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

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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, 63 (1). pp. 37-56.

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Abstract

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 • Tel.: 61 362262819; Fax: 61 362667662; E-mal: ross.large@utas.edu.au; p.mcgoldrick@utas.edu.au 0375-6742/98/$19.00 © 1998 Elsevier Science B.Y. All rights reserved. PH: SO 375 - 6 7 4 2 (98) 000 I 3 - 2 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
Keywords: Sedex Zinc-Lead, manganese Carbonate, Manganese halo, siderite, mineral exploration, Sedex alteration index
Journal or Publication Title: Journal of Geochemical Exploration
Page Range: pp. 37-56
Identification Number - DOI: 10.1016/S0375-6742(98)00013-2
Additional Information: The definitive version is available online at http://www.sciencedirect.com/
Date Deposited: 14 Jul 2008 13:10
Last Modified: 18 Nov 2014 03:45
URI: http://eprints.utas.edu.au/id/eprint/7057
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