Applied Geology Lecture 9 Stratiform Deposits of Sedimentary and Volcanic Environments

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Applied Geology - Lecture 9Stratiform Deposits
of Sedimentary and Volcanic Environments

• Concordant massive sulfide deposits have 2 basic
  classes
• Sediment hosted - where sedimentary processes
  were important (SedEx)
• Volcanic associated Massive Sulfide (VMS) dep
  where volcanic processes important
• However, there may be overlap between these 2
  classes eg a weak link to volc activity within a
  dominantly exhalative (sed) environm

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Sedimentary Hosted Sulfide Deposits

• Sedimentary exhalative deposits (abbreviated as
  SEDEX from SEDimentary EXhalative) are ore
  deposits which are interpreted to have been
  formed by release of ore-bearing hydrothermal
  fluids into a water reservoir (usually the
  ocean), resulting in the precipitation of
  stratiform ore.http//en.wikipedia.org/wiki/Sedime
  ntary_exhalative_deposits
• Two groups
• 1) Sediment hosted Cu deposits
• 2) Sediment hosted Pb-Zn deposits (often referred
  to as SedEx deposits)

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SEDEX Process
The process of ore genesis of SEDEX
mineralisation is varied, depending on the type
of ore which is deposited by sedimentary
exhalative processes. Broadly, the process
involves generation of ore-bearing fluids in a
source region, which may vary in its chemical
composition, lithology and nature according to
the ore type in question. The ore fluids
generated in the source region is conducted,
usually by faults, toward the seafloor and then
discharges onto the seafloor. Upon mixing of the
ore fluids with the seawater, dispersed across
the seafloor, the ore constituents and gangue are
precipitated onto the seafloor to form an orebody
and mineralisation halo which are congruent with
the underlying stratigraphy and are generally
fine grained, finely laminated and can be
recognised as chemically deposited from solution.
http//en.wikipedia.org/wiki/Sedimentary_exhalativ
e_deposits
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Mineralisation types
SEDEX mineralisation is best known in lead-zinc
ore deposit classification schemes as the vast
majority of the largest and most important
deposits of this type are formed by
sedimentary-exhalative processes. However, other
forms of SEDEX mineralisation are known The vast
majority of the world's barite deposits are
considered to have been formed by SEDEX
mineralisation processes The scheelite
(tungsten) deposits of the Erzgebirge in
Czechoslovakia are considered to be formed by
SEDEX processes The gold deposits of Nevada are
considered to be stratiform chert formed by SEDEX
processes on the seafloor The immense Broken
Hill, Century Zinc, Lady Loretta, and Mt Isa
deposits in Australia, the sullivan, Red Dog and
Jason deposits of North America and the Hindustan
zinc belt in India are all SEDEX type deposits.
http//en.wikipedia.org/wiki/Sedimentary_exhalativ
e_deposits
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Stratiform Sulfide Deposits of Sedimentary
Affiliation

• Occur in non-volcanic marine or deltaic environm
• Widely distributed in time and space
  Proterozoic-Tertiary
• Vary in size up to several hundred million tonnes
• Usually lensoid to stratiform in shape and
  length, atleast 10x breadth
• Often more than one ore layer present
• Feeder zones have been identified beneath some
  deposits
• Degree of metmsm and defm varies across
  individual deposits indicating a pre-metm
  formation
• Frequently organic rich host rock (black shale)
• Sulfides are fine-grained

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Mount Isa Stratiform Pb-Zn Ore
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Tectonic Setting of Sedimentary Sulfide Deposits

• Geological setting is generally intracratonic
  including
• First marine transgressions over continental
  deposits (Kupferschiefer, Zambia White Pine, USA
• Carbonate shelf sequence (Ireland)
• Fault controlled sedimentary basins or
  aulocogens failed rift valleys (Selwyn Basin,
  Yukon Belt-Purcell Basin British Columbia)
• Important source of Cu, 2nd only to porphyry Cu
  deposits the worlds most important source of
  cobalt, with Ag also becoming an important
  by-product

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Sediment-hosted Copper Deposits

• Grades vary between 1-5 Cu
• Tonnages can be enormous eg Lubin, Poland 2600Mt
  _at_ 2 Cu, 30-80 g/t Ag 0.1g/t Au
• Most deposits occur in reduced, pyritic,
  organic-rich, calcareous shales with the
  remaining 30 occurring in sandstone
• Host rocks form in anoxic, saline, lacustrine
  sedimentary environm, usually above red,
  oxidised, continental clastic seds
• Commonly interbedded with evaporites
• The mineralized beds contain the following
  minerals hematite, native copper, chalcocite,
  bornite, chalcopyrite, galena, shalerite and
  pyrite

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Sediment-hosted Lead-Zinc Deposits (sedex
deposits)

• Avg size 70Mt _at_ PbZn 12 with important
  by-products Ag, Au, Cd, Cu, Sn barite
• Form a distinct group of ores formed in local
  basins on the sea floor due to hydrothermal
  activity accompanying continental rifting
• Host rocks are shales, siltstones and carbonates
• In Ireland at the Silvermines and Tynagh deps
  fossil hydrothermal chimneys and feeder zones
  have been found indicating these deposits
  probably formed by hydrothermal fluids venting
  into restricted basins on the sea floor
• However, the environm was not deep ocean but more
  like the Gulf of California, where sulfide and
  barite deps are forming today in axial rifts and
  are contemporaneous with sedimentation

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Sediment-hosted Lead-Zinc Deposits (sedex
deposits)

• How were these metal bearing, hydrothermal fluids
  generated?
• 2 models
• Mississippi Valley-type fluids originated
  within and were expelled from clastic sed basins
• Exhalative fluids were formed by sea-water
  convection cells, which dissolved metals from
  sediments they passed through before being heated
  and forced to the surface
• Shallow penetration leads to leaching of Fe, Mn
  SiO2, which accounts for Mn enrichment in
  footwall
• Deeper circulation results in leaching of Pb Zn
  if this is the extent of leaching it will form
  Cu-free deps
• Even deeper circulation will leach Cu which may
  be added to Pb-Zn minzn at a later stage

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Tectonic Setting
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Sediment-hosted Lead-Zinc Deposit Mississippe
Valley Formation
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Sediment-hosted Lead-Zinc Deposit Exhalative
Formation (California)
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Volcanic-associated Massive Sulfide (VMS) Deposits
http//oceanexplorer.noaa.gov/explorations/02fire/
background/plumes/media/fig2.html
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Volcanic-associated Massive Sulfide (VMS) Deposits

• These are an important group of hydrothermal
  deposits that form on the sea-floor often
  referred to as VMS (volcanic-associated massive
  sulfide)
• They are typically conformable bedded in their
  upper sections and epigenetic (stockwork) or
  stratabound in their lower parts
• Deposits of this type have observed forming as
  hydrothermal vents (black smokers) on the
  sea-floor at spreading centres

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VMS Deposits

• Usually small 0.1-10 Mt but occasionally can be
  very big rich eg Rio Tinto, Spain 500Mt _at_ 1.6
  Cu, 2 Zn, 1 Pb, 67 g/t Ag, 1 g/t Au Rosebury,
  Tasmania 19 Mt
• Mineralogy usually consists of gt90 iron sulfide,
  mostly pyrite with some pyrrhotite. Chalcopyrite,
  sphalerite galena may be important constituents
  depending on deposit class along with bornite,
  chalcocite, arsenopyrite, magnetite
• Most VMS deps are zoned. Galena and sphalerite
  being more abundant in the upper half whereas
  chalcopyrite increases toward the
  footwall-staockwork zone

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VMS-associated Wallrock Alteration

• Usually confined to the footwall-stockwork zone
• Chloritization and sericitization are the 2 most
  common forms
• Pipe shaped alteration usually containing the Cu
  minzn
• Diameter of alteration increases upward

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VMS Environment

• Cyprus-type Cu - Assoc with basalts at spreading
  centres
• Kuroko-type - assoc with intermediate-acid volc
  rocks in island-arc environ termed
  (Cu-Pb-Zn-Ag-Au). Occur with lg amounts of qtz,
  barite and gypsum eg Woodlawn NSW, Hellyer Tas.
• Besshi within plate back-arc rifting

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VMS Classification
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Black Smokers
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VMS Formation
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VMS Fluid Circulation
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Stages of VMS Develop-ment Zoning
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Spreading Centres (Cyprus-type VMS)
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Tectonic Settings