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A K-Feldspar Breccia from the MO-Cu Stockwork Deposit in the Galway Granite, West of Ireland

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A K-Feldspar Breccia from the MO-Cu Stockwork Deposit in the Galway Granite, West of Ireland Journal ofthe Geological Sociefy, London, Vol. 145, 1988, pp. 661-667, 4 figs, 2 tables. Printed in Northern Ireland A K-feldspar breccia from the MO-Cu stockwork deposit in the Galway Granite, west of Ireland J. M. DERHAM & M. FEELY Department of Geology, University College, Galway, Ireland Abstrart: A K-feldspar breccia, spatially associated with the MO-Cu mineralization of a stockwork in the Late Caledonian Galway Granite at Mace Head, is described for the first time. Detailed mapping reveals a network of breccia pods and veins over an area of approximately 6000 m’. The breccia is clast-supportedand is composed of sub-angular fragments of perthiticK-feldspar megacrysts (<10cm), granite and microgranodiorite clasts (<25 m) set in a matrix of quartz (<l cm), biotite (<5 cm) and apatite (<3 mm). Field and textural studies indicated that the feldspar megacrysts and granite clasts were brecciated and silicified as they were carried (to the present structural level) by hydrous K- and Si0,-rich fluids. The residue of these fluids crystallized to form the breccia matrix. The formation of the breccia predates the mineralized quartz veins of the MO-Cu stockwork. It is concluded that the breccia formation is genetically related to ore-forming processes in the Galway Granite. Breccia lithologies with a wide spectrum of characteristics of the batholith. It is composed of concentric arcs of are associated worldwide with molybdenumand other metal K-feldspar-rich and K-feldspar-poor varieties of theCarna concentrations especially in mineralized graniteterrains granodiorite (Fig. 1). (Sham 1978; Norman & Sawkins 1985; Scherkenbach et al. 1985;- Warnaars et al. 1985). Molybdenite,chalcopyrite, The MO-Cu mineralization at Mace Head pyrite and galena occur at a number of localities widespread in the Galway Granite. Mineralization commonly occurs in The MO-Cu mineralization at Mace Head in theCarna late-stage quartz veins and joints but is also to be found as Dome extends NE from thecentral K-feldspar-poor disseminations in aplite veins and within the host granites. granodiorite to a zone of K-feldspar-rich granodiorite along Fluorite veins containing minor sulphides occur throughout theNE shore of L. Bunnacliffa (see Fig. lb).The thebatholith, in particular within the leucogranites.A mineralization is controlled by stronga NE-trending noteworthyoccurrence of low-grade molybdeniteand structural grain present in the region.NE-trending faults chalcopyrite mineralization is located at Mace Head in the contain disseminated chalcopyrite andpyrite, whereas western end of the batholith(see below). This is a small molybdeniteconcentrations (0.02-0.11% MO) occur ina MO-Cu stockwork depositwhich has been exploredby Anglo quartz vein stockwork, although minor amounts are found Unitedand Canadian Johns-Manville. Geochemical inves- disseminated in the hostgranodiorite (see Talbot & Max tigations by thesecompanies in the area are reported by 1984; Max & Talbot 1986). Field studies of the Talbot (1973) and Talbot & Max (1984). Detailed mapping mineralization in the area of Mace Head made by Derham by JMD has demonstrated an extension of the mineralized (1986) show that there are two distinct paragenetic sub-types zone, and a result of these field studies was the discovery of of molybdenite in the stockwork. The first of these is an the K-feldspar breccia at Mace Head. It is the first of its ‘intra-vein’ molybdenite, the second is a ‘selvedge-type’ type to be described from the Galway Granite. molybdenite which occurs as coatings onthe interface between the quartz veins and the adjacentwall rock. In general all mineralized quartz veins strike approxim- Geological setting of the Galway Granite ately NEand dip NW. However, thequartz veins with The Galway Granite occupies anarea of approximately ‘intra-vein’ molybdenite dipmore steeply than the 600 km2 along the north shore of Galway Bay in the west of ‘selvedge-type’ molybdenite-bearing quartz veins. Derham Ireland. It is a late Caledonian ‘I-type’ intrusion emplaced at (1986) states ‘this suggests that there were two phases of approximatley 400 Ma (Leggo et al. 1966; Pidgeon 1969). molybdenitemineralization, associated with different tec- The countryrocks arethe synorogenic ortho-and tonic phases’. The mineralized quartz veins are transected para-gneisses of the Dalradian Massif in the north, and, in by laterbarren quartz veins (15 cm wide) and by the south, rocks of the South Connemara Groupassumed to NW-trending faults. be of Ordovician age. It is a composite batholith composed of asuite of comagmaticgranodiorites, adamellites and leucogranites. Four domalunits have been recognized by Field relationships Max et al. (1978), the Carna, Spiddal, Galway-Kilkieran and The breccia is exposedover an area of approximately Roundstone Domes which are shown on the geological map 6000m’ on the foreshore to thewest of Mace Pier (see Fig. (Fig. la). lb). Detailed mapping at a scale 1cm to 3 m of exposures in The MO-Cu stockworkdeposit at Mace Head occurs the breccia outcrop has defined a network of breccia veins within the Carna Dome which is situated at the western end and pods intersecting the Carna granodiorite (Fig. 2). The 661 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/661/4889484/gsjgs.145.4.0661.pdf by guest on 25 September 2021 662 J. M. DERHAM & M. FEELY Fig. 1. (a) Geological setting of the Galway Granite showing the location of the four granite domes. (b) The geology of the Carna Dome (adapted from Max et al. 1978) at Mace Head and the location of the mineralized zone. breccia occurs on the concave side of an arcuate intrusion of foliation. This foliated margin was formed during emplace- microgranodiorite. No exposures of breccia have been ment of the breccia. observed within the body of the microgranodiorite, however The centralparts of the breccia podsare commonly the breccia occursalong part of the contactbetween the occupied by quartz knotsmeasuring up to 0.5 m across. Carna granodiorite and the microgranodiorite. These also occur outside the breccia within the host granite. On horizontal exposures, pods of breccia reach 5 m in Some of these knotscontain clots of magnetite <20cm longest dimension andthe interconnecting breccia veins across. The exposures of breccia are cut by the commonly range in thickness from 10 to 25 cm (see Figs 3a molybdenite-bearing quartz veins which form an integral & b). Neither the pods nor the veins show any preferred part of the stockwork system in this area and are described orientation. Vertical exposures in the breccia zone show that by Max & Talbot (1986) and Derham (1986). breccia veins dipsteeply, being separated by screens and Sub-angular clasts of salmon pink K-feldspar megacrysts columns of host granite. Locally, more extensive exposures (2-5 cm in length) are the most common clast type in the show that these veins are feeders to thepods of breccia seen breccia (see Fig. 3c & d).Indeed one K-feldspar clast on horizontalsurfaces. Elsewhere, vertical sectionsreveal observed measured 10 X 20 cm. The feldspar fragments are largeblocks of granite,up to 3m in length,that are usually mantled and transected by long (5 cm), thin (2 mm) disrupted and engulfed in the breccia matrix which fills the foliae of biotite.Sub-angular torounded clasts (2mm- ramifying fractures. The contacts between the breccia and 0.5 m) of the Carna granodiorite and the microgranodiorite thegranite are usually sharpand rectilinear. However, are also present throughout. The mineralogy of the matrix is locally the breccia fluids have reacted with the host granite difficult to resolve in the field but on favourable surfaces it is andpartial resorption leads to gradationalcontacts up to seen to consist of a darkand relatively fine-grained 10cm inwidth. The margin of the breccia is sometimes crystalline aggregate of quartz and biotite. The petrography marked by abiotite-rich foliated zone (10-25 cm wide) of this matrix is described in greater detail below. consisting of thin plates of biotite (<5 mm wide) together Disseminations of molybdenite are occasionally observed with quartzand feldspar (<2mm). Clasts of granite at the contact between the breccia and the host rock, and (0.5-2 cm) and K-feldspar (1-5 cm) form augen within the also in the breccia matrix.Discontinuous quartz veinlets Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/661/4889484/gsjgs.145.4.0661.pdf by guest on 25 September 2021 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/661/4889484/gsjgs.145.4.0661.pdf by guest on 25 September 2021 Fig. 3. (a) Plate showing a typical breccia pod (outlined). Note vertical feeder vein (lower part of plate) and the centrally disposed quartz knots (outlined). Hammer length: 40 cm. (b) Plate showing typical breccia vein. Note sharp contactswith host granodiorite and late cross-cutting quartz vein. Hammerlength: 40 cm. (c) Plate showing angular clasts of K-feldspars and host granodiorite in the breccia. Lens cap: 5 cm. (a) Plate showing theclast supported nature of the breccia. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/661/4889484/gsjgs.145.4.0661.pdf by guest on 25 September 2021 Fig. 4. (a) Plane polarized light photomicrograph showing matrix mineralogy of quartz, biotite and apatite occurring betweentwo K-feldspar clasts. (b) Plane polarizedlight photomicrograph showing growth of apatite along a fracturewithin a K-feldsparclast. Note also the invasion of the feldspar clast by matrix quartz. (C) Photomicrograph (crossed polars)showing silicification textures in a K-feldspar clast. (a) photomicrograph taken in plane polarisedlight showing small clasts of K-feldspars in a channelof matrix quartz and biotite. Abbreviations: K, K-feldspar; Qtz, quartz;Bi, biotite; Ap, apatite. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/661/4889484/gsjgs.145.4.0661.pdf by guest on 25 September 2021 666 DERHAM J. M. FEELY & M.
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