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 degrees to 500 degrees C), confining pressure (500 to 2,000 bars), and strain rate (constant 7.2 X 10 (super -5) sec (super -1) ) 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} and {112} . Polysynthetic deformation twinning, also on {112}, begins at 100 degrees C (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 brittle-ductile transition in chalcopyrite takes place between 1,000 and 1,500 bars confining pressure at room temperature and below 500 bars at 200 degrees C. 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 500 degrees C. 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 24 degrees C but are not visible in these same faces after polishing and etching. At temperatures of 100 degrees C 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 deformed. If, under simplified laboratory conditions, these sulfides undergo pronounced changes and even reversals in strength as one variable, temperature, is changed, it is unreasonable to expect any single ranking of strength, mobility, or plasticity to be universally applicable to all naturally deformed sulfide deposits.