Metal remobilization and ore-fluid perturbation during episodic replacement of auriferous pyrite from an epizonal orogenic gold deposit

Abstract

Mineral-scale episodic replacement of auriferous pyrite by texturally-complex pyrite, marcasite and minor arsenopyrite occurred in breccia ores from the Daqiao epizonal orogenic gold deposit, West Qinling Orogen, China. This study uses a novel combination of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), Nanoscale secondary ion mass spectrometry (NanoSIMS), and secondary ion mass spectrometry (SIMS) to investigate the remobilization and re-concentration of gold and other trace elements during this complex replacement process and the probable mechanism. Several lines of evidence including some degree of preservation of external morphology, sharp contacts and compositional differences between the parent pyrite and product pyrite and marcasite, and reaction-induced porosity suggest that the replacement of parent pyrite proceeds via a two-step replacement via a dissolution and reprecipitation mechanism, plus an additional marcasite overgrowth. During the replacement of euhedral pyrite, depletion of gold and other trace elements (Te, Se, Zn, Co, Tl, Ni, W, and As) in porous product pyrite relative to its precursor indicate exsolution and remobilization of these metals from crystal lattice of the original pyrite. In the subsequent replacement of porous pyrite by two types of marcasite and minor arsenopyrite, euhedral product marcasite contains low contents of trace elements, possibly due to high metal solubility in the acidic fluids favorable for marcasite precipitation. The complex-zoned marcasite significantly enriched in gold and other metals relative to porous pyrite (W, Tl, As, Sb, Ag, Se, and Zn) is thought to have formed via precipitation triggered by further oxidation and/or immediate reduction in threshold supersaturation. Dissolution of the impurity-rich pyrite and precipitation of new pyrite and marcasite generations could have occurred at low pH plus high concentrations of dissolved Fe 2+ condition caused by partial oxidation of aqueous H2S and/or S 2in ore fluids. The fluid oxidation is evidenced by a general decreasing trend of δ 34 S values from the parent euhedral pyrite, to product porous pyrite, euhedral marcasite, and complex-zoned marcasite. The isotopic results are consistent with ore fluid oxidation controlled by pressure fluctuations during multistage hydraulic fracturing in a fault-valve regime at Daqiao deposit. This quantitative study emphasizes that the pressure-driven hydrothermal process plays a key role in the micron-to nano-scale redistribution and re-enrichment of gold and other trace metals during episodic replacement of auriferous pyrite in brittle rheological zones from epizonal orogenic gold systems.

Auriferous pyrite formed by episodic fluid inputs in the Akeshi and Kasuga high-sulfidation deposits, Southern Kyushu, Japan

“…portion of the Au and Cu in high-sulfidation deposits (Vennemann et al 1993;Arribas 1995;Heinrich 2005). This interpretation may also be consistent with findings in other hydrothermal Au deposits that pressure-driven, episodic hydrothermal fluid movements in brittle rheological zones are recorded in pyrite grains, sometimes bearing CDR textures (Peterson and Mavrogenes 2014;Rottier et al 2016;Sanchez-Alfaro et al 2016;Wu et al 2019a).…”

Supplemental Material: Decoupling of Au and As during rapid pyrite crystallization

“…NanoSIMS analysis was performed on polished epoxy mounts at the Centre for Microscopy, Characterisation and Analysis (CMCA) at the University of Western Australia (UWA), using a CAMECA NanoSIMS 50L optimized as described by Wu et al (2019). A Cs + ion source with a spot size of approximately 50 nm was employed.…”

“…The LA-MC-ICP-MS analysis was conducted using a Resonetics-S155 excimer ArF laser ablation system coupled to a Neptune II multicollector ICP-MS, housed at the t State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences (CUG), Wuhan. Detailed analytical conditions and procedures were described by Li et al (2019). Pyrite was ablated at a spot size of ~30 micrometers with a laser repetition rate of 10 Hz, and the ablation process was set to last for 40 s. Standard-sample bracketing (SSB) was used to determine the δ 34 S values of samples during the analytical sessions.…”

Discovery of SEDEX gold in the Youjiang basin, SW China: Implications for a new type Au mineralization

“…前人研究显示, 白铁矿的形成主要是在低pH(<2.5) 和低温条件下(<220°C)由黄铁矿氧化和溶蚀形成并稳 定存在 [26,27] . 对白铁矿扫描电子显微镜分析显示(图5, BSE图), 白铁矿中元素存在不均一性, 这种成分不均 一的白铁矿被认为由过饱和流体瞬间氧化或者是还原 形成 [28] . 事实上, 大多数金属的溶解度随着pH的降低而 增加, 因此在白铁矿稳定的低pH条件下, 金属的活动性 增强但不同金属之间的相容性较低 [29] , 这也导致了白 铁矿中微量元素含量较低, 与本研究白铁矿微量元素 低含量的结果一致.…”

“…这种 流体释压沸腾进一步导致H 2 S的逃逸, 致使流体瞬间氧 化, 溶蚀黄铁矿形成白铁矿. 因此, 在流体过饱和的情 况下, 瞬间的氧化非常有利于复杂成分白铁矿的形 成 [28] , 同样这种沸腾作用也非常有利于自然Au的沉淀 (如Higashi-Aogashima海底热液区 [21] ), 而微量元素(如 Cu, As, Zn等)因具有高溶解度在白铁矿中少量沉淀. 另外, 刘家军等人 [30] 对黔西南卡林型金矿床容矿硅质 岩的研究认为具有喷流沉积特征, 为本次含Au硫化物 的SEDEX成因提供了佐证.…”