Morphology, Mineralogy, and Chemistry of Ocherous Precipitate Aggregates Downstream of an Abandoned Mine Site

Mitsuo Manaka

doi:10.3390/min11010032

Abstract

Mineral precipitates forming downstream of abandoned and/or uncontrolled mine sites generally act as scavengers for heavy metals, such as As and Sb, leaking from the sites. This study reports the morphology of ocherous precipitate aggregates downstream of Ayuta, an abandoned antimony mine site in Tochigi Prefecture, Japan, because its morphology differs significantly from those reported previously. The morphology of this aggregate consists of stacked, small terraces enclosed by numerous connected rimstone dams, although on a smaller scale compared to typical terrace landscapes. The rimstone pools contained ocherous spheroids precipitates at the bottom. Additionally, stream water and ocherous aggregates collected from the site were investigated for mineralogy and chemistry. As (0.07–0.17 μg/L) and Sb (0.02 μg/L) levels in the stream water were determined, and the distributions of As and Sb in the mineral phases of the precipitate were estimated using a sequential extraction procedure. The investigations revealed that As was adsorbed by iron-bearing ocherous precipitate aggregates, especially ferrihydrite that formed on the stream bank at concentrations, comparable to those reported by other studies (85 mg/kg). This adsorption contributed to the natural attenuation of As in the stream. Sb in the aggregate consisted of ultra-fine silt and clay-size particles of stibnite ore transported from the surrounding area and/or secondary minerals transported by groundwater and surface water.

Highlights

Precipitates formed by the dissolution and oxidation of iron and aluminum-containing minerals often occur downstream of abandoned and/or uncontrolled mine sites
Its morphology consists of dish-like ocherous products (~10 mm in diameter) stacked in layers and appearing as numerous connected rimstone dams that could be observed on the surface of the aggregate upon detailed inspection
The internal morphology of the ocherous aggregate can be observed from the computed tomography (CT) images, shown in Figure 3, of slices cut parallel to the stream surface

Summary

Introduction

Precipitates formed by the dissolution and oxidation of iron and aluminum-containing minerals often occur downstream of abandoned and/or uncontrolled mine sites. Precipitates occur on river sediments [1,2,3,4], below and/or on the sides of the dam water outflow [5,6,7], on riverbanks [8], and in cemented layers, such as hardpans [9]. The precipitates effectively adsorb heavy metal ions, such as As and Sb, and inhibit the outflow of these ions downstream. Sb concentration was found to be reduced to background levels [7] Such natural attenuation can be helpful for removing heavy metal contaminants from the environment. Numerous studies have focused on the adsorption of these ions by precipitates based on experimental results on natural and synthetic materials; the natural attenuation properties of iron- and/or aluminum-bearing materials are not well understood in riverbanks [8]

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Conclusion