Geology and genesis of the F-Sn-W (-Be-Zn) skarn (wrigglite) at Moina, Tasmania

The Moina skarn deposit, with its associated Sn-W-F veins and greisen, occurs at the margin of the Dolcoath leucogranite. The skarn occurs as a thick horizontal plate approximately one km in its longest dimension and up to 100 m thick and is separated from the granite9s upper near-horizontal contact by approximately 200 m of the Moina sandstone. The necessary plumbing system for access of mineralizing fluids is probably a series of east-west-trending tension fractures, now Sn-W quartz veins, associated with a major northwest-southeast-trending fault known as the Bismuth Creek fault. Emplacement of the granite was at shallow depths ( 80 vol % wollastonite); and (e) a granular garnet-pyroxene-vesuvianite-fluorite skarn overlying the other units. Unit (e) is relatively enriched in boron ([asymp] 600 ppm).The skarn unit carries up to 25 weight percent F; 0.6 percent Sn, 0.5 percent W, 0.2 percent Be, 27.5 percent Zn, and 4.5 ppm Au. Sn, Be, and Fe values increase toward the upper part of the skarn sequence whereas Zn, Cu, and Mo values are erratic. Secondary Zn-Cu-In-Cd-Au sulfide-amphibole alteration of the primary F-Sn-Be oxide skarn is related to the Bismuth Creek fault. When the primary wrigglite skarn is altered, Sn is largely lost from that part of the skarn.