Geology and ore deposits of the Landusky Mining Dustrict, Phillips County, Montana

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Authors Richardson, George Lusk, 1942-

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GEOLOGY AND ORE DEPOSITS OF THE

LANDUSKY MINING DISTRICT, PHILLIPS COUNTY, MONTANA

by

George Lusk Richardson

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE WITH A MAJOR IN GEOLOGY

In the Graduate College THE UNIVERSITY OF ARIZONA •

19 7 3 STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ ment the proposed use of the material is in the interests of scholar­ ship. In all other instances, however, permission must be obtained from the author.

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

'\ JOHN M./GflLBERT / ] Date ACKNOWLEDGMENTS

While I was 9,000 miles away in South Africa, Dr. J„ M.

Guilbert reminded me that I had not fulfilled an obligation to myself until I had completed this thesis. For that reminder alone I acknow­ ledge a deep indebtedness. Dr. Guilbert's encouragement and direction as thesis advisor are also sincerely appreciated. Appreciation is ex­ tended to Drs. J. M. Guilbert, D. E. Livingston, W. J. McLean, Mr.

J. E. Kinnison, and my father. Dr. J. K. Richardson, who read the manuscript and offered many helpful suggestions.

Particular thanks are due Dr. J. J. Durek, Dr. T. F. O'Neill, and Kaiser Exploration and Mining Company for allowing me to expand the Landusky project into a thesis. I also wish to express thanks to

Mr. R. E. Legg, president of Niseka Mining Limited of Vancouver,

B. C ., who allowed the release of certain material for use in this thesis, and to Edward Wieglanda of Oracle, Arizona, formerly of Lewistown,

Montana, who provided samples and information about the Landusky d istric t.

An indebtedness is acknowledged to the patient encouragement of my wife, mother, and father. TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS ...... vi

LIST OF TABLES ...... v iii

ABSTRACT ...... ix INTRODUCTION ...... 1

Methods of Study ...... 3 Topography and Accessibility ...... 4 History and Production ...... 5 Previous Work ...... 10

REGIONAL GEOLOGIC SETTING...... 12

GEOLOGY OF THE LANDUSKY DISTRICT ...... 15

Rocks ...... 15 Precambrian Rocks ...... 15 Paleozoic Rocks ...... 16 Flathead Sandstone...... 16 Emerson Formation ...... 16 Bighorn Dolomite...... 17 Tertiary Rocks ...... 17 Mission Peak Porphyritic Syenite...... 18 Platy Syenite Porphyry ...... 19 Syenite Porphyry ...... 20 Big Eye Porphyry ...... 22 Possum Porphyry ...... 22 Structure ...... 26 VEIN MINERALIZATION OF THE LANDUSKY DISTRICT...... 29

August-Niseka Vein ...... 29 Mineralogy of the August-Niseka Vein ...... 30 New Discovery Zone ...... 30 Mineralogy of the New Discovery Z one ...... 31 Gold Bug Vein ...... 34 M ineralogy of the Gold Bug Vein ...... 36 Wall-rock Alteration . 36 Paragenesis ...... 40

SUPERGENE ENRICHMENT ...... 43

iv V TABLE OF CONTENTS—Continued

Page GEOTHERMOMETRY OF THE LANDUSKY DISTRICT ...... 46

GEOLOGIC HISTORY OF THE LANDUSKY MINING DISTRICT .... 48

A COMPARISON OF THE LANDUSKY, CRIPPLE CREEK, AND BOULDER COUNTY TELLURIDE DISTRICTS ...... 52

ECONOMIC EVALUATION OF THE NEW DISCOVERY ZONE. .... 58

SUMMARY...... 60

REFERENCES. . 63 LIST OF ILLUSTRATIONS

Figure Page

1 „ Location Map of the Little Rocky Mountains , Phillips County, Montana 2 1 2 „ Geologic Map of. Little Rocky Mountains and Encircling Foothills, Montana ...... in pocket

3. Reconnaissance Geological Map of the Landusky Mining District, , Phillips County, Montana ...... in pocket

4. Geologic Cross Sections, Landusky Mining District, Phillips County, Montana ...... in pocket

5. Geology of the Underground Workings, August—Little Ben Zone, Landusky Mining District, Phillips County, Montana ...... in pocket

6. Geology and Assay Plan of the Niseka Crosscut, Landusky Mining District, Phillips County, Montana ...... in pocket

7. Block Diagram of the New Discovery Zone from Sections B-B' and C -C , Landusky Mining District, Phillips County, Montana ...... in pocket

8 .■ Assay Comparison at the Elevation of the Niseka Tunnel, Landusky Mining District, Phillips County, Montana ...... in pocket

9o Section along D.D.H.-5 Bearing S54E at -45°, Section Looking NE, Landusky Mining District, Phillips County, Montana ...... in pocket

10. Topography of the Landusky Mining District ...... 6

11. Core Specimen and Photomicrograph of Thin Section of Syenite Porphyry (subtype Tsp-1). .... 21

12. Hand Specimen and Photomicrographs of Thin Section of Big Eye Porphyry ...... 23

13. Photomicrographs of Thin Section of Possum Porphyry ...... 25 vi v ii

LIST OF ILLUSTRATIONS— Continued

Figure Page 14. Photomicrographs of Polished Section of Vein Filling from the Niseka Crosscut ...... 33 LIST OF TABLES

Table Page

1 „ Summary of the Mineralogy of the Landusky Telluride District ...... 37 2. Comparison of Characteristics of Cripple Creek, Landusky, and Boulder County Telluride D istricts ...... 56

I

v iii ABSTRACT

The Landusky mining district, Phillips County, Montana, is an area containing a series of epithermal gold-silver telluride veins in a syenite and quartz monzonite laccolith of Tertiary age „ The laccolith has domed the overlying Paleozoic and Mesozoic sedimentary strata.

Erosion has exposed the alkalic igneous cores of the domes which make up the laccolith„ Vein deposits of the Landusky district occur in min­ eralized reverse faults and shears spatially related to late-stage phono- lite dikes „ All vein deposits of economic importance lie wholly within igneous rocks of the laccolith. Hypogene ore minerals present in the vein deposits of the Lan­ dusky district include: sylvanite, hessite, empressite, native gold, freibergite, acanthite, and minor amounts of galena and sphalerite. The gangue vein minerals present are botryoidal and vein pyrite, calcite, alpha quartz, and earthy to banded purple fluorite. Wall-rock alteration immediately associated with the veins consists of quartz + sericite + kaolinite „

Based on the physical, chemical, and mineralogical character­ istics of the vein deposits and the spatially associated phonolite dikes, it is concluded that the laccolith and its related domes are intrusive, the veins epithermal„ The alkalic rocks and associated telluride min­ eralization of the Little Rocky Mountains are an example of several small ranges in central Montana showing similar Tertiary igneous rock associ­ ations and, in one other range, telluride mineralization.

ix INTRODUCTION

The Landusky mining district is located in the Little Rocky

Mountains in Phillips County, north-central Montana (Fig. 1). It is one mile north of the small settlement of Landusky (Fig. 2, in pocket),

190 road miles northeast of Great Falls, and 160 miles north of Billings, the two largest cities in Montana.

The primary purpose of this study is to preserve and consolidate past geologic data on the Landusky mining district and to augment it with the extensive new geologic data‘gathered during 1970 and 1971. From this assembled material a picture emerges of the geologic, geochemical and genetic relationships of the gold and silver deposits in syenite in­ trusions of the Landusky district. Of prime interest is that the Landusky mining district contains classic early Tertiary vein deposits related to subvolcanic mineralization. The relative obscurity of the district as a cited example of telluride vein mineralization is due to a lack of detailed study. This thesis by presenting a better description of the district should help to establish it as typical locality for telluride deposits.

Also of interest is the apparent analogy between disseminated porphyry copper deposits and the silver mineralization of the Landusky district.

The study develops a coherent picture of epithermal vein min­ eralization in the Landusky district and the relationship of the telluride mineralization to alkalic igneous rocks of the Little Rocky Mountains laccolith. The telluride mineralization of the Landusky district is very similar to that of the two major North American telluride districts,

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Figure 1. Location Map of the Little Rocky Mountains, Phillips County, Montana

to Boulder County and Cripple Creek, Colorado. Furthermore, the Little

Rocky Mountains with their association of alkalic igneous rocks and telluride mineralization are one example of a number of small ranges in central Montana showing like alkalic igneous rocks and, in one other range, telluride mineralization. Tertiary syenitic intrusions are extreme­ ly common in central Montana, and it is proposed that the central Mon­ tana area is a Tertiary syenitic province like no other in North America.

Several aspects of disseminated sulfides and pervasive alteration of the

Little Rocky Mountains syenitic laccolith will be considered.

Methods of Study

Both field and laboratory methods were used to gather quanti­ tative data on the Landusky district. Field work consisted of .the prep­ aration of a reconnaissance geologic map of the district, using both unpublished data of early workers in the district and surface mapping by me (Fig. 3, in pocket). Much of the surface mapping was based on float because of the paucity of outcrops away from roadcuts „ The topographic base for the reconnaissance map was supplied by Niseka Mining Ltd.

Composite cross sections (Fig. 4, in pocket) were constructed to the scale of the reconnaissance map, using the information from the eight diamond drill holes, surface contacts, underground mapping data (Fig. 5, in pocket), and one underground composite map of the August vein sur­ veyed by the Little Ben Mining Company (not included) . Underground mapping of the two workings in the vicinity of the August and Little Ben veins (Fig. 5, in pocket) was done with tape and Brunton at a scale of

1 inch to 50 feet, except in the Niseka crosscut (Fig. 6, in pocket), where detail on a scale of 1 inch to 20 feet was recorded. A channel

sample 364 feet long, cut from the Niseka crosscut, was assayed, and

4,284 feet of drill core ware examined with a binocular microscope for

details of lithology, alteration, mineralization, and structure.

Methods of descriptive geometry were employed to give better understanding of the dimensions of the district. An isometric block dia­ gram (Fig. 7, in pocket) was constructed from the above sources to show the shape of a minor ore body lying between the two major controlling veins of the district. A final construction (Fig. 8, in pocket) was com­ piled to compare assay values for samples derived from diamond drilling versus percussion drilling and to determine whether these assays would indicate a zone of enrichment near the Niseka adit. Detailed sections were drawn for each of the eight diamond drill holes shown on the recon­ naissance geologic map of the district (Fig. 3, in pocket). One of these sections (Fig. 9, in pocket) is included to show the amount of detail generated.

The minerals present in the veins of the district were identified microscopically from polished section made from core samples and se­ lected ore samples supplied by Ed. Wieglenda, Continental Materials,

Oracle, Arizona, and F. T. Graybeal, American Smelting and Refining

C o., Tucson, Arizona. Two samples had been sent to Lakefield Research of Canada Limited, Lakefield, Ontario, for mineral identification by X-ray diffraction and fluorescence techniques.

Topography and Accessibility

The district is located south and west of the central uplift of the Little Rocky Mountains, which rise 2,500 feet above the surrounding grass-covered rolling plains. The mountains have a northeasterly elon­ gation of 8 miles and are 5 miles wide „ There are twelve principal peaks, all with an elevation of about 5,500 feet; the relief in the central portion of the mountains is 600 to 800 feet. Slopes, ranging from 20 to 50 de­ grees, are covered with loose rock debris, 5 to 10 feet of soil, and a dense growth of young pines (Fig. 10). Creep has been very active on many slopes.

The Landusky district is accessible by automobile from Lewis- town, which is 87 miles south, or from Malta, 68 miles north, via paved

U.S. 181 to its intersection with Montana 3 76, 7 miles east of the set­ tlement of Landusky. A secondary county road leads north from Landusky and winds through the district. This county road is impassable during the winter and spring and is graded by the county road maintenance crew once during the summer. During the first months of this study, acces­ sibility was by cross-country skiing; later, the road was maintained by daily plowing with a D-4 Caterpillar.

History and Production

Precious metal activity has spanned a century in the Landusky district. Prospecting for gold had been surreptitiously carried out with­ in the Fort Belknap Indian Reservation since its establishment in 1874.

This clandestine work resulted in unpleasant incidents between prospec­ tors and the Indians as well as with federal authorities (Bryant, 1953).

In 1888, Pike Landusky made seemingly significant discoveries of gold mineralization, and immigration pressures began to mount, which cul­ minated with the withdrawal of certain mineral-bearing lands from the 6

Figure 10. Topography of the Landusky Mining District A. Background is east slope of Mission Peak, one of the main domes exposed by erosion of overlying sedimentary strata. B. This panoramic view of the southern slope of Gold Bug Hill shows the greater part of the Landusky mining district. C. The telephoto view shows the site of D.D.H.-8 seen in the right center of B. The drill hole is near the entrance of the Niseka tunnel. reservation, thus opening them to mining „ Landusky , the original dis­ coverer of gold, realized only modest results by today's standards. It appears that he grossed about $35,000 in gold ores mined from reserva­ tion lands prior to his shooting death by Kid Curry in 1894 (Bryant, 1953).

The town of Landusky was later named for him. Immediately after Lan­ dusky' s death, the Gold Bug claim group was purchased from his heirs by the Mission Peak Mining Company. This new company immediately authorized the building of a ten-stamp mill equipped with amalgamating plates and two concentrating tables. The mill was built in Landusky about one mile from the mine. The telluride ore from the Gold Bug mine did not lend itself to either amalgamation or gravity concentration, and the mill was closed shortly after its construction. The Gold Bug mine lay idle for 30 years until 1924, when it was purchased by the Little Ben

Mining Company for $8,000 (Bryant, 1953). The August vein, 1,650 feet northwest of the Gold Bug vein, . was discovered in 1907, and a newly formed company, called the August

Gold Mining Company, purchased the claims on the August vein from several of the parties associated with the Mission Peak Mining Com­ pany. The August Gold Mining Company purchased the old stamp mill in

Landusky from the Mission Peak Mining Company and replaced the stamps with two sets of 15 x 26-inch long rolls. Four cyanidation tanks, each with an ore capacity of 100 tons, were added. Between 1907 and 1914, the August Gold Mining Company mined and milled about 75,000 tons of ore valued at $20 per ton in combined gold and silver. From this $20 per ton, $12 per ton were lost to the tailings, chiefly as silver tellurides which were not amenable to cyanidation. Over this 7-year period from 1907 to 1914, the August Gold Mining Company grossed $600, 000 of which the owners of the company pocketed $42,000 in dividends in one year (Bryant, 1953). By 1915, more severe problems began to hamper the company „ On the 500-foot level of the August vein, large volumes of water had been encountered in a shaft. This flow, amounting to 1,000 gallons per minute, was difficult to handle with existing pumps. To complicate matters further, the August vein contained progressively more silver than gold in the deeper parts of the mine and the mill had to be redesigned to accommodate larger volumes of silver ore. The cost of transporting the crude ore about 1.5 miles by teams from the August vein to the mill in Landusky was $1 per ton. In the summer of 1915, a deci­ sion was made to build a tramway from the mine to the Landusky mill.

This proved to be a costly move because the tramway never worked prop­ erly and was beset with breakdowns. Finally, with the accruing of larger expenses on the mill and tramway, a strike in 1916 caused the closing of the mine (Bryant, 1953).

A second nonproductive period extended from 1942 through 1953. In 1954, the district was purchased by the Little Rockies Mining and

Development Company, organized by residents of Lewistown, Montana.

Mining began at the Gold Bug mine and a 20-ton per day flotation mill was erected in 1956. A 50-percent recovery was reported from heads assaying from $5 to $20 per ton in gold and silver. Declining grade caused operations to be suspended in 1958. Smelter shipments of 359 tons of crude ore and 10 tons of concentrates were reported for the 2- year period. 9 A 10-year lease upon the district was negotiated in 1961 by

Northern Continental Incorporated, a uranium producer in Grand Junction,

Colorado, and a subsidiary of Continental Materials Corporation of

Chicago„ Northern Continental explored and developed 1,200 feet of the

Gold Bug vein, shipping 5,266 tons of selected ore that averaged 1.06 ounces of gold and 37.06 ounces of silver per ton. They concluded that insufficient tonnage existed to justify further operations at the existing price of gold and silver, and activities terminated in 1964. The major patented and unpatented claims in the district were optioned in 196 7 by

Little Rocky Mining Limited of Vancouver, British Columbia. Exploration work included channel sampling, trenching, and core drilling, principal­ ly in the Gold Bug area. In 1968, Iso Mines Limited of Toronto paid a fee for participation and opened a core-drilling program near the August mine. This program was stopped prior to completion of the second hole.

Little Rocky Mining Ltd. thereupon relinquished their option, and a new option on the district was negotiated by several of the same principals in the name of Niseka Mining Limited. Underground development was begun between the August and Gold Bug veins, with percussion drilling being done both underground and from the surface. This work was stopped in August 1970, when Kaiser Exploration and Mining Company, a sub­ sidiary of Kaiser Aluminum and Chemical Corporation, assumed the di­ rection of further exploration.

The Permanent Individual Mine Record kept by the Albany,

Oregon, office of the U.S. Bureau of Mines reports total production for the Landusky district from 1900 to 1942 as 495,365 tons yielding 116,998 ounces of gold and 912,841 ounces of silver. Production since 1941 has 10 been minor. From September 1970 through July 1971, Kaiser Exploration and Mining Company prepared the reconnaissance map of the Landusky mining district, drilled the eight diamond drill holes totaling 4,284 feet, and channel sampled the 364 feet of underground workings mentioned earlier. Accessible underground workings were mapped. During this last period, I had the opportunity, as project geologist, to assemble the data for the following study of the district's economic geology.

Previous Work '

Published authoritative accounts of the geology of the Landusky mining district are limited. Weed and Pirsson (1896) of the U.S. Geo­ logical Survey published an account of the geology and ore deposits of the Little Rocky Mountains. These men were sent as emissaries of the federal government to determine the mineral resource potential of the area with the objective of withdrawing potentially productive lands from the Indian reservations and opening them to mining.

After the withdrawal of the Landusky mining district from the

Fort Belknap Reservations, Emmons (190 8) published an account of the existing mines in the district. His mineralogic descriptions of the upper portions of the major veins are excellent and are the only published record of this mineralogy. Emmons speculated on the form and origin of the Little Rocky Mountains, describing them as having resulted from intrusion and subsequent doming of the sedimentary rocks by a syenite porphyry laccolith. Emmons' conclusions were based on Weed and

Pirsson's (1896) earlier observations of crystalline schist in deeply eroded portions of the central portions of the Little Rocky Mountains, which they had suggested were the floor of a laccolith. 11 In 1938, Dyson published an article on the Ruby Gulch mining district, now known as the Zortman district, which is approximately 2 to 3 miles northeast of the Landusky district. From his descriptions of the mineralogy and economic geology , the Ruby Gulch district appears to be very similar to the Landusky district. Dyson interpreted the crystalline schists in the central Little Rockies as roof pendants rather than the floor beneath a laccolith. This conclusion is probably incorrect but may have some merit, and it will be discussed in a later chapter.

The field work for the last major geological study of the Little

Rocky Mountains was done by Knechtel,.who mapped the geology and described the stratigraphy of the Little Rockies during the period of 1936 to 1939. His map and report were not published until 1959 (Fig. 2, in pocket). Knechtel1 s work has been considered by field geologists to be the most authoritative geologic data on the range.

Bryant (1953) published a first-hand account of the history, development, and production of the Landusky district. Through his long and full career as a mining engineer in Montana during the early 1900's, he had experienced much of the colorful history of the Landusky district that his article relates. REGIONAL GEOLOGIC SETTING

The Landusky mining district lies well within the Little Rocky

Mountains (Fig „ 2, in pocket). The origin and character of these moun­ tains and several other ranges in central Montana have a bearing on the type of rocks exposed and the type of mineralization represented by the veins of the Landusky district. The Little Rocky Mountains consist of a cluster of twelve peaks, which have been interpreted to be intrusive domes (Weed and Pirsson

1896). The cores of these domes consist almost wholly of syenite por­ phyry. The domes are surrounded by quaquaversally tilted sedimentary rocks, ranging in age from pre-Belt (Precambrian) to Tertiary (Fig. 2) „

The syenite porphyry intrusive unit is thought to be laccolithic and to be underlain by pre-Belt metamorphic rocks (Weed and Pirsson, 1896), although the Precambrian rocks appear to have been elevated in the cen­ tral portion of the mountains (Dyson, 1938). It is now commonly aigreed by geologists who have worked in the district that the alkalic mass which makes up the central portion of the Little Rockies and the cores of the minor domes within them is laccolithic (Knechtel, 1959) rather than stock-like as Dyson (1938) originally concluded.

The Bearpaw Mountains to the northwest are covered by exten­ sive flows of tr achy tic to phonolitic composition and are thought to be derived from syenitic laccoliths (Weed and Pirsson, 1896; Pecora, 1962). The Bearpaw range also contains carbonatites related to the

12 13 alkalic intrusions (Pecora, 1962) „ The Little Rocky and Bearpaw Moun­ tains are thus of similar age and origin. The Judith Mountains, 60 miles southwest of the Little Rockies and northeast of Lewistown (Fig. 1), are little more than an eroded al­ kalic syenitic laccolith (Fenneman, 1931). The Judith Mountains contain telluride mineralization similar to that in the Landusky district. A further parallel between the Little Rocky and Judith Mountains is the occurrence of tinguaite, a rare phonolite, in dikes and along contacts in both ranges

(Weed and Pirsson, 1896). Furthermore, a similar Tertiary age is recog­ nized for the two ranges (Fenneman, 1931).

The volcanic Highwood Mountains, 36 miles east of Great Falls (Fig. 1), were originally described by Weed and Pirsson (1901). Up­ dated information has more reccently been provided by Hyndman (1972), who describes these mountains as classic examples of potassium-rich basaltic rocks with related syenite laccoliths and phonolite dikes.

Hyndman generalizes that the Highwood Mountains are a group of lacco­ liths, stocks, dikes, and related extrusive rocks, intruding and cover­ ing flat-lying Cretaceous sedimentary rocks . Following the intrusive and extrusive phase, mid-Tertiary volcanic eruptions of potassium-rich quartz latite composition built up a volcano several thousand feet high. The original volcano was deeply eroded, and then renewed volcanism rebuilt the volcano. The second-generation igneous rocks consist of leucite, analcite, and pseudoleucite phonolites of a potassium-rich basaltic suite, which are rich in mafic minerals (Hyndman, 19 72).

The most famous of the nine laccoliths in the Highwood Moun-. tains is the Shonkin Sag, which is about 250 feet thick in the center 14 and thins to 125 feet on its margins (Hurlbut and Griggs, 1939). This laccolith exhibits a layering related to differentiation and marginal chill­ ing. The Shonkin Sag displays three main rock types: shonkinite, pseudo- leucite, and syenite (Hurlbut and Griggs, 1939). The northern Crazy Mountains, about 45 miles northeast of Bozeman (Fig. 1), are prime examples of nepheline syenites and soda- rich intrusive rocks (Hyndman, 19 72). Here Tertiary igneous rocks in­ trude Cretaceous-Paleocene sandstones and shales. The exposed ig- ( neous rocks consist of an alkali-calcic group overlapping in time with an alkalic group (Wolff, 1938; Simms, 1966). The alkalic to subalkalic groups are characteristic; of the northern part of the Crazy Mountains.

These rock types occur as laccoliths, sills, and dikes of syenite, neph­ eline syenite, augite-rich nepheline syenite, trachyte, leucocratic mon- zonite, latite, rhyolite, basalt, and lamprophyre (Wolff, 1938; Simms, 1966). Larsen (1940) looked at the entire region of central Montana—

Yellowstone Park volcanic region, Crazy Mountains, High wood Moun­ tains, and Bearpaw Mountains—as part of a Tertiary alkalic province.

I would also include the Little Rocky Mountains and the Judith Mountains in this category.

The conclusion may be drawn from the preceding discussion that the region of central Montana is a type locality for Tertiary alkalic igneous rocks in North America. The ranges mentioned previously are of similar age, origin, and compositions, and at least two of them, the

Little Rocky and Judith Mountains, contain similar telluride vein min­ eralization. GEOLOGY OF THE LANDUSKY DISTRICT

The most detailed description of the entire sedimentary column exposed in the Little Rocky Mountains is given by Knechtel (1959). Most of the lithologic descriptions, rock and formation names, and thicknesses of the sedimentary and metamorphic rocks given below are adapted from his report„ The Tertiary alkalic igneous rock descriptions and differen­ tiations are based in part on the microscopic and megascopic descrip­ tions of Weed and Pirsson (1896). The reports of Knechtel and Weed and

Pirsson are of exceptional clarity and perception. The discussion of the geology of the Landusky district that follows is restricted to the area of

Gold Bug Hill and the south slope of Mission Peak.

Rocks

Precambrian Rocks '

The oldest rocks exposed in the mapped area (Fig. 3, in pocket) and throughout the Little Rocky Mountains are believed to be older than the Precambrian Belt series. Most of these rocks are metasediments and metavolcanics. All pre-Belt rocks show moderate foliation, and the de­ gree of regional metamorphism is upper greenschist facies or lower am- phibolite facies. Rocks of sedimentary origin are now biotite schists and gneisses interbedded with metaquartzites, locally with rounded quartz grains. The metavolcanics are now hornblende gneisses and foli­ ated amphibolites. Knechtel (1959) suggests from descriptions of some of the pre-Belt layers that contain augen structures and quartz veins along planes of foliation that the gneisses are in part injection gneisses.

15 Paleozoic Rocks The Paleozoic rocks exposed in the Landusky district range in age from Middle and Upper Cambrian to Upper Ordovician. The basal member of the Paleozoic section, represented by the Flathead Sandstone, nonconformably overlies the pre-Belt metamorphic rocks. The strati- graphic break between the Cambrian and Ordovician units is based on

the disconformity between the Emerson Formation and the Bighorn Dolo­

mite . I Flathead Sandstone. The Flathead Sandstone is the oldest Paleozoic formation exposed in the Landusky district. Typically, it is

concealed by soil, talus, and alluvium. The best exposures are near

the mine buildings in the southernmost area mapped (Fig. 3, in pocket).

The unit consists of argillaceous tan, green, and gray sandstone, inter­

bedded throughout with fine-grained conglomerates. The Flathead Sand­

stone does not exceed 50 feet in thickness, and no fossils have been

found in it. The Flathead Sandstone nonconformably overlies the pre-

Belt metamorphic rocks and is itself conformably overlain by the Emerson . Form ation.

Emerson Formation. The Emerson Formation is included with the

Flathead Sandstone on Figure 3 (in pocket) because of the scarcity of out­

crops. Most of the Emerson Formation, where exposed, is disrupted by

faults or partially covered by superficial debris. Knechtel (1959) de­

scribed the formation and determined its thickness based on the type

section in Phillips County. Where exposed in the Landusky district,

the Emerson consists of greenish-gray shale with thin, intercalated beds

of shale, limestone, dolomite, and edgewise intraformational 17 conglomerates. The thin intercalated beds increase in abundance near the top of the formation. The measured thickness of the Emerson Forma­ tion in its type area is between 950 and 1,100 feet (Knechtel, 1959).

Bellefontia fauna of Early Ordovician age is present in the upper Emer­ son (Knechtel, 1959).

Bighorn Dolomite. The Bighorn Dolomite represents the most prominent ridge-forming stratum and the most consistent unit in the Lan- dusky district. The Bighorn disconformably overlies the older Emerson

Formation. The dolomitic unit in the Little Rocky Mountains is correlated by Knechtel (1959) with the type section of the Bighorn Dolomite in north- central Wyoming because of the similarity of lithology and faunal content.

The lower half of the formation is almost entirely massive dapple-gray dolomitic limestone with a distinctly bluish cast. This part of the forma­ tion contains numerous tiny segments of crinoid stems and less abundant remains of corals, brachiopods, mollusks, trilobites, and spongelike forms. The upper half of the formation is a thinly bedded grayish-white dense dolomite. According to logs from early oil and gas exploration, the Bighorn Dolomite is about 2 75 feet thick (Knechtel, 1959) .

Tertiary Rocks

The Tertiary rocks exposed in the Landusky district are lacco­ lith ic igneous rocks, generally syenitic and monzonitic, with associated dikes of phonolite porphyry. These rocks constitute about 14 percent of the Little Rocky Mountains. While the syenite porphyries, designated as subtypes Tsp-1 (syenite porphyry), Tsp-2 (platy syenite porphyry), and Tsp-3 (Mission Peak porphyritic syenite) on Figure 3 (in pocket), are . 18 visually compositionally identical, they are differentiated on the basis of textural and structural characteristics „ The Tertiary igneous rocks will be discussed in the order of oldest to youngest.

Mission Peak Porphyritic Syenite. The Mission Peak porphyritic syenite (Tsp-3, Fig. 3) is typical of the coarse facies of alkalic rocks present in the cores of many exposed domes in the Little Rockies. This unit is restricted to Mission Peak within the confines of the Landusky mining district. Weed and Pirssoh (1896) first described this rock type based on megascopic and microscopic examination of several specimens from Mission Peak and Antoine Butte (Figs. 2 and 3, in pocket). They named the rock a granite-syenite porphyry because minor quartz was present in the rock and by early definition the presence of quartz pre­ cluded usage of the word syenite alone.

In hand specimen, the rock consists of a lavender-gray two- feldspar porphyry. Thirty percent of the rock is composed of equidimen- sidnal phenocrysts of white opaque orthoclase, 1 to 3 mm in length „

Large, 10 to 15 mm crystals of pale flesh-colored glassy orthoclase are present in volumes ranging from 10 to 15 percent, and they are the most prominent constituent due to color contrast. Both types of feldspar phenocrysts are euhedral. The rock is evenly stippled with minor dark specks of hematite from weathered biotite books.

Weed and Pirsson (1896) report that the smaller white feldspar phenocrysts found in thin section consist predominantly of orthoclase with minor oligoclase. The groundmass is microcrystalline and consists of turbid orthoclase plus minor albite and subordinate amounts of quartz.

A chemical analysis of the granite-syenite porphyry is reported by Weed and Pirsson (1896, p. 414) as follows:

S i0 2 68.65 A12°3 18.31 Fe203 .56 FeO .08 MgO .12 CaO 1.00 NaoO 4.86 k 2o 4.74 Li20 trace CO t o 2 o MnO trace BaO .13 SrO .10 HgO above 110° .83 H2C below 110° .27 FI trace 00 Cl o s o 2 trace P2 °5 trace

Total 99.88 Coupled with this analysis is the following norm calculation on the same granite-syenite porphyry by Emmons (1908, p. 108):

quartz 20 orthoclase 28 albite - 41 anorthite 5 others __ 6 100

General considerations from. Emmons' (1908) and Weed and Pirsson1 s

(1896) observations lead to the conclusion that the major domes are

truly alkalic igneous masses because of the overwhelming predominance of orthoclase, albite, and oligoclase in the rocks and the almost total

lack of mafic constituents except for very minor biotite. Free silica is

in the form of quartz.

Platy Syenite Porphyry. Platy syenite porphyry was mapped as a separate unit (Tsp-2, Fig. 3) because of its platy nature in weathered 20 outcrops, but careful megascopic examination of hand specimens and microscopic examination of thin sections suggest that it is equivalent to the syenite porphyry designated as subtype Tsp-1. Subtype Tsp-2 is found only along contacts between Mission Peak porphyritic syenite

(Tsp-3) and syenite porphyry (Tsp-1), and its platy nature indicates that it is a structural variant rather than a true mineralogic or petrographic variant of subtype Tsp-1 „ The detailed mineralogic description of sub- type Tsp-1 which follows is equally applicable to Tsp-2.

Syenite Porphyry. The subtype Tsp-1 syenite porphyry is the most widespread syenite unit in the Landusky district. It is composi- tionally identical to the Mission Peak porphyritic syenite (Tsp-3), but the size of the white orthoclase phenocrysts is smaller, averaging 2 mm with very little size variation. Essentially all phenocrysts are euhedral, equidimensional, and evenly distributed. A population of larger pale flesh-colored orthoclase phenocrysts is present in volumes of 2 to 5 percent, averaging 5 mm in length. Syenite porphyry subtype Tsp-1 has a cryptocrystalline groundmass consisting of kaolinized turbid feldspar, probably orthoclase. Very rare, 1 mm in diameter, quartz phenocrysts occur (Fig. 11). In the area of the Landusky mining district, diamond drill core samples of this unit show evidence of flow deformation and mylonitization of feldspar phenocrysts parallel to jointing directions.

Throughout the diamond drill core samples, the feldspars are strongly kaolinized near the veins and a greenish cast is prominent in the matrix.

The centers of the larger flesh-colored orthoclase phenocrysts remain unaltered. Near the mines, the syenite contains pyrite both as dissemi­ nated discrete grains and in veinlets. The pyrite occurs as cubes and 21

Figure 11. Core Specimen and Photomicrograph of Thin Section of Syenite Porphyry (subtype Tsp-1) A. Core sample of syenite porphyry (subtype Tsp-1) shows large pink tabular orthoclase and smaller white tabular plagioclase phenocrysts. B. The photomicrograph shows twin lamellae in plagio­ clase phenocrysts (p) in an equigranular microcrystalline orthoclase (o) groundmass. Crossed nicols, X35. 22 pyritohedrons and ranges from 2 to 5 percent by volume. The veinlets occur along joints and contain pyrite that is dominantly euhedral. Big Eye Porphyry. The name Big Eye porphyry derives from local usage. This unit is a micrphaneritic porphyry with quartz and feldspar phenocrysts 2 to 6 mm in diameter and is tentatively classified as a quartz monzonite porphyry. Megascopically, the Big Eye porphyry is dark gray to green containing 10 to 15 percent by volume quartz phenocrysts in a cryptocrystalline groundmass of feldspar. Microscopically, the ground- mass consists of turbid untwinned feldspar with less than 1 percent by volume chlorite after biotite. The feldspar is orthoclase (Fig. 12).

This rock type has not been described in the literature, although it is exposed over more than a third of the Landusky mining district (Fig. 3, in pocket). The Big Eye unit has long been recognized by miners in the district who gave it.its name. It occurs as radial dikes cutting Mis­ sion Peak porphyritic syenite in the vicinity of Mission Peak, but the largest mass of the Big Eye unit occurs as a sill southeast of the peak.

In the Landusky mining district, the Big Eye porphyry is mineralized and altered. Like the subtype Tsp-1 syenite porphyry, the Big Eye porphyry contains up to 5 percent by volume disseminated euhedral pyrite as pyritohedrons and cubes; the pyrite veinlets occur along joints. The contacts between the Big Eye porphyry and the Tsp-1 syenite porphyry in the Landusky mining district are generally gradational.

Possum Porphyry. The Possum porphyry represents a departure from the Tertiary rocks previously described. It was first recognized and described by Weed and Pirsson (1896), who thought that it was a contact form of the main igneous mass at its contact with the basal quartzite of Figure 12. Hand Specimen and Photomicrographs of Thin Sec­ tion of Big Eye Porphyry

A. Hand specimen of Big Eye porphyry breccia from the 550- foot level of the Gold Bug vein shows successive layers of alpha quartz, microcrystalline sphalerite in goethite, and a final layer of botryoidal p y rite . B. Photomicrograph of thin section from hand specimen shows euhedral orthoclase phenocrysts (o) in a turbid kaolinized nonequigranu- lar microcrystalline groundmass of orthoclase with minor quartz. Cubic disseminated pyrite occurs throughout section. Crossed nicols, X35.

C. Another field of the same thin section shows typical quartz phenocryst (q) and alpha quartz veinlet. Crossed nicols, X35. 23

Figure 12. Hand Specimen and Photomicrographs of Thin Sec­ tion of Big Eye Porphyry 24 the Cambrian Flathead Sandstone. Weed and Pirsson found a similar rock type associated with the Spotted Horse mine in the Judith Moun­ tain s .

The Possum porphyry is a variety of phonolite called tinguaite,

"a dike rock composed of alkalic feldspars, nepheline, and alkalic pyroxene and amphibole. The rock is commonly porphyritic and the mafic constituents have a characteristic crisscross orientation in the ground- mass" (Glossary of Geology and Related Sciences, American Geological

Institute, 1960, p. 299). The Possum porphyry is a dense dark-green porphyry which appears to be a resinous aphanitic dike rock upon cursory megascopic examination underground. In good light, the abundance of light-brown glassy phenocrysts of sanidine is obvious. As much as 70 percent of the rock is composed of laths and prisms of sanidine, ranging in length from 2 to 13 mm. Where the rock occurs in dikes, the laths show flow banding subparallel to the enclosing walls of porphyry. Rare black prisms of aegirine-augite crystals are present as phenocrysts.

The following description of the Possum porphyry in thin sec­ tion by Weed and Pirrson (1896) is confirmed by my Figure 13. The rock shows large phenocrysts of sanidine and smaller ones of augite in a fine groundmass of alkali feldspar arranged in a trachytoid structure with evident flow structures. Short stout microlites of aegirite appear through­ out the groundmass. Nephelite acts as a cement between the lath-like feldspars, and the rock is best described as a sanidine porphyry. The unit gives off a fetid odor when broken, presumably the origin of the name Possum. 25

Figure 13. Photomicrographs of Thin Section of Possum Porphyry The thin section was made from a hand specimen of Possum porphyry collected from a dike near the August vein. A. Field shows subparallel arrangement of large sanidine laths and nonequigranular trachytoid groundmass. Short stout prisms in groundmass are aegerite crystals. B. Another field of the same thin section shows strong kaolinization. Black cubic disseminations are hematite pseudomorphs after pyrite. Crossed nicols, X35. I

26 In the mapped area (Figs. 3 and 4, in pocket), the Possum por­

phyry occurs only as dikes, which intrude the subtype Tsp-1 syenite porphyry and the Big Eye porphyry along the major faults in the district. • All contacts between the dikes and the porphyries are sharp. As a gen­

eral rule, dikes of Possum porphyry are only slightly mineralized ex­

cept where they have been subjected to repeated movement and fracturing

in the main fault zones, and only then has open-space mineralization occurred.

{

Structure

The Landusky mining district is bounded by two major miner­ alized fault zones, the August-Niseka and the Gold Bug. These struc­ tural boundaries define a district which is 1,500 feet wide and approxi­

mately 3,000 feet long (Fig. 3). On a macroscopic scale (Fig. 2, in

pocket), the main controlling faults of the district are high-angle re­

verse faults which trend northeast and dip steeply northwest. The major

faults become arcuate to the northeast as if they were defining a struc­

tural and intrusive dome centered near Mission Peak. These major faults

in the Landusky district, shown on Figure 2, are interpreted by Durek

(1971) to have resulted from very local structural events. With some reservations, he proposed that the friction between adjacent sequentially

formed domes caused tangential faults to form between domes. The U.S.

Geological Survey investigators for petroleum exploration (Collier and

Cathcart, 1922) concluded that the faults formed during the emplacement

of the younger domes are reverse faults.

Between the August-Niseka fault zone on the northwest of the

district and the ill-defined mineralized Gold Bug fault zone on the 27 southeast are at least four sympathetic subparallel veins (Figs. 3 and 4).

The average strike of the veins is N. 50° E ., and the average dip is be­ tween 75° and 80° NW. No economically significant veins have been discovered outside the confines of the district. Prior to this study, only limited exploration had been carried out on the veins between the two major fault zones. The mineralization to which the faults were subjected will be dealt with in detail under the topic of ore deposits. The major structural difference between the controlling structures and the minor veins is the relative amount of movement which has taken place along the structures.

The August-Niseka fault zone can be traced along strike for

4,000 feet (Bryant, 1953) and is the most prominent structure in the district (Fig. 3). It varies from 1 to 65 feet in width. The fault zone is sheeted, and a well-developed gouge is locally present in underground workings „ Local dip reversal occurs, causing rolls or cymoidal loops in the fault plane (Fig. 4, Section B-B', in pocket). The hanging wall of the mineralized portion of the August-Niseka fault zone consists of syenite porphyry and Possum porphyry. The Possum porphyry dike pres­ ent in the main August workings snakes back and forth-subparallel.to the overall strike of the August-Niseka fault zone. Emmons (1908) reported striations trending N. 35° E. in both the hanging wall and footwall of the August vein. Applying a simple rule developed by Hulin (1929) to the planimetric shape and thickness of the Possum dikes (Fig. 3) and to that portion of the August vein mapped in underground workings (Figs. 5 and

6, in pocket), it is possible to conclude that both major fault zones in the Landusky district had a left-lateral component of movement. In 28 addition, most of the larger stopes on the August vein occur on the flatter parts of the fault plane, indicating by the same rule reverse faulting. Therefore, more throw than heave exists on the tangential fa u lts .

The Gold Bug fault zone which defines the southeastern limit of the district is structurally different from the August zone. Rather than a well-defined sheeted zone with local gouge, the Gold Bug fault zone is a breccia zone trending northeast within Big Eye porphyry. The breccia zone is a result of more than just minor rotation of fragments. The walls of the zone are ill defined and fractured, and abundant open space was developed. The zone can be traced on the surface for 2,000 feet before it is lost under cover, but it reappears on the crest of Gold Bug Hill

(Fig. 3).

A finding of importance is that the trend of the fault zones and veins in the Landusky district is completely different from the trend of the mineralized zones in the nearby Zortman district (Fig. 2, in pocket). This observation emphasizes that the structural control of the telluride district is local rather than regional. VEIN MINERALIZATION OF THE LANDUSKY DISTRICT

Known mineralization in the Landusky district is confined to the area bounded on the north by the August-Niseka vein and to the south by the Gold Bug vein. Between these major veins are at least three subparallel veins which constitute the New Discovery zone. '

I August-Niseka Vein

Historically, the August vein was the major producer of gold in the district. An estimate, based on a few surviving underground sur­ vey maps inspected by me, is that at least 5 miles of drifts, crosscuts, shafts, and other workings were in existence at one time or another. Three thousand feet of strike length of the vein was mined and stoped to a depth of 600 feet below the crest of Gold Bug Hill (Figs. 3 and 4, in pocket). Many of the old stopes along the trend of the August-Niseka vein are open and accessible to a depth of 200 feet or more. The vein was mined oh six major levels approximately 100 feet apart vertically.

The upper levels were ultimately stoped to the surface.

Structurally, the August-Niseka vein shifts from a reddish- brown sheeted zone several feet in width on its extremities to a well- developed mineralized fault.zone along its interior. The open stopes available for study are up to 40 feet wide. The well-defined walls of the fault are here exposed, and slickensides are present on these walls.

The old underground workings of the August-Niseka vein are not acces­ sible except through the open stopes which intersect the surface and

29. 30 these are not considered safe. The vein does not appear to pinch ver­ tically within the depths to which it was mined. Gold values decreased vertically and silver values increased (E. Wieglenda, oral communica­ tion, 1973) „

Mineralogy of the August-Niseka Vein

No underground samples were collected for this study, and the following mineralogy is based on hand samples provided by E. Wieglanda, on several samples still tagged with their locations within the August vein that were found in old mine buildings, and on diamond drill core samples. Examination was with binocular microscope. All samples examined are of slightly brecciated and sheeted syenite porphyry (sub- type Tsp-1) and contain abundant open-space filling and common hydrous oxides. Gangue minerals are alpha quartz, calcite, and banded purple fluorite. Opaque minerals are native gold psuedomorphs after sylvanite, argentite, galena, and abundant late-stage botryoidal pyrite.

New Discovery Zone

The New Discovery zone (Figs. 3, 4, 5, and 7, in pocket) was the area of most intense recent study because it was the area of great­ est economic potential. The zone consists of a series of at least three subparallel veins lying between the August-Niseka and Gold Bug veins.

Most of the diamond drilling, underground mapping, and channel sam­ pling was done to evaluate this zone.

Structurally, the veins which make up the New Discovery zone are less persistent horizontally and vertically than the two major veins.

The veins in the New Discovery zone (Figs. 4, 5, and 9, in pocket) are 31 sheeted zones of coarsely brecciated syenite porphyry (Tsp-1) and Big Eye porphyry. The zones are strongly oxidized and reddish brown to black. Where mapped underground (Figs. 5 and 6, in pocket), the veins range from 6 inches to 5 feet in width. The veins are the areas of most intense sheeting and show only minor movement and less brecciation than the August and Gold Bug veins. Silver and gold values exist only where the sheeted zones and individual shears are coated with dark- brown to black iron and manganese oxides. One vein exposed in the southeast end of the Niseka crosscut (Fig. 5) was unoxidized, and the primary or hypogene textures of the mineralization appear intact. Two impregnated polished sections were made of the vein filling, and a bulk sample from the vein was collected for mineral identification by Lake- field Research of Canada Limited. The assay of this sampled vein was

1,804 ounces of silver and 7.3 7 ounces of gold per ton (Lakefield Re­ search of Canada Limited, written communication, 19 70).

Mineralogy of the New Discovery Zone

: The detailed mineralogy of this zone is of importance because the zone is the most accessible mineralized area within the Landusky district and has not been extensively exploited. The polished sections studied by me came from the fir able pulverulent vein filling in the south­ east end of the Niseka crosscut. The vein is 2 to 6 inches in width and does not contain visible iron oxide staining. Megascopically, the mass of intergrown euhedral alpha quartz prisms line the interior of the vein in a comb structure. The quartz mass contains euhedral pyrite cubes. 32 Coating the quartz terminations is a dull black film 1 to 2 mm thick with visible euhedral pyrite cubes dispersed throughout.

A polished section of the black coating (Fig. 14) was examined at 160X. The section shows a fine mosaic of rounded anhedral freibergite or argentiferous tetrahedrite grains with acanthite filling the small spaces between the grains of freibergite. Several subhedral crystals of hessite are present, one of them partially replaced by native gold. The texture is a breccia showing rotation and grinding of freibergite fragments.

The gray material filling the void space around the mosaic of freibergite is acanthite. No cusp and carle relationships exist between freibergite and acanthite, but small veinlets of acanthite crisscross the fractured freibergite but do not appear to be replacing it. The acanthite is definite­ ly younger than the freibergite. Figure 14A shows an anhedral mass of hessite which contains two veinlets of native gold partially Replacing the hessite. The hessite shows less brecciation than the freibergite.

Figure 14B shows the massive nature of hessite in the polished section.

Both cusp and carie structure and replacement veinlets of acanthite are present in the hessite. Again, the acanthite is younger than the hessite.

The relative age relationship between the hessite and freibergite is not apparent from textural interpretation, but the thermodynamic stability of these two minerals (Kelly and Goddard, 1969) precludes their having . been deposited in equilibrium and indicates freibergite to be older than the hessite. In summary, Figure 14 indicates the minerals,from oldest to youngest, are freibergite, hessite, gold, and acanthite. No evidence of replacement of acanthite by younger telluride minerals was found; therefore, it is concluded that the acanthite is supergene. 33

Figure 14. Photomicrographs of Polished Section of Vein Filling from the N iseka C rosscut

A. The center of the field shows brecciated freibergite (fb) with interstitial acanthite (ac). Hessite (hs) is being replaced by native gold (au) and acanthite. B. Massive hessite being replaced by acan­ thite is shown in lower center of another field of the same polished sec­ tion. Brecciated freibergite is shown in right center. Reflected light, X160. 34 Lakefield Research of Canada Limited examined a bulk speci­ men from this vein by X-ray diffractometry and fluorescence and identi­ fied the following minerals: argentite (acanthite), hessite, empressite, polybasite, freibergite, native silver, and possibly petzite, in order of decreasing abundance,(Lakefield Research of Canada Limited, written communication, 1970).

Gold Bug Vein

The Gold Bug vein (Fig. 3) at the southern limit of the Lan- dusky district, is structurally different from the August-Niseka vein and the New Discovery zone, although mineralization is much the same.

The Gold Bug vein occurs wholly in sheared and brecciated Big Eye por­ phyry. No Possum porphyry dikes are associated with the vein at either the surface or underground (Bryant, 1953). The vein is traceable 2,000 feet along strike and has been mined to a depth of 500 feet below the highest point of its outcrop on the southwest slope of Gold Bug Hill.

Two open adits provide access to the old workings, but flooding of the old drifts precluded detailed mapping. The vein was persistent in the floor of the old 550-foot level. This drift was driven by Northern Con­ tinental Incorporated, the last mining company to pursue mineable ore on the Gold Bug vein. Presently, more than 1,000 gallons of water per minute flow out of the caved adit to this level, which is located near the old mine buildings (Fig. 3). This water flow is the main source for

Montana Creek, Landusky's water supply.

The Gold Bug vein was inspected during December 1970, and observations were made for several hundred feet before access became 35 too difficult. From these observations, it was concluded that the vein zone of the Gold Bug is a coarse tectonic shatter breccia where the main pipelike lodes were mined. Bryant (1953) examined one of these pipelike lodes near the northeast end of the 100-foot level and reported that it continued vertically to the 300-foot level. Generally, the stopes are a few inches to several feet wide. The Gold Bug vein is 1 to 2 feet wide where it is well defined and con­ sists of a sheared, iron-stained zone. Very little fault gouge is present.

E. Wieglenda (oral communication, 19 70) told me that several mineable lodes occurred in the Gold Bug vein where it intersected, engulfed, and displaced shale beds (Emerson?). enclosed in the. Big Eye porphyry. I also observed this same phenomenon. The shale occurs as undistorted subhorizontal beds engulfed in Big Eye porphyry. Other than for minor silicification, the shale appears megascopically unaltered by the por­ phyry.

Following a suggestion made by R. Legg, president of Niseka

Mining Limited, I made a curious find north of the intersection of the upper road with the Gold Bug vein (Fig. 3). The area for approximately

100 feet north of this intersection contains abundant float of coarsely brecciated Big Eye porphyry cemented by quartz veinlets containing a blackish, sticky, tarlike material, definitely a natural hydrocarbon.

The coarse brecciation occurs along the north side of the Gold Bug vein for 20 to 50 feet. The short vein shown on Figure 3 on top of Gold Bug

Hill along the strike of the Gold Bug vein to the northeast is a lenticular breccia zone rather than a true vein. Quartz-cemented, brecciated Big

Eye porphyry from this zone also contains black tarlike material. 36 Mineralogy of the Gold Bug Vein

The mineralogy of the Gold Bug vein is based on observations

by Emmons. (1908) / Bryant (1953), and my examination of five specimens

of Gold Bug ore from the 550-foot level. The gangue minerals of the vein

are alpha quartz prisms, which line fractures in the Big Eye porphyry,

and associated earthy purple fluorite. Banded tabular masses of purple fluorite line open spaces on the 550-foot level. Sulfides consist of mas­

sive botryoidal pyrite in open fractures and euhedral pyrite desseminated in the Big Eye porphyry (Fig. 12). One sample (Fig. 12) shows brecciated Big Eye porphyry with successive layers of quartz, microcrystalline sphalerite in a goethite matrix, and botryoidal pyrite. Tellur ides occur megascopically in discrete dull-black earthy coatings on shears. So- called rusty gold occurs in the upper portions of the vein where the oxi­ dation of tellurides has produced fine gold in spongy iron oxides (Kelly and Goddard, 1969; E. Wieglanda, oral communication, 1970).

Table 1 is a summary of .the native elements and minerals that were identified from the veins in the Landusky district both by Lakefield

Research of Canada Limited and by me. The tabulation includes the elements and minerals identified, their possible source—hypogene or

supergene—and their geographic occurrence.

Wall-rock Alteration

All veins in the Landusky district show the same type of wall- rock alteration associated with vein mineralization. A pervasive sericite envelope is the most conspicuous megascopic feature of the wall-rock 37 Table 1. Summary of the Mineralogy of the Landusky Telluride District (x - actually found; x? = strongly suspected)

Geographic Occurrence Possible Origin August- New Gold Hypo- Super- N iseka D iscovery Bug Elements and Minerals gene gene Vein Zone Vein

Gangue Minerals -

Alpha quartz XX < X X Purple fluorite X X X C alcite X X X Gold and Silver Minerals

Native gold X x l X XX Acanthite XXX?X X? He s site X X?XX? Em pressite XX?XX? Sylvanite X XX Freiberg ite X X?XX?

Sulfides of Minor Importance

Pyrite Botryoidal pyrite X XX Pyrite veins X X X ' X Disseminated _ euhedral pyrite X XXX G alena X XX X? Sphalerite X x 2

Oxides and Oxidation Products

G oethite Colloform goethite • x^ XX Spongy goethite with native gold XXX Hem atite XX X C erussite X X X X Wad XXX X Selenite X X

1. By oxidation of tellurides„ 2 „ X-ray identification, 3„ Kelly and Goddard, 1969„ 38 alteration. The fine sericite envelopes range in width along either side

of veins from 0.5 inch in the Niseka crosscut (Fig. 6, in pocket) to 6 inches along larger veins in the August and Gold Bug workings. The wall rock of the porphyries between the veins contains microcrystalline irregular quartz both as disseminations and void fillings in mylonitized porphyry. During channel sampling of the Niseka crosscut, the rock hardness increased as veins were approached. Upon inspection with the hand lens, I noted the contrast between the strong vitreous luster and the dull appearance of the nonsilicified syenite porphyry and remapped one area in the Niseka adit to determine if a different rock type existed. Thin sections and core samples of the syenite porphyries in the mine areas show kaolinization of the orthoclase and oligoalase.

The specific zones of alteration associated with the veins of the Landusky district, from the vein outward, are:

1. A fine sericite envelope from 0.5 to 6 inches thick is developed

which has totally destroyed the texture of the porphyries in

which it occurs. Both K-feldspar and oligoclase plagioclase

are destroyed in this interval with the production of a fine matte of sericite,quartz and minor kaolin. Both alteration silica and minor primary quartz are evident in the altered syenite.

2. Beyond the sericite envelope a zone of silicification of the

groundmass of the porphyries involved is present. The texture

of the original rock remains. The thickness of this zone varies

- from 1 to 2 feet. Disseminated euhedral pyrite and pyrite vein­

lets are present in this zone. Only moderate to trace amounts

of alteration silicates are noted. 39 3„ Beyond the silicification subzone, the porphyries are weakly kaolinized. Orthoclase phenocrysts contain microveinlets of sericite and quartz, and the K-feldspar crystal margins have

been kaolinized. The plagioclase phenocrysts are kaolinized throughout. The groundmass has been kaolinized, but the relic

texture remains. Disseminated pyrite is present in lesser

amounts than in the silicified zone closer to the structure.

In summary, wall-rock alteration associated with the vein mineralization in the Landusky district consists of sericite + quartz, then outwardly kaolinite. This alteration is presumed to be hypogene and part of the ground preparation of the veins. Alteration silicates do not appear to be a necessary precursor to precious metallization on a local scale, although no firm evidence exists which would either asso­ ciate or dissociate the alteration with the metallization event.

Regional propylitic alteration of the laccolith is present. The laccolith is composed of porphyries of syenite, quartz syenite, and quartz monzonite. All of these rocks are very low in mafic constituents at less than a volume percent. Biotite is the only mafic constituent identified, and the regional effects of alteration have transformed it into chlorite. Partialkaolinization of orthoclase and plagioclase phenocrysts present throughout.the district may be either hypogene or supergene.

Two sulfide disseminations are very conspicuous from drill hole infor­ mation. Euhedral pyrite in volumes up to 5 percent is present in chlor- itized portions of the laccolith. The drill holes which intersected large masses of Big Eye porphyry quartz monzonite (Fig. 4) contained more 40 abundant chloritic alteration presumably after biotite „ A higher mafic content in the Big Eye porphyry is thought to be present at depth. Dis­

seminated galena is locally present in D.D.H. 3 and D.D.H, 5. The galena occurs in vugs rimmed by anhedral pyrite, No fractures or vein­ lets are associated with the disseminations. Sections of core from the two holes were assayed for minor substitution of silver, but none was found„ In only one drill hole, D.D.H. 1, were discrete occurrences of chalcopyrite grains 2 to 5 mm in diameter occurring as disseminations. No holes deeper than 1,000 feet have been drilled in the Lan- dusky district to determine if vertical zoning is present in the laccolith, nor have any of the diamond drill holes intersected its floor. The ques­ tion of a silver analog to the disseminated copper deposits of the South­ west still remains unanswered at depth.

Paragenesis

Three facts are basic to any model for a paragenetic sequence for the minerals present in the Landusky district: (1) no one specimen showed all of the primary minerals listed in Table 1; (2) accessibility of the old levels in the August-Niseka and Gold Bug veins is such that only samples of the ore were examined rather than good cross sections of the veins; and (3) the veins in the New Discovery zone, which is best understood, may be younger than the August-Niseka and Gold Bug veins, as indicated by gangue mineralogy of the New Discovery zone which lacks fluorite and calcite. The Zortman district (Dyson, 1938), mentioned earlier under previous work, is similar geologically, and Dyson's (1938) paragenesis of the district mineralization is in part 41 referred to. The framework of the mineralization sequence of the Lan- dusky district is:

1. The initial ground preparation of the district consisted of tan­

gential shearing and faulting between the individual domes of

the laccolith (Durek, 19 71). The faulting created dilatant

areas of low pressure and open space for later hydrothermal flu id s.

2. Following faulting., the open faults, shear zones , and brec- ciated zones were invaded by hydrothermal fluids which altered the wall rocks of the structures to sericite accompanied by a

silicification and kaolinization of feldspars.

3. The major structural zones of the district, the August and Gold

Bug vein zones, were initially invaded by hydrothermal solu­

tions that precipitated quartz, pink calcite, minor amounts of

galena, sphalerite, and the sulfosalts .freibergite and poly-

basite. Textures of this phase of mineralization are typical of

open-space filling; minerals are euhedral and crystal faces of quartz and calcite are well developed in vugs . Pyrite accom­

panied this early phase as well as all other phases of mineral­

ization. The pyrite of the early phase occurs as anhedral

disseminations and in contact with galena. The New Discovery

zone was mineralized during this phase with quartz, sphalerite,

galena, freibergite, polybasite, and euhedral pyrite.

4. Brecciation in the August and Gold Bug vein zones and minor

brecciation in the New Discovery zone followed phase 3 . The

precipitation of fluorite followed movement along the Gold Bug 42 and August veins, and the brecciated textures were cemented

by fluorite and quartz. The New Discovery zone contains no visible fluorite. 5. Closely following phase 4 in time was the precipitation from hydrothermal fluids of sylvanite, hessite, empressite, native

gold and pyrite. This phase of mineralization is present

throughout the district.

The mode of transport of metals has been investigated for years without being totally explained. Kelly and Goddard (1969) have investi­ gated in detail the transport of tellurium at Boulder County, Colorado.

They concluded that Eh-pH relationships at room temperature preclude any important role of bitelluride ion transport because of its stability under extremely reducing conditions. These investigators go on to say that the soluble complex telluride ions are unstable over the ranges of acidities and oxidation potentials present in a.hydrothermal environment.

Kelly and Goddard favor the transport of tellurium and other metallic ions as a soluble chloride complex, such as TeClg=, AuCl^", AgCl2" , and

AgCl4 =. No experimentation in the system Te-HgO at elevated tempera­ tures. has been done. SUPERGENE ENRICHMENT

The following evidence concerning supergene enrichment is based on polished section examinations and a comparison of assay values in the plane of the Niseka adit (Pig. 8, in pocket). The conclu­ sions are offered with caution.

In polished section (Fig. 14), the only mineral identified that may be supergene is acanthite. No definitive replacement boundaries were observed between acanthite and freibergite, but small irregular acanthite replacement veinlet and cusp-and-carie structures occur in massive hessite and may be supergene. Some of the black oxide mapped underground may be similar to that that Kelly and Goddard (1969) have described as acanthite grease in the telluride veins of Boulder County,

Colorado. Gold and primarily silver assays are higher where the black grainy material coats minor shears.

Figure 8 shows the assay values in the area of the New Dis­ covery zone. All drill holes intersecting sections of the veins in the zone have been projected into the plane of the Niseka tunnel. Compari­

sons of three pairs of drill holes and their assay values are noteworthy.

D.D.H.-5 and D.D.H.-8 cut sections of the New Discovery veins at an average elevation of 4,650 feet, which is approximately 100 feet be­ low the plane of Figure 8. The assay values for both drill holes are very nearly the same. D.D.H.-2 and P.D.H.-4 penetrate appreciably above the plane of the construction and show similar assay values for silver and gold, which are higher in value than those of the first pair.

■ 43 44 P.D .H .N .-l and the channel sample cut in the Niseka crosscut (Figs. 6 and 8) are at almost the same elevation, the plane of Figure 8. The assay values for this pair are much higher than those for the first two pairs, and the assay values for silver in this pair are within two-tenths of one another„ It is therefore suggested that the Niseka tunnel at its elevation of 4, 765 feet represents an enriched perched pocket above the oxidized-unoxidized line shown on cross section B-B' (Fig. 4), which is based on the disappearance of goethite and the appearance of pyrite.

Whether the enrichment is due to supergene chemistry is unanswered, and it must be recalled that precious metal deposits of epithermal af­ finity are notoriously not persistent.

Telluride deposits, such as Cripple Creek, Colorado (Lindgren and Ransome, 1906) and Boulder County, Colorado (Kelly and Goddard,

1969), contain appreciable amounts of rusty gold in the upper oxidized portions of the veins, which is a finely divided mixture of goethite and gold. The same association was abundant in the upper levels of the

August vein where a sample was collected by E. Wieglenda by panning the oxidized friable material from the August vein. I examined the papned material microscopically and found that it was composed of native gold and goethite. Whether this type of gold can be considered supergene enrichment is a matter of usage. Ransome's original defini­ tion, as cited in the Glossary of Geology and Related Sciences (American

Geological Institute, 1960, p. 287), pertains to "ores or ore minerals that have been formed by generally descending water. Ore or minerals formed by downward enrichment." Experimental work by Kelly and God­ dard (1969) indicates that oxidation of such minerals as sylvanite and hessite in the presence of pyrites will produce native gold such as that described as rusty gold. Redistribution of gold is not necessarily indi­ cated. The amount of gold in the rock remains the same, but the specific

\ gravity of the rock has been decreased by the removal of silver, tellurium, and pyrite. Therefore, the ounces of gold per ton has been increased, which may be considered a form of enrichment. GEOTHERMOMETRY OF THE LANDUSKY DISTRICT

Information on the temperatures and pressures during formation of the veins in the Landusky mining district can be obtained primarily from the telluride mineralogy of the New Discovery zone. Only tentative conclusions on the temperatures and pressures of formation can be drawn from the sample material because most of it was iron stained, oxidized, and friable. Most of the ore minerals were identified by X-ray methods. Kelly and Goddard (1969) list a summary of useful geothermom­ eters applicable to telluride and associated mineralization derived from their work on the telluride deposits of Boulder County, Colorado. Based on their work, several conclusions can be drawn with respect to the temperatures and pressures which prevailed during mineralization of the veins of the Landusky district.

Kelly and Goddard state in their summary that an empressite- bearing mineral suite like that present in the Landusky district crystal­ lized below 210°C, the maximum temperature of empressite (AgTe) stability. Empressite was identified only in the New Discovery zone.

Low temperatures of formation are also indicated by the presence of frac­ tures containing colloform goethite (Lakefield Research of Canada

Limited, written communication, 1970) in the Niseka crosscut. The colloform goethite of the New Discovery zone is much like that described by Kelly and Goddard (1969) as hypogene material in some of the Boulder

County ores, and its occurrence thus provides another useful

46 47 geothermometer in the Landusky district. The maximum temperature placed on hypogene goethite by these investigators lies between 132°C and 148°C. They also place the maximum temperature for the associa­ tion sylvanite-hessite below 170°C; this association is almost certainly present in the New Discovery zone. Polished sections examined did not show equilibrium mutual boundary associations of hessite and sylvanite, but these minerals are present as the two major tellurides identified by me in the district. The wall-rock alteration in the New Discovery zone (sericite- quartz-kaolinite) indicates temperatures of formation below 250°C

(Kelly and Goddard, 1969), although kaolinite stability up to 350°C is indicated by experimental work by Hemley and Jones (1964) . The probable minimum temperatures at depths of 2,600 to 4,600 feet, using a thermal gradient of 50°C/km and a mean surface temperature of 32°C, would be between 70°C and 100°C, which gives a lower limit for the temperatures of formation for the New Discovery vein mineralization. Therefore, the thickness of the sedimentary cover could not have been much greater than 4,600 feet. GEOLOGIC HISTORY OF THE LANDUSKY MINING DISTRICT

The preintrusive history of the Little Rocky Mountains area is

described in detail by Knechtel (1959) and will not be reviewed here

directly so that the complex history of the formation of the present

Little Rocky Mountains and the Landusky mining district can be covered in detail „ ,

Following the deposition of the Cretaceous Bearpaw Shale in central Montana, the Laramide orogeny of the main Rocky Mountains province may have created foreland stresses in the pre-Tertiary sedi­ mentary and metamorphic pile to the east of the main Rocky Mountains.

It is further proposed that the stresses created by the main Laramide orogeny were distributed and released along major regional lineaments

(Durek, 1971) and associated local and regional structural arching. One of the local arches was centered near the present site of the Little Rocky Mountains. A local structural arch in the vicinity of the present

Little Rocky Mountains may explain the demonstrated accumulation of hydrocarbons in the vicinity of the Gold Bug vein, the methane, trapped in the Possum porphyry dikes (J. J. Durek, oral communication, 1973), and natural gas accumulations in minor domes associated with the main

Little Rocky Mountains (Collier and C athcart,. 1922; Knechtel, 1959) .

The hypothesis of a local structural arch or dome would partially answer two other difficult problems associated with the history of the Little Rocky Mountains: (1) the partial erosion and removal of 7,000 feet of

48 49 pre-Tertiary sedimentary rocks (Collier and Gathcart, 1922; Knechtel, 1959) overlying the present site of the Little Rockies prior to the intru­ sion of the laccolith; and (2) the presence of a partially void space in a zone of structural weakness which permitted the intrusion that produced the laccolith.

A quartz syenite porphyry intruded the pre-Belt Precambrian floor through several conduits and formed multiple domes under the pre­ viously arched overlying sedimentary rocks. This intrusion formed the laccolith (Weed and Pirrson, 1896; Knechtel, 1959). Prior to the com­ plete cooling of the quartz syenite porphyry, a second phase of quartz monzonite porphyry intruded the semi-cooled syenite porphyry laccolith, probably through the same conduits, and formed dikes in the cooler por­ tions of the intrusion and irregular sill-like bodies within the uncooled interior of the syenite domes. This phase caused arcuate fractures in the overlying sedimentary rocks and tangential shearing between com­ peting domes within the laccolith (Durek, 1971). The Landusky district appears to be one of those zones of reverse faulting along a shear zone which was tangential to a pair of competing domes. The episode of Pos­ sum porphyry dike intrusion closely followed the formation of tangential shearing. The Possum porphyry intruded the open shears forming dikes and in some places small bosses lying between the top of the laccolith and the base of the Cambrian sedimentary section (Weed and Pirsson,

1896; Knechtel, 1959). The Possum porphyry dikes that occur in tan­ gential shear zones between competing domes of the laccolith sealed these shear zones. Shrinkage due to cooling of the laccolith.must have reopened these shear zones and precipitated movement along them. 50 Mineralization by hydrothermal fluids of the reopened tangential shear

zones, spatially associated with the Possum porphyry dikes, was the last phase of the Tertiary igneous history.

From considerations of geothermometry and depth of burial of the Landusky vein deposits, it may be concluded that at least 4,000 to

5,000 feet of sediments had been removed from the top of the main

Little Rocky Mountains by erosion and gravity faulting (Knechtel, 1959) prior to vein mineralization „ Furthermore, I reconnoitered the Milk River Valley and the Missouri River Basin, historically the two main drainages

\ for the Little Rocky Mountains (Hauptman and Todd, 1953), where more than 1,000 feet of Tertiary and Quaternary sediments are deposited. My observations of these deposits coupled with Knechtel's (1959) mapping of gravity faulting on the basinward margins of the Little Rocky Mountains again urges the conclusion that 4,000 to 5,000 feet of sediments reason­ ably have been eroded from the Little Rockies after Laramide uplift and prior to mid-Tertiary vein mineralization in the Landusky district. The flanks of the Little Rocky Mountains were invaded by the

Keewatin ice sheet at the Illinoian or Iowan stage of glaciation (Haupt­ man and Todd, 1953). The estimated thickness of the ice sheet in the

Milk River Valley north of the Little Rocky Mountains was about 2,500

(Hauptman and Todd, 1953). This was the final geologic event in the evolution of this area of the Little Rocky Mountains.

In summary, a good working hypothesis which will explain the age relationships and compositions of the rock types making up the Ter­ tiary igneous phase of intrusion in the Little Rocky Mountains is develop­ ing. According to Hyndman (1972), the differentiation of an alkali 51 olivine basalt magma derived from partial melting of the deep crust or upper mantle explains both the rock compositions and their sequence of intrusion. The presence of carbonatite (Pecora, 1962) in the nearby

Bearpaw Mountains (Fig. 1) implies that the Little Rocky Mountains lac­ colith and many of the previously mentioned alkalic ranges in central

Montana are related to deep crustal breaks and partial melting of the crust or upper mantle. A COMPARISON OF THE LANDUSKY, CRIPPLE CREEK, AND BOULDER COUNTY TELLURIDE DISTRICTS

Two classic epithermal gold-silver telluride districts of Cripple

Creek and Boulder County, Colorado (Lindgren and Ransome, 1906; Kelly and Goddard, 1969) show many similarities to the Landusky district. The petrologic associations within the three districts being considered pre­ sent some similarities and some dissimilarities among the magmas from which the telluride-rich hydrothermal solutions evolved. Because of the relative depths of erosion at each of the districts, there is a significant difference in the outcrop abundance of Tertiary igneous rocks. For ex­ ample, the Boulder County district contains only remnants of Tertiary intrusive breccias and dikes of biotite latite. All extrusive phases have been removed by erosion (Kelly and Goddard, 1969). The Cripple Creek district proper contains no extrusive flows but rather is characterized by outcrop of part of a complex intrusive neck of a Miocene volcano (Lind­ gren and Ransome, 1906). In the Landusky district and the Little Rocky

Mountains proper, only the very top of the Tertiary igneous mass associ­ ated with the telluride veins has been exposed by erosion.

The Boulder County telluride district contains early Tertiary intrusions in the form of stocks and dikes. The early Tertiary porphyries of the Front Range mineral belt grade in composition from diabase to sodic granite, but the telluride deposits are considered to be associated with the youngest of the Tertiary rocks, which are small dikes and ir­ regular bodies of biotite latite and intrusive breccias. These intrusive

52 53 breccias contain fragments of basement granite and pegmatite (Kelly and

Goddard, 1969; Levering and Goddard, 1938). Most of the telluride veins occur within the Precambrian Boulder Creek granite intruded by the dikes and intrusive breccias. The biotite latite porphyries described by

Kelly and Goddard (1969) are dense, dark-gray rocks which contain X phenocrysts of biotite, hornblende, and andesine in a microcrystalline groundmass of oligoclase and orthoclase. The biotite latite intrusive breccias have a groundmass of microcrystalline laths of feldspar in glass. The rocks associated with the Cripple Creek telluride district are Tertiary intrusive volcanic rocks and breccias of phonolite, latite- phonolite, and alkalic syenite „ The overall composition of the Tertiary

iptrusive rock in this district is close to that of phonolite (Lindgren and

Ransome, 1906). The basic Tertiary rock types that are associated with the telluride veins of the Landusky district are leucocratic syenite, leucocratic quartz monzonite, and tinguaite phonolite. Texturally and compositionally, the phonolites of the Cripple Creek and Landusky dis­ tricts are similar, but the latite-phonolites and alkalic syenites of the

Cripple Creek district are more mafic rich than the alkalic rocks of the

Landusky district. The biotite latite porphyry dikes and the intrusive breccias of the Boulder district are texturally similar to the phonolite dikes of the Landusky district but are compositionally similar to the quartz monzonite of the Landusky district. Spatially, the phonolite dikes of the Landusky district are as­

sociated with the telluride deposit, but the mineralization of the veins definitely occurred after the intrusion of the phonolite dikes. I found no indications that suggest that the Possum porphyry dikes were directly 54 responsible for the mineralization of the telluride veins. It is concluded

from the descriptions of Kelly and Goddard (1969) and Lindgren and Ran­

som e (1906) that the same spatial relationships between dikes and veins

are present in all three districts—the veins closely follow the phonolite or biotite latite dikes, but the mineralization postdates the dike intru­ sions. I consider the phonolites in the Cripple Creek and Landusky

districts and the biotite latite of the Boulder district to be textural indi­

cators of near-surface intrusion and compositional indicators of the end

stage of differentiation of the different magmas.

The hypogene mineralogy of the veins and the paragenetic se­

quence in the Landusky district are comparable to both the Cripple Creek

and Boulder County occurrences. The Landusky district shows early sul-

fosalts associated with galena and sphalerite mineralization followed by late-stage tellur ides and native gold. This same general sequence is

found in Cripple Creek (Lindgren and Ransome, 1906) and portions of the

lead-zinc veins in the Boulder County district (Kelly and Goddard, 1969).

Generally, the Boulder County telluride veins are devoid of the lead and

zinc sulfides, but a small portion of the district contains these minerals

in veins which were later subjected to telluride mineralization. At

Cripple Creek, the main telluride vein minerals are calaverite, sylvanite,

and krennerite. The sulfosalt tetrahedrite (freibergite?) is also present. The mineralogic differences between the Cripple Creek and Landusky districts, in my opinion, may be due to differences in the

magmas that generated the mineralizing hydrothermal solutions in each

district and differences between depth of burial of the intrusions. Recon­

struction of the old Cripple Creek volcanic cone by Lindgren and Ransome 55 (1906) places the present surface of the Cripple Creek district well within 2,000 feet of the original surface at the time of eruption. Several accessory minerals present in the veins of the Cripple Creek volcanic neck, such as cinnabar and stibnite, indicate, at least qualitatively, that the mineralized district was a near-surface phenomenon. A 2,000- foot depth is completely consistent with the predominantly low temperature-pressure mineral assemblages present.

The Boulder County and Landusky telluride districts show marked similarities. The Boulder County district is an erosional remnant of what must have been a very rich gold-silver telluride district (Kelly and Goddard, 1969). The chief ore minerals of the Boulder district are sylvanite, petzite, hessite, and native gold, so the primary hypogene vein mineralogy is more like that of the Landusky district than is that of the Cripple Creek district. Minor sulfide and sulfosalts associated with the telluride stage of mineralization match those found by me in the Lan­ dusky district. From physiographic evidence, the Boulder County veins were formed under rock cover of 2,600 to 4,600 feet. It has been pre­ viously concluded that the depth of burial for the Landusky veins was could not have been much greater than 4,600 feet.

Table 2 is presented as a summary of the pertinent character­ istics of the three telluride districts. Table 2. Comparison of Characteristics of Cripple Creek , Landusky, and Boulder County Telluride Districts

Characteristics Cripple Creek District Landusky District Boulder County District

Rock types phonolite, latite- synenite, quartz syenite, biotite latite, intrusion brec­ phonolite, alkalic quartz monzonite, cias of biotitic latite + syenite phonolite basement materials Structural near-vertical discontin­ high-angle reverse faults steep fissures in granite, control uous fissures, roughly and shear zones, con­ continuous for up to 1.5 radial in plan, occur in tinuous for 3,000 to miles in length; sense of ancient volcanic neck; 4,000 feet along strike movement unknown sense of movement unknown Ore mineralogy calaverite, sylvanite, sylvanite, hessite, sylvanite, petzite, hessite, krennerite, molybdenite, native gold, freibergite, native gold, calaverite, sphalerite, galena, stib- acanthite, empressite, krennerite, empressite, nite, tetrahedrite, sphalerite, galena minor sulfosalts and sulfides huebnerite, cinnabar Gangue quartz, chalcedony, opal, quartz, calcite, quartz, roscoelite, ankerite, mineralogy fluorite, calcite, dolo­ fluorite, botryoidal calcite, fluorite, barite, mite, rhodochrosite, pyrite, collOform colloform goethite barite, celestite, goethite wavellite, adularia, sericite, roscoelite Depositional open-space filling, vug- open-space filling, open-space filling, vuggy, character gy, coarse euhedral vuggy, coarse euhedral coarse euhedral crystals crystals; some replace­ crystals ment textures Wall-rock sericite, adularia; sericite + quartz + sericite + quartz + kaolinite alteration potassic alteration,(?) kaolinite + chlorite yielding outward to propylitization Table 2 „ Comparison of Characteristics—Continued

Characteristics Cripple Creek District Landusky District Boulder County District

Pertinent (not studied) empressite, hypogene empressite, hypogene geothermpmeters colloform goethite, colloform goethite, altera­ alteration assemblage tion assemblage of sericite of quartz + sericite + + quartz + kaolinite; kaolinite; sylvanite- mineral pairs of sylvanite- hessite pair petzite, sylvanite-hes site; quartz-fluid inclusion tem­ peratures ECONOMIC EVALUATION OF THE NEW DISCOVERY ZONE

A calculation of the potential tonnage and value of the area designated as the New Discovery zone (Fig. 7) was made. The follow­ ing approach was followed: 1. The mineable strike length of the New Discovery zone extends

from point F to point G along the line shown on Figure 3; the

measured length is 1,380 feet.

2. The width of the mineable zone was taken as 122 feet,, which

is the normal distance across which channel sampling in the

Niseka crosscut delineated a mineable ore grade (Fig. 6).

3. The average depth of the ore zone was determined from a longi­

tudinal section whose.top is the, line F-G (Fig. 3) and whose base is a line connecting the elevations of the points repre­

senting the pxidized-unoxidized boundaries shown on cross-

. sections A-A', B-B', and C-C (Fig. 4). The average thickness

is 250 feet. .

4. The grade was determined by averaging the assays of three

oblique intercepts: (a) a 164-foot section of P.D.H.N.-4

drilled by Niseka Mining Ltd; (b) d d 22-foot section of the

Niseka crosscut; and (c) a 120-foot section of D.D.H.A.U.-8 drilled by Niseka Mining Ltd. All three intercepts cross the

New Discovery zone. The average silver value was 2.0 ounces

per ton, and the average gold value was 0.03 ounces per ton.

58 59 5. The factor 12.0 used to determine cubic feet per short ton was

determined from core recovered.

Using these parameters and a price of $1.70 per ounce for silver and $40.00 per ounce for gold, it was calculated that the mineralized area of the New Discovery zone contains 3.5 million tons of probable and inferred ore valued at $4.60 per ton. This tonnage estimate is a maximum figure. SUMMARY

It is my strong conviction that the Little Rocky Mountains and

the Landusky telluride district are the result of multiple shallow intru­

sions that produced doming in the irregular ceiling of a resulting

laccolith . Observations in the Little Rocky Mountains by previous in­

vestigators have been limited to the surface and very near surface erosional expressions of the alkalic igneous core of the range and the

resultant domed sedimentary rocks. Surface observations and the infor­

mation extrapolated from diamond drill core samples strongly suggest

that the intrusive mass forming the laccolith contains disseminated

mineralization, alteration, and areas of discrete epithermal vein min­

eralization . Both the Landusky and the nearby Zortman telluride districts

are examples of local structural events related to the formation of the

multiple-domed ceiling of the alkalic igneous mass of the Little Rocky M ountains. The tangential stresses imposed on the area between two ig­

neous domes were released along marginal shear zones which developed

into major high-angle reverse faults with the continued elevation of one

or both of the competing embryonic domes. The reverse faults became

arcuate on their extremities, curving around the more highly elevated

dome „ These arcuate zones of reverse faulting allowed the rapid release of both internal rock stress and confined pressure. The release of the pressure allowed the rapid migration of phonolitic magma that invaded the reverse faults and formed the dikes of Possum porphyry that sealed

60 the fault zones „ The Possum porphyry did not break through the overlying sedimentary rocks to form extrusive flows, although extrusive flows do occur in many of the surrounding Tertiary ranges of central Montana.

The residual hydrothermal fluids associated with the end stage of the alkaline olivine basalt magma proposed by Hyndman (1972) were trapped at depth in the laccolith by the healing of the fault zones until further movement, caused either by shrinkage due to cooling of the lac­ colith or by a slight reelevation of the domes, reactivated the areas of reverse faulting and allowed the slow escape of the hydrothermal fluids along the reopened fault zones. These zones were altered and mineral­ ized during several such stages of movement and slow release of pres­ sure.

In retrospect, the domes of the Little Rocky Mountains are little more than aborted volcanoes which were never quite forceful enough to break through the overlying sedimentary rocks. The energy of the intrusion was dissipated through several conduits leather than con­ centrated in one conduit as at Cripple Creek. The multiple conduits and their associated competing domes of rising magma dispersed the energy by internal friction over a broad area, and the intrusion therefore never reached the surface or even appreciably broke into the overlying arched sedimentary rocks.

The Landusky district and the Little Rocky Mountains are of importance to the understanding of shallow laccolith formation and as­ sociated vein mineralization. The Landusky district will undoubtedly be revisited and reevaluated and a better understanding of the processes of formation should evolve. . 62 This study produced no data from which to conclude that a silver analog to porphyry copper deposits exist in the area, but no deep drilling has been done in the Landusky district. Therefore, no determin­ ation can be made as to the extent at depth of the disseminated sulfides and propylitic alteration observed in the district or whether these phe­ nomena are associated with a major disseminated mineral deposit. REFERENCES

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63 64

Knechtel, M. M ., 1959, Stratigraphy of the Little Rocky Mountains and encircling foothills, Montana: U.S. Geol. Survey Bull. 1072-N, p. 723-729.

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Pecora, W. T., 1962, Carbonatite problem in the Bearpaw Mountains, Montana, in Petrologic studies: Geol. Soc. America, p. 83-104. Simms', F. E., Jr., 1966, The igneous petrology, geochemistry, and structural geology of part of the northern Crazy Mountains, Montana: Ph.D. dissertation. University of Cincinnati; D issert. Abs,, sec. B, Sci. Eng. 27, no. 6, p. 1991B.

Weed, W. H. and L. V. Pirsson, 1896, The geology of the Little Rocky Mountains: Jour. Geology, v. 4, p. 399-428.

Weed, W. H. and L. V. Pirsson, 1901, Geology of the Shonkin Sag and Palisade Butte laccoliths: Am. Jour. Sci.., v. 12, p. 1-17.

Wolff, J. E. , 1938, Igneous rocks of the Crazy Mountains, Montana: Geol. Soc. America Bull. 49, p. 1569-1628. 7 3 55 5000 FIGURE-9 SECTION ALONG D.D.H.-5 trench BEARING S54E AT -45°, SECTION LOOKING NE,

LANOUSKY MINING DISTRICT, 4950 D.D.H.-5 (27702 N 28687 E) PHILLIPS COUNTY, MONTANA

IVew Discovery Tunnel

4900

T sp- EXPLANATION 4850

Tp Possum porphyry 4th Level New 400 Level Tb Big Eye porphyry

4800

Tsp- Tsp-I Syenite porphyry

Tsp- Niseko Tunnel X Veins 4750 Tsp-I Underground □ Workings Diamond drill hole showing intense X jointing 4700

shear zon

shear zone 4650 Tsp-I

Tsp-

4600

Total Depth 483 feet

50 100 FEET

SCALE George L. Richardson, 197 3, Geology Thesis

FIGURE-7 BLOCK DIAGRAM OF ASSAY COMPARISON AT THE ELEVATION FIGURE-8 THE NEW DISCOVERY ZONE OF THE FROM SECTIONS B-B* AND C-C', NISEKA TUNNEL, LANDUSKY MINING DISTRICT, LANDUSKY MINING DISTRICT, PHILLIPS COUNTY, MONTANA PHILLIPS COUNTY, MONTANA

001 - 0 9

D.D.H.-5 "

0.02 - 2. 8

70

— 5000 D.D.H.-5 D.D.H.-2

o - 4900

— 4800 chonnel 85 — 4700 0 03 - 4 8 49C0 — D.0.H.-8 4600 4800 J P.D.H. N-4 «-IO

4 7 0 0 -

4600 45 85 7/5' EXPLANATION EXPLANATION

Little Ben Projected vein A vein 0 03-2 3 intercepts

0 200 400 FEET New Discovery Gold-silver ossoys B i— vein resp. (0%/ton) 0-01-2 3 SCALE Niseko Mining Ltd. ossoys D.0.H.-2 El. 4765 C Unnomed vein Diomond drill hole

D.D.H.-5 RO.H. N-l 001- 0-7 1 Diomond drill hole Percussion drill * 10° hole " k D.D.H.-8

0 50 100 FEET SCALE

FIGURES 7a 8

George L. Richordson, 1973, Geology Thesis 1 4 7 3 /

FIGURE - 4 GEOLOGIC CROSS SECTIONS, LANDUSKY MINING DISTRICT, PHILLIPS COUNTY, MONTANA

Position of sections shown by letters on figure 3. Sections Looking Northeast Explanation some as figure 3.

5400 D.D.H.-6 August vein 5 3 0 0 - New Discovery 1 zone Tb 5 2 0 0 - D.D.H.-7 August zone New Discovery , . zone I 50 0 0 - Tsp-1 5000 — D.D.H.-5 D.D.H.-2 0.0.H.-4 D.D.H.-3 -August New Discovery 5100 - zone I zone , 4900 — Tp-L I New Disc, D.D.H.-8 Little Ben adit 5000— vein 4900 — Tb ox Tsp-I 4800 - Tb unox 4th Little Ben 11 Little Ben iji open New40i " . -v»m . . — vein 4900— 1.1 stope 4800— Niseko 4 7 0 0 - Niseko vn. shear. Tp_L// vein Tsp-I zone 4 7 0 0 - 4800— ox ox 4600— unox ■5th level unox 4 7 0 0 - 4600— 1—Drain tunnel Tsp- 4500 -

200 400 FEET

SCALE

George L. Richardson, 1973, Geology Thesis M73 FIGURE - 5 GEOLOGY OF THE UNDERGROUND WORKINGS, AUGUST - LITTLE BEN ZONE, LANDUSKY MINING DISTRICT, PHILLIPS COUNTY, MONTANA

flaky Mn0 2

"D " "E" banded lim. plastic MnO NEW DISCOVERY TUNNEL NISEKA TUNNEL In vein PORTAL ELEVATION 4925 FEET PORTAL ELEVATION 4765 FEET

70 60 2" alteration rims ® on fractures

27500 N

weak mineralization

thrust plate ^ghec-r.ng

El. 4765

EXPLANATION no occess X x Possum 50 porphyry n Z 7 b ocky y. N v 8ig Eye porphyry no access N

ilicified 4- Syenite 27500 N + 4 porphyry

Joints h El. 4925 Sheors-dk. 100 FEET v\ when ox. X5 Veins xx Contact George L. Richardson, 1973, Geology Thesis » _ f f ; f / # 7 3

veinX 6

V V

ASSAY PLAN ASSAY v ,z in face O and breccia' and 70 'st shearing 'st O OF OF THE AND AND NISEKA CROSSCUT, LANDUSKY MINING DISTRICT, PHILLIPS COUNTY, MONTANA GEOLOGY GEOLOGY George L. Richardson, 1973, Geology Thesis L. Richardson, Geology 1973, George st ox along ox st shears - t Tsp-h i 7r_ 7r_ T T 1 1 *

FIGURE - 6 FIGURE 0'°2-4.W / ” 0'°2-4.W and and ox sheared s ; . o '- it !9 *- st st ox shears m

/

vein at at base vein df shear/becomei walls and and shear Possum porphyry Possum ton) (Oz/ respectively Big Eye Eye Big porphyry Syenite porphyry Fault Fault Vein: Geologic contact Geologic Joints Oxioized shear Oxioized Gold and silver assays silver and Gold Brecciution EXPLANATION lTr-0-31 °'06'*'< »-9,

1 1 Tsp-I ox ox shears 0-07-42 0-031-1-0

blacky blacky with st silic EL. EL. 4765 2 7 5 0 0 N

26500 E :)413 «p»y X sfO oo EXPLANATION

TERTIARY

Phonolite porphyry - dork groy dikes or sills; generally "POSSUM" fine, accicular texture of chill facies; sonodine laths 1/8-1/2 inch; elsewhere, fetid odor when broken.

Monzonite porphyry (?)-fine texture with quartz "big e y e " phenocrysts 1/16-I/o Inch. Radial dikes on Mission Peak; increasing exposure southward.

Syenite porphyry - fine texture with feldspar SYENITE phenocrysts, only. Irregular silicification* typically shattered with shear deformation.

Syenite porphyry-intermediate facies; ploty. SYENITE Limited exposure; relationship uncertain.

MISSION PEAK MISSION Porphyritic syenite — coarse texture with 1/2 inch feldspar phenocrysts. Restricted to Mission Peak, PEAK but may have aphanite porphyry facies.

PRECAMBRI AN-ORDOVICIAN

Lower half: dolomitic limestone, massive, groy with \ BIGHORN bluish cost in some exposures. Upper half: dolomite, thinly bedded.

PI atlic a n Flathead Sandstone (Cambrian) — mainly sandstone, ' light gray, green and ton. Emerson Formation (Cambrian and Early Ordovician (?))-chiefly gray to greenish-gray r t M t n b U N shale with thin intercalated beds. 'O 4950 o. * P R E -B E L T Metasediments — chiefly biotite schist and gneiss. - Metavolcanlcs -chiefly hornblende gneiss and 4 8 50 Mb I AMUKrHICb amphibolite; and younger pre-Belt rocks.

4900 SYMBOLS

<(/ 20 — Vein Dip and strike of foliation o ' ( o ' / — Fault Dip and strike of bedding 4 9 50 El Shaft Dip and strike of jointing

Diminished confidence of contacts indicated by solid, dashed, and 5000 D.O.H.O Diamond dotted lines. drill holes

5050 cP

TSP-I X,

Tsp-I

'O. oo

/

GOL0/-8UG

f

D.D.H. DATA

1 S52E -4 5 ° 483*

■

4 00 FEET

Contour interval 10 feet Datum is mean sea level / X

RECONNAISSANCE GEOLOGICAL MAP

OF THE FIGURE-3 LANDUSKY MINING DISTRICT, PHILLIPS COUNTY, MONTANA

George L. Richardson, 1973, Geology Thesis

UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY BULLETIN 1072 PLATE 52 EXPLANATION

R 2b E cr R 2 4 ER^ 23 E R 24 ER^ R 25 E Qal UJ > h- cr • < < =D Z / Younger alluvium o f filarial fiepntit* not tho.m J ~ > ? r v i cr < ] > z QTal tr I " ’ f : - ! £ UJ h-

/ U 1 Lamprophyre dike T ra c h y te Aegirine trachyte Syenite porphyry or quartz cr or small boss porphyry dike porphyry dike monzonite porphyry dike <

Tsp

Syenite porphyry

Kb 7 7 Bearpaw shale Ita rk-grau marine thalr. weathering tight grag, a few bede of bentonite and of *and»tone,and [ / / ■ r j t oiling thin bedn and lennen oj chertg material and caUareou* ronrretionii i Kjr

Judith River formation Inti rbedded light-grag to buff toft fine-grained mmdntonr. thole, and grag impure ring A few thin riml hedt in upper part »;■ Mm>xxxX '--VO ("laggett shale A Dnrk-grag ahale and niltHtone. u eathenng to browninh grag. a few bedn of bentonite and bedn containing large hnulderlikc eeptarian ronrretionii m ’7 \ Eagle sandstone, undivided I'pper member rhiefly gragrag thale thale with with many many thin thin bedi bedn o f tilt atone. Mindy ahale. and friable aand- atone that weather a reddiah■ddith fan,tan. Virgelle Virgelle tnndttmaandatone member yellow to buff aandatone. gray Q T al / aHtatone, and gray ahale

Cal ' Kwu * Kw

Warm Creek shale ! =3 ni/iment Warm Creek ahale. undivided. K * I 'p p e i her. Kwu mainly hlmsh-aray ahale containing many large ralrareoua concretiona.'etiona. eaoecialty eayerially in lowerhalf, half. .\fiddle Middle ralrareoua ralrareoua member. member. KwgK 52 (Greenhorn limeatone eguivalent/ inrludea a banal unit, the Monbyaandatone aandatone unit, whichunit, whirh ron- c Mmc siats o f slabby to olaty aandatone containing lenticular bodies ofnhferoua foasiliferous limeatone, limeatone. and andan an overlying unit, which is largely ralrareoua ahale that weathera lightgray gray Lower Lower ahale ahale member. member. < K *b (Belle Fourrhe shale earn valent) mainlydu clay and and ahale ahale with with < ...... a little aandatone and many UJ limestone concretions,I HM weathera ...... light „ gray and I lo w e lou r pa err t opart f m enof member contains many nodulea of heavy black slightly manganiferous siderite '5

K m Qa, MoufTtain X ' pCu Mowry shale Shale, mostly papery to slaty, medium- to dark-gray; weathers bright silvery gray, a thin bed of bentonite in uppermost part 4 Butte Ktc Kt

Thermopolis shale B eavtr Chiefly dark-bluiah-gray shalecontaminanumeroua small ferruginous concretions Sear middle,in Mountain south western part o f area mapped, is rgprian aandatone member. Ktc. made up rhiefly o f sand-

Kcc

\ u First Cat Creek sand of drillers I’pper member dark-gray ahale. siltatone. and argiUareoua sandstone Lower member mostly S A t f t f f i : 7 sandstone, somewhat arkoair. roarae grained, massively bedded j!, Saw rrvi'^ • 3 / / > ' Cha , „

A L A 8 A Ju

X € 1 1 ^ . \M m c\ g Upper Jurassic rocks, undifferentiated Comprises three formations, uppermost of whirh is the Morrison formation largely light-gray mudstone and friable glauconitic sandstone, with a coaly layer at the top Swift formation I ' X 't : I | ZD i (Ellis group), which underlies Morrison, is mostly shale, gypaiferoua. light and dark gray, with BLAINE CO \ ‘.'A1, A ! O Allpark numerous large broum calcareous concretions, layers of glauconitic aandatone. sandy mudstone, dark ahale.and impure limestone in upper part Kierdon formation (Ellis group) is mostly light- to dark-gray marly limestone u-eathering fsilr gray, with some ahale -O Tal

Q T al' Zurtma

K*b ^ Mission Canyon limestone Limestone, mostly coarse grained and massively bedded, locally crossbedded, with nodules and ii I lenses of cherty material, numerous solution rarities in upper part Q _ LU

P e a k 1 I

1 QTal Lodgepole limestone 5 Limestone, richly foasiliferous. thin bedded, with some massive ledges, many small lenses of chert, QTal and thin partings of shale, mostly dark to light gray, but two mnes m upper half are predom­ inantly red. at base is thin black mnodont-hen ring shale o f Little Chief Canyon member S Du

pCu/ Q_ Devonian and Mississippian rocks, undifferentiated Z CL r , Includes three formations Uppermost (poorly esposed! is Three Forks shale (') (Devonian and C r o w n Lookout M ississippian) consisting of calcareous cloy and siltatone, light gray to light green, resting on B utte QXX XA y \ } Mountain/ ) Z __ Jefferson limestone (Deinmani The Jefferson limeatone consists of an upper member lime­ p stone and dolomite, light gray to buff, partly sandy, massively bedded but slobby where greatly O in weathered, a middle member shale, siltstone.and thinly laminated dolomite and limestone, and > in a lower member chiefly limestone, finelu crystalline, dark gray and brou-nish gray, weathering West light gray to whitish lowermost is Maywood formation (Devonian), consisting of shale, silt- mi i ) //r; • Cobu stone. limestone, and dolomite, uoper two-fifths largely bright red. lower three-fifths mainly Stage Roi>te Butte light gray, light green, yellow, and brown, lower 5 fl is pinkish dolomite, silty, platy B utte i r a z < reek Ob u

► > QTal Bighorn dolomite o Q rer half dolomitic limestone, massive, i dopie gray with bluish cast in some exposure ‘athered surfaces commonly pitted. I'pperhalf dolomite, thinly bedded, hard, gray to whit cr O

Joplin e^nc^i < z n Flathead sandstone and Emerson formation, undifferentiated The Emerson formation (Cambrian and Early Ordovician (?)) is chiefly gray to greenish-gray I i shale with thin intercalated beds, increasing in number upward, of shale, limestone, dolomite, oo O and edgewise i ntra format nmal conglomerate The lower formation, the Flathead sandstone (Co in brio n I is mainly so nd.-iom . I ght gray, green and tan. with some interbedded fine-grained conglomerate i§ 1 : U iD pCu 5 is Pre-Belt metamorphic rocks, undifferentiated Q Meta sedimentary rucks, rhiefly biatite srhist_ a nd gneiss, meta volcanic rocks, chiefly hornblende > Kcl gneiss and amphibolite; and younger pre-Belt I*) ferromagnesian rocks forming a few dikes and 1 1 'i No exposure I

C o n ta c t V Dashed where approximately located, dotted where concealed M ine ' \ O k Normal fault, approximately located S p r in g i ^ U. upthrown side. D. down thrown side, dotted where concealed .•SSfiS^f A 1'- Thrust fault, approximately located ! D ry hole T. upper plate, dotted where concealed O

Probable fault G as well

A — H S h a ft Locality referred to in text R 23 E R 24 E R 25 E R 26 E

1 09" 108 Tl NIOR GEOl OGlCAL SUR . t ' WASHINGTON D C Base from plats of the Bureau of Land Management Geology mapped by Maxwell M Knechtel. 1 9 3 6 -3 9 - i ? y Topographic contours for areas within the Fort Belknap GEOLOGIC MAP OF LITTLE ROCKY MOUNTAINS AND ENCIRCLING FOOTHILLS, MONTANA FIGURE - 2 assisted by S R Srockumer. 1936. and by S. W Hobbs. Indian Reservation are copied from plats of the Bureau 1937 and 1938 of Land Management Those for areas outside Reser Scale vation were drawn photogrammetncally m 1938 by E A 4 M iles MONTANA Schuster of the Geological Survey i Contour interval 200 feet 1 0 9" 108 Patnni is Hint ii snt It rt I George L. Richordson, 1973, Geology Thesis INDEX MAP SHOWING AREA MAPPED