1. SULFIDE REMOBILISATION FROM SULFIDE ORE AT HIGH
TEMPERATURES AND DIFFERENTIAL STRESSES:
AN EXPERIMENTAL APPROACH
2. INTRODUCTION:
Remobilisation is the process of ore components being transported from
pre-existing ore bodies (Mookherjee, 1976;Marshall et al., 2000a, b).
Radical modification of primary ore bodies due to mechanical and
chemical remobilization (Mookherjee, 1976; Marshall & Gilligan, 1987).
Marshall and Gilligan (1993) classified remobilisation into internal and
external cases.
Previous experiments commonly separated mechanical from chemical
remobilisation, and hence the simulations did not address the interactions
between the two factors.
3. STARTING MATERIAL:
Collected from -707 m level in the main ore zone of the Hongtoushan
deposit, Liaoning Province, NE China.
It is a volcanic-hosted massive sulfide deposit metamorphosed at upper
amphibolite facies at temperatures between 600 and 650°C during 3.0–2.8
Ga (Zhang et al., 1984; Liu et al., 1994).
The ore consists of pyrite (32 vol.%), chalcopyrite (9%),pyrrhotite (8%), and
sphalerite (6%).
Gangue minerals are quartz (26%) and other silicates (19%, including
hornblende, garnet, biotite, plagioclase, and muscovite
Average metal contents are 4.06 wt.% Cu, 2.48 wt.% Zn, 0.03 wt.% Pb, 0.36
ppm Au, and 94 ppm Ag which were observed by atomic absorption
spectrometry (AAS)
4. TEXTURE:
Pyrite occurs as euhedral to subhedral porphyroblastic crystals.
Pyrrhotite grains in the matrix are mostly hexagonal (Gu et al., 2001b).
Chalcopyrite and sphalerite also occur as subhedral to anhedral grains in
the matrix
Most gangue minerals form globular textures, 2 to 5 mm in size.
5. INSTRUMENTATION AND
METHODOLOGY:
It was conducted in a horizontal compression chamber on a Changjiang
500 triaxial rock stress machine at the Guiyang Institute of Geochemistry,
Chinese Academy of Sciences.
Argon gas provides the confining pressure, which is measured by means of
a Heise gauge with a maximum capability of 600 Mpa with <1%
uncertainty.
Auto-regulator transformer
The axial load was applied to the sample through a hardened and
polished steel piston, 1.9 cm in diameter.
6. CONTINUED….
The magnitude of the axial load is measured outside of the chamber by
means of a load cell containing a resistance bridge of four foil strain
gauges.
Ore sample for each run was cut into a cylinder 40 mm in length and 17
mm in diameter, and immersed in solution of 20 wt.% NaCl for 260 h.
The sample was enveloped in an aluminium foil and then mounted in a
graphite tube.
At the termination of each experiment, power for the furnace was turned
off, but pressures were held constant while the specimen cooled to room
temperature.
9. DEFORMATION AND MECHENICAL
REMOBILIZATION
Set of release fractures: Perpendicular to sample length
Become much better developed with increasing temperature
PYRITE:
Cataclastic deformation
Rotation shows plastic behavior of surrounding material
11. CHEMICAL REMOBILIZATION: SULPHIDE
VEINLETS
Cross-cut Pyrite-porphyroclasts
Show no displacement along strike
Abundance: Chalcopyrite > Pyrrhotite > Spherite
Chemical remobilization:
Discontinuity outside the grains
Change in composition of Sphalerite
12. SULPHIDE VEINLETS: EFFECT OF
TEMPERATURE
Remobilization becomes more intense with increasing
temperature
At 362o C :
> 2vol%
55% Chalcopyrite, 45% Pyrrhotite, 1% Sphalerite
At 682o C :
5%vol%
80% Chalcopyrite, 20% Pyrrhotite, 1% Sphalerite
15. MECHANISMS OF DEFORMATION
Minerals exhibit various degrees of cataclastic deformation in lowtemperature (362C)
The strength of minerals decreases markedly with increasing temperature.
brittle-ductile transition of pyrite
brittle-ductile transition of pyrrhotite, chalcopy- rite, and sphalerite
Annealing and recrystallisation
Annealing of pyrrhotite.
16. MECHANISMS FOR CHEMICAL
REMOBILISATION
All the present experiments were performed at temperatures below the
melting points of all the sulfides in the samples (Vaughan & Craig, 1997;
Stevens et al., 2005; Sack & Ebel, 2006)
Most of the veinlets cutting pyrite porphyroclasts were formed by chemical
remobilisation
17. THIS CONCLUSION IS SUPPORTED BY
THE FOLLOWING EVIDENCE:
1.
the amout of remobilised chalcopyrite in most veinlets markedly exceeds
the remobilised pyrrhotite, particularly in the high temperature runs
2.
sulfides in many veinlets are not connected to the same sulfides outside
the pyrite cracks
3.
pyrite grains are replaced by chal- copyrite in some of the runs
4.
chemical composition of remobilised sphalerite in the veinlets deviates
from that of the residual sphalerite.
18. GEOLOGICAL APPLICATIONS
Syn-tectonic remobilisation of sulfides in the natural system is a complex
process, and would be affected conspicuously by varying factors, such as:
strain rate,
temperature,
fluid property,
composition of the original ore, and;
time length for the process
19. CONT.
The present experiments show, however, that the extent of mechanical
translocation is limited in comparison to chemical remobilisation.
The present experiments show that sulfides vary in ability of chemical
remobilisation, particularly at higher temperatures.
The present experiments show that remobilisation could substantially
modify the geometry and distribution pattern of syn- genetic orebodies,
and produce a range of textures characteristic of epigenetic
mineralisation
20. CONT.
The present experiments demonstrate that remobilised sul- fides select
preferentially pre-existing sulfide minerals, particularly pyrite, as substrate
for overgrowth
Experiments and geology research suggest that pyrite is a favourable
substrate for gold precipitation (Chen & Fu, 1992; Larocque & Hodgson,
1995; Zhang et al., 1996; Simon et al., 1999; Zhou et al., 2000; Gu et al.,
2001a, 2007)
21. ACKNOWLEDGMENTS
This research was jointly supported by Natural Science Foundation of China
(no. 40872064) and Foundation for Doctorial Research, Ministry of Education
of China (no. 20060284013). We wish to thank Prof. Sun Yuan for his helpful
discussion with the authors. The authors are also grateful to Yasushi Watanabe
and one anonymous RGE reviewer for their constructive comments and
language corrections on the early manuscript for this paper.
22. REFERENCES
Barton, P. B. Jr. (1973) Solid solution in the system Cu-Fe-S Part I: the Cu-S and Cu-Fe-S joins.
Econ. Geol., 68, 455–465.
Bellot, J. P. (2004) Shear zone-hosted polymetallic sulfides in the south Limousin area,
massif central, France: remobilized sulfide deposits related to Variscan collisional tectonics
and amphibolite facies metamorphism. Econ. Geol., 99, 819–827.
Berger, A. and Stu ̋nitz, H. (1996) Deformation mechanisms and reaction of hornblende:
examples from the Bergell tonalite (Central Alps). Tectonophysics, 257, 149–174.
Bourcier, W. H. and Barnes, H. L. (1987) Ore solution chemistry: VII. Stabilities of chloride
and bisulphide complexes of zinc to 350oC. Econ. Geol., 82, 1839–1863.
Bursnall, J. T. (1989) Mineralization and shear zones. Geology. Ass. Can., Short Course
Notes, 6, 1–299.
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