Geological setting, paragenesis and fluid history of the Walford Creek Zn‐Pb‐Cu‐Ag prospect, Mt Isa Basin, Australia*

Abstract

The low‐grade Walford Creek Zn‐Pb‐Cu‐Ag deposit is located on the faulted northern margin of the Proterozoic Mt Isa Basin. Mineralisation is localised by the Fish River growth fault and its intersection with reduced organic‐rich shale units within the Mt Les Siltstone. The Mt Les Siltstone (1640 ± 7 Ma) is a fine‐grained, mixed clastic‐carbonate sequence deposited predominantly below storm wave‐base in a basin controlled by synsedimentary faulting. Four sulfide generations are recognised. Stage I consists of synsedimentary pyrite and marcasite deposited within three lenses as microbially precipitated mounds from subaqueous springs that vented from the Fish River Fault, with evidence for extensive detrital reworking. Early diagenetic stratiform and stratabound, primary porosity cavity‐fill sphalerite, galena with or without pyrite and quartz mineralisation (Stage II) has overprinted Stage I sulfides. Fluid‐inclusion data are consistent with metal transport by >180°C NaCI‐rich fluids (6–7 wt% eq. NaCI), and with precipitation by mixing with cool, low‐salinity (1 wt%), dilute fluids (<50°C). Mineralisation formed shortly after sediment deposition from connate lacustrine water, or during uplift and erosion of the succession prior to deposition of the overlying Doomadgee Formation. In the second case, the low‐salinity fluids are meteoric groundwater. Stage III sulfides consist of cavity‐filling and vein sphalerite, galena and chalcopyrite. Stage III was deposited by cooling and mixing of hypersaline (150–170°C) MgCI2 brines (∼25 wt% eq.) with ambient (<70°C) CaCI2‐MgCI2 brines. Salinities for all Stage III fluids range between 18 to 25 wt%. Mineralisation post‐dates stylolitisation and probably formed at a depth of 1–2 km. Stage IV mineralisation comprises vein chalcopyrite, sphalerite and galena, which were deposited by conductive cooling in association with reduction of hypersaline (125°C) CaCI2 brines (∼30 wt% eq.). Stages III and IV formed after deposition of the Doomadgee Formation, but prior to maximum burial. The Walford Creek mineralisation shows affinities with both early sediment‐hosted stratiform and late Mississippi Valley‐type styles of mineralisation. The diversity of mineralisation styles are a product of multiple mineralisation events that extended from sedimentation through to deep burial. The prolonged mineralisation history has resulted from the Fish River Fault episodically tapping deep‐seated hydrothermal fluids of varying compositions. The local sedimentary fades have been an efficient base‐metal trap from the time of sedimentation to late diagenesis or later.