The Navan Orebody is a world-class carbonate-hosted Zn-Pb deposit comprising complex tabular lenses within Lower Carboniferous limestones and excluding depletion, would be over 130Mt in size. Detailed descriptions of the geology are available in a number of publications of which Ashton et al., (2015) is the most recent. The discovery of the Tara Deep satellite and outline geology are summarized in Ashton et al., (2018).

Central Ireland comprises generally flat lying sequences of Lower Carboniferous limestones with common inliers of sedimentary Lower Palaeozic and Devonian rocks. The limestones are cut by numerous, locally syn-depositional NW to ENE trending major normal faults and these control the location of several carbonate hosted Zn-Pb deposits, of which Navan is by far the largest.

In eastern Ireland, the Carboniferous Limestones are part of the Dublin Basin, a significant feature that after extensional basin-margin faulting and later Hercynian inversion, exposes some large Lower Paleozoic inliers at its margins and exhibits some outliers of Namurian and later Permo-Triassic sediments.

The Navan Orebody is located on the footwall (northern) side of a major south-dipping normal fault that constitutes a basin margin controlling feature. The orebody itself is controlled by a complex array of Lower Carboniferous normal faulting and slides on the uplifted footwall of this major fault. The orebody generally dips at about 10-15 degrees to the WSW and comprises several, locally stacked, tabular stratiform to stratabound lenses, oriented in general concordance with the host limestones. The mineralisation ranges from a few meters to over 70m in vertical thickness. A major slide and overlying debris flow cuts the orebody obliquely and is also mineralized. The orebodies are effectively masked from the surface by a thick succession of deep-water calc-turbidites that comprise the Dublin Basin.

Although there are number of significant lenses and fault blocks at Navan, >95% of the mineralisation occurs as sphalerite and galena in partly dolomitized limestones as complexly-textured replacements, veining and open-space infill where Zn:Pb ratios are typically around 4 or 5 to 1. Gangue mineralisation comprises subsidiary calcite, pyrite, marcasite, dolomite and barite. The remainder of the mineralisation occurs as massive pyritic lenses containing sphalerite and galena hosted by debris-flow conglomerates that overlie the deposit. This material contains often abundant fine-grained pyrite which has the potential of degrading the normally excellent metallurgy if not blended with normal run of mine ore.

The Boliden Tara Mines in Navan started development in 1974 with production commencing in 1977. Over the years, the mining methods evolved from top-slice and bench to various types of open stoping.

In most areas of the mine open-stopes are designed parallel to the strike direction of the orebody such that in-stope development is sub-horizontal, whereas in the upper-near surface sections the open stopes are along dip and may necessitate slightly more complex development. In both cases, ore thicknesses up to about 20m can be mined via a single footwall drift with blast-hole drilling drilled upwards. Thicker ore necessitates hanging-wall drifts. In both cases, development is advanced to the end of the stope, and a slot crosscut created from where a raise is blasted using longhole techniques to the orebody hanging wall. Subsequent longhole blasting opens up a slot over full stope width and subsequent long-hole blasting creates a blasted ore muck pile in the developing stope which is mostly mucked-out using remote control and increasingly tele-remote operations. Areas of weak ground, often related to faulting may occur in the back and/or sidewalls of stopes. These are secured using various cable bolts patterns. Once a stope is complete, it is backfilled with hydraulically placed cemented sand-fraction tailings and then the intervening pillars are mined employing broadly similar methods. Stopes vary from 12 to 20m in width. Areas of drift height ore are extracted by low profile open stoping and referred to as ‘room and pillar’ stopes.

In the underground mining operations, the ore is crushed in the primary crushing stations before hoisting to the surface and is transferred by a conveyor system to the coarse ore storage building. Ore processing in the mill is achieved by grinding, flotation, and dewatering. These processes are automated, monitored and controlled by a process control system. The grinding circuit, including an Autogenous mill, is designed to reduce the ore particle size to a size range suitable for separation, typically in the 10 to 75 micron range, and a maximum size less than 120 microns.

Ore production encompasses the drilling, blasting and removal of the ore from underground deposits. Broken ore is delivered to one of five underground primary crushers and reduced in size to <150 millimetres before being hoisted to the surface. Ore is then fed to an Autogenous grinding mill which grinds the ore to a fine powder which is then pumped as aqueous slurry to metallurgic flotation cells in the main Processing plant.

Within the flotation cells, galena (Lead sulphide) and sphalerite (Zinc sulphide) are differentially separated, while undesirable minerals such as pyrite are depressed. Once the target minerals have been extracted the waste stream, otherwise known as tailings is cycloned to separate the coarse sand fraction from the finer slimes fraction. The coarse fraction of tailings is stabilised, chemically and physically, by mixing with cement and is pumped underground to backfill mined out areas.

The remaining fines fraction tailings is pumped as an a ........