Copper, gold and silver enrichment in ore mylonites within massive sulphide orebodies at Hongtoushan VHMS deposit, N.E. China

Abstract

The Hongtoushan Archaean Cu–Zn Volcanic-hosted Massive Sulphide (VHMS) deposit occurs in the Hunbei granite–greenstone terrane, Liaoning Province of NE China. The ores were metamorphosed (3.0–2.8 Ga) to upper amphibolite facies at temperatures between 600 and 650 °C, and underwent three phases of deformation. The deformation has transposed the stratiform orebodies into a Y-shaped configuration. Ore minerals in the deposit were originally composed of pyrite, pyrrhotite, chalcopyrite and sphalerite. Prograde metamorphism obliterated almost all the sedimentary textures in the ores, and the present ore textures were formed substantially during peak and post-peak metamorphism. Intensive remobilisations have occurred in the Hongtoushan deposit at various scales. There are more than 30 oreshoots inside the main massive sulphide orebodies. The oreshoots range from 2 to 30 m in length, 0.1 to 1.5 m in width, and 2 to 30 m in depth extension. Most run parallel or sub-parallel to the strike of the massive sulphide ore layers with only a few being developed in metamorphic rocks in the immediate vicinity of the sulphide orebodies. Almost all the sulphide and gangue minerals in the oreshoots have been affected by intense shearing. Pyrite is characterised by cataclastic flow; pyrrhotite, chalcopyrite and sphalerite exhibit strong plasticity. Hydrothermal alteration minerals in the oreshoots are dominated by deformation-free hornblende, actinolite, epidote, clinozoisite, biotite, muscovite, albite, carbonate and quartz. Cu, Au and Ag in the ore mylonites average 11 wt.%, 1.74 and 235 g/t, respectively. Compared to the massive sulphide ores, these oreshoots show remarkable enrichment of these elements with enrichment coefficients of 5.3, 5.0 and 4.6, respectively. We draw the conclusion that these oreshoots are actually ore mylonites overprinted by late-stage fluids. The presence of annealing textures in pyrrhotite and the absence in pyrite suggest that formation and succeeding annealing of the ore mylonite took place over the temperature range from 550 to 450 °C. Such a temperature range, combined with the geology and mineralogy, indicates that these ore mylonites and the derivative oreshoots were formed during upper-greenschist-facies retrograde metamorphism. Similarity of the ore mylonites to the massive ores in metal varieties and lead isotope ages suggest that Cu, Au and Ag of the overprinting fluids on the ore mylonites were derived mainly from the nearby massive sulphide ores. However, lead isotope compositions of the ore mylonites also suggest that an external source, such as the metamorphic host rocks, has also contributed to their metal budget.