fINERALOGIST. \ 'OI, 56. SEPTEMBER OCTOBER, 1q71

MINERALOGY OF THE SLOAN DIATREME,A KIMBERLITE PIPE

IN NORTHERN LARIMER COUNTY, COLORADO

M. E. McCerwwr, Colorado State Unittersity, Forl Collins, Colorado

AND

D. H. Eccr.Br., Te*as A €f M Uniaersity, Col,lege Station, Teras.

ABSTRACT'fhe fault-controlled kimberlitic Sloan diatreme penetrates Precambrian granitic rocks

near Prairie Divide in northern Colorado. The kimberlite is predominantly an intrusivebreccia in which clasts consist of serpentine pseudomorphous aJter olivine and pyroxene,with variable amounts of magnesian ilmenite, perovskite, pyrope, chrome diopside,phiogopite, biotite, chromite, picotite, and magnetite. The matrlr of the breccia consists offinely crystalline serpentine, calcite, dolomite, phlogopite, hematite, chlorite, and talc.

Chemical analyses of the kimberlite show low concentrations of SiOz, A1zOr, and KzOand relatively high amounts of MgO and HrO. These chemical trends and a low phlogopitecontent suggest a basaltic kimberlite affinity.

Included in the pipe are numerous xenoliths of Precambrian felsic rocks, Upper Ordovi-cian to Silurian (?) carbonates, kimberlite and lherzolite nodules (and mineral inclusions),and phlogopitic carbonate nodules. Stable carbon and oxygen isotope analyses of carbonateinclusions suggest a magmatic origin for the phlogopitic and a few other carbonates, whichprobably represent a carbonatite liquid that was associated with the original kimberliticmagma.

A very Late Silurian to Early Devonian age of emplacement is postulated.

INtnooucrroN

Several kimberlitic diatremes that penetrate Precambrian crystallinerocks and contain inclusions of sedimentary rocks have recently beenrecognized in the northern Front Range of Colorado and southern Wyom-ing (Chronic et a1,., 1965; D. H. Eggler, 1967 , Ph.D. thesis, Univ. Colo-rado, p. 115; McCal lum and Eggler , 1968; Chronic et a1. ,1969). The Sloandiatreme, largest of the structures yet discovered in the region, is locatednear Prairie Divide on the Sloan ranch, about 40 miles northwest of FortColl ins, Colorado (Fig. 1). It has been quarried intermittently for manyvears as a source of building materials, soil additives, and rock collectors'curios. The pipe penetrates granite of the Precambrian Log Cabinbatholith. Both the pipe and the granite were truncated by a LaLeTertiary to Pleistocene erosion surface (Sherman surface). Because thekimberlite is rather easily weathered, the pipe surface area, which isapproximately 1800 feet by 350 feet, is somewhat lower than that oI themore resistant adjacent granitic rocks.

Emplacement of this steep-walled pipe was partially fault-controlled,

1735

I / J O M. E. MCCALLUM AND D. II. EGGLER

l-rc. 1. Location maD of the Sloan diatreme.

although prominent joint sets also played an important role. The long

dimension coincides with an east-west fault zone (Copper King fault) thatis offset at the western edge oI the diatreme by a northwest-trending

fault zone (Prairie Divide fault) (Fig. 1). Intrusion of volcanic material

into the intersection of the fault zones has produced the crudely bilobate

surface outl ine of the pipe. Smaller projections from the main body par-

allel a set of northwest-trending vertical joints.

v i. ETROGRAPIIY

Serpentirrized rock in the Sloan diatreme is very similar to ultramafic rocks in pipes

from Arkansas (Miser and Ross, 1922), Lrizota (Malde, H. E., 1954; Shoemaker el ol.,

tr962; Watson, K. D., 1967), Utah (McGetchin and Silver, 1970), Kansas (Byrne el a1,',

1956; Rosa and Brookins, 1966), Montana (Hearn, B. C. Jr., 1968), Africa (Dawson, J. B',

1962; Edwards and Howkins, 1966; Nixon et al., 1963), Canada (Watson, K. D., 1955),

Czechoslovakia (Kopecky, L., 1967), and Russia (Moor, G. G., 1941; Davidson, C. F.,

1957 ,1967 ; Smirnov, G. I., 1959) and has been classified accordingly as kimberlite.

The kimberlite is a light- to dark-green or gray-green rock that typically is decomposed

at the surface. The poorly consolidated, highly oxidized pale-green to yellow-green surface

material closely resembles the well known "yellow ground" oI several famous kimberlite

Iocalities. Relatively undecomposed rock is exposed in numerous prospect pits, a 70-foot

shaft, a 130 foot drift and in the face of a small_quarry (Fig. 2).

Both intrusive breccia and massive varieties of kimberlite are present. No beddeC

breccia has been recognized. The intrusive breccia (Figs. 3a and 3b), by far the most abun-

dant t'?e, consists of subangular to subrounded mineral and rock ftagments (average size

about 1 to 5 cm.) set in a fine- to medium-grained matrlx. Rock fragments (xenoliths) are

chiefly limestone, dolomite, granite, gneiss, and schist with lesser amounts of early genera-

tion kimberlite and phlogopitic carbonate, and a few altered lherzolite nodules. Mineral

fragments and crystals (xenocrysts or phenocrysts) are mainly derived from deep-sdated

ultramafic sources, which were cllmost certainly peridotite or early generation kirnberlite,

although a few fragmental grains of country rock are also present. Serpentinized olivine

\ l J tKlMBERL]TE IN CALORADO

o

ola

ct)

Cd

obo

N

N

I

{,5 [;.E "= uE

1738 M. E. MCCALLAM AND D. H. EGGLER

and pyroxene are most abundant; some of the serpentine pseudomorphs are zoned, suggest-

ing former pyroxene (enstatite) coronas on olivine. Lesser amounts of magnesian ilmenit.,

perovskite, pyrope, chrome diopside, phlogopite, biotite, chromite, picotite, and magnetite

are also present as xenocrysts. Some of the biotite crystals and rare grains of quartz and

microcline may be derived from the surrounding granite and felsic gneisses and schists.The

breccia matrlr consists of a finely-crystalline mosaic of serpentine, calcite, and dolomite

with minor amounts of phlogopite, hematite, perovskite, ilmenite, chlorite, talc, and a few

tiny grains of zircon and apatite.

The less abundant massive kimberlite is porphy'ritic (Figs. 3c and 3d) and contains

very little fragmental inaterial. Serpentine pseudomorphs after olivine and pyroxene

xenocrysts or phenocrysts comprise up to 60 percent of the rock, and euhedral to anhedra'l

crystals of phlogopite, perovskite, ilmer:ite, pyrope' chrome diopside, spinels, and iron ore

minerals are usually present. The matrix of the massive kimberlite is similar to that of the

intrusive breccia.Preliminary chemical analysis and density determinations of tlre two kimberlite types

show,a basic similarity, although abundant fragmental material in the breccia produces

slightly lower density values and higher CaO and CO2 contents. Density determinations on

five samples using a balance and Beckman air-comparison pycnometer range f rom2.66-2.72

g./cc. Two whole-rock analyses (Table 1) show low SiOz, AlzOr, and KzO values and rela-

tively high Mgo and Hzo contents, which are indicative of basaltic kimberlites (Dawson,

1962,p.553). Low phlogopite content also suggests a basaltic rather than micaceous kim-

berlite affinity.

MtNBnerocv

The mineralogy of the Sloan diatreme kimberlite has been studied b-v

petrographic and X-ray diffraction methods. Unit cells were refined by

the least-squares method of Evans et al. (1963). Microprobe studies were

conducted on a few selected samples of garnet, chrome diopside, and

ilmenite (Table 2). Several grains of each sample were analyzed on an

ARL-AMX electron microprobe arralyzer. Dead-time, background, and

matrix corrections were made utilizing the Bence and Albee (1968)

method. Standards, which are synthetic unless otherwise noted, in-

cluded: SiOz and AlrOr, natural kyanite; MgO, enstatite glass and peri-

clasel FeO, fayalite and basalt glass; TiOz, sphene glass; CaO, diopsideglass; CrzOr, natural chromite; MnO, rhodonite glass; NarO, nepheline

and natural albite; KzO, natural orthroclase. FeO and TiOz were deter-

mined on ilmenite with a natural ilmenite standard. Other analyses done

by this procedure and analyses of analyzed glasses indicate that results

are within five percent of the amount present.Additional microprobe analyses of several mineral groups are in prog-

rESS.

Primary Minerols

Pyrope, ilmenite, chrome diopside, spinels, and phlogopite are the

most abundant primary minerals preserved in the Sloan kimberlite. All

Kl MBERLITE ]N COI,ORADO r739

Fro. 3. Hand sample (a) and photomicrograph (b) of kimberlite breccia. Subroundedxenocrysts of serpentine (s) (aJter olivine and enstatite) and pyrope (p) (with dark kel-yphitic rims) and fragments of limestone and dolomite (c) in a serpentine-calcite-dolomite-phlogopite matrix. Opaque grains are magnesian ilmenite, picotite, and chromite.

(c) and (d): Hand sample and photomicrograph respectively of massive porphyritickimberlite. Subrounded xenocrysts (phenocrysts) of pseudomorphically serpentinizedolivine and enstatite in a fine-grained matrix similar to that of kimberlite breccia. Mottledblack subrounded grains are serpentinized olivine with abuDdant finely disseminatedmagnetite. Small opaque grains are magnesian ilmenite, picotite, chromite, and perovskite.

1740 M. E. MCCALLUM AND D. H. EGGLER

Table 1. Chemical Analysirs of Sloan Diatreme Kirnberl i te anCAverage enaly-es for Base"ltlc and "Mlcaceoue., KlmberlLtes

3-2 3b 3 - 5 0 0

slo2

frg2

A l2O3

Cr2o3

".19,F::.

MnO

M9o

Cao

Na2O

K2o

n z u '

H2o-

P 2 0 5

ca2

3 L . 4 2

0 . 8 5

2 . 2 0

0. ' ls

5 , . 5 8

r . s 8

0:"r3

2 7 . 9 4

e . e ? ,I

0 . 1 0

L . 2 7

L 0 . 4 2

v . 2 3

0 . t 2

5 . 8 4

32.2:1

I : 0 8

3 . 0 5

0 . 1 6. . .

7 . 4 f . 1 "..-...f

'

1.1}9

0 . 1 5

2 1 . L 3

' . ' 9 . 6 3 : ..

ii:ti-O . 0 6 . i " '

i i, 0 . 8 4

9.7 i1 ,5 .

2 . 6 5

o . 5 4

5 , 2 0

, 3 5 . 0 2

S'er. ZZ

: i . , : : .

R I R

f , \0 . 0 6 ,

3 1 . 2 9

. &r. ' trff %.'.:-Br *t " i r . o5

? {*&-!** #T*'i u' F -

o; , 's .872 . 7 3

3 6 . 3 3

1 . 8 9

s . 0 9r :,.*

s

7 . 4 3

3 . 4 0a * FF f f i O . I O

2 6 . 5 3

" 6 . 7 8 . :_. rq

0 . 3 7

z . l l

I a 4

0 . 6 5

r . 6 4

, , - 1 0 0 . 0 1 1 9 9 . 9 7

n F:S .F3 -23b . K i r i r L ' e i t ' I i t e , i n t i - us i vc b recc i a , S loa r r t r i a t r tXpe , Co lo rado '

, Ana l ys t , t i i r osh i Asa r i .

3 -500 . K imbe r l l t e , mass i ve , S loan d l a t r eme , Co lo rado .Ana l ys t , i i i r osh l Asa r i .

I . Average cheruical compostr t lon of basal t ic k imber l i te,, 10 analyses ( t tockolds ' !93.4, p. L023)

2 . Ave rage chem ica l compos i t i on o f m i caceous k i nbe r l i t e ,

, _ ; i . 4 a n a l y s e s ( N o c k o f t i s , 1 9 5 4 , p . 1 0 2 3 )

occur as subangular to tounded xenocrysts; pebble-shaped fragments ofpyrope and ilmenite up to two inches in diameter have been found. Someof the ilmenite and pyrope show surface polishing, pitting, and striationscaused by fragment abrasion during emplacement. The mineral inclusions

K]MBERL]TE ]N COLORADO I74I

Tab le 2 . Mic roprobe Ana lyses o f Se lec tedMinera ls f rom the S loan K imber l i te

* To ta l Fe as FeO.

are apparently disaggregation products from a previously-consolidatedkimberlite or possibly from lherzolite and eclogite (?).

Pyrope grains are highly fractured and range in color from deep red toreddish-black and rarely pale purple to orange. Fractures are fi,lled withcalcite, chlorite, andf or chrysotile, and smaller grains ;Lre usually rimmedby kelyphite. Kelyphite rinds were apparently stripped off most of theIarger pyrope xenocrysts during intrusion of the pipe. Refractive indicesand unit cell dimensions of several garnet grains range frop 1.746 to1.755 and 11.547 to 11.584 (A) respectively, indicating an averagepyrope-molecule content in excess of 75 percent (Sriramadas, 1957, p.297). Microprobe data for the two garnet samplqs analyzed show highCrzOa contents (Table 2) characteristic of chrome pyropes (Nixon el ol.,1963 , p . 1105 ) .

Garnet D iops ide I Imeni- te

S D L 3 - 2 9 c S D - P I S D 2 O S D 5 O I

s io2

TiO2

Al2O3

Ct 2o3

FeO*

MnO

CaO

Mgo

Naro

K2o

t

3 9 . 8 4 0 . 1

. 5 8 . 8 4

1 9 . I 2 t . 3

6 . 5 3 . 0 3

7 . 7 8 . 2

. 3 9 . 2 9

7 . 6 5 . 0

1 9 . 3 2 t . 3

. 0 0 . 0 0

. 0 1 . 0 1

1 0 0 . 9 8 1 0 0 . 0 7

. 5 4 . 8 , 5 5 . 1

. 2 0 . 1 6

l . 3 5 1 . 1 9

1 . 3 9 r . 4 7

2 . 0 2 2 . 7 0

. 13 , . r r

2 0 . 9 2 0 . 2

. 1 8 . 0 1 9 . I

1 . 1 1 r . 1 4

. 0 2 . 0 4

9 9 . 9 2 1 0 t . 2 1

. 0 2

5 0 . 6 "

. 6 2

3 . t 7

3 0 . 9

. 2 4

. 0 1

1 3 . 4

n . d .

n . d .

1 98 9'6

1742 M. E. MCCALLUM AND D. H. EGGLER

Ilmenite occurs as common breccia fragments and as abundant micro-crysts and megacrysts in kimberlite matrix. Leucoxene coatings aretypical, particularly on smaller grains, which also may be surroundedwith granular perovskite. X-ray powder data for Sloan ilmenite samplescompare favorably with those of magnesian ilmenite from Basutolandkimberlites (Nixon et al., 1963, p. 1121). The d-spacings and the refinedunit cell dimensions indicate moderate substitution of Fe2+ b,v Mg2+,and density values of Sloan ilmenite also indicate the presence of ap-preciable geikielite molecule. Cell dimensions and density values fori lmenite-geikielite are as follows:

Ilmenite.Geikielite'Sloan ilmenite

. After Deer, Howie, and Zussman, 1962,p.3l

Microprobe analysis of a Sloan ilmenite sample shows an MgO con-tent of 13.4 percent (Table 2). Analysis of another i lmenite sample from anearby diatreme (Schaffer diatreme-Chronic el al., 1969) shows an MgOcontent oI 4.5 percent.

Numerous xenocyrsts of chrome diopside, an emerald-green clino-pyroxene, are present in the kimberlite; they are particularly abundant indump material from a shallow shaft near the south margin of the pipe(Fig. 2). Grains are typically subrounded, range up to an inch in diame-ter, and commonly are rimmed by thin rinds of a finely-crystallineserpentine-calcite mixture. Most of the chrome diopside shows little in-ternal alteration, although in a few grains minor amounts of serpentineand chlorite (?) were observed along well-developed planes of cleavageand parting. The chromian clinopyroxene is normally nonpleochrcic,although a faint pleochroism from light gray-green to yellow-green ma-v-be observed. The optic axial angle (2V") ranging from 58 to 600 tends tobe slightly higher than for non-chromian diopsides, and Z /\c is 38 to 40".Preliminary microprobe analyses indicate CrrOa contents of approxi-mately 1 to 1.5 percent (Table 2).

Small euhedral to anhedral crystals of spinel-group minerals t1'picallvoccur in kimberlite matrix. Magnetite and reddish-brown picotite up t,rabout 0.5 mm are most common, and a few grains of brownish-black togray-black chromite have been recognized. Some of the magnetite may bea secondary product of serpentinization. Perovskite is associated vriihand occasionally mistaken for groundmass spinel. Perovskite occurs inabundance as tiny dark brown grains (commonly cubes) and semi-opaquebrownish-black aggregates. Individual crystals range up to about 0.8 mni

d(A)

5 . 0 95 . 1 05.062

'(A)t4 .16t4 .12t3.922

p

4 . 7 24 . 0 54.43

KlMBERLITE IN COINRADO t743

and may be zoned. Both crystals and aggregates are usually leucoxenized,and the aggregates commonly contain corroded grains of ilmenite.

Phlogopite occurs as medium-sized (several mm.) euhedral to sub-hedral crystals in the kimberlite groundmass and is generally at Ieastpartially altered to chlorite. The phlogopite absorption formula isnormal, Z:Y) X with pleochroism of Y:pale yellow orange, I/ andZ:reddish orange. X-ray data suggest a (lM) phlogopite with appreci-able Fe2+ replacing Mg2+. Phlogopite is also an important constituent ofa number of carbonate xenoliths.

Minor amounts of biotite and zircon are also present in the kimberlite.These may be accidental inclusions from adjacent granitic and schistoserocks, but close association of many grains with phlogopite suggests aprimary kimberlitic origin for at least some.

Only a few small crystals of apatite have been recognized in the Sloankimberlite. However, a significant submicroscopic content is inferredfrom appreciable quantities of PzOs in chemical analyses (Table 1).

Carbonate minerals in the Sloan diatreme are clearly derived fromseveral sources. Excluding xenoliths, carbonates occur as finely-crystal'line matrix constituents, secondary alteration products, and vein fillings.Although some of the matrix material may have been derived from localcalcareous rocks, most of it and many carbonate xenoliths are magmaticin origin (see section on stable isotopes). Calcite is the predominant car-bonate, but dolomite is also abundant, and magnesite has been tenta-tively identified. Aragonite is also present, but appears to be exclusivelya secondary vein-filling mineral.

Secondary Minerals. Secondary or alteration minerals comprise thegreatest proportion of the Sloan diatreme kimberlite. Of these, serpen-tine-group minerals are by far the most important.

Serpentine minerals occur chiefly as pseudomorphs after olivine andorthopyroxene xenocrysts and phenocrysts, as fine-grained lamellae andplatelets in the groundmass, and as secondary rims on many xenocrysticand xenolithic fragments. Several types of serpentine have been recog-nized by optical and X-ray data, but the most common appears to be amixture of both single- and double-layered lizardite-chrysotile (Whitakerand Zussman, 1956). Aluminian serpentine (Bailey and Tyler, 1960) isalso present, and antigoritc pseudomorphic after enstatite (bastite) isfairly abundant. Fibrous chrysotile is a common vein-filling constituent,and fine-grained serpophite has been identified in the kimberlite ground-mass. X-ray data for some of the serpentines are compiled in Table 3.

Much of the serpentine that is pseudomorphous after olivine containsabundant small grains of magnetite formed during the serpentinization

o

O{tl

E

o. o

( l ) O r

.Fl o> c

..{ .r{

F.{ .p

o c ( utr O ..{o t {lJ O .'{|+] O Fld o

o o o5 l | Ao 5 0s + r . cO X P r

. L | ' . 1 gX O E O

r - i 6 o 6+ , 6 . t J 6d 5 . . . r 5E O E O

o l r aO ' g r d O .{ J X N. r . { . . { O . . { CF{ t . l dr+ Ol . | $ ' r { |o f d + r ( u ( 0. a E c l r gE o . - t o..1 (l) Qr tr t{A 1 + ) } ] O O

.Fl O Ut C)C - l . U l . - l

.,{ g +J .lJ( l ) o . . d C )

o . o > o r d + ,l] E tr tr ...{5 . ' { O 5 0. | J } 1 O ! . . l . lx r : l x o 6.- r t r o. - r c NE., . r x E ' . r ' . r

' $ r l

o o o o c I+ J C + J t J q J O..1 . ' l 'r l ..{ O{ +J6 ! d d t r ' . rl l C l { r . r O !( d o r d d o oN o ]N N . b ' l

..r H ..{ .r{ o o.,r

.-.{ o r-l Fl..t tr .lrI O | | + r ' r { CO O O ' r { > r d

Fl C.-l +r + --l -'d rd ..1 ..1 d r-l! . . r r J s g o oo t r o o ( t ) + rm. . r O U ' -q H. .1> E >..r 0) rrl r 5 l | . l J r o l J o

-CFl -C tr Otl-{ (uu < u < t r d o

O F l c \ c a- l N r l - l - { F {

n n n n n

a a t h a a u )

M. E. MCCALLUM AND D. II . EGGLER

o F o E E B F B F B B B F

O B E E o o F o o F s F F

9E ' sE O E B

@ oO Fl (')\0 =l Fl c\l (,o 1--o ('| sf @@ (fr o c- or rnrn $ FIC)F- tO Ln =f Fl Fl

c ! N N N r { F { r l F { F l F l

6 N r Oc \ 1 r ) \ o

F. <. (n

\O Ol ( ' ) ( \ t<f F- ' - { O NLn@ F) sf O\ O rO l-- OrF{ (f) tO @ Ln<fN tn \g \O tn sf N C)CO tO S Fl Fl O

F - < ( f ) N c { N c { c \ t F - l F l - l r l r { n

A A A E E E o E A Q } B

N - | . { r n N6 C O N O O T O T O I @ @ c 4 @ < fN s f . \ O @ \ O t n = f N O t O r { O

F - s f d t N c { N N N N F l r + F l

A > A E O B 3 } F E E

lO tO .-{ O\ N CO \gt O F r O c n O r @ L O @ O r r D ON L n \ O \ O

a ' - < i c O N N C \ N N F I - l r +

> E F

\O sf r-lrf) F{ f-r n r ) s f

'-i Fl r-.1

A 3 : r / l F E E

N \ . ot'- tO Fl rn O f*Fl rO \O \O rO st

t ' - = l c O N N N

--l tO Or t-- (n N (f)\O tO N =l tO Or LnF- CO c') F rON ro \o co \o sf $N o ro s! r+

. tF - V r f ) N N N N N c . , l r J r l /

r744

o<

C D H

H

c { H-

(,)o{

F-{ l-t

U)o<

F{

(no<

d

HN

C/) o<d

rJ

(n

04ro

o t rF-{ od ' . {l{ .lJo ( '

..{ dE d

tl

2v..{ vD A

H Oo + )U) '.1

g t ro 0 )q{ .q( d . i.tr 14(d

Gt r OOl Fld ( n7o c )A t r

P>rd Et'{ oI t {

X t{-{

(o

c)

(d

Er

KIMBERL]TE IN COLORADO 1745

reaction. A deep-red variety is apparently also an alteration product ofolivine and derives its color from finely disseminated hematite concen-trated along cleavage planes. Some of the serpentine pseudomorphs arerimmed by an optically-inhomogeneous serpentine with pronouncedcleavage (or parting) and parallel extinction. Rims appear to be moreantigoritic than cores and probably reflect chemical differences betweenformer olivine crystals and enstatite coronas.

Thin, second-generation serpentine rinds on many xenocrysts andxenoliths appear to consist of mixed serpentine minerals, with chrysotilegenerally predominant.

Serpentine xenocrysts are typically partially replaced by carbonateminerals, particularly by dolomite and, to a lesser extent, calcite. Manycarbonate xenoliths display a thin surface rind in which magnesiancarbonate has replaced (dolomitized) the more calcic parent material.

Other secondary minerals reported from the Sloan kimberlite are thephyllosilicates chlorite, talc, and montmorillonite. These minerals aremost prevalent in the deeply-weathered "yellow ground" areas of thediatreme surface. Small amounts of limonite, chalcedonic chert, andmixed clays are also present locally.

XnNor,rrns

Xenoliths in the Sloan diatreme include Lower Paleozoic carbonates,Precambrian crystalline rocks, and kimberlite, lherzolite, and carbonatitenodules (commonly phlogopitic). Alteration of most xenoliths is minorand chiefly involves surface hydration andfor carbonation of constituentminerals. Many of the xenoliths (as well as xenocrysts) have thin serpen-tine and carbonate rinds, and carbonate inclusions commonly have par-tially dolomitized border zones.

Sedimentary carbonate xenoliths are most abundant and consist ofsmall subrounded fragments to large angular blocks of limestone, dol-omite, and dolomite breccia. Three major chaotic block concentrationshave been mapped (Fig. 2) where individual xenoliths range up to 10 feetacross. AII of the sedimentary carbonates represent formations that havebeen stripped away by erosion subsequent to kimberlite emplacement.Fossils indicate an Upper Ordovician and Silurian age for the sedimen-tary xenoliths (Chronic et al., t969, p. 15a).

Abundant smaller carbonate inclusions which are not obviously sedi-mentarv in origin are found in kimberlite, together with nodules ofphlogopitic carbonatite. These rounded to subrounded carbonate noduleshave been observed at many localities in the diatreme. No equivalentrocks are known in nearby bedrock outside of the pipe, and isotopestudies suggest that they are chiefly carbonalites derived from a magmarelated to the kimberlite.

L746 M. D. MCCALLAM AND D. E. EGGLER

Crystalline rock xenoliths consist of pebble- to boulder-sized blocksof granite, gneiss, and schist, similar to Precambrian rocks in the immedi-ate vicinity. These inclusions are generally not significantly altered, al-though some show saussurit ization of plagioclase and/or oxidation andchlorit ization of biotite and hornblende.

A few small nodules (up to 10 cm.) of serpentinized lherzolite havebeen found in the quarried area of the diatreme (Fig. 2). These consistchiefly of coarselv crystall ine, serpentinized olivine and orthrpyroxene(enstatite) with abundant unaltered emerald green chrome diopside.Most of the lherzolites contain small to moderate amounts of dark brownto black spinel, and some contain garnet andf or phlogopite. Severalsmall rounded cognate kimberlitic nodules, similar in composition to theenclosing kimberlite, are also present but are much less abundant thankimberlit ic mineral xenocrysts.

No nodules of eclogite have yet been found at the Sloan locality; how-ever, a well preserved four inch eclogite inclusion was recently collectedfrom the Schaffer 3 diatreme several miles to the north.

Sreer,r IsoropBs ol CARBoNAm INcr,usro-lis

Stable carbon and oxvgen isotope ratios have been determined oncalcite from several samples of Sloan diatreme rocks by Dr. Peter Deines.Nine carbonate inclusions from serpentinized kimberlite group closelyabout an average value of 6018 (o/oo, relative to SMOW, standard meanocean water) :+13, 6C13 (" /oo, re lat ive to PDB, Pee Dee belemni tecalcite) : -4. Two samples of calcite-phlogopite marble (carbonatite)average 6OrE:+17, dC13:-6.5, and a la te carbonate vein has DO1E: + 2 7 , 6 C 1 3 : - 3 . 5 .

Samples of obvious Paleozoic limestone from the pipe have very' dif-ferent isotopic composi t ion: DOtE: +23, DC13:0. Carbonate inc lus ionsanalyzed cannot, therefore, be xenoliths of unaltered l imestone. Theycould be limestone which was recrystallized during upward transport inthe pipe. Ilowever, not only recrystallization, but cornplete isotopicexchange with a fluid phase would be necessary to produce the ratiosobserved. Partial exchange should produce a range of values. The DC13ratio is particularly diagnostic, since exchange with carbon-free countryrocks lower in the pipe would not change dC13. Because the range of ratiosfound in inclusions so far sampled is restricted (0C13: -7.5 to -4.5), wetentatively conclude that most carbonate inclusions are primary, possiblyfrom a carbonatite magma. The calcite-phlogopite inclusions, whichfall into a different range, are more conclusively magmatic, and mayrepresent a different carbonatite stage.

Stable isotope ratios for all these non-sedimentarl. carbonate inclusionslie within the ranse of values from other carbonatite and kimberlite

KIMBERLITE IN COLORADO 1747

sequences, in particular the exceptionally well-studied Oka carbonatite-alkalic complex (Deines, 1967). The values from the Sloan diatreme arehcavier in both carbon and oxygen than material Taylor et al. (1967)consider to be primary carbonatite. These heavier values may reflectdifferentiation of a primary carbonatite magma before intrusion orincorporation in a kimberlite pipe.

Narunn ann Acr oF EMPLAcEMENT

Mechanisms for the emplacement of kimberlitic diatremes have beensubject to controversy for many years. However, most current workersseem to favor some form of fluidization process. An excellent review of thesubject of emplacement mechanisms has been compiled by J. B. Dawson(1967 , p. 246-247).

In the initial stages of kimberlitic magma intrusion, upward migrationwas apparently controlled by some deep-seated zone of crustal weaknesssuch as a system of tension fractures as suggested by Dawson (1967,p.246). The site of surface penetration of the gas-charged magma, orfragments of consolidated magma, is in turn determined by near-surfacestructures such as joint sets and faults. Emplacement of the Sloan dia-treme was determined by the presence of intersecting fault zones, thegeometry of which is well expressed by the surface configuration of thepipe (Figs. t and2).Intrusion apparently occurred through a gas-solidstreaming mechanism, which we infer from the presence of abundantrounded to subrounded rock and mineral fragments, and from thesurface polishing, striations, and pitting of many pyrope and ilmeniteinclusions. Surface velocities of such a stream can approach Mach 3(McGetchin , 1969). A relatively low temperature of intrusion is indicatedby the nearly complete lack of contact metamorphism (pyrometa-morphism) of wall rocks and included xenoliths.

Since the kimberlite contains sedimentary rock xenoliths of UpperOrdovician and Silurian age, the intrusion must have penetrated a LowerPaleozoic rock sequence that was subsequently removed by erosion.Subsidence of the sedimentary rock slabs and fragments several hundredsto thousands of feet into the pipe, where they were preserved as xenolithsin the kimberlite, has provided the sole remaining record of EarlyPaleozoic sedimentation in the Northern Front Range of Colorado(Chronic, et al., 1969).

The time of kimberlite emplacement is uncertain, but paleontologicalevidence and local geology indicate a post-Silurian or very Late Silurian,pre-Pennsylvanian age. Since no Upper Paleozoic rocks appear to bepresent in the diatreme, and since the Lower Paleozoic cover must havebeen stripped off well before local Pennsylvanian arkoses were depositedand very possibly was removed before deposition of Devonian sedi-

1748 M. D. MCCALLUM AND D. H. EGGLER

ments, emplacement of the Sloan diatreme most likely occurred priorto the pre-Devonian erosional cycle (Chronic et al., 1969, p. 155). A verylate Silurian or Early Devonian age of emplacement is postulated.

AcxNowr.noclmNrs

The study was supported in part by Colorado State University, FIC Grant No. 5301-80. The authors wish to acknowledge the assistance of R. K. Corbett, J. W. Creasy, W. M.Oriel, and E. M. Warner in the plane table preparation of topographic and geologic maps ofthe area. Appreciation is also expressed to Mr. and Mrs. George Gibbs of the Sloan Ranchfor their cooperation and hospitality during the course of our field studies, and to Mr.Frank Yaussi for his assistance in the area. Thanks are also extended to B. C. Hearn, Jr.,B. F. Leonard, S. A. Schumm, and G. L. Millhollen for critical reading of the manuscript.We gratefully acknowledge the use of the electron microprobe in the Mineral ConstitutionLaboratories, College of Earth and Mineral Sciences, Pennsylvania State University. Theisotope analyses were very kindly performed by Peter Deines in the Pennsylvania StateUniversity mass spectrographic laboratories.

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Manuscript receizted, February 18, 1971; accepled. {or publ'ication, April, 19, 1971.