Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn–Pb–Ag deposits

The giant stratiform Zn–Pb–Ag HYC deposit displays a broad Zn, Pb and Tl halo which extends laterally along the favourable pyritic black shale facies of the Barney Creek Formation for at least 15 km west of the deposit. A ferroan dolomite/ankerite halo overlaps with the Zn–Pb–Tl halo extending up to 250 m into the immediate stratigraphic hangingwall, and 50 to 100 m into the footwall sediments close to the deposit. A manganese carbonate halo is offset from the ferroan dolomite/ankerite halo, being concentrated in the immediate footwall of the deposit and extending laterally along the W-Fold Shale Member. Manganiferous carbonate forms the most pronounced and laterally extensive halo at HYC extending well beyond the Zn–Pb–Tl and ankerite halos. The geometry and extent of the halos described above are based on sampling and analyses from two drill holes reported in this study and a further six drill holes reported previously by I.B. Lambert and K.M. Scott [J. Geochem. Explor. 2, 307–330, 1973]. The SEDEX alteration index previously proposed as a vector for the Lady Loretta deposit [R.R. Large and P.J. McGoldrick, 1998, J. Geochem. Explor. 63, 37–56] has been shown to have applications in the McArthur Basin for defining the favourable stratigraphic unit hosting stratiform Zn–Pb–Ag mineralisation. A modified alteration index (AI Mark 3) has also been developed which eliminates the effect of the shale/dolomite ratio on the index and thus highlights the control of carbonate chemistry and its relationship to stratiform mineralisation. The application of a group of indices including Zn, Pb, Tl, SEDEX alteration index, AI Mark 3 and manganese content of dolomite (MnOd) is proposed for defining the most favourable stratigraphic units for stratiform Zn–Pb–Ag mineralisation within carbonate-bearing sedimentary basins.