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
ore textures.
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.

Sediment-hosted lead-zinc deposits: A global perspective

Sediment-hosted Pb-Zn deposits can be divided into two major subtypes. The first subtype is clastic-dominated lead-zinc (CD Pb-Zn) ores, which are hosted in shale, sandstone, siltstone, or mixed clastic rocks, or occur as carbonate replacement, within a CD sedimentary rock sequence. This subtype includes deposits that have been traditionally referred to as sedimentary exhalative (SEDEX) deposits. The CD Pb-Zn deposits occur in passive margins, back-arcs and continental rifts, and sag basins, which are tectonic settings that, in some cases, are transitional into one another. The second subtype of sediment-hosted Pb-Zn deposits is the Mississippi Valley-type (MVT Pb-Zn) that occurs in platform carbonate sequences, typically in passive-margin tectonic settings. Considering that the redox state of sulfur is one of the major controls on the extraction, transport, and deposition of Pb and Zn at shallow crustal sites, sediment-hosted Pb-Zn ores can be considered a special rock type that recorded the oxygenation of Earth’s hydrosphere. The emergence of CD and MVT deposits in the rock record between 2.02 Ga, the age of the earliest known deposit of these ores, and 1.85 to 1.58 Ga, a major period of CD Pb-Zn mineralization in Australia and India, corresponds to a time after the Great Oxygenation Event that occurred at ca 2.4 to 1.8 Ga. Contributing to the abundance of CD deposits at ca 1.85 to 1.58 Ga was the following: (1) enhanced oxidation of sulfides in the crust that provided sulfate to the hydrosphere and Pb and Zn to sediments; (2) development of major redox and compositional gradients in the oceans; (3) first formation of significant sulfate-bearing evaporites; (4) formation of red beds and oxidized aquifers, possibly containing easily extractable Pb and Zn; (5) evolution of sulfate-reducing bacteria; and (6) formation of large and long-lived basins on stable cratons. Although MVT and CD deposits appeared for the first time in Earth history at 2.02 Ga, only CD deposits were important repositories for Pb and Zn in sediments between the Great Oxygenation Event, until after the second oxidation of the atmosphere in the late Neoproterozic. Increased oxygenation of the oceans following the second oxidation event led to an abundance of evaporites, resulting oxidized brines, and a dramatic increase in the volume of coarse-grained and permeable carbonates of the Paleozoic carbonate platforms, which host many of the great MVT deposits. The MVT deposits reached their maximum abundance during the final assembly of Pangea from Devonian into the Carboniferous. This was also a time for important CD mineral deposit formation along passive margins in evaporative belts of Pangea. Following the breakup of Pangea, a new era of MVT ores began with the onset of the assembly of the Neosupercontinent. A significant limitation on interpreting the secular distribution of the deposits is that there is no way to quantitatively evaluate the removal of deposits from the rock record through tectonic recycling. Considering that most of the sedimentary rock record has been recycled, most sediment-hosted Pb-Zn deposits probably have also been destroyed by subduction and erosion, or modified by metamorphism and tectonism, so that they are no longer recognizable. Thus, the uneven secular distribution of sediment-hosted Pb-Zn deposits reflects the genesis of these deposits, linked to Earth’s evolving tectonic and geochemical systems, as well as an unknown amount of recycling of the sedimentary rock record.

Sediment-Hosted Lead-Zinc Deposits in Earth History

https://www.ess.uci.edu/~bwalker/Brett_Walker/Publications_files/Miller_Geochim_Cosmochim_Ac_2011.pdf

Re-assessing the surface cycling of molybdenum and rhenium

Sustainable development and the transition to a clean-energy economy drives ever-increasing demand for base metals, substantially outstripping the discovery rate of new deposits and necessitating dramatic improvements in exploration success. Rifting of the continents has formed widespread sedimentary basins, some of which contain large quantities of copper, lead and zinc. Despite over a century of research, the geological structure responsible for the spatial distribution of such fertile regions remains enigmatic. Here, we use statistical tests to compare deposit locations with new maps of lithospheric thickness, which outline the base of tectonic plates. We find that 85% of sediment-hosted base metals, including all giant deposits (>10 megatonnes of metal), occur within 200 kilometres of the transition between thick and thin lithosphere. Rifting in this setting produces greater subsidence and lower basal heat flow, enlarging the depth extent of hydrothermal circulation available for forming giant deposits. Given that mineralization ages span the past two billion years, this observation implies long-term lithospheric edge stability and a genetic link between deep Earth processes and near-surface hydrothermal mineral systems. This discovery provides an unprecedented global framework for identifying fertile regions for targeted mineral exploration, reducing the search space for new deposits by two-thirds on this lithospheric thickness criterion alone. Major sediment-hosted base metal deposits are located within 200 km of the border between thick and thin lithosphere, according to statistical comparisons between global lithospheric thickness and known deposit locations.

/pdf/global-distribution-of-sediment-hosted-metals-controlled-by-j6a8rvpwky.pdf

Global distribution of sediment-hosted metals controlled by craton edge stability

The Red Dog Zn-Pb deposits are hosted in organic-rich mudstone and shale of the Mississippian Kuna Formation. A complex mineralization history is defined by four sphalerite types or stages: (1) early brown sphalerite, (2) yellow-brown sphalerite, (3) red-brown sphalerite, and (4) late tan sphalerite. Stages 2 and 3 constitute the main ore-forming event and are volumetrically the most important. Sulfides in stages 1 and 2 were deposited with barite, whereas stage 3 largely replaces barite. Distinct chemical differences exist among the different stages of sphalerite. From early brown sphalerite to later yellow-brown sphalerite and red-brown sphalerite, Fe and Co content generally increase and Mn and Tl content generally decrease. Early brown sphalerite contains no more than 1.9 wt percent Fe and 63 ppm Co, with high Mn (up to 37 ppm) and Tl (126 ppm), whereas yellow-brown sphalerite and red-brown sphalerite contain high Fe (up to 7.3 wt %) and Co (up to 382 ppm), and low Mn (<27 ppm) and Tl (<37 ppm). Late tan sphalerite has distinctly lower Fe (< 0.9 wt %) and higher Tl (up to 355 ppm), Mn (up to 177 ppm), and Ge (426 ppm), relative to earlier sphalerite. Wide ranges in concentrations of Ag, Cu, Pb, and Sb characterize all sphalerite types, particularly yellow-brown sphalerite and red-brown sphalerite, and most likely reflect submicroscopic inclusions of galena, chalcopyrite and/or tetrahedrite in the sphalerite. In situ ion microprobe sulfur isotope analyses show a progression from extremely low δ 34S values for stage 1 (as low as –37.2‰) to much higher values for yellow-brown sphalerite (mean of 3.3‰; n = 30) and red-brown sphalerite (mean of 3.4; n = 20). Late tan sphalerite is isotopically light (–16.4 to –27.2‰).

The textural, chemical, and isotopic data indicate the following paragenesis: (1) deposition of early brown sphalerite with abundant barite, minor pyrite, and trace galena immediately beneath the sea floor in unconsolidated mud; (2) deposition of yellow-brown sphalerite during subsea-floor hydrothermal recrystallization and coarsening of preexisting barite; (3) open-space deposition of barite, red-brown sphalerite and other sulfides in veins and coeval replacement of barite; and (4) postore sulfide deposition, including the formation of late tan sphalerite breccias. Stage 1 mineralization took place in a low-temperature environment where fluids rich in Ba mixed with pore water or water-column sulfate to form barite, and metals combined with H2S derived from bacterial sulfate reduction to form sulfides. Higher temperatures and salinities and relatively oxidized ore-stage fluids (stages 2 and 3) compared with stage 1 were probably important controls on the abundances and relative amounts of metals in the fluids and the resulting sulfide chemistry. Textural observations and isotopic data show that preexisting barite was reductively dissolved, providing a source of H2S for sulfide mineral formation. In stage 3, the continued flow of hydrothermal fluids caused thermal alteration of organic-rich mudstones and a build-up of methane that led to fluid overpressuring, hydrofracturing, and vein formation. Barite, red-brown sphalerite, and other sulfides were deposited in the veins, and preexisting barite was pervasively replaced by red-brown sphalerite. Hydrothermal activity ceased until Jurassic time when thrusting and large-scale fluid flow related to the Brookian orogeny remobilized and formed late tan sphalerite in tectonic breccias.

Textural, Compositional, and Sulfur Isotope Variations of Sulfide Minerals in the Red Dog Zn-Pb-Ag Deposits, Brooks Range, Alaska: Implications for Ore Formation

The Red Dog sediment-hosted deposit in the De Long Mountains of northern Alaska is the largest Zn producer in the world. Main stage mineralization is characterized by massive sulfide ore and crosscutting subvertical veins. Although the vein mineralization is clearly younger than the massive ore, the exact temporal relationship between the two is unclear. Re-Os geochronology of pyrite is used to determine the absolute age of main stage ore at Red Dog. A 10-point isochron on both massive and vein pyrite yields an age of 338.3 ± 5.8 Ma and is interpreted to represent the age of main stage ore. The Re-Os data indicate that both massive and vein ore types are coeval within the resolution of the technique. Formation of the Red Dog deposit was associated with extension along a passive continental margin, and therefore the Re-Os age of main stage ore constrains the timing of rifting as well as the age of the host sedimentary rocks. Sphalerite from both massive and vein ore yields imprecise ages and shows a high degree of scatter compared to pyrite. We suggest that the Re-Os systematics of sphalerite can be disturbed and that this mineral is not reliable for Re-Os geochronology.

Re-Os sulfide geochronology of the Red Dog sediment-hosted Zn-Pb-Ag deposit, Brooks Range, Alaska

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Re-Os sulfide geochronology of the Red Dog sediment-hosted Zn-Pb-Ag deposit, Brooks Range, Alaska