SULFIDE REMOBILISATION FROM SULFIDE ORE AT HIGHTEMPERATURES AND DIFFERENTIAL STRESSES:AN EXPERIMENTAL APPROACH

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.

7. CONDITIONS FOR THE 4 RUNS:

8. RESULTS

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

10. CHALCOPYRITE-PYRRHOTITESPHALERITE

Plastic deformation with only local cataclastic textures

Deformation bands and twins

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

13. CHEMICAL REMOBILIZATION:
SULPHIDE MICROCRYSTALLITES

Space fillings


Aggregates of worm-like bodies


Same minerals welded by same mineral e.g Chalcopyrite
Pure pyrite growth along fissures of Chalcopyrite
Lower in relief

14. DISCUSSION

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.

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
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
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
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
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
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
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