Download arehart-draft - The Nevada Seismological Laboratory

Collaborative Research: Deep crustal structure of the Carlin trend: Crustal-scale
controls on the location of large-scale mass transfer in hydrothermal systems
Introduction
The origin of linear arrays of fossil hydrothermal systems around the world has been the
subject of much speculation. Over the past 50 years, as our understanding of plate tectonics has
expanded, some of those arrays are explainable. For example, the Abitibi gold belt in Canada is
widely considered to have formed in an ancient crustal suture, a zone of crustal weakness that
allowed hydrothermal systems to develop and redistribute metals in the crust. In the case of the
Abitibi belt, there are sufficient exposures at and near the surface to allow clear delineation of the
structure. For other linear trends of mineral belts, the deep crustal structure is not so obvious.
One of the best examples of such a linear belt that is poorly understood at depth is the Carlin
Trend, in northeastern Nevada.
The Carlin trend, located near the center of
the Great Basin (Figure 1) and had pre-mining
resources in excess of 150 million ounces of
gold. Current production from this group of
mineral deposits comprises approximately 8% of
the global production of gold, making it by far
the richest plot of ground in the world. The
Carlin trend is a striking linear array of deposits,
with intervening ground having significant
mineralization present (the only difference
between the deposits and intervening ground is
one of economics, not geology). In these
deposits, gold is present primarily at the atomic
scale within the structure of pyrite that is
disseminated within the host rocks. These host
rocks comprise lower Paleozoic sedimentary
rocks that have been intruded by plutons and
dikes of Mesozoic to Tertiary age. Additional
controls on the distribution of gold include
structures and permeable stratigraphic units, both
of which provided pathways for mineralizing
fluids. The temporal and spatial coincidence of
igneous rocks with gold mineralization is
striking, leading many researchers to conclude
that magmatism is a critical component in their
genesis. It is still debatable whether magmas
contributed metals and fluids to the
hydrothermal systems.
An alternative
hypothesis is that the deposits represent
deeply-sourced fluids that have moved into the
upper crust along a large-scale structural
Figure 1. Terrane map of part of the western US
showing sources of igneous components. The SrI
= 0.706 is approximately at the boundary between
volcanic (green) and sedimentary (light yellow)
terranes in central Nevada. Large dashed lines
discontinuity in the crust, the nature of which is still under debate.
The question to be addressed by the research proposed herein is: Why is the Carlin trend
such a striking linear array of mineral deposits? Our hypothesis is that it represents a
long-lived crustal-scale structure that focused igneous and hydrothermal activity. Prior to
~45 Ma, this structure was in a generally compressional tectonic regime, but at 45 Ma (onset of
extension in the Great Basin) because it was a weakness in the crust, it opened up to facilitate
intrusion and associated hydrothermal activity and metal mobilization. This hypothesis will be
tested by a combination of geological, geochemical and geophysical approaches.
Crustal Architecture of the Great Basin
The Great Basin is underlain by several major crustal blocks, each of which has distinctive
geological characteristics as elucidated by geophysical, geochemical, and geological data (Figure
2). Although there are some uncertainties in the exact location of their boundaries, these blocks
include the Wyoming craton (>2.5 Ga), Proterozoic accreted terranes (<2.0 Ga); and the Mojavia
block (2.0 - 2.3 Ga) (Zartman, 1974; Condie, 1981; Bennett and DePaolo, 1987). Based on Nd
isotopic mapping, the Mojavia block may be a part of the Archean craton shifted in a left-lateral
sense in mid- to late-Proterozoic time, thus opening up a rift basin which was filled with late
Proterozoic sedimentary rocks (terrigenous detrital sequence [TDS] of Stewart, 1980).
During early- to mid-Paleozoic time, much of the western margin of North America was a
passive margin, with continued accumulation of a thick section of dominantly fine-grained clastic
and carbonate “eastern assemblage” rocks in the region that is currently the east-central Great
Basin. Further offshore in the ocean basin, time-equivalent “western assemblage” rocks
comprise deep-water sediments, predominantly cherty rocks with minor amounts of mafic
volcanic rocks. In late Paleozoic and Mesozoic times, compressional tectonics associated with
the Antler and Golconda orogenies resulted in the development of east-vergent thrust plates such
as the Roberts Mountains and Golconda thrusts. Deep-water “western assemblage” Paleozoic
rocks were displaced eastward over the time-equivalent “eastern assemblage” rocks.
Following the Antler and Golconda orogenies, the craton margin changed from passive to
active at the end of the Triassic and the earliest pulse (Nevadan-Elko) of the Cordilleran
orogeny began (Speed et al., 1988). Continued compression in the Cretaceous was expressed by a
second pulse of contraction in the Great Basin region, the Sevier orogeny (Thorman et al., 1991).
The third pulse of contraction, the Laramide orogeny, is poorly represented in the Great Basin,
but was well-developed in areas outside of the Great Basin.
As the result of tectonic reorganization, decreased convergence rates between the North
American and Pacific plates caused foundering of the subducting slab in the Eocene. This
resulted in the development of an extensional environment in the Great Basin beginning at ~45
Ma. Since that time, the Great Basin has been undergoing extension, although at different rates
in different locations within the region.
Geological data
Surface and near-surface geology at local and regional scales is well-known because of
mapping in three dimensions including pits, underground exposures, and drilling (to 2 km depth).
The recent compilation edited by Thompson et al. (2002) summarizes the detailed geology of the
Carlin Trend, including stratigraphy, structure, geochronology, with limited geochemical and
geophysical data. Maps and cross-sections of the area are summarized from detailed maps of
the mining industry. These data provide an important surface base that will ground truth any
hypotheses developed during the proposed research. The surface geology of necessity will have
to be compatible with the inferred crustal structure.
Should we summarize the regional geology in a paragraph here?
Geochemical data
Compressional tectonism of the active margin was accompanied by plutonic igneous activity
in the Great Basin, beginning in the Jurassic. Associated with the Nevadan-Elko phase of this
protracted event was the emplacement of a suite of plutonic rocks. Following a decrease in
convergence between ~145 and ~120 Ma, plutonic igneous activity again became abundant in the
Great Basin during the Sevier phase of this orogenic event in the Cretaceous. The third major
period of igneous activity commenced in the Eocene, at the same time as the tectonic regime
changed from compressional to extensional. Although igneous activity in the Great Basin
continues today, the youngest plutons of any significant size that are exposed at the surface are
Eocene in age.
These igneous rocks (both plutonic and volcanic) currently exposed at the surface provide a
mechanism for sampling of the crust (upper and lower) and possibly the mantle. Insights into
crustal structure are available through the use of major and trace elements, and radiogenic and
stable isotope measurements on plutons across the Great Basin as well as complementary
geophysical data. In particular, the isotope systematics of Rb/Sr, Sm/Nd, U/Pb and O have
provided important constraints on the broad structure of the crust.
Kistler and Peterman (1973) and Kistler (1978) were among the earliest to recognize and
utilize Sr isotope ratios in western North America to examine the crust. Magmas derived from
older crustal material (i.e., having a relatively longer residence time in the crust) should generally
have higher initial Sr ratios. Kistler utilized initial strontium isotope ratios for plutons to define
what he considered a major crustal boundary, where the 87Sr/86Srinitial = 0.706 line was considered
the western margin of older continental crust (Figure 1). Similarly, Nd isotopic data were
utilized to infer older vs. younger crustal material. Farmer and DePaolo (1983) and Bennett and
DePaolo (1987) constructed isotopic maps of western North America from Nd and Sr data.
They observed a similar boundary between cratonic North America and accreted terranes but, in
addition, could distinguish some of the older cratonic structures, i.e., the Archean (N) province
from the Proterozoic (S) province. Similar refinements to the Precambrian crustal architecture
are provided by Pb isotope data (e.g., Doe and Stacey, 1974; Doe and Zartman, 1979; Wright and
Wooden, 1991, Fleck and Wooden, 1997, Tosdal et al., 2000, among others).
In the Sierra Nevada and Great Basin, oxygen isotopes have been used to assess the degree of
interaction between magmas and crust. Where significant magma-crust interaction has taken
place, oxygen values often are generally higher, reflecting the input of 18O from sedimentary (or
metasedimentary) rocks, either as partial melts or as assimilated materials. This was clearly
documented in the southern Sierra Nevada by Taylor and Silver (1978) and Taylor (1980). In a
traverse across the Great Basin, Solomon and Taylor (1989) and King et al. (2004) documented
the change in whole-rock and zircon oxygen isotope signatures from west to east. They showed
Figure 4. Bouger gravity map of a portion of
northern Nevada, with basin effects removed.
Carlin-type deposits shown by purple dots. From
Grauch et al., 2003.
that plutonic rocks in the western part of the crust are dominated by mantle-derived materials,
having oxygen isotope values below +9‰, whereas in the central and eastern portion of the Great
Basin, magmas have been influenced by crustal material. King et al. also presented evidence
suggesting that plutons of different ages within the central-eastern zone had interacted with
crustal material to different degrees.
Geophysical Data
There are a number of geophysical data that suggest that the location of major mineral belts
containing Carlin-type deposits overlie large-scale crustal structures. Gravity data presented by
Grauch et al. (2003; Figure 4) indicate that the Carlin trend lies on the eastern boundary of a
horst block, and the Battle Mountain-Eureka trend lies on the western margin. Supporting data
for this interpretation are present in the magnetic model, although those data are less robust.
Need more input here from geophysics community.
Proposed Research
The question to be addressed by the research proposed herein is: Why is the Carlin trend
such a striking linear array of mineral deposits? Our hypothesis is that it represents a
long-lived crustal-scale structure that focused igneous and hydrothermal activity.
Geological. The proposed research will focus on integrated studies of surface geology,
moderate and deep geophysical techniques, and geochem. The geological portion of the
research is requisite because the surface geology undoubtedly reflects the underlying crustal
structure, which reflects the geological evolution of this part of the world. Any understanding
developed from geophysical and geochemical techniques must be consistent with the observed
geological relations. Many man-years of effort have been expended in mapping the Carlin trend
by Barrick and Newmont geologists. There is an extensive database of geological maps and
sections that cover the entire trend. Although we anticipate that little additional geological
mapping will be required as part of this research, we will make extensive use of the extant data
set in understanding the structural evolution of the Carlin trend, as each event over the history of
the trend will have left an imprint on the region. Our task will be primarily one of compilation
of the data and integration of those data into a larger picture.
Geophysical. The proposed geophysical research will focus on imaging of the Carlin trend,
from the surface into the mantle. This will be accomplished by utilizing a number of techniques
that will generate complementary datasets. Seismic imaging.... etc.
Need geophysics group input here....
Geochemical. The proposed geochemical research will focus on utilizing stable and
radiogenic isotopes to assess the changes in magmatic events in both space (on the Carlin trend
and laterally from it) and time (intrusions in the same space at different times). The primary
vehicle for this sampling will be igneous rocks (mostly plutons), which will have recorded
information about their origin and magma-crustal interaction. Spatial imaging will comprise
sampling and analysis of plutons of different ages within the Carlin trend, and lateral to the trend.
Figure 3 shows proposed sample locations in and around the Carlin trend; at least 100 potential
sample locations.
In samples of the same age, it is likely that igneous rocks will have a similar region of origin
vertical position in the crust/mantle), and will differ only in their post-origin history, that is, their
interaction with the crust during their ascent. Therefore, if the Carlin trend represents a
crustal-scale structure that guided emplacement of magmas, it is likely that igneous rocks in the
Carlin trend will have passed through the crust will have interacted differently with the crust than
magmas of similar origin that were not emplaced within that structure. These differences are
expected to be evident in the isotopic signatures (Sr, Nd, Pb, O, S) of the plutons. The extant
data set for the Great Basin is sufficiently diffuse that the Carlin trend cannot be identified using
these criteria, that is, the Carlin trend is a smaller-scale feature than can be recognized with the
existing density of data. Therefore, our proposal is to increase the density of data in and around
the Carlin trend sufficiently to be able to detect differences in the isotopic signature of plutons
within the crustal structure vs. lateral to that structure. Our interpretation will be guided in part
by the results of geophysical investigations that will help to more clearly image the Carlin trend
below the surface (see below).
In samples in the same location but of different ages, isotopic data should provide important
information on the origin of the magmas, although such data may be somewhat more difficult to
interpret. If the Carlin trend is a long-lived crustal feature as we suggest, then magmas coming
through the crustal column should have interacted similarly with that column, but may differ in
the isotopic signature of their origin, compounded by their degree of interaction with the crust.
Preliminary data from the CT (M. Ressel, unpublished data) suggest significant differences in Pb
isotope signatures of Eocene vs. Mesozoic plutons.
Oxygen stable isotope analyses will be done for whole-rock, zircon, and quartz. Analyzing
these three sample types will allow assessment of the effects of hydrothermal alteration and
post-crystallization tectonic processes on the signal of the pluton; typically, whole-rock samples
will reflect alteration before quartz, and zircon appears extremely resistant to changes from
magmatic values (e.g., King et al., 1997). The oxygen isotope signature of the plutons should
reflect in part their origin (as mantle or crustal melts) and in part their subsequent history of
interaction with the crust during ascent. Sulfur stable isotope analyses will be done for
whole-rock samples and for apatite mineral separates. Although the sulfur isotope signature of
plutons may be complex, preliminary data for a NS traverse through eastern Nevada (including
the Carlin trend) suggest systematic variations in the sulfur isotope signature with latitude, likely
reflecting interaction of the magmas with crustal sulfur. An obvious problem with sulfur
isotopes in altered, mineralized plutons is the introduction of sulfur that is post-crystallization.
Efforts will be made to avoid such (whole-rock) samples, and analyses done of trace sulfate from
apatite where possible; it is likely that apatite sulfate will be more representative of magmatic
sulfur than bulk rock analyses of altered rocks. Stable isotope measurements will be carried out
at the Nevada Stable Isotope Laboratory (http://www.mines.unr.edu/isotope/) under the direction
of the PI Arehart and Co-I Poulson.
Radiogenic isotope analyses will comprise Sr, Nd, and Pb of whole-rock samples.
Radiogenic isotope data will provide an important complement to the stable isotope data, and can
be compared to extant data for the region. As shown in Figure 1, large crustal blocks have been
outlined in western North America using reconnaissance radiogenic isotope data. The proposed
research will focus more closely on elucidating the details of the area around the Carlin trend.
Multiple samples of the same plutonic suite will be analyzed as necessary to obtain the most
useful data. Neodymium has been chosen because of the relatively minimal effect of
hydrothermal alteration on the Nd isotope composition of plutonic rocks; this resistance to
alteration will be critical in obtaining useful data from samples of rocks in altered areas of the
Carlin trend. The ages of the rocks to be analyzed are generally well-known so that appropriate
age corrections to the isotope ratios can be made; where necessary, additional age determinations
will be made. Radiogenic isotope analyses, coupled with concentration measurements of the
parent and daughters, will be conducted in the Radiogenic Isotopes Laboratory, Ohio State
University (http://www.geology.ohio-state.edu/ril/) under the direction of Co-I Foland as outlined
in Foland and Allen (1991).
Logistics and timeline
A preliminary timeline is outlined in Table 1. We expect to begin the research with a
strategy meeting in June, 2005, to be followed on an annual basis by a research meeting in Elko,
NV. Because of the academic and geographic diversity of researchers, such a meeting is
requisite to keep all participants abreast of progress and plans.
Broader impacts of the proposed research
If trends of the past two decades continue (Einaudi, 1994; J. Dilles, personal communication,
2002), the number of geoscientists in the US who are working on mineral deposits research will
soon be vanishingly small. In a society critically dependent on minerals (per capita consumption
of minerals in the US is approximately 25 tons annually; minerals contribute directly to about
$125 billion of US GDP: US Census Bureau, 2001), this decline represents the likely loss of
critical intellectual mass necessary for the understanding of commodities vital to our national
security.
The research proposed here will help to counter these trends by providing opportunities for
several PhD students focused on developing careers in or related to minerals research. In
addition, the research effort will involve multiple undergraduate hourly students (at least 3) who
will get exposed to the opportunities in mineral deposits research through participation in the
project. This combination of applied science supported in part by industry and in part by
governmental agencies such as NSF provides students exposure to both aspects of mineral
deposit research, and is critical in training the upcoming generation of mineral geoscientists for
positions in industry, academia, or government. Students involved in this research will gain an
appreciation for a wide variety of geoscience subdisciplines and ....
The results of the proposed research will be broadly disseminated in the scientific literature,
as have previous studies of the PIs and their students. In addition, because of our close links to
the mineral industry, our research is generally applied to mineral problems shortly following
publication or presentation, thereby providing an important linkage between discovery and
societal benefit. For example, the recent publications by the PI regarding the age of Carlin-type
deposits has been widely utilized to guide mineral exploration in Nevada. This interaction will
be fostered formally by our proposed annual meetings in Elko, NV to exchange data and ideas.
In addition, we are in a special position to disseminate the results of our research through
meetings of the Geological Society of Nevada (GSN), a very active bridge organization between
academia and industry. Such presentations often precede formal publication by 12 to 24
months, and thus are a critical early communication to the stakeholders in the mining industry
Support for this research will help to maintain and enhance the infrastructure for research and
education in several locations, as well as foster inter-institutional cooperation and collaboration.
It is expected that students involved in the research will visit other institutions to present and
discuss research results.
Results of Prior NSF Support (past 5 years)
Greg B. Arehart
EAR-9805384 - Age of Formation of Carlin-Type Gold Deposits and Relationship to
Tectonic Environments (1998-2000). New radioisotopic dates were generated, including the
first successful dating of a clearly ore-related sulfide mineral (Tretbar et al., 2000), which
indicate an Eocene (~40 Ma) age for magmatism and gold deposits of the Carlin Trend during the
transition from a compressional to extensional tectonic environment (Arehart et al., 2000, 2003).
Funds supported two MSc theses at UNR (Chakurian, 2001, Tretbar, 2004).
EAR-9977209 Acquisition of a Stable Isotope Facility to Study Earth and Environmental
Systems and Processes (1999-2001). Funds from this grant were used to install two mass
spectrometers and associated preparation systems, including a laser fluorination line. This
equipment has been in use since December, 2001, and is being extensively used by visiting
researchers, faculty, and graduate students in the Department of Geological Sciences.
EAR 0309935 Stable Isotope Constraints on Source, Contamination, and Alteration of Great
Basin Igneous Rocks, and Implications for Metallogeny (2003-2006). This research focuses on
stable isotope signatures (primarily S) of plutons across the Great Basin that have interacted with
the upper crust during emplacement. Preliminary data suggest systematic changes in S
signatures in a NS transect in east-central Nevada, reflecting changes in sedimentary rock
geochemistry. One MS and one PhD student are being supported.
EAR 0412883 Mass Redistribution in Continental Magmatic-Hydrothermal Systems
(2004-2005). Partial support for a GSA Penrose Conference to be held in September, 2004.
Simon R. Poulson
EAR-9977209 and EAR 0309935 (see above)
EAR- (with Noble)