title: Tara deep zinc-lead deposit in Navan, Co. Meath, Ireland: ore and carbonate depositional processes
Creators: name: Drummond, Drew Alexander
Publisher: name: University of Glasgow
Year: 2021

description: The Tara Deep Zn-Pb deposit (currently 26.2 Mt @ 8.4% Zn, 1.6% Pb) is the latest major discovery by Boliden Tara Mines (first announced in 2016) which significantly adds to the existing world-class Navan deposit. Located 2 km south of the Navan deposit in Co. Meath, Ireland, economic mineralization is hosted by upper Tournaisian carbonates (Pale Beds; 87% of the total economic resource), within a degraded footwall of a major south-dipping normal fault, and also within lower Viséan sedimentary breccias (S Fault Conglomerates; 13% of the total economic resource). Sphalerite and galena are the dominant sulfides, with massive, cavity fill and brecciated textures dominating. These textures attest complex, subsurface, episodic mineralization events that display considerable reworking, fracturing, dolomitization, open-space infill and selective replacement. Lower Viséan syn-rift sliding, erosion, and deposition of thick debrites and calc-turbidites at Tara Deep record basin margin processes near extensional faulting associated with formation of the Dublin Basin. These sedimentary breccias host detrital sulfide-rich clasts and offer unambiguous evidence that the onset of mineralization occurred during the upper Tournaisian. Conventionally drilled 34S values of base metal sulfides have a bimodal distribution suggesting both bacteriogenic (-13.5 to -3.6 ‰) and hydrothermal sulfur sources (+3.4 to +16.2 ‰). Both textural and sulfur isotope data reveal the dynamic nature of mineralization at Tara Deep and infer fluid mixing. Conventional lead isotope analyses display remarkably homogenous 206Pb/204Pb of 18.23±0.006 (2σ, n=25), which is consistent with Pb isotope data across the Navan deposit. Subsequently, Tara Deep and Navan are isotopically similar, showing both a statistically identical Pb isotopic signature and a bimodal sulfide S isotopic distribution and homogeneous sulfate signature. In particular, the Pb isotopes and the hydrothermal S signature, correlate with Navan and agree that base metals were leached from the underlying Lower Palaeozoic basement, this suggests that similar deep, circulating metalliferous fluids were also involved at Tara Deep. However, despite these similarities, key differences can be recognized within the S isotope data; around 5‰ shifts to higher 34S in the surface-derived S isotope signatures (both bacteriogenic sulfide and sulfate) indicate that Tara Deep’s sulfur was sourced from a distinct seawater/connate fluid signature. Floatation concentrate samples were obtained on representative drillholes from across the Tara Deep deposit by Boliden Tara Mines’ laboratory to inform recovery data for future financial modelling. The concentrates produced were analysed for S and Pb isotopes. The samples reveal limited variation with mean δ34S = -6 ± 5.9 ‰ (n = 38), and galena concentrate mean 206Pb/204Pb=18.22 ± 0.004 (n = 19). Mass balance calculations suggest that ~90 percent of the Tara Deep sulfides were derived from bacteriogenic reduction of contemporaneous Lower Carboniferous seawater sulfate. In contrast, metals were acquired from a local, orogenic crustal source beneath the orebody. These concentrate isotopic signatures differ from those associated with the Main Navan Orebody, in particular the bacteriogenic mode, reiterating that Tara Deep should be considered as an isolated deposit in space, and potentially time, and not an allochthonous block of the Navan deposit. As was reported for the world-class Navan deposit, enhanced bacterial activity was fundamental to ore deposition at Tara Deep, providing significant reassurance for continuing exploration. Tara Deep is located at depths of between 1.2-1.9 km within fault-controlled terraces on the degraded footwall of a major normal fault system. The system formed as a result of a rapidly subsiding basin margin during the upper Tournaisian extension. The region’s long lived and dynamic evolution has resulted in a complex architecture of large normal and strike slip faults. The complexity of the Dublin Basin margin evolution at Navan can be divided into three distinct phases: pre-rift ramp sedimentation in the Lower Carboniferous (Mississippian), syn-rift abrupt detachment and destructive debris flows in the lower Viséan, and finally late-rift basin infill during the Viséan. Diagenetic processes include dolomitization, mineralization and extensive pressure solution, which complicate the lithofacies reconstruction by occluding pre-existing textures. Dolomitization occurs in four phases: D1 – D4. D1 predates mineralization and represents early replacement by non-ferroan euhedral dolomite (30 μm), this dolomite typically has cloudy cores (often associated with Mn), with clear ferrous rims. D2 represents interparticle infill by anhedral (50 μm) ferroan dolomite. D3 occurs as late, coarse, non-ferroan saddle dolomite cement which occurs in vugs. D4 occurs as the last phase throughout the Tara Deep succession, is nanocrystalline, and is related to burial dolomitization witnessed by its close association with pressure solution seams. Pressure solution is extensive at Tara Deep and is partially responsible for the generation of common and distinct stylo-nodular textures in the Pale Beds, and can also lead to occlusion of the original host rock fabric in the Thin Bedded Unit. Oxygen and carbon isotopes are used to constrain these interpretations. Mineralization is also intimately linked to basin margin development and architecture and is recorded across early syn-rifting textures to late-rifting textures, highlighting the complexity and longevity of the mineralizing system within the Lower Carboniferous (Mississippian) carbonates. Together these processes speak of a dynamic sedimentary, tectonic, and microbiological environment which has culminated in an exceptional accumulation and preservation of base metals during the Lower Carboniferous. Mineralization initiated during an early phase of the developing Dublin Basin (syn-diagenetically) and kept pace with rifting and subsequently an evolving basin. The Tara Deep deposit has many similarities with the neighbouring Navan deposit reflecting comparable controls on the mineralizing processes in terms of host rocks, Pb and S sources, and tectonic environment.