Sapphirine in 

Precambrian Rocks Associated with 

Stratabound Sulfide Deposits, 

Custer County, Colorado 

GEOLOGICAL SURVEY BULLETIN 1513 

~apphirine in 

:Precambrian Rocks Associated with 

Stratabound Sulfide Deposits, 

Custer County, Colorado 

By WILLIAM H. RAYMOND, PETER A. LEIGGI, 

and DOUGLAS M. SHERIDAN 

GEOLOGICAL SURVEY BULLETIN 1513 

Mineralogic relationships between sapphirine and 

sulfide-bearing Precambrian rocks 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1980 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Raymond, William H. 

Sapphirine in Precambrian rocks associated with stratabound sulfide deposits, 

Custer County, Colorado. 

(Geological Survey Bulletin 1 513) 

Bibliography: p. 20 

Supt. of Docs. no.: I 19.3:1513 

1. Sapphirine-Colorado-Custer County. 2. Geology, Stratigraphic-Precambrian. 

3. Sulphides. 4. Geology-Colorado-Custer County. I. Leiggi, Peter A., joint author. 

II. Sheridan, Douglas M., 1921- joint author. III. Title. IV. Series: United States 

Geological Survey Bulletin 1513. 

QE75.B9 no. 1513 (QE391.S25] 557.3s 80-607763 (549'.6] 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
CONTENTS 

Page 

Abstract -------------------------------------------------------------------------------------------------------------------- 1 

Introduction ------------------------------------------------------------------------------------------------------------- 1 

Acknowledgments ---------------------------------------------------------------------------------------------------- 3 

Geologic setting -------------------------------------------------------------------------------------------------------- 3 

Sapphirine ----------------------------------------------------------------------------------------------------------------- 5 

Mineral assemblages and petrography ------------------------------------------------------------------- 8 

Discussion of metamorphic history ------------------------------------------------------------------------- 16 

References cited -------------------------------------------------------------------------------------------------------- 20 

ILLUSTRATIONS 

Page 

FIGURE 1. Location map of the San Isabel area----------------------------------------------- 2 

2-7. Photomicrographs: 

2. Tabular grain of sapphirine in a sample from the Amethyst 

prospect ------------------------------------------------------------------------------------- 6 

3. Sector twinning in sapphirine in a sample from the Amethyst 

prospect ------------------------------------------------------------------------------------- 8 

4. Polysynthetic twinning in the right half of a simple twin of 

sapphirine in a sample from the Amethyst prospect -------------- 9 

5. Sapphirine and orthopyroxene in a sample from the unnamed 

prospect ------------------------------------------------------------------------------------- 11 

6. The common assemblage sapphirine-amphibole-cordieritezincian spinel in a sample from the Amethyst prospect--------- 12 

7. An inter growth of sapphirine and zincian spinel in a sample 

from the Amethyst prospect---------------------------------------------------- 15 

TABLES 

Page 

TABLE 1. Optical data for sapphirine from the southern Wet Mountains, Colo. - 7 

2. Modes of sapphirine-bearing gneisses, southern Wet Mountains, 

Colo. ------------------------------------------------------------------------------------------------------ 14 

III 

SAPPHIRINE IN PRECAMBRIAN 

ROCKS ASSOCIATED WITH 

STRATABOUND SULFIDE DEPOSITS, 

CUSTERCOUNTY,COLORADO 

By WILLIAM H. RAYMOND, PETER A. LEIGGI, 

and DOUGLAS M. SHERIDAN 

ABSTRACT 

Sapphirine, a rare magnesium-iron-aluminum silicate found only in widely scattered 

occurrences in the world in very high grade contact or regionally metamorphosed 

rocks, has been identified at three abandoned mines and prospects in the southern 

Wet Mountains, Colo. These occurrences of sapphirine, believed to be the first 

reported in the Rocky Mountains of western North America, are unlike most other occurrences reported elsewhere because the mineral is in the host rocks of Precambrian 

stratabound sulfide deposits. The sulfide deposits and their host rocks occur in very 

large xenoliths, as much as a square kilometer in surface area, within a small batholith 

of the San Isabel Granite of Boyer. Sapphirine, in grains as large as 3.8 mm 

(millimeters) and in amounts of as much as 25 percent of individual samples, occurs in 

fine- to medium-grained gneisses that are characterized at all three localities by 

magnesian amphiboles and green zincian spinel. At one locality the sapphirinebearing gneisses extend along a strike length of at least 150 m. Coexisting with sapphirine in various assemblages are cordierite, orthorhombic and monoclinic amphiboles, phlogopitic biotite, zincian spinel, corundum, plagioclase (calcic andesine to 

labradorite), orthopyroxene, and the sulfide minerals, including sphalerite, 

chalcopyrite, and galena. Interlayered with the sapphirine-bearing gneisses are impure marble and calc-silicate gneiss containing forsterite, clinopyroxene, monoclinic 

amphibole, garnet, clinohumite, and sulfide minerals. Although the occurrences are in 

very large xenoliths, the intrusion of the San Isabel Granite about 1,400 m.y. ago is 

not believed to have played a causative role in the formation of sapphirine. Instead, 

presently available data indicate that the Precambrian sulfide deposits and their host 

rocks were regionally metamorphosed about 1, 700 m.y. ago. Sapphirine is believed to 

have crystallized then with the coexisting silicates and zincian spinel under local 

pressure-temperature conditions of the amphibolite-granulite transitional facies .. 

INTRODUCTION 

Sapphirine, a rare magnesium-iron-aluminum silicate, has been 

identified in the host rocks of Precambrian stratabound sulfide 

deposits at three localities approximately 4 km (kilometers) west of 

San Isabel in the southern Wet Mountains, Custer County, Colo. The 

1 
2 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

index map of the San Isabel area (fig. 1) shows four abandoned mines 

and prospects that were examined as part of a study of Precambrian 

deposits of zinc, copper, and lead sulfides in Colorado. Sapphirine is 

abundant at the Amethyst prospect and occurs in lesser amounts at 

the unnamed prospect 150 m (meters) west-northwest of the 

Amethyst prospect and at the upper arlit of the Marion mine, but 

none was found in samples collected at the Dewey prospect (fig. 1). 

These appear to be the first reported occurrences of sapphirine in the 

Rocky Mountains of western North America (Raymond and others, 

1980). Sapphirine is rarely found in metamorphosed stratabound 

base-metal sulfide deposits. The only other known occurrences 

described in the literature are in gneisses containing stratabound 

deposits of chalcopyrite, sphalerite, and galena in central Australia 

(Stewart and Warren, 1977, p. 26). 

Boyer (1962) published a report and a geologic map of the southern 

Wet Mountains in which he discussed the petrology and structure of 

the region. Boyer (1963) also published a brief description of the 

suffide deposits at the Marion mine, the Amethyst prospect, and the 

Dewey prospect. Boyd (1934, p. 58-60) and Gabelman (1953, p. 193) 

also briefly described geologic features at the Marion mine. This mine 

is the largest of the old mining operations in the area and was opened 

in the early 1900's for the mining of sulfide ores of zinc, copper, and 

lead (Parmelee, 1910; Babbitt, 1911). 

34 35 

1-----+--/-//-/'-) --.:l..,-------+------/-... -/-f,+---~-------+1~ ;~~ 

- / 

i 

"·· "· \ 

FIGURE I.-Location map of the San Isabel area, Custer County, Colo. 
ACKNOWLEDGMENTS 3 

Fieldwork for the present study consisted of reconnaissance examination and sampling of host rocks and deposits. Laboratory work 

included petrographic studies of standard and polished thin sections 

of rocks and ores and X-ray diffraction studies of mineral concentrates. 

In this report we describe the sapphirine in these occurrences, the 

assemblages in which it occurs, and the petrography of the 

sapphirine-bearing rocks. 

ACKNOWLEDGMENTS 

We are grateful to many U.S. Geological Survey colleagues for help 

during the preparation of this report. Fred Barker gave helpful advice during petrographic studies of the sapphirine-bearing rocks and 

acquainted us with pertinent references in the literature. George A. 

Desborough made a microprobe analysis of spinel from the Marion 

mine. Leslie J. Cox and Arthur J. Gude, 3d, provided information, obtained from studies using a scanning electron microscope, concerning 

the chemical compositions of spinel samples from the sapphirine 

localities. Richard B. Taylor and Ogden Tweto provided useful information concerning the areal geology of the Wet Mountains. Joseph T. 

Airey, formerly of the U.S. Geological Survey, provided welcome 

assistance during a brief backpacking trip to the sapphirine-bearing 

localities in September 1978. We are grateful to William N. Sharp for 

his help in improving the photographic illustrations. 

GEOLOGIC SETTING 

Precambrian sulfide deposits occur in many localities in Colorado 

(Sheridan and Raymond, 1977, fig. 1) in metamorphic rocks known to 

be more than 1,700 million years old (Tweto, 1977, p. D1). The progenitors of these metamorphic rocks were sedimentary, volcanic, and 

subvolcanic intrusive rocks. Available isotopic data (Hedge, 1969) indicate that no great amount of time elapsed between the original 

deposition of the rocks and the time of metamorphism. The major 

period of Precambrian folding and regional metamorphism in Colorado reached its peak about 1, 700 m.y. ago, according to radiometric 

age determinations (Hedge and others, 1967; Hedge and others, 1968; 

Hansen and Peterman, 1968; Silver and Barker, 1968; Stern and 

others, 1971). Lead isotope studies on galena from Precambrian 

sulfide deposits in Colorado gave the same general 1,700-m.y. age 

(Bruce Doe, oral commun., 1979). The age data together with the 
4 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

metamorphic textures of the ore-bearing rocks indicate that the 

sulfide deposits were metamorphosed with their host rocks during 

regional metamorphism about 1,700 m.y. ago. 

In the southern Wet Mountains the Precambrian metamorphic rocks 

were intruded by the San Isabel Granite batholith (Boyer, 1962, pl. 1, 

p. 1056) and associated smaller granite plutons. Tweto (1977, p. D8) included the San Isabel Granite in his group of granitic rocks of about 

1,400-m.y. age on the basis of uranium-lead age determination. 

Boyer (1963, p. 147) used the term "mineralized xenoliths" for the 

sulfide deposits at the Marion mine and the Amethyst and Dewey 

prospects; he described them as lens-shaped zones within the granite 

and noted that the sulfide mineralization is concentrated in calcsilicate rocks associated with paraschist and paragneiss. Although 

our field studies are of reconnaissance type, we accept Boyer's 

designation of these sulfide-bearing bodies of metamorphic rocks as 

xenoliths. Observations for this study indicate, however, that even 

the smallest of these is actually much larger than the term ''xenolith'' 

commonly denotes. Thus, the body of metamorphic rocks containing 

a stratabound sulfide deposit at the Marion mine is about 25 m thick 

and at least 60 m long, trends N. 50 o E., and dips about 30 o NW. 

Babbitt (1911, p. 548), based on study of underground workings at 

the Marion mine, reported that the ore body extends downdip from 

the surface for at least 225ft (feet) (69 m). The total plunge length of 

the Marion xenolith, therefore, probably exceeds the longest dimension at the surface. Reconnaissance studies in the area to the 

southwest indicate that the Amethyst and Dewey prospects are not 

in relatively small xenoliths as depicted by Boyer (1963, pl. 2). Instead, our studies indicate that the entire area encompassing the 

Amethyst prospect, the unnamed prospect, and the Dewey prospect 

is composed of metamorphic rocks containing several stratabound 

sulfide deposits. Although not continuously exposed, this body of 

metamorphic rocks appears to be on the order of one square kilometer 

in size. Such a large body may indeed be a xenolith, but it also could 

be either a roof pendant or even a part of the basement projecting 

through the floor of the granite batholith. 

Sapphirine was first identified in samples from the Amethyst prospect and subsequently identified in samples from the unnamed prospect and from the upper arlit at the Marion mine. Metamorphic rocks 

at the Marion mine and the Amethyst prospect consist of an interlayered sequence of amphibole-rich gneisses, calc-silicate gneiss, 

and impure marble. At the unnamed prospect the amphibole-rich 

rocks also contain substantial -amounts--oforthopyroxene~Zincfiui 

spinel is abundant at all three localities. Sphalerite is the dominant 

sulfide at both the Marion mine and the Amethyst prospect. The ores 

at both of these workings also contain appreciable amounts of 
SAPPHIRINE 5 

chalcopyrite and galena; pyrite and pyrrhotite were noted in the 

sulfide ores at the Marion mine. Sulfide minerals also are present at 

the unnamed prospect. 

The Amethyst prospect consists of an inclined adit, now caved, and 

a small open pit. Sapphirine-bearing rocks were collected from the 

dump near the portal of the adit, from sparse outcrops to the west 

and east, and from abundant float that is distributed along the gentle 

hillside for about 100 m west and about 50 m east of the dump. 

Because the general strike of layering in the metamorphic rocks 

in this area is N. 50 °-70 o E., and the dip is moderately northwest, 

the strike length of the sapphirine-bearing rocks is probably at least 

150m. 

The unnamed prospect consists of three closely spaced adits, all 

now caved. Grab samples of rock rich in zincian spinel were found to 

contain abundant orthopyroxene and minor sapphirine. 

The Marion mine consists of a main adit, opening at the portal in 

San Isabel Granite, which is caved a short distance from the portal. 

Babbitt's description (1911, p. 548) noted that this crosscut tunnel 

was driven 225ft (69 m) to the ore body, from which point an incline 

was sunk along the ore body for 100 feet (30 m); drifts were driven 

along the ore body for about 200 feet (61 m) at both the tunnel level 

and at the bottom of the incline. Sapphirine was not found on the 

main dump at the Marion mine, but it occurs in amphibole-rich gneiss 

in the back at the portal of a short upper adit, approximately 27 m 

above the main adit. A layer of impure marble about 2m thick, containing clinopyroxene and scattered grains of galena, occurs about 

1 m below the sapphirine-bearing rock. 

SAPPHIRINE 

Sapphitine from the three occurrences in the southern Wet Mountains, Colo., is pale blue to blue in hand specimen and is as much as 25 

percent by volume of individual rock samples. On weathered surfaces 

aggregates of sapphirine, commonly intergrown with zincian spinel 

and other minerals, stand out as resistant knots 1 to 7 mm in maximum dimension. In thin section, individual grains of sapphirine 

range in size from 0.02 to 3.8 mm, but most are in the range of 0.3-

2.0 mm. Whereas most of the grains are anhedral, some are subhedral 

to euhedral, and tabular (fig. 2). The sapphirine is biaxial negative; 

the optic angle measured in one grain .(table 1) is 61 °± 1 °. Inclined 

dispersion r < v is strong. The indices of refraction show a range of 

values (table 1): a=l.711-1.717; {1=1.716-1.722; 1'=1.718-1.724. 

The birefringence ()'-a), calculated from refractive-index determinations, ranges from 0.005 to 0.007. The mineral is strongly pleochroic: 

a, colorless to pale yellowish or pale tan; {J, blue;)', deep blue to Pros-
6 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

0.25 mm 

FIGURE 2.-Photomicrograph of a tabular grain of sapphirine (sa) associated with 

anhedral grains of sapphirine in a sample from the Amethyst prospect. Other 

minerals are biotite (B) and amphibole (am). Plane-polarized light. 

sian blue. The absorption is a< {3 < 'Y· Cleavage is very poorly 

developed; in one grain 'Y:[OOl] about 5°. Grains of sapphirine in thin 

section commonly show sector twinning (fig. 3) and simple twinning. 

Although less common, polysynthetic twinning is shown in some of 

the grains (fig. 4). In a manner similar to thatcommonly observed in 

altered olivine, some grains of sapphirine are crisscrossed in a braided or anastamosing pattern by thin stringers of an unidentified, very 

fine grained, white, talclike alteration product. 

Merlino (1973) has reported that there are two types of sapphirine: 

(1) monoclinic (2M), a type that can be untwinned, simply twinned, or 

sector twinned; (2) triclinic (1 Tc), a type that has been observed in 

polysynthetic twins. Preliminary work directed toward the deter-

.mination of the unit cell of sapphirine from one sample from the 
Sample 

No. 

S-51A-77 ,---

grain 1. 

S-51A-77,---

grain 2. 

s.:.51Y-77----

S-30AX1-78--

TABLE I.-Optical data for sapphirine from the southern Wet Mountains 

{Leaders (-), not determined/ 

Locality 

Amethyst prospect,---

dump. 

--do-----------------

Outcrop east of------

Amethyst prospect. 

Portal of short------

adit about 27 m 

above main adit 

leve 1, Marion 

mine. 

Indices of refraction (±0.002) 1 Birefringence Optic angle 

(measured) 

(-)2V 

2 1.714 1. 718 l. 721 0.007 361°±1° 

2 1. 716 1. 719 1. 721 .005 

2 1.717 1.722 1.724 .007 ---

4 1.711 1. 716 1. 718 .007 ---

1optical data were obtained on single grains that were selected from crushed splits of each 

sample; the grains were studied petrographically by spindle-stage procedures described by Wilcox 

(1959). 

2Refractive indices were determined by focal masking techniques described by Cherkasov 

(1955a, b; 1957). 

3The grain of sapphirine was mounted so that the optic plane was normal to the spindle 

axis; measurement of 2V was then made directly by rotation of the spindle from one melatope to 

the other. 

4Refractive indices were determined by the Becke line method; an interference filter was 

used that passes a wavelength near the D-line of the spectrum. 

r:n 

> ""C 

""C 

::r: 

~ 

:::0 

~ 

z 

t?=l 

-:J 
8 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

Amethyst prospect indicates that it is the monoclinic type. Polysynthetically twinned grains of sapphirine in some of the thin sections 

suggest that triclinic sapphirine also may be present. 

MINERAL ASSEMBLAGES AND PETROGRAPHY 

Sapphirine-bearing, fine- to medium-grained gneisses at all three 

localities in the San Isabel area are characterized by light-colored amphiboles and green zincian spinel. Other minerals typically associated 

with sapphirine in these rocks are pale brown biotite, cordierite, 

1.0 mm 

FIGURE 3.-Photomicrograph showing sector twinning in sapphirine (sa) in a sample 

from the Amethyst prospect; each of four sectors is labeled with the letter symbol 

(sa). Other minerals are amphibole (am) and zincian spinel (zs). Crossed polars. 
MINERAL ASSEMBLAGES AND PETROGRAPHY 9 

0.5 mm 

FIGURE 4.-Photomicrograph showing polysynthetic twinning in the right half of a 

simple twin of sapphirine (sa) in a sample from the Amethyst prospect. Other minerals are amphibole (am), biotite (B), sphalerite (sl), and zincian spinel (zs). Crossed 

polars. 

plagioclase, and sulfide minerals; a less common associate is corundum. Orthopyroxene is present in the rocks at the unnamed prospect. 

Quartz has not been found in any of the sapphirine-bearing gneisses 

but is fairly common in some of the associated sapphirine-free rocks. 

Sapphirine has not been found in the impure marble and calc-silicate 

gneiss at the Marion mine and the Amethyst prospect, although zincian spinel occurs in these rocks as well as in the sapphirine-bearing 

rocks. 
10 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

Sapphirine-bearing assemblages of minerals observed in thin sections of rocks from the three localities are listed as follows: 

AMETHYST PROSPECT 

Sapphirine-amphibole-cordierite-zincian spinel 

Sapphirine-amphibole-cordierite± plagioclase± biotite± sulfides 

Sapphirine-amphibole-plagioclase-zincian spinel± biotite 

Sapphirine-amphibole-zincian spinel± cordierite± biotite± sulfides 

Sapphirine-amphibole-zincian spinel± biotite± corundum 

Sapphirine-cordierite-plagioclase-zincian spinel 

Sapphirine-cordierite-zincian spinel-biotite 

Sapphirine-zincian spinel-biotite± corundum± sulfides 

UNNAMED PROSPECT 

Sapphirine-amphibole-orthopyroxene± sulfides 

Sapphirine-orthopyroxene-zincian spinel-biotite± sulfides 

Sapphirine-amphibole-cordierite± sulfides 

Sapphirine-amphibole-zincian spinel± sulfides 

UPPER ADIT, MARION MINE 

Sapphirine-amphibole±zincian spinel± biotite 

Each of the assemblages includes minerals that are in mutual contact. With microprobe analyses and more detailed petrographic 

studies at a later time, an evaluation of equilibrium conditions can be 

made. 

The general term ''amphibole'' is used in the list of mineral 

assemblages because more than one species are present in some of the 

rocks. All are very pale gray to colorless in thin section. Orthorhombic amphibole appears to be the most common type present. Optical 

data and X-ray studies indicate that some is anthophyllite and that 

some probably is another species in the anthophyllite-gedrite series. 

In addition to orthorhombic amphibole, some of the rocks also 

contain one or more monoclinic amphiboles, some of which are well 

twinned. 

In a sapphirine-free amphibole-cordierite-orthopyroxene gneiss 

from the unnamed prospect, the orthopyroxene is distinctly 

pleochroic (pink to pale greenish gray), is biaxial negative, and has a 

large 2V. These aspects plus X-ray data indicate that the pyroxene 

is hypersthene. In the sapphirine-bearing gneiss from this locality 

(fig. 5) the pleochroism of the orthopyroxene is much less distinct, 

although it also is biaxial negative and has a large 2V. X-ray data indicate that the peaks are shifted toward enstatite, so we have used 

the general term ''orthopyroxene'' in the mineral assemblages. 
MINERAL ASSEMBLAGES AND PETROGRAPHY 11 

Most of the biotite in these assemblages as viewed in thin section is 

very pale brown, and some is almost colorless. It is probably 

phlogopitic biotite, but, as with the amphiboles and pyroxenes, more 

precise terminology will require microprobe data. 

Spinel that contains significant but variable amounts of zinc is 

characteristic of the three sapphirine localities. Its color in hand 

specimens ranges from pale bluish green in the vicinity of the upper 

adit of the Marion mine to green almost everywhere else. In some of 

0.25 mm 

FIGURE 5.-Photomicrograph showing both sapphirine (sa) and orthopyroxene (op) 

in a sample from the unnamed prospect. Other minerals are cordierite (co), amphibole (am), biotite (B), zincian spinel (zs), and chalcopyrite (cp). Plane-polarized 

light. 
12 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

0.5 mm 

FIGURE 6.-Photomicrograph showing the common assemblage sapphirine (salamphibole (am)-cordierite (co)-zincian spinel (zs) in a sample from the Amethyst 

prospect. Other minerals are an intergrowth (sl-cp) of sphalerite and chalcopyrite. 

Plane-polarized light. 

the rocks at the unnamed prospect it is very dark green to nearly 

black. Microprobe data (G. A. Desborough, written commun., 1977) 

for spinel from calc-silicate gneiss at the Marion mine are: 12.0 
MINERAL ASSEMBLAGES AND PETROGRAPHY 13 

weight percent magnesium, 10.0 weight percent zinc, and 2.0 weight 

percent iron. Green spinel from the Amethyst prospect was found to 

have a refractive index of 1.741±0.002, a value intermediate in the 

spinel-gahnite series (Winchell and Winchell, 1951, p. 83). Qualitative 

studies using a scanning electron microscope indicate that the 

relative amounts of magnesium, zinc, and iron vary considerably in 

spinels from the three sapphirine localities. Although no zinc was 

detected in one sample, in the other samples the amounts of zinc, 

estimated from peak intensities, range from low or moderate to dominant, suggesting probable compositions ranging from zincian spinel 

to gahnite. For these reasons the general term "zincian spinel" has 

been used in this report; we recognize, however, that the composition 

probably ranges considerably. 

Cordierite is a prominent constituent of some of the sapphirinebearing gneisses. It occurs in the common assemblage sapphirineamphibole-cordierite-zincian spinel (fig. 6). In some of the sapphirinebearing . gneisses, plagioclase, ranging from calcic andesine to 

labradorite, occurs to the exclusion of cordierite, whereas in other 

samples both cordierite and plagioclase are present. In some thin sections the rocks show thin plagioclase-rich layers separated from 

cordierite-rich layers by layers rich in amphibole and biotite. Some of 

the cordieri te is well twinned. 

Corundum occurs in some of the sapphirine-bearing gneisses at the 

Amethyst prospect and in a sapphirine-free sillimanitic cordieriteamphibole-plagioclase gneiss at the Marion mine. Corundum was not 

observed, however, in samples of orthopyroxene-bearing gneiss from 

the unnamed prospect. 

Modes of five representative samples of sapphirine-bearing 

gneisses were determined by the point-count method (1,000 points 

each) and are reported in table 2. Modes 1 and 2, representing 

samples from the Amethyst prospect, contain nearly equal amounts 

of amphibole but variable amounts of cordierite, sapphirine, zincian 

spinel, and biotite. In mode 3, also from the Amethyst prospect, 

plagioclase is abundant, cordierite is absent, and the amphibole content is much lower than in the other two samples from the Amethyst 

prospect. A small amount of corundum occurs in this plagioclase-rich 

gneiss. Mode 4 characterizes the orthopyroxene-rich gneiss at the unnamed prospect. Mode 5, a sample from the upper adit of the Marion 

mine, illustrates a rock in which the principal minerals are amphibole 

and sapphirine. Sulfide minerals, commonly sphalerite and 

chalcopyrite, are present in each of the samples described in table 2. 

Other accessory minerals observed are: zircon, causing pleochroic 

halos in biotite and cordierite; tiny acicular crystals that probably are 
14 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

TABLE 2.-Modes of sapphirine-bearing gneisses, southern Wet Mountains, Colorado 

[Values in volume percent. Tr, trace; .... indicates mineral not present] 

Mode ------------------------

Amphibole -------------------

Cordierite ------------------

Sapphirine ------------------

Zincian spinel --------------

Biotite ---------------------

Orthopyroxene ---------------

Plagioclase1 ---------------

Corundum --------------------

Sillimanite(?) --------------

Prismatic mineral2 ---------

Sulfide minerals and 

their oxidation 

products ------------------

Rutile(?) -------------------

Carbonate mineral -----------

·zircon ----------------------

Total ---------

1Partly sericitized . 

55.5 

23.8 

14.8 

3.5 

1.9 

.5 

Tr. 

tr. 

100.0 

2 

51.1 

11.7 

9.8 

10.8 

15.7 

.9 

Tr. 

100.0 

4 

10.7 23.7 70.2 

15.1 

9.8 . 7 18.5 

13.7 15.7 6.6 

17.9 7.9 3.3 

35.8 

44.3 

2.9 

.4 

.5 

.2 . 7 1.4 

Tr. 

tr. 

tr. Tr. Tr. 

100.0 100.0 100.0 

2unidentified pale-brownish prismatic mineral: commonly 0.08-0.16 mm long and 

0.02-0.03 mm thick; length-slow; parallel extinction; low birefringence and high 

refringence. 

Mode Sample No. 

1-- S-25F1-78 

2-- S-25J2-78 

3-- S-25K2-78 

4-- S-26C1-78 

5-- S-30Al-78 

Locality and rock type 

Amethyst prospect: zincian spinel-bearing amphibole-cordieritesapphirine gneiss. 

Amethyst prospect: sapphirine-bearing amphibole-biotitecordierite-zincian spinel gneiss. 

Amethyst prospect: sapphirine- and corundum-bearing 

plagioclase-biotite-zincian spinel-amphibole gneiss. 

Unnamed prospect: sapphirine-bearing biotitic orthopyroxeneamphibole-zincian spinel-cordierite gneiss. 

Upper adit, Marion mine: zincian spinel-bearing amphibolesapphirine gneiss. 

sillimanite; tiny yellowish grains that are probably rutile; a carbonate 

mineral; and an unidentified pale-brownish prismatic mineral. 

Textpral relations observed in the sapphirine-bearing gneisses are 

closely similar to the metamorphic textures observed in many of the 

host rocks of Precambrian sulfide deposits elsewhere in Colorado. In 

all of these rocks, including those from the San Isabel area, silicate 

minerals, sulfide minerals, and zincian spinel are complexly intergrown; in many of the rocks elongate grains and aggregates of the 

various minerals are alined along the gneissic and compositional 
MINERAL ASSEMBLAGES AND PETROGRAPHY 15 

layering. Although sapphirine was observed rimming zincian spinel 

in some thin sections, this is by no means the dominating textural 

relationship shown by these two minerals. Instead, sapphirine and 

zincian spinel occur just as commonly as independent individual 

grains; aggregates of each of these minerals are commonly alined 

along the gneissic layering. In some of the rocks sapphirine and zincian spinel are intergrown (fig. 7). Examples of sapphirine 

poikiloblastically intergrown in zincian spinel appear to be as com0.5mm 

FIGURE 7.-Photomicrograph showing an intergrowth of sapphirine (sa) and zincian 

spinel (zs) in a sample from the Amethyst prospect. Other minerals are biotite (B) 

and amphibole (am). Plane-polarized light. 
16 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

monas the reverse relation. Orthopyroxene occurs in elongate grains 

and aggregates alined parallel to the layering shown by the 

associated cordierite, amphibole, biotite, and zincian spinel (see, for 

example, fig. 5). 

Sapphirine-free rocks from the Marion mine and the Amethyst 

prospect commonly range from impure marble to calc-silicate gneiss. 

These rocks are commonly fine to medium grained, although locally 

some are coarse grained. Silicate minerals present in these rocks include forsterite (commonly altered partly or almost entirely to 

serpentine), clinopyroxene, garnet, monoclinic amphibole, and 

clinohumite. Brucite also was identified in one of these rocks. Quartz 

is fairly common in some of the calc-silicate gneisses and also occurs 

at the unnamed prospect in an amphibole-cordierite-plagioclase 

gneiss. A mineral of the humite group occurs in an amphibole-rich 

gneiss containing abundant dark-green grains of zincian spinel at the 

unnamed prospect. 

DISCUSSION OF METAMORPHIC HISTORY 

The rarity of sapphirine and its unexpected discovery in the 

southern Wet Mountains merit an appraisal of data pertinent to the 

geologic history and conditions that caused its development. An 

assessment of presently available data suggests that sapphirine and 

orthopyroxene probably formed in these rocks during the regional 

metamorphism that prevailed in much of Colorado about 1,700 m.y. 

ago. Although the intrusion of the San Isabel Granite about 1,400 

m.y. ago may have played a causative role in the formation of sapphirine and orthopyroxene, the present data suggest that such an 

origin relating to thermal metamorphism during this later event is 

not likely. 

Except for the presence of sapphirine and orthopyroxene, the 

mineralogy of the host rocks of Precambrian sulfide deposits in the 

San Isabel area is notably similar to the mineralogy of host rocks at 

many other localities in Colorado. Many Precambrian stratabound 

sulfide deposits and their host rocks have been metamorphosed to 

the upper amphibolite facies (sillimanite zone of regional metamorphism) elsewhere in the Wet Mountains, the Salida area, the Front 

Range, and the northern Park Range (Sheridan and Raymond, 1977, 

p. 9-10). In these widespread localities gahnite and(or) zincian spinel, 

cordierite, orthorhombic amphibole, and phlogopitic biotite very 

commonly characterize the host rocks of the sulfide deposits. Corundum is present in at least one of these other localities. In the impure 

marbles and calc-silicate gneisses commonly interlayered with these 

magnesium-rich amphibole and cordierite gneisses, the mineralogy 

also is duplicated by the common presence of clinopyroxene, gahnite 

and(or) zincian spinel, forsterite, minerals of the humite group, 
DISCUSSION OF METAMORPHIC HISTORY 17 

monoclinic amphibole, garnet, and sulfide minerals. The only differences we noted are that sillimanite, quartz, and garnet, which are 

abundant in some of the host rocks elsewhere in Colorado, are not 

that abundant in the host rocks sampled in the San Isabel area. 

In addition to the mineralogic similarities noted above, the 

metamorphic textures observed in thin sections of rocks from 

localities in the San Isabel area are closely similar to those observed 

in thin sections of rocks from the other localities in Colorado. These 

textural features are described in some detail in the preceding 

description of mineral assemblages and petrography. Important in 

this regard is the fact that both sapphirine and orthopyroxene, the 

only minerals not observed in the widespread other localities, appear 

to fit into the same pattern of metamorphic textures. Both are commonly in elongate aggregates alined along the gneissic compositional 

layering. Although sapphirine occurs as a rim on zincian spinel in 

some of the rocks, as noted in the descriptions, such relations are not 

a dominating textural feature. Occurrence as independent individual 

grains and aggregates appears to be just as common, and, where the 

two minerals are intergrown, poikiloblastic inclusions of sapphirine 

in zincian spinel are as common as zincian spinel in sapphirine. The 

rimming relation to the spinel, therefore, does not appear to be as 

prevalent in the rocks of the San Isabel area as it is in some other sapphirine localities. Thus, S~renson (1955, p. 26) found that sapphirine 

in West Greenland occurs most commonly around centers of spinel 

and(or) corundum, and Nixon and others (1973, p. 421-422) found 

that sapphirine in granulites in Uganda occurs as rims on aggregates 

of ilmenite, green spinel, and corundum. 

The close similarities in mineralogy and texture noted above are 

the principal reasons for the suggestion that sapphirine and orthopyroxene formed in these rocks during the regional metamorphism 

about 1,700 m.y. ago. That event clearly was responsible for the principal features of mineralogy and texture of host rocks of the majority 

of Precambrian sulfide deposits elsewhere in Colorado. Consequently, 

this same metamorphism probably caused the mineralogy and textures of the host rocks of the Precambrian sulfide deposits in the San 

Isabel area, although the grade of metamorphism was probably 

somewhat higher there. If the presence of sapphirine and orthopyroxene was caused by thermal metamorphism by the San Isabel Granite 

about 1,400 m.y. ago, these minerals should have prominent textural 

features that would indicate their occurrence as a late overprint on 

previously existing minerals and textures. It seems reasonable to 

assume also that lead-lead ages for galena occurring in the ore at the 

Marion mine would have been adjusted to something younger than 

1,700 m.y. if the temperature had reached a sapphirine-forming level 

at the time of intrusion of the San Isabel Granite. Except for the rimming of zincian spinel by sapphirine, a feature that can be as readily 
18 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

explained as a stage In progressive regional metamorphism, there is 

no clear-cut evidence presently available that would date sapphirine 

and orthopyroxene as relatively late contact metamorphic minerals 

in these occurrences. On the contrary, the available data suggest that 

they formed essentially at the same time as the associated metamorphic minerals in a regional metamorphic environment. 

If the interpretation given above is correct, the following question 

remains to be answered: Why did sapphirine and orthopyroxene form 

in the host rocks of these particular Precambrian sulfide deposits? 

The question is pertinent because elsewhere in Colorado the highest 

metamorphic grade recognized in host rocks of Precambrian sulfide 

deposits is the upper amphibolite facies. Yet sapphirine and orthopyroxene are minerals that commonly connote very high grade 

metamorphic conditions at or in pressure-temperature conditions of 

the granulite facies. In the remainder of this discussion we attempt to 

answer this question in relation to the voluminous literature regarding regional metamorphism. 

In describing the granulite facies, Turner (1968, p. 330) noted that 

sapphirine is "a rare but seemingly characteristic phase in silicadeficient assemblages, rich in both Mg and AI." And Seifert (1974, 

p. 201) noted that sapphirine is a mineral requiring high 

temperatures and that high pressures will favor its appearance. 

Despite these connotations, the research over the years has not really 

restricted sapphirine to the granulite facies. Thus, Ramberg (1948, p. 

21-22) concluded from work on sapphirine-bearing rocks from West 

Greenland that the physical stability field of sapphirine is a narrow 

pressure-temperature field between the granulite facies and the amphibolite facies. And S~renson (1955, p. 26), who also worked on rocks 

from West Greenland, concluded that sapphirine is found in associations "typical for the low granulite- or the high amphibolite facies." 

Following more recent work on the occurrences in West Greenland, 

Herd and others (1969, p. 2, 41) found that sapphirine is stable within 

a wide range of pressure and temperature conditions (from amphibolite facies to hornblende-granulite facies) but within a limited 

range of chemical environments. 

Orthopyroxene commonly is mentioned as being characteristic of 

the granulite facies. Turner (1968, p. 328-329) listed orthopyroxene, 

more particularly hypersthene, in several assemblages characteristic 

of the metamorphic rocks and related charnockitic rocks belonging to 

the granulite facies. Although orthopyroxene occurs locally in the 

area described in our present report, it is not a common mineral in 

Colorado's Precambrian metamorphic rocks. R. B. Taylor (oral commun., 1979) pointed out that in regional mapping of the metamorphic 

rocks surrounding the San Isabel Granite batholith he recognized 

only those assemblages characteristic of the upper amphibolite 

facies. However, about 30 km northwest of the San Isabel area, 
DISCUSSION OF METAMORPHIC HISTORY 19 

Brock and Singewald (1968) have mapped charnockitic gneisses 

characterized by hypersthene in the Mount Tyndall quadrangle. 

In arbitrarily drawing a line that divides the amphibolite facies, in 

a strict sense, from the granulite facies, Turner (1968, p. 320) accepted Buddington's following definition (1963, p. 1163-1168) of the 

critical assemblage characterizing the granulite facies: plagioclaseclinopyroxene-hypersthene-hornblende. Turner also recognized the 

fact that in many regions it is possible to trace in the field a gradational transition from the amphibolite facies toward or into the 

granulite facies, and, for that reason, Turner listed in his classification the "Amphibolite-granulite transitional facies" (1968, p. 

320-328). The mineral assemblages of sapphirine-bearing rocks and 

associated granulitic rocks from Sonapahar, Assam, India, were 

studied by Lal and others who concluded (1978, p. 1 77) that the 

mineral parageneses suggest "metamorphism within amphibolegranulite transition facies of Turner (1968)." 

When the mineral assemblages observed in the sapphirine-bearing 

localities of the San Isabel area are assessed in relation to the foregoing references from the literature, we conclude that the regional 

metamorphism probably was within the amphibolite-granulite transitional facies of Turner (1968, p. 320-328) rather than the granulite 

facies. Although both orthopyroxene and clinopyroxene have been 

identified in rocks from these localities, we have not observed these 

two minerals in mutual contact as in Buddington' s critical 

assemblage cited by Turner (1968, p. 320) for the granulite facies. 

Moreover, the abundance of phlogopitic biotite in many of the rocks 

suggests that the metamorphic conditions were not sufficiently intense to produce the ideal anhydrous assemblages of the granulite 

facies. 

The occurrences of sapphirine in Colorado are closely similar in 

some respects to certain occurrences in Australia and India. In the 

southern part of the Arunta Block in central Australia, Stewart and 

Warren (1977, p. 26) reported that among the rock types associated 

with stratabound deposits of chalcopyrite, sphalerite, and galena are 

gneisses containing abundant magnesium and aluminum, and 

characterized by anthophyllite, cummingtonite-gedrite, magnesiumaluminum spinel, enstatite, and sapphirine. In assemblages described 

by Lal and others (1978, p. 171) at Sonapahar, Assam, India, sapphirine coexists with cordierite, orthorhombic amphibole, corundum, 

phlogopite, and spinel. Interestingly, Lal and others (1978, p. 171) 

noted that pyrrhotite and pyrite occur in some of the associated 

rocks, but they do not mention any sphalerite, chalcopyrite, or 

galena. And Lal and others (1978, p. 172) also reported that a few 

samples of spinel contain as much as 7 mole percent of gahnite, 

although most of their analyses range between ordinary magnesian 

spinel and hercynite. 
20 SAPPHIRINE IN PRECAMBRIAN SULFIDE DEPOSITS, COLORADO 

Until the more detailed studies currently in progress are completed, a tentative conclusion is made that the pressure-temperature 

conditions in the San Isabel area during the regional metamorphism 

about 1,700 m.y. ago locally exceeded conditions of the upper amphibolite facies but were not as intense as those set for the granulite 

facies proper. Thus, under such conditions, sapphirine formed where 

the host rocks of the Precambrian sulfide deposits provided the proper silica-poor, magnesia-rich chemical environment favorable for this 

uncommon mineral. Locally in the same area and more abundantly 

elsewhere in the Wet Mountains, as in the Mount Tyndall 

quadrangle, orthopyroxene formed in response to these same 

elevated conditions of pressure and temperature. 

Although sapphirine may be a rare mineral, experience in the San 

Isabel area suggests that it can easily be overlooked even where it is 

relatively abundant. Apparently the mineral was not recognized by 

previous workers in that area because no reports of its occurrence 

were published. It could easily have been missed during this study 

had not samples been taken of rocks having a somewhat unusual 

bluish color as well as of rocks that contain a more noticeable zincian 

spinel. 

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