Sediment hosted lead-zinc deposits: A global perspective

Abstract

Sediment-hosted Pb-Zn deposits contain the world's greatest lead and zinc resources and dominate world
\nproduction of these metals. They are a chverse group of ore deposits hosted by a wide variety of carbonate and
\nsiliciclastic roch that have no obviolls genetic association with igneous activity. A nmge of ore-fortl1ing
\nprocesses in a vmiety of geologic and tectonic environments created these deposits over at least two billion
\nyears of Earth history. The metals were precipitated by basinal brines in synsedimentary and early diagenetic
\nto low-grade metamorphic environments. The deposits display a broad range of relationships to enclosing host
\nrocks that includes stratiform, strata-bound, and discordant ores.
\nThese ores are divided into two broad subt)1Jes: Mississippi Valley-type (MVT) and sedimentmy exhalative
\n(SEDEX), Despite the "exhalative" component inherent in the term "SEDEX," in this manusclipt, direct evidence
\nof an exhalite in the ore or alteration component is not essential for a deposit to be classified as SEDEX.
\nThe presence of laminated sulfides parallel to bedding is assumed to be permissive evidence for exhalative ores.
\nThe chstinction between some SEDEX and MVT depOSits can be quite subjective because some SEDEX ores
\nreplaced carbonate, whereas some MVT depOSits formed in an early diagenetic environment and display laminated
\nore textures.
\nGeologic and resource information are presented for 248 depositS that provide a framework to describe ,mel
\ncompare these deposits. Nine of tlle 10 largest sediment-hosted Pb-Zn deposits are SEDEX, Of the deposits
\nthat contain at least 2.5 million metric tons (Mt), there are 35 SEDEX (excluding Broken Hill-type) deposits
\nand 15 MVT (excluding Iris-type) deposits. Despite the skewed distribution of the deposit size, the two deposits
\ntypes have an excellent correlation between total tonnage and tonnage of contained metal (Pb + Zn),
\nwith a fairly consistent ratio of about lO/l, regardless of the size of the deposit or district. Zinc grades are approximately
\nthe same for both, whereas Pb and Ag grades are about 25 percent greater for SEDEX deposits.
\nThe largest difference between SEDEX and MVT deposits is their Cu content. Three times as many SEDEX
\ndeposits have reported Cu contents, and the median Cu value of SEDEX deposits is nearly double that of MVT
\ndeposits. Furthermore, grade-tonnage values for MVT deposits compared to a subset of SEDEX deposits
\nhosted in carbonate rocks are virtually indistinguishable.
\nThe distribution of MVT deposits through geologic time shows that they are mainly a Phanerozoic phenomenon.
\nThe ages of SEDEX deposits are grouped into two major groups, one in the Proterozoic and another in
\nthe 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,
\nand rifted continental margins. The ages of MVT ores are generally tens of millions of years younger than their
\nhost rocks; however, a few are close <~5 m.y.) to the age of their host rocks. In the absence of direct dates for
\nSEDEX deposits, their age of formation is generally constnuned by relationships to sedimentary or diagenetic
\nfeatures in the rocks. These studies suggest that deposition of SEDEX ores was coeval with sedimentation or
\nearly diagenesis, whereas some deposits formed at least 20 m.y. after sedimentation.
\nFluid inclusion, isotopic studies, and deposit modeling suggest that MVf and SEDEX deposits formed from
\nbasin brines with similar temperatures of mainly 90° to 200°C and lO to 30 wt percent NaCI equiv. Lead isotope
\ncompositions for MVT and SEDEX deposits show that Pb was mainly derived from a variety of crustal
\nsources. Lead isotope compositions do not provide critelia that distinguish MVT from SEDEX subtypes. However,
\nsulfur isotope compositions for sphalerite and galena show an apparent difference. SEDEX and MVf sulfur
\nisotope compositions extend over a large range; however, most data for SEDEX ores have mainly positive
\nisotopic compositions from 0 to 20 per mil. Isotopic values for MVf ores extend over a wider range and include
\nmore data with negative isotopic values.
\nGiven that there are relatively small differences between the metal character of MVT and SEDEX deposits
\nand the fluids that deposited them, perhaps the most significant difference between these deposits is their depositional
\nenvironment, which is determined by their respective tectonic settings. The contrasting tectonic setting
\nalso dictates the fundamental deposit attributes that generally set them apart, such as host-rock lithology,
\ndeposit morphology, and ore textures.
\nBlief discussions are also presented on two controversial sets of deposits: Broken Hill-type deposits and a
\nsubset of deposits in the MVT group located in the Irish Midlands, considered by some authors to be a distinct
\nore type (Irish type). There are no Significant differences in grade tonnage values between MVT deposits and
\nthe subset that is described as Irish type. Most features of the Irish deposits are not distinct from the family of
\nMVT deposits; however, the age of mineralization that is the same as or close to the age of the host rocks and
\nthe anomalously high fluid inclusion temperatures (up to 250°C) stand out as distinctly different from typical
\nMVT ores. The dominance of bacteriogenic sulfur in the hish ores commonly ascribed as uniquely hish type
\nis in fact no different from several MVT deposits or districts.
\nA comparison of SEDEX and Broken Hill-type deposits shows that the latter deposits contain signiflcantly
\nhigher contents of Ag and Pb relative to SEDEX deposits. In terms of median values, Broken Hill-type deposits
\nare almost three times more ellliched in Ag and one and a half times more enriched in Pb compared to
\nother SEDEX deposits. Metamorphism is a charactelisoc feature but not a prerequisite for inclusion in the
\nBroken Hill-type category, and IGlown Broken Hill-type examples appear to occur in Paleo- to Mesoproterozoic
\nterranes. Broken Hill-type deposits remain an enigmatic grouping; however, there is sufficient evidence to
\nsupport their inclusion as a separate category of SEDEX deposits.