Retrograde Ag-enrichment in fahlores from the Coeur d'Alene mining district, Idaho, USA
| Autor: | S. M. Kuehner, L. S. Hardy, R. O. Sack |
|---|---|
| Rok vydání: | 2002 |
| Předmět: |
Pyrargyrite
education.field_of_study 010504 meteorology & atmospheric sciences Chemistry Tetrahedrite Population Mineralogy engineering.material Bournonite 010502 geochemistry & geophysics 01 natural sciences Crystallography Sphalerite Thermodynamic database Geochemistry and Petrology Galena engineering education 0105 earth and related environmental sciences |
| Zdroj: | Mineralogical Magazine. 66:215-229 |
| ISSN: | 1471-8022 0026-461X |
| Popis: | Tetrahedrite fahlores from the Coeur d’Alene mining district (Idaho) have been found to be enriched in Ag by the Ag–Cu exchange reaction \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \begin{eqnarray*}&&{\frac{1}{10}Cu\_{10}(Zn,Fe)\_{2}Sb\_{4}S\_{13}\_{Cu-fahlore}^{}}\ +\ {AgSbS\_{2}\_{in\ galena}^{}}\ +\ {Pbs\_{galena}^{}}\ =\ \\&&{CuPbSbS\_{3}\_{bournonite}^{}}\ +\ {\frac{1}{10}Ag\_{10}(Zn,Fe)\_{2}Sb\_{4}S\_{13}_{Ag-fahlore}^{}}\end{eqnarray*} \end{document} which occurred during cooling following galena mineralization. This solid–state reaction resulted in quantitative removal of Ag (in a AgSbS2 component) from galena and development of bournonite coronas on fahlore grains. The reaction produced a distinct population of high-Ag fahlores found in galena-rich samples and accounts for all of the bournonite mineralization. The most argentian of these high-Ag fahlores examined in this study (molar Ag/(Ag+Cu) = 0.303±0.011 and 0.336±0.011) are found in samples which achieved saturation with respect to other Ag-sulfosalts, namely pyrargyrite and polybasite and diaphorite, respectively. Multiple lines of evidence for the Ag-Cu exchange reaction are presented in this paper which applies mass-balance constraints and a thermodynamic database for sulfides/sulfosalts to microprobe analyses and textural observations, and to bulk production data. This solid-state reaction may explain why previous district studies have been unable to demonstrate convincingly primary fahlore zoning. Based on the Ag/(Ag+Cu) of fahlores coexisting with other Ag-sulfosalts and Fe-Zn partitioning between fahlore and sphalerite, we estimate that fahlore compositions were frozen in by 175°C. Examining the composition data for other Ag tetrahedrite fahlores found in the Coeur d’Alene district and elsewhere, we conclude that the thermodynamic database provides an accurate description of phase equilibria, except possibly for very Fe-rich systems where further studies of the Fe-Zn partitioning between fahlore and Fe-rich (Zn,Fe)S phases and of the thermodynamics of Fe-rich (Zn,Fe)S phases are warranted. |
| Databáze: | OpenAIRE |
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