The Timing and Duration of Hydrothermal Events
Holly J. Stein, Department of Earth Resources, Colorado State University,
Fort Collins, CO 80523-1482 USA [hstein@cnr.colostate.edu]
Larry M. Cathles, Department of Geological Sciences, Cornell University,
Ithaca, NY 14853 USA [cathles@geology.cornell.edu]
Why would anyone spend their time dating or modeling ore deposits? In
a society where "timing is everything" and "computers are
king", it seems a bit curious to even ask this question. As geoscientists,
however, we know the value of dating in determining the genesis of ore
deposits. For example, we are well aware how our long-standing difficulties
in dating Mississippi Valley-type Pb-Zn deposits have limited our ability
to causally relate them to other geologic events. And, although we all
probably still tend to the intuitive view that a fully adequate amount
of time must always be available to make something as valuable as an ore
deposit, as this issue will emphasize, we now must recognize that ore-forming
episodes may be but brief punctuations within much longer-lived geologic
cycles.
This century has seen the development, but not necessarily the confluence
of two technologies that powerfully address the timing and duration issues:
radiometric dating and computer modeling. The measurement and calculation
of absolute geologic time became possible with the invention of mass spectrometry.
For the first time, geologic events could be discussed in an absolute sense
relative to one another. Initially, knowledge of the correct decay constants
for parent isotopes was a limitation in providing accurate age information.
More recently, we have been able to produce not only accurate, but also
highly precise ages that can resolve even the most subsidiary perturbations
within the larger geologic scheme. This thinner and thinner slicing of
geologic time has been possible with low blank laboratories and continually
advancing mass spectrometry. Our appetite for precision will probably always
exceed our technological capabilities, but the precisions now possible
with modern techniques are truly astounding, particularly to those who
have not closely followed (and been frustrated by involvement in) rapidly
developing fields in geochronology. As an example pertinent to this volume,
one need only look at the recent explosion of papers in the literature
demonstrating the direct and highly precise dating of ores using the Re-Os
method. The Re-Os method is the only radiometric dating tool that can be
applied to suites of cogenetic sulfide and/or oxide minerals, typically
characterized by highly variable Re/Os ratios, thereby permitting better
isochron resolution and more precise age determinations. At the same time,
computer modeling techniques (with their own set of internal annoyances)
have developed to the point where we can now realistically incorporate
into models precise radiometric ages and the complicated patterns of intrusion
and faulting provided by geological mapping. These numerical models will
be particularly effective in establishing and testing causative connections
in ore-forming processes.
It is within this context that the papers in this special issue of Economic
Geology are offered. The papers are based largely on presentations
made at an SEG symposium on "The Timing and Duration of Hydrothermal
Events", held November 8, 1995 in New Orleans. The symposium sought
to assess and compare estimates of the duration of hydrothermal events
derived from numerical modeling with results derived from radiometric age
determinations. The symposium theme arose in part from the relatively long-held
perception that modeling estimates for the duration of hydrothermal events,
which spanned tens of thousands of years, were incompatible with radiometric
estimates for ore-forming systems, which spanned several millions of years.
The conference broadly addressed the duration and episodicity of ore-forming
activity in the intrusive, diagenetic (sedimentary basin), and metamorphic
environments. The papers in this issue show that the full array of dynamic
geologic events associated with hydrothermal and mineralizing episodes
may, in fact, encompass several millions of years, but the actual duration
of ore-forming pulses is on the scale of only tens to hundreds of thousands
of years (i.e., about 10,000 to 200,000 years). This represents an important
consensus between laboratory and computer results.
The first group of six papers addresses the duration and episodicity
of mineralization processes associated with igneous events. The first two
papers in this set use numerical modeling to estimate duration; the next
three papers approach the duration question using radiometric dating at
specific mineral districts; the last paper examines the effects of plume-related
magmatism on the global scale, with specific reference to changes in the
marine environment.
In the first paper, Cathles et al., explore the question: "How
long can a near-surface hydrothermal system be sustained by emplacement
of a single intrusion?" System duration is maximized if a large volume
of magma intrudes deep in a host environment just permeable enough to allow
convection. A 40 X 2 km thick sill, emplaced between 16 and 18 km depth,
can sustain geothermal activity for ~800,000 years at a single, near-surface
site. Since in this model all parameters were chosen at the end of the
geologically plausible range that maximizes hydrothermal duration, the
authors consider one million years a reasonable estimate for the longest
period of time that a hydrothermal system could be sustained by a single
intrusion.
In the next paper, Mizuta and Scott, applying experimental data on iron
diffusivity to sphalerite containing chalcopyrite and pyrrhotite inclusions,
devise a "sphalerite speedometer" to estimate the cooling time
of skarn deposits. This is achieved by modeling depletion in the iron content
of sphalerite within 70 microns of pyrrhotite inclusions which ceased exsolving
from the sphalerite host at ~350C. Iron diffusion ceased at 245C. From
the distinctive error function form of the iron depletion halo near the
sphalerite laths, Mizuta and Scott calculate a cooling rate of 0.5C/1000
years, and show that the time needed to cool from 350C to 245C took no
longer than 210,000 years.
In the first paper utilizing radiometric dating, Marsh et al. obtained
high resolution 40Ar-39Ar ages for primary phenocrysts
and related hydrothermal minerals from seven intrusive centers in the Potrerillos
Cu-Au-Ag district in Chile. They show that although porphyry and related
mineralization district-wide occurred over a period of more than 8 million
years, the emplacement, mineralization, and cooling, associated with individual
porphyry stocks was of short duration, spanning only about 10,000 to
100,000 years. Dating by the 40Ar-39Ar method is
sufficient to definitively tie together individual intrusions with their
associated mineralization. They also note that when using the 40Ar-39Ar
method on relatively young (Eocene-Oligocene) rocks, the percent uncertainty
in the analytical method translates to an absolute time interval that is
very near the time required to complete a single intrusive-hydrothermal
cycle. In other words, we can barely resolve the duration of ore-forming
episodes for young deposits. This emphasizes the need to further improve
our dating resolution.
The paper by Henry et al. shows that the 16 million oz Au deposit at
Round Mountain, Nevada formed in an environment where the hydrothermal
activity associated with Au deposition lasted 50,000 to 100,000 years.
Gold was deposited within 500 m of the paleosurface along the ring fractures
of a caldera which had collapsed 500,000 years earlier. The Au mineralization
at Round Mountain was likely produced by an as yet unidentified intrusion
into the ring fracture. Again, the intrusion of a stock, mineralization,
and cooling sequence, was completed within a time interval that approaches
the analytical uncertainty in the 40Ar-39Ar dating
method. As a result, the intrusion-mineralization-cooling sequence could
be significantly less than 50,000 years.
The paper by Stein et al. presents highly precise Re-Os ages from two
molybdenite deposits in the East Qinling molybdenum belt, Shaanxi province,
China. A highly unusual carbonatite-hosted Mo-Pb deposit is exposed in
the Qinling belt, and seven replicate analyses yield a Re-Os age of 221.5
0.3 (0.15%), which is known to be a time of regional compression. A Re-Os
age of 138.4 0.5 Ma, based on two analyses, was obtained for a Climax-type
granite-molybdenum system located within a few km of the carbonatite Mo-Pb
deposit. Stein et al. suggest that the East Qinling molybdenum belt provides
a rare glimpse of the mantle's role in producing a molybdenum-fertile lower
crust which can be subsequently tapped during periods of tectonic extension.
This model may be applied to other regions containing well-developed Climax-type
molybdenum mineralization, for example, the Colorado mineral belt. This
study emphasizes that regional scale preparatory geologic events may take
tens of millions of years to set the stage for very short episodes of ore
deposition.
Finally, Sinton and Duncan investigate the consequences of the submarine
extrusive eruption of a volume of flood basalt similar to the sill volume
modeled in the first paper by Cathles et al. (~10,000 km3 in
both cases). Single flows with volumes of 1000's of cubic kilometers have
been mapped in continental flood basalts. Three major ocean plateaus were
constructed from plume-related basalts that were extruded near the Cenomanian-Turonian
(C-T) boundary at ~92 Ma: the Caribbean plate (93-87 Ma), the Ontong-Java
plateau (second phase of construction 94-84 Ma), and the Madagascar flood
basalt province with a mean age of 87 0.6 Ma. Sinton and Duncan argue that
the fertilizing effects of the sudden discharge of 10,000 km3
of Fe-rich, reducing hydrothermal fluids could have driven the world's
oceans anoxic, thus accounting for marine extinctions and the increase
in the accumulation and preservation of organic-rich sediments (black shales)
that is observed at the C-T boundary.
The next group of two papers addresses mineralization and episodic fluid
movements in sedimentary basins. Tompkins et al. present a detailed study
of the Cadjebut Mississippi Valley-type Pb-Zn deposit on the Lennard shelf,
western Australia. In this example, the regional stress shifted from weak
extension to weak compression (reverse faulting) as mineralization occurred.
From this structural examination, as well as from stable isotopic evidence,
the authors argue that mineralization occurred over a 35 million year period,
starting close to the time of maximum basin development (Late Devonian)
and extending into a subsequent period of uplift and erosion (mid-Carboniferous).
They document that at least six pulses of basin brine contributed metals
to the Cadjebut deposit. The ores became progressively more hydrocarbon-rich,
and the mineralizing fluids became increasingly overpressured with time.
They suggest that heating of the basin, resulting from uplift and passing
through the ocean thermocline, may have led to rapid gas generation, to
the episodic expulsion of overpressured basin brines, and to pulses of
hydrocarbon-rich mineralization.
Some of the best data for understanding the architecture and behavior
of sedimentary basins comes from observation of basins that are currently
active. Roberts and Carney provide this perspective with their paper on
episodic fluid, gas, and sediment venting in the northern Gulf of Mexico.
The most rapid venting, consisting of a slurry-like mixture of sediment,
gas, water, and unaltered crude oil at >2000 m depth, occurs at mud
volcanoes ~35 m high and ~500 m in diameter. Mud volcanoes are aligned
along and clearly controlled by faults. The venting rate and sediment instability
strongly inhibit biologic activity and as a result, mud volcano sites are
usually barren except for bacterial mats. Swirls in these mats sometimes
reveal convection in the muds filling the axial craters of the volcanoes.
Following the mud volcano stage of rapid venting, slower but sustained
leakage feeds clathrate accumulations, and together both the venting and
clathrates sustain methane- and sulfide-based chemosynthetic organisms
(e.g., mussels, tube worms, bacterial mats). Hydrocarbon oxidation at this
stage provides CO2 for carbonate mounds and hardgrounds; hydrocarbon
reduction of sulfate provides H2S for sulfide-based tubeworms.
Finally, as venting wanes to levels that can sustain only Beggiatoa mats,
the vents become sites for carbonate and barite deposition. Little is known
about the temporal variability of venting, but water temperature and clathrate
stability are likely controlled by the passage of Gulf Stream loop currents
several times a year. The venting cycle, from active mud volcanoes to the
quiet venting that deposits mainly carbonates, probably lasts hundreds
to thousands of years. Overall, control is exerted by the 100,000 year
glacial cycle which produces cyclic changes in sea level, sedimentation,
and salt diapirism.
The volume concludes with a numerical modeling paper by Hanson that
provides a fascinating look at metamorphically-produced fluid movements
on the continental scale. He demonstrates that metamorphic reactions can
expel 106 kg/m2 of water over the ~30 million years
following continental overthrusting. The pattern of expulsion is independent
of crustal permeability because the fluids are able to hydrofracture their
environment, creating the permeability they need to escape. The magnitude
of venting is less than the 108 kg/m2 that others
have suggested is necessary to account for observed alteration, but focusing
of expelled fluids might overcome this difference. Hanson's figures clearly
show how any chemical signature (ore deposit) associated with metamorphic
venting would tend to be erased by topography-driven meteoric water flow
in the more permeable upper few kilometers of the crust.
The papers in this volume provide a brief but hardly complete overview
of the capabilities and potential of modern radiometric dating and computer
modeling. In the intrusive, diagenetic, and metamorphic setting, the total
duration of the geologic events associated with mineralization is tens
of millions of years, but in all three settings there is ample evidence
that episodic fluid venting and ore formation are short-lived, on the scale
of tens to hundreds of thousands of years or less. Remarkably, the very
different geologic settings covered in this volume provide quite a consistent
story, undoubtedly for very different reasons. In contrast to the perception
that motivated the symposium, the papers in this volume find no discrepancy
between estimates of the duration of hydrothermal activity and/or ore deposition
derived from modeling and derived from radiometric dating. Indeed, both
approaches indicate pulses of fluid expulsion (magmatic, hydrocarbon, or
aqueous) in a much longer-lived framework of intrusions and volcanism,
sedimentation, or continental scale mountain building and erosion. The
old view that it takes many, many millions of years to create an ore deposit
is correct in the broadest sense, but we now can appreciate the brevity
of the actual ore-forming episodes themselves.
The examples given in the papers containing radiometric ages illustrate
the importance of precise dating (small uncertainties), and also the remarkable
accuracy possible with modern analytical techniques. It is self-evident,
but worth emphasizing, that specific knowledge of the age of ore formation
provides a vital key to determining the geologic and genetic relationships
of mineral deposition. We believe that the papers in this volume also illustrate
the complimentary nature of field, laboratory, and modeling studies. Geologic
mapping and radiometric dating provide the geometry and timing for realistic
fluid flow modeling, which can then predict the time duration, cumulative
fluxes, and alteration central to any exploration or resource assessment.
Modern analytical and computer techniques have just become adequate to
the task of determining the timing and duration, and simulating ore-forming
systems. The confluence and application of these techniques to ore deposits
in the next decades should prove particularly instructive and exciting.
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