Rapid orthopyroxene growth induced by silica assimilation: constraints from sector-zoned orthopyroxene, olivine oxygen isotopes and trace element variations in the Huangshanxi Ni–Cu deposit, Northwest China

Abstract

The Permian Huangshanxi Ni–Cu-hosted intrusion, located in the southern Central Asian Orogenic Belt in NW China, is dominated by cumulus olivine and orthopyroxene. Various types of compositional zonings in cumulate phases were studied using high-resolution synchrotron X-Ray fluorescence imaging, electron microprobe, laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectrometry, to shed light on the relationship between the orthopyroxene-rich cumulate and sulfide accumulation. Normal, oscillatory, and sector zonings of Cr in orthopyroxene from both lherzolites and olivine websterites were noted, likely due to the different cut orientation of orthopyroxene crystals composed of sector-zoned cores and reverse-zoned rims. Significant Fe–Mg–Al–Si variations found in the exterior regions of the grains do not correspond to the Cr zoning. However, Cr content is positively correlated with Ti–Al contents in interior regions of orthopyroxene, reflecting coupled charge substitution during the development of sector zoning, and indicating diffusion-modified Fe–Mg–Al–Si variation in the exterior regions. Primary Cr-sector zoning in the core is likely the result of rapid in situ growth in a boundary layer at the cumulus stage, probably induced by Si addition from wall rocks and crystallization under supersaturated conditions. In addition, some olivine shows Mg–Fe–Mn–Ca–Cr variations in the rim but not in the core, and both core and rim of olivine show a uniform O isotope composition, illustrating that they have not been modified by post-cumulus processes. Given the distinct difference in O isotopic signatures between mantle-derived melts and crustal materials, olivine O isotope composition could be used to decipher the relative degree of Si addition at crystallization. The Fo value, O isotopic signature and trace element content in the cores of olivine grains from different depths show systematic variations, dividing the sulfide-bearing cumulate into two intervals; the lower and upper lherzolite zones with harzburgitic rocks at the contact. In the lower zone, the olivine δ18O values increase upward from ~ 6 to ~ 7‰ and the oxygen fugacity decreases from QFM to QFM-2, suggesting increasing contamination by more reduced crustal materials. Over the same interval, the modal proportion of orthopyroxene and modal orthopyroxene/olivine ratio increase with increasing depth with little variation in whole-rock Mg#, clearly illustrating that the non-cotectic orthopyroxene proportions resulted from Si addition from the wall rock. In the upper zone, the decrease in δ18O values, and increase in olivine nucleation density, Fo value and oxygen fugacity suggest there was a pulse of sulfide- and olivine-charged magma into the magma chamber, that mixed with resident magma to form a hybrid bottom layer in contact with the early-crystallized orthopyroxene-rich cumulate pile. The sulfide content decreases up-section in the lower zone, and then increases again towards the top of the cumulate sequence. This, combined with the compositional variations with depth, suggests that sulfides in the Huangshanxi Ni–Cu deposit settled downward during at least two sulfide-loaded magma pulses. The fast-growing orthopyroxene-rich cumulate may have acted as a permeability barrier, preventing sulfide percolation into the lower cumulate formed during the first magma pulse. Overall, sector zoning in orthopyroxene and compositional variations in olivine suggest that Si and reduced material addition during contamination is of great importance in forming magmatic Ni–Cu deposits hosted by olivine- and orthopyroxene-dominated intrusions.

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Acknowledgements

This study has been financially supported by Grants from the Nature Science Foundation of China (Grant 41872094, 41830430, 41472075). SJB acknowledges support from the CSIRO Research Plus Science Leader program. JdLC GeoHistory Facility instruments were funded via an Australian Geophysical Observing System grant provided to AuScope Pty Ltd. by the AQ44 Australian Education Investment Fund program. We thank Jun-Hui Xie, Guan-Liang Ren, Shengchao Xue, Ye Tian, Ding-Min Guo for their assistance in field work, Michael Verrall, Bradley McDonald, Di Zhang and Malcolm Roberts for their assistance in XRF, LA-ICP-MS, and EMPA laboratory work. The synchrotron X-ray fluorescence maps were collected on the X-ray fluorescence microscopy beamline at the Australian Synchrotron, Clayton, Victoria, Australia, operated by ANSTO. We acknowledge the assistance of Daryl Howard and David Paterson for synchrotron XRF mapping. The manuscript was significantly improved by constructive comments from Rais Latypov and Chris Ballhaus.

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Appendix

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See Figs. 14 and 15.

Fig. 14
figure14

Optical and XRF images for a harzburgitic rock from the Huangshanxi deposit. a Optical image; b combined XRF image of Cr (red), Ca (green) and Fe (blue); c phase map; d combined XRF image of an area denoted in (b) and (c) showing Cr, Ca and Fe in the various mineral phases. e Cr zoning in orthopyroxene across sections shown in white in (d). Both Cr-rich and Cr-poor rims are shown in the combined XRF images (b, c). The phase map shows that olivine crystals included in orthopyroxene oikocrysts are considerably smaller than those included in clinopyroxene and plagioclase oikocrysts (d). The CSD results of these two olivine groups are plotted in Fig. 11a

Fig. 15
figure15

Images for a lherzolite from the Huangshanxi deposit: a optical image; b combined XRF image of three elements; Cr (red), Ca (green), Fe (blue); c processed image of manually separated olivine grains. The olivine grains shown in (c) were used to calculate the CSDs of olivine shown in Fig. 11b

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Mao, Y., Barnes, S.J., Qin, K. et al. Rapid orthopyroxene growth induced by silica assimilation: constraints from sector-zoned orthopyroxene, olivine oxygen isotopes and trace element variations in the Huangshanxi Ni–Cu deposit, Northwest China. Contrib Mineral Petrol 174, 33 (2019). https://doi.org/10.1007/s00410-019-1574-6

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Keywords

  • Huangshanxi magmatic Ni–Cu deposit
  • Orthopyroxene sector zoning
  • Olivine oxygen isotopes
  • Contamination
  • Oxygen fugacity
  • Crystal size distribution