New Geology Articles Published Online Ahead of Print
Boulder, Colo., USA: Article topics and locations include submarine
landslides; a fossil oceanic lithosphere preserved inside a continent;
crater formation during the onset of mud volcanism; and kill dates from
re-exposed black mosses in the northern Antarctic Peninsula. These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent.
Slab damage and the pulsating retreat of the Ionian-Apennines
subduction
Eugenio Carminati; Claudio Chiarabba
Episodes of slow and fast plate subduction, slab rollback, and backarc
opening are widely documented; e.g., in the central Mediterranean region.
Pervasive damage by fluids is emerging as a possible weakening mechanism
that could lead to slab segmentation and breakoff. We show that
low-velocity anomalies within the Ionian slab present in along-dip seismic
tomography profiles are traces of past damaging events generated by water
penetration into the oceanic lithosphere when it was at the trench. We
propose that the stepwise evolution of the central Mediterranean over the
past 21 m.y. has been governed by strength changes of the Ionian slab
induced by damage. More generally, we speculate that the cyclicity of such
a process in space and time in the different circum-Mediterranean
subduction events may have led to the present shape of the Mediterranean
basin.
Mantle serpentinization and associated hydrogen flux at North
Atlantic magma-poor rifted margins
Zhonglan Liu; Marta Perez-Gussinye; Javier García-Pintado; Leila Mezri;
Wolfgang Bach
Mantle serpentinization influences the rheology of altered peridotites and
the global fluxes of energy and volatiles, the generation of seafloor and
sub-seafloor chemolithotrophic life, and the carbon cycle. As a by-product
of serpentinization, molecular hydrogen (H2) is generated, which
supports chemosynthetic communities, and this mechanism may have driven the
origin of life on early Earth. At continent-ocean transition zones (COTs)
of magma-poor rifted margins, the mantle is exposed and hydrated over
hundreds of kilometers across the rift, but the H2 fluxes
associated with this process are poorly known. Here, we coupled a
thermomechanical model with serpentinization reaction equations to estimate
associated H2 release during mantle exhumation at COTs. This
reproduced a tectonic structure similar to that of the West Iberia margin,
one of the best-studied magma-poor margins. We estimated the rate of H2 production from mantle hydration at (7.5 ± 2.5) × 10 7 mol/(yr × km). By estimating the area of exhumed mantle from
wide-angle seismic profiles at North Atlantic magma-poor margins, we
calculated that the accumulated H2 production could have been as
high as ~4.3 × 1018 mol (~8.6 × 1012 metric tons)
prior to opening of the North Atlantic Ocean, at a rate of ~1.4 × 10 17 mol/m.y. This is one quarter of the total predicted flux
produced by the global system of mid-ocean ridges, thus highlighting the
significance of H2 generation at magma-poor margins in global H 2 fluxes, to hydrogenothropic microbial life, and, perhaps, as a
potential energy source.
Slab window–related magmatism as a probe for pyroxenite
heterogeneities in the upper mantle
Malcolm J. Hole; Sally A. Gibson; Matthew C. Morris
New high-precision trace-element analyses of magmatic olivines point to a
pyroxenite-dominated source for recent alkali basalts erupted above slab
windows formed along the Antarctic Peninsula. Melting occurred at ambient
mantle temperature, and basalts have geochemical compositions that are
indistinguishable from ocean-island basalts (OIBs). We propose that the
pyroxenite component originally resided in the upper mantle beneath the
subducted slab; formation of a slab window allowed limited decompression
and the generation of melts of garnet-pyroxenite, but little or no melting
of mantle peridotite. The pyroxenite component in the mantle formed ca. 550
Ma, an age that does not require long-term recycling of subducting slabs to
the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the
adjacent extinct Phoenix Ridge owes its enriched trace-element compositions
to mixing between small melt fractions of pyroxenite and peridotite during
a period of decreased spreading rate prior to the death of the ridge ca.
3.3 Ma. It is likely that the variable trace-element enrichment seen in
East Pacific Rise E-MORB distal from hotspots results from the same process
of interactions between small-melt-fraction (<~5%) melts of pyroxenite
and peridotite.
Unravelling biotic versus abiotic processes in the development of
large sulfuric-acid karsts
D. Laurent; G. Barré; C. Durlet; P. Cartigny; C. Carpentier ...
In carbonate rocks, natural production of sulfuric acid can form karstic
cavities. Where both epigenic and hypogenic speleogeneses have taken place,
these processes are challenging to constrain, especially if there is more
than one source of sulfur involved. Thanks to an innovative approach
coupling geomorphology with measurements of multiple sulfur, oxygen, and
strontium isotopes, our study of two French Pyrenean caves quantifies the
relative influence of both microbial and thermochemical processes implied
in sulfuric-acid production. Multiple sulfur isotopes reveal that sulfate
speleothems derived from a mixing of microbial H2S in
hydrothermal water and fossil thermochemical H2S previously
trapped within the cave host rock. We quantify the percentages of biotic
and abiotic sulfuric-acid speleogeneses that have taken place in these
caves, paving the way for similar studies of other sulfuric-acid caves
where usually only microbial activity has been considered.
Diamonds reveal subducted slab harzburgite in the lower mantle
Nicole A. Meyer; Thomas Stachel; D. Graham Pearson; Richard A. Stern;
Jeffrey W. Harris ...
Characterizing compositional heterogeneity in Earth’s lower mantle is
critical to understanding its dynamics. Three low-nitrogen diamonds from
Koffiefontein (South Africa), containing inclusion assemblages of
ferropericlase ± orthopyroxene ± magnesite, constrain diamond formation in
an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg#
82.7–88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg#
95.0–95.1 and mantle-like δ18O of +5.6‰ ± 0.2‰. Magnesite
included in one diamond implicates carbonated fluids in diamond formation.
High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted
meta-harzburgitic environment, in contrast to more fertile compositions
expected for primitive lower mantle. Extremely low Ca in orthopyroxene
inclusions may reflect a combination of melt depletion and low
equilibration temperatures at the time of trapping. Inclusion compositions
implicate subducted oceanic slab meta-harzburgite as the host for diamond
growth. Mantle-like δ18O of the orthopyroxene inclusions
indicates unaltered oceanic lithosphere. Similar melt-depleted
characteristics in lower-mantle inclusion assemblages worldwide support
that meta-harzburgite is the dominant host of lower-mantle diamonds.
Concordant changes in late Holocene hydroclimate across southern
Patagonia modulated by westerly winds and the El Niño–Southern
Oscillation
Julie Loisel; Kristen Sarna; Zhengyu Xia; Yongsong Huang; Zicheng Yu
The southern westerly winds influence weather patterns and water resources
across the southern high-latitude regions, with important socioeconomic
impacts. The strengthening and poleward migration of these winds since the
late 20th century also have implications for regional
environmental change, including drought, wildfire, and sea-ice loss.
However, it is challenging to recognize the natural variability of the
westerlies and predict their future behavior, as those recent changes have
been influenced by anthropogenic factors. We present a 4200-yr-long record
from a southern Patagonian peatland in a location that is sensitive to
changes in the position and/or strength of the westerlies. Our δ 13C record shows a 6‰ increasing trend from 4200 to 1200 cal. yr
B.P., indicating a progressive, millennial-scale increase in peatland
moisture. This long-term trend is attributed to an increase in moisture
induced by strengthening southern westerly winds associated with a change
in the mean state of the El Niño–Southern Oscillation (ENSO) system.
Superimposed on this millennial trend, centennial-scale shifts in
hydroclimate persist into modern times. We suggest that a “paleo”–Southern
Annular Mode, which is linked to tropical Pacific climate, with dry events
contemporaneous with positive phases and La Niña–like conditions, is
responsible for this enhanced hydroclimate variability. Overall, our
results point to millennial- and centennial-scale changes in hydroclimate
during the late Holocene that link tropical Pacific climate variability
with the Southern Annular Mode and the southern westerlies, with
far-reaching implications for future changes in the southern high
latitudes, including CO2 ventilation from the Southern Ocean.
Kill dates from re-exposed black mosses constrain past glacier
advances in the northern Antarctic Peninsula
Dulcinea V. Groff; David W. Beilman; Zicheng Yu; Derek Ford; Zhengyu Xia
Glaciers are receding in the northern Antarctic Peninsula and exposing
previously entombed soils and plants. We used 39 black (dead) mosses
collected from rapidly retreating ice margins at four sites along the
Antarctic Peninsula to determine the kill dates using radiocarbon
measurements and to constrain the timing of past glacier advances over the
last 1500 yr. We established strict new criteria for sample collection to
promote robust estimates of plant death. We found distinct phases of ice
advance during ca. 1300, 800, and 200 calibrated years before 1950 (cal yr
B.P.). We report estimates of the rate of glacier advance at ca. 800 cal yr
B.P. at Gamage and Bonaparte Points (southern Anvers Island) of 2.0 and 0.3
m/yr, respectively. Although the range of kill dates is relatively narrow
within a region, suggesting multiple glaciers advanced simultaneously, the
rates of local advances can vary by almost an order of magnitude and are
much less than retreat rates. Our kill dates coincide with evidence for
glacier advances from other studies in the northern Antarctic Peninsula at
ca. 1300, 800, and 200 cal yr B.P. and for penguin colony abandonment at
several sites in the region ranging from 450 to 0 cal yr B.P. The
combination of our new terrestrial evidence for glacier advances with other
lines of evidence shows the regional synchroneity of glacial dynamics and
cryosphere-biosphere connections during rapid climate shifts and the
sensitivity of terrestrial ecosystems to climate cooling.
A power-based abrasion law for use in landscape evolution models
D.D. Hansen; J.P. Brooks; L.K. Zoet; N.T. Stevens; L. Smith ...
Subglacial abrasion drives erosion for many glaciers, inundating forefields
and proglacial marine environments with glaciogenic sediments. Theoretical
treatments of this process suggest that bedrock abrasion rates scale
linearly with the energy expended through rock-on-rock friction during
slip, but this assumption lacks an empirical basis for general
implementation. To test this approach, we simulated abrasion by sliding
debris-laden ice over rock beds under subglacial conditions in a cryo-ring
shear and a direct shear device. Miniscule volumes of erosion that occurred
during each run were mapped with a white-light profilometer, and we
measured the rock mechanical properties needed to constrain the energy
expended through abrasion. We find that abraded volume per unit area
increases linearly with average shear force at the bed and that abrasion
rates increase linearly with basal power for plane beds. Lastly, only a
small percentage (1%) of the energy partitioned to basal slip is dissipated
by abrasion. These results confirm the basal-power abrasion rule is viable
to implement in landscape evolution models.
Crater formation during the onset of mud volcanism
Evan Pryce; Chris Kirkham; Joe Cartwright
Three-dimensional (3-D) seismic imaging was used to reveal >2.5-km-wide
and >150-m-deep craters at the basal surface of 64 mud volcanoes out of
a suite of 86, offshore Egypt. The craters were infilled soon after they
formed by successive mud extrusions that combined to build mud volcanoes,
as evidenced by onlap fill geometries of the earliest mud flows. We propose
that the craters formed as the earliest manifestation of mud volcano
formation. We infer that the energy required to excise in situ clays and
sands buried and consolidated to depths over 150 m below the seafloor was
provided by the highly vigorous venting of a dominantly gas and water mix
during the initial eruption, in which gas column height was the critical
factor. This primary phase of mud volcanism is rarely observed, and the
findings presented here have significant implications for interpretations
of the dynamism during this fundamental stage of mud volcano genesis.
Fragmentation, rafting, and drowning of a carbonate platform margin
in a rift-basin setting
Alexander Petrovic; Thomas Lüdmann; Abdulkader M. Afifi; Yannick Saitz;
Christian Betzler ...
High-resolution bathymetric and shallow seismic data along the northeast
Red Sea margin reveal a previously disregarded mechanism for carbonate
platform drowning at a steep-flanked rift basin. At the seafloor, salt
extrusions highlight the influence of extensional salt tectonics, with a
salt flow from the southern flank of the Al Wajh carbonate platform that
likely originates from below. Salt-flow direction, morphology, and
kilometer-sized slumps and rotated blocks indicate platform-margin
disintegration and rafting of platform blocks toward the southwest. The
outlines of several smaller detached or semi-detached carbonate platforms
to the south of the main platform can be refitted to the larger platform
margin by counter-moving the direction of mass wasting. Several platforms,
reaching heights above the seafloor of up to 650 m, are partially or fully
submerged in the mesophotic zone and appear to be in danger of drowning. We
conclude that the southern outer rim of the Al Wajh platform is breaking
apart owing to salt withdrawal, which indicates that carbonate platforms on
top of salt sequences grow on mobile ground, leading to platform
disintegration, basinward rafts, and the demise of broken-off, smaller
pieces of platform. Salt displacement also controls the growth geometries
of individual platform rafts, with keep-up reef growth (growth rate equal
to sea-level rise) and drowning occurring in close spatial proximity.
Therefore, the interplay between salt diapirism and platform growth is not
limited to platforms growing on the apexes of diapirs and is more complex
than previously thought.
Large-scale submarine landslide drives long-lasting regime shift in
slope sediment deposition
Marina Dottore Stagna; Vittorio Maselli; Arjan van Vliet
Submarine landslides and associated mass-transport deposits (MTDs) modify
the physiography of continental margins and influence the evolution of
submarine sediment routing systems. Previous studies highlighted the
control of landslides and MTDs on subsequent sedimentary processes and
deposits at spatial scales ranging from tens of centimeters to few
kilometers, leaving a knowledge gap on how and for how long large-scale
submarine landslides (i.e., headscarps wider than 50–100 km) may affect the
stratigraphic evolution of continental margins. To fill this gap, we used
three-dimensional seismic reflection data tied to an exploration well to
investigate the impact of one of the largest submarine landslides
discovered on Earth, the Mafia mega-slide (Mms) offshore Tanzania, on slope
sediment deposition. Seismic data interpretation indicates that turbidite
lobes/lobe complexes and coalescent mixed turbidite-contourite systems
formed the pre-Mms stratigraphy between 38 and ca. 21 Ma (age of the Mms),
whereas coarser-grained sheet turbidites and debrites accumulated after the
Mms for ~15 m.y., primarily on the topographic lows generated by the
emplacement of the landslide. We interpret this drastic and long-lasting
regime shift in sediment deposition to be driven by the increase in
seafloor gradient and the capture and focus of feeding systems within the
broad failed area. We propose that the extensive evacuation zones
associated with such giant landslides can generate major “conveyor belts”,
trapping land-derived material or sediments transported by along-slope
processes such as bottom currents. During the progressive healing of the
landslide escarpments, which may last for several million years, sand-prone
facies are deposited primarily in the upper slope, filling up the
accommodation space generated by the landslide, while deeper-water
environments likely remain sediment starved or experience accumulation of
finer-grained deposits. Our study provides new insights into the long-term
response of slope depositional systems to large-scale submarine landslides,
with implications for the transfer of coarse-grained sediments that can be
applied to continental margins worldwide.
The role of surface processes in basin inversion and breakup
unconformity
Luke S. Mondy; Patrice F. Rey; Guillaume Duclaux
In the context of continental extension, transient compressional episodes
(stress inversion) and phases of uplift (depth inversion) are commonly
recorded with no corresponding change in plate motion. Changes in
gravitational potential energy during the rifting process have been invoked
as a possible source of compressional stresses, but their magnitude,
timing, and relationship with depth inversions remain unclear. Using
high-resolution two-dimensional numerical experiments of the full rifting
process, we track the dynamic interplay between the far-field tectonic
forces, loading and unloading of the surface via surface processes, and
gravitational body forces. Our results show that rift basins tend to
localize compressive stresses; they record transient phases of
compressional stresses as high as 30 MPa and experience a profound depth
inversion, 2 km in magnitude, when sediment supply ceases, providing an
additional driver for the breakup unconformity, a well-documented phase of
regional uplift typically associated with continental breakup.
Quantitative record of the Neoarchean water cycle from a 2.67 Ga
magmatic-hydrothermal system, Fennoscandian Shield
D.O. Zakharov; D.R. Zozulya; D.P. Colòn
Given the scarcity of reliable paleoclimate record, the surface
temperatures of the first half of Earth’s history remain poorly
constrained. Here we show how the climate-sensitive δ18O value
of surface precipitation recorded in Archean igneous and hydrothermal
formations can help to resolve the state of early Earth climate. The Keivy
complex, Kola craton (Fennoscandian Shield), formed via the intrusion of
granitic and mafic magmas in the shallow crust at 2.67 Ga, where
circulation of meteoric water created a distinct archive of the
contemporaneous water cycle. Using whole rock data, mineral separates, and in situ zircon δ18O measurements, we disentangle the
reaction mechanisms between the shallow magma and local precipitation.
Syn-emplacement hydrothermal alteration produced near-contact lithologies
with δ18O values as low as –8‰ recorded in amphiboles, while
igneous zircons from granites crystallized from melts with δ18O
from +1‰ to +4.5‰. High-precision U-Pb geochronology constrains the granite
intrusion at 2673.5 ± 0.3 Ma. Using the Δ17O approach, these
rocks reveal that the precipitation had a δ18O value 18‰ lower
than the hydrosphere, providing one of the earliest quantitative records of
continental precipitation generally compatible with a cold climate at high
latitudes.
Plumbing the depths of magma crystallization using 176
Lu/177Hf in zircon as a pressure proxy
Hugo Moreira; Anda Buzenchi; Chris J. Hawkesworth; Bruno Dhuime
Extensional tectonics are marked by shallow magma crystallization depths,
whereas compressional tectonics are associated with deeper crystallization
depths. This implies that variations in crystallization depths can be used
to track changes in Earth’s dominant tectonic regimes through time. We
therefore developed a new “pressure of crystallization” proxy based on the
variation of the 176Lu/177Hf ratio in zircon. This
ratio is controlled by zircon fractionation and residual garnet, and it can
be used to monitor the evolution of a crystallizing magma ascending within
the crust. The secular evolution of the 176Lu/177Hf
ratio in zircon is characterized by cyclical oscillations that are broadly
in tune with the δ18O record in zircon and with periods of
continental collision and supercontinent amalgamation. The apparent mean
depth of crystallization of zircon-bearing igneous rocks has decreased with
time over the last ~3.0 b.y. This can be linked to shallowing of the
primary crystallization depths and/or to the effect of time-integrated
erosion in the geologic record. Prior to ca. 3.0 Ga, crystallization depth
maxima and oscillations in apparent depth are less clear, perhaps
suggesting that the nature of tectonic interactions was different in the
Mesoarchean and earlier.
Triggering of episodic back-arc extensions in the northeast Asian
continental margin by deep mantle flow
Yu Dong; Shuai Xiong; Feng Wang; Zheng Ji; Yi-Bing Li ...
Back-arc extension has been well documented in subduction plate tectonic
regimes. However, the reasons why back-arc extensions are associated with
some subduction systems but not others have remained elusive. Here,
spatio-temporal variations in the composition of Cenozoic basalts in the
northeast Asian continental margin are used to constrain the mechanism for
episodic back-arc extensions. Using geochemical data sensitive to tectonic
affinity, we show that typical volcanic arc compositions are located in the
eastern margin of northeast Asia, whereas coeval intraplate volcanic
compositions are located in the western part of northeast Asia, and that
the intraplate and arc volcanism exhibit two eastward shifts, from 52 Ma to
33 Ma and from 33 Ma to 21 Ma. Intraplate basalts dated at ca. 11 Ma
display a weak, arc-like geochemical signature, which suggests that the
upwelling of asthenospheric mantle resulted in the remelting of previously
melt-extracted lithospheric mantle modified by slab-derived fluids and the
cessation of back-arc extensions. Thus, we propose that the eastward mantle
flow resulted in eastward shifts of back-arc extensions that led to the
development of extensive Cenozoic arc and intraplate volcanism in the
northeast Asian continental margin.
Ancient deep ocean as a harbor of biotic innovation revealed by
Carboniferous ophiuroid microfossils
Ben Thuy; Larry Knox; Lea D. Numberger-Thuy; Nicholas S. Smith; Colin D.
Sumrall
Fossil-informed molecular phylogenies have emerged as the most powerful
tool for correlating biotic evolution and Earth history. The accuracy of
these trees, however, depends on the completeness of fossil sampling. For
most organismal clades, the available fossil record is insufficiently
sampled. This is especially true for groups with a multi-element skeleton,
such as echinoderms and vertebrates, where sampling efforts focus largely
on rare finds of intact skeletons. For these groups, inconspicuous but
informative skeletal fragments are commonly neglected. This sampling bias
excludes the numerous paleoenvironments in which preservation of intact
skeletons is extremely unlikely, in particular deep-water settings. We
describe new finds of brittle-star, or ophiuroid, fossils retrieved from
sieving residues of sediments deposited during the Atokan (Upper
Carboniferous) on the deep shelf to upper slope of the Ardmore Basin in
present-day southern Oklahoma, USA. Although preserved as disarticulated,
microscopic ossicles, the pristine preservation of the skeletal
microstructure allows for precise identification of the remains.
Comparative anatomical and phylogenetic analyses confirm the presence of
basal representatives of the extant ophiuroid orders Ophioscolecida and
Amphilepidida. Our finds provide the first unambiguous fossil evidence that
the early crown-group diversification of the Ophiuroidea was well under way
long before the end-Permian mass extinction, and that a significant part of
this diversification took place in deep-water settings, as previously
predicted by molecular evidence.
Fibrous calcite veins record stepwise, asymmetric opening and
episodic hydrocarbon expulsion from organic-rich shales
Miao Wang; Yong Chen; Richard A. Stern; Ashley Went; Yaoqi Zhou ...
Episodic fluid expulsion through fractures is widely expected during
hydrocarbon generation, yet direct evidence for this process is lacking in
the case of organic-rich shales. We investigated the formation of
antitaxial, bed-parallel fibrous calcite veins hosted in organic-rich
shales of the Eocene Dongying Depression, Bohai Bay Basin, China. Our
results from detailed, in situ geochemical traverses show that
while some symmetric veins exhibit broadly synchronous and steady-state
opening, other asymmetric veins consist of two geochemically distinct
generations of calcite to either side of the median zone, suggesting
asymmetric and asynchronous growth in two discrete episodes during
hydrocarbon expulsion. Thus, we argue that each asymmetric vein recording
two stages of opening implies that hydrocarbons were expelled from shales
episodically.
Phanerozoic cratonization by plume welding
Xi Xu; Hanlin Chen; Andrew V. Zuza; An Yin; Peng Yu ...
Deformation-resistant cratons comprise >60% of the continental landmass
on Earth. Because they were formed mostly in the Archean to
Mesoproterozoic, it remains unclear if cratonization was a process unique
to early Earth. We address this question by presenting an integrated
geological-geophysical data set from the Tarim region of central Asia. This
data set shows that the Tarim region was a deformable domain from the
Proterozoic to early Paleozoic, but deformation ceased after the
emplacement of a Permian plume despite the fact that deformation continued
to the north and south due to the closure of the Paleo-Asian and Tethyan
Oceans. We interpret this spatiotemporal correlation to indicate
plume-driven welding of the earlier deformable continents and the formation
of Tarim’s stable cratonic lithosphere. Our work highlights the Phanerozoic
plume-driven cratonization process and implies that mantle plumes may have
significantly contributed to the development of cratons on early Earth.
A fossil oceanic lithosphere preserved inside a continent
Shucheng Wu; Yingjie Yang; Yixian Xu; Juan Carlos Afonso; Anqi Zhang
The recycling of oceanic lithosphere into the deep mantle at subduction
zones is one of the most fundamental geodynamic processes on Earth. During
the closure of an ocean, ancient oceanic slabs are thought to be consumed
entirely in subduction zones due to their negative buoyancy. Yet, it is
recently suggested that small pieces of oceanic slabs could be trapped
along paleo-subduction zones. What remains far more enigmatic is whether
significant portions of paleo-oceanic lithosphere could eventually avoid
the fate of subduction and be accreted to continental lithosphere, thus
contributing to continental growth through time. We present seismic
evidence for a preserved paleo-oceanic lithosphere beneath the Junggar
region in northwestern China. We show that unsubducted oceanic lithosphere
in the West Junggar has been preserved beneath the Junggar Basin, becoming
a piece of the Eurasian continent. This scenario is likely to have occurred
in other continents throughout Earth’s history, providing an additional and
commonly underestimated contribution to the growth of continental
lithosphere.
Natural growth of gold dendrites within silica gels
Thomas Monecke; T. James Reynolds; Tadsuda Taksavasu; Erik R. Tharalson;
Lauren R. Zeeck ...
High-grade ores in low-sulfidation epithermal precious metal deposits
include banded quartz veins that contain gold dendrites. The processes by
which dendrite growth takes place have been subject to debate for decades,
especially given that these deposits are known to form from dilute thermal
liquids that contain only trace amounts of gold. It is shown here that
growth of gold dendrites in epithermal veins at the McLaughlin deposit in
California (western USA) originally took place within bands of gel-like
noncrystalline silica. The gel provided a framework for the delicate
dendrites to form. The high permeability of the gel allowed the diffusion
and advection of gold from the thermal liquids flowing across the top of
the silica layers to the sites of crystal growth within the gel. Over time,
the gel hardened to form opal-AG. This silica phase is thermodynamically
unstable and recrystallized to quartz that has a distinct mosaic texture.
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