Criteria for the recognition of Archean calc-alkaline lamprophyres: examples from the Abitibi Subprovince

Abstract

Lamprophyres are minor intrusions with atypical sources and crystallisation sequences. Among lamprophyres, the calc-alkaline (CAL) type on which this study focuses has the least distinctive chemistry and petrology. CAL correspond to small-volume mafic intrusions characterised by the early fractionation of amphibole and (or) biotite. In the Archean Superior Province (Canada), CAL are temporally and spatially related to several gold deposits and may thus be relevant to mineral exploration. This study focuses on several altered and metamorphosed intrusions of the Abitibi and La Grande subprovinces, which were designated lamprophyres based on field observations. Several criteria established from thin sections, whole-rock chemical analyses, and SEM data are applied to the studied rocks to distinguish CAL from other types of magma. As a result, only one of the studied dykes has the morphology, chemistry, and petrology typical of CAL, while the other intrusions are either too altered to be classified or may correspond to metamorphosed and metasomatized gabbro and diorite. This study shows that thin sections and whole-rock chemical analyses are not always sufficient to unequivocally classify an altered and metamorphosed intrusion as a CAL. Also, intrusions as challenging to recognise as CAL should not be used by exploration geologists to prospect for orogenic gold deposits. Much remains to be done to document the distribution and volume represented by lamprophyres in Archean greenstone belts and to confirm their spatial dependence with gold deposits.