Regional oxygen isotope zonation at Broken Hill, New South Wales, Australia: Large-scale fluid flow and implications for Pb-Zn-Ag mineralization

Metasedimentary and metavolcanic rocks at Broken Hill, Australia, show regi onal-scale lowering of delta(18)O values from as high as 16 per mil in Para gon Group metasedimentary rocks to values as low as 7 per mil within a few hundreds of meters of Pb-Zn-Ag orebodies. Such large-scale oxygen isotope r esetting cannot be achieved by closed-system processes (such as partial mel ting or devolatization), implying that the rocks were affected by fluid flo w The preservation of peak metamorphic O-18 fractionations between coexisti ng minerals. and a lack of correlation between delta(18)O values and the in tensity of retrogression, suggests that oxygen isotope resetting occurred a t, or prior to, the peak of regional metamorphism. Fluid flow during the pe ak of regional metamorphism is unlikely due to widespread fluid-absent part ial melting and internal buffering of volatile activities at that time. Thu s, fluid flow most probably predated regional metamorphism. The association of rocks with low delta(18)O values with the Pb-Zn-Ag orebodies suggests a link with mineralization. The delta(18)O values of rocks adjacent to the o rebodies are similar to those recorded in volcanic-hosted massive sulfide d eposits where convective circulation of ocean water has occurred. Base meta l mineralization at Broken Hill may have occurred at hydrothermal vents at, or close to, the sea floor producing localized low delta(18)O values with later kilometer-scale fluid circulation, possibly driven by the same or rel ated intrusions, causing regional resetting of oxygen isotopes. However, mi neralization in skarn systems associated with pre-regional metamorphic gran ites is also a plausible model that could explain the regional delta(18)O t rends and much of the other geochemical data. Alternatively: the large-scal e resetting of oxygen isotopes may be unrelated to mineralization. While th e stable isotope data may not unambiguously constrain the origin of the bas e metal orebody they indicate that mineralization most probably occurred pr ior to regional metamorphism.