Redmond P.B.,Stanford University |
Redmond P.B.,Teck Ireland Ltd. |
Einaudi M.T.,Stanford University
Economic Geology | Year: 2010
The Bingham Canyon porphyry copper-gold-molybdenum deposit is one of the largest and highest-grade porphyry orebodies in the world. This study focused on the northwest side of the deposit where quartz monzonite porphyry (QMP), the first and largest porphyry intrusion, hosts the bulk of the high-grade copper-gold ore (>1.0% Cu, >1.0 ppm Au). The north-northeast-trending, high-grade zone had pre-mining dimensions of 1,500 m strike, >300 m vertical, and 500 m width and contained more than 500 million tonnes (Mt) of ore associated with potassic alteration and abundant quartz veins. The lack of superimposed sericitic alteration yielded ideal exposures in which to study the early, high-temperature stages of ore formation, a style of mineralization that in many porphyry deposits represents the major period of copper introduction. We mapped multiple porphyry dikes in the sequence: (1) QMP, (2) latite porphyry (LP), (3) biotite porphyry (BP), (4) quartz latite porphyry breccia (QLPbx), and (5) quartz latite porphyry (QLP). Porphyry dikes, faults, and quartz veins are steeply dipping and have two dominant orientations; north-northeast- and northweststriking. Dikes have a north-northeast strike but they thicken and develop northwest-trending apophyses and host high-grade copper-gold zones at intersections with northwest-faults, indicating that magmatic-hydrothermal fluids were focused by these structural intersections. Each porphyry intrusion was accompanied by a similar sequence of veins, potassic alteration, and sulfides. Biotite veinlets were followed by fractures with early dark micaceous (EDM) halos of sericite, K-feldspar, biotite, andalusite, and local corundum containing disseminated bornite-chalcopyrite-gold. EDM halos are cut by multiple generations of A-quartz veins representing the main Cu-Au ore-forming event. Postdating all intrusions are quartz-molybdenite veins followed by quartz-sericite-pyrite veins. Cathodoluminescence (CL) petrography identified distinct A-quartz veinlets consisting of dark-luminescing quartz filling fractures and dissolution vugs in earlier A-quartz veins and adjacent porphyry wall rock. These veinlets contain abundant bornite and chalcopyrite and minor K-feldspar and are closely linked in time to the introduction of the bulk of the copper and gold. Although a similar sequence of veins was repeated on emplacement of all porphyry intrusions, the vein density and intensity of potassic alteration declined with time. The youngest porphyry, QLP, is mostly weakly mineralized and locally unaltered. These observations indicate that magmatic-hydrothermal fluids underwent a similar physiochemical evolution during and immediately following emplacement of each of several porphyry dikes. The relationship between EDM veins and A-quartz veins requires that the flux of magmatic fluid from the magma chamber occurred in an episodic manner as opposed to a continuous discharge. Vein truncation relationships coupled with abrupt changes in copper-gold grades, sulfide ratios, and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive porphyry intrusive-hydrothermal cycles, presumably due to depletion of metals and volatiles in the underlying magma chamber. ©2010 Society of Economic Geologists, Inc.
de Morton S.N.,University of Melbourne |
Wallace M.W.,University of Melbourne |
Reed C.P.,Teck Resources Chile Limitada |
Hewson C.,Teck Ireland Ltd. |
And 3 more authors.
Sedimentary Geology | Year: 2015
Recently acquired seismic reflection data, combined with detailed subsurface stratigraphic analysis (core analysis and gamma ray logs) reveal a new view of Lower Carboniferous stratigraphy and tectonism in Ireland. Seismic stratigraphic relationships and stratal thickness variations within Tournaisian units indicates that the Ballinalack High (and associated faulting) was produced by tectonism during the mid to late Tournaisian (Moathill Event, ~ 348 Ma). A second major tectonic event, dominated by regional subsidence (rather than faulting), occurred during the Lower Viséan (Tober Colleen Event, ~ 345 Ma). Each of these tectonic events was associated with major subsidence in the basin, producing strong transgressions within the stratigraphy. We suggest that the Late Tournaisian Moathill Event was responsible for producing the structural setting of the Ballinalack and other Zn-Pb deposits in the Irish Midlands. The suggested earlier timing of fault movement in the basin has implications for arguments about the origin of Irish-type Zn-Pb deposits and the necessity (or not) for having active faulting during mineralization. © 2015 Elsevier B.V.
McCusker J.,Teck Ireland Ltd. |
Reed C.,Teck Ireland Ltd.
Mineralium Deposita | Year: 2013
The Stonepark Prospect is located in County Limerick, south-central Ireland. Multiple zones of Zn-Pb mineralisation have been identified at Stonepark and these are approximately 5 km west of the Pallas Green Prospect. At Stonepark, the sulphide bodies are hosted within the Waulsortian Limestone and closely resemble other Irish-type deposits. The mineralisation is composed of pyrite-marcasite, sphalerite and galena with gangue Fe-dolomite and calcite cements. A key difference at Stonepark is the presence of Chadian-aged volcanic rocks (Knockroe Volcanics) that intrude into and overlie the Waulsortian Limestone. Subsequent hydrothermal brecciation of the Waulsortian Limestone and Knockroe intrusions resulted in the formation of tabular polymict breccia bodies containing mixed carbonate and clasts of intrusive rocks. These have then been overprinted by massive sulphide mineralisation. Further syn-mineralisation brecciation has overprinted the earlier breccias. Drilling has demonstrated a spatial relationship between the volume of intrusive rocks (dykes and polymict breccias) and Zn-Pb mineralisation. This association suggests that the intrusive rocks provided a mechanism for the introduction of the mineralising fluids into the breccia bodies. This is significant as to date no large controlling fault has been identified, as is seen at other Irish-type deposits. Further work is required to understand the alteration process of the intrusive rocks and how this may relate to the mineralising process. © 2013 Springer-Verlag Berlin Heidelberg.
Elliott H.A.L.,UK National Oceanography Center |
Gernon T.M.,UK National Oceanography Center |
Roberts S.,UK National Oceanography Center |
Hewson C.,Teck Ireland Ltd.
Bulletin of Volcanology | Year: 2015
Lead-zinc exploration drilling within the Limerick Basin (SW Ireland) has revealed the deep internal architecture and extra-crater deposits of five alkali-basaltic maar-diatremes. These were emplaced as part of a regional north-east south-west tectonomagmatic trend during the Lower Carboniferous Period. Field relationships and textural observations suggest that the diatremes erupted into a shallow submarine environment. Limerick trace element data indicates a genetic relationship between the diatremes and extra-crater successions of the Knockroe Formation, which records multiple diatreme filling and emptying cycles. Deposition was controlled largely by bathymetry defined by the surrounding Waulsortian carbonate mounds. An initial non-diatreme forming eruption stage occurred at the water-sediment interface, with magma-water interaction prevented by high magma ascent rates. This was followed by seawater incursion and the onset of phreatomagmatic activity. Magma-water interaction generated poorly vesicular blocky clasts, although the co-occurrence of plastically deformed and highly vesicular clasts indicate that phreatomagmatic and magmatic processes were not mutually exclusive. At a later stage, the diatreme filled with a slurry of juvenile lapilli and country rock lithic clasts, homogenised by the action of debris jets. The resulting extra-crater deposits eventually emerged above sea level, so that water ingress significantly declined, and late-stage magmatic processes became dominant. These deposits, largely confined to the deep vents, incorporate high concentrations of partially sintered globular and large ‘raggy’ lapilli showing evidence for heat retention. Our study provides new insights into the dynamics and evolution of basaltic diatremes erupting into a shallow water (20–120 m) submarine environment. © 2015, Springer-Verlag Berlin Heidelberg.