Iceland GeoSurvey 18

19 Main results of exploration Base metals Hjalti Franzson Iceland GeoSurvey 19

20 Thormodsdalur prospect Hjalti Franzson Iceland GeoSurvey 20

21 Thormodsdalur anomaly Low sulphide quartz-adularia-sericite Au concentration up to 400 ppm Hjalti Franzson Iceland GeoSurvey 21

22 Our accomplishment so far We have proven that low- and highsalinity geothermal systems in Iceland precipitate gold Iceland has as yet no proven Au-minable resource but several low- to high value prospects have been located Most of these have, however, only been studied in a preliminary way Hjalti Franzson Iceland GeoSurvey 22

23 NordMin This presentaion has aimed at introducing the ore forming environment in Iceland This environment is in many respects different from that of Scandinavia and Greenland BUT It has important similarities Hjalti Franzson Iceland GeoSurvey 23

24 Ore formation is closely linked to magmatic processes and geothermal activity. Iceland is a natural laboratory of magmatic and geothermal processes. These are both present as active ones as well as fossil and eroded, some all the way down to the magmatic heatsource of the geothermal system. We can see projects both as part of student training or as part of specific research projects Hjalti Franzson Iceland GeoSurvey 24

25 Epithermal environment of ore formation We have eroded sections with ore formation in fossil geothermal systems where we can supply the background data We have the natural laboratory of the active Reykjanes saline high-temperature system which is precipitating high concentrations of gold and base metals. We have the same opportunity with other freshwater high-t systems Hjalti Franzson Iceland GeoSurvey 25

26 We would need input such as: Research answers on the hydrothermal evolution of the known epithermal ore locations. What would be the best Au extraction processes e.g. in the Thormodsdalur prospect? What are the environmental issues in the mining processes that would best apply to Icelandic environment Hjalti Franzson Iceland GeoSurvey 26

27 Thank you Hjalti Franzson Iceland GeoSurvey 27