Mercury in the Broken Hill (N.S.W., Australia) lead-zinc-silver lodes |
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| Authors: | W R Ryall |
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| Institution: | C.S.I.R.O., Division of Mineralogy, P.O. Box 136, North Ryde, N.S.W. 2113 Australia |
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| Abstract: | In the Pb-Zn-Ag lodes at Broken Hill, significant Hg remains located in sphalerite and in tetrahedrite inclusions in galena, despite prograde granulite facies regional metamorphism that has resulted in recrystallization of the ore. The stratigraphically uppermost No. 3 lens has the highest Hg content, with sphalerite and galena concentrates containing up to 149,000 ppb and 198,000 ppb Hg, respectively, in Ag-Sb-rich parts of the lode.The No. 2 and 1 lenses and A lode have generally low Hg contents, but narrow tetrahedrite-rich sections contain up to 83,000 ppb Hg in sphalerite concentrates and 82,000 ppb in galena concentrates. The stratigraphically lowermost B lode has a moderately high Hg content with sphalerite concentrates containing from 4400 to 41,000 ppb Hg and galena concentrates from 30 to 21,000 ppb Hg.Programmed heating determinations showed Hg in sphalerite concentrates to be contained principally in the sphalerite structure, with subordinate amounts in tetrahedrite inclusions. Mercury in galena concentrates is almost wholly contained in tetrahedrite inclusions, with lesser amounts in dyscrasite and pyrargyrite.Piercement bodies show increased Pb, Ag, Sb, As and Hg contents due to preferential migration of galena and tetrahedrite. The Hg content of sphalerite in these bodies is approximately doubled relative to the source lode, and the Hg content of galena concentrates is increased sevenfold, due to an increase of tetrahedrite inclusions.Coarse-grained sulfide-gangue pods and pegmatites, which reflect partial melting and remobilization of parts of the lode, have a mean Hg content less than the same volume of host lode. Within these bodies Hg is strongly partitioned into sphalerite, which contains from 14,000 to 33,000 ppb Hg, whereas galena contains only from 10 to 570 ppb Hg, due to the absence of Hg-bearing tetrahedrite inclusions.Within the lode environment, trace amounts of Hg in veins formed during retrograde metamorphism are located in sphalerite which contains up to 29,000 ppb Hg.The presence of significant amounts of Hg in Ag-Sb-As minerals in a siderite-rich vein within the main lode and in the Consols lode (located 700 m from the main lode) indicates that Hg remains associated with these elements over a wide range of conditions.In the wall rocks, there is no Hg halo which is exclusive of disseminated sulfide minerals. Under high-grade regional metamorphism, trace amounts of Hg in the Broken Hill lode are retained within pre-metamorphism host minerals. However, mobilization of Hg does take place in veins associated with retrograde metamorphism after localized breakdown of tetrahedrite. |
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