Docstoc

FORMATION OF MVT DEPOSITS

Document Sample
FORMATION OF MVT DEPOSITS Powered By Docstoc
					Carbonate and Sandstone
 Hosted Pb-Zn Deposits
                 Outline
• Intro (What, Where, size, tectonic setting)
• Mineralization in deposits
• Using Isotopes to date deposits
• Proposed mechanisms of fluid flow. Metals
  transported to site of deposition in
  hydrothermal brines
• Models for sulfide deposition
• Some useful exploration guides
• Pine Point, NWT, an MVT example
  What Are Carbonate and Sandstone
       hosted Pb-Zn Deposits?
• More often called MVT Pb-Zn deposits
• MVT stands for Mississippi Valley-type lead-zinc
  deposits.
• Named after famous MVT districts within the
  drainage basin of the Mississippi River in the US.
• They are a family of epigenetic ores precipitated
  from basinal brines.
• They are the product of regional or sub-
  continental-scale hydrological processes. These
  large scale processes cause a great diversity
  among MVT deposits to occur.
          Important Characteristics
• 1. Occur principally in Dolostone, sometimes in limestone
  or sandstone.
• 2. Epigenetic and strata-bound.
• 3. Not associated with igneous activity.
• 4. Commonly occur at shallow depths at flanks of basins.
• 5. Occur in platform carbonate sequences, bordering
  foredeeps or in foreland thrust belts.
• 6. Deposits occur in districts that cover hundreds of km.
• 7. Districts localized by geologic features that permit
  upward migration of ore fluids.
• 8. Ore fluids are dense basinal brines (10-30wt% salts)
• 9. Temp. of deposition low (75-200 C)
• 10. Mineralogically simple (sphalerite, galena, pyrite,
  marcasite, dolomite, calcite, Qtz., sometimes
  chalcopyrite, barite and fluorite)
  Where Are MVT Deposits Found in the
               World?
• Major Deposits
  located in Central
  US.
• Western Australia
  hosts significant
  deposits
• Austria, Slovenia
  and Italy host
  notable deposits
  Canada has several
  deposits
• All large deposits
  are located near the
  shelf of a major
  sedimentary basin.
In Canada
16 MVT districts identified
in Canada.
Largest group of deposits
located in the Mackenzie
Mountains of the Yukon
and NWT
Several large deposits
found on the flanks of the
WCSB.
A few deposits in Eastern
Canada: hosted in
deformed carbonate rocks
of the Appalachian foreland
thrust belts
All deposits are located
near a major sedimentary
basin
Size of Canadian and Worldwide MVT
          Deposits/Districts
Ratio of Zn to Pb
           • Most deposits are Zn
             rich relative to Pb
           • Exceptions include the
             Missouri District which
             is almost all Pb
           • Combined Pb+Zn
             grades seldom exceed
             10%
          Tectonic Setting
• Form from large basin wide hydrothermal
  systems.
• Fluid drive provided by plate tectonic
  (orogenic) processes.
• Deposits occur in platform carbonate
  sequences in 2 settings:
  1. Undeformed flat-lying rocks of an
  orogenic forebulge. (Pine Point, Central
  Tennessee districts)
  2. Foreland thrust belts. (Appalachains,
  Western Canada districts)
           District Controls on
              Mineralization
• 1. Shale Edges: Shale units act as aquitards which
  control the migration of ore fluids. Shale units
  underlying carbonates focus fluids upwards. Overlying
  shales confine fluids to the carbonates.
• 2. Limestone to Dolostone Transition: Significant
  increase in intergranular and fracture controlled
  permeability with transition from limestone to pre-ore
  dolostone. This concentrates ore fluids in dolostone.
• 3. Reef and Barrier complexes: reefs provide abrupt
  changes in sedimentary facies. Produces large
  permeability contrasts. This creates changes in fluid
  transmissivity and mixing allowing steep
  physiochemical gradients to form. Results in sulfide
  precipitation.
                   Cont’d

• 4. Pre-ore solution collapse breccias:
  Dissolution of limestone or dolomite pre-ore
  creates cavities where overlying dolomite
  can collapse. This focuses fluid flow in the
  area and allows ore depositional processes
  to operate. Always located beneath
  unconformity or disconformity.
• 5. Basement Topography: Basement highs
  focus fluid flow upward to P&T conditions
  suitable for sulfide deposition. Also controls
  development of facies tracts.
     Deposit Characteristics
• Ores in MVT deposits are extremely varied in
  character and form.
• They range from massive replacement zones,
  to open space fillings of fractures and breccias,
  to disseminated clusters of crystals in pore
  spaces.
• In some instances the zones are arranged in
  linear patterns, suggesting tectonic control.
• Altered solid or liquid H/C of different amounts
  are often found associated with MVT deposits.
  The alteration may be due to thermochemical
  sulfate reduction, a sulfide depositional
  process.
Deposit Characteristics

                    •   Typical breccia
                        body found in
                        MVT deposits.
                    •   3 breccia
                        types/textures:
                        crackle breccia,
                        rock-matrix
                        breccia, ore-
                        matrix breccia.
                    •   Pre-, syn- or
                        post ore
                        hydrothermal
                        dolomite can
                        create
                        alteration halos
                        around
                        deposits.
Ore Textures
Ore Textures
         Transport of Metals
• Metals originate in basin or are
  dissolved along fluid route.
• Although metal-bisulfide and
  organometallic complexes have been
  proposed, metal-chloride complexes are
  considered most reasonable base metal
  transporting species.
• Cl comes from abundance of dissolved
  salt in brines.
• PH of 4.5 or less required to effectively
  transport metals in complexes.
                         Isotopes

• δ34S isotopic
  values indicate the
  sulfur originated in
  connate seawater
  in adjacent
  sedimentary rocks.
• Pb isotopes plot in
  linear array. Means
  multiple sources of
  Pb in MVT
  deposits. Can
  mean multiple
  source rocks or
  source brines.
Age ranges of Deposits
         Mechanisms of Fluid Flow

• The widespread deposition of MVT mineralization at
  shallow depths, Temps higher then local basement-
  controlled geothermal gradients, and lack of igneous
  activity put constraints on mechanisms of fluid flow.
• 3 different models have been proposed to explain the
  drive of ore fluids:
   – 1. The topographic or gravity-driven fluid flow
     model.
   – 2. The sedimentary and tectonic compaction
     model.
   – 3. The hydrothermal convection model.
   Topographic or Gravity-Driven Fluid
              Flow Model
• Involves flushing of
  subsurface brines out of a
  sedimentary basin by
  groundwater flow from
  recharge areas in elevated
  regions.
• Subsurface flow is driven
  away from uplifted areas
  towards the basin’s flanks
  by higher hydraulic head.
  The brines are then
  concentrated in more
  permeable units away from
  the uplifted area.
• This model has been
  proposed for the Pine Point
  deposit.
     Sedimentary and Tectonic Compaction
•   Compaction of
                   Model
  sediments in a
  subsiding basin
  drives a continuous
  outward flow of
  pore fluids laterally
  along aquifers.
• The movement of
  brines out of the
  compacted layers
  leaches the metals
  out of the source
  rocks.
• This model requires
  rapid rates of
  sedimentation (1
  km/m.y.) in order to
  drive fluids out of
  pores.
• Deposits in Western
  Australia are
  believed to have
  formed from this
  model.
    Hydrothermal Convection Model

• Involves deep convection circulation of
  hydrothermal brines due to buoyancy
  forces related to temperatures and salinity
  variations.
• subsurface solutions can be circulated and
  recycled many times through the rock
  mass.
• This model has been proposed to explain
  MVT deposits of the northern Canadian
  Rocky Mountains as well as extensional
  settings (Nanisivik, NWT)
            Deposition of Sulfides

• After transport of the brines specific chemical
  reactions must occur to deposit sulfides in
  economic quantities. 3 models have been
  proposed for this process and they take into
  account the nature of the ore fluids (whether
  or not sulfur travelled with the minerals or
  was introduced at depositional site).
• Possible models are:
  1. The reduced sulfur model
  2. The sulfate reduction model
  3. Various fluid mixing models

*usually a combination of these models is responsible
  for ore deposition
 Back To Chloro-Complexes For A Sec


• Metals transported in brines as
  Chloro-Complexes:
• ZnCl2 (aq) + H2S = ZnS + 2HCl(aq)
• This reaction releases acid which
  dissolves the surrounding carbonate
  forming solution collapse, open-
  spaced structures.
        The Reduced Sulfur Model

• Assumes that metals and reduced sulfur
  (HS-, H2S) are transported together in the
  same fluid.
• Ore deposition is achieved by means of
  cooling, dilution of the brine by meteoric
  waters or less saline groundwater, or by
  changes in pH.
• pH values of 4.5 or lower required to
  transport reduced sulfur and metals in same
  fluid. When the brines encounter a carbonate
  aquifer a quick increase in PH can occur. This
  will cause the sulfides to quickly be deposited
  out.
         The Sulfate Reduction Model

• Involves transport
  of base metals and
  sulfate in the same
  solution. The brines
  pick up the sulfate
  when they
  encounter
  evaporites.
• Organic matter
  (methane) present
  in the carbonates
  reduces the sulfate
  to H2S which
  reacts with lead
  and zinc to form
  the sulfides.
                  Fluid Mixing Models
• 2 different brines are
  mixed at the site of
  deposition. Usually a
  metal-rich brine and
  a H2S rich brine are
  mixed.
• The mixing of fluids
  will produce high
  degrees of
  supersaturation,
  large chemical
  and/or concentration
  gradients and
  relatively rapid rates
  of precipitation of
  fine grained,
  colloform sulfides.
             Exploration Guides
• Great diversity in local and regional controls of
  mineralization make it hard to create a list of
  universally applicable features.
• Within districts features like basement highs, facies
  changes and unconformities can be used to explore
  for additional deposits.
• When looking for new districts exploration should be
  directed toward platformal carbonate sequences
  cratonward of foreland basins and fold belts.
• High priority zones include: areas of rapid facies
  change, highly permeable zones such as reefs and
  faults, large areas of sparry dolomite and collapse
  breccias
• Geophysical methods can locate potential MVT ore
  bodies, but usually only find deposits with significant
  Pb content
• Pb and Zn can be found in soil and stream sediments
  to varying degrees near ore bodies.
             Genetic Model
• Range of geological and geochemical
  processes involved in transport and
  deposition of the ore components make it
  difficult to construct a genetic model.
• constants in MVTs:
  1. Temp of ore fluids- fluid inclusion
  temps range from 75˚-150˚C.
  2. Salinity of ore fluids- exceed 10 wt.%
  salts.
  3. Isotopes- indicate Pb derived from
  crustal source, S derived from seawater.
  4. Widespread deposition of sulfides over
  large area coinciding with tectonic uplift.
               The Pine Point District
• During production at
  Pine Point (1964-1988),
  approximately 26
  percent of the nation's
  lead and 17 percent of
  its zinc were extracted
  on this single property.
  The value of Pine Point
  ore milled during the
  final year of significant
  production (1987) was
  near $500 million.
• The mine produced and
  shipped 10,785,000 tons
  of lead and zinc
  concentrates after
  mining and milling
  69,416,000 tons of ore
  material. Contained
  metal was
  approximately 2 million
  tons of lead and 7
  million tons of zinc.
Location
Ore-body Distribution
Ore-Body Size and Grades
              Pine Point Ore and Textures




Note the rounded collapse fragment surrounded by   COLLAPSED DOLOMITIC CAVE SEDIMENTS
high grade sphalerite and galena mineralization.   RIMMED BY SPHALERITE AND GALENA.




  Coarse sphalerite and dolospar (Zebra banding)
  suggests that there were cyclic periods of       NOTE THE WIRE GALENA PARTLY ENCLOSED
  mineralization.                                  BY THE CALCITE CRYSTAL.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:350
posted:9/26/2010
language:English
pages:35