Mississippi Valley Type Mineral Deposits

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Mississippi Valley TypeMineralDeposits
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Why Study Mineral Deposits?

• History Economics. Mining was a vital part of
  early Midwest
• economy
• 1. Lead mining began in the Upper Mississippi
  Valley about 1820.
• 2. Mineral Point was founded in 1827.
• 3. Territory of Wisconsin was created in Mineral
  Point in 1836.
• 4. First Territorial Governor (later US Senator)
  was Henry Dodge,
• a mine owner from Mineral Point.
• B. Science.
• 1. Minerals are chemicals.
• 2. Mineral deposit formation involves chemical
  and
• physical processes that concentrate minerals.
• 3. Studying ore deposits is like working in an
  outdoor chemistry
• laboratory

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What are Mississippi Valley Type Ore Deposits? A.
Deposits of Lead (Galena (PbS)), Zinc
(Sphalerite (ZnS)), and Copper (chalcopyrite
(iron/copper sulfide)) in sedimentary rocks
(commonly carbonates). B. Host rocks are often
significantly older than ore minerals. C.
Generally there is no nearby igneous source of
heat or fluids. Why is this important? 1.
Mineral deposit components must be transported to
a site, then precipitated. 2. Most substances
are more soluble (more easily transported) in
hot fluids than in cold ones. D. Various mineral
thermometers indicate that deposition took
place at relatively low temperature (a little
over 100C) and at only a km or so depth.
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In detail, the chemistry of Mississippi Valley
Type (MVT) is fairly complicated. A. Did the
metal ions and the sulfur travel together? 1.
Objection PbS and ZnS are very insoluble. 2.
However, perhaps the sulfur was transported as
sulfate ion (SO4-2) and reduced by organic
matter near the deposits. 3. Or, maybe lead and
sulfide formed as a soluble complex so that they
could travel together. B. Perhaps a sulfide-rich
solution mixed with a metal-rich solution at the
site of deposition. Problem There are matching
precipitation bands over long distances. C. No
model fits all the observations comfortably. D.
But, the deposits EXIST, therefore there must be
an explanation!
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How the Mineral Components Got There
Mineral Deposit
Source Region
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basinal brine hypothesis

• According to the basinal brine hypothesis of ore
  formation, hot saline fluids similar to oilfield
  brines migrated out of sedimentary basins and
  along aquifers, eventually forming ore deposits
  in sedimentary host rocks at distances of the
  order of 100 km from the basins.
• This hypothesis explains why the major element
  compositions, high salinities, D/H and 18O/16O
  ratios and temperatures of Mississippi
  Valley-type ore-forming fluids are remarkably
  similar to those of oil-field brines found in
  present-day sedimentary basins

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MVT ConceptText Fig. 3.33
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GENESIS OF MISSISSIPPI VALLEY-TYPE LEAD-ZINC
DEPOSITSDirnitri A. Sverjensky

• The most important characteristics of Mississippi
  Valley-type deposits are the following
• 1. They occur principally in limestone or
  dolostone that forms a thin cover over an igneous
  or highly metamorphosed Precambrian basement.
• 2. They consist of bedded replacements, vuggy
  ores, and veins, but the ore is strongly
  controlled by individual strata.
• 3. They contain galena, sphalerite pyrite,
  marcasite, fluorite, barite, chalcopyrite,
  dolomite, calcite, and quartz.
• 4. They are not associated with igneous rocks,
  except in the case of the Kentucky-Illinois
  district.
• 5. They always occur in areas of mild
  deformation, expressed in brittle fracture, broad
  domes and basins, and gentle folds.
• 6. The ore is never in the basement rocks, but
  its distribution is often spatially related to
  basement highs, with the ore located within
  sandbanks, ridges, and reef structures that
  surround the basement highs.
• 7. The ore is at shallow depths, generally less
  than 600 m relative to the present surface, and
  was probably never at depths greater than about
  1500 m.
• 8. There is always evidence of dissolution of the
  carbonate host rock, expressed by slumping,
  collapse, brecciation, or thinning of the host
  rock, that provides clear proof that the ores are
  epigenetic.

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More! (Just note that there are some pretty
cool ways of studying these rocks)

• 9. The carbon and oxygen isotopic compositions of
  the host rocks are normal for such rocks but are
  lowered adjacent to ore, which suggests that the
  host rocks were recrystallized in the presence of
  a fluid.
• 10. Fluid inclusions in sphalerite, fluorite,
  barite, and calcite always contain dense, saline,
  aqueous fluids and often oil and/or methane. The
  total dissolved salts range from 10 to 30 wtg/o
  and are predominantly chloride, sodium, and
  calcium, with much smaller amounts of potassium
  and magnesium. Homogenization temperatures are
  generally in the range 50-200?C.
• 11. Reconstruction of the total sediment
  thickness over the ore, together with normal
  geothermal gradients, suggests temperatures much
  lower than the fluid inclusion homgenization
  temperatures.
• 12. The hydrogen and oxygen isotopic compositions
  of the water in the fluid inclusions are similar
  to those of the pore fluids in sedimentary
  basins.
• 13. The ranges of sulfur isotopic values and the
  degree of approach to isotopic equilibrium
  between sulfides are different for each district.
  In some districts, the source of the sulfur could
  not have been magmatic and thus must have been
  sedimentary.
• 14. The isotopic composition of the lead in
  galena is extremely radiogenic and thus yields
  future model ages, which suggest sources in the
  upper crust. The lead isotopic values are often
  zoned across whole districts, within individual
  deposits, and even within single crystals of
  galena such zoning suggests multiple sources of
  lead, a long period of
• mineralization, or both.

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Tectonic setting and other factors (Ref
Introduction to Ore-Forming Processes, Laurence
Robb, Blackwell, 2004)

• I. The majority of MVT deposits worldwide formed
  in the Phanerozoic Eon, but more specifically, in
  Devonian to Permian times related to formation
  of Pangea. some deposits also formed during
  the Cretaceous-Paleogene period related to the
  Alpine and Laramide orogenies associated with
  compressional tectonic regimes.
• II. Other factors
• A. Carbonate host rocks in hydrologic contact
  with orogenic belts
• B. Low latitude (at time of formation)
• 1. High rainfall (at time of formation)
• 2. Association with sabkhas producing high
  salinity solutions
• C. Transport problematic (It is difficult to
  pinpoint where the MVT materials came from and
  how they travelled.)

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Additional material
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Geochemical models for transportation and
precipitation of metals and sulfur
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Questions to resolve

• 1. What were the mechanisms of fluid flow and the
  pathways during migration?
• 2. How long did the flow systems persist, and how
  much fluid passed through the site of ore
  deposition?
• 3. Did the brines become ore-forming fluids
  before, during, or after migration?
• 4. What were the sources of the ore-forming
  constituents, their mechanisms of transport, and
  their concentrations in the brines?
• 5. What chemical reactions were responsible for
  the precipitation of the sulfide ore minerals?
• McLimans study of Upper Mississippi Valley
  deposits
• Relatively high fluid inclusion temperatures (to
  220?C)
• Distinctive color bands in sphalerite traceable
  for distances of km in some cases
• C. Repeated deposition and dissolution of
  sphalerite
• McLimans et al used B. and C. to argue for a
  solution that carried both metals and sulfur
  (rather than mixing at the site). Models II. or
  III. of Table

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Relationships between aquifer lithologies, states
of saturation of migrating fluids, and metal
abundances in resultant ores
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Fletcher Mine, Viburnum TrendText Fig. 2, p.209
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Summary of the characteristics of three
Mississippi Valley-type districts
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Age and duration of Mississippi Valleytype
ore-mineralizing events M. T. Lewchuk D. T. A.
Symons

• The ore-magnetization ages for the six districts
  span gt150 m.y., and these ores are emplaced in
  sediments that are as much as 200 m.y. older, but
  within each district the host-ore magnetization
  pairs differ by only a few million to a few tens
  of millions of years. This suggests a genetic
  link between host-rock remagnetization and ore
  precipitation. Fluid-inclusion data and conodont
  alteration indices (Sangster et al., 1994) for
  the host rocks indicate mineralization or later
  temperatures that are far too low to thermally
  reset an existing remanence (Pullaiah et al.,
  1975). Thus, any remagnetization of the host
  rocks must have been the product of chemical
  interaction involving fluids. The distinct
  host-rock and ore characteristic remanent
  magnetization directions indicate that the fluid
  responsible for the host-rock remagnetization did
  not simultaneously precipitate the ore minerals.
  Either the fluid and/or its environment changed
  or a second fluid passed through each district.

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• Mean pole positions, plotted on Paleozoic
  apparent polar wander path 1990), for ores
  (circles) and host rocks (diamonds) from six
  North American MVT ore districts. Stars indicate
  published mean poles for those districts where
  new data differ. Su, Dl, Dm, Du, Cl, Cu, Pl, Pu,
  and Trl refer to Late Silurian through Early
  Triassic periods.

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St. Peter Sandstone unconformity. St. L.
represents the St. Lawrence group and T.C.
represents the Tunnel City Formation. The
vertical channel depicts a paleo-river valley
later filled in by the St. Peter Sandstone
(after Heyl et al., 1959 Ostrom, 1967 Arnold et
al., 1996). This area was never buried more than
1 km. The project, to test the formation of
quartz overgrowths, indicates that these were
unrelated to MVT ore deposition however, the
article summarizes MVT literature for UMV.
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The model of MVT fluids envisioned by Arnold et
al. (1996) predicts that overgrowths increase in
d18O by about 9 if fluid composition is
approximately constant and temperatures
decrease from 110?C in the south to 50?C in
the north. Bottom line Overgrowths Are low
T Meteoric.
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SULFUR ISOTOPES FROM MISSISSIPPI VALLEY-TYPE
MINERALIZATION IN EASTERN WISCONSIN John Lucjaz
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INTRODUCTION TO GEOENVIRONMENTAL MODELS OF
MINERAL DEPOSITS R. R. Seal II, Nora K. Foley,
and R. B. Wanty
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Naturally occurring isotopes of lead

• Isotope Atomic mass Natural abundance
• (ma/u) (atom )
• 204Pb 203.973020 1.4
• 206Pb 205.974440 24.1
• 207Pb 206.975872 22.1
• 208Pb 207.976627 52.4

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SULFUR ISOTOPES FROM MISSISSIPPI VALLEY-TYPE
MINERALIZATION IN EASTERN WISCONSIN John Lucjaz
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Mineral Stability vs pH, fO2Text Fig. 3.34