Asahiko Sugaki

Solubility of sphalerite in aqueous sulfide solutions at temperatures between 25 and 240°c

Abstract In order to assess the role of zinc sulfide complexes in ore-forming solutions, the solubility of sphalerite was measured in NaOHH 2 S aqueous solutions of 0.0 to 3.0 m NaHS concentration at temperatures of 25 to 240°C. Solubilities vary with temperature, activities of H 2 S(aq) and HS − , total reduced sulfur concentration (∑S), and pH. From the solubility data, the main reactions that form zinc sulfide complexes were determined as follows: ZnS ( s ) + H 2 S ( aq ) = Zn ( HS ) 2 0 , ZnS ( s ) + H 2 S ( aq ) + HS − = Zn ( HS ) − 3 , ZnS ( s ) + H 2 S ( aq ) + 2 HS − = Zn ( HS ) 4 2− , ZnS ( s ) + H 2 O (1) + HS − = Zn ( OH )( HS ) − 2 , and ZnS ( s ) + H 2 O (1) + 2 HS − = Zn ( OH )( HS ) 2− 3 . Their equilibrium constants (log K ) are 25°C: −5.3, −3.3, −3.4, −4.4, −4.9; 100°C: −5.2, −3.5, −3.2, −4.1, −5.0; 150°C: −4.7, −3.8, −3.1, −4.7, −5.2; 200°C: −5.1, −3.4, −3.1, −4.6; 240°C: −4.9, −3.3, −3.1, −4.9, respectively. Zn(OHXHS) 2− 3 is not stable at temperatures higher than 200°C. Zinc sulfide complexes predominate over chloride complexes in relatively low temperature hydrothermal solutions which have high ∑S, low ∑C1 − , and high pH values. In these solutions, ZnS is precipitated in response to changes of temperature, pH, and ∑S. Among them, decrease of ∑S is more effective than that of temperature and pH. Zinc sulfide complexes do not transport significant zinc in those ore-forming solutions responsible for economic zinc sulfide deposits. However, they become predominant zinc species in certain geothermal solutions and ore-forming solutions responsible for some epithermal precious metal deposits.

Bulk compositions of intimate intergrowths of chalcopyrite and sphalerite and their genetic implications

To determine the bulk chemical compositions of chalcopyrite containing starlike sphalerite and sphalerite including dotlike chalcopyrite, specimens from various types of ore deposits in Japan were used for modal and electron microprobe analyses. According to the analytical results, most of the measured zinc contents in chalcopyrite containing starlike sphalerite are less than 0.8 at%, corresponding to the maximum solubility of zinc in chalcopyrite as determined experimentally at 400°C. However, specimens from the Maruyama deposit in the Tsumo mine contain 1.2–1.4 at% Zn, which are within the solubility limit of an intermediate solid solution (ISS) above 400°C. It is therefore concluded that starlike sphalerite in chalcopyrite are exsolution products derived from primary chalcopyrite solid solution and/or zincic ISS. Measured copper contents in sphalerite including dotlike chalcopyrite yield considerably higher values, i.e., 1.5–6.0 at%, which exceed the solubility limits of copper in sphalerite solid solution as determined experimentally. This result suggests that not all the chalcopyrite dots were exsolved from sphalerite, but that most of them are the product of some other mechanisms.

Fluid inclusion studies of the polymetallic hydrothermal ore deposits in bolivia

Homogenization temperature and salinity were determined for fluid inclusions in mostly quartz and partly sphalerite, cassiterite, and barite from the 28 tin-polymetallic ore deposits in Bolivia. Generally, the homogenization temperatures and salinities of these fluid inclusions are comparatively high for ore deposits formed by cassiterite mineralization, such as Morococala and Avicaya in the Oruro district, frequently indicating a temperature higher than 300°C and salinity higher than 20 equiv. wt% NaCl. Particularly, it is quite possible that tin deposits associated with the W-Bi and tourmaline mineralizations such as Viloco and Caracoles have been produced by such high-temperature hypersaline fluid ranging up to 500°C and 56 equiv. wt% NaCl, similar to the porphyry copper type. This feature reveals that the hydrothermal fluid related to the Sn-W-Bi mineralization may be of magmatic origin. Homogenization temperatures for the Pb-Zn deposits with no tin minerals are low, mostly ranging 170°–300°C. At the Avicaya-Bolivar mining area in the Oruro district as well as at the Tasna and Chocaya-Animas mining areas in the Quechisla district temperature gradients consistent with the zonal distributions of ore minerals were confirmed.