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

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.

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)

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.

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.

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.

CONDITIONS FOR THE 4 RUNS:

RESULTS

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

CHALCOPYRITE-PYRRHOTITE-SPHALERITE

Plastic deformation with only local cataclastic textures

Deformation bands and twins

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

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

CHEMICAL REMOBILIZATION:SULPHIDE MICROCRYSTALLITES

Space fillings Same minerals welded by same mineral e.g Chalcopyrite

Aggregates of worm-like bodies Pure pyrite growth along fissures of Chalcopyrite

Lower in relief

DISCUSSION

MECHANISMS OF DEFORMATION

Minerals exhibit various degrees of cataclastic deformation in low-temperature (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.

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

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.

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

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

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)

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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