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.
IDENTIFICATION
SYNONYMS: Shuswap-type,
Ammeburg-type Zn-Pb, Jervois-type.
COMMODITIES (BY-PRODUCTS):
Pb, Zn, Ag, (Cu, Au, barite).
EXAMPLES (British Columbia (MINFILE #) -
Canada/International): Cottonbelt (082M086), River Jordan
(082M001), Ruddock Creek (082M082-084), Big Ledge? (082LSE012), Colby?
(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).
GEOLOGICAL
CHARACTERISTICS
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.
EXPLORATION GUIDES
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.
ECONOMIC FACTORS
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.
REFERENCES
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.
Beeson, R. (1990): Broken Hill-type
Lead-Zinc Deposits - An Overview of their Occurrences and Geological
Setting; The Institution of Mining and Metallurgy, Transactions,
Section B, Volume 99, pages 163-175.
Höy, T. (1987): Geology of the
Cottonbelt Lead-Zinc-Magnetite Layer, Carbonatites and Alkalic Rocks in
the Mount Grace Area, Frenchman Cap Dome, Southeastern British Columbia;
B.C. Ministry of Energy, Mines and Petroleum Resources, Bulletin
80, 99 pages.
Fyles, J.T. (1970): The Jordan River
Area near Revelstoke, British Columbia; B.C. Ministry of Energy, Mines
and Petroleum Resources, Bulletin 57, 64 pages.
Hedström, P., Simeonov, A. and
Malmström, L (1989): The Zinkgruvan Ore Deposit, South- central
Sweden: A Proterozoic, Proximal Zn-Pb-Ag Deposit in Distal Volcanic
Facies; Economic Geology, Volume 84, pages 1235-1261.
Mackenzie, D.H. and Davies, R.H. (1990):
Broken Hill Lead-Silver-Zinc Deposit at Z.C. Mines; in Geology of the
Mineral Deposits of Australia and Papua New Guinea, Hughes, F.E., Editor,
The Australasian Institute of Mining and Metallurgy, pages
1079-1084.
Parr, J.M and Plimer, I.R. (1993):
Models for Broken Hill-type Lead-Zinc-Silver Deposits; in Mineral Deposit
Modelling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M.,
Editors, Geological Association of Canada, Special Paper 40, pages
253-288.
Plimer, I.R. (1986): Sediment-hosted
Exhalative Pb-Zn Deposits - Products of Contrasting Ensialic Rifting;
Geological Society of South Africa, Transactions, Volume 89, pages
57-73.
van der Heyden, A. and Edgecombe, D.R.
(1990): Silver-Lead-Zinc Deposit at South Mine, Broken Hill; in
Geology of the Mineral Deposits of Australia and Papua New Guinea, Hughes,
F.E., Editor, The Australasian Institute of Mining and Metallurgy,
pages 1073-1078.
Walters, S. (1995): Broken Hill-type
Pb-Zn-Ag Deposits - Characteristics and Exploration Models; Mineral
Deposits Research Unit, The University of British Columbia, Short
Course notes, Metallogeny of Proterozoic Basins, 56 pages. |