Sediment-hosted lead-zinc deposits: A global perspective
Leach, D and Sangster, D and Kelley, K and Large, RR and Garven, G and Allen, C and Gutzmer, J and Walters, S (2005) Sediment-hosted lead-zinc deposits: A global perspective. Economic Geology, 100th Aniversary . pp. 561-607. ISSN 978-1-887483-01-8
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Sediment-hosted Pb-Zn deposits contain the world's greatest lead and zinc resources and dominate world
production of these metals. They are a chverse group of ore deposits hosted by a wide variety of carbonate and
siliciclastic roch that have no obviolls genetic association with igneous activity. A nmge of ore-fortl1ing
processes in a vmiety of geologic and tectonic environments created these deposits over at least two billion
years of Earth history. The metals were precipitated by basinal brines in synsedimentary and early diagenetic
to low-grade metamorphic environments. The deposits display a broad range of relationships to enclosing host
rocks that includes stratiform, strata-bound, and discordant ores.
These ores are divided into two broad subt)1Jes: Mississippi Valley-type (MVT) and sedimentmy exhalative
(SEDEX), Despite the "exhalative" component inherent in the term "SEDEX," in this manusclipt, direct evidence
of an exhalite in the ore or alteration component is not essential for a deposit to be classified as SEDEX.
The presence of laminated sulfides parallel to bedding is assumed to be permissive evidence for exhalative ores.
The chstinction between some SEDEX and MVT depOSits can be quite subjective because some SEDEX ores
replaced carbonate, whereas some MVT depOSits formed in an early diagenetic environment and display laminated
Geologic and resource information are presented for 248 depositS that provide a framework to describe ,mel
compare these deposits. Nine of tlle 10 largest sediment-hosted Pb-Zn deposits are SEDEX, Of the deposits
that contain at least 2.5 million metric tons (Mt), there are 35 SEDEX (excluding Broken Hill-type) deposits
and 15 MVT (excluding Iris-type) deposits. Despite the skewed distribution of the deposit size, the two deposits
types have an excellent correlation between total tonnage and tonnage of contained metal (Pb + Zn),
with a fairly consistent ratio of about lO/l, regardless of the size of the deposit or district. Zinc grades are approximately
the same for both, whereas Pb and Ag grades are about 25 percent greater for SEDEX deposits.
The largest difference between SEDEX and MVT deposits is their Cu content. Three times as many SEDEX
deposits have reported Cu contents, and the median Cu value of SEDEX deposits is nearly double that of MVT
deposits. Furthermore, grade-tonnage values for MVT deposits compared to a subset of SEDEX deposits
hosted in carbonate rocks are virtually indistinguishable.
The distribution of MVT deposits through geologic time shows that they are mainly a Phanerozoic phenomenon.
The ages of SEDEX deposits are grouped into two major groups, one in the Proterozoic and another in
the Phanerozoic, MVT deposits dominantly formed in platform carbonate sequences typically located within extensional zones inboard of orogenic belts, whereas SEDEX deposits formed in intracontinental or failed rifts,
and rifted continental margins. The ages of MVT ores are generally tens of millions of years younger than their
host rocks; however, a few are close <~5 m.y.) to the age of their host rocks. In the absence of direct dates for
SEDEX deposits, their age of formation is generally constnuned by relationships to sedimentary or diagenetic
features in the rocks. These studies suggest that deposition of SEDEX ores was coeval with sedimentation or
early diagenesis, whereas some deposits formed at least 20 m.y. after sedimentation.
Fluid inclusion, isotopic studies, and deposit modeling suggest that MVf and SEDEX deposits formed from
basin brines with similar temperatures of mainly 90° to 200°C and lO to 30 wt percent NaCI equiv. Lead isotope
compositions for MVT and SEDEX deposits show that Pb was mainly derived from a variety of crustal
sources. Lead isotope compositions do not provide critelia that distinguish MVT from SEDEX subtypes. However,
sulfur isotope compositions for sphalerite and galena show an apparent difference. SEDEX and MVf sulfur
isotope compositions extend over a large range; however, most data for SEDEX ores have mainly positive
isotopic compositions from 0 to 20 per mil. Isotopic values for MVf ores extend over a wider range and include
more data with negative isotopic values.
Given that there are relatively small differences between the metal character of MVT and SEDEX deposits
and the fluids that deposited them, perhaps the most significant difference between these deposits is their depositional
environment, which is determined by their respective tectonic settings. The contrasting tectonic setting
also dictates the fundamental deposit attributes that generally set them apart, such as host-rock lithology,
deposit morphology, and ore textures.
Blief discussions are also presented on two controversial sets of deposits: Broken Hill-type deposits and a
subset of deposits in the MVT group located in the Irish Midlands, considered by some authors to be a distinct
ore type (Irish type). There are no Significant differences in grade tonnage values between MVT deposits and
the subset that is described as Irish type. Most features of the Irish deposits are not distinct from the family of
MVT deposits; however, the age of mineralization that is the same as or close to the age of the host rocks and
the anomalously high fluid inclusion temperatures (up to 250°C) stand out as distinctly different from typical
MVT ores. The dominance of bacteriogenic sulfur in the hish ores commonly ascribed as uniquely hish type
is in fact no different from several MVT deposits or districts.
A comparison of SEDEX and Broken Hill-type deposits shows that the latter deposits contain signiflcantly
higher contents of Ag and Pb relative to SEDEX deposits. In terms of median values, Broken Hill-type deposits
are almost three times more ellliched in Ag and one and a half times more enriched in Pb compared to
other SEDEX deposits. Metamorphism is a charactelisoc feature but not a prerequisite for inclusion in the
Broken Hill-type category, and IGlown Broken Hill-type examples appear to occur in Paleo- to Mesoproterozoic
terranes. Broken Hill-type deposits remain an enigmatic grouping; however, there is sufficient evidence to
support their inclusion as a separate category of SEDEX deposits.
|Additional Information:||© 2005 Society of Economic Geologists, Inc.|
|Keywords:||Sedex, MVT, Pb-Zn-Ag, stratiform, HYC, Red Dog|
|Deposited By:||Mrs Hlaing Nilar|
|Deposited On:||28 Apr 2009 10:28|
|Last Modified:||13 Jul 2009 09:44|
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