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

Abstract

Metasedimentary and metavolcanic rocks at Broken Hill, Australia, show regional-scale lowering of 8•sO values from as high as 16 per mil in Paragon 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 resetting cannot be achieved by closed-system processes (such as partial melting or devolatization), implying that the rocks were affected by fluid flow. The preservation of peak metamorphic •sO fractionations between coexisting minerals, and a lack of correlation between 81sO values and the intensity of retrogression, suggests that oxygen isotope resetting occurred at, or prior to, the peak of regional metamorphism. Fluid flow during the peak of regional metamorphism is unlikely due to widespread fluid-absent partial melting and internal buffering of volatile activities at that time. Thus, fluid flow most probably predated regional metamorphism. The association of rocks with low 8•sO values with the Pb-Zn-Ag orebodies suggests a link with mineralization. The 8•sO values of rocks adjacent to the orebodies are similar to those recorded in volcanic-hosted massive sulfide deposits where convective circulation of ocean water has occurred. Base metal mineralization at Broken Hill may have occurred at hydrothermal vents at, or close to, the sea floor producing localized low 8•sO values with later kilometer-scale fluid circulation, possibly driven by the same or related intrusions, eausing regional resetting of oxygen isotopes. However, mineralization in skarn systems associated with pre-regional metamorphic granites is also a plausible model that could explain the regional 8•sO trends and much of the other geochemical data. Alternatively, the large-scale resetting of oxygen isotopes may be unrelated to mineralization. While the stable isotope data may not unambiguously constrain the origin of the base metal orebody, they indicate that mineralization most probably occurred prior to regional metamorphism.

Timing and nature of fluid flow and alteration during Mesoproterozoic shear zone formation, Olary Domain, South Australia

“…(2000) inferred a mixed reservoir of fluids for calcsilicate alteration sourced from the devolatilization of a rift related volcano‐sedimentary sequence and the intrusion of the regional S‐type granites during the later stages of the D3 event. This is in contrast to the findings of Cartwright (1999) who concluded that fluids involved in the resetting of δ 18 O values pre‐dated regional metamorphism and granite generation because of fluid‐absent partial melting occurring during peak regional metamorphism. Cartwright (1999) further suggests that lack of correlation between δ 18 O values and widespread retrograde metamorphic assemblages indicates it is unlikely that retrogression involved large volumes of accompanying fluid flow.…”

“…Previous studies (Ashley, 2000; Skirrow & Ashley, 2000; Teale & Fanning, 2000) have focused on the isotopic signatures of and timing of fluid flow with respect to the mineralization in the region. Cartwright (1999) and Doyle & Cartwright (2000) have carried out an investigation of stable isotopic systematics of fluid flow in shear zones in the Broken Hill Domain, in relation to the Broken Hill Pb‐Zn‐Ag mineralization system. These studies have proposed a hybrid magmatic–metamorphic source of fluids with an either syn‐ or post‐orogenic timing for alteration.…”

Stable isotopic constraints on Kuroko-type paleohydrothermal systems in the Mesoproterozoic Serra do Itaberaba group, São Paulo State, Brazil

“…8). A similar pattern also is observed in most VMSDs (Green et al, 1983;Ohmoto, 1996;Gemmell et al, 1998;Cartwright, 1999;Shikazono, 2003). Alteration of CZ2 rocks must have been produced by higher temperature fluids compared with those responsible for the alteration of rocks from the CZ1, which caused oxygen isotopic shifts toward lower values.…”

“…The d 18 O halos are opposite those of most Archean and Phanerozic VMSDs, including Kuroko-type deposits, in which 18 O values decrease toward the mineralized zone and increase outward from this zone (Barrett and MacLean, 1994;Ohmoto, 1996;Vásquez et al, 1998;Gemmell et al, 1998;Cartwright, 1999;Shikazono, 2003). However, at the Kidd Creek VMSD, a reverse d 18 O pattern, similar to that of the Serra do Itaberaba Group, is observed at both a decimeter scale beneath the mineralization zone and a regional scale with higher whole-rock 18 O values near the mineralized zone.…”

Mineralization zoning in Yindongzi-Daxigou barite-siderite, silver-polymetallic deposits in the Qinling orogen, China

“…Interest in the ore-forming history of sedimentary rocks and basins has grown rapidly since 1960 and is now intensified ( Cartwright, 1999 ;Bechtel and Elliott, 1999 ;Raymond and Peter, 1999 ;Dooley and McClay, 1997) . The main reason behind the acceleration is the increasing awareness that the key factors responsible for the formation of metal deposits in a sedimentary basin are the source rocks beneath the basin and intensive hydrothermal activity in it .…”