The mechanisms and implications of hydrothermal mineral replacement reactions in carbonate-hosted ore deposits

Knorsch, Manuel (2021) The mechanisms and implications of hydrothermal mineral replacement reactions in carbonate-hosted ore deposits. PhD thesis, Murdoch University.

Abstract

Mineral replacement reactions commonly occur during the formation and alteration of ore deposits. In such settings, primary minerals are often dissolved, with new phases forming as products in the presence of a fluid phase. Particularly within carbonate-hosted epigenetic ore deposits, mineral replacement reactions represent a fundamental ore-forming mechanism whereby primary carbonates are replaced by secondary minerals. These replacement processes generate porosity and facilitate ore mineral precipitation. This study compares observations of natural, ore-forming mineral replacement reactions with laboratory-derived hydrothermal replacement experiments to decipher the metasomatic processes of ore formation from the deposit scale to the nanoscale. This thesis presents insights on (i) the ore-forming mechanisms and post-ore alteration of the Artemis Zn-Cu-Au prospect (Chapters 2 and 3), (ii) the pseudomorphic replacement of calcite by siderite (Chapter 4), and (iii) the replacement of bastnaesite by rhabdophane and monazite (Chapter 5). These chapters focus on the mechanisms, porosity formation, and element redistribution of mineral replacement reactions, typically observed in carbonate-hosted mineral deposits.

Chapter 2 details the importance of mineral replacement reactions during the formation and post-depositional alteration of a carbonate replacement deposit. The study was carried out on the Artemis Zn-Cu-Au prospect, which is located in the Cloncurry district, NW Queensland, Australia. The high-grade polymetallic mineralization displays a complex association of massive sulfides and carbonates hosted in a vertical marble lens. Petrographic analyses suggested three major stages: (i) pre-ore stage marble; (ii) ore stage mineralization involving the dissolution of calcite from the marble and precipitation of sulfides and secondary carbonates; and (iii) a post-ore alteration stage involving the replacement of sulfides by calcite and garnet. The replacement of Co-rich arsenopyrite (Fe0.75Co0.26As1.13S0.83) by relatively Co-poor arsenopyrite (Fe0.86Co0.15As0.99S0.99) revealed formation temperatures of 650 ± 50 and 450 ± 70 °C, respectively. In the post-ore stage, pyrrhotite Fe(1-x)S, (x = 0.091–0.119) was pervasively replaced by an almandine-spessartine-rich garnet. The microscale investigations of the mineralization processes in the Artemis orebody document the impact of hydrothermal mineral replacement reactions on metal sequestration. Mineral replacement reactions facilitate the formation of ore deposits and subsequent alteration processes, which may lead to the late-stage loss of economic value of the deposit.

Chapter 3 discusses the importance of conducting nanoscale investigations of mineral replacement reactions to constrain their physicochemical conditions, exemplified by the suggested replacement of pyrrhotite by garnet at the Zn-Cu-Au Artemis prospect. Nanoscale examinations by transmission electron microscopy (TEM) of the grain boundary between pyrrhotite and garnet reveal textural and compositional complexity at the reaction front. The single crystal of pyrrhotite has a <100 nm thick Bi-rich rim (~2.7 at.% of Bi). The pyrrhotite rim is bounded by a 5–20 nm wide hematite layer, while pores (<10 nm thick) are present between the two phases. The garnet single crystal and the hematite layer are separated by a gap (10–30 nm). These observations indicate that the replacement of pyrrhotite by hematite proceeded via the coupled dissolution reprecipitation (CDR) mechanism, while Bi precipitated from the hydrothermal solution. The newly formed porosity was occupied by subsequent precipitation of garnet. This study highlights the importance of nanoscale characterization for revealing detailed mechanisms of sulfide alteration.

Chapter 4 details experiments on the replacement of calcite by siderite in Fe-rich hydrothermal fluids, investigating the effects of temperature (60–200 °C), solution pH (1.8–10), time (2–1680 h), and calcite precursor (marble and Iceland spar). Two contrasting replacement processes were identified: At 60 °C, the replacement was slow at a transformation of 89% after 1680 h for Iceland spar experiments. Siderite precipitated preferentially along the twin boundaries of calcite, and porosity developed in-between the twins. At 200 °C, the reaction was much quicker at a transformation of 94 % after 8 h. Several concentric 5–15 μm thick epitaxial siderite layers, separated by gaps (<15 μm) that were partly occupied by akaganeite [Fe3+O(OH,Cl)], replaced the calcite grains. Two siderite populations were identified: (i) metastable, pristine and Ca-rich siderite-1 and (ii) microporous, Ca-poor siderite-2. With time, siderite-1 equilibrated with the Fe-rich bulk fluid and transformed into siderite-2, marking the second replacement step. The formation of microporosity created permeability as a result of molar volume change between siderite-1 and -2 (-4.8%). The concentric layer formation is interpreted as an “oscillatory” CDR mechanism, which forms due to the interplay of passivation, fluid with nonequilibrium composition at the reaction front, and secondary porosity formation. Both textures may be used to interpret reaction conditions in natural mineral replacement reactions, especially within carbonate-replacement deposits.

Chapter 5 focuses on the replacement of the rare earth element (REE) fluorocarbonate mineral bastnaesite (REECO3F), which is frequently affected by fluid-induced alteration processes throughout all crustal levels. The experiments studied the effect of epithermal phosphate-bearing solutions on bastnaesite, which led to its replacement by monazite (REEPO4) and rhabdophane (REEPO4∙nH2O). At 90 °C, fibrous metastable rhabdophane replaced bastnaesite, which was then gradually replaced by monazite. At 220 °C, only monazite formed, and the reaction initiation was much quicker. Spot analyses showed that REE patterns were similar between bastnaesite and monazite. However, rhabdophane preferentially scavenged Nd to Ho. The presented results provide new insights into the formation conditions of rhabdophane and monazite and detail contrasting REE fractionation processes in the epithermal environment, which are critical to our understanding of the genesis of REE deposits.

Item Type:	Thesis (PhD)
Murdoch Affiliation(s):	Harry Butler Institute
Supervisor(s):	Xia, Fang, Deditius, Artur, Pearce, M. and Uvarova,, Y.
URI:	http://researchrepository.murdoch.edu.au/id/eprint/64693

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