Studies of Mineralized Intrusive 

Complexes in North-Central Montana

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1301
Studies of Mineralized Intrusive 

Complexes in North-Central Montana

Mineralized Breccias in Intrusive Complexes of

Late Cretaceous and Paleocene age, North-Central Montana

By DAVID A. LINDSEY and FREDERICK S. FISHER

Relation Between Igneous Intrusion and Gold 

Mineralization in the North Moccasin Mountains, 

Fergus County, Montana 

By DAVID A. LINDSEY and CHARLES W. NAESER

A Gold-Mineralized Breccia Zone at Kendall,

North Moccasin Mountains, Fergus County, Montana

By DAVID A. LINDSEY

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1301-A,B,C

Geology, nature of hydrothermal alteration, and 

genesis of mineralized zones in and near intrusive 

igneous complexes of Late Cretaceous-Paleocene 

age in north-central Montana

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1985
UNITED STATES DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Studies of Mineralized Intrusive Complexes in North-Central Montana.

(Geological Survey Professional Paper ; 1301-A,B,C)

56 p.

Bibliography: p. 20, 41, 55-56

Supt. of Docs. No.: 119.16:1301-A-C

1. Contents: Mineralized breccias in intrusive complexes of Late Cretaceous and Paleocene age, north-central 

Montana / by David A. Lindsey and Frederick S. Fisher Relation between igneous intrusion and gold 

mineralization in the North Moccasin Mountains, Fergus County, Montana / by David A. Lindsey and Charles 

W. Naeser A gold-mineralized breccia zone at Kendall, North Moccasin Mountains, Fergus County, Montana 

/ by David A. Lindsey.

1. Intrusions (Geology) Montana Addresses, essays, lectures.

I. Lindsey, David A. Mineralized breccias in intrusive complexes of Late Cretaceous and Paleocene age, northcentral Montana. 1985. II. Lindsey, David A. Relation between igneous intrusion and gold mineralization 

in the North Moccasin Mountains, Fergus County, Montana. 1985. III. Lindsey, David A. A goldmineralized breccia zone at Kendall, North Moccasin Mountains, Fergus County, Montana. 1985. IV. Series.

QE611.5.U6S88 1985 551.8'8'09786292 83-600214

For sale by the Branch of Distribution

U.S. Geological Survey

604 South Pickett Street

Alexandria, VA 22304
CONTENTS

[Letters designate the chapters.]

Page

(A) Mineralized breccias in intrusive complexes of Late Cretaceous and Paleocene age,

north-central Montana, by David A. Lindsey and Frederick S. Fisher . . 1

(B) Relation between igneous intrusion and gold mineralization in the North Moccasin 

Mountains, Fergus County, Montana, by David A. Lindsey and Charles W. 

Naeser ...................................... 35

(C) A gold-mineralized breccia zone at Kendall, North Moccasin Mountains, Fergus

County, Montana, by David A. Lindsey ................... 43

III

Mineralized Breccias in 

Intrusive Complexes of 

Late Cretaceous and Paleocene Age, 

North-Central Montana

By DAVID A. LINDSEY and FREDERICK S. FISHER

STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN 

NORTH-CENTRAL MONTANA

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1301-A
CONTENTS

Abstract ............................................. 3

Introduction ........................................... 3

Acknowledgments ........................................ 5

South Moccasin Mountains ................................... 5

Plum Creek, North Moccasin Mountains ........................... 7

Limekiln Canyon, Judith Mountains .............................. 11

Judith Peak-Red Mountain area, Judith Mountains ..................... 11

Gold Bug Butte and vicinity, Little Rocky Mountains ................... 13

Comparison of alteration and geochemical features ..................... 15

Discussion and conclusions ................................... 18

References cited ......................................... 20

ILLUSTRATIONS

Page 

FIGURE 1. Map showing geologic setting of mineralized intrusive breccias in north-central Montana and principal geologic structures . 4

2. Geologic map showing intrusive breccias in the South Moccasin Mountains ........................ 6

3. Photographs of breccias of the South Moccasin Mountains ................................. 7

4. Geologic map showing intrusive breccia of Plum Creek, North Moccasin Mountains ................... 8

5. Photographs of breccia of Plum Creek, North Moccasin Mountains ............................ 9

6. Sketch maps showing alteration patterns and geochemical anomalies in the Plum Creek breccia pipe and adjacent syenite

porphyry, North Moccasin Mountains .......................................... 10

7. Geologic map showing intrusive breccia of Limekiln Canyon, Judith Mountains ..................... 12

8. Geologic map showing intrusive breccia of Judith Peak-Red Mountain area, Judith Mountains ............. 14

9. Geologic map showing intrusive breccias of the Little Rocky Mountains ......................... 16

10. Photographs of breccias at Gold Bug Butte, Little Rocky Mountains ........................... 17

11. Histograms showing the frequency distributions of elements in 164 samples of breccias and wall rocks in Late CretaceousPaleocene intrusive complexes of north-central Montana ................................ 21

TABLE

Page

TABLE 1. Summary of alteration and geochemical features of breccias in north-central Montana .................. ig
STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN 

NORTH-CENTRAL MONTANA

MINERALIZED BRECCIAS IN INTRUSIVE COMPLEXES OF

LATE CRETACEOUS AND PALEOCENE AGE,

NORTH-CENTRAL MONTANA

By DAVID A. LINDSEY and FREDERICK S. FISHER

ABSTRACT

Mineralized breccias are found in most of the intrusive porphyry 

complexes of Late Cretaceous-Paleocene age in north-central Montana. Breccias have been mapped and studied in the central part 

of the South Moccasin Mountains and at the Republic claim south 

of the mountains, at the head of Plum Creek in the North Moccasin 

Mountains, at Limekiln Canyon and the Judith Peak-Red Mountain 

area in the Judith Mountains, and at Gold Bug Butte in the Little 

Rocky Mountains. Porphyry host rocks, which range mostly from 

syenite to quartz monzonite in composition, were emplaced as laccoliths, stocks, sills, dikes, plugs, and complex combinations of these 

forms at shallow depths (1,100-2,700 meters). The breccias probably 

were formed by hydraulic ramming of residual fluids and magma during cooling of the porphyry intrusions. Emplacement of magma continued after breccia formation in the Judith Peak-Red Mountain area. 

Mineralization preceded formation of breccias in the South Moccasin 

Mountains, but accompanied and followed formation of most of the 

other breccias.

The breccias intrude porphyry in all places studied except one; they 

generally form pipe-shaped bodies, although irregular masses and 

dikes are present also. Most breccias are composed of large to small 

fragments of syenite and related porphyry in a fine matrix of particles 

derived from the same rocks. Locally, Precambrian and Paleozoic 

rocks make up most of the fragments, and commonly are present 

in minor amounts where porphyry fragments predominate. Contacts 

between some of the breccias and the host porphyry generally are 

well defined within a few meters, but other contacts are gradational 

through fragment-rich breccia and crackle zones. Dikes of igneous 

rock cut intrusive breccia in the Judith Peak-Red Mountain area, 

and reaction rims surround breccia fragments at one locality in the 

Little Rocky Mountains; these features and evidence of mineralization 

indicate that the breccias were emplaced during the waning stages 

of intrusive activity.

Alteration and mineralization took place both before and after 

emplacement of the breccias, indicating a close tie between the two 

processes. Most of the breccias show evidence of incomplete hydrothermal alteration, ranging from minor kaolinite in breccias of the 

South Moccasin Mountains and at Limekiln Canyon in the Judith 

Mountains, to kaolinite and quartz-illite-sericite at Plum Creek in the 

North Moccasin Mountains, to quartz-sericite in the Gold Bug Butte 

area of the Little Rocky Mountains, and to quartz-potassium feldspar 

in the breccias of the Judith Peak-Red Mountain area in the Judith 

Mountains. Quartz-sericite and quartz-potassium feldspar altered 

breccias contain abundant limonite, jarosite, and, locally, pyrite. 

Spectrographic analyses of breccias from each area show no major

chemical changes in the formation of kaolinitic breccias; analyses show 

that calcium, magnesium, sodium, and manganese have been leached 

and that iron and titanium have been redistributed during alteration 

to quartz-sericite and quartz-potassium feldspar breccias. Anomalous 

amounts of gold, silver, and vanadium are most widespread in quartzsericite altered breccia and to a lesser extent in quartz-potassium 

feldspar breccia; anomalous amounts of copper, lead, and molybdenum 

are found in both.

INTRODUCTION

Intrusive breccias were emplaced during two periods 

of igneous activity in north-central Montana (fig. 1). 

The earliest period of igneous activity, during Late Cretaceous and Paleocene time, consisted of emplacement 

of intrusive porphyry complexes, including breccias, in 

the Moccasin, Judith, and Little Rocky Mountains about 

69-60 m.y. (million years) ago (Marvin and others, 

1980). The second period of igneous activity, during 

Eocene time, consisted of both volcanism and emplacement of plutons in the Bearpaw and Highwood Mountains 54-50 m.y. ago (Marvin and others, 1980) and 

emplacement of plutons 56-41 m.y. ago in the Little 

Belt Mountains (Marvin and others, 1973). Eocene igneous activity was accompanied by emplacement of breccias in the Little Belt Mountains, and also by eruption 

of the Missouri Breaks diatremes 52-47 m.y. ago 

(Hearn, 1979). In addition to differences in age and 

mode of emplacement, the Eocene igneous rocks of 

north-central Montana contain many examples of mafic 

alkalic and feldspathoidal rocks (Weed and Pirsson, 

1896; Hurlbut and Griggs, 1939; Larsen, 1941; Witkind, 

1973). In contrast, the intrusive complexes of Late Cretaceous and Paleocene age contain only minor volumes 

of mafic alkalic and feldspathoidal rocks these are the 

tinguaite dikes of the Judith and Little Rocky Mountains (Weed and Pirsson, 1898; Brockunier, 1936; Wallace, 1953). The intrusive breccias associated with the 

Upper Cretaceous-Paleocene porphyry complexes are 

the subject of this report.
STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

110° 109° 108°

10 20 30 40 KILOMETERS

11 I

«£ Intrusive rocks (Eocene)

Volcanic rocks (Eocene) 

d, diatreme occurrence

Intrusive rocks

(Paleocene and Upper 

Cretaceous)

Locality of intrusive breccia

SM, South Moccasin Mountains; 

R, Republic claim;PC, Rum Creek; 

LC, Limekiln Canyon; 

JP-RM, Judith Peak-Red Mountain; 

GB, Gold Bug Butte

l 

10 20 MILES

EXPLANATION

4OOO Structure contours in feet to base of 

Colorado Shale (restored); contour 

interval 1000 ft (from Dobbin and 

Erdmann, 1955)

107°

Index map

FIGURE 1. Map showing geologic setting of mineralized intrusive breccias in north-central Montana and principal geologic structures.
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE

The large complexes of Upper Cretaceous-Paleocene 

intrusive porphyry (fig. 1) are mostly silica-saturated 

rocks of syenite, monzonite, and quartz monzonite composition and were emplaced at shallow depths (1,100- 

2,700 m). They take the form of laccoliths, stocks, sills, 

dikes, plugs, and complex combinations thereof (Weed 

and Pirsson, 1898, p. 572-587; Goddard, 1950; Knechtel, 

1959; Miller, 1959; Lindsey, 1982). Some intrusive centers, notably that at Judith Peak in the Judith Mountains (Wallace, 1956), are composed of a varied assemblage of rocks, including syenite, quartz monzonite, 

alkali granite, and tinguaite, and probably resulted 

from multiple episodes of differentiation and emplacement. Intrusive activity was accompanied by formation 

of breccias and by mineralization of the porphyry complexes and their country rocks. Mineralizing fluids pervaded the intrusive breccias, circulated along fissures 

in porphyry (Weed and Pirsson, 1898, p. 592-593; 

Dyson, 1939), invaded country rocks along contacts 

with porphyry (Weed and Pirsson, 1898, p. 594; Forrest, 1971), and invaded fault breccias formed during 

intrusion of porphyry complexes (Lindsey, 1982). 

Mineralization of a fault breccia is discussed in chapter 

C of this paper.

Our study was made to provide a comprehensive description and comparison of the breccias and to interpret their origin and significance with respect to mineral resources. We have relied on the previously established geologic framework of north-central Montana, 

but have provided additional details on the geology of 

the breccias and new data on their petrographic and 

chemical composition. Published information on the 

breccias is scarce, although most of them have been 

prospected for gold and other metals. Those in the Moccasin Mountains were mapped and described recently 

(Lindsey, 1982); the intrusive breccias of the Judith 

Mountains had been mapped (Goddard and Wallace's 

map in Wallace, 1953) but not described; and those in 

the Little Rocky Mountains were recognized and described briefly by Weed and Pirsson (1896, p. 411^412), 

but have not been mentioned since that time. Geologic 

maps showing the breccias in the Moccasin Mountains 

were taken from the map of Lindsey (1982); a detailed 

map of Limekiln Canyon and vicinity in the Judith 

Mountains was compiled from the mapping of E. N. 

Goddard and S. R. Wallace; unpublished mapping of 

the Judith Peak-Red Mountain area of the Judith Mountains, by Goddard and Wallace, was adapted for the 

present report; and a map showing breccias in the Little 

Rocky Mountains was prepared by F. S. Fisher from 

his mapping and the geologic map of Knechtel (1959). 

All the breccias were sampled for petrographic study

and chemical analyses. Thin-section and X-ray diffraction studies were conducted to characterize the degree 

and type of alteration of the breccias, and chemical 

analyses by spectrographic methods and gold analyses 

by fire assay and atomic absorption were made to identify anomalous concentrations of metals.

ACKNOWLEDGMENTS

C. A. Brannon, C. N. Culver, G. D. May, and M. 

R. Pawlowski assisted in fieldwork; Ezekiel Rivera prepared X-ray diffractograms; R. E. Phillips studied thin 

sections and made mineral identifications; and S. J. 

Ashe prepared photographs and compiled data; all are 

employees of the U.S. Geological Survey. Chemical 

analyses were done in laboratories of the U.S. Geological Survey in Denver, Colo.; six-step semiquantitative 

spectrographic analyses were done by L. A. Bradley, 

N. M. Conklin, and J. C. Hamilton; fire assay-atomic 

absorption analyses for gold were made by J. G. Crock, 

A. W. Haubert, and Joseph Haffty.

SOUTH MOCCASIN MOUNTAINS

Breccia pipes are found in syenite porphyry and 

quartz monzonite porphyry near triangulation station 

South in the central part of the Moccasin Mountains 

(fig. 2) (Lindsey, 1982). Both syenite porphyry and 

quartz monzonite porphyry of the South Moccasin 

Mountains have been dated radiometrically at 64-65 

m.y. (Marvin and others, 1980). The breccias are poorly 

exposed in the center of the mountains except where 

cut by the access road leading to radio facilities at triangulation station South; where exposed, their contacts 

with syenite porphyry can be defined within 1^4 m 

(meters). Breccia in quartz monzonite porphyry is 

poorly exposed and little studied; in part, it appears 

to be "crackle" breccia, containing many fissures and 

unrotated fragments of the host rock. Additional occurrences of breccia may have gone undetected in 

poorly exposed areas of the South Moccasin Mountains. 

A bleached zone in syenite porphyry extends north 

from the breccias near triangulation station South.

A breccia pipe also occurs in the Lower Cretaceous 

Skull Creek Member of the Colorado Shale at the Republic claim south of the mountains (fig. 2). The Republic breccia crops out along a normal fault that probably 

formed during intrusion of syenite porphyry and accompanying uplift of the Hanover dome. Contacts of the 

breccia at the Republic claim can be located within a 

few meters; the host shale shows no obvious effects 

of alteration.
STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

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Geology from Lindsey 119821

1 MILE

CONTOUR INTERVAL 200 FEET 

FIGURE 2 (above and facing page). Geologic map showing intrusive breccias in the South Moccasin Mountains, Mont.
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE

EXPLANATION

Intrusive breccia (Paleocene?)

Quartz monzonite porphyry 

(Paleocene)

Syenite porphyry (Paleocene)

Undivided sedimentary rocks (Cretaceous and 

Jurassic) Cretaceous Colorado Shale and 

Kootenai Formation, Jurassic Morrison Formation 

and Ellis Group

Undivided sedimentary rocks (Mississippian)

Mississippian Big Snowy and Madison Groups

Undivided sedimentary rocks (Devonian and 

Cambrian) Devonian Jefferson Formation and 

Cambrian Pilgrim Limestone

Bleached area in syenite porphyry

Contact

Fault, ball and bar on downthrown side

Sample locality

Open pit

The breccias near triangulation station South are 

composed mainly of angular fragments and crystals derived from the host syenite porphyry and are set in 

a finely pulverized matrix shown by microscopic study 

to be composed of the same rock (fig. 3A). A few fragments of sandstone, quartzite, metamorphosed carbonate rock, and mineralized carbonate rock containing 

galena and pyrite, were found in some of the breccias. 

Minor amounts of kaolinite, identified in thin section 

and by X-ray diffraction, partly replace feldspar crystals and matrix in the breccias and in the adjacent zone 

of bleached syenite porphyry. Anomalous zinc ((500 

ppm (parts per million)), probably from sphalerite in 

mineralized fragments, was detected in some of the 

breccias, but in general the breccias are not highly 

mineralized.

Breccia at the Republic claim contains a diverse suite 

of fragments (fig. 35), including syenite porphyry, 

quartz, carbonate rocks, shale, sandstone, coal from the 

Lower Cretaceous Kootenai Formation, schist and 

gneiss of Precambrian age, and fragments containing 

galena, sphalerite, chalcopyrite, and pyrite that were 

mineralized before brecciation. Fragments are as much 

as 0.3 m across and are subround to round. The matrix 

of some of the breccia has a sandy appearance, but 

under the microscope is seen to be composed of small 

broken crystals and fragments derived from syenite 

porphyry. Elsewhere in the breccia mass, the matrix 

is composed of comminuted black shale of the host Skull 

Creek Member of the Colorado Shale. Little alteration 

of the breccia is visible, but small (0.02-0.1 mm (millimeters)) euhedral pyrite is abundant locally in the maFIGURE 3. Photographs of breccias of the South Moccasin Mountains, Mont. A, Breccia from south of triangulation station South, 

showing small fragments of mostly syenite porphyry in finely 

ground matrix of same material. Light rims around some syenite 

porphyry fragments are due to weak hydrothermal alteration or 

weathering. B, Breccia from the Republic claim, showing subround 

to round fragments of syenite porphyry (P) and Precambrian gneiss 

(GN) in matrix of shale and syenite fragments.

trix. Highly anomalous quantities of copper, gold, lead, 

molybdenum, silver, and zinc were detected by chemical analysis, mainly in mineralized fragments in the 

breccia.

PLUM CREEK, NORTH 

MOCCASIN MOUNTAINS

The large (0.3 km2 (square kilometers)) breccia pipe 

at the head of Plum Creek, in the North Moccasin 

Mountains, intrudes a laccolith of syenite porphyry 

dated radiometrically at 66 m.y. (fig. 4) (Lindsey, 1982; 

Marvin and others, 1980). Contacts between the breccia
STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

109°27

47019'

1 MILE

1 KILOMETER 

CONTOUR INTERVAL 20D FEET

FIGURE 4 (above and facing page). Geologic map showing intrusive breccia of Plum Creek, North Moccasin Mountains, Mont.
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE

EXPLANATION

Intrusive breccia (Paleocene?)

Syenite porphyry (Paleocene)

Undivided sedimentary rocks (Cretaceous and 

Jurassic) Cretaceous Kootenai Formation, 

Jurassic Morrison Formation and Ellis Group

Brecciated and silicified limestone in Madison Group 

(Mississippian)

Undivided sedimentary rocks (Mississippian)

Mississippian Kibbey Formation and Madison 

Group

Undivided sedimentary rocks (Devonian and 

Cambrian) Devonian Jefferson Formation, 

Cambrian Pilgrim LJmestone, Cambrian 

unassigned strata, and Cambrian Flathead 

Sandstone

Schist (Precambrian)

Contact

Fault, ball and bar on downthrown side

Sample locality

Shaft

Inclined shaft

Open pit

Adit

and host porphyry can be located within a few meters 

where exposures permit; along the southern contact, 

a large (0.3 km2) zone of crackle breccia in syenite porphyry adjoins the pipe. Contacts between the crackle 

zone and host syenite porphyry are gradational over 

a few tens of meters. Two small (<20 m2 (square 

meters)) breccia pipes were found north and northeast 

of the main pipe; other small breccia pipes may be present. Three large xenoliths of Paleozoic and Mesozoic 

rocks crop out within and near the southern boundary 

of the crackle zone.

The breccia is composed mainly of angular fragments 

of syenite, a few millimeters to centimeters across, a 

few fragments of Precambrian schist and Paleozoic 

sandstones, carbonate rocks and shale, and many broken crystals; all these are set in a fine-grained matrix 

of mostly altered particles derived from syenite porphyry (fig. 5A). Limonite, of probable supergene origin 

from oxidation of pyrite, and sericite or illite form reaction rims around many rock fragments and replace 

mafic minerals in rock fragments and in breccia matrix. 

Small amounts of kaolinite, illite, and sericite are widespread. (Sericite was distinguished from illite by its relatively sharp X-ray diffraction peak at 10A (angstrom) 

and by its well-crystallized form in thin section.) Pyrite 

and limonite pseudomorphs after pyrite are disseminated in fragments and in the matrix of some of the 

breccia. Minerals that are the products of alteration appear to have formed both before and after transport 

of breccia fragments.

The crackle zone is composed of bleached, ironFIGURE 5. Photographs of breccia of Plum Creek, North Moccasin 

Mountains, Mont.: A, Breccia composed mostly of small fragments 

of syenite porphyry in finely ground matrix of same material; B, 

crackle breccia in syenite porphyry south of breccia pipe, showing 

abundant quartz veinlets.

stained syenite porphyry that is traversed by many 

closely spaced fractures as much as 0.5 cm (centimeters) 

wide (fig. 5#). Both open and quartz-filled fractures are 

common. Fresh specimens reveal sparse to abundant 

disseminated pyrite. Thin-section and X-ray diffraction 

studies reveal small amounts of kaolinite and locally 

abundant illite and sericite, mostly replacing ferromagnesian and feldspar phenocrysts. In contrast to 

the breccia and crackle zone, much of the nearby host 

porphyry shows only the effects of propylitic alteration. 

Alteration and mineralization were more intense in 

part of the breccia pipe and adjoining crackle zone than 

in surrounding rocks, as shown by the distribution of 

sericite (fig. 6A) and anomalous copper (<150 ppm), 

gold (<4.11 ppm), lead (< 1,000 ppm), molybdenum
10 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

,/Tb

Tb'

Tsp

Au 0.68 

Pb 500

r

Mu

KJu

-Au 3.09 

Mo 30 

Cu 50 

u 100

Mo 15

Ag 15

Au 4-11

Tsp

EXPLANATION

Tb Intrusive breccia (Paleocene?)

Tsp Syenite porphyry, (Paleocene) 

(includes crackle zone)

KJu Undivided sedimentary rocks 

(Cretaceous and Jurassic)

Mu Undivided sedimentary rocks 

(Mississippian)

DCu Undivided sedimentary rocks 

(Devonian and Cambrian)

Sample locality

Illite present (A)

Sericite (well-crystallized) present (A)

Metal anomaly, in parts per million (B)

Area of most intense sericite alteration 

and mineralization (C)

1 KILOMETER

J

MILE

/Tb

KJu

/Tb

Tsp

FIGURE 6. Sketch maps showing alteration patterns and geochemical anomalies in the breccia 

of Plum Creek and adjacent syenite porphyry, North Moccasin Mountains, Mont.: A, Distribution of illite and sericite; B, metal anomalies; and C, area of most intense alteration and 

mineralization.
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 11

(<70 ppm), and silver (<30 ppm), (fig. 6£). The area 

of most intense alteration coincides approximately with 

that of anomalous metal values (fig. 6C). Most of the 

rock sampled has been oxidized, with pyrite visible in 

only a few samples, so that many of the metal values 

may have been reduced from their original concentrations. Downstream from the breccia, fine particles of 

gold were recovered by panning sediment in Plum 

Creek, and placer gold has been mined both from Plum 

Creek and from Iron Gulch, which also drains part of 

the crackle zone. A gold-mineralized zone in fault-brecciated limestone 2 km southeast of the mineralized breccia pipe is described in chapter C of this paper.

LIMEKILN CANYON, 

JUDITH MOUNTAINS

Breccia pipes occur in a laccolith of quartz monzonite 

porphyry of probable Late Cretaceous and Paleocene 

age in the upper part of Limekiln Canyon, located in 

the southwestern part of the Judith Mountains (fig. 7) 

(Goddard and Wallace in Wallace, 1953). Contacts with 

the host porphyry can be located within a few meters, 

but outcrops are poor. The breccia crops out mainly 

over one crescent-shaped area of 0.1 km2; smaller pipes 

crop out a few hundred meters south, 1 km southwest, 

and 1 km east-southeast of the main pipe. No sizable 

zones of crackle breccia or altered rock more than a 

few meters beyond the margin of the pipes have been 

detected, but such zones may have escaped detection 

owing to generally poor exposures.

The breccia consists mostly of angular fragments of 

quartz monzonite porphyry as much as 0.5 m across 

in a matrix of broken crystals and fine particles of the 

host quartz monzonite porphyry; fragments of Precambrian schist and Paleozoic rocks are abundant locally. 

Thin-section and X-ray diffraction studies reveal that 

most of the breccia has been partly altered to kaolinite, 

minor illite, and around the northeastern side of the 

main pipe, to sericite (fig. 7). Much of the kaolinite appears to be restricted to the matrix, and probably postdates emplacement of the breccia. At a few places, illite 

and sericite replace fragments of porphyry, feldspar 

phenocrysts, and ferromagnesian phenocrysts as well 

as matrix. Secondary quartz was noted at only a few 

places. Limonite is common, but it appears to be derived from partial alteration of ferromagnesian minerals 

and magnetite instead of pyrite; iron-oxide 

pseudomorphs after pyrite were observed at one locality in the northeast part of the main pipe, and jarosite 

was noted at only one locality. In contrast to the 

kaolinitic alteration of the breccia, the host porphyry 

is propylitically altered; porphyry about 0.3 km east 

of the main pipe containsonly chlorite, calcite, and iron

oxide replacing ferromagnesian minerals and matrix. 

Anomalous concentrations of metals were found in only 

one sample; no samples were taken from the two small 

pipes 1 km east-southeast of the main pipe.

JUDITH PEAK-RED MOUNTAIN AREA, 

JUDITH MOUNTAINS

The Judith Peak-Red Mountain area, mapped by E. 

N. Goddard and S. R. Wallace (Wallace, 1953), is a 

complex intrusive center (fig. 8) composed of quartz 

monzonite, syenite, and rhyolite porphyries, all of 

which are intruded by coarse-grained alkali granite on 

Judith Peak, by dikes of fine- and coarse-grained granite and granite porphyry, and by dikes of green and 

gray tinguaite (called trachyte by Hall, 1976) and tinguaite porphyry. Detailed descriptions of some of the 

intrusive rocks are given by Wallace (1956). The porphyry complex of Red Mountain and vicinity probably 

was emplaced about 65-67 m.y. ago, although many discordant radiometric dates make this estimate tentative 

(Marvin and others, 1980). The coarse-grained alkali 

granite of Judith Peak probably was emplaced about 

62 m.y. ago (Marvin and others, 1980).

Breccia pipes intrude the complex of quartz monzonite, syenite, rhyolite, and related porphyries on Red 

Mountain, at the head of Lincoln Gulch, and in the 

upper part of Collar Gulch (fig. 8). Granite breccia, composed of fine- and coarse-grained alkali granite, intrudes two phases of alkali granite porphyry on Judith 

Peak; this breccia was not studied by us. The breccias 

of Red Mountain and nearby altered porphyries are intruded by dikes of alkali granite and tinguaite; the granite dikes probably emanated from the same magma as 

the alkali granite of Judith Peak. Inasmuch as the breccias contain fragments of granite as well as numerous 

intact dikes of this rock, it follows that formation of 

breccia was more or less contemporaneous with 

emplacement of the alkali granite of Judith Peak about 

62 m.y. ago.

Contacts between the breccias and their host porphyries range from sharp to gradational. The crescentshaped mass of breccia on the west flank of Red Mountain has generally well-defined contacts. At the north 

end of the breccia, a dike of granite porphyry crosses 

a sharp contact between breccia and porphyry (fig. 8); 

the dike contains pyrite but is not as highly altered 

as its wall rocks. The contacts of other breccias farther 

south are located with difficulty because of poor exposure and pervasive alteration of both breccia and host 

porphyry. Contacts of the breccia mass in Collar Gulch 

are generalized; unpublished mapping by E. N. Goddard and S. R. Wallace shows more than one interpretation of the outline of this breccia. The breccia at the
12 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

47'W

47°B8'

1 KILOMETER

Geology ny c. n. uoodard and 

S. R. Wallace, 1940,1941,1948, 

1949, 1951, and 1952

MILE

CONTOUR INTERVAL 250 FEET

FIGURE 7 (above and facing page). Geologic map showing intrusive breccia of Limekiln Canyon, Judith Mountains, Mont.

head of Lincoln Gulch is poorly exposed, and the relationship of the rhyolite porphyry there with other 

rocks is not understood.

The breccias that intrude the porphyry complex of 

the Judith Peak-Red Mountain area are composed 

mostly of angular fragments and broken crystals of the

host porphyry; fragments range in size from a few centimeters to a few millimeters and are set in a finely 

comminuted, altered matrix of the same material. 

Large angular fragments of porphyry that do not appear to have been transported far are found in the breccia at Collar Gulch. More than one type of porphyry
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 13

EXPLANATION

Intrusive breccia (Paleocene or Upper 

Cretaceous)

Quartz monzonite porphyry (Paleocene or 

Upper Cretaceous)

Undivided sedimentary rocks (Cretaceous 

and Jurassic) Cretaceous Kootenai 

Formation, Jurassic Morrison Formation 

and Ellis group

Undivided sedimentary rocks (Mississippiari) 

Mississippian Big Snowy and Madison 

Groups

^^Bi: j Undivided sedimentary rocks (Devonian and

Cambrian) Devonian Jefferson Formation, 

Cambrian Pilgrim Limestone, unassigned 

Cambrian strata, and Cambrian Flathead 

Sandstone

 Contact

"I Fault, ball and bar on downthrown side

 Sample locality

is seen commonly in the breccia, and fragments of sandstone and other sedimentary rocks from country rocks 

adjacent to the porphyry complex are seen rarely. 

Fragments of granite porphyry and green and gray tinguaite porphyry were noted locally in the breccias of 

Red Mountain and Collar Gulch.

A large zone of post-breccia silicic and pyritic alteration, as mapped by E. N. Goddard and S. R. Wallace 

(fig. 8), pervades the porphyry complex and its associated breccias. Pyrite is present nearly everywhere 

where fresh rock can be found; well-crystallized jarosite 

and limonite of supergene origin both are widespread 

and attest to the former abundance of pyrite. Thin-section and X-ray diffraction studies of the altered rocks 

show that they contain abundant fine-grained potassium 

feldspar in the matrix, minor quartz in the matrix and 

as overgrowths on quartz crystals, and, locally, wellcrystallized sericite in matrix and replacing breccia 

fragments. Calcite, quartz, fluorite, galena, and barite 

occur in sparse veins that cut altered rock (Hall, 1976). 

Chemical analyses show anomalous copper (<700 ppm), 

gold (<0.93 ppm), lead (<3,000 ppm), and molybdenum 

(<300 ppm). Chemical precipitates of limonitic iron are 

found along Collar Gulch and Armell Creek, both of 

which drain the Red Mountain area. An analysis of 

limonite precipitate from Collar Gulch revealed 1,500 

ppm lead but no other anomalous concentrations of metals. Evidence of extensive late supergene alteration of 

the altered zone at Judith Peak-Red Mountain suggests 

that the metal values now found in rocks of the altered 

zone have been much reduced from their original 

amounts.

GOLD BUG BUTTE AND VICINITY, 

LITTLE ROCKY MOUNTAINS

In the Little Rocky Mountains, irregular stocks and 

laccoliths of syenite porphyry intruded Precambrian 

basement rocks and Paleozoic and Mesozoic strata during Paleocene time (67-60 m.y.), lifting them into a 

broad complex of domes about 15 km in diameter (fig. 

1) (Brockunier, 1936; Knechtel, 1959; Marvin and 

others, 1980). Breccia pipes and breccia dikes are present in a few scattered localities in the Little Rocky 

Mountains and are especially well developed for about 

1.5 km along the crest of Gold Bug Butte and the hill 

to the northeast (fig. 9). The breccia pipes have been 

emplaced mainly in altered syenite porphyry, but one 

pipe also cuts Precambrian schist and gneiss. In a few 

places the breccias form knobs a few meters high (fig. 

10A) but there is little topographic difference between 

most breccias and their host rocks. Individual pipes 

generally crop out over small areas (<1/16 km2) and 

are poorly exposed.

Contacts between the breccia pipes and the country 

rocks are mostly gradational over a few tens of meters, 

but in a few places the contact zones are much narrower 

(less than 1 m). The outermost parts of the contact 

zones are characterized by slightly to moderately brecciated country rock (crackle breccias). In these outermost zones there has been no rotation of the fragments 

and there is little matrix. The amount of rotation of 

fragments and the amount of matrix increases toward 

the center of the pipes. Also, in most pipes the breccias 

are more heterolithologic towards the center.

The fragments in the pipes can be as large as 30 

cm and are typically about 8 cm in diameter (fig. WB). 

They are mainly subangular to angular and are composed of Precambrian schist and gneiss, Cambrian 

Flathead Sandstone (locally a quartzite), and several 

varieties of syenite porphyry. The relative abundance 

of rock types is highly variable and any of the just-listed 

rocks may comprise more than 95 percent of the fragments in an individual pipe. Usually one lithic type is 

predominant and makes up 75-90 percent of the breccia 

fragments. Matrix generally is sparse and is composed 

of fine-grained (<1 mm) particles of syenite and unidentified fragments.

At one locality on Gold Bug Butte and at the Hawkeye Mine (fig. 9), discontinuous breccia dikes about 2 

m wide cut the syenite porphyry. The relationship between the breccia pipes and breccia dikes is uncertain; 

the dikes may be either offshoots or feeders for the 

pipes, but there is no field evidence to establish these 

relationships. Fragments make up about 20-30 percent
14 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

Geology by E. N. Goddard and S.R. Wallace, 

1940, 1941, 1948, 1949, 1951, and 1952

2 KILOMETERS

1 MILE

CONTOUR INTERVAL 250 FEET

FIGURE 8 (above and facing page). Geologic map showing intrusive breccia of Judith Peak-Red Mountain area, Judith Mountains, Mont.

of the breccia dikes and are composed mainly of porphyritic syenite and sparse to rare Precambrian schist and 

gneiss. Most of the fragments are about 5 cm in diameter but may be as large as 40 cm, are subrounded to 

angular, and in the breccia dike at Gold Bug Butte, 

have well-developed reaction rims (fig. 10C). The matrix of the breccia dikes is composed mostly of fine (<1

mm) fragmental grains of quartz and potassium feldspar 

from the wall rock as well as small amounts of sericite, 

limonite, magnetite, and pyrite. Some of the pyrite also 

occurs as larger (1-2 mm) euhedral grains. Rock fragments 1-2 mm in size are also a common constituent 

in the matrix. Secondary quartz and limonite are the 

major cements.
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 15

EXPLANATION

Intrusive granite breccia (Paleocene)

Alkali granite porphyry (Paleocene) 

Coarse-grained phase; Tgd, dikes

Tinguaite and tinguaite porphyry 

(Paleocene?) ltd, dikes

Intrusive breccia (Paleocene?)

Intrusive rhyolite (Paleocene or 

Upper Cretaceous)

Porphyry (Paleocene and Upper

Cretaceous) Quartz monzonite; 

monzonite, quartz diorite, diorite 

and syenite porphyry

Undivided sedimentary rocks (Cretaceous 

and Jurassic) Cretaceous Colorado 

Shale and Kootenai Formation 

Jurassic Morrison Formation and 

Ellis Group

Undivided sedimentary rocks (Mississippian) 

Mississippian Big Snowy and Madison 

Groups

Contact

Fault

 Sample locality 

>- Adit

Breccia fragments and adjacent wall rocks (especially 

the syenite porphyry) on Gold Bug Butte have been 

altered to varying degrees and contain abundant wellcrystallized sericite, quartz, pyrite of hypogene origin, 

and jarosite and limonite of probable supergene origin. 

These minerals replace feldspar and ferromagnesian 

minerals, fill small cracks, and are disseminated in the 

groundmass of syenite porphyry fragments and in wall 

rock. Secondary quartz is abundant locally and occurs 

as fracture fillings and cavity fillings. Secondary potassium feldspar was observed in a few samples. Kaolinite 

is present locally in moderate amounts, but is confined 

mainly to breccia fragments of feldspathic schist and 

gneiss. Irregular patches of limonite (in places as 

pseudomorphs after pyrite) and jarosite are more abundant than pyrite; pyrite occurs as euhedral grains and 

as interstitial fillings. The matrix of the breccias contains small fragments of wall rock, shredded masses 

of sericite, and rounded limonite grains (after pyrite?) 

that probably predate transport of fragments to their 

present position. The matrix is traversed by fissures

filled with limonite, jarosite, quartz, and rarely pyrite, 

that are clearly post-breccia. All the breccias have been 

mineralized and contain anomalous amounts of copper 

(<700 ppm), gold (<5 ppm), molybdenum (<700 ppm), 

lead (<3,000 ppm), and silver (<300 ppm). However, 

in hand specimens pyrite is the only sulfide mineral observed.

COMPARISON OF ALTERATION AND 

GEOCHEMICAL FEATURES

The type and degree of alteration of the breccias can 

be related to a single sequence (table 1), although 

whether this sequence represents the effects of duration or intensity of alteration, thermal gradients, or 

variation of the concentrations of chemical species in 

the altering fluids is unclear. In general, the alteration 

sequence in breccias and their adjoining host rocks begins with fresh or propylitized rocks and proceeds 

through the sequence kaolinite, quartz-illite or quartzsericite, and quartz-potassium feldspar. Generally, only 

two members of this sequence can be observed in any 

one breccia pipe or cluster of pipes. Thus, kaolinite and 

quartz-illite-sericite assemblages were observed at 

Plum Creek and Limekiln Canyon, and quartz-sericite 

and quartz-potassium feldspar were observed at Gold 

Bug Butte and the Judith Peak-Red Mountain area. 

Most of the alteration appears to have been incomplete; 

although ferromagnesian minerals were altered completely in most breccias, potassium feldspar that formed 

during crystallization of the host porphyries was altered 

only partly in most rocks studied. Pyrite, commonly 

indicated only by its oxidation products limonite and 

jarosite, probably was introduced during quartz-sericite 

alteration. The alteration sequence kaolinite, quartzsericite, and quartz-potassium feldspar differs from that 

of illite (sericite?), kaolinite, and quartz observed in 

breccia pipes of the San Juan Mountains of Colorado; 

the latter sequence has been interpreted as the product 

of hydrogen ion metasomatism (Fisher and Leedy, 

1973). The quartz-sericite alteration assemblage observed in the breccias of Gold Bug Butte also is found 

along gold telluride-bearing fissure fillings in porphyry 

of the Little Rocky Mountains (Bailey, 1974), where 

it is reminiscent of sericite alteration found in wall rocks 

of the gold telluride-bearing veins of Boulder County, 

Cob. (Kelly and Goddard, 1969, p. 25).

Alteration of the breccias began during formation of 

crackle zones, as seen by intense alteration of these 

breccias and of fragments but not matrix in some breccias with rotated and transported clasts; alteration continued after emplacement of breccias, but varied greatly in intensity. Alteration during the crackle stage of 

brecciation clearly is evident at Gold Bug Butte; minor
16 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

108°37 108°36' :i)8°3S'

Geology modified from Knechtel 119591

2 KILOMETERS

J

CONTOUR INTERVAL 200 FEET

0

EXPLANATION

Intrusive breccia (Paleocene?)

Syenite porphyry (Paleocene)

Includes syenite porphyry of Antone, alaskite 

porphyry of Silvertip mine and quartz syenite 

porphyry of Old Scraggy (Brockunier, 1936) and 

several types of syenite porphyry with trachytic 

texture that occur as dikes and small plugs 

(Brockunier, 1936)

Kootenai Formation (Lower Cretaceous) 

Madison Group (Mississippian)

Undivided sedimentary rocks (Mississippian, 

Devonian, Ordovician, and Cambrian) Mississippian and Devonian Three Forks Formation, 

Devonian Jefferson and Maywood Formations, 

Ordovician Bighorn Dolomite, Ordovician and 

Cambrian Emerson Formation, and Cambrian 

Flathead Sandstone

1 MILE

Schist and gneiss (Precambrian)

Dike of intrusive breccia

 Contact

1 Fault, ball and bar on downthrown side

 Sample locality

 Adit

FIGURE 9. Geologic map showing intrusive breccias of the Little Rocky Mountains, Mont.
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 17

FIGURE 10. Photographs of breccias at Gold Bug Butte, Little Rocky 

Mountains, Mont. A, West end of southernmost breccia pipe, cutting syenite porphyry on Gold Bug Butte. This is one of the few 

places in the Little Rocky Mountains where the breccia pipes have 

significant topographic expression. B, Detail of breccia showing 

abundant fragments of syenite porphyries. C, Detail of breccia dike 

showing reaction rims around fragments.

alteration, including filling of fissures with quartz, limonite, and jarosite, followed emplacement of these breccias. Alteration and mineralization of fragments in the 

South Moccasin and Republic breccias took place almost 

entirely before emplacement. Major alteration at Plum 

Creek began before emplacement of breccia with abundant matrix and continued after emplacement. At the 

Judith Peak-Red Mountain area, post-breccia alteration 

was intense, depositing quartz, potassium feldspar, and 

pyrite in the breccia matrix. Such post-breccia alteration may have accompanied intrusion of alkali granite 

on Judith Peak.

Geochemical anomalies in the breccias are closely related to the type and degree of alteration (table 1); 

these anomalies were interpreted from study of histograms for each element analyzed (fig. 11, at end of this 

chapter). Breccias with weak kaolmitic alteration, as 

in the South Moccasin Mountains and at Limekiln Canyon in the Judith Mountains, do not have significant 

geochemical anomalies. Breccias characterized by 

quartz-sericite and quartz-potassium feldspar alteration, seen at Plum Creek in the North Moccasin Mountains, Judith Peak-Red Mountain in the Judith 

Mountains, and at Gold Bug Butte in the Little Rocky 

Mountains, show significant depletion of readily leached 

alkalis and alkaline earth elements except potassium, 

and they show significant enrichment of potassium and 

of base and precious metals.

Although the chemical composition of the host rock 

may vary among areas somewhat, such differences may 

be largely ignored in comparing the effects of alteration 

and mineralization because these processes have 

brought major changes in the chemical composition of 

the altered rocks. Using the relatively weakly altered, 

unmineralized breccias at Limekiln Canyon as a basis 

for comparison, the parallel effects of alteration and 

mineralization can be described. The average content 

of iron and titanium does not appear to have changed 

much with weak kaolinitic alteration, but the two metals have been redistributed extensively in more highly 

altered quartz-potassium feldspar and quartz-sericite 

breccias, as indicated by the wide variation in iron and 

titanium content at Judith Peak-Red Mountain and Gold 

Bug Butte as compared with Limekiln Canyon. Manganese has been depleted in highly altered breccias and 

also, to a lesser extent, in the quartz-sericite altered 

zone at Plum Creek. Calcium, magnesium, and sodium 

are most depleted in the highly altered breccias of 

Judith Peak-Red Mountain and depleted to a lesser extent in breccias of the Little Rocky Mountains. Potassium is enriched at Judith Peak-Red Mountain and at 

Gold Bug Butte. Barium is enriched at Judith Peak-Red 

Mountain but nowhere else; it may be present as barite 

reported in calcite-fluorite-quartz veins by Hall (1976).
18 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

TABLE 1. Summary of alteration and geochemical features of breccias in north-central Montana

[Leaders ( ) indicate no change in chemical composition; dominant alteration assemblage is underlined]

Locality of South Limekiln 

mineralized Moccasin Canyon 

breccia Mountains

Alteration Kaolinite Kaolinite,

assemblage quartz-sericite,

and illite

alteration

Mn 

Au, Ag, V 

Cu, Mo, Pb 

Co, Cr, Ni 

Plum 

Creek

Kaolinite,

quartz-sericite

and illite

Slightly enriched 

Gold Bug 

Butte

Quartz-sericite,

quartz-potassium

feldspar

Strongly enriched 

Enr i ched 

 do 

Judith PeakRed Mountain

Quartz-sericite ,

quartz-potassium

feldspar

Do. 

Do.

Do.

Gold, silver, and vanadium were enriched moderately 

at Plum Creek and Judith Peak-Red Mountain, but 

strongly enriched in breccias at Gold Bug Butte; copper, molybdenum, and lead appear to have been enriched about equally at these three localities. Anomalous concentrations of beryllium, cobalt, chromium, and 

nickel are found only in the Little Rocky Mountains; 

some of these anomalies may represent differences in 

mineralization or merely the presence of abundant fragments of Precambrian schist and gneiss of mafic ancestry in some of the breccias there. The concentration 

of most other elements does not vary appreciably 

among breccias having differing degrees of alteration 

(fig. IDSome elements, notably iron, magnesium, calcium, 

sodium, and manganese, have distinct bimodal or 

polymodal frequency distributions that probably represent sampling of both slightly altered and highly altered 

breccia; for example, the most altered rock of the Judith 

Peak-Red Mountain breccia pipes contains 0.5-0.7 percent sodium, but some less-altered parts of the breccia 

complex still retain as much as 3 percent sodium. The 

frequency distributions of some metals, such as chromium in the breccias of Gold Bug Butte, show many 

modes distributed in every other class of abundance; 

this may represent an analytical problem. Other 

polymodal distributions, such as those of silver and 

gold, may represent multiple pulses of mineralizing 

fluids or other complexities in the process of mineralization.

DISCUSSION AND CONCLUSIONS

The breccia pipes of north-central Montana evidently 

were formed by the activity of residual vapor and partially solidified magma that collected in the upper regions of cooling intrusive bodies. Breccias in the North 

and South Moccasin Mountains, and at Limekiln Canyon 

in the Judith Mountains, are centrally located within 

host porphyries, and thus are below the projected apex 

of the porphyries; those of the Judith Peak-Red Mountain area and at Gold Bug Butte and vicinity in the 

Little Rocky Mountains are located in large intrusive 

complexes where the apex of intrusion is not evident. 

At Judith Peak-Red Mountain, breccias were formed 

before intrusion of late granitic magmas into the porphyry complex, but the source of the breccias has not 

been determined. All the breccias of north-central Montana contain abundant fragments of the host syenite, 

quartz monzonite, and related porphyries, and contain 

lesser quantities of sedimentary rocks of Paleozoic and 

Mesozoic age and of Precambrian schist and gneiss. The 

host porphyries were evidently the principal site of 

breccia formation. No fragments of alkalic ultramafic 

rocks, or peridotites from the upper mantle, such as 

those found in the Eocene diatremes of north-central 

Montana (Hearn, 1968, 1979), were found in the breccias associated with Upper Cretaceous-Paleocene porphyry intrusions.

The depth of breccia formation, and emplacement of 

the host porphyry, can be estimated directly from the 

thicknesses of horizontal sedimentary strata (assuming
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 19

no thinning above the intrusions) that overlay each intrusive complex during early Paleocene time; this 

method is simple and yields independent estimates in 

good agreement with each other. Estimates of depth 

are as follows: North and South Moccasin Mountains, 

1,700-2,300 m; Limekiln Canyon, Judith Mountains, 

1,600-2,700 m; Judith Peak-Red Mountain, Judith 

Mountains, 1,100-1,900 m (see also Wallace, 1956, p. 

370); and Gold Bug Butte, Little Rocky Mountains, 

1,400-2,300 m. The ranges in depth represent the lowest and highest stratigraphic levels of emplacement of 

each main mass of porphyry. Intrusive rocks in each 

complex contain sanidine phenocrysts and an aphanitic 

groundmass, and country rocks show little evidence of 

contact metamorphism; all these features imply a shallow depth of emplacement and rapid cooling, and are 

thus in general agreement with the estimates of depth 

of porphyry emplacement and brecciation.

Most of the breccias of north-central Montana have 

characteristics attributed by Rents (1961, 1964) to be 

the result of hydraulic ramming by hydrothermal and 

magmatic fluids. After emplacement and partial cooling 

of the main mass of magma, residual fluids that collected in the apical parts of the intrusion were subjected 

to hydraulic pressure from surges of magma below. 

Under pressure, the residual fluids shattered the overlying cap of chilled intrusive porphyry and country 

rocks over areas of several hundred to a thousand 

meters, forming rupture breccias (also known as crackle 

breccias) marked by a myriad of fissures and unrotated 

angular fragments. Part of the breccia at Plum Creek 

and the margins of breccias at Gold Bug Butte are 

crackle breccias. The fragments were altered and 

mineralized by hydrothermal fluids, and the fissures 

commonly were filled with quartz. Withdrawal of 

magma from below may have caused collapse of the 

crackle zone, forming a breccia of rotated angular fragments derived from overlying rocks and containing 

abundant open spaces; but this seems not to have occurred to a noticeable extent in the Montana breccias. 

Continued or repeated surges of magma from below 

also could have caused the crackle breccia to disintegrate into a breccia of rotated and transported fragments 

surrounded by a matrix of fine-grained fragments. 

Eventually, these processes led to formation of well-defined pipes of intrusive breccia dominated by matrix 

and breccia containing well-rounded fragments 

(kneaded and milled breccias of Rents, 1964). Breccias 

composed of small fragments and dominated by matrix 

are well developed in the South Moccasin Mountains, 

at Plum Creek, Limekiln Canyon, and at Judith Peak. 

The Republic breccia contains rounded clasts and 

clearly extends above its source; it must have been

forced up through the host shale formation. In a few 

places the breccia columns were then intruded by igneous dikes (Judith Peak-Red Mountain area) and by breccia dikes (Gold Bug Butte area).

Considering stratigraphic evidence for shallow 

emplacement of the breccias, some of the breccias may 

have broken through the surface. Possible detritus from 

such an eruption in the Little Rocky Mountains has 

been found in the lower part of the Fort Union Formation (Marvin and others, 1980, p. 21-22). Evacuation 

of the pipe and concomitant release of pressure might 

have been followed by withdrawal of breccia material 

into the resulting partial vacuum; such up and down 

movement would further aid production of breccia and 

matrix and would account for the occurrence together 

of fragments from stratigraphically higher and lower 

levels.

Other mechanisms for breccia emplacement were considered but some, such as postmagmatic hydrothermal 

solution and collapse (Sillitoe and Sawkins, 1971), and 

collapse by the exsolved magma vapor mechanism (Norton and Cathles, 1973), seem to apply best where there 

is evidence for substantial downward movement of 

breccia fragments, and where the breccias lack a fine 

matrix. The widespread presence of fragments of Precambrian schist and gneiss, which must have been derived from below the porphyry intrusions, is strong evidence that the breccias of north-central Montana were 

not formed entirely by collapse. Most of the Montana 

breccias have abundant fine matrix and little open space 

between fragments. The possibility that the breccias 

were emplaced into voids formed by faulting (Mitcham, 

1974) was dismissed also, because most of the breccias 

are not near faults.

Hydrothermal fluids permeated the breccias during 

and after their formation, altering and mineralizing 

them to varying degrees. In general, alteration was incomplete and followed the sequence kaolinite, quartz-illite and quartz-sericite, and quartz-potassium feldspar. 

Raolinitic alteration was extremely weak. During alteration, iron and titanium were redistributed; calcium, 

magnesium, sodium, and manganese were leached; and 

potassium and base and precious metals were concentrated. The highest concentrations of gold and silver 

were deposited in areas of most intense quartz-sericite 

alteration. Feldspathic alteration does not seem to have 

been accompanied by continued deposition of gold and 

silver, but quartz-sericite alteration and gold-silver 

deposition may have occurred on the fringes of 

feldspathic alteration. Using the model of alteration and 

mineralization developed here, the breccias most favorable for gold and silver, in decreasing order of favorableness, are (1) Gold Bug Butte and vicinity, Little
20 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

Rocky Mountains, (2) Plum Creek, North Moccasin 

Mountains, and (3) the fringes of the Judith Peak-Red 

Mountain altered area, Judith Mountains.

We believe that alteration and gold-silver mineralization accompanied closely the formation of breccia pipes 

during final stages of cooling of intrusive porphyry. 

This was true undoubtedly at Judith Peak-Red Mountain, where less-altered dikes cut altered breccia. If alteration and igneous activity were contemporaneous or 

nearly so, then alteration and mineralization must have 

taken place at depths comparable to the 1,100-2,700 

m depth range estimated for the cooling porphyry.

The occurrence of mineralized (copper, lead, and zinc) 

fragments in breccias that otherwise show only weak 

effects of alteration and mineralization, as in the South 

Moccasin Mountains and at the Republic claim nearby, 

indicate that base-metal mineralization took place prior 

to formation of some of the breccias. Mineralized (chalcopyrite-pyrite) inclusions have been found in quartz 

monzonite porphyry in the South Moccasin Mountains 

(Lindsey, 1982) and in unmineralized porphyry south 

of Judith Peak in the Judith Mountains (Marvin and 

others, 1980). These inclusions were derived from deposits of metal sulfides that formed prior to or during 

emplacement of the porphyry complexes. The significance of these mineralized inclusions, in both breccia 

and porphyry, is that base-metal sulfide mineralization 

began early during Late Cretaceous-Paleocene intrusive activity and continued after the formation of breccias, as indicated by widespread anomalies of copper, 

lead, and molybdenum in the breccias and altered wall 

rocks. Gold and silver mineralization, and accompanying 

quartz-sericite alteration of intrusive breccia, appears 

to have been confined more closely to the time of breccia formation.

REFERENCES CITED

Bailey, R. L., 1974, Geology and ore deposits of the Alder Gulch 

area, Little Rocky Mountains, Montana: Bozeman, Mont., Montana State University M.S. thesis, 81 p.

Brockunier, S. R., 1936, Geology of the Little Rocky Mountains, Montana: New Haven, Conn., Yale University Ph. D. thesis, 

130 p.

Dobbin, C. E., and Erdmann, C. E., 1955, Structure contour map 

of the Montana Plains: U.S. Geological Survey Oil and Gas Investigations Map OM-178B.

Dyson, J. L., 1939, Ruby Gulch gold mining district, Little Rocky 

Mountains, Montana: Economic Geology, v. 34, no. 2, p. 201-213.

Fisher, F. S., and Leedy, W. P., 1973, Geochemical characteristics 

of mineralized breccia pipes in the Red Mountain district, San 

Juan Mountains, Colorado: U.S. Geological Survey Bulletin 1381, 

43 p.

Forrest, R. A., 1971, Geology and mineral deposits of the Warm 

Springs-Giltedge district, Fergus County, Montana: Butte, 

Mont., Montana College of Mineral Science and Technology M.S. 

thesis, 191 p.

Goddard, E. N., 1950, Structure of the Judith Mountains, Montana

tabs.]: Geological Society of America Bulletin, v. 61, no. 12, pt. 

2, p. 1464.

Hall, R. J., 1976, Petrology of diamond drill core from Judith PeakRed Mountain area, Fergus County, Montana: Cheney, Wash., 

Eastern Washington State College M.S. thesis, 38 p.

Hearn, B. C., Jr., 1968, Diatremes with kimberlitic affinities in northcentral Montana: Science, v. 159, no. 3815, p. 622-625.

 1979, Preliminary map of diatremes and alkalic ultramafic intrusions in the Missouri River Breaks and vicinity, north-central 

Montana: U.S. Geological Survey Open-File Report 79-1128.

Hurlbut, C. S, Jr., and Griggs, D., 1939, Igneous rocks of the Highwood Mountains, Montana Pt. I, The laccoliths: Geological Society of America Bulletin, v. 50, no. 7, p. 1043-1112.

Kelly, W. C., and Goddard, E. N., 1969, Telluride ores of Boulder 

County, Colorado: Geological Society of America Memoir 109, 

237 p.

Rents, Paul, 1961, Brief outline of a possible origin of copper porphyry breccias: Economic Geology, v. 56, no. 8, p. 1465-1471.

 1964, Special breccias associated with hydrothermal developments in the Andes: Economic Geology, v. 59, no. 8, p. 1551- 

1563.

Knechtel, M. M., 1959, Stratigraphy of the Little Rocky Montains 

and encircling foothills, Montana: U.S. Geological Survey Bulletin 

1072-N, p. 723-752.

Larsen, E. S., Jr., 1941, Igneous rocks of the Highwood Mountains, 

Montana Pt. 2, The extrusive rocks: Geological Society of 

America Bulletin, v. 52, no. 11, p. 1733-1752.

Lindsey, D. A., 1982, Geologic map and discussion of selected mineral 

resources of the North and South Moccasin Mountains, Fergus 

County, Montana: U.S. Geological Survey Miscellaneous Investigations Map 1-1362.

Marvin, R. F., Hearn, B. C., Jr., Mehnert, H. H., Naeser, C. W., 

Zartman, R. E., and Lindsey, D. A., 1980, Late Cretaceous-Paleocene-Eocene igneous activity in north-central Montana: Isochron/West, no. 29, p. 5-25.

Marvin, R. F., Witkind, I. J., Reefer, W. R., and Mehnert, H. H., 

1973, Radiometric ages of intrusive rocks in the Little Belt Mountains, Montana: Geological Society of America Bulletin, v. 84, 

no. 6, p. 1977-1986.

Miller, R. N., 1959, Geology of the South Moccasin Mountains, Fergus County, Montana: Montana Bureau of Mines and Geology 

Memoir 37, 44 p.

Mitcham, T. W., 1974, Origin of breccia pipes: Economic Geology, 

v. 69, no. 3, p. 412-413.

Norton, D. L., and Cathles, L. M., 1973, Breccia pipes Products 

of exsolved vapor from magmas: Economic Geology, v. 68, no. 

4, p. 540-^546.

Sillitoe, R. H., and Sawkins, F. J., 1971, Geologic, mineralogic, and 

fluid-inclusion studies relating to the origin of copper-bearing 

tourmaline breccia pipes, Chile: Economic Geology, v. 66, no. 

7, p. 1028-1041.

Wallace, S. R., 1953, The petrology of the Judith Mountains, Fergus 

County, Montana: U.S. Geological Survey Open-file report, 

189 p.

 1956, Petrogenetic significance of some feldspars from the 

Judith Mountains, Montana: Journal of Geology, v. 64, no. 4, 

p. 369-384.

Weed, W. H., and Pirsson, L. V., 1896, The geology of the Little 

Rocky Mountains: Journal of Geology, v. 4, no. 4, p. 399-428.

 1898, Geology and mineral resources of the Judith Mountains

of Montana: U.S. Geological Survey 18th Annual Report, pt. 3,

p. 437-616. 

Witkind, I. J., 1973, Igneous rocks and related mineral deposits of

the Barker quadrangle, Little Belt Mountains, Montana: U.S.

Geological Survey Professional Paper 752, 58 p.
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 21

FIGURE 11. Histograms showing the frequency distributions of elements in 164 samples of breccias and wall rocks in Upper Cretaceous 

and Paleocene intrusive porphyry complexes of north-central Montana. Separate histograms are shown for breccias at Plum Creek 

in the North Moccasin Mountains, Limekiln Canyon in the Judith Mountains, the Judith Peak-Red Mountain area in the Judith Mountains, 

and Gold Bug Butte and vicinity in the Little Rocky Mountains. Gold analyzed by fire assay-atomic absorption spectrometry; all other 

elements analyzed by six-step semiquantitative spectrographic method. N, not found.
22 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

IRON MAGNESIUM

60-|

40-

20-

20-,

LOCALITY AND NUMBER 

OF SAMPLES (N)

OOOOOOOOOOOOOO'-'-CNIol

10-

All Samples 

IM=164

Plum Creek 

N=32

20-i

LU 10- 

m

20-1

Limekiln Canyon 

N=20

10-

20- 

10- 

n

r

n

7m ir~i I It-1 n^i

40- 

20-

n

P^! 

p:8

i

i=3 n

 CN co in r^ in

60606006666666'- ^ CM CO

Judith PeakRed Mountain 

IM=33

Gold Bug Butte 

N=66

Fe, IN PERCENT Mg, IN PERCENT
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 23

CALCIUM

30-i

SODIUM

LOCALITY AND NUMBER 

OF SAMPLES (N)

0 _kp

All Samples 

N=164

OOOOOOOOOOOOOOOO*- ^ CN CO inr-O

OOOOOOOO.-.-cNr>Lnr--

n i i i i i i i i i i r

Plum Creek 

N=32

O

cc

uj 10-

m , r~i n

Limekiln Canyon 

N=20

20-1

10-

oJudith PeakRed Mountain 

N=33

25 - 

20-

n

popopppppO'-'-tNC'JLnr-- in 

0000000000000000"-^-

Ca,IN PERCENT Na, IN PERCENT

Gold Bug Butte 

N=66
24 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

POTASSIUM TITANIUM

70- 

60-

I I I I I CM co in r^ o

LOCALITY AND NUMBER 

OF SAMPLES (N)

70- 

60-

40-

I I I I

oooooooooooooo

All Samples 

N=164

15- 

10-

15- 

10-

Plum Creek 

N=32

"-20-i 

O

20-

10-

r\ FR1

I ';

Limekiln Canyon 

N=20

20 -I

10-

Judith PeakRed Mountain 

N=33

40^

I I I I I

CM CO LT3 r^ O

K, IN PERCENT

"TT.j i

ID Is- ID

O O »- * CNCOiOr^ ID

6. dodo ddooddddd 

TUN PERCENT

Gold Bug Butte 

N=66
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 25

MANGANESE SILVER

LOCALITY AND NUMBER 

OF SAMPLES (N)

10-

I T 

r~ 

V

r- eg co in F^ o

25

20-rsr

10-

El ^

niiiinir

v- .- CN co

All Samples 

N=164

Plum Creek 

N=32

\ \ TT ii i i i i i i i i i i in i i r w i i i i i i i i i i i i i i r r i i r

IP

IIP i i i i i T r i i i i i i i i i i i

Limekiln Canyon 

N=20

Judith PeakRed Mountain 

N=33

10-

I I I I I I T

eg co in i~» O

Mn, IN PARTS PER MILLION

\\\ i i i i i i i \ i i r i

in in i^ in

O O o *~~ « csjcoinr^OLnoooooooO

w » ^-CNcr)Lnr^OinoO v - r- CM m

Ag, IN PARTS PER MILLION

Gold Bug Butte 

N=66
26 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

BARIUM BERYLLIUM LOCALITY AND NUMBER

OF SAMPLES (N)

4O20-

0

i Ti i i iTi i i IT

OOLOOOOOOOOOO 

T- CN roirir-OLnooo

\y « *- rsj ro uD

r

o

0

-

o

0

o

1

o

0

T

1

o

0

o

rTrV i T

o o o o o o 80 o o o o

o o o o o

 - <- CM n ID r~ o

20-

,- ^ ^ CM n

10-

0 _Lii

All Samples 

IM=164

Plum Creek 

IM=32

1 1 I I I I

20-,

DC

LLI 

m

=3 0-

10-

1

I I I I I I I I I I I

Limekiln Canyon 

IM=20

20-1

Judith PeakRed Mountain 

IM=33

40-i

20-

I I I riii i i i [i iiiiiir

oomooooooooooooooooooo ^ - -CMMlDt^OtnOOOOOOOOOOOOO

Ba, IN PARTS PER MILLION

20-1

i i t i i i i i i i i

in

"-"-T-cNininr^oiDOO 

V «- - CM n

Be. IN PARTS PER MILLION

Gold Bug Butte 

IM=66
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 

COBALT CHROMIUM LOCALITY AND NUMBER 

OF SAMPLES (N)

27

Q'

i i i i i i i i i i

pjpjinr-OinOOOO

V «- T- CM pj in iv

JEL

All Samples 

IM=164

Plum Creek 

IM=32

Limekiln Canyon 

IM=20

Q_Ei i i i i i i i i i r i i i i i i

Judith PeakRed Mountain 

IM=33

r

Co, IN PARTS PER MILLION

r-i-i-tNoini^OinOOOOOOOO 

V i-T-CNcoinr-OmOO

r- «- CN P3

Cr, IN PARTS PER MILLION

Gold Bug Butte 

IM=66
28 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

COPPER GALLIUM LOCALITY AND NUMBER

OF SAMPLES (N)

20-

10-

All Samples 

N=164

Plum Creek 

N=32

DC

UJ 10-

m

20-

Limekiln Canyon 

N=20

20-i 20-1 26

Judith PeakRed Mountain 

N=33

I I I I I I I

10-

,-^r-cNcoior^oinoooooooooooo

Gold Bug Butte 

N=66

Cu, IN PARTS PER MILLION

in in o O O O O

«- - CM tn 10 i^ o

V -

Ga, IN PARTS PER MILLION
? 00- 

H 30-

* 70-^

w 100-

m 150 ~1 

3) 200 - 2 300-3 

F 500- 

O 700 - NUMBER OF SAMPLES 3 O O I I I I as H » > 

r

hHN & O bdgH Od

O

2.

f?

O) m

o 

og O O 

H

as

II H aj

H

II

CD

m 31
30 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

NIOBIUM

62

10-

O O in O

i- - ,- CM

V

NICKEL

81

LOCALITY AND NUMBER 

OF SAMPLES (N)

All Samples 

N=164

n

Plum Creek 

N=32

C/3 

UO 10-

CL 

LLJ 

00

20-i

10-

Limekiln Canyon 

N=20

I I I I I I I I I I I I I I

10-

Judith PeakRed Mountain 

IM=33

O O lf> O 

.- <- - CM

v 

Mb, IN PARTS PER MILLION

0^

CMcMcoinrvomOOOOOOO 

v .-.-cMcoinrvOLDO

 - t- CM

Ni,IN PARTS PER MILLION

Gold Bug Butte 

N=66
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE

LEAD SCANDIUM LOCALITY AND NUMBER

OF SAMPLES (NY

10-

All Samples 

IM=164

1 I i I I I i i i r F i r i IT

inmoooooooooooooo

31

20-i

ID- 10-

Plum Creek 

IM=32

20 -

10-

n

20 J

^B] ~~

 ,10- S^-l

In

w

i

m

Limekiln Canyon

IM=20

TT

20-i

I I I I

10-

Judith PeakRed Mountain 

IM=33

miDoooooooo oooooo

«- «- CM co 

Pb, IN PARTS PER MILLION Sc, IN PARTS PER MILLION

Gold Bug Butte 

IM=66
32 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA 

STRONTIUM

40-

20-

^

;3

Jt

~i

UM

58

D

:,'i

A

r

Dl

i

UM

^

LOCALITY AND NUMBER 

OF SAMPLES (N)

All Samples 

IM=164 ,

Plum Creek 

N=32

20-i

10-

Limekiln Canyon 

N=20

20-1 20-1

Judith PeakRed Mountain 

IM=33

20-1

\ i i i i i i i i i i i i i i i n i r

inini^omooooooooooooooo V T-«-cMnir)i^

o "i iTinTTTiTl-'rViT

oomoooooooooooo

Gold Bug Butte 

IM=66

Sr,IN PARTS PER MILLION V. IN PARTS PER MILLION
MINERALIZED BRECCIAS OF LATE CRETACEOUS AND PALEOCENE AGE 33

YTTERBIUM

237837

20-1

LOCALITY AND NUMBER 

OF SAMPLES (N)

All Samples 

IM=164

sal 10-

' Plum Creek 

IM=32

^ 20- 

o_

^

w

"- 10-

O

CC

UJ 

CO

 t r\ 

20-

Erlffr

C

 t;^: i

 I

10-

- ^ 

:fts[

m

Limekiln Canyon

N=20

EH

f&--*!

>fi

^

SM

1

I

\ \ \ I i I I f 1

10-

Judith PeakRed Mountain 

N=33

20-i

10-

T \ I I I I I I I

20-i

10-

Gold Bug Butte 

N=66

Y, IN PARTS PER MILLION Yb, IN PARTS PER MILLION
34 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

ZIRCONIUM GOLD

i ( 1 I \ 1 \\ 

oooooooo

rn

r^-,90

10 -

LOCALITY AND NUMBER 

OF SAMPLES (N)

All Samples 

IM=164

Plum Creek 

IM=32

cc

LJJ 

CO

D 0

Limekiln Canyon 

IM=20

Judith PeakRed Mountain 

IM=33

-

10-

0 

FL

:

a

g

hj

I

fl

oooooooo

I^F^OlfiOOOO V «- «- CN n in r~

Zr, IN PARTS PER MILLION

i i i i i r

OO'~'~cNroiDr^ LO

v

Au, IN PARTS PER MILLION

Gold Bug Butte 

IM=66
Relation Between Igneous Intrusion and 

Gold Mineralization in the 

North Moccasin Mountains, 

Fergus County, Montana

By DAVID A. LINDSEY and CHARLES W. NAESER

STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN 

NORTH-CENTRAL MONTANA

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1301-B
CONTENTS

	Page

Abstract ............................................ 37

Introduction and acknowledgments .............................. 37

Geology ............................................. 37

Methods of radiometric dating ................................ 37

Results and discussion .................................... 39

References cited ........................................ 41

ILLUSTRATIONS

Page

FIGURE 1. Map showing geologic setting and localities from which samples were taken for radiometric dating, North Moccasin Mountains, Fergus County, Mont. ................................................. 38

2. Graph comparing radiometric dates of fresh and hydrothermally altered syenite porphyry, North Moccasin Mountains,

Fergus County, Mont. .................................................... 40

TABLE

TABLE 1. Analytical data for new fission-track dates of syenite porphyry in the North Moccasin Mountains, Fergus County, Mont. . 40

36
STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

RELATION BETWEEN IGNEOUS INTRUSION AND

GOLD MINERALIZATION IN THE 

NORTH MOCCASIN MOUNTAINS, FERGUS COUNTY, MONTANA

By DAVID A. LINDSEY and CHARLES W. NAESER

ABSTRACT

Gold mineralization in the North Moccasin Mountains probably 

closely followed emplacement of syenite porphyry. Radial and concentric faults were formed during intrusion of a central laccolith, and 

these faults became pathways for intrusion of small apophyses of 

syenite porphyry and for hydrothermal fluids that altered and 

mineralized the apophyses and fault breccias. The age of emplacement 

of the main laccolith of syenite porphyry is 66 million years as estimated from seven concordant radiometric dates; this age is only 

slightly older than the average zircon age of 59 million years for 

three small apophyses of altered, mineralized porphyry. The zircon 

age of altered porphyry indicates either (1) the age of intrusion and 

cooling of apophyses from the main laccolith, or (2) the age of alteration and mineralization by hydrothermal fluids. Either interpretation 

is consistent with field evidence for control of mineralization by faults 

formed during intrusion of the main laccolith and hydrothermal alteration and mineralization soon after intrusion. Alteration and mineralization probably did not occur during a thermal event much later than 

intrusion of the main laccolith.

INTRODUCTION AND 

ACKNOWLEDGMENTS

The North Moccasin Mountains contain one of three 

centers of gold mineralization associated with Late Cretaceous-Paleocene intrusive activity in central Montana; 

the other centers of intrusion and mineralization are 

the Little Rocky Mountains and the Judith Mountains 

(Corry, 1933; Marvin and others, 1980; chapter A, this 

paper). Gold deposits in each area occur as disseminations in intrusive breccia, as fissure fillings in porphyry, 

in contact zones around porphyry, and in brecciated carbonate rocks. In this report, the results of radiometric 

dating, mostly by the fission-track method, are discussed in relation to the temporal and spatial relationship between intrusion and mineralization in the North 

Moccasin Mountains. We acknowledge the help of 

Ezekiel Rivera, who prepared mineral separates, and 

Patty Billings, who processed them for fission-track 

dating; both are employees of the U.S. Geological Survey.

GEOLOGY

The core of the North Moccasin Mountains is a laccolithic dome of syenite porphyry intruded into carbonate rocks below the Mississippian Madison Group (Blixt, 

1933; Lindsey, 1982) (fig. 1). Above the Madison Group, 

a major sill and many small dikes and sills of syenite 

porphyry intruded strata of Jurassic and Cretaceous 

age. Small intrusions of syenite porphyry cut the Mississippian Madison Group and connect the main laccolith 

with the upper zone of intrusions, but the massive limestone of the Madison Group acted in part as a barrier 

to intrusion. Intrusion of syenite porphyry was accomplished by doming and development of concentric 

and radial normal faults in the overlying sedimentary 

rocks. Breccia zones in faults that formed during intrusion served as pathways for intrusion of small sills and 

dikes of syenite porphyry and for ascent of hydrothermal fluids that altered country rocks and deposited 

gold. The most extensive alteration and mineralization 

occurred along a zone of fault breccia and small apophyses of syenite porphyry near the townsite of Kendall. 

That gold mineralization is described in detail in 

chapter C.

METHODS OF RADIOMETRIC DATING

The ages of intrusion and gold mineralization were 

estimated by comparing dates of hydrothermally altered (kaolinized) and mineralized syenite porphyry 

with those of fresh syenite porphyry (reported by Marvin and others, 1980). Dates of sanidine in fresh rocks 

were determined by the potassium-argon method and 

dates of accessory apatite, sphene, and zircon were determined by the fission-track method (Naeser, 1976). 

Details of dates of fresh syenite porphyry and of one 

date of altered syenite porphyry have been reported 

by Marvin and others (1980). New dates of zircon from 

three samples of altered and mineralized syenite porphyry, and of apatite from two samples of fresh syenite 

porphyry, are reported here (table 1).

37
109C30'

STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

109°2S' 1D9°28' 109°27'

MILE

1 KILOMETER 

CONTOUR INTERVAL 200 FEET

FIGURE 1 (above and facing page). Geologic setting and localities from which samples were taken for radiometric dating, North Moccasin

Mountains, Fergus County, Mont.
RELATION BETWEEN IGNEOUS INTRUSION AND GOLD MINERALIZATION 39

EXPLANATION

Intrusive breccia (Paleocene?)

Syenite porphyry (Paleocene)

Undivided sedimentary rocks (Cretaceous and 

Jurassic) Cretaceous Kootenai Formation, 

Jurassic Morrison Formation and Ellis Group

Brecciated and silicified limestone in Madison Group 

(Mississippian)

Undivided sedimentary rocks (Mississippian)

Mississippian Kibbey Formation and Madison 

Group

Undivided sedimentary rocks (Devonian and 

Cambrian) Devonian Jefferson Formation, 

Cambrian Pilgrim Limestone, Cambrian 

unassigned strata, and Cambrian Flathead 

Sandstone 

I Schist (Precambrian)

Contact

' Fault, ball and bar on downthrown side 

Sample locality for radiometric date 

Shaft

Inclined shaft 

Open pit 

Adit

RESULTS AND DISCUSSION

The age of emplacement of the central laccolith of 

syenite porphyry in the North Moccasin Mountains is 

estimated at 66 m.y. (million years), based on two previously published potassium-argon dates of 64.1 ±1.5 

m.y. and 65.8 ± 1.6 m.y. on sanidine, and five previously 

published fission-track dates ranging from 62.1 ±3.9 

m.y. to 70.9 ±6.5 m.y. (Marvin and others, 1980) (fig. 

2). All the dates are concordant within the ranges of 

overlap that may be attributed to analytical uncertainty, and the age of emplacement is well established.

An eighth radiometric date, of 39±11 m.y., determined by the fission-track method on apatite from fresh 

syenite porphyry, is strongly discordant with dates of 

the other three minerals dated from the same fresh 

syenite porphyry sample (K48, fig. 2). The date evidently reflects a local, post-emplacement thermal event. 

South of the mountains, there is evidence for postemplacement thermal activity that may have reset ages; 

warm water flows from springs south of The Park and 

large travertine deposits of Pleistocene age (Lindsey, 

1982) crop out in The Park.

Three small plutons of altered and mineralized syenite porphyry, which are probably apophyses of the 

main laccolith, yield zircon dates that are slightly 

younger than the estimated age of emplacement. The 

age of zircon from mineralized porphyry of the small 

plutons is estimated at about 59 m.y. by averaging concordant fission-track dates of 57.6±3.2, 59.4±3.6, and 

59.8±3.1 m.y. A somewhat younger date of 53.5 ±3.7 

m.y. on zircon was reported for one of the plutons (Marvin and others, 1980), but a second determination on

the same sample yielded a date of 59.8±3.1 m.y., which 

is consistent with the other zircon dates of altered porphyry (fig. 2). The 59-m.y. zircon age of altered syenite 

porphyry is slightly less than, but close to the average 

of 63 m.y. for zircon from fresh porphyry, based on 

three dates of 60.5 + 3.1 m.y. on the south sill, and 

62.1 ±3.9 and 65.5 ±3.4 m.y. on the main laccolith 

(fig. 2).

The results permit two interpretations.

1. The zircon dates of altered porphyry in the satellite 

plutons are primary and therefore represent the 

age of emplacement of apophyses from the main 

laccolith. This interpretation is compatible with 

the close agreement between the age of altered 

porphyry and the youngest zircon date (60.5 ±3.1 

m.y.) from fresh porphyry, which is from the 

large sill in the southern part of the mountains. 

If the dates are primary, thermal effects of postemplacement hydrothermal fluids were insufficient to reset zircon fission-track dates, and little 

can be concluded about the age of hydrothermal 

alteration and gold mineralization in the North 

Moccasin Mountains except that it followed 

emplacement of syenite porphyry apophyses 

about 59 m.y. ago.

2. Alternatively, the zircon dates are secondary, reset 

by the thermal effects of hydrothermal fluids that 

deposited gold in the North Moccasin Mountains. 

If the dates are secondary, they suggest that hydrothermal alteration and mineralization closely 

followed emplacement about 66 m.y. ago of the 

main laccolith of syenite porphyry and its apophyses. In order to evaluate this alternative, independent estimates of alteration temperatures are 

required; such estimates are unavailable. Search 

of thin sections for fluid inclusions that might be 

studied to estimate temperatures yielded none 

suitable for study.

Both interpretations are consistent with field evidence which suggests that alteration and mineralization 

were structurally controlled by emplacement of syenite 

porphyry. Faults that formed during intrusion of the 

main laccolith provided pathways for intrusion of small 

plutons and for movement of hydrothermal fluids that 

altered and mineralized both syenite porphyry and adjacent sedimentary rocks. Hydrothermal alteration and 

gold mineralization probably closely followed intrusion 

of syenite porphyry. This conclusion is consistent with 

the sequence of events suggested by radiometric dating 

of intrusive rocks and gold deposits in the Judith Mountains: the age of most intrusive activity in the Judith 

Mountains is estimated at about 69-60 m.y., and the 

age of gold mineralization at the Spotted Horse Mine 

in the Judith Mountains is estimated at about 59 m.y.
40 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

TABLE 1. Analytical data for new fission-track dates of syenite porphyry in the North Moccasin Mountains, Fergus County, Mont.

[Zircon dates by external detector method; apatite age by population method. Neutron dose determined by C. W. Naeser. cm2, square centimeters; m.y., million years]

Sample

No.

2K2

3K48

K216A

K217

K2A8

Rock 

description

Kaolinized

syenite

porphyry.

Fresh

syenite

porphyry.

Kaolinized

syenite

porphyry.

Fresh

syenite

porphyry.

Kaolinized

syenite

porphyry.

Sample locality Mineral Number of Induced tracks Neutron dose Age ±2 

(see also dated grains Fossil tracks in detector (neutrons/cm ) (m.y.)

fig.

SW1/4NE1/4

T. 18 N.,

1) Number Density Number Density 

counted (tracks/cm ) counted (tracks/cm )

sec. 31, Zircon 6 795 1.38xl07 408 7.08xl06 1.03xl015 59.8±3.1

R. 18 E.,

Kendall open pit.

SW1/4SW1/4

T. 18 N. ,

SE1/4NE1/4

T. 18 N. ,

prospect.

NW1/4NW1/4

T. 18 N. ,

SE1/4NW1/4

T. 18 N. ,

prospect

Horseshoe

^ 5 1S 

sec. 30, Apatite 100 641 2.97xl03 917 4.25x10 9.36xlO i:) 39±11

R. 18 E.

sec. 36, Zircon 7 965 1.49xl07 514 7.93xl06 1.03xl0 15 57.6±3.2

R. 17 E.,

sec. 31, Apatite-- 50 235 2.18xl0 15 218 2.02xl05 1.03xl0 15 66±15

R. 18 E.

sec. 29, Zircon 7 856 1.37xl07 442 7.08xl06 1.03xl0 15 59.4±3.6

R. 18 E.,

west of

mine.

Computed using D=l.551x10 /yr and F=7.03x10 /yr. n , Total decay constant for %; p , decay constant for spontaneous 

fission of ZJbU.

9 Zircon (fission-track) date from same sample, different split, reported as 53.5±3.7 m.y. (Marvin and others, 1980).

o

Discordant age; other dates on same sample are 64.1±1.5 m.y. (potassium-argon method on sanidine), 62.1±3.9 m.y. (fission-track 

method on zircon), and 67.4±4.2 m.y. (fission-track method on sphene) (Marvin and others, 1980).

Syenite porphyry

intrusion alteratio^ Sample No' A9e' mineral dated ' and method

Satellite^ 

plutons

Centra^ 

laccolith

Kaolinized-' 

Unaltered-1

( 

III 

1 153.5 ±3. 7, zircon, f.t. ** 

III 

K2 4 

(. ' '59. 8 ±3.1, zircon, f.t.*

K216A i 157. 6 ±3. 2, zircon, f.t.*

K16 i 160. 5 ±3.1, zircon, f.t.*

I 164.1 ±1.5, sanidine, K-Ar**

64 ±29, apatite, f.t.**

f ' '

K51 < 

1 165. 8 ±1.6, sanidine, K-Ar

I I I I I I I I

20 30 40 50 60 70 80 90 1C

AGE, IN MILLIONS OF YEARS

FIGURE 2. Graph comparing radiometric dates of fresh and hydrothermally altered syenite porphyry, North Moccasin Mountains, Fergus 

County, Mont. ±2a values of dates reported by Marvin and others (1980) have been recalculated according to newer statistical methods, 

f.t, dated by fission-track method; K-Ar, dated by potassium-argon method; *, date reported in this paper; **, date reported by Marvin 

and others (1980).
RELATION BETWEEN IGNEOUS INTRUSION AND GOLD MINERALIZATION 41

(Marvin and others, 1980). Late hydrothermal events, 

such as those that deposited travertine south of the 

mountains, cannot be excluded as causes of mineralization at Kendall, but there is no evidence connecting 

such events with mineralization.

REFERENCES CITED

Blixt, J. E., 1933, Geology and gold deposits of the North Moccasin 

Mountains, Fergus County, Montana: Montana Bureau of Mines 

and Geology Memoir 8, 25 p.

Corry, A. V., 1933, Some gold deposits of Broadwater, Beaverhead, 

Phillips, and Fergus Counties, Montana: Montana Bureau of 

Mines and Geology Memoir 10, 45 p.

Lindsey, D. A., 1982, Geologic map and discussion of selected mineral 

resources of the North and South Moccasin Mountains, Fergus 

County, Montana: U.S. Geological Survey Miscellaneous Investigations Map 1-1362.

Marvin, R. F., Hearn, B. C., Mehnert, H. H., Naeser, C. W., 

Zartman, R. E., and Lindsey, D. A., 1980, Late Cretaceous-Paleocene-Eocene igneous activity in north-central Montana: Isochron/West, no. 29, p. 5-25.

Naeser, C. W., 1976, Fission-track dating: U.S. Geological Survey 

Open-File Report 76-190, 68 p.

A Gold-Mineralized Breccia Zone at 

Kendall, North Moccasin Mountains, 

Fergus County, Montana

By DAVID A. LINDSEY

STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN 

NORTH-CENTRAL MONTANA

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1301-C
CONTENTS

Abstract ............................... 45

Introduction .................... 45

Acknowledgments .................. 45

Geologic setting .................. 45

Description of mineralized rocks ...............- 47

Oxidized limestone breccia ................ 47

Unoxidized limestone ................. 48

Altered syenite porphyry .................. 48

Geochemistry of oxidized limestone breccia .............. 48

Sampling methods ................-. 48

Chemical composition and variation ............... 49

Factor analysis of geochemical data ............. 51

A model for gold mineralization ................-. "4

References cited ................... ^5

ILLUSTRATIONS

FIGURE 1. Map showing geologic setting of the Kendall area ...................................... 46

2. Photograph of oxidized mineralized limestone breccia ..................................... 47

3. Sampling plan for open pits in mineralized breccia zone at Kendall ............................ 48

4. Eigenvalues and cumulative proportion of total variance explained by the first 10 principal components ....... 54

TABLES

Page

TABLE 1. Summary statistics for chemical composition of 48 samples of mineralized limestone breccia .............. 49

2. Gold values in mineralized limestone breccia ......................................... 50

3. Estimates of total variance and components of variance for major oxides and trace elements in mineralized limestone

breccia ............................................................. 51

4. Correlation coefficients between major oxides and trace elements in mineralized limestone breccia ........... 53

5. A five-factor geochemical model for mineralized limestone breccia ............................. 54

44
STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

A GOLD-MINERALIZED BRECCIA ZONE AT KENDALL, 

NORTH MOCCASIN MOUNTAINS, FERGUS COUNTY, MONTANA

By DAVID A. LINDSEY

ABSTRACT

Kendall is one of three gold-mineralized areas associated with laccolithic porphyry complexes in north-central Montana. Most gold production (about 450,000 ounces) at Kendall was from limestone fault 

breccia at the top of the Mississippian Madison Group. This breccia 

was formed by doming and faulting of sedimentary rocks during 

emplacement of a syenite porphyry laccolith about 66 million years 

ago. Syenite porphyry magma and hydrothermal fluids from the laccolith invaded the fault zone; hydrothermal fluids leached syenite porphyry, formed silicified limestone fault breccia, and deposited disseminated gold and metal sulfides in both. Low gold contents are widespread in the breccia zone.

The gold-mineralized breccia zone at Kendall contains anomalous 

arsenic, mercury, and antimony, a geochemical association present 

in the disseminated gold deposits of northern Nevada. Statistical analysis of geochemical data on oxidized mineralized limestone breccia 

indicates that mineralizing fluids introduced three separate suites of 

elements: (1) gold, mercury, vanadium, and antimony; (2) arsenic, 

iron, zinc, and antimony, probably as sulfides and accompanied by 

Si02; and (3) copper and silver in trace amounts. Gold occurs separately from sulfides and silica, so that the latter two, although conspicuous in outcrop, are not specific indicators of gold ore. Gold 

mineralization, subsidization, and silicification by hydrothermal fluids 

were followed by oxidation of sulfides and concentration of trace lead 

and manganese, probably during weathering of the mineralized brecINTRODUCTION

About 450,000 oz of gold was produced near Kendall 

in the North Moccasin Mountains between 1900 and 

1923 (Blixt, 1933; Robertson, 1950). Kendall was one 

of three gold-producing areas associated with laccolithic 

igneous complexes in north-central Montana; other 

areas were in the Judith and Little Rocky Mountains 

(Corry, 1933). The gold-mineralized area at Kendall was 

studied in detail, with the ultimate goal of developing 

a model for use in mineral exploration and assessment 

in north-central Montana. Details of the geologic setting 

of the Kendall area have been set forth in a map and 

report (Lindsey, 1982) and the relationship between 

igneous activity and gold mineralization has been discussed in chapter B of this paper. This last chapter 

(C) presents a statistical analysis of the geochemistry 

of the mineralized zone at Kendall, and integrates these

data with information on the geologic setting and associated igneous rocks to provide a model of gold 

mineralization in country rocks of the laccolithic intrusive complexes of north-central Montana.

ACKNOWLEDGMENTS

C. N. Culver assisted with field mapping of the North 

Moccasin Mountains in 1977; Ezekiel Rivera prepared 

X-ray diffractograms and mineral separates; R. A. 

Schaefer assisted in statistical analysis; B. F. Leonard 

provided advice and help in identifying minerals in 

polished sections; and R. P. Christian provided electron 

microprobe analyses. Other members of the U.S. 

Geological Survey performed chemical anaylyses; they 

are acknowledged in table 1. James Webb and A. W. 

Tipton of GeoSurveys Inc. provided information about 

the Kendall area and samples from the Abbey mine.

GEOLOGIC SETTING

The gold-mineralized zone at Kendall was formed by 

the intrusive-hydrothermal event that uplifted the 

North Moccasin Mountains. Uplift began with emplacement of a laccolith of syenite porphyry about 66 m.y. 

ago (Lindsey, 1982; Marvin and others, 1980). Intrusion 

of the laccolith was accompanied by emplacement of intrusive breccias, by formation of concentric and radial 

normal faults in overlying sedimentary rocks, and by 

intrusion of small plutons along some of these faults. 

Intrusive breccias in the central part of the laccolith 

were mineralized by gold-bearing hydrothermal fluids 

as discussed in chapter A. The mineralized zone at Kendall townsite formed by faulting and intrusion of small 

dikes and sills along the east side of the range in response to intrusion of the central laccolith. The fault 

breccias at Kendall were mineralized by gold-bearing 

hydrothermal fluids, probably soon after intrusion of 

the central laccolith, as discussed in chapter B.

45
46

109°30'

STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

109°29' 1QQ09R- 109°27'

1 MILE 

J

1 KILOMETER 

CONTOUR INTERVAL 200 FEET

FIGURE 1. Map showing geologic setting of the Kendall area, North Moccasin Mountains, Fergus County, Mont., and location of

open pits sampled for this study.
A GOLD-MINERALIZED BRECCIA ZONE AT KENDALL 47

EXPLANATION

Intrusive breccia (Paleocene?)

Syenite porphyry (Paleocene)

Undivided sedimentary rocks (Cretaceous and 

Jurassic) Cretaceous Kootenai Formation, 

Jurassic Morrison Formation and Ellis Group

Brecciated and silicified limestone in Madison Group 

(Mississippian)

Undivided sedimentary rocks (Mississippian)

Mississippian Kibbey Formation and Madison 

Group

Undivided sedimentary rocks (Devonian and 

Cambrian) Devonian Jefferson Formation, 

Cambrian Pilgrim Limestone, Cambrian 

unassigned strata, and Cambrian Flathead 

Sandstone

Schist (Precambrian)

Contact

Fault, ball and bar on downthrown side

Shaft

Inclined shaft

Open pit

Adit

The mineralized zone at Kendall extends as much as 

3.5 km along a normal fault that is concentric to the 

syenite porphyry laccolith and mostly parallel to bedding in adjacent sedimentary rocks (fig. 1) (Lindsey, 

1982). Surface exposures of the mineralized zone are 

oxidized and consist mainly of silicified, rusty-weathering limestone breccia in the uppermost part of the Madison Group and of minor rusty-weathering kaolinized 

porphyry. Black, unoxidized limestone ore is reported

FIGURE 2. Photograph of oxidized, mineralized limestone breccia, 

showing fragments of silicified limestone and coarsely crystalline 

calcite in a matrix of calcite and quartz.

at depth (Blixt, 1933, p. 21). Blixt (1933) believed that 

the mineralized zone was controlled by bedding-plane 

faults and by favorable beds in the uppermost part of 

the Madison Group; new mapping (Lindsey, 1982) 

showed that the zone is fault-controlled at the surface, 

although the dip of the fault and its relation to underground workings are unknown. Underground workings 

at Kendall are reported to follow argillaceous and carbonaceous limestone in the uppermost Madison Group 

(Blixt, 1933); however, such beds crop out at the surface only in the lower part of the Jurassic Piper Formation, and these are mineralized in the Abbey mine. The 

structure and stratigraphy of the subsurface need to 

be re-examined before these inconsistencies can be resolved, but a reasonable supposition is that the 

mineralized zone follows fault breccia and extends into 

adjacent favorable strata such as carbonaceous limestone in the lower part of the Piper Formation.

DESCRIPTION OF 

MINERALIZED ROCKS

OXIDIZED LIMESTONE BRECCIA

Silicified rusty limestone breccia is the most abundant 

rock in the mineralized zone (fig. 2). It consists mostly 

of angular fragments of limestone from the Madison 

Group, fragments of coarsely crystalline calcite derived 

from fracture fillings, and, locally, fragments of sandstone from the Mississippian Kibbey Formation all are 

set in a cement of mostly calcite and quartz. Limestone 

fragments in the breccia are variably silicified and some 

contain limonite, magnetite, and traces of pyrite. The 

breccia cement varies from clear, coarsely crystalline 

calcite to a mixture of calcite and quartz (both microcrystalline and chalcedonic forms) and generally contains traces of dolomite, fluorite, barite, and clay, as 

shown by X-ray diffraction and thin-section study. 

Some quartz cement is quite late in the paragenetic 

sequence because it fills cavities and appears to replace 

calcite; other quartz is confined only to silicified limestone fragments and appears to have been deposited 

prior to brecciation. Likewise, coarsely crystalline calcite appears as both cement and fragments, indicating 

that calcite cement was deposited in open spaces both 

before and after brecciation. Limonite; a few partially 

oxidized remnants of pyrite, traces of goethite after pyrite, and traces of sphalerite all identified in polished 

sections are found mainly in fractures, interstices of 

the cement, and along the margins of clasts; no gold 

was observed in polished sections. Qualitative electron 

microprobe analyses showed the pyrite to be nearly 

pure, and the sphalerite to contain abundant iron and 

arsenic. Accessory heavy minerals, concentrated from 

breccia containing about 10-11 ppm gold, proved to be
48 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

mainly limonite, fluorite, and barite; gold was not visible in the mineral concentrates, but gold was detected 

in a fluorite-rich concentrate by energy dispersive Xray spectrometry.

UNOXIDIZED LIMESTONE

Unoxidized rock from the lower part of the breccia 

zone was not available for study, but unoxidized goldbearing limestone from the Abbey mine, located north 

of the mineralized zone, was examined as a possible 

analog to the unoxidized part of the breccia zone. Unoxidized gold-bearing rock from the Abbey consists of 

thin-bedded black, microcrystalline limestone from the 

lower part of the Piper Formation; three samples contained 0.32-1.43 ppm gold and almost no other ore metals (Lindsey, 1982, table 3). Study of polished thin sections revealed finely crystalline calcite, scattered quartz 

silt, minor dispersed fluorite, and fine-grained (less 

than 10 microns across) interstitial pyrite having both 

euhedral and ragged forms; in one specimen, very fine 

grained pyrite crystals less than 1 micron across and 

organic carbon outline oval, calcite-filled voids and form 

flattened, discontinuous laminae; these features may be 

relict shell fragments. No gold was seen. Microprobe 

analysis of interstitial pyrite shows most to be nearly 

pure; as much as 4 percent arsenic was detected in a 

few grains. No metals except iron were detected by 

microprobe analysis of very fine pyrite in cavities.

ALTERED SYENITE PORPHYRY

Hydrothermally altered syenite porphyry occurs as 

small irregular sills and dikes in and near the fault zone 

of limestone breccia at Kendall; these are believed to 

be apophyses of the main laccolith. Some of the syenite 

porphyry is brecciated near its contact with limestone 

breccia. Altered porphyry is tan to white and is composed mainly of kaolinite (identified by X-ray diffraction), partly altered feldspar phenocrysts, and rusty 

specks of iron oxides after pyrite. Porphyritic texture 

is evident; feldspar phenocrysts are partly altered to 

kaolinite and ferromagnesian phenocrysts are completely altered to sericite and iron oxides. Altered porphyry has low gold values (Lindsey, 1982; chapter A, 

this report). Chemical analyses of altered and fresh 

syenite porphyry (samples K-2 and K-51, Lindsey, 

1982, table 1) show that altered porphyry has lost most 

of its former content of iron, alkalis, and alkaline earth 

metals, leaving a residue of mainly quartz and kaolinite.

GEOCHEMISTRY OF OXIDIZED 

LIMESTONE BRECCIA

SAMPLING METHODS

The fault zone of mineralized limestone breccia at 

Kendall was sampled to determine the nature of alteration and geochemical anomalies in the district. Six open 

pits (fig. 1) in mineralized breccia were sampled by random methods according to a barbell design (fig. 3). In

3 ITK

-300 m

Strike of breccia zone

 3 m

.3 m

m 

OPEN PIT 1

3 m

OPEN PIT 2

FIGURE 3. Sampling plan for open pits in mineralized breccia zone at Kendall, Mont. Six open pits were sampled. Sites at which

two samples were collected 0.3 m apart are designated by "8."
A GOLD-MINERALIZED BRECCIA ZONE AT KENDALL 49

all, 48 samples of limestone breccia were collected; each 

sample consisted of about 0.5 kg (kilogram) of rock 

chips taken from an area of about 0.1 m2 . Twenty-four 

of the samples were selected at random and split. These 

splits, together with the other 24 samples (76 samples 

total), were placed in random order prior to chemical 

analysis.

Sampling of the freshest mineralized rock available 

from surface exposures may provide reliable information about the entire mineralized zone at Kendall, but 

sampling of the subsurface parts of the zone will ultimately be needed to test the data and conclusions presented here. With respect to estimating the gold content of the Kendall deposits, the sampling is biased in 

that (1) sampling was restricted to open pits where 

mineralized rock could be obtained, (2) early mining 

probably removed the best ore, and (3) only oxidized 

rock was available for sampling. Most of the rock sampled proved not to be of ore grade, but to be altered 

rock with low gold contents.

CHEMICAL COMPOSITION AND VARIATION

The overall chemical composition of the mineralized 

limestone breccia varies widely from nearly pure calcite 

(as much as 58.5 percent CaO) to about half calcite (24.3 

percent CaO) and half quartz (as much as 52.9 percent 

Si02) (table 1). Other major rock-forming elements generally make up less than 1-2 percent of the breccia. 

Total iron as Fe203 , representing mostly the presence 

of pyrite and its oxidation products, and MgO, present 

in calcite, are the most abundant minor constituents. 

The quantity of Si02 and iron in breccia is a function 

of the degree of alteration of originally nearly pure 

limestone.

Much of the mineralized breccia contains small 

amounts of gold; of 48 samples, 34 contained gold in 

quantities at or above the limit of analytical detection 

(0.05 ppm) (table 2). Typical gold values are extremely 

low (geometric mean of 0.12 ppm) as is the average 

grade (arithmetic mean of 0.7 ppm), but variation of 

gold content in the mineralized breccia is large, with 

a measured maximum concentration of 11.7 ppm; in 

comparison, the ores mined previously contained about 

4-10 ppm (Blixt, 1933, p. 20).

The breccia contains a number of trace metals and 

elements that might be expected to be associated with 

gold and sulfide ores; compared with unmineralized 

limestone, arsenic, mercury, antimony, zinc, silver, vanadium, lead, thallium, and possibly barium are present 

in anomalous but trace quantities (table 1). The presence of anomalous concentrations of arsenic, mercury, 

antimony, and thallium is noteworthy; this geochemical 

association is characteristic of disseminated gold deTABLE 1. Summary statistics for chemical composition of 48 samples of mineralized limestone breccia, Kendall area, Northern 

Moccasin Mountains, Mont.

[SiO2 , A12O3 , total Fe as Feos, CaO, TiO2 , MnO, P2O6> Cl and total S by X-ray fluorescence 

by J. S. Wahlberg; MgO, Na20, and K2O by atomic absorption by V. M. Merritt; As 

by graphite furnace-atomic absorption method by J. G. Crock; Au by fire assay-atomic 

absorption method by A. W. Haubert, L. M. Lee, and Claude Huffman, Jr.; Hg by 

wet oxidation-atomic absorption method by J. A. Thomas and G. O. Riddle; Sb by 

Rhodamine-B method by G. T. Burrow; Zn by atomic absorption by J. G. Crock; all 

other elements by six-step semiquantitative spectrographic method by H. G. Neiman; 

elements looked for but not found, with lower limits of detection are: P2Os (0.10 pet), 

Cl (0.010 pet), total S (0.04 pet), Bi (10 ppm), Cd (20 ppm), Ce (200 ppm), Eu (100 

ppm), Ga (5 ppm), Ge (10 ppm), Hf (100 ppm), In (10 ppm), La (30 ppm), Nb (10 ppm), 

Pd (1 ppm),Pt (30 ppm), Re (100 ppm), Sn (10 ppm), Ta (200 ppm), Te (2000 ppm), 

Th (200 ppm), U (1000 ppm), and W (100 ppm); leaders ( ) indicate not calculated because of insufficient values above limit of detection]

Number of

samples below Lower limit

limit of of detection

detection

Geometric 

deviation

Range

Si02 

A1 20 3 - 

Total Fe as 

Fe 20 3 

MgO 

T102 -

Hg 

Sb 

Ag 

Sr 

V 

Chemica

0

0

0

23

Chemical

24

0

0

0 

40

45

1 composition

0.2

0.02 

0.01

0.02

composition,

0.1

1.5

5

7 

10

1

i, in weight percent

8.4 2.23

0.28 2.32 

0.31 1.53

0.02 2.04

in parts per million

5 2.26

64 4.39

184 1.75

28 1.82

2.1-52.9

0.04-1.55 

0.10-1.09

<0. 02-0. 12

0.4->15

3-5,000

<3-7

30-500

10-200 

<10-10 

<1-1

Geometric mean for censored distributions computed by method of A. C. 

Cohen (Miesch, 1967).

Four samples above limit of detection of 15 ppm.

posits in northern Nevada (Erickson and others, 1966; 

Wells and others, 1969; Radtke and others, 1972; 

Radtke and Dickson, 1974).

The spatial variation in chemical composition of the 

breccia, particularly of the content of gold and other 

metals, is of interest in exploring and evaluating the 

gold-mineralized zone at Kendall and similar occurrences elsewhere. Variation in the composition of the 

breccia is defined by a five-level nested analysis-of-variance design (Krumbein and Graybill, 1965, p. 205-218)
50 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

TABLE 2. Gold content in mineralized limestone breccia at Kendall, Mont.

[Analyses by fire assay-atomic absorption method by A. W. Haubert, L. M. Lee, and Claude Huffman, Jr. Sample numbers ending in "B" are splits 

of those ending in "A". Open-pit localities are shown on figure 1. ppm, parts per million]

Sample 

No.

Open pit 1: 3111A

3112A

3112B

3121A

3121B

3122A

3211A

3212A

3221A

3222A

Open pit 2: 4111A

4112A

4112B

4121A

4121B

4122A

4211A

421 IB

4212A

4221A

4221B

4222A

Au 

(in ppm)

0.87

0.23

0.24

0.37

0.33

0.10

0.30

0.30

<0.05

0.15

11.70

10.90

11.00

0.25

0.22

0.50

0.17

0.17

0.25

0.17

0.23

0.60

Sample 

No.

Open pit 3: 5111A

5111B

5112A

5121A

5121B

5122A

5122B

5211A

5212A

5221A

522 2A

5222B

Open pit 4: 6111A

6112A

6121A

6121B

6122A

6122B

621 1A

6211B

6212A

6212B

6221A

6222A

Au 

(in ppm)

0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

0.07

<0.05

0.15

0.13

0.10

0.06

0.08

0.08

<0.05

<0.05

0.35

0.30

0.29

0.31

0.07

0.41

Sample

No.

Open pit 5: 711 1A

7112A

7112B

7121A

7122A

7122B

7211A

7211B

7212A

7212B

7221A

7222A

7222B

Open pit 6: 8111A

8112A

8112B

8121A

8122A

8211A

8211B

8212A

8212B

8221A

8221B

8222A

8222B

Au 

(in ppm)

1.50

0.30

0.30

1.20

0.90

0.87

0.14

0.13

0.18

0.15

0.21

0.22

0.21

<0.05

<0.05

<0.05

<0.05

0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

<0.05

that follows the sampling plan of figure 2. The model 

for variation in the mineralized zone is:

where Xijkmn is the nth value on the rath sample taken 

of a pair 0.3 m apart, in the &th sampling site of a 

pair 3 m apart, in the jih sampling area of a pair 30 

m apart, and in the ith open pit in the mineralized zone. 

Each value (Xykmn) is equal to the sum of the grand 

mean (IJL) of all values modified by deviations representing compositional variation among open pits about 300 

m apart (04), between sampling areas 30 m apart (p^), 

between sampling sites 3 m apart ( yij/c), between samples 0.3 m apart (§ijlcm), and between splits (eijkmn); 

where i is as high as 6 and j, k, ra, and n are as high 

as 2.

The preceding analysis-of- variance design was used 

to partition the variance into four distance-related components. The analysis assumes that xi9 xy-, xijk, xijkm, 

and Xijkmn are random variables with means of zero and 

variances of sa2, sp2 , sy2 , ss2 , and se2 , that are additive:

where sT2 is the total variance; sa2 is the variance among open pits 300 m apart; 

Sp2 > s-y2 , and s82 are the variances that represent sampling 30 m, 3 m, and 0.3 m apart, respectively; and 

se2 is the variance due to laboratory procedures such 

as splitting, grinding, and analysis. The variance components sa2 , Sp2 , sy2 , and ss2 provide insight to the scale 

of variation in the mineralized zone.

Estimates of the variance in chemical composition of 

the mineralized zone show that most constituents vary 

a great deal at distances of about 30 m or less, but 

that four elements manganese (as MnO), gold, mercury, and chromium have relatively large variance components among the open pits, which average about 300 

m apart (table 3). This conclusion is somewhat tentative, however, because the variance components estimated for these elements are not quite significant at 

the 0.05 level. For gold, 44 percent of the variance reflects differences among open pits; most of the remaining variance (41 percent) is distributed among sites 30 

m apart and 3 m apart. The analysis shows that gold 

is the most likely of all elements studied to be concentrated in blocks of ground greater than 30 m across; 

such blocks might have a reasonable chance of being 

detected by a sampling density that takes such largescale variation into account. The large-scale component 

of variance is readily apparent from a scan of the gold 

content (table 2); localities 1, 2, 4, and 5 (fig. 1) account
A GOLD-MINERALIZED BRECCIA ZONE AT KENDALL 51

TABLE 3. Estimates of total variance and components of variance for major oxides and trace elements in

mineralized limestone breccia at Kendall, Mont.

[Analysis of variance performed after transformation of all data to logarithms; five-level analysis of variance design. Asterisk (*), variance of MnO 

among open pits is significant at the 0.13 level; of Au, at the 0.07 level; of Hg, at the 0.23 level; and of Cr, at the 0.22 level, pet, percent; 

ppm, parts per million; T-Fe2Og, total Pe as Fe20g]

____Components of variance as percent of total variance______

Oxide or trace Total Among open Among areas Among sites Among samples Among

element variance pits ( 300 m (30 m apart) (3 m apart) (0.3 m apart) sample

apart) splits

o

as (log pet)

Si0 2 0.117 0 24 6 69* 1

A1 2°3 0.146 0 27 1 30* 42

T-Fe 2 0 3 0.169 0 65* 0 34* <1

MgO 0.038 0 61* 7 32* <1

CaO 0.007 1 0 13 85* 1

Na 2 0 0.014 0 25 30* 14 31

K2 0 0.098 0 54* 0 45* 1

Ti0 2 0.063 0 49* 0 47* 4

MnO 0.113 39 42* 2 17* <1

as (log ppm) 2

As 0.305 0 64* 4 26* 6

Au 0.441 44 19 22* 14* <1

Hg 0.142 21 32 21* 21* 5

Sb 0.160 0 63* 12 20* 5

Zn 0.140 0 69* 0 31* <1

Ag 0.296 0 70* 3 21* 6

Ba 0.447 0 36* 7 55* 2

Cr 0.068 16 24* 0 48* 12

Cu 0.189 0 63* 2 31* 4

N1 0.037 0 14 0 70* 16

Pb 0.137 0 59* 0 34* 7

Sr 0.069 9 2 18 0 71

v 0.083 1 48* 14 21* 16

Zr 0.081 0 61* 3 27* 9

for most of the high values. The variation in readily 

observable features such as degree of silicification 

(Si02) is only local (69 percent at a scale of 0.3 m), 

so that the proportion of quartz (Si02) in the breccia 

appears to be a poor guide to ore. Variance of trace 

elements that might serve as pathfinders for gold is 

also mainly local, with the possible exception of manganese and mercury, which have large-scale variance components of 39 and 21 percent, respectively.

The variance due to analytical uncertainty is of importance in evaluating covariation among elements, a 

topic to be examined next. Variance among splits is 

large only for A1203 , strontium, and perhaps, Na20; 

correlations among these elements must be interpreted 

with caution because of the high component of error 

variance.

FACTOR ANALYSIS OF GEOCHEMICAL DATA

The chemical composition of the mineralized breccia 

zone, and the variation within that zone, is the result 

of several geochemical processes. A complete list of 

these processes and the effects that they have had on 

the composition of the rock would serve as a geochemical model of the breccia, and would serve to identify 

the effects of hydrothermal mineralization on chemical 

composition. Such processes may be thought of as factors, and a general model for the effect of the factors 

on the concentration of an element measured in the 

breccia zone is:

xji=ajlFli+oLj2F2i+- +ajpFpl +ajUji 

where Xi is the chemical concentration of the ith sample
52 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

for the jth element, and is assumed to be a linear function of p common factors (F) operating ^with intensities 

ofaj! through a.jp. Uji is a unique factor introduced to 

account for the variation unexplained by the model 

(Cooley and Lohnes, 1962, p. 160). To illustrate, Xji may 

represent the observed amount of gold in a sample i; 

the amount of gold in the sample is assumed to be a 

function of various factors (F's) operating at various 

intensities (a 's). In this study, the parameters (a' s, 

F's) in the model are estimated by .R-mode factor analysis.

The method of .R-mode factor analysis (Harman, 1960; 

Cooley and Lohnes, 1962, p. 151-172; Miesch and 

others, 1966) involves four steps; (1) an mxm correlation matrix CR-matrix) is computed, where ra is the 

number of measured variables in this case oxides and 

elements; (2) the 72-matrix is solved for m roots (eigenvalues) and a number of roots (p) is selected as the 

minimum number needed to account for a reasonable 

percent of the total variance; (3) an initial matrix of 

factor loadings defines the principal (uncorrelated) components of the system; and (4) a final matrix of factor 

loadings is derived from the initial matrix by the Varimax criterion. These loadings are then taken as the 

intensities (a's) of the equation for the model.

Principal-components analysis (step 3) may be visualized in terms of m-dimensional space, where m is the 

number of original variables. Each variable defines an 

axis in the m-space and the position of each of N samples can be plotted in terms of the m-axes. Commonly, 

the N samples will tend to cluster as an elongate swarm 

of points in the m-space. The object of the principalcomponents analysis is to define new, uncorrelated variables (axes). The first principal component is positioned 

along the major axis of the sample cluster so that the 

sample projections onto it are minimized. This axis defines a new variable that describes the largest proportion of the variance in the data. A second axis is located 

at right angles to the first but in a direction such that 

the next largest proportion of variance is explained, and 

so on. Although as many as m axes are possible, some 

number p<m generally will account for most of the variance. The variances of the new variables, the principal 

components, are their eigenvalues. The relationship between the principal components and the original variables is defined by a matrix of factor loadings (a's).

Commonly, the principal-components axes in p-space 

are rotated in order to interpret the new variables (step 

4). The method of rotation used here is the Varimax 

criterion (Harman, 1960, p. 301), which requires that 

the variance of the squared factor loadings be 

maximized. Rotation simplifies interpretation of the factors because it tends to yield large positive or negative 

loadings for only a few variables on each Varimax axis;

the rest of the variables tend to have extremely small 

loadings. The resulting Varimax factor loadings specify 

the relationship of the original variables to the p Varimax axes, which are then interpreted as factors. Because only p<m axes are retained in the Varimax solution, less than 100 percent of the total variance for each 

variable is accounted for; the percentage accounted for 

is called the communality (h2) and is a measure of how 

well the solution explains the original data. The remaining, unexplained variance (1-fe2) is the unique component (Uji); it consists of variance caused by unrecognized factors and by error.

A correlation matrix was computed for those 23 

oxides and elements for which the majority of samples 

were above the limit of analytical detection (table 4). 

The 23x23 ^-matrix (table 4), with unities in the 

diagonal, was used for the factor analysis. Three 

criteria were used to select the number of principal 

components for rotation and interpretation. (1) The 

eigenvalues for the first 10 principal components were 

compared to determine which components were relatively large; the first five eigenvalues appear to be 

markedly larger than the others and the first five principal components account for 77 percent of the cumulative variance (fig. 4). As a first approximation, a solution containing five principal components was considered for rotation. (2) The sum of the non-error variances (table 3) was compared with the communalities 

of various solutions: a five-factor model (table 5A) has 

high communalities that explain most of the non-error 

variance of the system. The communalities of most variables, except A1203 and strontium, do not exceed the 

non-error variance. (3) Finally, the five-factor solution 

appears amenable to geochemical interpretation.

A five-factor model accounts for a reasonably high 

proportion of the variance of all chemical oxides and 

elements except nickel and barium (table 5A). The 

model is interpreted in genetic terms (table 5B) and 

contains the following factors or processes.

Factor 1 (Gold, mercury, vanadium and antimony). 

Concentration of these elements probably occurred during hydrothermal alteration and mineralization of the 

limestone breccia. Arsenic, which occurs in anomalous 

quantities, is not strongly correlated with gold (r=0.27) 

and hence does not appear in factor 1. The strong association of vanadium with gold at Kendall may be due 

to its presence in roscoelite, a vanadium mica that occurs also in gold-bearing fluorite veins at the Spotted 

Horse and other mines in the Judith Mountains (Corry, 

1933).

Factor 2 (K20, Ti02 , zirconium, chromium, A12O3 , 

nickel, and Na20). These elements are among the 

main constituents of clastic minerals such as feldspar, 

clay, mica, iron-titanium oxides, and zircon; chromium
A GOLD-MINERALIZED BRECCIA ZONE AT KENDALL 53

TABLE 4. Correlation coefficients between major oxides and trace elements in mineralized limestone breccia, Kendall, Mont.

[Linear correlation coefficients were computed from logarithms of original data; T-FegOg, total Pe as Fe^J

Si02 Al2°3

Si02 I. 00 0.57

A1 2°3 1 '°° 

T-Fe 203 

MgO 

CaO 

Na 20 

Ti02

As 

^S

T-Fe 20 3 MgO CaO Na 20 K20 TiOC, MnO As Au Hg Sb Zn Ag

0.66 -0.60 -0.84 0.39 0.52 0.43 0.02 0.61 0.37 0.56 0.55 0.58 0.43 

0.43 -0.10 -0.51 0.56 0.82 0.73 0.12 0.29 0.36 0.43 0.30 0.41 0.47 

1.00 -0.36 -0.53 0.21 0.48 0.44 0.30 0.89 0.31 0.48 0.72 0.81 0.32 

1.00 0.46 -0.11 0.04 0.20 0.09 -0.45 -0.13 -0.33 -0.33 -0.27 -0.28 

 LOO -0.52 -0.38 -0.42 0.18 -0.49 -0.44 -0.54 -0.44 -0.47 -0.35 

 LOO 0.53 0.44 -0.45 0.15 0.28 0.22 0.00 0.04 0.14

 LOO 0.16 0.33 0.37 0.37 0.31 0.43 0.29

 LOO 0.27 0.49 0.72 0.77 0.31

Ba

0.11 

0.31 

0.01 

0.35 

0.11 

0.08

0.03

0.00

Cr

0.47 

0.60 

0.33 

-0.07 

-0.34 

0.56

0.60

0.26 

0.25

Cu

0.61 

0.49 

0.55 

-0.41 

-0.49 

0.25

0.37

0.52

Ni

0.46 

0.49 

0.59 

-0.08 

-0.45 

0.30

0.47

0.49

P

0. 

0. 

0. 

-0. 

-0. 

-0.

0.

0.

b

21 

26 

43 

05 

18 

28

24

41

S

-0. 

0. 

-0. 

0. 

0. 

0.

0.

-0.

r

17 

14 

17 

47 

19 

19

28

29

V

0.57 

0.49 

0.56 

-0.27 

-0.44 

0.30

0.56

0.55

Zr

0.44 

0.59 

0.41 

0.07 

-0.32 

0.33

0.80

0.31

and nickel would be expected to be present in iron-titanium oxides. Concentration of these minerals probably 

occurred by accumulation of terrigenous detritus in lime 

mud of the Mississippian Madison sea; if so, then elements having high loadings on factor 2 have little relationship to mineralization.

Factor 3 (MgO, CaO, and strontium versus arsenic, 

total iron as Fe203 , Si02 , zinc, and antimony). MgO, 

CaO, and strontium are components of the carbonate 

breccia which, when mineralized, was depleted in these 

elements and enriched in silica and sulfides containing 

iron (as pyrite), arsenic, zinc (as sphalerite), and antimony. Copper also was concentrated somewhat by factor 3. Subsequent oxidation of mineralized breccia produced the rusty-brown silicified breccia of the 

mineralized zone. High positive and negative loadings 

of major rock-forming components on factor 3 suggest 

that this factor largely represents closure (Chayes, 

1960) in the data, a situation that exists when an inverse relationship is produced by two or more components that tend to sum to 100 percent. Nevertheless, 

the association of total iron, Si02 , zinc, and antimony 

represents the process of silicification and sulfidization

during hydrothermal alteration of the limestone breccia.

Factor 4 (copper, silver, and barium). Traces of 

these three metals evidently were concentrated during 

hydrothermal mineralization; lead also was concentrated to some degree by factor 4. These elements evidently were concentrated by chemical processes somewhat different but unknown from those that deposited 

gold and associated metals of factor 1 and the sulfides 

of factor 3. Barium, present as barite at Kendall, evidently tends to occur separately from other effects of 

mineralization, as shown by its low communality and 

by low correlation coefficients with all other elements 

analyzed. Its appearance in factor 4 is regarded as 

spurious.

Factor 5 (Na^O versus lead and MnO). This factor 

evidently represents the effect of weathering on the 

mineralized breccia. Weathering could be expected to 

leach Na^O and to concentrate manganese as oxides, 

and manganese is well known for its ability to scavenge 

lead and other heavy metals (perhaps zinc and antimony, which are weakly associated with factor 5). Presumably, factor 5 was responsible for oxidation, but not 

for appreciable concentration or depletion, of iron.
54

10r

STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

III IV v VI VII VIII

PRINCIPAL COMPONENT

IX

FIGURE 4. Eigenvalues and cumulative proportion of total variance 

explained by the first 10 principal components.

A MODEL FOR GOLD MINERALIZATION

Studies of field and geochemical relationships at Kendall indicate that gold was deposited in limestone fault 

breccia by hydrothermal fluids that emanated from or 

circulated near cooling syenite magma. The host breccia 

occurs along a concentric fault that formed in response

TABLE 5. A five-factor geochemical model for mineralized limestone 

breccia, Kendall, Mont.: A, Varimax factor loadings, and B, interpretation of model

[Loadings greater than 0.50 shown in boldface type]

Communality 

(h 2)

Au

Hg

V

Sb

Ni

A1 203

Cr

Zr

TiO 2

K 2O

MgO

CaO

Sr

Zn

Si0 2

T-Fe203

As

Cu

Ag

Ba

Na2O

Pb

MnO

0.94

0.84

0.59

0.57

O.Z7 

0.24 

0.10

-0.08 

0.19 

0.2Z

-0.17

-0.40 

0.11 

0.14 

0.32 

0.17 

0.17 

0.24 

0.23

-0.07 

0.25 

0.17 

0.23

-0.17

-0.18

-0.48

-0.13

-0.60

-0.74

-0.77

-0.84

-0.88

-0.89

-0.25 

0.32 

=0.48

-0.39

-0.40

-0.41

-0.25

-0.39

-0.29

-0.14

-0.55

-0.12

-0.02

-0.05

-0.32

-0.29

-0.54

-0.24

-0.16

-0.07

-0.18

-0.09

-0.06 

0.70 

0.63 

0.58

-0.60

-0.7Z

-0.76

-0.83

-0.41

-0.19

-0.02

-0.12

-0.17

-0.02

-0.03

-0.11

-0.14 

0.04

-0.16

-0.30

-0.35 

0.15 

0.09

-0.19 

0.41 

0.16 

0.06

-0.23

-0.19 

0.03 

0.02

-0.55

-0.67

-0.75

-0.07

-0.44 

0.14

-0.09

-0.15

-0.15

-0.48

-0.16 

0.05 

0.12

-0.02

-0.10

-0.03

-0.23

-0.30

-0.04

-0.48 

0.14

-0.32

-0.30

-0.23

-0.28 

0.12 

0.64

-0.75

-0.80

0.93

0.88

0.70

0.87

0.54

0.72

0.75

0.77

0.83

0.88

0.80

0.77

0.58

0.81

0.83

0.89

0.87

0.73

0.71

0.58

0.79

0.84

0.71

B Factor 1 (Au, Hg, V and Sb) Hydrothermal gold mineralization.

Factor 2 (K 2O, TiO 2, Zr, Cr, A1 2O3 , Ni and Na2O) Concentration of clastic

minerals (feldspar, clay, mica, iron-titanium oxide minerals and zircon) in

limestone. 

Factor 3 (MgO, CaO and Sr versus As, Total Fe as Fe2O3 , SiO 2, Zn and Sb) 

Hydrothermal sulfide (pyrite and sphalerite) mineralization and silicification.

Inverse relation between major constituents CaO and SiO2 reflects closure in

data (Chayes, 1960).

Factor 4 (Cu, Ag, Ba) Trace mineralization. 

Factor 5 (Na2O versus Pb and MnO) Alteration by weathering, characterized by

concentration of Pb in manganese oxides.

to doming of sedimentary rocks during emplacement of 

a syenite porphyry laccolith about 66 m.y. ago 

(Lindsey, 1982). Emplacement of the main laccolith and 

attendant faulting probably was followed closely by 

emplacement of small dikes and sills of syenite porphyry along the fault zone and by hydrothermal alteration of these small plutons and of the fault breccia 

(chapter B).

Hydrothermal fluids leached iron, alkali metals, and 

alkaline earth metals and deposited finely disseminated 

gold and pyrite in syenite porphyry plutons along the 

fault; the fluids deposited abundant silica (as quartz), 

finely disseminated pyrite, and gold in the limestone 

fault breccia. Three distinct suites of elements were deposited by hydrothermal fluids: (1) gold, mercury, vanadium, and antimony; (2) arsenic, iron, zinc, and antimo-
A GOLD-MINERALIZED BRECCIA ZONE AT KENDALL 55

ny, probably in sulfides and with Si02; and (3) copper 

and silver in trace amounts. The three suites tend to 

occur separately in the mineralized breccia, but in close 

proximity to one another; their depositional sequence 

is unknown. A fourth suite of lead and MnO probably 

was concentrated by weathering. Weathering oxidized 

the sulfides deposited by hydrothermal fluids, but evidently did not redistribute them far from their original 

site of deposition.

The occurrence of gold and iron oxides after pyrite 

in fine-grained, disseminated form in silicified calcareous rocks as well as in intrusive igneous rocks and the 

nearby breccia pipe in Plum Creek (chapter A), and 

the general association of gold . with geochemical 

anomalies of arsenic, mercury, antimony, and thallium 

suggests that the gold deposits at Kendall belong to 

the same class as the disseminated gold deposits (for 

example, Carlin) of northern Nevada, which share these 

features. Perhaps the geologically similar deposits of 

the Giltedge mine in the Judith Mountains (Forrest, 

1971, p. 140-148) are also of the Carlin type. Surface 

exposures of Nevada gold deposits are oxidized, 

silicified limestone containing free gold and iron oxides 

replacing pyrite (Hardie, 1966; Erickson and others, 

1966; Wells and others, 1969); beneath the oxidized 

zone, gold occurs in disseminated pyrite in some of the 

Nevada deposits (Wells and Mullens 1973; Radtke and 

others, 1980). A noteworthy feature of both the Kendall 

deposit and the Nevada deposits is the occurrence of 

arsenic, mercury, antimony, tungsten (not analyzed at 

Kendall), and thallium with gold (Erickson and others, 

1966; Wells and others, 1969; Radtke and Dickson, 

1974). The strong statistical correlation between gold 

and mercury, noted at Kendall, is present at Carlin, 

also (Harris and Radtke, 1976). Evidence for a gold 

suite (concentration of gold, mercury, vanadium, and 

antimony by factor 1) and a separate base-metal suite 

(concentration of sulfides by factor 3) at Kendall is 

analogous though not identical to that reported for some 

Nevada deposits (Wrucke and Armbrustmacher, 1975; 

Harris and Radtke, 1976).

The Kendall deposit differs from those in northern 

Nevada in containing disseminated fluorite and anomalous amounts of vanadium that probably are derived 

from roscoelite. Although no roscoelite was observed 

at Kendall, the mineral occurs with fluorite and the 

gold-silver telluride mineral sylvanite in mines of the 

Judith Mountains (Corry, 1933; Forrest, 1971, p. 87-93) 

and free gold occurs with fluorite and sylvanite in the 

Little Rocky Mountains (Corry, 1933; Dyson, 1939); all 

these might be found below the oxidized zone at Kendall or in nearby mineralized zones in syenite porphyry. 

The gold-tellurium-vanadium-fluorine association is best 

developed in fissure-filling veins and replacement deposits adjacent to or within intrusive igneous rocks in 

the Judith and Little Rocky Mountains, thus indicating 

that this association is more characteristic of the intrusive environment. The Kendall deposits, and perhaps 

gold deposits of the Giltedge Mine in the Judith Mountains (Forrest, 1971), evidently represent the effects 

of mineralizing fluids more distant from the intrusive 

porphyry.

Differences between the gold deposits of north-central Montana and those of northern Nevada may represent differences in the depth of mineralization, source 

of hydrothermal fluids, and composition of source rocks. 

The Kendall gold deposits probably formed at a depth 

of about 1,700-2,300 m (chapter A). In contrast, 

Nevada gold deposits such as Carlin are believed to 

have formed in a shallow hydrothermal environment 

(Radtke and others, 1980). The source of metals in the 

Kendall gold deposits is not known but may have been 

the associated intrusive rocks; the source of metals in 

the Nevada gold deposits is thought to have been the 

host sedimentary rocks (Dickson and others, 1979). 

Studies of fluid inclusions and stable isotope composition of some of the Montana gold ores should be undertaken to answer these questions.

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56 STUDIES OF MINERALIZED INTRUSIVE COMPLEXES IN NORTH-CENTRAL MONTANA

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