Kinetics of iron depletion near pyrrhotite and chalcopyrite inclusions in sphalerite; the sphalerite speedometer

The textures and chemical compositions of sphalerite containing chalcopyrite and pyrrhotite inclusions from some skarns, together with experimental data on diffusivity, have permitted rates of exsolution (speedometry) and cooling histories of the skarns to be calculated. These calculations are based upon Fe depletion profiles obtained by electron microprobe transverses across those portions of sphalerite grains that are adjacent to chalcopyrite lamellae and pyrrhotite laths. The lamellae 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 chalcopyrite disease textures.Two types of iron depletion profiles are observed in iron-rich sphalerite near sulfide inclusions: one type indicates a slight decrease in the Fe contents adjacent to chalcopyrite blebs, the other exhibits a larger Fe depletion (e.g., 3-6 mole % FeS over 70mu m) near chalcopyrite lamellae and pyrrhotite laths.Experimentally determined tracer diffusion rates of (super 59) Fe in sphalerite (D (sub Fe) = 5.6 x 10 (super -4) exp[-38 + or - 2 kcal/mole/RT], with a pyrrhotite + pyrite buffer) have been applied as a test to determine if Fe depletion can be 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 isothermal 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 estimated temperature at which such a process may have taken place. Because of the slow Fe diffusivity in sphalerite, this time is unreasonably long, exceeding the geologic age (Mesozoic) of the deposits under consideration, so exsolution of chalcopyrite blebs from homogeneous sphalerite is unlikely. In contrast, the profiles of sharp Fe depletion in sphalerite are presumed to have developed during reequilibration of Fe-rich sphalerite attending the exsolution of pyrrhotite laths in response to decreasing temperature and rising sulfur fugacity. The measured profiles near pyrrhotite laths have been simulated by finite difference approximations to Fick's diffusion equation. For an exsolution process 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 245 degrees C.