Domingo Aerden - PhD Thesis (1992)
MACRO- AND
MICROSTRUCTURAL CONTROLS ON THE ROSEBERY AND HERCULES PB-ZN ORE DEPOSITS,
WESTERN TASMANIA.
For my PhD I studied the Rosebery and Hercules ore deposits of
the Cambrian "Mount Read" Volcanic belt of Western Tasmania. These
massive sulphides were generally considered to be "volcanogenic", in
other words, to have formed as chemical sediments around hydrothermal vents on
the (Cambrian) sea floor. This model appeared consistent with the prominent
compositional layering exhibited by the ores reminiscent of sedimentary bedding
and similar mineralogical zoning as observed in other ore deposits of
undisputed 'VMS' (volcanogenic massive sulphides) origin such as
"Kuroko" in Japan. During the Devonian Tabberabberan orogeny Western
Tasmania experienced pervasive folding and the sedimentary host rock of both
mineral were converted in fine-grained schists. The microstructural
relationships I documented showed that the ore minerals actually formed late
during development of the schistosity by chemical replacement rehabilitating
the ideas of F. Stilwell (1934). This famous geologist concluded syntectonic
replacement from the presence in the ore of numerous schist fragments (see
microphotographs further down): he interpreted these as unreplaced host-rock
relics. My work further confirmed Stilwell's view by showing that:
(1)
The ore lenses are discordant to bedding but parallel to cleavage.
(2)
The compositional layering of the ores is inherited from the foliation.
(3)
The main ore minerals (sphalerite and galena) sharply cross-cuts cleavage in
host-rock fragments and must have formed mainly after the cleavage.
(4)
The mine-scale geometry of the ore lenses and their mineralogical zoning patterns
are consistent with mineralization controlled by foliation boudinage.
Download Powerpoint Presentation
- Aerden,
D.G.A.M., 1991. Foliation boudinage control on the formation of the
Rosebery pb/zn orebody. Journal of Structural Geology 13, 759–776.
- Aerden,
D.G.A.M., 1993. Formation of massive–sulphide lenses by replacement
of folds; the Hercules Pb–Zn Mine, western Tasmania. Economic
Geology 88, 377–396.
- Aerden,
D.G.A.M., 1994. Microstructural timing of the Rosebery massive sulphides,
Tasmania; evidence for a metamorphogenic origin through mobilization of
disseminated base metals.Journal of Metamorphic Geology 12, 505-522.
Boudinage
controlled ore locallization at thin-section scale (scale bar = 1 cm)
The
thin-section image above is about 4 cm across. It shows the localization of
brownish red sphalerite (ZnS2) in the fractured neck zones between boudins of
light coloured host-rock layers (sericite schist). It is easy to imagine how a
more advanced stage of the same process could be responsible for the ore
textures shown in the figure below with host-rock fragments completely
surrounded by massive sphalerite-rich minerallization
High-magnification
of a residual host-rock fragment (light) containing a spaced cleavage that is
sharply cut by the sphalerite-pyrite ore contact (dark mass). The geometry of
this ore contact is indicative of post-cleavage replacement.