Abstract

The ~216-Ma Piaohechuan no. 4 intrusion in northeastern China, situated in the easternmost part of the Central Asian orogenic belt, is composed of hornblende-olivine gabbro overlain by hornblende gabbro. The intrusion is about 630 m long, 40 to 250 m wide, and 50 to 150 m thick. Nickel sulfide mineralization is restricted to the lower part of the intrusion and contains 10,000 metric tons (t) of Ni with average PGE concentrations of 40 ppb Os, 30 ppb Ir, 36 ppb Ru, 26 ppb Rh, 255 ppb Pt and 161 ppb Pd in 100% sulfide.

The sulfide ores display some distinctive textures and can be divided into “globular ores,” which comprise millimeter- to centimeter-sized spherical to irregular patches of sulfide in a gabbroic matrix, “network ores” in which irregular sulfide aggregates with cuspate margins are intergrown at a scale of millimeters to centimeters with roughly equal proportion of gabbro, and “breccia ores” in which centimeter- to micron-sized fragments of gabbro or broken crystals from the gabbro are enclosed in a sulfide matrix. The silicate portions of the three types of sulfide ores are almost totally altered and only some fragments in the breccia ores are fresh. Pyrrhotite and pentlandite are the dominant sulfides in the ores and chalcopyrite is largely confined to finer fractures and patches in the silicate rocks. The Cu-rich ores are sparsely distributed in the orebody and are richer in Pt and Pd than other ores, consistent with fractionation of Cu and PGE during crystallization of sulfide liquid. Breccia and network ores have Pt ranging from 1.72 to 205 ppb, with positive and negative Pt anomalies in primitive mantle-normalized chalcophile element patterns. The high Pt concentrations of some ores are manifested by the sporadic occurrence of sperrylite.

Nickel in olivine decreases rapidly (2,258–393 ppm) with decreasing Fo (86–72). Small sulfide droplets are enclosed in olivine, indicating that olivine crystallized from sulfide-saturated magma. The rocks of the intrusion have restricted εNd(t = 216 Ma) (4.4–4.6) and variable γOs(t = 216 Ma) values (39–67), consistent with the assimilation of external crustal sulfide in a shallow magma chamber. We propose that sulfide saturation was achieved by olivine fractionation and the addition of crustal sulfide. Early segregated sulfide liquid settled toward the base of the magma chamber and then migrated through partially molten silicates, along small channelways. The migrating sulfide liquid may have displaced and disaggregated the silicate melt or cumulus crystals into small fragments, creating the network and breccia ores. The globular ores formed when the coalesced sulfide droplets were largely solid before the final solidification of silicate minerals. Although the distance of migration is unconstrained, such migration of sulfide liquid had significant controls on the distribution of ores in the intrusion. At lower temperatures, metamorphism transformed part of the intrusion to low-grade hydrous assemblages and resulted in weak redistribution of sulfides and secondary silicate minerals.

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