Skarn development from limestone adjacent to the Glenrock Granodiorite, Marulan Batholith, New South Wales, Australia
Titel:
Skarn development from limestone adjacent to the Glenrock Granodiorite, Marulan Batholith, New South Wales, Australia
Auteur:
Chenhall, B. E. Mazaheri, S. A.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 40 (1993) nr. 5 pagina's 485-500
Jaar:
1993-10
Inhoud:
Skarns are developed over two temperature-time intervals in calcite limestone adjacent to the southern extension of the Glenrock Granodiorite, a pluton of the Marulan Batholith, Southern Highlands, New South Wales. The initial volumetrically-dominant prograde phase of skarn formation produced a suite comprising bimetasomatic skarn, including pyroxene endoskarn, potassic endoskarn and wollastonite-bearing exoskarn, together with mineralogically-zoned vein skarn, massive garnet-pyroxene skarn and calcite-vesuvianite skarn. Retrograde replacement is manifested by the development of hydrous silicate minerals, carbonate and cross-cutting sulphide veinlets. A genetic model is proposed to account for the development of bimetasomatic skarn in the deposit. Exoskarn geochemistry indicates addition of many components relative to an essentially 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 calculations at constant volume. Garnet and pyroxene compositions in the massive garnet-pyroxene skarn range from Gr30 to Gr66 and Hd61 to Hd87, respectively. Compositions from Gr67 to Gr95 are typical of the vein skarn garnets. Chemical zonation patterns in garnet, pyroxene and vesuvianite are generally characterized by rim Fe depletion relative to cores of grains. Prograde skarn probably formed at T = 500-580°C; P < 220 MPa. The massive garnet-pyroxene skarn evolved under conditions of log fO2 = -18.9 to -22.9 (assuming a constant fCO2 of 20 MPa) within the fS2 stability field of pyrrhotite. Retrograde skarn formed at T < 400°C, possibly under conditions of XH2O < 0.01. Vesuvianite plus wollastonite assemblages, present in exoskarn, probably attest to very water-rich conditions. The marble wall rocks, isolated from the source of skarn-forming fluids, probably evolved under conditions of minimum Xco2 >0.2. Low temperature CO2 -rich fluid inclusions and prehnite (stable at Xco2 <0.01), present in the marble and skarn, respectively, suggest that substantial differences in Xco2: XH2O were maintained during cooling. Observed mineralogical and chemical zonation within the skarn reflects the complex interaction of T, P, fO2, Xco2 and other chemical variables such as aSiO2 and aAl2O3 throughout the skarn system. No single variable can account adequately for the mineralogical diversity observed in the skarn deposit.
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