Abstract

With a resource of ∼ 2.8 Mt at 30%–35% wollastonite occurring at a depth of about 75 m, the Olden (formerly Hawley) prospect is the largest of a swarm of skarns hosted by amphibolite facies, dominantly calcitic marble that occurs adjacent to and as inliers within the Mountain Grove pluton. The post-kinematic intrusion comprises units with a wide compositional range from anorthositic gabbro to alkali-feldspar granite and syenite and widely exhibits megascopic fabrics recording magma comingling and mixing. Isotope dilution – thermal ionization mass spectrometry (ID–TIMS) dating of zircon separates from a hornblende diorite unit yields a 207Pb–206Pb age of 1153 ± 2 Ma, significantly older than the contiguous 1070 ± 3 Ma McLean granite. Al-in-hornblende geobarometry on several Mountain Grove units indicates that the intrusion crystallized at pressures of 250–470 MPa, equivalent to mesozonal depths of 10–15 km. The main exoskarn body, ∼200 m long and up to 50 m wide, is dominated by wollastonite, clinopyroxene (Di73–94Hd2–19), and calcic garnet (Gr52–83And12–37) and ranges from massive to podiform. The eastern termination exhibits rhythmically alternating wollastonite- and calcite-rich layers, 10 cm wide and locally with chevron-shaped crenulations. These layers bear no relationship to bedding or metamorphic foliation and are interpreted as “wrigglitic,” i.e., they are a record of a metasomatic front that migrated. Veins of garnet–pyroxene–vesuvianite cut the main exoskarn. Retrograde phlogopite-rich skarns, with erratic serpentine and brucite, contain variable sphalerite and pyrite. Restricted pyroxene–garnet (–wollastonite–scapolite) endoskarn is developed in intrusive rocks contiguous with the exoskarn. Skarn development is ascribed to H2O-rich (XCO2 < 0.3) magmatogene brines and high temperatures (T = 500–650 °C), which caused intense Si, Al, and Fe metasomatism of the marbles, hydrothermal activity taking place at considerable depth. The occurrence of wollastonite around the periphery of the small Long Lake sphalerite deposit, restricted to a marble roof pendant in the Mountain Grove pluton 2.1 km east-northeast of the Olden prospect, indicates that this base-metal mineralization may be an exoskarn, rather than metamorphosed Mississippi Valley type. Incremental-heating 40Ar–39Ar dating of hornblende and biotite from the Mountain Grove diorite yields plateau ages of 1058 ± 14 and 1047 ± 4 Ma, respectively, and an exoskarn phlogopite age of 1074 ± 5 Ma. A genetic relationship between hydrothermal activity and the McLean pluton cannot be ruled out, but a parental role for the older Mountain Grove pluton is favoured on the basis of the close areal relationships of skarn bodies and that intrusion.

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