HYDROTHERMAL DEPOSITS:

Mississippi Valley-type (MVT) deposits

Iron-Oxide Copper Gold (IOCG) Deposits
Porphyry Deposits

Dr. Devi K
Assistant Professor (C)
Department of Geology
Farook College (Autonomous) Kozhikode
 MVT Deposits
• Mississippi Valley-type (MVT) deposits are epigenetic
stratabound carbonate-hosted sulphide bodies composed
predominantly of sphalerite and galena with associated fluorite
and barite.
• These deposits account for approximately 25 percent of the
world's lead and zinc resources.
• They are so-named because several classic MVT districts are
located in carbonate rocks within the drainage basin of the
Mississippi River in the central United States (US).
• Important Canadian districts include Pine Point, Cornwallis,
Nanisivik, Newfoundland Zinc, Gays River, Monarch-Kicking Horse,
and Robb Lake
The sandstone-hosted MVT deposits characterized by Pb>Zn>>Cu
 MVT Deposits
• The deposits occur mainly in dolostone as open-space fillings,
collapse breccias and/or as replacement of the carbonate host
rock.
• Less commonly, sulphide and gangue minerals occupy primary
carbonate porosity. The deposits are epigenetic, having been
emplaced after lithification of the host rock. Metals can be
deposited tens of millions of years after sediment deposition.
• MVT deposits originate from saline basinal metalliferous fluids
at temperatures in the range of 75°-200°C.
• They are located in carbonate platform settings, typically in
relatively undeformed orogenic foreland rocks, commonly in
foreland thrust belts
• Their origin to fluid circulation and metal transport/ deposition
within sedimentary basins
 MVT - GENESIS
• causes of fluid flow and hydrothermal activity involved in their
formation.
• It is generally accepted that topographically driven fluid
pathways were critical to the development of large MVT ore
districts, and that the carbonate host rocks maintained a
hydrological connection to orogenic belts active during the
period of ore deposition
• Other features believed to conceptually link most MVT deposits
include a low-latitudinal setting, where high rainfall ensured an
adequate fluid reservoir, and the presence, somewhere in the
fluid flow system, of a high-evaporation sabkha (Brine)
environment that resulted in the high fluid salinities necessary
for viable levels of metal solubility and transport
• Regional fluid flow is stimulated by compressional orogeny that results in
thrust faulting and uplift and this, in turn, creates a topographic head and
fluid flow down a hydrological gradient.
• Fluid flow in these tectonic settings occurs over distances of hundreds of
kilometers and such fluids are implicated in the migration of hydrocarbons as
well as metals
• At the site of metal deposition MVT ores are sometimes bedded and focused
along conformable dolostone/limestone interfaces, but are more commonly
associated with discordant, dissolution-related zones of brecciation.
• The hydrothermal fluids that are linked to MVT deposit formation are
typically low-temperature (100-150 degree C) high-salinity (>15 wt% NaCl
equivalent) brines, with appreciable SO4 2−, CO2 and CH4 and associated
organic compounds and oil-like droplets (Gize and Hoering, 1980; Roedder,
1984).
• These compositional characteristics are not unlike oil-field brines and reflect
protracted fluid residence times in the sedimentary basin.
• In many MVT deposits ores occur as cement between carbonate breccia
fragments suggesting that brecciation occurred either before (perhaps as a
• Hydrothermal dissolution of carbonate requires
acidic fluids and could occur by a reaction such
as:

• The fluid itself might originally have been

acidic, or hydrogen ions were produced by
precipitation of metal sulfides according to the
reaction:
• The production of additional Ca2+, Mg2+, and
CO2 in the fluid as a result of carbonate
dissolution provides the ingredients for later
precipitation of calcite and dolomite, a feature
that is observed in some MVT deposits where
secondary carbonate gangue minerals cement
previously precipitated ore sulfides
• Under light load, slumping or gentle synclinal folding of brittle
sedimentary beds gives rise to a series of connected tension cracks or
openings collectively known as 'pitches and flats. Gentle open folding
also forms anticlinal tension cracks at the crests of anticlines (or
troughs of synclines).
• Pitches and Flats. Figures 5*4-21 and 5*4-22 show typical openings
of the upper Mississippi Valley. These openings occur in the Galena
dolomitic limestone and are encrusted or filled with zinc and lead ore
 References
• Laurence Robb, An introduction to ore forming
process, 2005 by Blackwell Science Ltd, 197-
209
• Allan Bateman Economic mineral deposits
Iron-Oxide Copper Gold (IOCG) Deposits
 IOCG
• Iron oxide copper-gold (IOCG) deposits (Hitzman et al.,
1992) are a diverse family of mineral deposits
characterised by the following features:
• (1) Cu with or without Au, as economic metals,
• (2) hydrothermal ore styles and strong structural controls,
• (3) abundant magnetite and/or hematite,
• (4) Fe oxides with Fe/Ti greater than those in most igneous
rocks, and
• (5) no clear spatial associations with igneous intrusions as,
for example, displayed by porphyry and skarn ore deposits
 IOCG
• Most IOCG deposits display a broad space-time
association with batholithic granitoids, occur in
crustal settings with very extensive and commonly
pervasive alkali metasomatism, and many are
enriched in a distinctive, geochemically diverse suite
of minor elements including various combinations
of U, REE, F, P, Mo, Ag, Ba, Co, Ni and As
• magmatic-hydrothermal, low-sulphur, iron oxide-
copper-gold (IOCG) deposits, characterized by large
masses of magnetite or haematite
 IOCG
• A distinctive feature of IOCG deposits is the presence of two
distinct fluids during deposit formation:
• (1) a highly oxidised fluid (e.g., meteoric/ground waters), and
• (2) deep-sourced high-temperature brines (magmatic-
hydrothermal fluids and/or fluids reacted with metamorphic
rocks).
• Fluid Pathways: Fluid flow is enhanced by juxtaposition of
earlier rift basins with this high-temperature melt province.
Pre-existing basinal structures and second-order cross
structures (e.g., conjugate fault sets) localise dilational
deformation, brecciation (at high crustal levels), and fluid flow.
The intersections of second-order faults with crustal-scale
terrane boundaries are favoured locations for IOCG systems.
 IOCG
• Plutonic (intrusion-related) deposits
• Magmatic-hydrothermal iron oxide-copper-
gold (IOCG) deposits were proposed as a new
class of deposits after the discovery of the
spectacular copper-uranium deposit at
Olympic Dam in South Australia in 1976
 IOCG-GENESIS
• Iron oxide copper gold deposits typically form within 'provinces' where several
deposits of similar style, timing and similar genesis form within similar geologic
settings. The genesis and provenance of IOCG deposits, their alteration
assemblages and gangue mineralogy may vary between provinces, but all are
related to;

• Major regional thermal event broadly coeval with IOCG formation, represented
by low to medium grade metamorphism, and/or mafic intrusions, and/or I- or
A-type granitoids
• Host stratigraphy is relatively Fe-enriched (BIF, ironstones), but have relatively
little reduced carbon (e.g.; coal, etc.).
• Regional-scale alteration systems, operating over tens or hundreds of
kilometres, involving admixture of at least two fluids
• Large-scale crustal structures which allow extensive hydrothermal circulation of
mineralising fluids
 IOCG –Ex: Olympic Dam
• The Olympic Dam deposit occurs within an anorogenic
oxidized potassic granite (dated to ~1590Ma), which is set
within a Palaeo-/Mesoproterozoic graben
• covered by 350 m of younger, unmineralized sedimentary
rocks.
• Host rocks are coarse haematite-rich granite breccias of
explosive volcanic and phreatomagmatic origin
• The breccia ore contains copper sulphide and by-product
grades of rare earth elements, uranium, gold and silver.
• Total resources are estimated to ~7700Mt of ore with 0.9%
Cu, 0.3 kg/t U3O8, 0.3 g/t Au and 1.6 g/t Ag
 IOCG –Ex: Olympic Dam
• The mineralization appears to be the product of mixing
of ascending hot magmatic brines (carrying reduced
sulphur species) with shallow highly oxidized haematite-
forming groundwater leaching uranium and LREE.
• The source of copper and gold can hardly have been the
host granite
• Mingling of mafic and silicic melt (Clark & Kontak 2004),
deep crust (Heinson et al. 2006) and fertile mantle
enriched by prior subduction (Groves et al. 2010,
Skirrow et al. 2007) may have contributed to the metal
endowment
 REFERENCE
• https://
www.ga.gov.au/data-pubs/data-and-publicatio
ns-search/publications/critical-commodities-fo
r-a-high-tech-world/iron-oxide-copper-gold
• Walter L. Pohl (2011), Economic geology :
principles and practice : metals, minerals, coal
and hydrocarbons introduction to formation
and sustainable exploitation of mineral
deposits , Wiley Blackwell publication pp 188
Porphyry Deposits
• Host rocks of copper (Au-Mo) porphyry deposits are shallow (<4km) subvolcanic and
mostly cylindric intrusions.
• The parent rocks are frequently calc-alkaline diorites (or the volcanic equivalent andesite-
dacite), monzonites (latite) or granites (rhyolite) of I-type that occur in volcanoplutonic
arcs above subduction zones, either on active continental margins or in island arcs
• Subduction of topographic and thermal anomalies appears to favour copper porphyry
genesis (Cooke 2005).
• Less frequent are Cu-Au porphyry and related epithermal gold deposits in continental
collision zones and post-subduction settings
• Their hydrous nature, elevated oxidation and SO2 contents (including anhydrite as a
magmatic phase) are the main differentiating characteristics in comparison to the
numerous barren intrusions of convergent plate margins.
• Porphyries on active continental margins are marked by elevated Sn and Mo
concentrations apart from copper, those of island arcs more often contain gold as the
second metal.
• Porphyry deposits older than Early Tertiary are rare (Cooke et al. 2005) but the oldest
date from the Archaean
• Several porphyry copper ore deposits display a strong epithermal signature (cf. “Volcanogenic
Ore Deposits”) and overprinting of earlier hydrothermal alteration, for example clays replacing
potassic zone minerals, as at the gold deposit
• Sources of the metals concentrated in porphyry copper ore deposits are probably deeper
mafic magmas but ultimately a fertile mantle.
• Melts and supercritical fluids that originate in the subducting oceanic crust are oxidizing and
dissolve chalcophile metals.
• Absence of reduced sulphur in the source region is a precondition because otherwise sulphide
melts would form, which must lag behind rising silicate liquid
• In epizonal staging chambers, however, an intermediate step of sulphide melt formation may
intervene
• Transfer of the metals from mafic melt into the subvolcanic felsic magma may be enacted by
highly metal-charged ore brines
• The shallow felsic magma is the apparent source of ore fluids that cause brecciation, alteration
and mineralization in the solid roof, concomitant with the dynamics of a rising stratovolcano
• The activity of individual porphyry systems may last for ~100,000 to several million years;
districts are active for 10–20 My
• Figure 1.34 Genetic
concept of deep
magmatic processes
preparing the
formation of a
porphyry copper
deposit
• The average porphyry copper deposit comprises 1000 million tonnes
(Mt) of ore.
• Common hypogene ore grades are 0.5–1.5% Cu, 0.01–0.04% Mo and 2
Mt, supergiants such as Chuquicamata (Figure 1.31), >24 Mt of copper
metal.
• The largest is El Teniente in central Chile, with >94 Mt of contained
copper (Cannell et al. 2005). Large copper porphyry open cut mines
expose orebodies that extend over several km2 .
• Alteration halos surrounding ore can be 10 to 20 times this size. Metal
contents are generally low but the large mass allows profitable mining.
• Apart from copper, by-product metals include significant tonnages of
Ag, Au, Mo, Re, Pb, Zn, Mn and minor amounts of As, Bi, Sn, W, U and
Pt. Clusters of giant deposits that far exceed average copper porphyry
size occur in central and northern Chile, and southwestern Arizona to
northern Mexico
• Porphyry copper systems develop above large parental magma
chambers at 5 to 15 km palaeodepth, which establish the heat and
flux regime of both the porphyry intrusions and the mineralizing
fluids. In a non-telescoped system, the potassic hydraulic fracturing-
mineralization-alteration is surrounded by propylitization due to
moderate-temperature hydration reactions.
• Above the porphyry level, advanced argillic (quartz-pyrophyllite-
kaolinite-alunite) and vuggy quartz lithocaps may be formed by
extreme base leaching. The vuggy quartz level is often the site of
epithermal high sulphidation gold mineralization (Sillitoe 2010).
• Intermediate sulphidation mineralization with higher contents of Pb,
Ag, Zn and Mn may be formed at a greater distance from the
porphyry centre
• Porphyry copper ore deposits display a hydrothermal alteration zoning that
is best characterized by the Lowell-Guilbert (1970) model (Figure 1.32a).
• Note that this sketch is time-integrated and displayed features originate at
different times.
• Typically, sericitization and the main ore precipitation overprint earlier
potassic (mafic minerals replaced by secondary biotite) and propylitic
alteration.
• In practice, there is much variance because of complexities such as
repeated intrusive activity and previous alteration of affected rocks (Sillitoe
2010).
• Hydrothermal systems that produce porphyry ores at depth can extend to
the surface where shallow veins and hot springs ore deposits may be
formed (Heinrich et al. 2004, Heinrich 2005).
• Skarn ore may be generated at contacts of the porphyric intrusion with
carbonate rocks.
• Ore formation and the hydrothermal alteration are
caused by magmatic fluids and vapours, at
temperatures between ~ 800 and <300 degree C
• More than 90% of the ore is hosted in a vein stockwork
in the outer potassic and/or the sericite zone
• some ore replaces silicate minerals (“disseminated
ore”).
• Upwards and outwards from the porphyry ore, a halo
of small but high-grade gold, silver and base metal vein
deposits may be present
• In most porphyries, main-stage potassic
alteration mineralization is connected with co-
existing immiscible brine and vapour. Brine is
dense (>1.3 g/cm3 ) and saline (35 to >70 wt.
% NaCl equivalent), with variable contents of
K, Na, Ca, Fe, Mo and Cu chlorides
• The same elements are found in the multitude of daughter minerals in fluid
inclusions (halite, sylvite, anhydrite, chalcopyrite, haematite, Fe-chloride, etc.) of
minerals precipitated at this stage.
• Early fluids have metal ratios that correlate with those calculated for the whole
deposit, which is a further argument for a magmatic derivation of the metals (Ulrich
et al. 1999).
• Another confirmation is the observation that melt and fluid inclusions co-exist in
early hydrothermal quartz (Harris et al. 2003).
• Cogenetic supercritical liquid brine and low-density vapour phase inclusions (D< 0.1
g/cm3)
• document boiling and unmixing of the fluids. Vapour collects acidic volatile species
(SO2, H2S, CO2, HCl, HF) and most Cu and Au, plus much of As, Ag, Sb, Te and B.
• Vapour flow dominates transport and precipitation. Cooling and contraction of
magmatic vapour to a liquid appears to dominate copper mineralization (Klemm et
al. 2007).
• Reduction can be induced by magnetite crystallization that triggers sulphate to
sulphide conversion.