Revisiting semi-massive Sulfides: Textures of sulfide-silicate interactions in magmatic sulfide deposits

Abstract

The interaction of sulfide melt with silicate melt or partially molten country rock produces a wide variety of textures that can appear similar to net-textured sulfides which originate from accumulation of sulfide melt within the host intrusion and therefore occur in different areas within the orebody. This similarity in textures can cause misidentification during exploration and could cause confusion and wrong financial decisions. Therefore, these semi-massive sulfide textures were investigated in detail and partly redefined. They comprise (1) net, (2) infiltration, (3) disaggregation, and (4) emulsion textures.

Net-textured sulfides form by sulfide melt infiltrating into existing silicate cumulate thereby replacing interstitial silicate melt resulting in a continuous matrix of sulfides enclosing magmatic cumulate silicates. Silicates like pyroxenes display the highest X_Mg values of all textures and are occasionally sector-zoned. Contrary, infiltration, disaggregation and emulsion textures form by the infiltration of sulfide melt into the country rock by partial melting. Infiltration textures form during the initial process and are therefore defined by refractory fragments of country rock with abundant peritectic phases with additional minor amounts of anatectic melt enclosed in a continuous matrix of magmatic sulfides. Disaggregation textures represent the refractory end-stage and are defined as single grains or clusters of high thermal stability silicate crystals derived from the country rock suspended in magmatic sulfides. Macroscopically, this texture can look identical to net-textured sulfides, but can be distinguished by the lack of magmatic silicates, such as olivine, by elevated Ni and Mn contents plus lower X_Mg values of pyroxenes and by different luminescence colours of plagioclase in the disaggregation texture. The newly formed buoyant and immiscible silicate melt is less dense than the sulfide melt resulting in upward counter migration and the formation of the emulsion texture when frozen in. Depending on the ratio of the two melts, this texture may appear as a fine-grained droplet emulsion or as a connected bicontinuous emulsion, but both types have the same defining characteristics, such as high levels of eutectic intergrowths, newly formed skeletal and euhedral silicate crystals, and they contain high levels of hydrous silicates.

The newly crystallized pyroxenes in the emulsion texture exhibit the highest Ni and Mn contents and lowest X_Mg values compared to all other textures, and in addition, plagioclase again shows different luminescence colours. Where the newly formed silicate melt is trapped, it can form pegmatoidal pockets displaying the same features as the emulsion textures, but with less sulfides.