KINETICS OF IRON DEPLETION NEAR PYRRHOTITE AND CHALCOPYRITE INCLUSIONS IN SPHALERITE - THE SPHALERITE SPEEDOMETER

The textures and chemical compositions of sphalerite containing chalco pyrite and pyrrhotite inclusions from some skarns, together with exper imental data on diffusivity, have permitted rates of exsolution (speed ometry) and cooling histories of the skarns to be calculated. These ca lculations are based upon Fe depletion profiles obtained by electron m icroprobe transverses across those portions of sphalerite grains that are adjacent to chalcopyrite lamellae and pyrrhotite laths. The lamell ae and laths occur within and along grain boundaries and polysynthetic twin planes in the host sphalerite. In addition, the samples contain 2- to 10-mu m chalcopyrite blebs that do not have the appearance of ch alcopyrite disease textures. Two types of iron depletion profiles are observed in iron-rich sphalerite near sulfide inclusions: one type ind icates a slight decrease in the Fe contents adjacent to chalcopyrite b lebs, the other exhibits a larger Fe depletion (e.g. 3-6 mole % FeS ov er 70 mu m) near chalcopyrite lamellae and pyrrhotite laths. Experimen tally determined tracer diffusion rates of Fe-59 in sphalerite (D-Fe = 5.6 X 10(-4) exp[-38 +/- 2 kcal/mole/RT], with a pyrrhotite + pyrite buffer) have been applied as a test to determine if Fe depletion can b e explained by the simple cooling histories of the skarns. If all the sulfide inclusions are assumed to have been originally dissolved in Cu -bearing sphalerite at a higher temperature, the duration for isotherm al precipitation of a chalcopyrite bleb from a sphalerite sphere of 50 -mu m radius is calculated to be ca. 870 m.y. at 400 degrees C, the es timated temperature at which such a process may have taken place. Beca use of the slow Fe diffusivity in sphalerite, this time is unreasonabl y long, exceeding the geologic age (Mesozoic) of the deposits under co nsideration, so exsolution of chalcopyrite blebs from homogeneous spha lerite is unlikely. In contrast, the profiles of sharp Fe depletion in sphalerite are presumed to have developed during reequilibration of F e-rich sphalerite attending the exsolution of pyrrhotite laths in resp onse to decreasing temperature and rising sulfur fugacity. The measure d profiles near pyrrhotite laths have been simulated by finite differe nce approximations to Fick's diffusion equation. For an exsolution pro cess starting at 350 degrees C and with a cooling rate of 0.5 degrees C/1,000 yr, the observed Fe depletion profiles would have been frozen in within 210,000 yr and with a closure temperature estimated to be 24 5 degrees C.