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Mineralogical Magazine; February 2004; v. 68; no. 1; p. 31-46; DOI: 10.1180/0026461046810169
© 2004 Mineralogical Society of Great Britain and Ireland
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Arsenopyrite and As-bearing pyrite from the Roudny deposit, Bohemian Massif

J. Zachariás1,*, J. Fryda2, B. Paterová2 and M. Mihaljevic1

1 Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic
2 Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic

* E-mail: zachar{at}natur.cuni.cz

The major- and trace-element chemistry of pyrite and arsenopyrite from the mesothermal Roudny gold deposits was studied by electron microprobe and laser ablation ICP-MS techniques. In total, four generations of pyrite and two of arsenopyrite were distinguished. The pyrite is enriched in As through an Fe (AsxS1–x)2 substitution mechanism. The As-rich zones of pyrite-2 (up to 4.5 wt.% As) are also enriched in gold (up to 20 ppm), lead (commonly up to 220 ppm, exceptionally up to 1500 ppm) and antimony (commonly <600 ppm, rarely up to 1350 ppm). Positive correlation of As and Au in the studied pyrites is not coupled with an Fe deficiency, in contrast to Au-rich As-bearing pyrites in Carlin-type gold deposits. The As-rich pyrite-2 coprecipitated with the Sb-rich (1–4.2 wt.%) and Au-rich (40–150 ppm) arsenopyrite-1. The younger arsenopyrite-2 is significantly less enriched in these elements (0–70 ppm of Au).

The chemical zonality of pyrites in the Roudny gold deposits reflects the chemical evolution of ore-bearing fluids that are not observed in any other mineral phases. The data available suggest relatively high activity of sulphur and low activities of arsenic and gold during crystallization of the older pyrite generation (pyrite-1). Later, after particular dissolution of pyrite-1, Au-rich As-bearing pyrite-2 and arsenopyrite precipitated. These facts suggest a marked increase in the arsenic and gold activities in ore-bearing fluids. The As-content of pyrite-2 decreases in an oscillatory manner from the core to the rim, reflecting changes in the As activity or/and in the P-T conditions. The As-bearing pyrites were formed at temperatures of at least 320–330°C, based on arsenopyrite thermometers and fluid inclusion data.

KEYWORDS: arsenic, pyrite, invisible gold, mesothermal gold deposits, LA-ICP-MS






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