G. Rizzi et Cjr. Braithwaite, SEDIMENTARY CYCLES AND SELECTIVE DOLOMITIZATION IN LIMESTONES HOSTINGTHE GIANT NAVAN ZINC-LEAD ORE DEPOSIT, IRELAND, Exploration and mining geology, 6(1), 1997, pp. 63-77
The Navan zinc-lead ore deposit (70 million tonnes) is hosted principa
lly (97%) by the Meath Formation (Lower Carboniferous, Courceyan, Nava
n Group), comprising a lower, mainly carbonate mudstone unit, the Stac
kallan Member, and an upper grainstone-dominated unit. The Stackallan
Member, about 60 m thick, comprises about 35 peritidal cycles includin
g an oolitic grainstone interval. Grainstones forming the upper part o
f the formation, about 150 m thick, comprise at least six shallowing-u
pward ramp cycles. Emergence horizons are indicated by subaerial disso
lution surfaces, in situ breccias, and a single palaeosol. Most dolomi
tization at Navan is confined to the Meath Formation. The linear dolom
ite body, trending NE-SW, has a flattened, laterally limited, tabular
geometry in cross-section, suggesting that dolomitizing fluids initial
ly rose vertically, cross-cutting stratigraphic and sedimentological b
oundaries, although fluid flow was subsequently controlled by these fe
atures. Three stages of dolomitization are indicated by textural relat
ionships, cathodoluminescence (CL), oxygen isotopes, and fluid inclusi
on data. Stage 1 crystals, with generally dully luminescing cores and
up to six overlying subsidiary zones, are the main replacive phase. Cr
ystal surfaces are commonly corroded and overlain by bright stage 2 rh
ombs (which also form new nuclei lining molds and fractures). Three su
bsets of zones are present in stage 2 crystals: bright, non-luminescen
t, and dull; these are locally separated by additional corrosion surfa
ces. Stage 3 comprises relatively large baroque crystals, generally du
lly luminescent but with up to seven sub-zones. Isotopic and fluid inc
lusion data suggest growth of successive stages from waters which beca
me progressively hotter (60 degrees-160 degrees C). The distribution o
f the ores follows the dolomite trend. Ores were precipitated between
Stages 1 and 2 of dolomite growth, following Stage 2, and also during
the nucleation and growth of Stage 3 crystals. Dolomitization and mine
ralization appear to have been temporally and genetically related. The
cyclic sedimentation of the host rocks, which accentuates lithologica
l and petrographical contrasts between depositional units, controlled
dolomitization. The relationship between dolomitization and mineraliza
tion, which is closely associated with faults, provides powerful explo
ration criteria. The recognition of these features in limestones elsew
here in the Irish ore field may point toward unrecognized zinc-lead pr
ospects. (C) 1997 Canadian Institute of Mining, Metallurgy and Petrole
um.