Polymetallic Mineralization at the Browns Deposit, Rum Jungle Mineral Field, Northern Territory, Australia

Abstract

The Rum Jungle mineral field, Northern Territory, Australia, forms part of the Paleoproterozoic Pine Creek orogen. The geology of the area comprises two Archean granitic basement domes, overlain by a Paleoproterozoic sequence of metasedimentary and subordinate metavolcanic rocks. The rocks have been deformed and metamorphosed to greenschist facies during the 1880 Ma Barramundi orogeny, but the original stratigraphic succession has been preserved. The Browns deposit (Browns and its extension Browns East), hosted within black carbonaceous pelites of the Paleoproterozoic Whites Formation, is the largest base metal deposit found to date in the Pine Creek orogen. Present resource estimates are 39 million metric tons (Mt) at 0.44 percent Cu, 3.61 percent Pb, 0.11 percent Co, and 0.09 percent Ni (Browns), and 31 Mt at 1.29 percent Cu, 1.28 percent Pb, 0.13 percent Co, and 0.13 percent Ni (Browns East). Major sulfide minerals in the ore zone are galena, sphalerite, chalcopyrite, pyrite, and the main carrier of Ni and Co, siegenite (Ni,Co) 3 S 4 . Digenite group minerals and bornite, Ca-bearing plumbogummite (PbAl 3 (PO 4 ) 2 (OH) 5 .H 2 O), arsenopyrite, and gersdorffite are present in minor quantities. Ore minerals occur as fine-grained wisps parallel to the main bedding-parallel foliation and as coarse-grained mono- and polyminerallic aggregates that crosscut the foliation. Total noncarbonate carbon contents of unaltered rocks range from 1.07 to 7.51 wt percent. Total sulfur contents range from 2.13 to 6.14 wt percent. Petrographic studies and Raman spectroscopy identify three varieties of carbonaceous matter: primary syngenetic; premetamorphic, epigenetic grain coating; and postmetamorphic, epigenetic, uraniferous migrated. Altered rocks are rarely encountered. Values of δ 34 S for sulfides range from –5 to +15 per mil and are interpreted to reflect a combination of inorganic thermochemical reduction of seawater and bacterial activity. No specific sulfur isotope patterns for individual minerals can be established indicating nonequilibrium conditions. Homogeneous lead isotope signatures of galena ( 206 Pb/ 204 Pb = 16.20–16.22, 207 Pb/ 206 Pb = 15.48–15.51, 208 Pb/ 207 Pb = 35.86–35.92) suggest that all Pb was derived from a single crustal source, and paragenetically late crosscutting galena appears to have been remobilized from bedding-parallel galena. Despite the occurrence of uranium in similar rocks nearby, uranium is quantitatively insignificant in the Browns deposit and is not genetically related to base metal mineralization. Instead, it is linked to brittle deformation in the waning stages of orogenesis. The Browns deposit differs from other well-known sediment-hosted stratiform ore deposits. Both the Zambian Copperbelt and the Polish Kupferschiefer deposits are copper dominant and hosted in sedimentary sequences with significant psammitic components. The Pb-Zn-Ag deposits of the Proterozoic Australian zinc province also have significant carbonate in their host rocks (e.g., carbonaceous dolomitic and siltstone at McArthur River) and lack economically interesting Cu, Co, and Ni. The nearby Woodcutters Pb-Zn-Ag deposit, in rocks of similar age, also differs in structural setting (veins), dominant metals, and Pb isotopes from the Browns deposit.