Primary geochemical and mineralogical dispersion in the vicinity of the Lady Loretta Zn-Pb-Ag deposit, Northwest Queensland |
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| Authors: | GR Carr |
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| Abstract: | Lady Loretta is a stratiform Zn-Pb-Ag deposit occurring within carbonate- and pyrite-rich argillaceous rocks of the southern part of the Lawn Hill Platform, northwestern Queensland. The deposit consists of a single massive sulphide lens within a steep-sided basinal structure and contains reserves of 8 690 000 tonnes of ore at 18.1% Zn, 6.7% Pb and 110 g t−1 Ag.The proportion of pyrite-rich layers in the host rocks increases progressively from about 2% at 150 m stratigraphically below the ore to > 90% in the top 10 m of the footwall rocks. Hanging-wall rocks contain up to 20% pyrite-rich layers for at least 100 m above the ores. Siderite is the carbonate phase present within the ore and within the host rocks in an aureole that extends from 75 m beneath to 50 m above the ore. Both older and younger rocks are dolomitic. The two carbonate phases do not occur together within unmineralized host rocks although they may coexist within veins. Siderite contains up to 13 mol% ZnCO3 and 32 mol% MgCO3. There is an antipathetic relationship between Zn and Mg, the latter increasing with distance from the ore.Zinc and, to a lesser extent, Hg, Pb, Ag and Ba show extensive primary dispersion within the host rocks. Within the plane of sedimentation, haloes vary in width from 50 m to 1.5 km and these dispersions are thought to be dependent on the shape of the sedimentary basin floor at the time of exhalative activity. Perpendicular to the sedimentary layering, Zn and Hg show the most extensive primary dispersion with anomalous values extending up to 100 m into the footwall and at least 50 m into the hanging wall. |
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