Abstract
A km-‐scale zone of enriched molybdenum (Mo) occurs where Cretaceous felsic-‐intermediate granitoids of Rahu Suite Berlins Porphyry intrude greenschist facies Greenland Group metasedimentary rocks in Cascade Creek, Westland. Three major granitoid units are identified in the area. Each is compositionally similar but texturally distinct; a grey-‐white granodiorite is most abundant and dark hyalodacites and grey-‐pink porphyritic granodiorites scarce. The intrusions are surrounded by up to 500 m wide zones of hornfelsed Greenland Group. The aureole comprises albite-‐epidote facies mineralogy at its margins and extent to hornblende hornfels facies adjacent to Berlins Porphyry. The rocks have been heated sufficiently that orthoclase spotting has developed within 40 m of the contact.
A 1500m x 500 m mineralised belt occurs at the contact of porphyritic granodiorite and Greenland Group. Molybdenite is most abundant and is accompanied by Cu, Zn, Fe, Bi, Ba, Pb-‐bearing minerals. The molybdenite occurs in three principal mineralisation styles: as ‘fill’ on Greenland Group joint planes; as inclusions within quartz veins/selvages within Greenland Group rocks; and disseminated within Greenland Group and porphyritic lithologies. Aside from Mo, the metals occur in alteration shells surrounding the porphyritic granodiorite. The most proximal zone contains propylitic pyrite, chlorite, aikinite and rutile veins. This shell grades into pervasive argillic-‐phyllic alteration that is characterised by the same mineral assemblage except that chlorite is replaced by muscovite. At the innermost boundary of this shell, the veins house symplectite intergrowths of chalcopyrite and sphalerite. From this position inwards, molybdenum increases in abundance with the greatest veining occurring at the intrusive contact.
The centripetal alteration halo is interpreted to represent an increase in pH and decrease in temperature outwards from the central intrusion. The propylitic and phyllic assemblages overprint the earlier-‐formed potassic core, suggesting that the alteration zones have telescoped upon thermal relaxation of the system. This has likely depleted the metal tenures and decreased the economic potential of the deposit. Although the intrusive rocks are chemically favourable for mineralisation, the quartzofeldspathic host lithology is chemically unfavourable due to a lack of carbonate material. Coupling this with the single pulse of intrusive activity has resulted in a singular pulse of hydrothermal activity that was ultimately insufficient to concentrate metals to levels of economic interest.