© MLI Ltd 2012

Broken Hill Type

Pb-Zn-Ag systems 

Characteristics
 

Occur in Middle Proterozoic Terranes

Size ranges from 30 to >250 Mt.

Large single deposits dominating a district are the norm.

Economic grades 10-20% Pb+Zn.

Strong Pb-Zn zonation trends.

High Pb:Zn and very high Ag credits (> 1OOppm)

High Cd, Sb, Mn and Fe, with localised elevated As, Cu, W, Bi, Au.

Synonyms:- Ammeburg-type Zn-Pb, Jervois-type.

Commodities:- Pb, Zn, Ag, (Cu, Au, barite).

Cited Examples:-  Broken Hill Main Lode and Pinnacles Mine (New South Wales, Australia), Cannington, (Queensland Australia), Zinkgruvan (Bergslagen  Sweden), Broken Hill and Black Mountain, Aggeneys-Gamsberg (South Africa), possibly Sullivan (Canada)

Other Probables:- Rampura Agucha (Rajastahn India),

Target Systems and Belts:-  Tjamotis (Sweden), Kolari (India), Mn Mines District (India), Buryatia (Russia)

Mineralisation:- Deposits comprise massive to semi massive galena, sphalerite, pyrrhotite and pyrite and/or magnetite layers or stacked lenses. 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 host rocks.

Deformation:- Strongly deformed and metamorphosed supracrustal rocks commonly referred to as ‘mobile belts’ which probably originated in an intracratonic rift or possibly continental margin setting.

Lithology:- 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:- Commonly Lower and Middle Proterozoic; some British Columbia deposits may be hosted by Late Proterozoic to Cambrian rocks.

Host:- 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.

Key Lithotypes:- 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 discontinuous, strongly deformed massive sulphide bodies. Thickening in fold hinges. Individual lenses vary from a metre to tens metres and extend hundreds of metres grading laterally into quartzite, quartz gahnite, garnet quartzite or pyrite/pyrrhotite

Texture/structure:- mineralization as  massive to disseminated stratabound sulphides banded, with locally coarse “skarn” textures; locally well layered or laminated sulphides and silicates.Gangue calcsilicate minerals, quartz or magnetite. Pegmatitic zones are present in some ore zones.

Ore mineralogy:- galena, sphalerite, magnetite, pyrrhotite, pyrite; chalcopyrite, tetrahedrite, molybdenite, arsenopyrite, löllingite. Magnetite can make up > 40% of the ore. Some deposits display zoning from siliceous zn-rich to distal carbonate-silicate pb-ag rich ore.

Gangue mineralogy:- 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. Footwall alteration pipes are generally not recognized, except for some of the Cu-rich deposits. Typically 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.  Increasing intensity of mineralization, Fe-Si-Mn systems 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 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.