TY - JOUR
T1 - Massive sulfides of Mount Jourdanne along the super-slow spreading Southwest Indian Ridge and their genesis
AU - Nayak, B.
AU - Halbach, P.
AU - Pracejus, B.
AU - Münch, U.
N1 - Funding Information:
We gratefully acknowledge the invitation to participate in the diving campaign in the Indian Ocean by JAMSTEC. We also gratefully acknowledge the Federal Ministry of Education and Research of Germany for the financial support (BMBF grant 03G0538A0 ). B. Nayak is grateful to the Alexander von Humboldt Foundation, Bonn for a ‘Humboldt Research Fellowship’. Ms. Elke Weiss and Dr. Andrea Koschinsky had helped with chemical analysis.
PY - 2014
Y1 - 2014
N2 - Modern massive sulfide deposits are known to occur in diverse tectonic settings and it is generally expected that hydrothermal deposits of similar geological settings shall have more or less similar mineralogical and geochemical signatures. However, the Mount Jourdanne sulfide deposits along the super-slow spreading Southwest Indian Ridge deviate from this common concept. These sulfide precipitates are Zn-rich (up to 35. wt.%) and are characterized by high concentrations of Pb (≤. 3.5. wt.%), As (≤. 1.1. wt.%), Ag (≤. 0.12. wt.%), Au (≤. 11. ppm), Sb (≤. 967. ppm), and Cd (≤. 0.2. wt.%) which are unusual for a modern sediment-free mid-oceanic ridge system. Therefore, we have reinvestigated the sulfide samples collected during the INDOYO cruise in 1998, in order to explain their unusual mineralogy and geochemical composition. The sulfide samples are polymetallic and are classified as: a) chimneys, b) mounds, and c) hydrothermal breccias. The chimneys are small tube-like symmetrical bodies (30-40. cm high; ~. 10. cm diameter) and consist mainly of sphalerite and less chalcopyrite, set in a matrix of late amorphous silica. The inner wall shows a late-stage colloform sphalerite containing co-precipitates of galena and/or Pb-As sulfosalts. In contrast, the mound samples are dominated either by Fe-sulfides (pyrite) or by a mixture of pyrite and chalcopyrite with less sphalerite, pyrrhotite, amorphous silica and barite. Both, the chimney and mound samples, are characterized by layering and mineral zonation. The hydrothermal breccias are highly altered and mineralogically heterogeneous. They consist of silicified basaltic material that are impregnated with sulfides and contain cm-sized chimney fragments within a matrix of low-temperature minerals such as sphalerite and pyrite. The latter fragments mainly consist of chalcopyrite with isocubanite lamellae. In addition, these breccias contain late-stage realgar, boulangerite, galena, Pb-As sulfosalts and barite that are mostly confined to vugs or fractures. At least five mineralogical associations are distinguished that indicate different thermal episodes ranging from black smoker mineralization conditions to cessation of the hydrothermal activity. Based on the mineralogical associations and established literature in this regard, it is inferred that the mineralization at Mt. Jourdanne occurred mainly in three temperature domains. Above 300. °C, the chalcopyrite (with isocubanite)-pyrrhotite association formed whereas the sphalerite dominated assemblage with much less chalcopyrite and pyrite formed around and below 300. °C. The late-stage mineralization (below 200. °C) contains colloform sphalerite, galena, Pb-As sulfosalts, realgar and barite. The unusual mineralogy and trace element chemistry for this modern VHMS deposit could be explained assuming hydrothermal leaching of some felsic differentiates underneath the basaltic cover and subsequent zone refining processes.
AB - Modern massive sulfide deposits are known to occur in diverse tectonic settings and it is generally expected that hydrothermal deposits of similar geological settings shall have more or less similar mineralogical and geochemical signatures. However, the Mount Jourdanne sulfide deposits along the super-slow spreading Southwest Indian Ridge deviate from this common concept. These sulfide precipitates are Zn-rich (up to 35. wt.%) and are characterized by high concentrations of Pb (≤. 3.5. wt.%), As (≤. 1.1. wt.%), Ag (≤. 0.12. wt.%), Au (≤. 11. ppm), Sb (≤. 967. ppm), and Cd (≤. 0.2. wt.%) which are unusual for a modern sediment-free mid-oceanic ridge system. Therefore, we have reinvestigated the sulfide samples collected during the INDOYO cruise in 1998, in order to explain their unusual mineralogy and geochemical composition. The sulfide samples are polymetallic and are classified as: a) chimneys, b) mounds, and c) hydrothermal breccias. The chimneys are small tube-like symmetrical bodies (30-40. cm high; ~. 10. cm diameter) and consist mainly of sphalerite and less chalcopyrite, set in a matrix of late amorphous silica. The inner wall shows a late-stage colloform sphalerite containing co-precipitates of galena and/or Pb-As sulfosalts. In contrast, the mound samples are dominated either by Fe-sulfides (pyrite) or by a mixture of pyrite and chalcopyrite with less sphalerite, pyrrhotite, amorphous silica and barite. Both, the chimney and mound samples, are characterized by layering and mineral zonation. The hydrothermal breccias are highly altered and mineralogically heterogeneous. They consist of silicified basaltic material that are impregnated with sulfides and contain cm-sized chimney fragments within a matrix of low-temperature minerals such as sphalerite and pyrite. The latter fragments mainly consist of chalcopyrite with isocubanite lamellae. In addition, these breccias contain late-stage realgar, boulangerite, galena, Pb-As sulfosalts and barite that are mostly confined to vugs or fractures. At least five mineralogical associations are distinguished that indicate different thermal episodes ranging from black smoker mineralization conditions to cessation of the hydrothermal activity. Based on the mineralogical associations and established literature in this regard, it is inferred that the mineralization at Mt. Jourdanne occurred mainly in three temperature domains. Above 300. °C, the chalcopyrite (with isocubanite)-pyrrhotite association formed whereas the sphalerite dominated assemblage with much less chalcopyrite and pyrite formed around and below 300. °C. The late-stage mineralization (below 200. °C) contains colloform sphalerite, galena, Pb-As sulfosalts, realgar and barite. The unusual mineralogy and trace element chemistry for this modern VHMS deposit could be explained assuming hydrothermal leaching of some felsic differentiates underneath the basaltic cover and subsequent zone refining processes.
KW - Hydrothermal
KW - Massive sulfides
KW - Mount Jourdanne
KW - SWIR
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U2 - 10.1016/j.oregeorev.2014.05.004
DO - 10.1016/j.oregeorev.2014.05.004
M3 - Article
AN - SCOPUS:84907370346
SN - 0169-1368
VL - 63
SP - 115
EP - 128
JO - Ore Geology Reviews
JF - Ore Geology Reviews
ER -