ENVIRONMENTAL RISK ASSESSMENT OF LEAD ZINC MINING CASESTUDY OF ADUDU METALOGEMIC PROVINCE MIDDLE BENUE TROUGH NIGERIA

CHAPTER ONE

1.0       INTRODUCTION

The study area, Adudu Metalogenic Province, lies in the Benue Trough. Although no economically viable discovery of hydrocarbon has being made in the Benue trough, it remains a site that receives constant influx of people owing to the large occurrence of various economic minerals present in its sedimentary piles. Some of the minerals that have being reported in the basin include barite, galena, sphalerite, salt etc.

Records show that organized mining started in Nigeria around 1939 through the privately owned foreign companies (Channda et al., 2010). However, no mining legislation was enacted until seven years later (1946) and this was not reviewed until 1999. Although the duo (1946 and 1999 mining acts) do not make provisions for artisanal mining, major pack-up of the mining companies in the 70s left many mine workers employed without any means of sustenance and future security. This scourge opened doors for massive illegal mining in various parts of the country including the Benue trough (Channda et al., 2010).

Various age groups, regardless of gender discrimination are involved in artisan mining, and the technology used by these majorly unskilled workers are dominantly primitive. Agricultural implements such as cutlass, hoe, digger and shovel are used by these groups for digging out the overburden to grant them access to the ore body (open cast mining). Underground channeling, lotto and the use of explosives are also being used, though at a regular frequency in comparison to the open cast mining. The hoary, open cast mining, which generates large amounts of sulphide-rich tailings (Bhattacharya et. al. 2006), has a serious environmental impact on the quality of soils and surface water due to pollution (Igwe et. al., 2014).

According to Nriagu and Pacyna (1988), the metal content in soil is a product of metals originating from natural processes and human activity. It is estimated that the contribution of metals from anthropogenic sources in soil is higher than the contribution from natural ones. Anthropogenic activities such as mining and smelting of metal ores have increased the prevalence and occurrence of heavy/trace metal contaminations and pollutions at the earth’s surface. In general, mined soils are mechanically, physically, chemically and biologically deficient (Vega et al. 2006), characterized by instability and limited cohesion, with low contents of nutrients and organic matter and high levels of heavy metals (He et al. 2005). Apart from the local disturbance of the physical properties, potential toxic metals (PTMs) can cause a more widespread contamination of soil, sediments and food crops leading eventually to a loss of biodiversity and a potential health risk to inhabitants in the vicinity of the mining area (Verner and Ramsey 1996; Lee et al. 2001; Zhang et al. 2002).

The Lead-Zinc mineralization which occur in the form of veins and veinlets associated with the host rock, are localized along the Northern-Southern trending belt of slightly deformed Sedimentary Cretaceous sequences (Albian Asu River Group) that measure about 500m thick (Igwe et al., 2014). This mineralization is structurally controlled and localized in fissures, faults zones and gently dipping veins. The veins are steeply dipping and have a depth of over 150m. They vary in width from less than a meter to 20m and in length from 30m to 120m. The dominant ores in the areas were observed from the fissures and contain lodes of sphalerite (ZnS), and/or galena (PbS) in association with smaller quantities of copper.  Galena is lead-grey in colour in veins and lenses. Orazulike (1994) had reported that the three types recognized on the basis of crystal form include: fibrous, granular, and cubic galena. Fibrous galena is often found close to fault zones and the gneissic texture is as a result of shear strain association with movement on the fault. In hand specimen, galena in granular form consists of a mass of tiny granules. It typically occurs away from the fault zones. Sphalerite is opaque, dark brown in colour and is usually in intimate association with galena. Its crystals are large and tabular with some faces measuring up to 8cm across.

Metallic ores are characterized by occurrence of gangue alongside mineralization of interest. Identified gangues associated with the Pb-Zn deposits in the Benue aulacogen includes siderite (FeCO₃), pyrite (FeS₂), marcasite, quartz and barites with other secondary minerals such as sulphates, carbonates and oxides. Marcasite is a ubiquitous gangue mineral though much less in abundance than siderite and quartz. Chalcopyrite is a minor mineral component, occurring generally in association with siderite and galena. It is massive and structure-less in hand specimen. Siderite is massive and has beige colour when fresh. On exposure to air, it tarnishes to dark brown. The crystal faces are poorly formed suggesting deposition at relatively high temperatures. Siderite is found in the main veins, minor fractures and vein-lets. In addition to galena and sphalerite, barite deposits in the trough as well as saline groundwater are of economic importance. Although the environmental impacts of the Pb-Zn mineralization in the lower Benue trough, which is being mined in the Enyingba district has received numerous attentions from researchers (e.g. Ezeh et al.,(2007), Ezeh and Anike (2009), Igwe et al., (2014)); there is no published work that has examined the effects of mining activities in the Adudu province found by me. It is therefore the focus of this research to critically examine the potential risks associated with the harvesting of these economic minerals in stated location.

There are two mining methods carried out in the area: open cast mining and underground mining. In the open cast mining, weathered materials known as overburden are removed by excavators to gain access of the minerals (fig.1.1). In the underground mining, the overburden is not removed but shafts are dug to meet the minerals underground where they are removed and brought by conveyors (fig.1.2). In both mining methods, groundwater is constantly pumped out to access the minerals.

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