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Goff.Laffner.1998.ultramafic_rock_sequester

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					Carbon Dioxide Sequestering Using
Ultramafic Rocks
FRASER GOFF * and K.S. LACKNER ‡
*
EES-1, MS D462; Los Alamos National Laboratory; Los Alamos, NM 87545
‡
T-3, MS B216; Los Alamos National Laboratory; Los Alamos, NM 87545         (Environmental Geosciences, Volume 5, Number 3, 1998 89-101)




ABSTRACT                                                               magnitude greater than the annual production of CO2 by vol-
   Fossil fuels continue to provide major sources of energy            canoes and metamorphic processes and an order of magni-
to the modern world even though global emissions of CO2                tude greater than the consumption rate of CO2 by natural
are presently at levels of ≥19 gigatons/year. Future antipol-          geologic processes (Kerrick et al., 1995). During the last two
lution measures may include sequestering of waste CO2 as               centuries, CO2 levels in the atmosphere have increased ~30%
magnesite (MgCO3) by processing ultramafic rocks. Com-                 (Ramanathan, 1988) raising legitimate concerns about global
mon ultramafic rocks react easily with HCl to form MgCl2               warming and the terrestrial carbon cycle (Sabine et al., 1997;
which is hydrolyzed to form Mg(OH)2. CO2 would be trans-               Weart, 1997).
ported by pipeline from a fossil fuel power plant to a seques-            Various schemes have been devised to reduce CO2 emis-
tering site and then reacted with Mg(OH)2 to produce ther-             sions while allowing continued consumption of fossil fuels
modynamically stable magnesite. Huge ultramafic deposits               (Blok et al., 1992). In one of these schemes, Lackner et al.
consisting of relatively pure Mg-rich silicates exist                  (1995) described two chemical processes to sequester CO2 b y
throughout much of the world in ophiolites and, to a lesser            formation of carbonate minerals. Sequestering CO2 involves
extent, in layered intrusions. Peridotites and associated ser-         reaction with divalent cations (principally Mg and/or Ca)
pentinite are found in discontinuous ophiolite belts along             derived from natural mineral deposits by either direct car-
both continental margins of North America. Serpentinites               bonation at high temperature or by reaction in aqueous solu-
and dunites comprise the best ores because they contain the            tion. Both sequestering processes are thermodynamically
most Mg by weight (35 to 49 wt-% MgO) and are relatively               favorable, but details of actual implementation require much
reactive to hot acids such as HCl. Small ultramafic bodies             further investigation (Lackner et al., 1995; 1997; Butt et al.,
(~1 km3) can potentially sequester ~1 gigaton of CO2 or                1996).
~20% of annual USA emissions. A single large deposit of                   Mg and Ca comprise ~2.0 and 2.1 mol-% of the earth’s
dunite (~30 km3) could dispose of nearly 20 years of current           crust, respectively, primarily bound in silicate minerals
USA CO2 emissions. The sequestering process could provide              (Brownlow, 1979). Although molar abundances are similar,
Mg, Si, Fe, Cr, Ni, and Mn as byproducts for other industrial          Mg silicates contain more reactive material per ton of rock
and strategic uses. Because "white" asbestos (chyrsotile) is a         due to the lower molecular weight of Mg. Two types of Mg
serpentine mineral, CO2 sequestering could dispose of some             silicate minerals occur in relatively pure deposits and have
waste asbestos. The cost and environmental impact of ex-               thermodynamic and chemical properties desirable for CO2
ploiting ultramafic deposits must be weighed against the               waste processing. These minerals, forsterite and serpentine,
increased costs of energy and benefits to the atmosphere and           occur as peridotites and serpentinites in ultramafic rocks.
climate.                                                               The object of this paper is to briefly describe CO2 sequester-
                                                                       ing by magnesite formation, discuss the distribution and
Key Words: carbon dioxide, environmental geology, geo-                 geochemistry of ultramafic deposits, estimate the CO2 se-
chemistry, global warming, mining, ultramafic rocks, waste             questering potential of some Unites States resources, and
disposal.                                                              consider some industrial and environmental impacts that
                                                                       would result from large-scale ultramafic mining.
INTRODUCTION
   Numerous resource evaluations show that worldwide re-               CO2 SEQUESTERING IN SOLID FORM
serves of fossil fuels can provide mankind’s energy needs for            In the presence of CO2, serpentine and forsterite are both
many centuries (e.g., United Nations, 1995). The major ob-             thermodynamically unstable. Given time, they are trans-
stacle to consuming these resources is growing emissions of            formed into magnesite plus additional materials that contain
CO2 to the atmosphere. The United States produces ~5 giga-             the silica and water that made up the stoichiometric balance
tons of CO2 annually, which is ~25% of the present global              of the original minerals. These carbonation reactions are
output of ≥19 gigatons. These quantities are an order of               exothermic and lower the free energy of the system. The en-



                                                                   1
ergy release is 64 and 95 kJ per mol of CO2 for serpentine and           disassociates into Mg(OH) 2 and MgCl2 in aqueous solution.
forsterite, respectively (Robie et al., 1979). This should be            In this cycle, it is possible to recover virtually all of the HCl.
compared to the heat of combustion of 394 kJ that is released            The most serious loss mechanism is the formation of alkali
in the formation of one mol of CO2. Because they are ther-               chlorides. However, the alkali content of peridotite and ser-
modynamically favored, magnesite and silica are common i n               pentinite rocks is very small, usually ≤0.5 wt-% (see below).
serpentinized ultramafic rocks (Barnes et al., 1973; O’Hanley,           A major design concern is to accomplish these processing
1996). The formation of these silica-carbonate rocks is pro-             steps by using only energy that is available from heat
moted by natural CO2-rich fluids permeating the mineral                  sources within the processing scheme.
deposits. The resulting magnesite is stable and is not likely               Fortunately, the carbonation reaction is quite exothermic
to release the bound CO2 again.                                          and, in principle, can provide all the energy necessary t o
   One approach to CO2 disposal is to accelerate this natural            recover the HCl. To harness this energy, the carbonation i s
process and form magnesite on a rapid scale. The resulting               performed in a gas-solid reaction between Mg(OH) 2 and CO2.
mineral is environmentally safe and provides a permanent                 In the aqueous alternative, the reaction rates may be faster,
storage for the large volumes of CO2 resulting from power                but because of the high degree of dilution, the heat of reac-
generation. The cost of such a process can be held low be-               tion would be lost for practical purposes.
cause the reaction is exothermic and, if properly designed,                 The CO2 would be pipelined to the disposal site from a
does not require the additional input of energy.                         power plant. Separating and shipping of CO2 has similarities
   The carbonation of serpentinite is broken up into several             with all other CO2 disposal methods. It is not further dis-
steps (Figure 1). First, the mineral ore is mined and ground             cussed here because its implementation will differ greatly
to a powder. To improve the carbonation reaction kinetics,               between various plant designs. The emphasis of this present
the magnesium is extracted from the ore and put in the form              work has been on the downstream disposal process. In par-
of magnesium hydroxide. The extraction is accomplished                   ticular, we have demonstrated experimentally that the gas-
with HCl, which dissolves the mineral forming MgCl2, and                 solid carbonation reaction goes virtually to completion i n
silica, which is readily precipitated along with iron oxide              <30 min at pressures of 50 bar and temperatures of ~500 t o
that was present in the original minerals. This process was              600°C. Such a process would already be economically feasi-
described in the 1940s and 1950s when Mg shortages were                  ble if one takes advantage of the fact that the pipelined CO2
driving research into alternative extraction technologies                is already pressurized. However, based on experience in the
(Houston, 1945; Barnes et al., 1950). The resulting MgCl2,               analogous sulfation chemistry of calcium oxide, it is ex-
which is always hydrated, can be hydrolyzed at 200 to 250°C              pected that a more careful investigation of the reaction
to form Mg(OH)Cl and HCl (Kelley, 1945). The Mg(OH)Cl                    mechanisms will lead to a significantly improved implemen-




                                Electricity
                                  1 GW



  Coal Mine                    Power Plant                    CO2
 earth moving      Coal            33%          CO2        collecting     Pipeline
  190 kt/day     9 kt/day      conversion     24 kt/day   compressing     24 kt/day
                                efficiency                                                       FIGURE 1: Processing stream for CO2
                                                                           <1000 km              disposal (Lackner et al., 1997); the upper
                                                                                                 branch shows the carbon flow from en-
                               MgCO3 46 kt/day                                Carbonation        ergy source (coal mine) through the
                                                                                                 power plant and the sequestration unit.
                                                                               Solid-Gas         The large amount of earth moved reflects
                                              497 MW                  513 MW    Reactor          the overburden of a typical surface mine.
                                              thermal                 thermal   550° C           It is assumed that the CO2 gas is delivered
                                                                                                 to the disposal site through a pipeline. The
                                                                                                 bottom half of the diagram represents the
  Serpentine       Rock                        MgCl2     HCl recovery Mg(OH)2                    disposal process (see text). Kt. Kilotons.
                                Serpentine
    Mine        48-62 kt/day                  52 kt/day  100-260° C 32 kt/day
                                  MgCl2
   35-45%          Silica                        HCl     endothermic       H2O
                                Extraction
    MgO         21-35 kt/day                   40 kt/day   608 MW        3 kt/day


                                                                                  H2O
                                  Fe2O3                      MgCl2              7 kt/day
                                 5 kt/day                  0.5 kt/day



                                                                  2
tation of the reaction (Lackner et al., 1997).
   The cost of mining and grinding the ore can be roughly es-        A
                                                                              AUST
timated from the analogous processing steps in copper min-
ing. It would amount to ~$8/ton of CO2. Very simple esti-                                                    EUR                                                  AFR
mates suggest that an additional $8/ton of CO2 would be a
reasonable goal to aim for in the cost of downstream process-
ing. To achieve such favorable costs would require that the
overall process does not demand significant amounts of ex-
ternal energy either in the form of heat or recompression of
gases. The $8/ton of CO2 would pay for a $300 million plant,
                                                                                                                             NA
plus operating and personnel costs, plus makeup of lost HCl.
To set the scale, for a power plant with 44% conversion effi-
ciency, $16/ton of CO2 amounts to $0.012 per kW hr (Lack-                                                                                                          SA
ner et al., 1995).

RESOURCES                                                                                                                  Ophiolite Belts

   The magnesium-rich, ultramafic rocks (primarily peri-
dotites and serpentinites) that are candidate ores in the se-
questering process are distributed throughout the world.             B                                                     Skaergaard


There are at least nine types of ultramafic rock associations,
but they occur in magmatic-tectonic settings too varied t o
document here (Coleman, 1977). The most voluminous and




                                                                                               Cord
                                                                                                   illera
widespread ultramafic rocks are the alpine (“metamorphic”)




                                                                                                                                            elt
                                                                                                         n




                                                                                                                                        n B
                                                                                                                      Sudbury
peridotites that form the basal sequence of ophiolites, slabs




                                                                                                                                   chia
                                                                                                              Stillwater




                                                                                                                                  ala
of oceanic crust uplifted and eroded along subduction zones




                                                                                                                                 App
                                                                                              Belt
and plate boundaries. The basal peridotites represent de-
tached slices of the Earth’s upper mantle exposed by these
                                                                                                                                                            an   Belt
                                                                                                                                                  Caribbe
tectonic processes (Dickinson et al., 1996). Because they
occur mostly along the upper plate of present and past sub-                                                                                Chuacus

duction zones, ophiolites are found as belts throughout most
of the world, having discontinuously exposed dimensions of
as much as 1000 × 100 km. Within North America, ophiolite            FIGURE 2: (A) Polar projection of the world showing generalized
belts are found along the Appalachian mountain chain                 locations of ophiolite belts (from Coleman, 1977). (B) Locations of
stretching from the southeast USA into Quebec and New-               ophiolite belts and major layered intrusions of North America. Example
                                                                     ophiolite bodies mentioned in text are B, Baltimore Complex; BM,
foundland and along the Cordilleran mountain chain stretch-          Belvidere Mountain; CM, Canyon Mountain; G, Green Mountain; TS,
ing from Alaska through British Columbia to California               Twin Sisters (from Coleman, 1977 and references in text).
(Figure 2). Smaller belts are found in Guatemala and in the
Caribbean.
   When examined more closely, the basal ultramafic rocks i n            the first formed minerals of crystallization (primarily Mg-
ophiolite belts are found to be elongate ribbons and frag-               rich silicates) settled by gravity toward the bottom of the
ments that parallel regional geologic structures (Figure 3).             intrusions. As a result, layers of peridotite as thick as a few
The tectonic processes that create ophiolites and expose                 hundred meters can be found in exposures as long as several
elongate fragments of the upper mantle are complex and usu-              tens of kilometers. The largest such body is the famous
ally take several million years to complete (Coleman, 1977;              Bushveld Complex in South Africa but other well-known
Harper, 1984). Individual exposures of ultramafic rock may               bodies occur at the Stillwater, Sudbury, and Skaergaard loca-
occupy hundreds of square kilometers or may be as small as               tions in North America (Figure 2). Of these three, the late
hand samples incorporated into fault zones.                              Archean (2.7 Ga (billion years ago)) Stillwater Complex i n
   The second most voluminous class of ultramafic rocks oc-              Montana has the largest exposures of gravity-settled peri-
curs in large, layered intrusions at many localities worldwide           dotite in North America (~48 km long; Czamanske and Zien-
(Hess, 1960; Cawthorn, 1997). These magma bodies gener-                  tek, 1985).
ally had initial compositions similar to mantle basalt and
were intruded into shallow levels of the Earth’s crust (usu-             CALIFORNIA STUDY REGION
ally ≤8 km). The larger bodies tend to be Precambrian age                   This present investigations focuses on California ultrama-
(≥650 Ma (million years ago)). Because of their great initial            fic bodies (Figure 3), because their basic geology is well-
volumes, these magmas cooled slowly within the crust and                 known, their distribution and volume is significant, their


                                                                 3
                    V                OREGON

                                 Klammoth-                                                        BM    Burro Mountain
                                  Trinity
           J                        Region                                                        DP    Del Puerto
                                 T                                                                J     Josephine
                                                                                                  KR    Kings River
                                                                                                  NI    New Idria
                                                                                                  PS    Point Sal




                                                          Sierra
                                                                                                  S     Smartville
                                 Co     S                                                         SM    San Mateo
                                  ast                               Lake                                                             FIGURE 3: Map of California (modi-
                                                                    Tahoe                         T     Trinity                      fied from Jennings, 1977) showing loca-
                                                                                                  V     Vulcan Peak                  tions of ultramafic provinces and some
                                                                                                  W     Wilbur Springs               example sites described in text. The
 PA




                                                           F
                                  W
                                                              oo
                                                                                                                                     Vulcan Peak peridotite in extreme
                                        Ra




                                                                   th
                     San                                                                                                             southern Oregon is actually part of the
                                                                    il
                                                                                             NE
                                            ng




                     Francisco                                                                                                       greater Josephine ophiolite, the largest
     CI




                                                                        ls
                                                                                               VA
                                                 es




                                                                                                  DA                                 in North America. The ophiolites have
                        SM              DP                                                                                           been deformed and exposed during
         FI




                                                                                                                                     prolonged subduction of the Pacific
                                                                    KR                                                               Plate (west) beneath the North Ameri-
                          Monterey                                                                                                   can Plate (east) during late Paleozoic to
            C




                                                 NI
                                                                                     B

                                        B                                                                                            early Tertiary time. Since ~30 Ma, the
                                                                                     el
                                                              B




                                            ig
                                                                   el




                                 BM
                                                                                         t

                                                                                                                                     rocks have experienced considerable
                                                                    t




                                                                                                                                     right-lateral shear along the present San
                 O




                                                      S
                                                          u
                                                              r                                                                      Andreas transform zone.
                   CE




                                                                        B
                                                 PS                         el
                                                                                 t
                        AN




                                                                                                                              A
            0   50 100 150                                                                                                  ARIZON
                  km                                                                                                O
                                                                                                               MEXIC




proximity to population and power manufacturing centers i s                                            1964). The body is part of an elongate slab of ophiolite
favorable, and their previous exploitation is well-established                                         whose ultramafic part (up to 300 m thick and ~40 km2) i s
(Coleman, 1996).                                                                                       variably serpentinized (Evarts and Schiffman, 1982).
   California ultramafic bodies occur in four locales: the                                                The contact between ultramafic rock and underlying ma-
Coast Ranges, Big Sur, Sierra Nevada foothills belts, and the                                          rine deposits of the Franciscan Complex (Jurassic to Eocene)
Klammoth-Trinity region (Saleeby, 1982; Harper, 1984;                                                  is relatively flat and sharp (Figure 4A). The extreme base of
Dickinson et al., 1996). It is well beyond the scope of this                                           the ultramafic body consists of strongly foliated antigorite
report to review the age and tectonic history of each province                                         schist grading upward into sheared, serpentinized harzbur-
in detail, but each formed during subduction-related events                                            gite and local zones of dunite. The western crest of the de-
that occurred from ~300 to 50 Ma (Paleozoic to Early Terti-                                            posit consists of material that is only 5 to 40% serpentinized
ary). Two ultramafic bodies in the Coast Ranges belt were                                              and contains ≥45 mol-% MgO (Table 1). The east portion of
chosen for preliminary study: the Del Puerto body because i t                                          the deposit is more highly serpentinized (Maddock, 1964;
contains a large mass of relatively unserpentinized ultrama-                                           Himmelberg and Coleman, 1968).
fic rock, and the Wilbur Springs body because it is mostly                                                Within the deposit, cross-cutting faults are pervasive. A
serpentinized.                                                                                         set of high-angle, northwest-trending faults and fractures
                                                                                                       host magnesite veins, pods, and eroded spring deposits
Del Puerto Ultramafic Body                                                                             (Bodenlos, 1950). Considerable geochemical research has
  The Del Puerto ultramafic body lies 60 km due east of San                                            shown that the magnesite forms from groundwater alteration
Jose in the California Coast Ranges (Himmelberg and Cole-                                              of the ultramafic rock and transport of Mg to favorable sites
man, 1968; Evarts, 1977). The ultramafic and surrounding                                               for precipitation (Barnes et al., 1967; 1973). Obviously, the
rocks are well explored as they have been mined for Mg                                                 host rocks are completely compatible with the magnesite
(magnesite), Mn (pyrolusite, MnO2), cinnabar (HgS), and                                                waste that would be generated by CO2 sequestering.
chromite (Hawkes et al., 1942; Bodenlos, 1950; Maddock,


                                                                                                  4
 A




B

                                                                                     FIGURE 4: (A) Photograph looking north of the
                                                                                     west side of the Del Puerto ultramafic body (UM)
                                                                                     overlying marine rocks of the Franciscan complex
                                                                                     (F). Note sharp vegetation contrast between units.
                                                                                     This boundary is a thrust fault developed during
                                                                                     late Cretaceous-early Teriary subduction. Ul-
                                                                                     tramafic rocks change from highly foliated
                                                                                     antigorite schist at the base to relatively massive
                                                                                     harzburgite and dunite near the top. Fracturing is
                                                                                     pervasive and serpentinization varies from 5 to 60
                                                                                     % within this zone. The ultramafic mass thickens
                                                                                     to the east (right). (B) Photograph looking north
                                                                                     of the Wilbur Springs serpentinite along Kilpepper
                                                                                     Creek, 1 km west of Complexion Spring. The
                                                                                     serpentinite, which is over 100 m thick at this
                                                                                     location, is sheared and fractured into blocky
                                                                                     rubble. Only ~5 % of the original peridotite min-
                                                                                     erals remain unaltered.




   Although the magnesite has been largely mined out, the           and “ranchettes” of single-family homes have been built i n
haul roads, shafts, pits, and dumps are still visible. Since        San Antonio Valley on the west margin of the ophiolite. The
World War II, the region has been mostly used for cattle            impact of renewed mining for carbonate waste disposal
ranching and hunting clubs. The ultramafic rocks are covered        would be carefully examined by the local public. Historic
primarily with brush of manzanita and live oak with scattered       (and dilapidated) mining infrastructure is visible all over the
pines. The area may be reached by paved roads from the west,        Del Puerto body.
east, and north. A small county park is located on the north-
eastern margin of the area. More recently, small subdivisions


                                                                5
TABLE 1. Chemical compositions (major elements in wt-%) of ultramafic rocks from the Del Puerto and Wilbur Springs ultramafic

––––––––––––––––––––––––––––
masses, California.

                                          Del Puerto Ultramafic Body
                                                                    DPS-AVEd   UM96-26a            Wilbur Springs               Serpentinite
                           a                b          c
Sample No. UM96-19              UMDP-AVE        66R22            c
                                                          66R20 Serpentinized Serpentinized UM96-3a UM96-13a                    UMWS-AVEe
Type        Dunite               Peridotite     Dunite Harzburgite Peridotite  Lherzolite Serpentinite Serpentinite              Serpentinite
SiO2        38.22              40.50 ± 1.89      39.0      44.9       38.8        40.8       39.75        41.73                  40.6 ± 1.2
TiO2         0.002              0.000± 0.001      0.02       0.02      0.013       0.011      0.080        0.065                  0.040± 0.02
Al2O3        0.71               0.40 ± 0.33       0.04       0.91      0.47        0.032      2.28         2.59                   1.99 ± 0.40
Fe2O3        4.54               4.10 ± 1.32       2.8        0.80      3.23        5.49       6.80         4.58                   5.06 ± 1.3
FeO          4.68               4.25 ± 1.15       5.0        7.0       4.74        1.88       1.38         3.53                   2.94 ± 1.0
MnO          0.159              0.134± 0.11       0.11       0.12      0.13        0.14       0.147        0.128                  0.138± 0.15
MgO         44.24              42.92 ± 1.18      46.1      43.0       42.5        34.7       34.61        36.85                  36.2 ± 1.1
CaO          0.00               0.00              0.00       1.50      0.53        5.76       0.17         1.87                   0.42 ± 0.1
Na2O         0.00               0.00              0.00       0.02      0.013       0.00       0.00         0.00                   0.00
K2O          0.00               0.00              0.23       0.08      0.15        0.00       0.00         0.00                   0.00
P2O5        <0.005             <0.005              0.03      0.03      0.051      <0.005     <0.005       <0.005                 <0.005
NiO          0.396              0.34 ± 0.03       0.35       0.32      0.27        0.116      0.34         0.30                   0.32
Cr2O3        1.18               0.53 ± .25        0.44       0.47      0.60        0.413      0.45         0.36                   0.40
CO2          ––– f              –––               0.21     <0.05       0.18        –––        –––          –––                    –––
H2O(+)       5.02               6.23 ± 2.42       5.6        1.0       8.43       11.3       13.76         8.50                  12.6 ± 1.1
H2O(-)       –––                –––               0.50       0.09      0.45        –––        –––          –––                    –––

TOTAL            99.15         99.40              100.4        100.3        100.6            100.9            99.77   100.5     100.7

MgO/SiO2        1.16             1.06                1.18        0.96            1.09           0.850         0.871     0.883     0.892
Mol-% MgO 52.5                 48.2                 51.1        52.7            45.6           36.7          35.9      37.8      37.6
ρ(g/cm3)        3.25             2.95                2.83        3.22            2.85           2.69          2.55      2.85      2.65
% Serpentinite 10.0            4 5 ± 30             58.5        10.0            56.1           90           100        60        95    ±5
a
  Analysis from Goff et al. (1997); LOI (loss on ignition) listed as H2O(+).
b
  Average of nine dunite and harzburgite samples with std. dev. (1s) from Goff et al. (1997); LOI listed as H2O(+).
c
  Analysis from Himmelberg and Coleman (1968).
d
  Average of eight peridotite samples from Himmelberg and Coleman (1968).
e
  Average of 15 samples with std. dev. (1s) from Goff et al. (1997); LOI listed as H2O(+).
f
  (–––). not analyzed.



Wilbur Springs Serpentinite                                                        tinite body and is the surface expression of a small geo-
   The Wilbur Springs serpentinite mass is located ≥200                            thermal reservoir (Goff and Janik, 1993). Drilling for
km NNE of San Francisco on the eastern side of the                                 geothermal resources in the 1960’s failed to find suffi-
Coast Ranges (McLaughlin et al., 1989). The serpen-                                ciently high temperatures for electrical generation (reser-
tinite is the preserved base of an extensive, north-trending                       voir temperature is ≤140°C at >2000 m). The presence of
sheet of ophiolite that is ~50 km long and averages 2 to 6                         low-temperature mineral springs, which occur sporadically
km wide. The thickness of the deposit varies from a few                            throughout most of the serpentinite, indicates that modern
tens of meters on the west to several hundred meters on                            day serpentinization is happening by reactions with
the east (Figure 4B).                                                              groundwaters. Complexion Spring (≤20°C), near the
   In contrast to the Del Puerto deposit, our examinations                         heart of our study area, precipitates brucite (Mg(OH)2) and
show that most outcrops have very little preserved peri-                           has a pH ≤12 (Barnes et al, 1972).
dotite minerals (≤5% overall), although original textures                             The southern margin of the serpentinite also hosts sev-
are occasionally well-preserved. The rocks are serpenti-                           eral small cinnabar mines, last worked in the early
nized harzburgites that are pervasively faulted and sheared                        1950’s, and one small gold mining district that has not
throughout much of the deposit. Typical samples from                               been worked since World War I (Peters, 1991; Goff and
15 widespread locations in the mass contain ~36 mol-%                              Janik, 1993). Homestake Mining Company re-explored
MgO (Table 1). The serpentinites overlie deformed Fran-                            the deposit in the 1980’s but decided to drop their lease
ciscan Complex rocks on the west and south and are over-                           due to low tonnage of gold-bearing rock. Wilbur Springs
lain by Great Valley sequence marine sediments (Jurassic-                          proper is a small but thriving hot spring resort first de-
Cretaceous) on the east.                                                           veloped before the turn of the century (Goff and Janik,
   A WNW-trending group of hot springs in an 8-km-long                             1993). The resort now caters to people who seek quiet,
zone occurs at the extreme southern edge of the serpen-                            natural surroundings.

                                                                            6
   Most of the land occupied by the serpentinite belongs                         wt-% and ≤5 wt-%, respectively. Total Ti, Na, K, and P con-
to the U.S. Bureau of Land Management or to a few cattle                         tents are usually ≤0.3 wt-%. The Fe is relatively reduced (Fe3+
ranches. Indian Valley on the west side of the mass con-                         /Fe2+ ≈ 0.3). Peridotites in layered intrusions generally con-
tains a reservoir that is used for recreational purposes,                        tain less magnesia and more of the other oxides due to higher
when there is water. The serpentinite hosts scrubby vege-                        proportions of pyroxenes and minor presence of plagioclase.
tation consisting of manzanita, buckthorn, live oak, scat-                       Contents of Cr, Ni, and Mn are roughly equivalent to alpine
tered pines, and rare cypress trees which can be nearly                          peridotites.
impenetrable to humans on foot. The area may be reached                             Most peridotites are partly to completely reconstituted
by dirt road from the south, east, and northwest.                                into hydrated Mg-rich silicates (serpentine and related min-
                                                                                 erals). The three serpentine minerals (lizardite, chrysotile,
GEOCHEMISTRY OF                                                                  antigorite) are isochemical with very similar sheet-like struc-
ULTRAMAFIC ROCKS                                                                 tures. The resulting serpentinites may contain some relict
   Mineral contents and chemical analyses of “fresh” peri-                       olivine and pyroxene, but more often they contain only ser-
dotite bodies (≥50 modal-% unaltered rock) are listed in nu-                     pentine minerals (Mg3(Si2O5) (OH)4), magnetite (Fe-rich
merous reports (e.g., Goff et al., 1997). Typical peridotites                    spinel), and residual chromite, plus brucite (Mg(OH)2), car-
are harzburgites containing ≤90 modal-% of forsteritic oli-                      bonates (usually magnesite, MgCO3), and free silica (SiO2).
vine and ≥10 modal-% of orthopyroxene with accessory                             Serpentinites may contain as much as 14 wt-% water. Tex-
chrome spinel ± chrome diopside. Most peridotites contain                        tural evidence in rocks shows that the hydration of forsterite
39 to 44 wt-% SiO2, 42 to 50 wt-% MgO, 7 to 9 wt-% FeO (as                       to form serpentine (and brucite) is accompanied by a volume
total Fe), ≤4000 ppm Cr, ≤3000 ppm Ni, and ≤1200 ppm Mn                          increase of as much as 53%. Thus, the serpentinites are low
(Tables 1 and 2). CaO and Al2O3 contents are generally ≤1.5                      density rocks (~2.5 g/cm3) relative to the original peridotites




––––––––––––––––––––––––––––
TABLE 2. Chemical compositions (major elements in wt-%) of ultramafic rocks from selected locations in the United States.

           Twin Sisters
               WA
                                Vulcan Peak
                                    OR
                                                        Belvidere Mtn
                                                              VT
                                                                           Cyn Mtn Balt. Cplx San Mateo
                                                                             OR           MD          CA
                                                                                                                Stillwater
                                                                                                                   MT.
               a        b       c                           d          e           f           g
Sample No. STD UNIM VP-AVE 19VP68 1VP68 BM-AV BMS-AVE                      CM-AVE      BC-AVE                 h
                                                                                                    FG96-312 ST-AVEi
Type      Dunite Dunite Peridotite Dunite Harzburgite Dunite Serpentinite Harzburgite Serpentinite Serpentinite Peridotite
SiO2       40.41 42.52    41.2     39.6      43.4     39.7       33.0       42.1         41.2        41.08        47.7
TiO2        0.005 –––j     0.035    0.03      0.02     0.0         0.0        0.04        0.05        0.054         0.12
Al2O3       0.19    0.19   0.38     0.07      0.25     0.4         0.6        1.70        1.33        1.76          4.82
Fe2O3       1.03    7.68   1.52     1.1       0.52     –––        –––         2.49        6.17        8.92          2.94
FeO         6.97    –––    6.93     9.8       7.8       8.9      14.1         4.79        2.43         –––          6.54
MnO         0.12    –––    0.12     0.14      0.14      0.23       0.2        0.13        0.12        0.133         0.17
MgO        49.59 48.01    45.5     47.4      45.3     48.3       38.0       35.6         35.09       33.06        29.0
CaO         0.17    0.02   0.70     0.30      0.91     –––        –––         5.58        1.67        1.76          2.44
Na2O        0.015 0.02     0.006    0.02      0.00     –––        –––         0.55        0.01        0.00          0.19
K2O         0.001 0.01     0.068    0.06      0.07     –––        –––         0.03        0.06        0.00          0.02
P2O5        0.002 –––      0.032    0.04      0.04     –––        –––         0.02        0.008       0.011         0.01
NiO         0.30    0.37   0.26     0.27      0.25     –––        –––         –––         0.18        0.33           –––
Cr2O3       0.58    0.13?  0.25     0.30      0.18     1.6         0.7        –––         0.16        0.37          0.48
CO2         0.08    –––   <0.06     0.05     <0.05     –––        –––         0.12        0.03         –––          0.11
H2O(+)      0.44    1.05   2.97     1.1       1.3       1.23     12.5         6.94       11.06       12.46          4.91
H2O(-)      0.06    –––    0.18     0.10      0.05     –––        –––         –––         0.31         –––          0.49
TOTAL         99.96 100.0        100.2      100.4       100.2       100.4         99.1          100.1           99.88         99.99          99.95

MgO/SiO2     1.23  1.13             1.10       1.20        1.04        1.22        1.15           0.845          0.852         0.804          0.608
Mol-% MgO 59.9    57.1             53.2       57.0        54.6        57.9        39.3           40.4           37.7          35.5           35.5
ρ(g/cm3)     3.32  3.3              3.18       3.28        3.27        3.27        2.63           2.95           2.71          2.63           3.07
% Serpentinite4    4               21          9          10          10         100             50             80           100             35
a
  Rock standard STD (Govindaraju, 1994).
b
  Analysis provided by Unimin Corp. (UNIM); Fe as Fe2O3 and LOI (loss on ignition) listed as H2O(+).
c
  Average of 13; all Vulcan Peak analyses from Himmelberg and Looney (1973).
d
  Average of three (Labotka and Albee, 1979); Fe as FeO and LOI listed as H2O(+).
e
  Average of three (Labotka and Albee, 1979); Fe as FeO and LOI listed as H2O(+).
f
  Average of 11 (Thayer, 1977); LOI listed as H2O(+).
g
  Average of four (Morgan, 1977).
h
  Average of eight homogenized pieces (total weight = 2 kg) from single outcrop (Goff et al., 1997); Fe as Fe2O3 and LOI listed as H2O(+).
i
 Average of 26 (Hess, 1960); no analyses for Ni reported.
j
 (–––). not analyzed.


                                                                           7
(~3.3 g/cm3).                                                                 tion causes the average oxidation state of iron to rise (Fe3+
   Chemical analyses of serpentinites are widely available i n                /Fe2+ ≈ 2). Contents of other transition metals are slightly
the literature (Tables 1 and 2). Because of their lower densi-                less than the original peridotites.
ties and high water contents, serpentinites contain substan-
tially less magnesia than peridotites, usually between 32 and                 Acid Dissolution Experiments
36 wt-% MgO. Formation of magnetite during serpentiniza-                        Acid dissolution experiments were conducted on various



           TABLE 3. Results of dissolution and analysis using 1 g of ultramafic rock sample mixed in hot 1:1 HCl or in
           HCl-HNO3-HF. The results are compared with X-ray fluorescence (XRF) analyses. The residues are mixtures of
           silica gel, spinels, and pyroxenes. No single reagent can effectively dissolve all components from ultramafic
           rocks, but HCl is more effective at dissolving Mg than is the mixed reagent and works better on serpentinites

           ––––––––––––––––––––––
           than peridotites.


           Sample Types
                                                                      Residue
                                                                      (wt-%)
                                                                                    MgO
                                                                                   (wt-%)
                                                                                               Mn
                                                                                              (ppm)
                                                                                                          Ni
                                                                                                        (ppm)
                                                                                                                      Cr
                                                                                                                    (ppm)
           Peridotites
             STD dunite (std value, Table 1)                           –––a         49.59     929       2360        3990
             STD (powder, 3 acids)                                     –––           8.04     916       2454         157
             STD (powder, hot HCl)                                     59.6         53.05?    951       2530          20

                 Three   Sisters   dunite (Unimin, ≤4% serp)           –––          48.01      –––      2907         886
                 Three   Sisters   dunite (XRF)                        –––          47.93     930       2830        3820
                 Three   Sisters   dunite (ore, hot HCl)               56.5         42.47     747       2524          21
                 Three   Sisters   dunite (crushed ore, hot HCl)       63.1         40.38     761       2430          27
                 Three   Sisters   dunite (powder, hot HCl)            53.0         46.89     830       2728         127

                 Green Mtn peridotite (Unimin, 10% serp)               –––          47.65      –––      3500        1710?
                 Green Mtn peridotite (XRF)                            –––          46.68    1005        140        3832
                 Green Mtn peridotite (powder, hot HCl)                48.1         41.28     738       2479         657

                 PCC-1 peridotite (std value)                          –––          43.43     930       2380        2730
                 PCC-1 (powder, 3 acids)                               –––           8.04     899       2484         804

                 JP-1 peridotite (std value)                           –––          44.72     930       2460        2970
                 JP-1 (powder, 3 acids)                                –––           7.73     914       2445         709

                 Del Puerto dunite (XRF, 40% serp)                     –––          44.31     951       2754        3160
                 Del Puerto dunite (powder, hot HCl)                   43.4         41.82     828       2398          25
                 Del Puerto dunite (powder, 3 acids)                   –––          11.46     822       2653          33

                 Del Puerto hzbgite (XRF, 50% serp)                    –––          41.63    1050       2560        2794
                 Del Puerto hzbgite (powder, hot HCl)                  46.7         34.64     784       2141         185
                 Del Puerto hzbgite (powder, 3 acids)                  –––          12.79     908       2285         435

           Serpentinites
             UB-N serpentine (std value)                               –––          35.21     929       2000        2300
             UB-N (powder, 3 acids, 5/96)                              –––           –––      981       2064        2145
             UB-N (powder, 3 acids, 2/97)                              –––          13.71     985       2102        2200

                 San   Mateo serpentinite (XRF, Table 1)               –––          33.06    1045       2514        2688
                 San   Mateo serpentinite (powder, 3 acids)            –––           –––     1014       2312        1998
                 San   Mateo serpentinite (powder, 60°C HCl)           48.8         30.92     765       2385        1440
                 San   Mateo serpentinite (powder, ≤128°C HCl)         46.6         32.75     865       2120        1595
                 San   Mateo serpentinite (powder, ≤208°C HCl)         47.5         32.66     840       2270        2135
                 San   Mateo serpentinite (60°C residue, 3 acids)      –––           –––      103         57        1481
                 San   Mateo serpentinite (60°C residue + leachate)    –––           –––      868       2442        2921

                 Wilbur Spgs serp (XRF, 85% serp)                      –––          35.41    1155       2253        3054
                 Wilbur Spgs serp (powder, hot HCl)                    42.2         34.04     985       1960        1992
                 Wilbur Spgs serp (powder, 3 acids)                    –––          15.19    1039       2137        2849
           a
               (–––). not analyzed.



                                                                       8
    peridotite and serpentinite samples to determine the relative          est such body in the USA is the Twin Sisters Dunite (Ragan,
    merits of HCl as opposed to a more complex rock reagent                1963), which occurs in the Cascade Range of northwestern
    such as HCl-HNO3-HF (Table 3). The results of these experi-            Washington (Table 2). This dunite body covers ~90 km2 and
    ments show that:                                                       is presently mined by open-pit methods for refractory (foun-
•   Hot HCl is better at dissolving Mg from ultramafic rocks               dry) sand. An examination of a 20-kg sample provided b y
    than the three-reagent mixture (≥35 wt-% versus ≤15 wt-%               the operator shows that it contains ≤4% serpentine and other
    Mg). Residual products (~45 to 60 wt-%) from HCl dissolu-              secondary reaction products. The relatively large, unserpen-
    tion include silica gel, spinels, and pyroxenes and additional         tinized Green Mountain Peridotite (≥5 km2) occurs in the
    silicates such as talc, amphiboles, chlorite and sericite. The         Appalachians of North Carolina and is also mined primarily
    three-acid mixture apparently precipitates MgF compounds               for foundry sand. An examination of this material shows that
    while dissolving the rock.                                             it contains ≥5% orthopyroxene and ~10% of high-grade
•   Hot HCl is slightly better at dissolving Mg from serpentinite          metamorphic products including talc and Mg-rich amphibole
    than from peridotite, including dunite, because serpentinites          (minerals not soluble in HCl). This material is less desirable
    contain less non-reactive silicates such as pyroxene. Much of          than dunite as a CO2 sequestering ore even though it con-
    the Fe in serpentinites occurs as microcrystalline magnetite           tains nearly comparable amounts of Mg (Goff et al., 1997).
    that is relatively easy to dissolve in HCl.                               Partially serpentinized peridotite and dunite in large
•   Hot HCl is less effective at dissolving many trace metals              masses (20 to 80% serpentine) are more common. Examples
    from ultramafic rocks than the three-acid mixture. This i s            include the Belvidere Mountain prospect in Vermont
    especially true for high field-strength elements like Cr and           (Labotka and Albee, 1979), the Canyon Mountain and Vulcan
    slightly true for Mn. On the other hand Co (not listed i n             Peak deposits in Oregon (Himmelberg and Loney, 1973;
    Table 2) and Ni mainly reside in olivine; thus, HCl dissolu-           Thayer, 1977), the Del Puerto and Burro Mountain bodies of
    tion works well for these elements.                                    California (Page, 1967; Himmelberg and Coleman, 1968;
•   HCl dissolution at 60°C and atmospheric pressure is nearly             Goff et al., 1997), and most peridotite in the Stillwater Com-
    as effective as HCl at 200°C and 15 bars for dissolving Mg             plex (Tables 1 and 2). The Belvidere, Vulcan Peak, Del
    from serpentinite (see results for San Mateo serpentinite,             Puerto, and Stillwater bodies contain small zones (≤4 km2) of
    Table 2). However, the gain in trace metal dissolution is too          relatively unaltered dunite.
    small to make this procedure worthwhile.                                  Completely serpentinized peridotite is exceptionally
                                                                           common in certain areas of eastern and western North Amer-
    Best Ultramafic Ores for Carbonate Disposal                            ica. Ultramafic rocks in the Baltimore Complex contain
       Because olivine and serpentine are the most reactive Mg-            ≥80% serpentinite and nearly all California deposits consist
    rich minerals in the HCl dissolution process, rocks com-               of ≥95% sepentinite (Rice, 1957; Morgan, 1977; Goff et al.,
    prised solely of these minerals would make the best ores. In           1997). Perhaps the largest body of continuous serpentine
    contrast, spinels and pyroxenes are not as reactive; thus,             outcrop in the USA occurs at the Josephine Ophiolite of
    these residual minerals (and silica) must be separated from            northwest California which extends over ≥800 km2 (Harper,
    primary reactants during processing. Although Cr-spinel                1984). These deposits, although some are huge, are mostly
    (and other potentially valuable metals) may contribute                 serpentinized harzburgite and are not as desirable as the un-
    greatly to the economics of the process, the pyroxenes con-            altered dunites.
    tribute virtually nothing of value. Thus, peridotites and
    serpentinites with little pyroxene comprise the most desir-            SEQUESTERING POTENTIAL OF
    able ores (Figure 5).                                                  TYPICAL ULTRAMAFIC BODIES
       Fresh dunite (or any unserpentinized peridotite for that              The CO 2-sequestering potential of some USA ultramafic
    matter) is relatively uncommon in large quantities. The larg-          bodies is compared in Table 4. Volume, bulk density, and
                                                     x
                             Op




                                                   Op




           50                                                                     50
                               x




                                   Harzburgite
                Lherzolite
          30                                                                            30        FIGURE 5: Diagram showing lower right
                  "Alpine"                                                                        corner of the Opx-Cpx-Ol ternary and
                  Peridotite                             Serpentinized "Alpine"                   lower half of the Opx-Ol-Serp ternary
                                     Dunite              Peridotite and Dunite
                 and Dunite                                                                       (modal-%). Because pyroxenes (and ac-
                                                                                                  cessory chromite) are not soluble in HCl,
            5                                                                                  5 dunite and thoroughly serpentinized dunite
                     Wehrlite                                   Best Ore
                                                                                                  or harzburgite comprise the best ores for
         Cpx                           Ol                                                    Serp CO2 sequestering. Numbers around the
                                                                                                  perimeter of diagram are percent Opx.


                                                                      9
TABLE 4. Physical-chemical and CO2-sequestering properties of example peridotite/serpentinite bodies (data from Czamanske and Zientek, 1985, Goff


––––––––––––––––––––––––––––
et al., 1997, Hess, 1960, Himmelberg and Coleman, 1968, Himmelberg and Loney, 1973, Lobatka and Albee, 1979, Morgan, 1977, and Ragan, 1963).

                            Twin
                           Sisters,
                                           Vulcan
                                            Peak,
                                                            Del
                                                           Puerto,
                                                                          Belvidere
                                                                           Mtn.,
                                                                                           Wilbur
                                                                                           Springs,
                                                                                                        Balttimore
                                                                                                        Complex,
                                                                                                                            San
                                                                                                                           Mateo,       Stillwater,
                            WA               OR             CA              VT               CA            MD               CA              MT
Volume-Density
  Area (km2)                 91             16              40               2.3           200            100               4?            26
  Depth (km)                  0.6            0.5             0.3            ≤1              ≥0.2            0.3             0.25           0.5
  Est. Vol. (km3)            54              8              12               2              40             30               1             13
  Density (g/cm3)             3.3            3.2             2.8             2.9             2.65           2.7             2.6            3.1

Wt-% Mg
  Peridotite                 29.9           27.4            27.2            29.1           –––            –––            –––              17.5
  Serpentinite             –––a            –––              20.9            23.1            21.8           21.2           19.9           –––
  Combined                   29             27              23              26              21             21             20              17

Sequestering Properties
  R(CO2)b                    1.91            2.05            2.40            2.13            2.63           2.61            2.76            3.25
  Mg (109 tons)             52.0             6.91            7.73            1.5            22.3           17.0             0.5             6.85
  CO2 (109 tons)            94.1            12.5            14.0             2.7            40.3           30.8             0.9            12.4
  USA (yr)c                 18.8             2.50            2.80            0.55            8.05           6.15            0.18            2.48
  World (yr)c                4.95            0.66            0.74            0.14            2.12           1.62            0.048           0.65
a
(–––). not significant.
b
R(CO2) is the calculated mass ratio of rock processed to CO2 disposed.
Assumes annual USA and World CO2 emission rates of ~5 × 109 and ~19 × 109 tons/yr. respectively.
c




Mg content for each body were estimated using published                       would generate ~0.15 tons dissolved Fe per ton of fixed CO2
geologic maps, reports, and chemical analyses. As men-                        or a total of 2 gigatons Fe for the entire deposit which would
tioned above, fresh peridotites, especially dunites, contain                  be precipitated as the oxide for feedstock in other industries.
the most Mg per unit mass of rock. Stillwater peridotite con-                 If completely mined, total Mg in the Del Puerto deposit
tains relatively more pyroxene plus some plagioclase but                      could fix 14 gigatons of CO2.
less olivine and is the least attractive ultramafic body evalu-                  Compared with a typical coal mine, the Del Puerto deposit
ated in terms of Mg content. All economic trade-offs for min-                 is quite large. By itself, the Del Puerto deposit could dispose
ing and processing various ultramafic rocks have not been                     of all the CO2 emissions associated with a population of 1 0
evaluated but some are discussed below.                                       million for ~70 years. Thus, it would suffice for the local
   The sequestering potential of small ultramafic bodies i s                  region. However, to absorb the CO2 output of the US many
considerable. For example the Belvidere Mountain ultrama-                     more such mines would be needed. Del Puerto could only
fic body, having an estimated volume of ~2 km3, is capable                    handle 2.8 years of total US output.
of handling the equivalent of 0.5 year of present total U.S.
CO2 emissions (~5 gigatons/yr; this value includes auto                       ADDITIONAL CONSIDERATIONS
emissions). The large dunite at Twin Sisters, Washington                      Strategic Minerals
could dispose of nearly 19 years of U.S. CO2 emissions and                       Ultramafic rocks contain many mineral resources.
~5 years of global emissions. From another perspective, the                   Chrome, platinum group metals, nickel, cobalt, and dia-
abundance of Mg in the Earth’s crust (2.0 mol-%) is nearly 6 0                monds come from various ultramafic rocks and their eroded
times greater than the abundance of C (0.035 mol-%; Brown-                    products, whereas manganese, copper, mercury, and other
low, 1979); thus, it is not surprising that there should be                   metals are sometimes obtained from within the bodies or
more than enough Mg in ultramafic deposits to sequester                       from enclosing rocks (Maddock, 1964). Metals such as these
global CO2 emissions.                                                         are of strategic importance during desperate political periods
   At a deposit such as the Del Puerto ultramafic body, Cali-                 but may be of environmental concern in large-scale mining
fornia (Evarts and Schiffman, 1982), a ton of sequestered CO2                 operations. The metal-bearing residues could be back-filled
would require on average mining, crushing, and dissolving                     into excavations or could be stockpiled for future use. Some
2.4 tons of ultramafic rock (Table 4, where R = mass ratio of                 metals such as Cr are bound primarily in chromite and minor
rock processed to CO2 sequestered). Every ton of CO2 would                    clinopyroxene which are relatively benign in the sequester-
precipitate 1.8 tons of magnesite that would be backfilled                    ing process. Most of the Ni and Mn is hosted in the olivine
into the existing mine. An additional 1.2 tons of silica and                  or products formed during serpentinization and are released
residual minerals such as pyroxenes and chromite would be                     by acid dissolution.
backfilled with the magnesite, although some could be                            Following the example used above, complete utilization of
stockpiled and sold for other industrial uses. The process                    the Del Puerto ultramafic body would yield at least 80 mega-



                                                                       10
tons of Cr, another 80 megatons of Ni, and perhaps 30 mega-              Because ultramafic rocks are exceptionally low in K and P,
tons of Mn. These quantities could dramatically impact the            and rich in Mg and Fe, they sustain a unique flora and fauna
economics of existing metals industries but would be a bene-          that are noticeable to even the most casual of observers
ficial by-product of CO2 sequestering.                                (O’Hanley, 1996). Most grasses grow with difficulty on ser-
                                                                      pentinites; instead one observes a restricted assemblage of
Chrysotile Asbestos                                                   thorny brush, pines and cypress trees (Figure 4A). Ultramafic
   Serpentinites host commercial deposits of chrysotile or            soils tend to be brick red to dark orange from oxidation of
"white" asbestos (Coleman, 1996; O’Hanley, 1996). Because             Fe. Near-surface outcrops combine these bright soil colors
most serpentinites contain appreciable chrysotile (usually i n        with the greens, grays, and blacks of the host rock. Sharp
noncommercial form), special environmental precautions                vegetation and soil contrasts occur along contacts between
may be required during mining. Chrysotile accounts for                ultramafic rocks and other formations. Because serpentinite
~90% of the asbestos that has been used historically in the           is the "state rock" of California, special care is generally
United States (Ross, 1981), and it is the primary type of as-         taken to restore or reseed areas where serpentinites are dis-
bestos used in insulation and many other construction mate-           turbed in this state (i.e., Dellinger, 1997).
rials. Chrysotile forms the bulk of asbestos-contaminated
waste resulting from removal activities.                              CONCLUSIONS
   The health risks associated with chrysotile have been the             Steady increases in global emissions will require new
focus of extensive scientific and public debate (Alleman and          technologies to capture and immobilize waste CO2. Conver-
Mossman, 1997). Many of the arguments deliberate whether              sion of CO2 into thermodynamically stable magnesite is one
chrysotile can cause mesothelioma (a rare type of cancer) i n         of many technologies under current examination. Abundant
humans. Although this issue is largely unresolved, the risk           resources of Mg-rich peridotite and serpentinite exist within
appears to be much less than that posed by amphibole asbes-           the United States and many other countries. Peridotite and
tos (Mossman et al., 1990; Hume and Rimstidt, 1992).                  serpentinite are relatively soluble in HCl; thus, huge quanti-
Chrysotile asbestos continues to be removed from buildings,           ties of Mg can be easily dissolved for further chemical uses.
albeit at a lower level than in years past. This asbestos-            Engineering and technology advances could lead to con-
containing material must be disposed of in landfills with             struction of coal- or gas-fired power plants in which waste
special precautions prescribed by the U.S. Environmental              CO2 is fed to a sequestering plant adjacent to an open-pit
Protection Agency (e.g., the use of a >15-cm cover of asbes-          ultramafic mine. Peridotite, serpentinite, and waste asbestos
tos-free material). Waste chrysotile asbestos could be used           would be consumed whereas magnesite and silica residues
as feedstock in the sequestering of CO2 if it was relatively          would be backfilled into the pit. Byproducts would include
uncontaminated by other materials.                                    Fe, Cr, Ni, Mn, and possibly other metals.
                                                                         Retrofitting all existing fossil fuel plants for benign CO2
Large-scale Open Pit Mining                                           disposal is surely an impractical task, but the global com-
  Open pit mining on the scale envisioned here would have             munity must eventually deal with the CO2 dilemma. CO2
profound economic and environmental impact. Projected                 sequestering in magnesite is one of many technologies that
costs would probably be similar to open-pit copper mining,            may eventually reduce or stabilize emissions. The environ-
as mentioned above. Precedents for open-pit mining of ser-            mental impact of large-scale ultramafic mining and CO2 se-
pentinite presently exist at New Idria and at other mines i n         questering, with associated increases in energy costs, would
the California Coast Ranges (Coleman, 1996).                          have to be weighed against the counterimpact of continued
  The McLaughlin gold mine (Homestake Mining Company)                 CO 2 emissions to the atmosphere and the risk of accelerated
was constructed in a similar geologic and physiographic               climate change.
environment to the Wilbur Springs serpentinite in the late
1970’s. The McLaughlin mine is located ~30 to 40 km south             ACKNOWLEDGMENTS
of the Wilbur Springs region and was over 100 m deep and                 We thank the following people: G. Guthrie (Los Alamos
1000 m long during maximum exploitation (Sherlock et al.,             National Laboratory [LANL]) for input on asbestos; D.
1995). The gold has now been mined out and the site is be-            Counce, E. Kluk, and M. Snow (LANL) for various analyses;
ing reclaimed. Processing of stockpiled ore will yield 19.2           D. Bergfeld (LANL) for graphics; A. Adams (LANL) for thin
tons of gold before Homestake abandons the site in the year           sections; P. Canelli (UNIMIN Corp.) for peridotite samples
2003 (Field, 1996). A large open-pit mine designed for car-           from two active mines; California Division of Mines and
bonate waste disposal would probably have considerable                Geology for maps and information; and R. G. Coleman (Stan-
community support in this economically depressed area.                ford University) for good advice. Initial reviews were pro-
However, the environmental impact would have to be fully              vided by M. J. Aldrich and B. Carey (LANL). Final reviews
evaluated before any development proceeded.                           were obtained from R. G. Coleman and two anonymous indi-
                                                                      viduals. This research was funded by the Program Directorate
Ultramafic Rocks and Ecology                                          Energy Technologies PDET (E. Joyce; LANL).


                                                                 11
                                                                          Goff, F. and Janik, C. J. (1993). Gas geochemistry and guide
                                                                            for geothermal features in the Clear Lake region, Califor-
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