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Phase relations of arsenian pyrite and arsenopyrite

journal contribution
posted on 2023-05-21, 00:58 authored by Stepanov, AS, Ross LargeRoss Large, Kiseeva, ES, Leonid Danyushevsky, Karsten GoemannKarsten Goemann, Sebastien MeffreSebastien Meffre, Zhukova, I, Ivan BelousovIvan Belousov
Arsenian pyrite containing above 1 wt% As plays a crucial role in deposition and deportment of Au and other chalcophile elements. The importance of arsenian pyrite led to theoretical and experimental studies that examined properties and genesis of the mineral; however, the interpretation of the phase relations between arsenian pyrite and arsenopyrite is conflicting. In this contribution, we present the results of a review of the experimental studies that have investigated the crystallisation of pyrite in As-bearing systems, a summary of As content in pyrite coexisting with arsenopyrite in 37 deposits and the composition of arsenian pyrite in deposits with little or no arsenopyrite. The review demonstrates that the previous experimental studies that attempted to achieve equilibrium between pyrite and arsenopyrite observed from <1 to 4.7 wt% As in pyrite. The literature survey of the assemblages of pyrite and arsenopyrite shows that pyrite crystallising together with arsenopyrite commonly has a very heterogeneous composition with As content varying from below detection to about 10 wt% As and no clear discontinuities were observed across this range. In the deposits without arsenopyrite, arsenic content in pyrite can reach 20 wt% As. We consider three principal scenarios of the relations of arsenian pyrite and arsenopyrite: (A) Pyrite with high As content is stable in equilibrium with arsenopyrite. Low-As pyrite coexisting with arsenopyrite is a product of disequilibrium crystallisation; (B) a scenario of control of As content in pyrite coexisting with arsenopyrite by thermodynamic parameters including temperature, pressure, the activity of components and fluid composition and (C) a scenario where arsenian pyrite is a metastable mineral. The observations are inconsistent with a model of 5–6 wt% As in pyrite coexisting with arsenopyrite in equilibrium (scenario A). The stability range of the assemblage of pyrite and arsenopyrite constrains the thermodynamic control on the composition of pyrite crystallising in equilibrium with arsenopyrite (scenario B). The scenario of metastable crystallisation of arsenian pyrite (C) proposes formation of the mineral by fast growth from oversaturated fluids with As content controlled by surface adsorption and can explain such features as sector zoning of the mineral and the apparent negative temperature dependence of the solubility. The data phase relations of arsenian pyrite highlight the need for new experimental studies, and suggest that the scenario of disequilibrium phase relations of arsenian pyrite should be considered for interpretation of natural assemblages.

Funding

Australian Research Council

AMIRA International Ltd

ARC C of E Industry Partner $ to be allocated

Anglo American Exploration Philippines Inc

AngloGold Ashanti Australia Limited

Australian National University

BHP Billiton Ltd

Barrick (Australia Pacific) PTY Limited

CSIRO Earth Science & Resource Engineering

Mineral Resources Tasmania

Minerals Council of Australia

Newcrest Mining Limited

Newmont Australia Ltd

Oz Minerals Australia Limited

Rio Tinto Exploration

St Barbara Limited

Teck Cominco Limited

University of Melbourne

University of Queensland

Zinifex Australia Ltd

History

Publication title

Ore Geology Reviews

Volume

136

Article number

104285

Number

104285

Pagination

1-15

ISSN

0169-1368

Department/School

School of Natural Sciences

Publisher

Elsevier Science Bv

Place of publication

Po Box 211, Amsterdam, Netherlands, 1000 Ae

Rights statement

© 2021 Elsevier B.V. All rights reserved.

Repository Status

  • Restricted

Socio-economic Objectives

Expanding knowledge in the earth sciences

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