"Rock of the Month # 56, posted February 2006" ---

Lamprophyres are a diverse clan of hypabyssal intrusive rocks. They typically display phenocrysts of biotite or amphibole in a cryptic groundmass, and are often rather fine-grained and/or extensively altered. They mostly occur as small dykes, which often weather recessively, and are thus volumetrically minor, rather enigmatic components of the map-areas in which they are found. These factors help to explain their determinedly obscure status amongst igneous rocks. Concise dictionary definitions for the lamprophyre family of rocks include the sense of "dark-coloured, fine-grained dyke rocks, commonly alkali-rich, calc-alkaline to ultramafic in bulk composition, porphyritic, with essential minerals a combination of biotite / amphibole / pyroxene plus feldspar or feldspathoids, plus accessory minerals which may range from iron-titanium oxides and apatite to quartz or olivine, the rock often highly altered to carbonates and sheet silicates". Rock (1987) subdivided lamprophyres into four branches: calc-alkaline lamprophyres, alkaline lamprophyres, ultramafic lamprophyres and lamproites. In economic terms, many would prefer to treat the latter separately, as lamproites and especially the somewhat similar suite of micaceous rocks known as kimberlites are the principal host rocks for diamonds on this planet! For this reason, if no other, we must leave these exotic cousins aside and focus on the widespread nature of lamprophyres of the first three groups.

This month's sample (CP88008R) was collected as a block of loose "float" from the Tiger Creek drainage, on the north side of the former Crown Point mine, a short-lived and very minor gold producer in South Belt area of the Rossland gold-mining camp at the close of the 19th century (Drysdale, 1915; Wilson et al., 1990). The hand specimen is strongly porphyritic, with coarse phenocrysts of white plagioclase feldspar and black amphibole in a fine-grained, medium grey groundmass flecked by fine-grained brown mica flakes.

A field term for such a rock, of evident intermediate composition, could be diorite porphyry or andesite porphyry, derived from a minor intrusion or porphyritic lava flow. In thin section the rock was classified as a calc-alkaline lamprophyre with a fine-grained matrix rich in carbonate, quartz, biotite, chlorite and Fe-Ti oxides. The groundmass encloses coarse and often-rounded crystals of, in decreasing order of abundance, alkali feldspar, plagioclase, pleochroic brown amphibole, biotite mica and quartz. Accessory minerals include ilmenite and magnetite, epidote and rare traces of tourmaline, pyrrhotite and chalcopyrite. The quartz in particular is rounded and deeply embayed, suggesting that it is not in equilibrium with the surrounding rock. The quartz, and some of the other minerals and rounded, mm-scale dark clasts may be xenocrysts and cognate xenoliths, derived from the dyke host rocks during emplacement. Such textures are described in lamprophyres, and are not unexpected from late intrusion of relatively primitive, reactive melts into chemically dissmilar host rocks.

However, some would say that the occurrence of feldspar as phenocrysts precludes this being a lamprophyre, where feldspar is common but generally or always as a groundmass and not a phenocryst phase (e.g., Harker, 1954; Rock, 1987). Serves me right for selecting an attractive rock which (being a loose float sample) lacks evidence of its primary field occurrence! On the basis of available evidence, this is a matter of textural classification, and the more generic term of "intermediate porphyry" could safely be substituted. Lamprophyres of varied compositions do nevertheless occur in the Rossland area, so we should conclude with a note on the wider significance of such rocks. Also, in local usage, Fyles (1984) describes a variety of dyke rocks as lamprophyres, and some of these contain feldspar phenocrysts. Around Rossland, the gold mineralization appears to be of Eocene age, long-postdating local Mesozoic monzonite, but closer in time to both local lamprophyres and other syenitic rocks of the Eocene-age, regional Coryell suite of intrusions (Gilbert, 1948; Fyles et al., 1973; Fyles, 1984; Wilson et al., 1990; Stinson and Simony, 1994; Sevigny and Theriault, 2003).

The Nature and Significance of Lamprophyres

For decades lamprophyres, like more abundant constituents of dykes and sills, such as diabase, were somewhat neglected in terms of economic geology, their occurrence serving more often to diminish the efficiency of mining operations in cases where the dykes cross-cut and offset older "pay rocks". If it can be said that every rock and mineral eventually finds an admirer then, in the case of lamprophyres, that person was the late Nick Rock who, in the decade before his untimely death in 1992, championed the humble lamprophyre more than anyone before or since, first at Cambridge University and then at the University of Western Australia. He published both comprehensive reviews of the diverse "clan" of lamprophyres (Rock, 1977, 1984, 1986, 1987, 1991), and also provocative speculations on their potential economic significance (Rock et al., 1987, 1989; Rock and Groves, 1988a,b). This work was conducted worldwide, in the British Isles, India (Rock and Paul, 1988) and Australia (Perring et al., 1989). Many lamprophyres have a decidely primitive chemistry, and besides the previous-noted association of some lamproites with diamond, lamprophyre dykes have often been noted in and around mesothermal lode gold deposits worldwide. The dyke rocks themselves are generally modest in volume and so, even if there is such a thing as a far-travelled lamprophyre magma, it may well be that these melts have served less as a significant transporter of metal, but instead are a useful indicator of crustal weakness, flagging terranes susceptible to later mineralization. Careful analyses of many lamprophyres suggests that lamprophyre melt is not appreciably gold-enriched relative to the continental crust, and that the spatial association of these curious rocks with gold deposits is related more to a common tectonic setting than to a genetic link (Wyman and Kerrich, 1988, 1989; Kerrich, 1990; Kerrich and Wyman, 1994, Wyman et al., 1995). Interest in lamprophyres will no doubt continue, and has spawned a large volume of both focused and incidental publications (of which the MINLIB database currently contains some 900 records, 1905-2006).

References

DRYSDALE,CW (1915) Geology and Ore Deposits of Rossland, British Columbia. GSC Mem. 77, 317pp. plus map folder.

FYLES,JT (1984) Geological Setting of the Rossland Mining Camp. BC MEMPR Bull. 74, 61pp. plus map folder.

FYLES,JT, HARAKAL,JE and WHITE,WH (1973) The age of sulfide mineralization at Rossland, British Columbia. Econ.Geol. 68, 23-33.

GILBERT,G (1948) Rossland camp. In `Structural Geology of Canadian Ore Deposits' (Gilbert,G editor), CIMM Jubilee Volume, 948pp., 189-196.

HARKER,A (1954) Petrology for Students. Cambridge University Press, 8th edition revised by Tilley,CE, Nockolds,SR and Black,M, 283pp., see pp.126-134.

KERRICH,R (1990) Mesothermal gold deposits: a critique of genetic hypotheses. In `Greenstone Gold and Crustal Evolution' (Robert,F, Sheahan,PA and Green,SB editors), GAC-MDD NUNA Conference Volume, 252pp., 13-31.

KERRICH,R and WYMAN,DA (1994) The mesothermal gold-lamprophyre association: significance for an accretionary geodynamic setting, supercontinent cycles, and metallogenic processes. Mineral.Petrol. 51, 147-172.

PERRING,CS, ROCK,NMS, GOLDING,SD and ROBERTS,DE (1989) Criteria for the recognition of metamorphosed or altered lamprophyres: a case study from the Archaean of Kambalda, Western Australia. Precambrian Research 43, 215-237.

ROCK,NMS (1977) The nature and origin of lamprophyres: some definitions, distinctions and derivations. Earth-Science Reviews 13, 123-169.

ROCK,NMS (1984) Nature and origin of calc-alkaline lamprophyres: minettes, vogesites, kersantites and spessartites. Trans.Roy.Soc.Edinburgh: Earth Sciences 74, 193-227.

ROCK,NMS (1986) The nature and origin of ultramafic lamprophyres: alnoites and allied rocks. J.Petrol. 27, 155-196.

ROCK,NMS (1987) The nature and origin of lamprophyres: an overview. In `Alkaline Igneous Rocks' (Fitton,JG and Upton,BGJ editors), Geol.Soc.Spec.Publ. 30, Blackwell Scientific Publications, 568pp., 191-226.

ROCK,NMS (1991) Lamprophyres. Blackie, Glasgow, 284pp..

ROCK,NMS and GROVES,DI (1988a) Do lamprophyres carry gold as well as diamonds? Nature 332, 253-255.

ROCK,NMS and GROVES,DI (1988b) Can lamprophyres resolve the genetic controversy over mesothermal gold deposits? Geology 16 no.6, 538-541.

ROCK,NMS and PAUL,DK (1988) `Lamprophyres', `lamproites' and `kimberlites' in India: a bibliography and preliminary appraisal. In `Alkaline Rocks' (Leelanandam,C editor), Geol.Soc.India Memoir 15, 311pp., 291-311.

ROCK,NMS, DULLER,P, HASZELDINE,RS and GROVES,DI (1987) Lamprophyres as potential gold exploration targets: some preliminary observations and speculations. In `Recent Advances in Understanding Precambrian Gold Deposits' (Ho,SE and Groves,DI editors), Geology Dept. and University Extension, Univ. of Western Australia, Publ. No. 11, 368pp., 271-286.

ROCK,NMS, GROVES,DI, PERRING,CS and GOLDING,SD (1989) Gold, lamprophyres, and porphyries: what does their association mean? In `The Geology of Gold Deposits: the Perspective in 1988' (Keays,RR, Ramsay,WRH and Groves,DI editors), Econ.Geol.Monograph 6, 667pp., 609-625.

SEVIGNY,JH and THERIAULT,RJ (2003) Geochemistry and Sr-Nd isotopic composition of Eocene lamphrophyre dykes, southeastern British Columbia. CJES 40, 853-864.

STINSON,P and SIMONY,PS (1994) The geology and structure of the Coryell batholith, southern British Columbia. In `Current Research: Cordillera and Pacific Margin', GSC Paper 1994-A, 243pp., 109-116.

WILSON,GC, RUCKLIDGE,JC and KILIUS,LR (1990) Sulfide gold content of skarn mineralization at Rossland, British Columbia. Econ.Geol. 85, 1252-1259.

WYMAN,DA and KERRICH,R (1988) Lamprophyres a source of gold. Nature 332, 209-210.

WYMAN,D and KERRICH,R (1989) Archean shoshonitic lamprophyres associated with Superior Province gold deposits: distribution, tectonic setting, noble metal abundances, and significance for gold mineralization. In `The Geology of Gold Deposits: the Perspective in 1988' (Keays,RR, Ramsay,WRH and Groves,DI editors), Econ.Geol.Monograph 6, 667pp., 651-667.

WYMAN,D, KERRICH,R and SUN,M (1995) Noble metal abundances of late Archean (2.7 Ga) accretion-related shoshonitic lamprophyres, Superior province, Canada. GCA 59, 47-57.