The role of vapour in the transport and deposition of metals in ore-forming systems /

The solubility of gold and copper chloride in liquid-undersaturated, HCl-bearing water vapour was investigated experimentally at elevated temperatures and pressures. Experimental results show that the solubility of gold and copper is significant, increasing at higher fHCl and fH2O for gold, and at higher fH2O for copper. These increases are attributed to the formation of hydrated gas species, with a metal:chlorine ratio of 1. Hydration numbers vary from 5 at 300°C to 3 at 360°C for gold, and from 7.6 at 280°C, to 6.0 at 300°C, and 6.1 at 320°C for copper; the results reflect the presence of trimer [Cu3Cl3•(H2O)n] or tetramer [Cu4Cl4•(H2O)n] species. Results indicate that solubility for both vapour species is retrograde, i.e., it decreases with increasing temperature, and formed via the reactions: Ausolid+m·HClgas+n·H2 Ogas=AuClm·H2 Ogasn+m 2·Hgas2 3CuClsolid+n·H2Ogas= Cu3Cl3·H2O gasnCalculations based on the solubility data indicate an economic high-sulphidation Au deposit (e.g., Nansatsu, Japan; 36 tonnes) could form in ~30,000 years, whereas a porphyry copper deposit (e.g., 50 million tonnes at 0.5% Cu) could form in as little as ~20,500 years, assuming transport only in the vapour phase.Precious- and base-metal-rich composite scales, containing up to 111 ppm Au and 628 ppm Ag, occur in surface pipes at the Momotombo geothermal field, Nicaragua. Polysulphide scale fragments, comprising chalcopyrite, sphalerite, galena, electrum and hessite grains in a matrix of amorphous silica, formed as a result of cooling and ligand loss induced by boiling, during fluid ascent in well MT-36. Secondary bornite, stromeyerite and chalcocite/digenite replaced chalcopyrite through the addition of Cu and Ag and an increase in fO2 . A drop in pH due to well closure resulted in replacement of primary and secondary sulphides by tetrahedrite.Reaction-path modelling using the program CHILLER simulates deposition of minerals from the reconstructed deep geothermal fluid, at temperature intervals (depths) along excess enthalpy and isoenthalpic boiling paths. These simulations accurately reproduce the paragenetic sequence of base- and precious-metal mineralization in the scales. The modelling indicates excess enthalpy boiling results in metal precipitation at greater depths than would be expected for isoenthalpic boiling, and that at Momotombo this occurs through the destabilisation of bisulphide complexes in response to loss of CO2 and H2 S during phase separation

Petrochemistry and hydrothermal alteration within the Tyrone Igneous Complex, Northern Ireland: implications for VMS mineralization in the British and Irish Caledonides

Although volcanogenic massive sulfide (VMS) deposits can form within a wide variety of rift-related tectonic environments, most are preserved within suprasubduction affinity crust related to ocean closure. In stark contrast to the VMS-rich Appalachian sector of the Grampian-Taconic orogeny, VMS mineralization is rare in the peri-Laurentian British and Irish Caledonides. Economic peri-Gondwanan affinity deposits are limited to Avoca and Parys Mountain. The Tyrone Igneous Complex of Northern Ireland represents a ca. 484–464 Ma peri-Laurentian affinity arc–ophiolite complex and a possible broad correlative of the Buchans-Robert’s Arm belt of Newfoundland, host to some of the most metal-rich VMS deposits globally. Stratigraphic horizons prospective for VMS mineralization in the Tyrone Igneous Complex are associated with rift-related magmatism, hydrothermal alteration, synvolcanic faults, and high-level subvolcanic intrusions (gabbro, diorite, and/or tonalite). Locally intense hydrothermal alteration is characterized by Na-depletion, elevated SiO2, MgO, Ba/Sr, Bi, Sb, chlorite–carbonate–pyrite alteration index (CCPI) and Hashimoto alteration index (AI) values. Rift-related mafic lavas typically occur in the hanging wall sequences to base and precious metal mineralization, closely associated with ironstones and/or argillaceous sedimentary rocks representing low temperature hydrothermal venting and volcanic quiescence. In the ca. 475 Ma pre-collisional, calc-alkaline lower Tyrone Volcanic Group rift-related magmatism is characterized by abundant non-arc type Fe-Ti-rich eMORB, island-arc tholeiite, and low-Zr tholeiitic rhyolite breccias. These petrochemical characteristics are typical of units associated with VMS mineralization in bimodal mafic, primitive post-Archean arc terranes. Following arc-accretion at ca. 470 Ma, late rifting in the ensialic upper Tyrone Volcanic Group is dominated by OIB-like, subalkaline to alkali basalt and A-type, high-Zr rhyolites. These units are petrochemically favorable for Kuroko-type VMS mineralization in bimodal-felsic evolved arc terranes. The scarcity of discovered peri-Laurentian VMS mineralization in the British and Irish Caledonides is due to a combination of minimal exploration, poor-preservation of upper ophiolite sequences, and limited rifting in the Lough Nafooey arc of western Ireland. The geological and geochemical characteristics of the Tyrone Volcanic Group of Northern Ireland and peri-Gondwanan affinity arc/backarc sequences of Ireland and northwest Wales represent the most prospective sequences in the British and Irish Caledonides for VMS mineralization

Distribution, mineralogy and geochemistry of silica-iron exhalites and related rocks from the Tyrone Igneous Complex: Implications for VMS mineralization in Northern Ireland

Iron formations, hematitic cherts (jaspers), ‘tuffites’, silica-iron exhalites and other metalliferous chemical sedimentary rocks are important stratigraphic marker horizons in a number of volcanogenic massive sulfide (VMS) districts worldwide, forming during episodes of regional hydrothermal activity. The VMS prospective ca. 484–464 Ma Tyrone Igneous Complex of Northern Ireland represents a structurally dissected arc-ophiolite complex that was accreted to the composite margin of Laurentia during the Grampian orogeny (ca. 475–465 Ma), and a potential broad correlative to the VMS-rich Buchans–Robert's Arm arc system of the Newfoundland Appalachians. Silica-iron-rich rocks occur at several stratigraphic levels in the Tyrone Igneous Complex spatially and temporally associated with rift-related basalts (e.g., Fe–Ti-rich eMORB, IAT, OIB) and zones of locally intense hydrothermal alteration. In the ca. 475–474 Ma lower Tyrone Volcanic Group, these rocks are characterized by massive, 1–5 m thick blood-red jaspers, hematitic siltstones and mudstones, and intensely silica-hematite altered tuffs and flows. Their mineralogy is dominated by quartz–hematite ± magnetite–(chlorite-sericite ± tremolite/actinolite), with Fe concentrations rarely exceeding 10 wt.%. Relict textures (including the presence of coalesced spherules of silica-iron oxides) in rocks exposed at Tanderagee NW, Creggan Lough and Tory's Hole are indicative of seafloor exhalation, whereas replacement of the original volcanic stratigraphy is evident to varying degrees at Tanderagee, Beaghbeg and Bonnety Bush. In the structurally overlying ca. 473–469 Ma upper Tyrone Volcanic Group, chemical sedimentary rocks include recrystallized: (i) thin and laterally-restricted jaspers in thick sequences of graphitic pelite at Boheragh; and (ii) laterally-persistent sulfidic cherts and ironstones dominated by quartz–hematite–magnetite–(chlorite) or quartz–pyrite–(chlorite) in sequences of tuff at Broughderg. Compared to chemical sedimentary rocks associated with VMS deposits worldwide, their geochemical characteristics are most similar to silica-iron exhalites of the Mount Windsor Subprovince (SE Australia) and jaspers of Central Arizona, Bald Mountain (Northern Maine), the Urals, Iberian Pyrite Belt and Løkken ophiolite (Norway). Positive Eu anomalies (at Slieve Gallion and Tanderagee NW), elevated Cu + Pb + Zn, Au, Fe/Ti, Fe/Mn, Sb, Ba/Zr and Fe + Mn/Al, together with low REE, Sc, Zr and Th, are indicative of a greater hydrothermal component and potentially more VMS-proximal signatures. Based on bulk ironstone geochemistry, Bonnety Bush, Tanderagee NW-Creggan Lough, Broughderg and Drummuck (Slieve Gallion) are considered the most VMS prospective areas in the Tyrone Igneous Complex and warrant further exploration

Episodic arc-ophiolite emplacement and the growth of continental margins: Late accretion in the Northern Irish sector of the Grampian-Taconic orogeny

In order to understand the progressive growth of continental margins and the evolution of continental crust, we must first understand the formation of allochthonous ophiolitic and island-arc terranes within ancient orogens and the nature of their accretion. During the early Paleozoic closure of the Iapetus Ocean, diverse sets of arc terranes, oceanic tracts, and ribbon-shaped microcontinental blocks were accreted to the passive continental margin of Laurentia during the Grampian-Taconic orogeny. In the northern Appalachians in central Newfoundland, Canada, three distinct phases of arc-ophiolite accretion have been recognized. New field mapping, high-resolution airborne geophysics, whole-rock and Nd-isotope geochemistry, and U-Pb zircon geochronology within the Tyrone Volcanic Group of Northern Ireland have allowed all three episodes to now be correlated into the British and Irish Caledonides. The Tyrone Volcanic Group (ca. 475–469 Ma) is characterized by mafic to intermediate lavas, tuffs, rhyolite, banded chert, ferruginous jasperoid, and argillaceous sedimentary rocks cut by numerous high-level intrusive rocks. Geochemical signatures are consistent with formation within an evolving peri-Laurentian island-arc/backarc, which underwent several episodes of intra-arc rifting prior to its accretion at ca. 470 Ma to an outboard peri-Laurentian microcontinental block. Outriding microcontinental blocks played a fundamental role within the orogen, explaining the range of ages for Iapetan ophiolites and the timing of their accretion, as well as discrepancies between the timing of ophiolite emplacement and the termination of the Laurentian Cambrian–Ordovician shelf sequences. Accretion of the Tyrone arc and its associated suprasubduction-zone ophiolite represents the third stage of arc-ophiolite emplacement to the Laurentian margin during the Grampian-Taconic orogeny in the British and Irish Caledonides