Ultramafic-hosted TalcMagnesite deposits

Gilpin R. Robinson, Jr, Bradley S. Van Gosen, 

and Nora K. Foley 

We thank Luzenac America for access to property sites and core 

U.S. Department of the Interior 

U.S. Geological Survey 

This presentation on the geology of ultramafic-hosted talc-magnesite deposits was given at 

the 42nd Forum on the Geology of Industrial Minerals, May 7-13, 2006 in Asheville, NC 

(USA). 

Talc is a soft inert industrial mineral commodity commonly used as a component or filler in 

ceramic, paint, paper, plastic, roofing, and electrical applications (Lindsey, 2003; 

Piniazkiewicz and others, 1994). Ultramafic-hosted talc-magnesite deposits are important 

sources of talc (Simandl and Ogden, 1999; Harben and Kuzvart, 1997). 

Luzenac America provided access to company property in the Proctorville, Vermont area 

and also provided site maps, drill core logs, and continuous core samples drilled in the 

Frostbite, Black Bear, and Kelly talc deposits in the study area. 
Characteristics of UM-hosted TalcMagnesite deposits

 Tectonic/Geologic setting:

 UM often serpentine in ophiolite setting 

 Talc-carbonate deposits located near UM-siliceous 

rock contacts or along faults cutting UM 

 Least compressive stress sites around UM bodies 

 Medium-pressure metamorphic belts in Fo+Atg 

and Atg+Tlc stability fields during prograde events 

 Formed by SiO2, CO2, and H2O metasomatism 

 Vermont deposits illustrate deposit features 

The ultramafic (UM) rocks that host talc-magnesite deposits, typically serpentine or 

serpentinized peridotite, occur in metamorphic belts in lithotectonic settings commonly 

associated with seafloor subduction (Simandl and Ogden, 1999). Individual ultramafic rock 

bodies occur as fault-bounded rocks that were tectonically-transported bodies or as olistolith 

blocks of serpentinized seafloor deposited in an accretionary subduction complex. 

Ultramafic-hosted talc-carbonate deposits occur near the contacts between ultramafic rocks 

and siliceous country rocks and along regional faults cutting ultramafic rocks 

(Piniazkiewicz and others, 1994; Simandl and Ogden, 1999). Large talc-carbonate rock 

bodies are structurally located in areas of least compressive stress around the larger 

ultramafic rock bodies. The talc (Tlc) and magnesite (Mgs) minerals formed during 

metamorphism of the ultramafic protolith, usually antigorite (Atg) with forsterite (Fo). 

Metasomatic (mass transfer) reactions accompanying metamorphism help form talc-rich 

deposits. 

The geological, mineralogical, and geochemical features of talc deposits in the Ludlow, 

Vermont area will be used to illustrate the important characteristics of ultramafic-hosted 

talc-magnesite deposits and provide data that constrain the physical conditions and 

processes that form talc deposits hosted by ultramafic rocks. 
Geology of Ludlow, Vermont 

 UM bodies (pink) in 

Moretown Fm at contact 

between granofels (blue) 

and carbonaceous 

schist (yellow) 

 Lithotectonic unit 

formed during 

Ordovician subduction 

 Talc-carbonate zones 

(green) at UM borders 

and faults 

Blackbear Mine 

Frostbite Mine 

The talc deposits in Ludlow Township, Vermont, are localized within part of a north 

trending band of scattered ultramafic rock bodies, primarily serpentine, that transect the 

New England states (Larrabee, 1971). The talc deposits in this area are associated with one 

of the most extensive belts of ultramafic rock found in western New England and large to 

small masses of ultramafic rock (shown in pink color) lie scattered along a 30 kilometer 

zone from Proctorsville to Townshend reservoir, Vermont (Ratcliffe, 1996; Ratcliffe, 

Walsh, and Aleinikoff, 1997). In this area, the ultramafic rocks are interspersed in the 

Moretown Formation (Ratcliffe, 1996; Ratcliffe, Walsh, and Aleinikoff, 1997) at or near the 

contact between dark gray carbonaceous sulfidic biotite schist (Omb unit, Yellow color on 

map) and laminated siliceous biotite granofels (Oml unit, blue color on map). The 

Moretown Formation has been interpreted as a lithotectonic unit formed as an allochthonous 

accretionary complex produced on the upper plate of an Ordovician oceanic subduction 

zone (Stanley and Ratcliffe, 1985; Ratcliffe, Walsh, and Aleinikoff, 1997). 

In Vermont, talc-rich parts of the ultramafic rock bodies are generally located along the 

periphery of the serpentine bodies where they are in contact with schist and siliceous 

granofels and along regional faults that have cut the serpentine bodies. The largest talcmagnesite lenses in the Proctorville, Vermont area are localized along the north and south 

margins of individual serpentine bodies, elongated parallel to the trend of regional foliation 

and faults (shown as talc-carbonate rock (green color on map)). 
Frostbite Mine 

The geology of the Frostbite mine area is shown in cross section (left figure) and map (right 

figure) views. The cross section occurs along line A-A’ of the map view and is based on 

surface geology and rock relations observed in drill core. 
Mineralogy of Rock Groups 

 BW: 

blackwall 

 TCA: talccarbonate 

group A 

 TCB: talccarbonate 

group B 

Dolomite 

Magnesite 

Forsterite 

Antigorite 

Talc 

Chlorite 

Albite 

Muscovite 

Biotite 

Quartz 

Mineralogy Schist Alt. Schist BW TCA TCB Serpentine

Minor Intermittant 

Major Intermittant 

Major Persistent 

Major > 20% > Minor > 5% 

Legend 

Mineral and chemical zones in and surrounding the ultramafic rock bodies with their 

characteristic mineral assemblages and modal abundances are shown above. 
Serpentine: Low Si and Fo-Atg groups 

0% 

10% 

20% 

30% 

40% 

50% 

60% 

70% 

80% 

90% 

100% 

FB-32-71-S150 FB-32-71-S90.5 FB-32-71-S188 FB-32-71-S109 FB-32-71-S122 

mineralogy

magnetite 

magnesite 

antigorite 

forsterite 

Serpentine: high Si group 

0% 

10% 

20% 

30% 

40% 

50% 

60% 

70% 

80% 

90% 

100% 

FB-32-71-S208 BB-84-10-

251.5 

FB-32-71-S242 BB-84-13-

283.5 

FB-32-71-S50 

mineralogy 

magnetite 

dolomite 

magnesite 

quartz 

biotite 

chlorite 

talc 

antigorite 

Mineralogy 

of 

Serpentine 

rocks 

 Atg+Mgt+/-

Mgs, Fo, Tlc 

 Low Si: 

+H2O, CO2 

 High Si: 

+H2O, SiO2 

The interior serpentine core zone consists of silicate-carbonate mineral assemblages of 

antigorite+forsterite+/-magnesite (Fo-Atg Group). Samples FB-32-71-S-109 and 122 shown 

in the upper figure illustrate the mineralogy of the interior serpentine core zone (Atg-Fo 

Serpentine Zone). Low Silica Serpentine that surrounds the core zone contains 

antigorite+magnesite assemblages. High silica serpentine (lower figure) contains 

antigorite+magnesite±talc assemblages. 

The serpentine rocks generally contain magnetite-spinel minerals, whereas the talccarbonate rocks generally lack magnetite. 
Talc-Carbonate A 

0% 

10% 

20% 

30% 

40% 

50% 

60% 

70% 

80% 

90% 

100% 

FB32-71-

S30.8 

BB84-5-

126.5 

BB84-5-

153 

BB84-13-

240 

BB84-13-

259.5 

BB84-10-

76 

BB84-5-

133 

FB32-71-

S30.6 

BB84-10-

89 

BB84-10-

81 

BB84-10-

100 

mineralogy 

dolomite 

magnesite 

quartz 

biotite 

chlorite 

talc 

antigorite 

Talc-Carbonate B 

0% 

10% 

20% 

30% 

40% 

50% 

60% 

70% 

80% 

90% 

100% 

FB32-71-

S-251 

FB32-71-

S47.5 

FB32-71-

S-41 

FB32-71-

S-38 

BB84-10-

138 

BB84-10-

244 

BB84-10-

108.5 

mineralogy

magnetite 

magnesite 

chlorite 

talc 

antigorite 

Mineralogy 

of TalcCarbonate 

rocks 

 TCA: 

Tlc rich 

Mgs+/-Do 

 TCB: 

Tlc ~ = Mgs 

little Atg 

Serpentine rocks transition, commonly abruptly, into talc-carbonate rocks: 1) 

talc+magnesite assemblages (Talc-Carbonate B) and 2) talc±magnesite±dolomite 

assemblages (Talc-Carbonate A) near the periphery of the serpentine bodies and along shear 

and fault zones. The carbonate minerals, magnesite and dolomite, incorporate the iron 

formerly hosted by magnetite in the serpentine rocks. 

The rocks in Talc-Carbonate B Zone contain talc and magnesite in roughly equal 

proportions by weight. The rocks in Talc-Carbonate A Zone have a higher weight 

proportion of talc to magnesite and frequently contain dolomite and minor amounts of 

chlorite. The Talc-Carbonate A Zone rocks are usually localized near lithologic contacts 

with siliceous metamorphic rocks and the calcium and aluminum components required to 

form dolomite and chlorite, respectively, are likely derived from these surrounding country 

rocks. 
Serpentine-Talc-Carbonate rocks 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

0.0 0.5 1.0 1.5 

molar Si/MF 

molar C/MF

Atg-Fo Serpentine 

Atg-Mgs Serpentine 

Talc-Carbonate B 

Talc-carbonate A 

minerals 

Mgs 

Mgt Fo Atg Tlc 

Serpentine & Talc-Carbonate rocks 

 Molar ratio diagram shows alteration path 

and characteristic mineral assemblages 

Geochemical relations between the ultramafic rock groups are illustrated in the above 

figure, that portrays the relative concentrations of magnesia, iron, silica, and carbonate, in 

terms of molar ratios, in the rock groups. Using molar ratios of Si/(Mg + Fe), shown as 

Si/MF, and C/(Mg + Fe), shown as C/MF, as graph variables allows both rock and mineral 

compositions to be shown in a way that illustrates the dominant mineralogy of the rock 

groups and the relative differences in silica, magnesia + iron, and carbonate compositions 

between the rock groups. 

The least altered serpentine rock in the cores of larger ultramafic rock bodies are the Atg-Fo 

(antigorite-forsterite) Serpentine group rocks. These rocks have a Si/MF value near 0.5, 

which is the composition expected from a dunite (olivine-rich) ultramafic rock protolith. 

Serpentinization by hydration of the precursor dunite oxidizes iron in the olivine forming 

antigorite-magnetite assemblages. Carbonation of the ultramafic rock protolith (increasing 

C/MF) forms rocks in the Ant-Mgs serpentine group; however two clusters of Atg-Mgs 

serpentine rocks can be seen in the figure above. The cluster with Si/MF values near 0.5 can 

be formed dominantly by carbonate addition, but the cluster with Si/MF values near 0.6 

require carbonation accompanied by a relative gain in silica and/or relative loss in 

magnesia. 

The ultramafic rocks in Talc-Carbonate B Zone indicate increased carbonation relative to 

the serpentine rocks (increasing C/MF) accompanied by small gains in silica relative to 

magnesia from the ultramafic rock protolith. Talc-carbonate rocks in zone A lie along the 

talc-magnesite join and exhibit moderate to large gains in silica relative to magnesia to form 

from the ultramafic rock protolith. 
Metasomatic reaction paths 

 Metasomatic reactions create bi- and monomineralic rock zone sequences. 

 Mgt→Sid 

The mineralogy and bulk composition of the ultramafic rock groups tend to lie along binary 

silicate-carbonate mineral joins. The small number of minerals present in these rock zones, 

the scale of the talc occurrences, and the significant and consistent mineral and chemical 

zoning developed between the ultramafic rock bodies and their enclosing metamorphic host 

rocks suggest that a fluid flow system driving metasomatic (mass transfer) alteration led to 

the formation of the talc deposits (Korzhinskii, 1970). The metasomatic reaction paths 

experienced by the ultramafic rocks and their enclosing siliciclastic metamorphic rocks are 

shown as blue lines in the above figure. The metasedimentary rocks, shown as the “pelite” 

composition lose silica and gain magnesia during metasomatism to form biotite and 

chlorite-rich alteration zones bordering the ultramafic rocks. 

Chemical zonation patterns in the metamorphic rocks surrounding the ultramafic rock 

bodies indicate that silica, magnesium, potassium, carbon, sulfur, arsenic, and water were 

all mobile components, to varying degrees, during metasomatism (Sanford, 1982). In 

addition to providing the silica needed to form talc from antigorite, the carbonation 

reactions in the talc-carbonate zones progressively destroy magnetite in the altering 

serpentine, forming ferroan magnesite. 
Metamorphic History, Ludlow, VT 

 Clockwise P-T-t 

path 

 Decompression 

in Atg field – 

MSH system 

 Least hydrated 

state: ~5-7 Kb 

The regional geologic setting of the serpentine bodies and the pressure-temperature-time 

evolution of regional metamorphic conditions are important controls on the formation of 

large high-grade talc deposits associated with ultramafic rocks. These metamorphic 

constraints can be used as guides for talc exploration. Peak metamorphic conditions need to 

occur in the stability zone of forsterite+antigorite and antigorite+talc assemblages, and the 

stability field for these mineral assemblages (Bucher and Frey, 1994) in the MgO-SiO2-H2O 

(MSH) system are shown in the above figure in relation to metamorphic conditions 

estimated for rocks in the nearby Chester-Athens Domes area of Vermont (Vance and 

Holland, 1993; Armstrong and Tracy, 2000). Metamorphic rocks in the Proctorville area 

equilibrated at slightly lower temperature and pressure conditions than the Chester-Athens 

Domes areas (Ratcliffe, Armstrong, and Aleinikoff, 1997, Figure 9), but are within the 

stability zone for antigorite+talc assemblages ranging from about 450 to 550 C at 5 to 10 

kilobars pressure. The metamorphic pressure-temperature-time conditions modeled for the 

Chester-Athens Domes area (Armstrong and Tracy, 2000), adjusted to the slightly lower 

pressure-temperature conditions most likely in the Proctorville area, indicate that 

metamorphic fluids related to dehydration reactions (Bucher and Frey, 1994) in the 

metapelitic rocks surrounding the ultramafic rock bodies are likely to be present during the 

metamorphic temperature-pressure-time path under conditions exceeding about 450 C and 5 

kilobars. 
T-XCO2 relations at 5 Kbar 

 Atg + Mgs and Tlc 

+ Do assemblages 

at 5 Kb : 

T < 490 C and 

XCO2 < 1.5 wt%. 

 ↑ P at T shifts 

reactions to ↓ CO2 . 

Mineral assemblage relationships as a function of fluid composition in the MgO-CaO-SiO2-H2O-CO2 system 

at 5 kilobars pressure are shown in the above figure, calculated using the thermodynamic data and TWQ 

program of Berman (1988; 1991), provide a framework to interpret the relative temperature and fluid 

composition relationships of the mineral reactions. 

At 5 kilobars (and higher pressures), antigorite is stable only with metamorphic fluids with very low 

concentrations of CO2 and antigorite-magnesite assemblages coexist with talc-dolomite assemblages only at 

temperatures below about 490 C with fluid compositions between 0.5 and 1.5 mole percent CO2 (figure 

above). At temperatures below 490 C, antigorite-forsterite-magnesite assemblages are stable at very low 

concentrations of CO2 (less than 0.4 mole percent). 

The high-grade massive talc-magnesite assemblages typically exhibit patchy textures and discontinuous zones 

of intersecting carbonate veins. The mineral reaction and mass balance relationships in the talc-carbonate 

alteration zones imply a negligible to small volume increase should develop in the Talc-Carbonate B zone 

rocks and a larger volume increase should occur in the Talc-Carbonate A zone rocks. The preferential 

development of the largest talc-carbonate alteration zones in areas of least compressive stress around 

ultramafic bodies is consistent with the structural setting expected to result from rock volume increase and 

associated fracture permeability caused by mass gain during metasomatism. The metasomatic alteration 

processes tend to concentrate talc in the outer part of the alteration zone where silica is likely to be more 

available from siliceous country rocks. Enrichments in arsenic, nickel, and sulfur also occur in the outer parts 

of the talc-magnesite alteration zones in the form of trace pyrite, nickel arsenide minerals, and nickel arsenate 

films on weathered fracture surfaces. Arsenic concentrations in talc and process waters need to be monitored 

during mineral processing and waste disposal. 
Graphite-H2O phase equilibria 

 Graphite buffered 

fluids are H2O-rich 

with little CO2 

 ↓ P = ↑ CO2. 

 Carbonation and 

dehydration reactions 

drive fluid from 

graphite saturation. 

 Carbonation reactions 

reduce fluid 

 Mgt →Sid 

The mineral buffers controlling, and relative timing of changes in, carbonate activity in the 

fluid phase during the metamorphic cycle influence the texture and abundance of talc in the 

alteration assemblage. In Vermont, the talc deposits occur in serpentine bodies bordering or 

enclosed in reduced (graphite-bearing) siliciclastic metamorphic host rocks that lack 

carbonate minerals. In these settings, the activity of CO2 in metamorphic fluids is controlled 

by graphite-water equilibria. 

Graphite-buffered metamorphic fluids in prograde siliciclastic rocks are H2O-rich with little 

CO2. Fluid compositions lie along the C-H2O tieline. Decreasing pressure increases the CO2 

concentration in the graphite buffered fluid. Carbonation of antigorite to form magnesite 

removes CO2 and adds H2O to the fluid, moving off graphite saturation toward highly 

reduced H2O-CH4-H2 fluids, that replace magnetite with a siderite component in magnesite. 
P-T-t-XCO2 & Talc deposits 

 ↓P ↑ CO2 in 

graphite-buffered 

H2O fluid 

 Clockwise P-T-t 

path = early Silica 

metasomatism 

followed by 

carbonation 

 Talc formation 

shown in green 

This figure shows the pressure-temperature-time (PTt) path of the Ludlow, Vermont rocks (red and 

green lines with arrows) in relation to the CO2 concentration in fluids buffered by graphite-water 

equilibria (Connolly and Cesare, 1993) as a function of pressure and temperature (black contours 

labeled in weight percent CO2). CO2 concentrations increase with increasing temperature and 

decreasing pressure. The area where talc- forming reactions are likely in Serpentine during the 

metamorphic temperature-pressure-time path of the Vermont rocks is shown in green. 

Serpentine bodies localized in siliclastic metamorphic rocks containing carbonate minerals tend to

have higher activities of CO2 in metamorphic fluids, with the result that serpentine reacts early in the 

metamorphic cycle to form listwanite (magnesite-quartz-talc rocks) or a fine-grained soapstone 

containing less than 50% of talc by weight. These rock types generally do not contain recoverable 

talc resources. The optimal metamorphic pressure-temperature-time path for the generation of highgrade talc deposits occurs where the early phase of prograde metamorphism occurs at high pressure 

and temperature conditions where the activity of CO2 is very low, fostering silica metasomatism

during the early phase of serpentine alteration. This metamorphic phase is followed by a high 

temperature decompression trend that fosters gradual carbonation of the alteration assemblage in

response to the increasing activity of CO2 in fluids buffered by the graphite-water system as pressure 

is reduced. 

Acknowledgements:

Luzenac America provided access to company property in the Proctorville, Vermont area and also 

provided site maps, drill core logs, and continuous core samples drilled in the Frostbite, Black Bear, 

and Kelly talc deposits in the study area. The authors particularly thank Mr. David Crouse and David 

Marek of Luzenac America for arranging our access to the mine sites, company property, and rock 

core storage sites that made this study possible. 
Black Bear Mine, 

Proctorsville, Vt 

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