Hollis, Steven P.
Hollis, Steven P.
Hollis, Steven P.
Now showing 1 - 10 of 16
- PublicationAge and Geochemistry of the Charlestown Group, Ireland: Implications for the Grampian-Taconic Orogeny and Regional Mineral PotentialHere we extend the analysis of the Irish Caledonides to the Charlestown Group, located geographically between the well - studied Grampian terranes of South Mayo and Tyrone.
- PublicationPreliminary structural constraints from the Tyrone Igneous Complex and its metamorphic basementThe Grampian orogeny of the British and Irish Caledonides records the progressive accretion of a series of arcs, ophiolites and microcontinental blocks to the Laurentian margin between the Late Cambrian and Middle Ordovician. Accreted oceanic tracts contain elevated, but sub-economic concentrations of the Platinum Group Elements in the Shetland and Ballantrae ophiolites, Scotland, and potentially economic Cu-Zn-Pb-Ag-Au deposits in volcanic arc sequences of counties Tyrone and Mayo. Orogenic collapse was associated with significant Au mineralization in the deformed passive margin sequences (=Dalradian Supergroup) of Northern Ireland, with the UKs largest gold deposit at Curraghinalt and only active gold mine at Cavanacaw. The Curraghinalt deposit contains at least 3.5 Moz of Au, with precious metals likely to have been sourced from the underlying Tyrone Igneous Complex (TIC) – a structurally dismembered Ordovician arc-ophiolite sequence that also locally contains high-grade Au mineralization. Despite its importance and extensive research on the TIC in the last ten years, no detailed study has been undertaken on its structural history. We present preliminary results of structural measurements made across the TIC and its underlying metamorphic basement (=Tyrone Central Inlier). These were made by mapping of the Corvanaghan and Fir Mountain quarries in the Tyrone Central Inlier, as well as mapping of outcrops in the Tyrone Volcanic Group (e.g. Carnanransy Burn, Tullybrick, Cashel Rock, Beaghbeg). The Tyrone Central Inlier exposed in the quarries is composed of amphibolite facies rocks – predominantly metabasite and psammite, and minor metapelites. Leucosome-rich zones occur in the metabasites. Dips of these leucosome fabrics, bedding dips and first foliations are either similarly dipping, or show rotations around an identical SW-NE-oriented axis. Two generations of faulting are apparent in the Tyrone Central Inlier, with high angle oblique-slip to strike-slip faults with dextral indicators, crosscut by a family of dip-slip planar fault zones, with a normal sense where measurable. Zones of gold-bearing pyrite and chalcopyrite (with malachite staining) were observed near veins and faults that are N-S oriented in Corvanaghan quarry. Pegmatite and muscovite granite intrusions crosscut deformation fabrics.
- PublicationA comparison between clumped C-O and fluid inclusion temperatures for carbonates associated with Irish-type Zn-Pb orebodiesIreland hosts the greatest concentration of discovered zinc per square kilometre on Earth, with past and current production from five Irish-type carbonate-hosted deposits, including the giant Navan deposit. Clumped C-O isotope analysis of carbonate phases offers a powerful new technique to deliver accurate fluid temperatures and fluid O isotope compositions, refining evolving genetic models and developing new tools for exploration.
- PublicationEnhancing current understanding of Irish Zn-Pb mineralization: a closer look at the Island Pod orebody, Lisheen depositIrish-type deposits are a series of Zn-Pb orebodies, formed from the carbonate replacement of Lower Carboniferous limestone, triggered primarily by fluid mixing. Current understanding of the complex fluid flow and mixing dynamics associated with mineralization is limited. By applying clumped O-C isotope analysis to these deposits, these processes can be constrained. Preliminary paragenetic studies of the Island Pod orebody (0.4 Mt @ 20% Zn & 1.6% Pb) have yielded textural evidence for early fluid mixing of sulphide-rich fluids, in a quiescent, far-from equilibrium environment, resulting in the rapid precipitation of dendritic galena and intergrowths of dolomite and sphalerite. Initial clumped O-C analysis has revealed temperatures of 100-170°C for hanging-wall white matrix breccias that accompanied ore formation. This technique will be used to constrain temperature variations across the orebody, thus yielding information on how the fluid evolved as precipitation continued. A more detailed paragenetic study is underway and will form the foundation of future clumped O-C isotope and Zn-Cu-S isotope analysis.
- PublicationClumped C-O isotope temperature constraints for carbonate precipitation associated with the Irish-type Lisheen and Navan Zn-Pb orebodiesMineral C-O isotope values are controlled by crystallization temperature and the isotopic composition of the fluid.
- PublicationPreliminary paragenetic studies of the high grade Island Pod Zn-Pb orebody, LisheenIrish-type deposits are a series of Zn-Pb orebodies which formed from the carbonate replacement of Lower Carboniferous limestone, triggered primarily by fluid mixing. This project aims to use isotopic (Zn-Cu-S and clumped O-C) techniques to identify geochemical halos and increase our understanding of hydrothermal fluid processes in these deposits.
- PublicationThe bimodal fluid evolution of the Nimbus Zn-Ag deposit: an Archean VHMS with epithermal characteristicsThe Nimbus Zn-Ag VHMS deposit represents an exceptional mineralised occurrence in the Yilgarn Craton. While other VHMS systems in the craton are restricted to paleo-rift zones, Nimbus is associated to a plume-related stratigraphy (Hollis et al., 2017). Furthermore, the epithermal characteristics resulting from low temperature and shallow water conditions allowed the development of an unusual mineralisation dominated by Ag-rich sulfosalts. In this study we take advantage of state-of-the-art in-situ techniques to investigate the fluid evolution of this peculiar VHMS system. From the trace element compositions and S-isotope signatures we suggest that Nimbus experienced a bimodal fluid evolution consisting of (i) an initial intense interaction between deep-magmatic fluids and seawater that developed barren pyritic lenses, and (ii) a subsequent closure of the hydrothermal system during which the Agrich ore formed sourcing sulfur almost entirely from a deepmagmatic source.
- PublicationClumped C-O isotope temperature constraints for carbonate precipitation associated with Irish-type Zn-Pb orebodiesIreland hosts the greatest concentration of discovered zinc per square kilometre on Earth, with production from 5 carbonate-hosted deposits, including the giant Navan deposit. Clumped C-O isotope analyses of carbonates offer a powerful new technique to directly deliver accurate fluid temperatures and precise calculations of fluid O isotope compositions, offering a significant opportunity to refine the evolving genetic models, and develop new vectoring tools for exploration. We present the first clumped C-O isotope results for parageneticallyconstrained carbonate generations from a number of Irishtype Zn-Pb deposits. Preliminary analysis of hanging-wall white matrix breccias from Lisheen show non-systematic temperature variation (~100 to 170°C), with fluid d18OV-SMOW increasing with temperature. Significant variations in temperature at the thin section scale may be indicative of fluid mixing and/or multiple phases of WMB dolomite brecciation. Post-ore pink dolomite at Lisheen, and crosscutting calcite veins formed at significantly lower temperatures (67 to 42°C). Temperatures of 61 to 110°C were obtained for sphalerite-bearing calcite veins in the hanging-wall of the Randalstown Fault near Navan. These veins contain coarse sphalerite interpreted to have been remobilised from the nearby Navan orebody by a single, cool fluid (Marks, 2015). Clumped C-O data will also be presented for samples from Galmoy, Kilbricken and Castlegard ("Pallas Green"), from which existing fluid inclusion constraints are available.
- PublicationTargeting VHMS mineralization at Erayinia in the Archaean Yilgarn Craton, Western Australia: geochemical and hyperspectral halosDespite having been a target for volcanic-hosted massive sulfide (VHMS) mineralization since the 1960s, few resources have been defined in the Archean Yilgarn Craton of Western Australia. Exploration challenges associated with regolith and deep cover exacerbate the already difficult task of exploring for small, deformed deposits in stratigraphically complex volcanic terranes.
- Publication2.7 Ga plume associated VMS mineralization in the Eastern Goldfields Superterrane: insights from the Ag-Zn-(Au) Nimbus depositEconomic volcanic - hosted massive sulfide (VHMS) deposits of the Archean Yilgarn Craton, Western Australia, are restricted to zones of juvenile crust as revealed through regional Nd, Pb and Hf isotopic variations and the geochemistry of felsic volcanic rocks. Interpreted as Archean paleo - rift zones, one of these runs N - S through the Eastern Goldfields Superterrane (broadly coincident with the Kurnalpi Terrane) and is associated with the high grade ca. 2690 Ma Teutonic Bore, Jaguar and Bentley deposits, plus sub - economic VHMS mineralization further south. To date, only small, historic Cu deposits (e.g. Anaconda) and barren pyritic lenses have been recognised in the older 2.7 Ga plume - dominated lower stratigraphy of the Eastern Goldfields.