Abstract
Six orogenic gold deposits constitute the Yangshan gold belt in the West Qinling Orogen. Gold is mostly invisible in solid solution or in the sulfide lattice, with minor visible gold associated with stibnite and in quartz-calcite veins. Detailed textural and trace-element analysis of sulfides in terms of a newly-erected paragenetic sequence for these deposits, together with previously published data, demonstrate that early magmatic-hydrothermal pyrite in granitic dike host-rocks has much higher Au contents than diagenetic pyrite in metasedimentary host rocks, but lower contents of As, Au, and Cu than ore-stage pyrite. Combined with sulfur isotope data, replacement textures in the gold ores indicate that the auriferous ore-fluids post-dated the granitic dikes and were not magmatic-hydrothermal in origin. There is a strong correlation between the relative activities of S and As and their total abundances in the ore fluid and the siting of gold in the Yangshan gold ores. Mass balance calculations indicate that there is no necessity to invoke remobilization processes to explain the occurrence of gold in the ores. The only exception is the Py1-2 replacement of Py1m, where fluid-mediated coupled dissolution-reprecipitation reactions may have occurred to exchange Au between the two pyrite phases.
Highlights
Gold commonly has a complex distribution in gold deposits [1,2]
Some authors have suggested that the visible native gold was remobilized from the invisible gold in sulfides [7,14,17,18,19,20], but it is unclear how gold could be first precipitated in pyrite and become soluble in that pyrite, without transformation to pyrrhotite, and subsequently be re-precipitated from essentially the same ore fluid just a short distance away in the same ore zone [20]
This study presents detailed textural and trace-element analysis of sulfides in terms of a newly erected paragenetic sequence, designed together with previously published data, to investigate the factors controlling the distribution of invisible and visible gold in the Yangshan ores to further improve genetic understanding of the deposits in the gold belt
Summary
Gold commonly has a complex distribution in gold deposits [1,2]. The gold that occurs in the structure of common sulfide minerals, or as discrete inclusions smaller than 100 nm, is referred to as ‘invisible gold’ [3]. Some authors have suggested that the visible native gold was remobilized from the invisible gold in sulfides [7,14,17,18,19,20], but it is unclear how gold could be first precipitated in pyrite and become soluble in that pyrite, without transformation to pyrrhotite, and subsequently be re-precipitated from essentially the same ore fluid just a short distance away in the same ore zone [20] In addition to this uncertainty, factors controlling the distribution of Minerals 2019, 9, 92; doi:10.3390/min9020092 www.mdpi.com/journal/minerals. This study presents detailed textural and trace-element analysis of sulfides in terms of a newly erected paragenetic sequence, designed together with previously published data, to investigate the factors controlling the distribution of invisible and visible gold in the Yangshan ores to further improve genetic understanding of the deposits in the gold belt
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