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Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn-Pb-Ag deposits, 1. Lady Loretta Deposit, Queensland

Large, RR and McGoldrick, PJ 1998 , 'Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn-Pb-Ag deposits, 1. Lady Loretta Deposit, Queensland' , Journal of Geochemical Exploration, vol. 63, no. 1 , pp. 37-56 , doi:

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Stratiform sediment hosted Zn-Pb-Ag 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 .
vectors useful to exploration. This study concentrates on the Lady Loretta deposit which is a typical example of the
class of Proterozoic SEDEXdeposits in northern Australia. We examined the l)1ajor 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 1.5 km 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 i,nner siderite halo
varies in composition from siderite to pistomesite (FeO.6MgOAC03), whereas carbonate in the outer ankerite halo varies from
ferroan dolomite to ankerite (Caa.sMgO.3Fea.2C03). Element dispersion around the stratiform ore lens is' variable with Pb,
Cu, Ba and Sr showing very little dispersion «50 m across strike), Zn and Fe showing moderate dispersion « 100 m) and
Mn and TI showing broad dispersion «200 m). Within the siderite halo Cu, Mg and Na show marked depletion compared
to the surrounding sediments. The magnitude of element dispersion mid 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: (I) SEDEX metal index = Zn + lOOPb + lOOTI; (2) SEDEX
alteration index = (FeO + lOMnO)IOO/(FeO + 10MnO + MgO); (3) manganese content of dolomite: MnOd = (MnO
x 30.41)/CaO. All three vectors increase to ore both across strike and along strike. The manganese content of dolomite
(MnOd) 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,
TI, metal index, alteration index, MnOd and MnOs ) is applicable in the exploration for stratiform Zn-Pb-Ag deposits in
dolomite-rich sedimentary basins generally. The indices defined can firstly assist in the identification of sedimentary units
favourable for SEDEX mineralisation, and secondly provide vectors along these units to ore. The alteration index and
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38 R.R. Large, P.J. McGoldrick/Journal of Geochemical Exploration 63 (1998) 37-56
MnOd' however, should only be used for exploration dolomitic sequences; they are not recommended for exploration in
clastic sequences devoid of carbonates. © 1998 Elsevier Science B.Y. All rights reserved.

Item Type: Article
Authors/Creators:Large, RR and McGoldrick, PJ
Keywords: Sedex Zinc-Lead, manganese Carbonate, Manganese halo, siderite, mineral exploration, Sedex alteration index
Journal or Publication Title: Journal of Geochemical Exploration
DOI / ID Number:
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