Oscillatory zoning of a discontinuous solid-solution series: sphalerite–stannite

Color zoning in sphalerite in a mineralized vug at the J. Cesar Mine, Cartagena, Spain, correlates with oscillatory variation in Fe, Cu, Sn, and In contents. The crystals show the following zonal units: (a) core of oscillatory-zoned sphalerite with stannite blebs, (b) conspicuous stannite zone, (c) oscillatory-zoned sphalerite with intercalated stannite zones, (d) oscillatory-zoned sphalerite with Cu,In-rich zones, (e) oscillatory-zoned sphalerite with decreased Fe, Cu, Sn, and In contents, and (f) rim zone of almost pure sphalerite.

The oscillatory zoning is explained by a crystallization model depicting cyclic supersaturation at crystal–liquid interfaces due to changes in diffusion and growth-controlled concentration gradients in the boundary layer adjacent to growing crystals during isothermal nonequilibrium crystallization of a supercooled hydrothermal solution. Rapid crystallization eliminates supersaturation at the crystal–liquid interfaces and causes a pause in crystal growth, during which renewed supersaturation of the interface hquid is attained by solute diffusion from the bulk liquid, resulting in a new growth cycle. The model involves true supercooling of bulk hquid as the driving force behind the oscillations and “constitutional supercooling” of interface hquid as the mechanism by which oscillations are accomplished.

Sphalerite and stannite form limited sohd solutions in the (Zn,Fe)S−Cu₂FeSnS₄–CuInS₂ system, and the pseudotemary diagram for hydrothermal crystallization presumably shows a transition curve at which sphalerite forms through a peritectic reaction involving stannite and residual solution. Oscillatory-zoned stannite–sphalerite in units (a), (b), and (c) is formed by isothermal oscillatory crystallization at a temperature near the stannite–sphalerite boundary temperature. Oscillatory-zoned sphalerite in units (d) and (e) is crystallized at lower temperatures. Indium enrichment in sphalerite in unit (d) is related to the intermediate stage of fractional oscillatory crystallization, whereas low Fe, Cu, Sn, and In in sphalerite in unit (e) and almost pure sphalerite in unit (f) are related to the later stages.