Download ES 3210 ECONOMIC MINERAL DEPOSITS Types of Natural Waters

ES 3210 ECONOMIC MINERAL
DEPOSITS
Types of Natural Waters
Stephen J. Piercey
Ryohei Morimoto (University of
Tokyo), 1952.
Explosion from the Bayonnaise Rocks submarine volcano in Japan's
central Izu Islands breaches the sea surface on September 23, 1952
ES 3210 ECONOMIC MINERAL
DEPOSITS
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Supplementary: Robb(2005)
Sections 3.1 & 3.2 [Natural Waters]
Natural Waters that Can Contribute
to Ore Forming Fluids
from Robb (2005)
METEORIC WATER
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Water with its most recent origin being from the
hydrological cycle (i.e., it has been in contact with the
atmosphere)
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Second largest reservoir of liquid water on Earth
In the context of ore deposit formation this is groundwater
infiltrated (downward into) the crust through seepage of
original surface waters
METEORIC WATER
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Exists in the interstitial pores and fracture structures of
rocks
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Can penetrate to very deep levels along faults and
fractures, and therefore circulates throughout the crustal
regime
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A meteoric water component has been detected in a wide
variety of ore-forming fluids
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Important in epithermal Au deposits
SEAWATER
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Seawater comprises 98% of Earth’s liquid water
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Seawater is weakly saline (3.5 wt.% total dissolved salts),
and quite consistent in its global composition
Substantial amounts of water have covered the Earth’s
surface (as oceans) for most of its history
SEAWATER
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Principal solutes are Na+, K+, Ca2+, Mg2+, Cl-, HCO3- SO42-
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It is the principal component of the fluids for
volcanogenic massive sulfides, and other important types
of ore deposit (e.g., SEDEX).
Seawater is often entrained and convected in
hydrothermal systems – particularly those associated
with submarine volcanism and magmatism
“Near surface” water budget of the Earth
Robb (2005); Figure 3.2
Seawater entrained in hydrothermal convention at mid-ocean ridges
Robb (2005); Figure 3.6c
CONNATE WATER
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Water included within the interstitial pore spaces of
sediments as they are deposited is connate (or
formational) water
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Connate water is therefore originally meteoric water or
seawater
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It is released by various processes during burial and
induration of sediments
CONNATE WATER
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Deep basinal waters commonly become quite saline (more
saline than seawater)
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Processes that increase salinity include “membrane”filtration by shales, and the assimilation of evaporitic
sequences
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The expulsion of connate brines by later processes of
basin dewatering are important in the genesis of MVTs
and related ore deposit types.
CONNATE WATERS IN
LARGE SEDIMENTARY
BASINS
VARIATION OF SALINITY WITH DEPTH
Robb (2005); Figure 3.2
MAGMATIC WATER
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Almost all magmas contain some amount of dissolved
H2O (and CO2, Cl and other volatiles)
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Processes of crystallization, cooling and decompression
during their emplacement can cause the exsolution of a
separate magmatic fluid phase
MAGMATIC WATER
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These fluids are hot (typically 600°-800°C) and can
develop high salinities and metal contents
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They play a primary role in the formation of, for example,
porphyry Cu deposits
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They are also detected as a component in the ore-forming
fluid of many other deposit types.
From Ridley (2013)
METAMORPHIC FLUIDS
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Fluids are often released during phase transformations
that affect the breakdown of hydrous minerals
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These fluids may contain H2O, CO2, CH4, sulphur species,
and other volatile components
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They tend to have low salinities, and low contents of
reduced sulphur
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Important for orogenic Au deposits
Summary
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Meteoric - e.g., impt in some ore deposits (epithermal Au).
Seawater - e.g., impt in VMS, SEDEX (+/-MVT).
Connate brines - e.g., impt in sediment-hosted deposits.
Metamorphic - e.g., impt in some gold deposits.