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

Abstract

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 (Bi 3 (AsO 4 ) 2 O(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 (Bi 2 S 3 ), which originally occurred with arsenopyrite to cement the breccias. These appear to have reacted with O 2 -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.

Physicochemical Conditions of Formation for Bismuth Mineralization Hosted in a Magmatic-Hydrothermal Breccia Complex: An Example from the Argentine Andes

“…At a deposit scale, the San Francisco de los Andes tourmaline-quartz-cemented breccia complex is spatially and genetically associated with the Tocota Pluton, and has cut the Carboniferous sedimentary host rocks (e.g., [2]; Figure 1). The breccia-hosted deposit is characterized by a complex Bi-Cu-Au-As-Fe-Zn-Pb-Ag mineral assemblage, which includes hypogene sulfides, sulfosalts, tellurides, and native elements, as well as supergene oxides, hydroxides, arsenates, sulfates, carbonates, and secondary sulfides and sulfosalts [5]. At a district scale, the study area is characterized by marine sedimentary rocks and granitoids ( Figure 1).…”

“…Preisingerite (Bi 3 (AsO 4 ) 2 O(OH)) was first documented in the San Francisco de los Andes deposit (type locality), and its crystal structure first described by Bedlivy and Mereiter (1982) [11]. The first occurrence of bismoclite (BiOCl) in a magmatic-hydrothermal breccia complex, and its use as an indicator phase in mineral exploration was reported by Testa et al (2016) [5]. Bismoclite rarely occurs as a pure phase, but is intimately associated with traces of preisingerite [5].…”