SKARN DEVELOPMENT FROM LIMESTONE ADJACENT TO THE GLENROCK GRANODIORITE, MARULAN BATHOLITH, NEW-SOUTH-WALES, AUSTRALIA

Skarns are developed over two temperature-time intervals in calcite li mestone adjacent to the southern extension of the Glenrock Granodiorit e, a pluton of the Marulan Batholith, Southern Highlands, New South Wa les. The initial volumetrically-dominant prograde phase of skarn forma tion produced a suite comprising bimetasomatic skarn, including pyroxe ne endoskarn, potassic endoskarn and wollastonite-bearing exoskarn, to gether with mineralogically-zoned vein skarn, massive garnet-pyroxene skarn and calcite-vesuviamite skarn. Retrograde replacement is manifes ted by the development of hydrous silicate minerals, carbonate and cro sscutting sulphide veinlets. A genetic model is proposed to account fo r the development of bimetasomatic skarn in the deposit. Exoskarn geoc hemistry indicates addition af many components relative to an essentia lly pure limestone precursor, including Si, Al, Fe, Zr, Zn, S, Mn and Cu, negligible transfer of K, Na and Rb and loss of CO2. Strontium and Ca loss from the parent limestone is indicated by mass balance calcul ations at constant volume. Garnet and pyroxene compositions in the mas sive garnet-pyroxene skarn range from Gr(30) to Gr(66) and Hd(61) to H d(87), respectively. Compositions from Gr(67) to Gr(95) are typical of the vein skarn garnets. Chemical zonation patterns in garnet, pyroxen e and vesuvianite are generally characterized by rim Fe depletion rela tive to cores of grains. Prograde skarn probably formed at T = 500-580 degrees C; P < 220 MPa. The massive garnet-pyroxene skarn evolved und er conditions of log fO(2) = -18.9 to -22.9 (assuming a constant fCO(2 ) of 20 MPa) within the fS(2) stability field of pyrrhotite. Retrograd e skarn formed at T < 400 degrees C, possibly under conditions of XH(2 )O < 0.01.