ENVIRONMENTAL RISK ASSESSMENT OF LEAD ZINC MINING CASE STUDY 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.   

Fig.1.1: A typical open cast mining pit in the study area.

Fig. 1.2: A typical underground mining in the study area.

1.1     OBJECTIVES

This study was carried out with the intent of assessing the environmental risk associated with the mining of galena and sphalerite in the study area, Adudu Mining Province. Specifically, the following objectives will be examined.

• Assessment of the conformity of past mining operations in the environment to existing mining laws.
• Determination of contaminants and pollutants levels in sediments and soil.
• Establishing the level of impacts of Pb–Zn mining on surface and groundwater in the area.

1.  STATEMENT OF PROBLEM

The large occurrence of economic minerals in the Benue trough has attracted several investors in the last five decades. The activities of artisan miners and the non-conformity of some investors to existing mining laws in Nigeria are common occurrences in the country; as such the environment is left degraded thereby posing possible threat to the ecosystems. The Adudu metalogenic province; which is located in the middle Benue trough is not exemption to these challenges. Although several works have been done to environmentally assess the impact of mining activities in the Benue trough, the Adudu province has received very insignificant research attention. The facts that the entire mining province is surrounded by farms and the deleterious nature of lead and zinc (which are the two minerals mined in the area coupled with the proximity of settlements to the mining sites necessitate the need to geochemically assess the impacts of the mining activities in the Adudu province.

1.  PREVIOUS WORK

The Cretaceous Benue trough has attracted significant research efforts in its Nigeria sector. Burke et al., (1971) investigated the relationship between the opening of the Gulf of guinea and the geologic history of the Benue depression and the petroliferous Niger Delta, basin. Olade, (1975) presented a tectonic model to explain the evolution of the Benue Aulacogen. In the year 1986, Benkhelil’s work on the origin and evolution of the trough was published. Petters, (1978) published on the Mid-Cretaceous paleoenvironments and biostratigraphy of the Benue Trough, Nigeria using evidences from fossils. These works unraveled the different formational and stratigraphic classifications that are currently in use in the basin. Although there are grey areas where contrary views have been held by different school of thoughts, the general evolution, stratigraphy as well as relationship to its contiguous basins, are well well-defined.

Olade, (1976) explained the genesis of lead-zinc deposits in the trough. The dispersion of Cadmium, Lead and Zinc in Soils and Sediments of the region was studied by Olade, (1987). The distribution of some heavy metals in soils in areas around the derelict Enyigba mines and its environmental implication was assessed by Ezeh et al., (2007). Ezeh and Anike (2009) carried out a preliminary assessment of the pollution status of streams and artificial lakes created by mining in the Enyigba mining district, Southeastern Nigeria.

Igwe et al., (2014) evaluated the potential risk associated with the mining of lead-zinc deposits in the Enyingbametalogenic province. They reported significance pollution level in the soil, stream sediments, mine tailings and rock types in the area.

It is noteworthy to state that the impacts of mining activities in the Benue trough had received some significant research attention; nevertheless, a closer look at all published works on the subject matter, revealed that virtually all are concentrated in the Lower Benue trough. The middle part of the trough where this research work is focused on has received very little attention in this aspect. As gathered from baseline studies, mining activities in the Adudu area starts over four decades ago, currently number of companies are still exploring and mining these metallic ores in the some parts of the province. This research is targeted at unraveling the contamination and pollution levels in the water and soil of the Adudu province; the motivation for this is the need to sensitize relevant authorities on the levels of impact produced by mining activities in the region.