Carbonate and Sandstone
Hosted Pb-Zn Deposits
• 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
• 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
• 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.
• 1. Occur principally in Dolostone, sometimes in limestone
• 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
• Major Deposits
located in Central
• Western Australia
• Austria, Slovenia
and Italy host
Canada has several
• All large deposits
are located near the
shelf of a major
16 MVT districts identified
Largest group of deposits
located in the Mackenzie
Mountains of the Yukon
Several large deposits
found on the flanks of the
A few deposits in Eastern
Canada: hosted in
deformed carbonate rocks
of the Appalachian foreland
All deposits are located
near a major sedimentary
Size of Canadian and Worldwide MVT
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
• Form from large basin wide hydrothermal
• Fluid drive provided by plate tectonic
• Deposits occur in platform carbonate
sequences in 2 settings:
1. Undeformed flat-lying rocks of an
orogenic forebulge. (Pine Point, Central
2. Foreland thrust belts. (Appalachains,
Western Canada districts)
District Controls on
• 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
• 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.
• 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
• 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
• Typical breccia
body found in
• 3 breccia
• Pre-, syn- or
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
• Cl comes from abundance of dissolved
salt in brines.
• PH of 4.5 or less required to effectively
transport metals in complexes.
• δ34S isotopic
values indicate the
sulfur originated in
• Pb isotopes plot in
linear array. Means
multiple sources of
Pb in MVT
source rocks or
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
– 2. The sedimentary and tectonic compaction
– 3. The hydrothermal convection model.
Topographic or Gravity-Driven Fluid
• Involves flushing of
subsurface brines out of a
sedimentary basin by
groundwater flow from
recharge areas in elevated
• 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
Sedimentary and Tectonic Compaction
• Compaction of
sediments in a
drives a continuous
outward flow of
pore fluids laterally
• The movement of
brines out of the
leaches the metals
out of the source
• This model requires
rapid rates of
km/m.y.) in order to
drive fluids out of
• Deposits in Western
believed to have
formed from this
Hydrothermal Convection Model
• Involves deep convection circulation of
hydrothermal brines due to buoyancy
forces related to temperatures and salinity
• subsurface solutions can be circulated and
recycled many times through the rock
• 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
• ZnCl2 (aq) + H2S = ZnS + 2HCl(aq)
• This reaction releases acid which
dissolves the surrounding carbonate
forming solution collapse, open-
The Reduced Sulfur Model
• Assumes that metals and reduced sulfur
(HS-, H2S) are transported together in the
• 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
The Sulfate Reduction Model
• Involves transport
of base metals and
sulfate in the same
solution. The brines
pick up the sulfate
• Organic matter
in the carbonates
reduces the sulfate
to H2S which
reacts with lead
and zinc to form
Fluid Mixing Models
• 2 different brines are
mixed at the site of
deposition. Usually a
metal-rich brine and
a H2S rich brine are
• The mixing of fluids
will produce high
relatively rapid rates
of precipitation of
• 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
• Geophysical methods can locate potential MVT ore
bodies, but usually only find deposits with significant
• Pb and Zn can be found in soil and stream sediments
to varying degrees near ore bodies.
• 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.%
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),
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
mining and milling
69,416,000 tons of ore
approximately 2 million
tons of lead and 7
million tons of zinc.
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.