Sulfide deformation studies; III, Experimental deformation of chalcopyrite to 2,000 bars and 500 degrees C

Abstract

Single crystals and aggregates of natural chalcopyrite were deformed under controlled conditions of temperature (24 ø to 500øC), confining pressure (500 to 2,000 bars), and strain rate (constant 7.2 X 10 -5 sec -•) in several series of experiments designed to test the deformational properties of this mineral under shallow tectonic conditions. At low temperature, chalcopyrite deforms by cataclasis combined with translation gliding on (112) (510) and (112) (0:•1). Polysynthetic deformation twinning, also on (112), begins at 100øC (at 500 bars) and becomes increasingly abundant at higher temperatures with a consequent loss of strength, increase in ductility, and decrease in cataclasis. True kinking is very rare in chalcopyrite. Likewise, no new grains or subgrains were observed in any of these deformation experiments. At room temperature, the strength and ductility of chalcopyrite increase with confining pressure, but at elevated temperatures the effects of confining pressure are greatly subordinate to those of the temperature itself. At the constant strain rate employed, the britfie-ductil.e transition in chalcopyrite takes place between 1,000 and 1,500 bars confining pressure at room temperature and below 500 bars at 200øC. As temperature increases, the ductility of chalcopyrite is enhanced and its strength is greatly reduced. For example, at 1,000 bars confining pressure the strength drops from nearly 5,000 bars at room temperature to less than 1,000 bars at 500øC. Even further reductions in strength would accompany the slower strain rates that prevail in nature (see Roscoe, 1975).Originally undeformed chalcopyrites were studied by both optical and scanning electron microscopy before and after experimental deformation for unambiguous evidence of deformation features. Chalcopyrite deformed at room temperature shows various types of shear and extension fractures whose orientation reflects the known stress field and whose intensity is strongly controlled by original twin and grain orientations. These cataclastic features persist but become subdued at higher temperatures. No deformation twins were seen in any room temperature run. Abundant (112) slip lines can be seen in crystal faces deformed at 24øC but are not visible in these same faces after polishing and etching. At temperatures of 100øC and higher, chalcopyrite develops polysynthetic (112) deformation twins that are readily distinguished from original (110) and (102) twins of nondeformational origin. Complex patterns and sequences of intragranular features arise where the (112) deformation twinning is superposed on a fabric of original growth twins, lensatic twins, and exsolution intergrowths. These features and their age relations could be clearly observed, and easily applied, in the analysis of naturally deformed sulfide deposits. These and earlier experiments (Clark and Kelly, 1973; Salmon et al., 1974) demonstrate that the relative strengths of chalcopyrite, pyrrhotite, sphalerite, and galena are dependent upon the conditions under which these minerals are...

Deformation and metamorphism of massive sulphides at Sulitjelma, Norway

“…17d). Pyrrhotite, chalcopyrite, sphalerite and galena in the sulphide bodies at Sulitjelma generally show evidence of ductile deformation, as would be predicted from experimental studies (Clark and Kelly, 1973;Kelly and Clark, 1975). The cases in which brittle fracturing of chalcopyrite and sphalerite has been observed are probably the result of post-D3 deformation under conditions below the brittle-ductile transition boundaries for these minerals.…”

“…These are shown in Fig. 6 onto which are also superimposed the brittle-ductile transformation boundaries for the main sulphide minerals based on experimental data from Kelly (1973), Salmon et al (1974), Atkinson (1975), Kelly and Clark (1975) and Cox et al (1981), together with the fields of greenschist and amphibolite grade metamorphism. The brittle-ductile boundaries for the sulphides are maximum values due to the disparity between experimental strain rates and geologically realistic strain rates which are several orders of magnitude lower.…”

“…The cases in which brittle fracturing of chalcopyrite and sphalerite has been observed are probably the result of post-D3 deformation under conditions below the brittle-ductile transition boundaries for these minerals. For chalcopyrite this requires temperatures below 200 ~ at pressures less than 500 bars with strain rates in the order of 7.2 x 10 -5 s e c t (Kelly and Clark, 1975). The continued mobility of pyrrhotite and chalcopyrite until late in the structural and metamorphic evolution is shown by their intergrowth with hydrous silicates of the retrograde assemblage (Fig.…”

The metamorphism of pyrite and pyritic ores: an overview

“…Hence, pyrite withstands deformation far better than most of the other typically associated sulphides, as shown in Fig. 2 Graf and Skinner (1970), Atkinson (1975), and Kelly and Clark (1975).…”

Ore-Mineral Textures and the Tales They Tell

“…The response of individual ore minerals to deformation varies widely, depending on the conditions of deformation and on their own physical properties (e.g., pyrite is much harder than most other sulfides and withstands deformation, whereas softer minerals flow plastically or are dissolved by pressure solution). Kelly & Clark (1975) showed that the strengths of several of the common sulfide minerals decrease significantly as temperature rises, making them more susceptible to plastic deformation at higher grades of metamorphism. More refractory sulfide and oxide minerals retain their strength in spite of rising temperature at most moderate metamorphic grades and are thus more likely to survive periods of deformation and to retain physical and chemical textures.…”