Emplacement of Viscous Mushes in the Jinchuan Ultramafic Intrusion, Western China

Abstract

The Jinchuan ultramafic intrusion in western China has an elongate surface expression of about 6000 by 350 meters. It is one of several mafic-ultramafic intrusions found in the fault-bounded Longshoushan uplifted terrane, of Proterozoic age, located along the southwestern edge of the Sino-Korea platform. Three boreholes from the western, central and eastern parts of the intrusion were sampled and studied. The rocks from the Jinchuan intrusion are olivine-orthopyroxene-chromian spinel cumulates with orthopyroxene, clinopyroxene, plagioclase and phlogopite as interstitial phases. On average, base-metal sulfides (pyrrhotite, pentlandite and chalcopyrite) constitute 5 wt% of the rocks. The samples are overprinted by low-grade metamorphism, hydrothermal alteration and near-surface oxidation. Minor chromian spinel and orthopyroxene preceded olivine in the crystallization sequence. Olivine composition varies between Fo 79 and Fo 85. This variation is negatively correlated with the amounts of trapped silicate liquid in the rocks and is ascribed to the effects of subliquidus and metamorphic re-equilibration. The complex and highly variable lithological structures, limited variation in Fo in the olivine and grain sizes suggest that the intrusion was formed by injection of high-yield-strength viscous crystal-mushes containing variable amounts of sulfide liquid and silicate melt of basaltic composition into planar fractures or faults. During mush emplacement at Jinchuan, flow differentiation controlled the proximal, central part of the intrusion, resulting in subconcentric lithological zonation in this regime, whereas chaotic flow prevailed in the distal, eastern and western parts of the intrusion, resulting in complex lithological banding and interfingering. The original emplacement and flow structures did not collapse because of the high viscosity and high yield-strength of the crystal mushes, strong enough to withhold the gravitational forces imposed on the mushes after flow eased. The observed sequence of crystallization in the rocks suggests that the crystal mushes were formed in a staging chamber at a depth between 4 and 9 km. In this staging chamber, olivine-poor magma occurred at the top and olivine-rich mushes containing minor orthopyroxene and chromian spinel, and variable amounts of silicate and sulfide liquids, were concentrated toward the base in response to gravitational settling. The stratified liquid-crystal materials were then squeezed out of the staging chamber into planar fracture or fault systems at higher levels to form a series of mafic-ultramafic intrusions, possibly induced by the collapse of the chamber roof. The roof collapse was probably triggered by regional extension. Early extraction of the crystal-poor magma from the top of the staging chamber formed the sulfide-poor mafic intrusions in the vicinity of the Jinchuan intrusion. Subsequent extraction of the sulfidebearing crystal mushes from the bottom of the staging chamber formed the Jinchuan intrusion. The overall high sulfide-silicate ratio in...

Multiple S isotopes, zircon Hf isotopes, whole-rock Sr-Nd isotopes, and spatial variations of PGE tenors in the Jinchuan Ni-Cu-PGE deposit, NW China

“…We propose that the three different pulses of magma were all charged with olivine crystals upon arrival at Jinchuan, because the bulk of the Jinchuan intrusion is composed of olivine cumulates (Chai and Naldrett 1992a;de Waal et al 2004;Li et al 2004). In addition, we suggest that minor immiscible sulfide droplets were present in the first and last pulses of magma before they arrived at Jinchuan, because the sulfide/olivine ratios in the related ore zones far exceed the cotectic sulfide/ ratios expected from sulfide-saturated magma that does not contain excessive sulfide (Li and Ripley 2011).…”

“…It contains more than 500 million metric tons of ores with 1.1 wt% Ni and 0.7 wt% Cu (Tang et al 2009). Recently, many studies on the deposit and its host intrusion have been carried out (e.g., Chai and Naldrett 1992a, b;Chen et al 2013;de Waal et al 2004;Li et al 2004;Lehmann et al 2007;Song et al 2006Song et al , 2009Song et al , 2012Su et al 2008;Tang 1993;Tonnelier 2010). These studies have improved our understanding of the genetic relationship between magma evolution and ore genesis.…”

Siderophile and chalcophile elemental constraints on the origin of the Jinchuan Ni-Cu-(PGE) sulfide deposit, NW China

“…The origin of such giant size ore bodies in such a small intrusion is still enigmatic. Tang and Li (1995), De Waal et al (2004), and Song et al (2006) proposed that the Jinchuan intrusion and sulfide ore bodies were formed by injection of pulses of sulfide-free and sulfide-bearing crystal-mushes from a magma chamber at a lower level.…”

Noble gas isotopic constraints on the origin and evolution of the Jinchuan Ni–Cu–(PGE) sulfide ore-bearing ultramafic intrusion, Western China

“…The ore-forming magma of the Jinchuan deposit is interpreted to have formed by a high-MgO (up to 18.5 wt.%) tholeiitic basaltic magma which were originated from a SCLM Naldrett, 1992a, 1992b;de Waal et al, 2004). It started to crystallize in a staging chamber Song et al, 2006bSong et al, , 2009.…”

“…Therefore, ASF related metasomatism (especially for the 400-1300°C fraction) could be responsible for the atmospheric component. Postmagmatic hydrothermal alteration may have been triggered by heat from large Paleozoic granite plutons formed during the collision between the North Qilian suture zone and the Langshoushan terrane at~450 Ma (Song et al, 2006a), and has affected the ultramafic rocks and associated sulfide ores of the Jinchuan intrusion (Chai and Naldrett, 1992a;Barnes and Tang, 1999;de Waal et al, 2004;Ripley et al, 2005;Lehmann et al, 2007), then were followed by subsequent regional metamorphism (Song et al, 2006bTang et al, 2009) as well as hydrothermal alteration , which are partly responsible to atmospheric component in 200-400°C fraction.…”