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Minerals 2016, 6(3), 62; doi:10.3390/min6030062

Bismoclite (BiOCl) in the San Francisco de los Andes Bi–Cu–Au Deposit, Argentina. First Occurrence of a Bismuth Oxychloride in a Magmatic–Hydrothermal Breccia Pipe and Its Usefulness as an Indicator Phase in Mineral Exploration

1
CODES-ARC Centre of Excellence in Ore Deposits, Private Bag 79, University of Tasmania, Hobart 7001, Australia
2
ARC Research Hub for Transforming the Mining Value Chain, Private Bag 79, University of Tasmania, Hobart 7001, Australia
3
INGEOSUR-CONICET, Departamento de Geología, Universidad Nacional del Sur, San Juan 670, Bahía Blanca 8000, Argentina
*
Author to whom correspondence should be addressed.
Academic Editor: Maria Economou-Eliopoulos
Received: 29 March 2016 / Revised: 15 June 2016 / Accepted: 16 June 2016 / Published: 28 June 2016
(This article belongs to the Special Issue Mineral Deposit Genesis and Exploration)
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Abstract

The rare bismuth oxychloride, bismoclite (BiOCl), has been identified in the weathered tourmaline–cemented, magmatic–hydrothermal breccia complex at the San Francisco de los Andes Bi–Cu–Au deposit, Argentina. A wide variety of supergene minerals were detected in the oxidized zone, but only preisingerite (Bi3(AsO4)2O(OH)) is intimately associated with bismoclite. Bismuth arsenate is present either as minor accessory phases or as traces in bismoclite-rich samples. This is the first documented occurrence of bismoclite in a porphyry-related, and magmatic–hydrothermal breccia pipe deposit. Bismoclite is interpreted to have formed by weathering of hypogene bismuthinite (Bi2S3), which originally occurred with arsenopyrite to cement the breccias. These appear to have reacted with O2- and HCl-bearing meteoric waters to produce pockets of supergene bismoclite–preisingerite assemblages. Bismoclite samples have been characterized by means of X-ray diffractometry (XRD), geochemistry, petrography, scanning electron microscopy (SEM), differential thermal analysis–thermogravimetry analysis (DTA–TGA) and infrared analysis (IR) providing useful insights and updated information regarding this rare bismuth oxychloride and associated arsenate mineral. The San Francisco de los Andes breccia complex shows similar geometry, morphology and internal organization as those found in traditional magmatic–hydrothermal breccias associated with Cu–Mo porphyry deposits. Bismoclite and preisingerite form due to the presence of hypogene Bi-bearing minerals followed by appropriate supergene conditions. These hypogene minerals commonly occur only as trace phases, or are entirely absent, in porphyry and related magmatic–hydrothermal breccia deposits. The scarcity of hypogene Bi–mineral phases in porphyry and related magmatic–hydrothermal breccia deposits is the main reason why bismoclite has not previously been reported in these types of deposits. The detection of bismoclite as a mineral phase in the oxidized zone of weathered deposits highlights hypogene Bi mineralization at depth, and associated metals. Bismoclite is an insoluble mineral of particular interest in those supergene profiles which have been completely leached out of distinctive, water-soluble phases, such as Cu sulfate minerals, which are diagnostic of Cu mineralization at depth. Consequently, bismoclite could potentially be the only indicator of hypogene and supergene mineralization in lower portions of a bismuth bearing ore deposit. View Full-Text
Keywords: bismoclite; magmatic–hydrothermal breccia pipe; porphyry-related; San Francisco de los Andes; Argentina bismoclite; magmatic–hydrothermal breccia pipe; porphyry-related; San Francisco de los Andes; Argentina
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MDPI and ACS Style

Testa, F.J.; Cooke, D.R.; Zhang, L.; Mas, G.R. Bismoclite (BiOCl) in the San Francisco de los Andes Bi–Cu–Au Deposit, Argentina. First Occurrence of a Bismuth Oxychloride in a Magmatic–Hydrothermal Breccia Pipe and Its Usefulness as an Indicator Phase in Mineral Exploration. Minerals 2016, 6, 62.

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