Stratiform sediment hosted Zn-PbAg deposits, often referred to as SEDEX deposits, represent an economically important class of ore, that have received relatively little attention in terms of defining lithochemical halos and geochemical vectosr useful to exploration. This study concentrates on the Lady Loretta deposit which is a typical example of the class of Proterozoic SEDEX deposits in northern Australia. We examined the major and trace element chemistry of carbonate-bearing sediments surrounding the deposit and defined a series of halos which extend for several hundred metres across strike and up to 1.5km along strike. The stratiform ore lens is surrounded by an inner sideritic halo [Carr, G.R., 1984. Primary geochemical and mineralogical dispersion in the vicinity of the Lady Loretta Zn-Pb-Ag deposit, North Queensland. J.Geochem. Expl. 22, 217-238], followed by an outer ankerite/ferroan dolomite halo which merges with low iron dolomitic sediments representative of the regional background compositions. Carbonate within the inner siderite halo varies in composition from siderite to pistomesite (Fe0.6Mg0.4CP3), whereas carbonate in the outer ankerite halo varies from ferroan dolomite to ankerite (Ca0.5Mg0.3Fe0.2CO3). Element dispersion around the stratiform ore lens is variable with Pb, Cu, Ba and Sr showing very little dispersion (<50m across strike), Zn and Fe showing moderate dispersion (<100m) and Mn and Tl showing broad dispersion (<200m). Within the siderite halo Cu, Mg and Na show marked depletion compared to the surrounding sediments. The magnitude of element dispersion and change in carbonate chemistry around the Lady Loretta orebody has enabled the development of three geochemical vectors applicable to exploration. Whole rock analyses are used to calculate the three vector quantities as follows: 1) SEDEX metal index = Zn + 100Pb + 100Tl; 2) SEDEX alteration index = (FeO + 10MnO)100/(FeO + 10MnO + MgO); 3) manganese content of dolomite: MnOd = (MnO × 30.41)/CaO. All three vectors incresae to ore both across strike and along strike. The manganese content of dolomite (MnO4) exhibits the most systematic pattern increasing from background values of about 0.2 wt% to a maximum of around 0.6 wt% at the boundary between the ankerite and siderite halos. Siderite within the inner halo contains considerably more Mn with MnO values of 0.4 to 4.0 wt%. It is suggested here that the basket of indices defined at Lady Loretta (Zn, Tl, metal index, alteration index, MnOd and MnOs) is applicable in the exploration for statiform Zn-Pb-Ag deposits in dolomite-rich sedimentary basins generally. The indices defined can firstly assist in the identification of sedimentary units favourable for SEDEDX mineralisation, and secondly provide vectors along these units to ore. The alteration index and MnOd, however, should only be used for exploration dolomotic sequences; they are not recommended for exploration in clastic sequences devoid of carbonates. | Stratiform sediment hosted Zn-Pb-Ag deposits, often referred to as SEDEX deposits, represent an economically important class of ore. Lady Loretta deposit is a typical example of the class of Proterozoic SEDEX deposits in northern Australia. The major and trace element chemistry of carbonate-bearing sediments surrounding the deposit was examined and a series of halos which extend for several hundred meters across strike and along strike was defined. The magnitude of element dispersion and change in carbonate chemistry around the Lady Loretta ore body has enabled the development of three geochemical vectors applicable to exploration. The three vector quantities are as follows: SEDEX metal index; SEDEX alteration index and manganese content of dolomite.