Hõy, T. (1996): Broken Hill-type Pb-Zn-Ag+/-Cu, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T., Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 117-120.
SYNONYMS: Shuswap-type, Ammeburg-type Zn-Pb, Jervois-type.
COMMODITIES (BYPRODUCTS): Pb, Zn, Ag, (Cu, Au, barite).
EXAMPLES (British Columbia (MINFILE #) - Canada/International): Cottonbelt (082M 086), River Jordan (082M 001), Ruddock Creek (082M 082, 082M 083, 082M 004), Big Ledge? (082LSE012), Colby 1 (082ESW062); Broken Hill and Pinnacles (New South Wales, Australia); Broken Hill and Black Mountain, Aggeneys district and Gammsberg area (South Africa), Knalla and Nygruvan, Bergslaggen district (Sweden).
CAPSULE DESCRIPTION: Deposits comprise massive to semimassive galena, sphalerite, pyrrhotite and pyrite and/or magnetite layers or stacked lenses hosted by thin- bedded, commonly calcareous paragneiss successions. A complex gangue mineralogy includes a variety of calcsilicate minerals. These stratabound deposits are typically thin, but laterally extensive and were deformed and metamorphosed together with their hostrocks.
TECTONIC SETTING: In strongly deformed and metamorphosed supracrustal rocks commonly referred to as ‘mobile belts’ which probably originated in an intracratonic rift or possibly continental margin setting.
DEPOSITIONAL ENVIRONMENT/GEOLOGICAL SETTING: Marine sediments and associated minor bimodal (?) volcanics (often felsic, possibly alkalic) reflect active extensional tectonics. Host successions include inferred evaporites and are generally interpreted as shallow marine. Underlying gneissic successions suggest some deposits formed on or along margins of tectonic highs. However, intense deformation and metamorphism have commonly masked relationships.
AGE OF MINERALIZATION: Commonly Lower and Middle Proterozoic; some British Columbia deposits may be hosted by Late Proterozoic to Cambrian rocks.
HOST/ASSOCIATED ROCK TYPES: Hosted by thin-bedded calcareous schists, impure marble, quartzites and, less commonly, graphitic schists. A common and important host rock is garnet quartzite which occurs as envelopes to the sulphide bodies; associated with well layered and heterogenous successions of quartzite, crystalline marble, quartzo-feldspathic gneiss, hornblende gneiss, and abundant pelitic and calcareous schist and gneiss; locally associated carbonatite and amphibolite. Banded iron formations, chert, gahnite quartzites and tourmalinites are common in the host stratigraphic succession as distal facies or in the footwall successions. Scapolite-rich units and sulphur isotopes suggest associated evaporites. Metamorphic grades vary from amphibolite to granulite.
DEPOSIT FORM: Stacked sulphide or sulphide/magnetite lenses are common; they are thin, irregular, discontinuous, strongly deformed massive sulphide bodies. Thickening in fold hinges is often critical to make economic thicknesses. Individual lenses vary from less than a metre to tens of metres and may extend hundreds of metres often grading laterally into quartzite, quartz gahnite, garnet quartzite or pyrite/pyrrhotite disseminated units that may persist for tens of kilometres.
TEXTURE/STRUCTURE: Mineralization occurs as discontinuous massive to semimassive sulphide lenses or as disseminated stratabound sulphides. Sulphides are massive to irregular banded, with locally coarse “skarn” textures; locally well layered or laminated sulphides and silicates occur. They are commonly medium to coarse grained and intimately intergrown with gangue calcsilicate minerals, quartz or magnetite; as well, there are occasional thin monominerallic sulphide layers. Disseminated sulphides are common in granular marble. Pegmatitic zones are present in some ore zones.
ORE MINERALOGY (Principal and subordinate): Galena, sphalerite, magnetite, pyrrhotite, pyrite; chalcopyrite, tetrahedrite, molybdenite, arsenopyrite, löllingite. In some deposits magnetite makes up more than 40% of the ore. Some deposits display zoning from siliceous Zn-rich to distal carbonate-silicate Pb-Ag rich ore.
GANGUE MINERALOGY (Principal and subordinate): Quartz, garnet, calcite, rhodonite, magnetite, siderite, pyroxenes and amphiboles, commonly manganiferous, fluorite; Mn olivine, apatite, gahnite, plagioclase, biotite, chlorite, ankerite, epidote, graphite, barite, hematite, wollastonite, sillimanite, staurolite, vesuvianite. The complex gangue mineralogy is a characteristic of Broken Hill-type deposits.
ALTERATION MINERALOGY: Original alteration assemblages are replaced by a complex variety of metamorphic minerals. Alteration envelopes and deposit zoning are common in larger deposits, but are generally not recognized in smaller ones. Footwall alteration pipes are generally not recognized, except for some of the Cu-rich deposits, which complicates their interpretation. Typically the alteration reflects enrichment of Fe, Si, Mn, Ca, P, F, K and CO3 and includes metamorphic silicates including amphiboles, olivine, biotite, phlogopite, sillimanite, orthoclase and clinozoisite as well as carbonates, fluorite and a variety of other minerals. Spessartine-quartz halos surround many deposits, with more regional silicification (quartz) and K (sillimanite) enrichment. In the Broken Hill area, Australia, with increasing intensity of mineralization, Fe-Si-Mn systems (typical of metamorphosed iron formations) are overprinted by extreme Ca-Mn-F enrichment with calcsilicate assemblages.
WEATHERING: Large gossans are not common; however, pyrrhotite and pyrite in some deposits locally produce rusted outcrops. Some Australian deposits have deep weathered zones: gossanous quartz-garnet-gahnite rocks, with abundant Mn and Fe oxides at surface, and secondary Ag enrichment at depths associated with oxides (goethite and coronadite) and carbonates (dolomite, cerussite and smithsonite). Leached sulphides mark the transition into underlying sulphide ore.
ORE CONTROLS: Not well understood; deposits appear to be restricted to Proterozoic “mobile belts”, generally interpreted to be intracratonic rifts. Oxidized shallow-marine basins, possibly developed due to extensional faulting above basement highs, and associated bimodal (?) volcanism are local controls.
GENETIC MODEL: Difficult to interpret due to high metamorphic grades. A sedimentary exhalite origin, with sulphide deposition in rapidly deepening rifts, is preferred because the deposits are associated with iron formations, chert and Mn-rich iron oxide facies. This environment, dominated by oxidized facies, contrasts with reduced, anoxic basins that commonly host sedex deposits. However, associated bimodal volcanics, ore and gangue chemistry and sulphide textures suggest similarities with volcanogenic massive sulphide deposition. Some workers have supported replacement models for the mineralization.
ASSOCIATED DEPOSIT TYPES: Sedimentary exhalative Pb-Zn deposits (E14), carbonatites (N01), nepheline syenites, polymetallic veins (I05) and W-Mo veins.
COMMENTS: Broken Hill-type deposits are a difficult exploration target due to their setting in strongly metamorphosed and deformed rocks.
GEOCHEMICAL SIGNATURE: Anomalous enrichments of Mn, Cu, Au, Bi, Sb, W, Co and As in the ore and some proximal exhalative units; high Ag:Pb ratios, Mn and K enrichment (with muscovite, K-feldspars and sillimanite) in alteration halos; elevated base metal values (particularly Zn) and Mn in more regional iron formations. In silt samples expect anomalous Pb, Zn, Ag, Mn and Ba.
GEOPHYSICAL SIGNATURE: Deposits with associated magnetite produce strong magnetic anomalies. Electromagnetic and induced polarization surveys may detect those deposits with pyrrhotite and pyrite massive sulphides lenses. Associated graphite in some (e.g., Big Ledge) may provide local targets.
OTHER EXPLORATION GUIDES: Main exploration guide is appropriate sedimentary/tectonic environment - thin-bedded succession of paragneisses with abundant carbonate. The mineralization may occur at, or near, the transition from quartzo- feldspathic basement rocks to fine-grained clastic metasediments. Rapid lithologic facies changes in the vicinity of deposits may indicate local hydrothermal systems. Associated volcanism is indicative of extension or rifting. In closer proximity to deposits, unusual mineral assemblages include garnet quartzites, gahnite quartzites and Mn-rich calcsilicates with skarn textures.
TYPICAL GRADE AND TONNAGE: Deposits frequently occur in clusters with numerous small, uneconomic deposits. Broken Hill-type targets average less than 5 to 20 Mt, but may be in excess of 100 Mt (Broken Hill, Australia: 280 Mt containing 10.0% Pb, 8.5% Zn and 148 g/t Ag, including approximately 150 Mt of more that 20% Pb+Zn). Grades are variable, commonly with 2 to 10 % Pb, 2 to 8% Zn and 10 to 150 g/t Ag. Some deposits contain no byproduct copper, others have 0.1 to 1% Cu. In British Columbia, known deposits range in size from less than one million to 6.5 Mt; geological reserves may be considerably larger. Grades range from approximately 2 to 5 % Zn and 2.5 to 6.5 % Pb with up to 50 g/t Ag. Ruddock Creek contains 5 Mt with 7.5% Zn, 2.5% Pb and Jordan River, 2.6 Mt with 5.6% Zn, 5.1% Pb and 35 g/t Ag.
ECONOMIC LIMITATIONS: Structural thickening is often critical to the genesis of economic deposits. Broken Hill-type deposits have not been mined in British Columbia, due mainly to their form - thin, though laterally persistent layers - and their location in remote, mountainous terrains.
IMPORTANCE: These deposit are an important source for lead, zinc and silver, and remain an attractive exploration target in British Columbia.
ACKNOWLEDGEMENTS: This description has drawn from information written up by Stephen Walters of BHP Exploration, Australia, and presented by Garry Davidson of CODES Key Centre, University of Tasmania at the 1995 MDRU short course, Metallogeny of Proterozoic Basins, in Vancouver.
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