Morphology, Mineralogy, and Composition of Sulfide Droplets in Picrodolerite from a Near-Bottom Apophysis of the Yoko-Dovyren Layered Intrusion

The paper presents data on sulfide globules in picrodolerites from a bottom apophysis of the Yoko-Dovyren massif, northern Transbaikalia. Textural features of the rocks indicate that they crystallized at rapid magma cooling. Their composition is similar to that of chilled olivine gabbronorites from the lower contact of the massif. Many droplet-shaped sulfides, which range from 2 to 15 mm, were identified in one of the samples using X-ray computed tomography. The largest of these globules was drilled out and studied in detail to clarify its inner structure, which consists of the main spheroid and a surrounding halo of disseminated sulfides. The main globule and surrounding halo significantly differ in composition (the halo is richer in Cu), as was calculated using the average compositions of the dominant sulfide phases and their relative proportions. The weighted mean (with regard for halo) composition can be assumed as an approximation of the composition of the initial sulfide liquid that was formed near the chill zone of the gabbronorite apophysis from the main intrusive chamber. The Cu content in the initial sulfide corresponds to the results of thermodynamic modeling (with the COMAGMAT-5 program package) of the geochemistry of primitive sulfides in ultramafite rocks from the basal zone of the massif. The differences in the composition of the halo and the main globule indicate the possibility of local migration (within a few millimeters) of the residual sulfide liquids enriched in Cu when the protosulfide melt differentiated. As a physical mechanism that produced the halo, we assumed that the latest sulfide fraction was pressed off into the pore space of rapidly crystallizing olivine orthocumulates.