The Geology and Geochemistry of the Summit Creek Molybdenum Prospect, Custer County, Idaho

Western Michigan University ScholarWorks at WMU

Master's Theses Graduate College

4-1983

Thomas Murray Hanna

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Recommended Citation Hanna, Thomas Murray, "The Geology and Geochemistry of the Summit Creek Molybdenum Prospect, Custer County, Idaho" (1983). Master's Theses. 1599. https://scholarworks.wmich.edu/masters_theses/1599

This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. THE GEOLOGY AND GEOCHEMISTRY OF THE SUMMIT CREEK MOLYBDENUM PROSPECT, CUSTER COUNTY, IDAHO

Thomas Murray Hanna

A Thesis Submitted to the Faculty of The Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Geology

Western Michigan University Kalamazoo, Michigan A pril 1983

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THE GEOLOGY AND GEOCHEMISTRY OF THE SUMMIT CREEK MOLYBDENUM PROSPECT, CUSTER COUNTY, IDAHO

Thomas Murray Hanna, M.S.

Western Michigan University, 1983

The Summit Creek stock, a molybdenum prospect located in the

Pioneer Mountains of central Idaho, is a 46.7 m.y. old medium

grained, slightly porphyritic quartz monzonite, which has intruded

the late Ordovician Phi Kappa formation. Trend surface analysis

shows that the stock is the product of one intrusive event. Perva

sive mineral alteration usually associated with such porphyry

(stockwork) molybdenum deposits is not present in this pluton, and

the alteration that is present is confined to local faults and

fractures.

Geochemical and petrographic evidence suggest that the pluton

is a fe ls ic d iffe re n tia te from a melt which originated at a depth of

220-260 kilometers along an east-dipping subduction zone. Whole

rock major element geochemical analysis indicates that the stock is .

a high potassium calc-alkaline in trusion . Trace element analysis

shows sim ilarities to other stocks associated with economic molyb

denum deposits. In addition, chemical analysis of a rg i11ic assem

blages are sim ilar to those of porphyry molybdenum deposits.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS

My thanks to Utah International for their financial and

logistical support during the field work on this project. In

particular, I would like to thank Richard J. Thompson who started

me in economic geology and fo r his guidance and stim ulating

discussion of the area. Also thanks to Miles G. Shaw for his

support and help in the field.

Additional thanks go to Chris Schmidt, Cynthia Wood fo r

their review of the manuscript, and Ron Chase for his discussions

in the field. I would like to thank John Grace for his critical

review of this paper and his help with the laboratory procedures.

Thomas Murray Hanna

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HANNA, THOMAS MURRAY

THE GEOLOGY AND GEOCHEMISTRY OF THE SUMMIT CREEK MOLYBDENUM PROSPECT, CUSTER COUNTY, IDAHO

WESTERN MICHIGAN UNIVERSITY M.S. 1983

University Microfilms International 300 N. Zeeb Road, Ann Arbor, MI 48106

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ACKNOWLEDGMENTS ...... i i

LIST OF FIGURES ...... v

LIST OF PLATES ...... vi

LIST OF TABLES ...... v ii

Chapter

I. INTRODUCTION ...... 1

Purpose o f Study ...... 1

Location ...... 1

Description of Study Area ...... 1

Geology o f the Pioneer Mountains ...... 3

Stratigraphy...... 3

Igneous Units . . 8

Structure ...... 10

Regional Ore D e p o sits ...... 11

Plate Subduction and Porphyry Molybdenum Deposits . . . 11

Porphyry Molybdenum Systems in North America ..... 14

Alteration ...... 15

Mineralization ...... 17

I I . ANALYTICAL PROCEDURES ...... 22

Sampling ...... 22

Petrographic Analysis ...... 23

Geochemical Analysis ...... 23

iii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter

I I I . GEOLOGY OF THE SUMMIT CREEK AREA ...... 28

Sedimentary Rocks ...... 28

Igneous Rocks ...... 31

Structure ...... 34

Breccia ...... 36

Mineralization ...... 36

Alteration ...... 38

IV. PETROGRAPHY ...... 40

Quartz Monzonite . . 40

Altered Quartz Monzonite ...... 42

Breccia ...... 43

V. STATISTICAL ANALYSIS ...... 44

Discussion of Trend Surface Analysis Method and Presentation o f Data ...... 44

VI. PETROGENESIS ...... 52

Petrogenesis of the Summit Creek Stock . 52

Emplacement ...... 52

V II. CHEMICAL ANALYSIS ...... 55

Whole R o c k ...... 55

Trace Element ...... 57

Geochemistry of Altered Rocks ...... 61

V III. CONCLUSION ...... 64

BIBLIOGRAPHY ...... 67

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

FIGURE

1. Location map o f the Pioneer Mountains and study area ...... 2

2. General geologic map of Idaho and location of the Pioneer Mountains ...... 4

3. General s tra ti graphic column of the Pioneer Mountains ...... 5

4. Depth to subducting slab for differentiated granitic stocks ...... 12

5. Spatial d is trib u tio n of magmatism and areas of different!'atied granitic intrusions associated with stockwork molybdenum deposits between 52 and 42 m.y. . 13

6. Schematic cross section of a porphyry molybdenum deposit ...... 16

7. Schematic cross section of the Summit Creek stock . . . 29

8. S iltstone o f the Phi Kappa formation ...... 30

9. Greywacke, part o f the Phi Kappa formation ...... 32

10. Quartz monzonite of the Summit Creek stock ...... 33

11. Andesite dike rock that intrudes the Summit Creek stock ...... 35

12. Breccia found in the lower portion of the stock .... 37

13. A rg il!ic altered quartz monzonite ...... 39

14. Photomicrograph of s lig h tly po rph yritic quartz monzonite ...... 40

15. Photomicrograph of a r g il!ic altered quartz monzonite ...... 42

16. Two-dimensional trend surface analysis fo r quartz . . . 47

17. Two-dimensional trend surface analysis for plagioclase ...... 48

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FIGURE

18. Two-dimensional trend surface analysis fo r potassium feldspar ...... 49

19. Two-dimensional trend surface analysis fo r color index ...... 50

20. Comparison of Rb concentrations associated with different whole rock compositions of porphyry molybdenum stocks ...... 58

21. Comparison of Nb concentrations associated with different whole rock compositions of porphyry molybdenum stocks ...... 59

22. Rb/Sr ratio for granite differentiates of porphyry molybdenum deposits ...... 60

LIST OF PLATES

PLATE

1. Outcrop geologic map of the Summit Creek stock and sample locations ...... pocket

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIS T OF TABLES

TABLE

1. Comparison of Molybdenum Deposits in Central Idaho . . . 20

2. Petrographic Modal Analysis Listed in Volume Percent o f Total Volume ...... 24

3. Values Obtained Using XRF, PIXE, and AAS of USGS ' Standard S ilic a te Rocks to Determine Composition. o f Rocks from the Summit Creek Stock ...... 26

4. USGS Standard S ilic a te Rocks Used to Determine Composition o f Unknown Rocks ...... 27

5. Chemical and Mineralogical Parameters fo r Classification of High K Calc-alkaline Rocks and Depth to Subducting Slab, with Comparison to the Summit Creek Stock ...... 53

6. Geochemical Composition of the Major Oxides of the Summit Creek Stock and Compositions from Vario.us Molybdenum Deposits . 56

7. Composition o f A rg il!ic Altered Rocks from the Summit Creek Stock, Henderson, and an Average of Porphyry Molybdenum Systems ...... 61

8. Chemical Change in Unaltered Stock (Table 6) Compared to A rgil lie Altered Assemblages (Table 7) and Change o f A rgil lie Assemblages of Various Porphyry Molybdenum Systems ...... 62

vi i

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I

INTRODUCTION

Purpose of Study

The purpose of th is study was to produce a geologic map of

the Summit Creek stock, the overlying Phi Kappa formation, the

associated structures, the post intrusive quartz veining, the

mineralization, and alteration in order to determine the interrela

tionships of these geologic units and events.

An additionaj purpose was to perform detailed petrographic and

geochemical analysis on the stock in order to compare it with other

stocks in the Rocky Mountain area and to evaluate its potential as

an economic molybdenum deposit.

Location

The Summit Creek stock is located in Custer County, Idaho, in

the Pioneer Mountains on the edge o f the Sawtooth Recreation Area,

55 kilometers east o f the Idaho b a th o lith . The area of study covers

portions of sections 16, 17, 20, 21, 25, 25 o f unsurveyed T6N, R19E

(Figure 1). This area is 24 kilometers northeast of Ketchum, Idaho

and is accessible in the summer by the Mackay-Ketchum road.

Description o f the Study Area

The Summit Creek stock is a slightly porphyritic quartz mon

zonite intrusion and has a K-Ar date of 46.7 m.y. (Armstrong, 1974).

1 '

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2

R18E R19E R20E

MacKay - 30 miles.

STUDY AREA

Hyndman a Peak 7s

17/Sun Valley KKetchum

T3N

Miles 93

km Hail ey T2N DAHO Bellevue

Twin Falls - 70 miles

Figure I. Location map of the Pioneer Mountains and study area. Modified from Dover (1969).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The intrusion crops out along the Tower elevations of the steep slopes

o f the g la c ia lly formed Summit Creek Valley. The stock is exposed on

the south side of Summit Creek. The southern exposure is approxi

mately 4.6 kilometers long and 1.6 kilometers wide and has a vertical

r e lie f of 460 meters.

Geology o f the Pioneer Mountains

The Pioneer Mountains, located in central Idaho, are flanked to

the west by the Idaho b a tho lith and. to the south and east by the

Snake River Volcanics (Figure 2). To the north and including an area

w ithin the Pioneer Mountains is an extensive region predominated by

Paleozoic sedimentary rocks.

Dover (1969) has documented the complex stratigraphy and struc

ture of the Pioneer Mountains. Many of the formations have now been

mapped and studied in detail, and new structural interpretations have

given a more comprehensive geologic history than e a rlie r studies.

Figure 3 is a generalized stratigraphic column for the Pioneer Moun

tain s, modified from Dover (1969).

Stratigraphy

The lowest structural unit containing the oldest rocks in the

Pioneer Mountains is the Precambrian Wildhorse Canyon Migmatic Gneiss

Complex which occupies 50 square kilometers o f the Wildhorse and Cave

Creek areas 8 kilometers southeast of the Summit Creek stock. This

sequence consists o f a basal quartzite u n it, a marble marker horizon,

a middle quartzitic to granitic gneiss unit, a mafic gneiss unit, an

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. K '.] Cretaceous Batholiths

Outcrops of Beltian Rocks

Early Precambrian Cores of Rocky Mountain Uplifts

Snake River Volcanics

P Pioneer Mountains

Metamorphism

and Thrusting

Figure 2. General geologic map o f Idaho and location o f the Pioneer Mountains. Modified from Dover (1969).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TERT CHALLIS VOLCANICS

PENN. WOOD RIVER FORMATION

M IS S . MILLIGEN FORMATION

TRAIL CREEK FORMATION SILURIAN

MIDDLE P H I KAPPA FORMATION ORDOVICIAN

MEMBER H

LOWER MEMBER G

PALEOZOIC MEMBER F

MEMBER E

MEMBER D M

LATEST PC MEMBER C OK AND/OR

CAMBRIAN MEMBER B

MEMBER A

FG WILD HORSE CANYON MIGMATIC GNEISS

UNCONFORMITY THRUST FAULT

Figure 3. General stra tig ra p h ic column of the Pioneer Mountains. Modified from Dover (1969).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6

upper quartzitic unit, and a granitic gneiss unit. These units were

subjected to high grade regional metamorphism, forming upper almandine-

amphibolite assemblages. Dover (1969) suggests a sedimentary o rig in

fo r the entire gneiss complex.

Overlying the Wildhorse Canyon Migmatic Gneiss Complex are

medium to high grade metasedimentary rocks that were divided into

the Hyndman and East Fork formations. The formations have subse

quently been subdivided in to eight units (Umpleby, Westage, & Ross,

1930) (Figure 3).

The Hyndman formation outcrops in the northeastern two-thirds

of the Hyndman-East Fork formation trend and is divided into four

units. Unit A is a pelitic schist, unit B a gneissic quartzite,

unit C is a green banded calc-silicate rock, and unit D is a gneiss-

ose quartzite. The Hyndman formation has a total thickness of

approximately 3,700 meters.

The overlying East Fork formation consists o f the upper portion

o f the metasediments and outcrops in the western th ird of the Hyndman-

East Fork formation trend. This formation is also divided into four

units. Unit E is a dolomitic marble, unit F is a vitreous quartzite,

u n it G is a marble, and u n it H is a q u artzite . The East Fork forma

tion has a total thickness of 410 meters.

Mineralogical and textural studies indicate that the metasedi

mentary rocks of the Hyndman and East Fork formations have undergone

synkinematic isochemical metamorphism in the upper almandine-

amphibolite facies during post-Ordovician regional, orogenic meta-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. morphism (Dover, 1969). The metasediments are considered to be

allochthonous uppermost Precambrian and/or lower Paleozoic.

The remainder of the Paleozoic sections (Figure 3) in the

Pioneer Mountains are represented by the Phi Kappa, T ra il Creek,

Mi 11igen, and Wood River formations. These weakly metamorphosed

Paleozoic argillites and quartzites overlay the crystalline core

of the Pioneer Mountains. The rocks are allochthonous and tectonic

emplacement postdates post-Ordovician metamorphism. Argillaceous

parts of these Paleozoic rocks have been converted to slates and

p h y llite s .

The Phi Kappa formation is exposed in a northwesterly trend

8 kilometers wide along the crest of the Pioneer Mountains. It

consists of black flinty argillite units interbedded with fine

grained quartzites that have a matrix of recrystallized quartz.

This sequence is 915 meters th ic k . The formation has abundant

graptolites which have been paleontologically dated at lower to

upper Ordovician (Charkin, 1963).

The Trail Creek formation outcrops on the west side of the

upper portion o f T rail Creek, and is S ilu rian in age. I t consists

of a siliceous a rg illite , quartz sandstone, and contains many

graptolites.

The Mi 11igen formation is composed mainly of grey, weathered,

phyllitic, argillaceous rocks. Due to the poor exposures, exten

sive deformation, and absence of fossils, the age of the Mi H i gen

formation has not been positively determined. Thomasson (1959)

considers the Milligen formation to be lower Mississippian in age

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8

by stratigraphic correlation. The formation lies in three irregular

belts that trend northwesterly.

The Wood River formation outcrops in the upper plate of an

extensive northwestern trending thrust belt on the west side of the

Pioneer Range. The Wood River formation has been dated Pennsylvanian

through Permian age by stratigraphic correlation (Dover, 1969). The

formation is approximately 2350 meters thick and is widely exposed in

the Pioneer Mountains. The formation contains a basal member o f

brownish quartz conglomerate, a unit of grey limestones interbedded

with conglomerates, and a sandy blue limestone unit containing corals

and brachiopods.

An unnamed formation o f coarse c la s tic and and calcareous Upper

Paleozoic rock outcrops northeast of the Pioneer Mountains ridge

crest. The age has been designated late Paleozoic by stratigraphic

correlation (Ross, 1934). I t consists of four units: 1) a chert-

quartzite pebble conglomerate and breccia, 2) g r itty quartzose to

greywacke sandstone, 3) dark shale, and 4) limestone.

Igneous Units

Synorogenic Jurassic in tru sive rocks form the core of the

Pioneer Mountains. They outcrop in a fan shaped belt that is

approximately 1.6 kilometers wide in the south, and broadens to

9.6 kilometers wide to the north of the Broad Canyon area. The

plutonic rocks have a range in composition ranging from clinopyroxene-

hornblende b io tite quartz d io rite to leucocratic hornblende b io tite

quartz monzonite. The rocks are coarse-grained and range texturally

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9

from nonfoliated to strongly gneissose and porphyritic. The intrusion

has a Pb-Alpha age of 144 m.y. (Dover, 1969).

During Cretaceous time, the Laramide fo ld and thrust b e lt formed

in eastern Idaho. Also at th is time the Cretaceous Idaho batholith

and Thompson Creek stock were emplaced northwest o f the Pioneer Moun

tains, and eugeosynclinal strata in western Idaho were deformed and

thrust against the plutonic rocks of the Cordilleran orogen (Burchfield

& Davis, 1972).

Porphyritic quartz monzonite, granodiorite, and quartz diorite

rocks make up the eastern part of the Atlanta lobe of the Idaho bath

o lith . These rocks form a composite pluton composed of composition-

a lly d iffe re n t phases, having K-Ar dates of 75-100 m.y. (Armstrong,

1974).

Post-orogenic quartz monzonites outcrop near the crest o f the

Pioneer Mountains. The largest of these intrusions.is located along

Summit Creek. This may be related to the 44 m.y. old (Armstrong,

1974) Sawtooth ba tho lith located on the eastern edge of the Idaho

ba tho lith . The Summit Creek stock consists of a medium grained

b io tite quartz monzonite. The quartz monzonite has been K-Ar dated

at 46.7 m.y. (Armstrong, 1974).

Porphyritic dikes with a composition ranging from granite to

quartz diorite cut through all units of the Pioneer Mountains. The

dikes are less than 3.5 meters thick. The dikes are designated

Tertiary in age, due to their spatial relationship with Tertiary

intrusions and the Challis Volcanics (Umpleby et a l., 1930).

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Small areas of T e rtia ry volcanics are described in -the Pioneer

Mountains by Dover (1969). These rocks are considered to be members

o f the Challis Volcanics (Umpleby, 1917; Ross, 1934). The Johnstone

Creek member consists of andesitic and la titic , red-purple to grey

porphyritic flow rocks with plagioclase and quartz phenocrysts. The

White Mountain member consists of a succession of greenish tuffaceous

beds with dark porphyritic flows. The volcanic rocks lie on top of

all other stratigraphic units and are thought to be Tertiary because

they truncate post-mid-Cretaceous thrust faults (Dover, 1969). To

the west o f the Pioneer Mountains the Challis Volcanics have a K-Ar

date of 49.2 * 1.3 m.y. to 43.8 * 1 m.y. (Armstrong, 1974).

F in ally sta rtin g in Miocene time a second episode of Cenozoic

volcanism began with the eruption of Columbia Plateau and Snake

River basalts to the south of the Pioneer Mountains (Figure 2). This

has continued until the present.

Structure

The thrust b e lt in the Pioneer Mountains covers an area that is

several thousand square kilometers. There are three major thrust

faults in the Pioneer Mountains. These are the Fall Creek thrust,

Wildhorse th ru st, and Summit Creek th ru st. Eastward movement of

several miles or more is assumed by Dover (1969). The thrust sheets

have been warped by post thrusting u p lift of the crystalline core of

the Pioneer Mountains in early T ertiary (Dover, 1969).

Post-ChaTlis high angle faults cut the thrust sheets and

Tertiary volcanics (Dover, 1969). These major late Tertiary faults

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. are the Kane Creek normal fa u lt, Pioneer reverse fa u lt, and White

Mountain normal fa u lt. The present r e lie f is largely a product of

Tertiary and later uplift along these faults.

Regional Ore Deposits

Mineral deposits in the Pioneer Mountains and nearby mountains

in central Idaho include: 1) high temperature molybdenum and tungsten

deposits that occur in plutons in quartz veinlet systems and associ

ated metamorphic halos, or sometimes disseminated in metamorphic halos

2) low and medium temperature lead, zinc, s ilv e r, and other metal

bearing veins in Paleozoic sedimentary rocks near igneous plutons,

and within the plutons; 3) zinc, lead, and silver bearing hydrothermal

replacement deposits in sedimentary rocks; and 4) placer deposits of

gold and heavy black sand metals (Cavanaugh, 1979).

Plate Subduction and Porphyry Molybdenum Deposits

Spatial and temporal relationships between plate subduction,

arc magmatism, and the formation of post Paleozoic porphyry molybdenum

deposits in the western United States suggest that in the western

C ordillera, magmatism and porphyry (stockwork) molybdenum deposits

are related (Westra & Keith, 1981). Geographic position o f arc

magmatism as well as the potassium geochemistry of magmas are produced

by partial melting as a function of depth along an inclined subducting

slab (Figure 4). The shifting magma chemistry patterns (Figure 5)

of the western United States are believed due to the changes in sub

duction angle with time (K eith, 1978).

Calc-alkalic type Climax type

Figure 4. Depth to subducting slab for differentiated granitic stocks (Westra & Keith, 1981). 12 IGH-K : a l c - ALKALIG ALKALI- CALCIC

SUMMIT CREEK STOCK

ALKALIC

5 0 0 Km 5 2 -4 5 m.y.

Figure 5. Spatial d is trib u tio n o f magmatism and areas of differentiated granitic intrusions associated with stockwork molybdenum deposits between 52 and 45 m.y. (Westra & Keith, 1981).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14

Potassium content can be used to determine depth at which a

melt has originated (Dickinson, 1975) by p a rtia l melting of subducted

oceanic lithosphere at successively greater depths (Figure 4). As

melts are generated deeper along the subducting slab the potassium

content increases (Figure 4). Stocks such as the Summit Creek stock

and stocks associated with porphyry molybdenum deposits have undergone

some modification on their way to the surface (Wyllie, 1979), but

molybdenum is most lik e ly derived from the subducted slab (Westra

& Keith, 1981).

Si 11itoe (1972) explains that molybdenum in post Paleozoic

deposits in the western United States was originally derived by

hydrothermal concentration of molybdenum through ocean water-basalt

interaction at the East P acific Rise. From the East P acific Rise

metals were carried toward the margins of the P acific Ocean basins

as components of basaltic crust, and thrust underneath the western

United States. Molybdenum was released from the underthrust oceanic

crust and sediments during partial, melting and incorporated in

ascending diapirs o f calc-alkaline magma (Si 11ito e , 1972).

Porphyry Molybdenum Systems in North America

Porphyry molybdenum deposits are the most economically important

type of molybdenum deposits. Ages o f porphyry molybdenum deposits in

North America range from 17-140 m.y. There are three d is tin c t prov

inces in North America: 1) Southern Rocky Mountain b e lt in Colorado

and New Mexico, 2) White Cloud-Canni van porphyry b e lt in Idaho and

Montana, 3) In te rio r b e lt o f B ritis h Columbia.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15

Host stocks of porphyry molybdenum deposits are related to

siliceous oversaturated rocks found in the three provinces. Petro

logic composition of intrusions range from granodiorite through quartz

monzonite to granite. Granodiorite type deposits are related in space

and time to emplacement of the younger phases of major calc-alkaline

Mesozoic batholiths, including the Boulder, Idaho, and Sierra Nevada

batholiths (Mutschler, Wright, Ludington, & Abbott, 1981). Quartz

monzonite type deposits are associated with small composite stocks

or late phases of batholiths. A few deposits are related to simple

single phase stocks (White, Bookstorm, Kami 11i , Ganster, Smith,

Ranta, & Steininger, 1981).

A1 teration

Hydrothermal a lte ra tio n is very frequently found in the host

intrusion and is occasionally found in the country rock. In classic

alteration assemblages there are halos or zones of alteration sur

rounding the ore body (Figure 6). Four zones of alteration commonly

found in porphyry molybdenum deposits in order of decreasing intensity

are:

1) Potassium feldspar zone--total replacement of piagioclase

by potassium feldspar.

2) Q u a rtz-se ricite -p yrite zo n e --se ricitiza tio n of potassium

feldspar and piagioclase and abundant p yrite .

3) Argillic zone--argillization of feldspars, replaced by

montmorillonite, kaolinite, and sericite. Biotite is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Greisen veins

<* V V I Late porphyry (coarse-grained) ' *> T* I / / I Base metal sulfide-rhodochrosite- I ^ I fluorite veins (late barren stage)

| , | Argillic alteration

Guartz-sericite alteration Molybdenite ore shell. Pervasive silicic alteration

Potassic alteration

Early porphyry (fine-grained)

Figure 6. Schematic cross section of a porphyry molybdenum deposit. Modified from Mutschler et a l. (1981).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17

replaced by muscovite, sericite, and small quantities of

rutile, leucoxene, pyrite, carbonate, and fluorite.

4) Propylitic zone-characterized by calcite, clay, and minor

sericite derived from piagioclase, minor chlorite, and

epidote occurring with biotite (White et a l., 1981

Mineralization

In porphyry molybdenum deposits, molybdenum is the only mineral

of economic importance wi.th the exception of Climax which recovers

the byproduct cassiterite, heubnerite, and pyrite. Next to molybde

n ite , p yrite is the most abundant su lfid e mineral present.- Other

sulfides that occur in only minor amounts are chalcopyrite, sphalerite,

galena, and pyrrhotite. Small amounts of tungstates may be present,

such as heubnerite, scheelite, and wolframite.

In porphyry molybdenum deposits the molybdenum introduced into

the hydrothermal system is derived from a c ry s ta lliz in g magma (Westra

& Keith, 1981). Moderately high temperatures of 300-400° C are usually

associated with the fluids in porphyry molybdenum systems (Lowell &

Guilbert, 1970). Molybdenum occurs early in the mineralizing epoch

associated with a plutonic event (Clark, 1972). It may be accompanied

by magnetite and the alteration mineral assemblages that usually

precede the mineralizing event. Pyrite and other sulfides may be

deposited contemporaneously, but usually appear a fte r the molybdenum.

Quartz is the earliest gangue mineral, being associated with early

ore minerals and alteration assemblages. Topaz, fluorite, and carbon

ates are usually deposited la te r.

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Ground preparation in porphyry molybdenum systems is important

as it is the primary means by which the fluids are moved and the

location where the ore is deposited. These six examples are the

major ore occurrences in porphyry molybdenum deposits (Clark, 1972).

1) Quartz veinlets containing molybdenite that are regular

spaced and fracture controlled. Replacement occurs along

irregular wall rock contacts and there is no offset where

veinlets cross.

2) Fissure-vein deposits, contain primary quartz, and molyb

denite.

3) Fine fractures, where molybdenum occurs as "paint" on

fracture or joint surfaces,

4) Molybdenum f illin g in openings in shear zones or in

mineralized fragments o f breccias, some of which have

been cemented by molybdenum bearing quartz.

5) Disseminated molybdenum that occurs as small flakes and

grains by replacement in host rocks.

6) Oxide molybdenum minerals tha t occur in weathered

deposits.

Table 1 compares geologic and economic characteristics of the

Summit Creek stock to other molybdenum deposits o f central Idaho.

A ll these deposits are located in the White Cloud-Cannivan porphyry

molybdenum b e lt o f Idaho and Montana (Armstrong, H o llis te r, & Hauakel,

1978). In al1 of the deposits molybdenite occurs in quartz veins, and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. there is a later set of barren quartz .veins; also, the deposits do

not appear to be deep.

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Table 1 Comparison of Molybdenum Deposits in Central Idaho (Modified from Cavanaugh, 1979)

Little Boulder Thompson Creek Little Falls Summit Creek Creek Deposit Prospect Stock

Proximity to 35 km 27 km 96 km Summit Creek NW North West Stock

Elevation 2592-2867 m 2318-2379 m 1022-1281 m 2562-2623 m

Host Rocks tactite adj. to porphyritic rhyolite dikes quartz monz. porphyritic quartz monz. and quartz and a p lite quartz monz. monz. pegmati te

Age in m.y. 83.6 85.9 post-Eocene Eocene 46.7

Structural ' in down-dropped in fa u lt zone in N 35°E in fault zone Controls block adj. to with N 60°W dike swarm with N 20-30°W N 15°E fa u lt trend trend

A lte ra tio n silicified, completely silicified, silicified, potassic zones, altered to at s e ric ite s e r ic ite , sericite near least propy- throughout, minor silic i- fault and late litic grade. potassic fic a tio n breccia veins Envelopes dikes along faults throughout and fractures include all grades (prop.- s i l i c . )

Average Deposit O '-300' O '-500' O' O' Burial Depth 0-91.5 m 0-152.5 m (to top)

Size in Meters 180x900x 300; 600 x 180 x 240 900 x 300 x ?a 1x12x7; (approx.) 150x 180 x 180; 3 x 90 x 7 1 2 0 x 6 0 x 120

Tonnage 167 m illio n 100 m illio n + "significant" information (e s t.) not available

Grade 0.15% Mo 0.15% Mo "subeconomic" information (e s t.) 1968 not available

aZone of visible Mo

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Table 1 (continued)

L ittle Boulder Thompson Creek Little Falls Sunni t Creek Creek Deposit Prospect Stock

Occurrence in fine quartz, in coarse in fine quartz in fin e to quartz-Kspar quartz veins v e in le ts coarse quartz ve inlets and veinlets veins and ve in lets

Molybdenite fine flakes coarse flakes fine flakes fine flakes Description on v e in le t in veins and in veinlets in veinlets margins s ilic if ie d zones

V e in le t gen. concord, •highly information N 20-30°W Orientation w/bedding— complex not available dip v e rt. N 15°E, dip 70°W

Late Barren present present present present Quartz Veins?

Pyri te about . 5 % in in veins, and "intense" in veins and Association veinlets and vnlts with ore in zone dissem. dissem. zone. Coarse 10,000' x No halo. No halo. c ry s ta ls . 2000'. Subtle halo. D is tin c t halo.

Scheelite in vnlts trace in ore information in tactites Association throughout ore zone v n lts . not available on stock zone. Distinct Ore grade in margins outer halo. veins ad j. to deposit

Magnetic on side of in magnetic information in magnetic Expression large magnetic high not available high high

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER II

ANALYTICAL PROCEDURES

Sampling

Field work was carried out during the 1979 and 1980 summer

field seasons. A 11:200" scale topographic map was used for

general field work. All outcrops in the study area were mapped

in detail (Plate 1).

Samples collected for the trend surface analysis were

collected as nearly as possible to equal intervals across the

study area to insure the integrity of statistical analysis (Plate

1). Of the fifty samples collected for the trend surface and

geochemical analysis, fo rty samples were used in the trend surface

analysis. The samples used for the trend surface analysis were

large enough to provide three representative thin sections and a

hand specimen. The other ten samples were taken fo r use in the

geochemical analysis.

Because of the excellent ve rtica l exposure of the stock, a

two-dimensional trend surface analysis was made to determine

mineralogical variation as a function of depth. This information

•is useful in determining if the stock was formed by multiple

intrusions and i f i t had undergone a change o f orientation since

its emplacement.

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Petrographic Analysis

The petrographic thin sections were stained with sodium

cobaltinitrate and rhodizonic acid dipotassium salt to aid in

identification of potassium feldspar and plagioclase feldspar.

The method used is outlined by Houghton (1980).

Calculation of volume percent of minerals was determined by

counting 1,000 points on three th in sections from each of the fo rty

samples cut in at least two different orientations (Table 2).

Chayes (1956) notes that the best results are attained by analyzing

three thin sections per sample.

Geochemical Analysis

Rock samples were powdered and fused into glass beads, then

powdered again to insure homogeneity as described in Hutchison

(1974) fo r X-Ray Fluorescence and Proton-Induced X-Ray Emission

(PIXE) analysis.

The powdered rock samples were combined in the ra tio of one

part rock mixed with ten parts HgBOg to increase cohesiveness, and

pressed into discs to be loaded in to XRF and PIXE.

Siand AlgOg were analyzed using a Phillips Universal X-Ray

Spectrograph. Potassium, iron, titanium, manganese, and calcium,

along with trace elements molybdenum, strontium , niobium, and

rubidium were analyzed in ppm by PIXE (Chapter V II). The rocks

were dissolved in hydrofluoric acid as outlined in Hutchison (1974)

and analyzed fo r sodium using the Beckman Atomic Absorption

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Table 2 Petrographic Modal Analysis Listed in Volume Percent of Total Volume

Potassium Color Sample Quartz Plagioclase Feldspar Feldspar Index

1 31.7 38.7 25.7 3.2

2 36.9 33.2 28.5 1.3

3 30.9 29.3 34.7 1.8

4 35.0 33.1 30.5 1.4

5 29.3 32.6 34.3 3.9

6 29.3 34.6 32.6 3,1

7 28.6 34.3 32.9 4.2

8 28.2 32.3 30.8 8.7

9 21.6 42.8 31.1 2.9

10 23.8 35.5 36.4 4.3

11 34.2 21.7 41.8 2.3 12 28.6 41.3 28.1 1.7

13 28.8 33:4 33.7 4.1

14 27.4 26.6 26.5 5.1

15 28.8 41.7 27.6 2.0

16 23.1 33.4 32.1 3.4

17 28.5 26.6 34.0 5.3

18 19.4 48.7 25.2 6.7

19 25.3 34.5 36.3 3.9

20 28.5 33.1 35.3 3.2

21 31.9 26.9 37.1 4.1

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Table 2 (continued)

Potassium Sample Quartz Plagioclase Color Feldspar Feldspar Index

22 41.3 35.3 19.8 3.8

23 31.2 36.2 31.4 1-2 24 32.3 31.5 33.1 3.2

25 32.2 35.3 31.3 4.1

26 32.1 30.7 34.8 2.3

27 34.5 31.5 28.3 5.0

28 33.1 31.5 32.6 2.7

29 32.6 31.1 34.7 1.4

30 30.9 26.1 36.0 7.1

31 28.1 32.8 32.5 6.6

32 25.0 33.3 36.9 4.7

33 26.9 38.1 31.1 3.9

34 34.8 36.2 27.8 11.7

35 48.6 0 48.7 1.0 36 26.1 31.3 38.5 4 .i

37 28.6 40.1 29.9 1.5

38 24.9 28.0 42.6 4.4

39 31.0 37.9 28.2 1.7

40 32.5 37.0 29.9 •5

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Spectrometer (AAS), Model DB-G.

USGS s ilic a te rock standards (Flanagan, 1967; Langmyhr & Paus,

1968) were used to create a working curve to determine concentra

tions of unknown element concentrations (Table 3). The USGS s ili

cate rock standards used are BCR-1, W-l, AGV-1, G-2, GSP-1.

Table 3 Values Obtained Using XRF, PIXE and AAS of USGS Standard S ilic a te Rocks to Determine Composition of Rocks from the Summit Creek Stock

XRF PIXE AAS Counts/10 i sec. ppm % Si Al Fe Mn Ca K Ti Na

G-2 265 234 11883 162 5456 8739 958 2.6

W-l 112 168 57223 743 37656 11780 2437 -

GSP-1 213 198 25838 424 9362 19245 1895 4.0

AGV-1 142 199 31126 422 13314 6442 2172 2.4

BCR-1 262 201 70937 893 24093 5117 5807 3.7

With the known concentrations of elements in the standards, a

plo t o f the element concentrations o f the abscissa and counts on the

ordinate produces a working curve. Using the equation of a line

which intersects the o rig in (y = mx), and determining the number of

counts of an unknown sample, the percentage of an element could be

determined by dividing the to ta l counts, minus background, by the

slope of the standard line (Table 4).

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Table 4 USGS Standard S ilic a te Rocks Used to Create a Working Curve to Determine Composition of Unknown Rocks

Si Al Fe Mn Ca K Ti Na

G-2 69.0 15.60 2.70 .034 1.86 4.51 0.43 4.0

W-l 52.65 15.27 11.20 .166 10.63 0.61 1.08 -

GSP-1 67.22 15.35 3.98 .05 2.07 5.50 0.66 2.79

AGV-1 59.00 17.10 6.42 .10 4.89 2.87 1.05 4.23

BCR-1 54.10 13.70 12.31 .19 6.91 1.69 2.25 3.26

Note. Values lis te d in percent of whole rock major oxides (Flanagan, 1967; Langmyhr & Paus, 1968).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I I I

GEOLOGY OF THE SUMMIT CREEK AREA

Sedimentary Rocks

Sedimentary rocks found outcropping along the ridges o f the

southern portion of the map area (Plate 1) belong to the lower portion

of the Phi Kappa formation based on exposures near the head of Kane

Creek where the complete formation is exposed (Dover, 1969). This

formation contains shales, siltstones, quartzites, and greywackes.

The beds dip to the west and southwest throughout the map area due

to doming by the intrusion of the Summit Creek stock (Figure 7).

A low angle thrust fa u lt-sp la y cuts the sediments into two d is tin c t

plates.

The lower plate is a 50 m thick series of siltstones and shales.

Individual beds are homogeneous. In hand specimen the siltsto n e is

dark grey to black, very fine-grained, and has a blocky fracture

(Figure 8). Silicification occurs along the contact between the

siltsto n e and in trusion . The silts to n e has undergone a low grade

contact metamorphism.

The upper plate consists mainly of siltstone with interbeds of

quartzite and greywacke .15 m to 2 m in thickness. The best exposure

of the upper plate is found in the cirque above L ittle Alpine Creek.

The quartzite is medium grained, brown to tan, quartz cemented, and

breaks in a blocky fashion. The greywacke has a very fine-grained

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Alluvium Aphanitlc Oik*# Figure 7. Schematic cross section o f the Creek Summit stock. Phi Phi Kappa Formation Braccla

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matrix, dark grey to black with chert fragments (1 mm) (Figure 9).

In outcrop, the greywacke has a blocky fracture. All rock units of

the Phi Kappa formation weather to a rust brown color along fractures.

Igneous Rocks

The in tru sive rock in the study area is a quartz monzonite to

granite composition. The intrusion is slightly porphyritic, medium to

fine crystalline, and in some places grades to very fine crystalline.

In hand specimen the rock is 20-30% K-spar, 30-40% plagioclase,

25-30% quartz, and 10% mafics (biotite) (Figure 10). Crystal shape

is subhedral to anhedral. The rock composition varies throughout the

stock, with a decrease in plagioclase and an increase in K-spar and

quartz in the lower portion of the exposed pluton. This change is

gradational and no contacts were found w ithin the intrusion. Close to

the contact with the country rock there is a decrease in mafic minerals.

The intrusion also becomes finer crystalline in this chill zone.

Small pegmatitic stringers of K-spar and quartz were noted in

the lower portion of the stock. The K-spar and quartz crystals are

approximately 4.5 cm in length and the stringers are 45 cm across.

Felsic aphanitic dikes are found throughout the intrusion . The

dikes are usually only 2 to 5 cm wide but may be as wide as 15 cm in

some areas. Some of the dikes have fragments of the host intrusion.

Many of the aphanitic dikes show a random orientation, but others

appear to follow a NW-SE and SW-NE set of fracture/joint sets. All

of the felsic dikes were formed during the same intrusive event. The

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dikes may be a later phase of the main intrusive event, due to a

residual wet melt that was super heated and intruded into existing

planes of weakness (Hyndman, 1972).

Quartz veining is prevalent throughout the study area. There

are at least two episodes o f quartz veining evident from cross cutting

re la tion s. The f i r s t episode appears to be contemporaneous with the

felsic dikes. A second set of quartz veins mainly fills pre-existing

fractures, and may be related in time with a breccia found in the

lower portion of the pluton.

One of the latest episodes of intrusive activity is the formation

of the andesite dikes that trend in a SW-NE direction and are vertical.

The andesite dikes are found mainly in the intrusion, and extend in

places into the sediments. The andesite dikes are found predominately

higher in the intrusion; the dikes appear to follow pre-existing

fractures. In hand specimen the andesite is fine crystalline, green

to grey, fresh, 70% mafics (hornblende and pyroxene), 20% feldspars,

and 10% quartz (Figure 11).

Structure

Three main systems of faults exist in the study area. A thrust

fa u lt—which may be a splay from the Summit Creek th ru st—cuts along

bedding planes 70 m above the intrusion (Plate 1). The fa u lt breccias

have been cemented with s ilic a and minor calcium carbonate. There is

no field evidence to indicate direction of thrusting, however Dover

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(1969) suggested a southwest thrust direction fo r the T ra il Creek

thrust system which may be genetically related to the Summit Creek

thrust.

The other two major fa u lt sets in the study area are high angle

normal fa u lts . One set o f fa u lts has a NE-SW trend, the other a NW-SE

trend. The NE-SW trending fa u lts appear to be the younger set, as

they cut the NW-SE trending set (Plate 1). Fracturing parallels the

fault systems. Some of the faults have slickensides that show

la te ra l movement as well as ve rtica l movement.

Breccia

A silicified breccia is located in the lower portion of the

stock and appears to be the top of highly silicic pipe (Plate 1). In

hand specimen the breccia is a highly s ilicifie d , massive rock with

angular and clay-sericite altered clasts 12-50 mm across. The clasts

are only detectable with polished rock slabs due to intense alteration.

The matrix is an iron stained mixture of very fine crystalline quartz

and clays (Figure 12).

Mineralization

Molybdenum mineralization is closely associated with quartz

veining. Booklets o f molybdenite are found in a small stockwork of

quartz veins near the contact o f the quartz monzonite and the Phi

Kappa formation. Near the center of the stock a 12 cm quartz vein

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with molybdenite is exposed. Also, a trace amount of molybdenite is

found in the breccia.

Pyrite and magnetite are usually found in trace amounts through

out the stock, and are occasionally found in quantities of up to one

percent in some areas o f the stock. There does not appear to be a

halo of these minerals, but a random distribution.

A lteration

The pervasive alteration associated with a typical porphyry

molybdenum deposit is absent in the Summit Creek stock. In contrast,

intense alteration in the Summit Creek stock is limited to narrow

zones associated with fractures. The dominant type of a lte ra tio n is

argi1 lie , with feldspars altering to clays and sericite, and biotite

altering to chlorite (Figure 13). Altered zones along these fractures

and faults range from .6 to 2 m in width. Amounts of alteration

increase closer to the sediment-intrusion contact. This is because

there is an increase in fracture density near the outside of the

stock.

Throughout the stock there is minor propylitic alteration,

plagioclase slightly altering to clay. This is not considered to be

deuteric but meteoric alteration because thin section analysis shows

that the alteration is predominately confined to grain boundaries.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IV

PETROGRAPHY

Quartz Monzoni te

The quartz monzonite is slightly porphyritic to equi-granular

throughout the pluton. The phenocrysts are 5 to 20 mm in size, and

are predominately euhedral to subhedral plagioclase and subhedral

orthoclase (Figure 14). The phenocrysts are slighly p o ikilitic where

the rock is fresh and saussuritized and sericitized in more altered

rocks. The orthoclase lacks twins and accounts for 32% o f the to ta l

Figure 14. Photomicrograph of s lig h tly po rph yritic quartz monzonite viewed under crossed nickels, magnification 35x.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41

rock. The plagioclase makes up 33% of the intrusion. Anorthite

content of the plagioclase ranges from An.^ to An^Q, and was deter

mined using a universal stage and determining angle of extinction by

observing the (010) axis (Kerr, 1977). Many of the grains have normal

zoning. Micrographic texture is found in the lower portions of the

pluton indicating a eutectic crystallization.

Twinning is extensive in the plagioclase. Most of the plagio

clase grains have albite, carlsbad, and pericline twins, and a few

of the grains exhibit carlsbad-albite twins. All of the feldspars

have straight to slightly undulatory extinction. Fracturing of the

feldspar grains is common.

The quartz is anhedral throughout the pluton and comprises 30%

of the rock. The quartz is inte rstitial, and some grains exhibit

sutured boundaries. The undulose to s lig h tly undulose quartz is '

smoky grey. Many fluid inclusions and feldspars are common in the

quartz. Fracturing in the quartz grains is prevalent, similar to

the feldspar.

The groundmass has a subautomorphic texture throughout the

stock, the crystals range in size from 1 to 17 mm. Biotite is

usually found in the groundmass and accounts fo r 3% of the rock. I t

is brown-green and occurs in laths 2 to 7 mm in length and is s lig h tly

altered to c h lo rite . Euhedral sphene crystals 1 mm in length account

fo r 1% of the rock in a few isolated areas o f the stock, but is

usually an accessory m ineral. Magnetite is also found in up to 1% in

the groundmass. Many of the iron bearing minerals have FegOg staining

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around them. Additional accessory minerals found in trace amounts are

zircon, a p a tite , and sphene.

Altered Quartz Monzonite

The altered quartz monzonite consists of quartz, plagioclase and

orthoclase that have been altered to clay-sericite, and biotite that is

altered to chlorite (Figure 15). Most of the alteration is argi11ic

alteration. Polygonal texture is lost due to alteration and quartz-

overgrowths. Hydrothermal quartz w ith undulose extinction and sutured

boundaries is found in the altered rock. Many of the feldspars have

been totally altered to-.clay-sericite.-and cannot be distinguished as.

either plagioclase or orthoclase. Fe203 staining is found surrounding

Figure 15. Photomicrograph of a rg il lie altered quartz monzonite viewed under plain lig h t, magnification 35x. Feldspars are totally altered to clays and undistinguishable as to type.

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fresher grain boundaries and within the altered grains. Biotite has

been completely altered to clays and cannot be identified in some of

the more intensely altered rocks.

Breccia

The breccia is composed of angular clasts of highly altered

quartz monzonite and a matrix of fine quartz and clay. They consist

of xenomorphic medium grained (1 mm) feldspar and quartz crystals.

The feldspars have been altered to clays and are indistinguishable

as either plagioclase or orthoclase in thin section. The quartz

grains have strong undulose e xtin ction with clays and Fe 20 3 rimming

the quartz and fillin g microfractures.

The matrix is composed o f 90% quartz, 10% clays, and Fe 203,

a trace amount of pyrite and molybdenite. The quartz is fine-grained

( . 1 to 1 mm) and is s lig h tly undulose and contains abundant flu id

inclusions. The quartz rich matrix is subautomorphic with some

interlocking of the quartz grains. The Fe 203 stained matrix gives

the breccia a red color in plain light.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V

STATISTICAL ANALYSIS

Discussion of Trend Surface Analysis Method and Presentation of Data

Trend surface analysis deals with the recognition, isolation,

and measurement of trends that can be represented by lines, surfaces

or four-dimensional hypersurfaces. Trend surface analysis separates

large scale variations or trends from local variations. The least

squares criterion is the method used in this study and has the trend

surface passing above, below, or through the actual data point to

form a "best f i t . "

The use of least squares f it t in g in trend surface analysis [as

developed by Grant (1957)] has been demonstrated by Whitten (1959) and

Krumbein (1959) as an e ffe ctive tool fo r analyzing two-dimensional

variations of g ra n itic plutons. Krumbein (1959) extended the method

to include non-orthogonally distributed data points. Whitten (1959)

has shown by u tiliz in g a least squares f it t in g method fo r trend

surface analysis that many chemical and mineralogical variation trends

can be detected both between and within individual granite plutons.

Trend surface analysis of non-orthogonally distributed data may

be computed fo r any degree, depending on the number of data points.

Surfaces analyzed for this study include firs t degree (linear), second

degree (linear + quadratic), third degree (linear + quadratic + cubic),

and fourth degree (linear + quadratic + cubic + quadratic) analysis.

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Sample sites were selected with the x coordinate being the hori

zontal, and the y coordinate being the vertical. This was chosen to

give a representative cross section through the intrusion. The depen

dent variable is the measured value at each x,y location. The surfaces

of "best fit" in this analysis were all third degree surfaces. A

th ird degree surface (Harbaugh & Merriam, 1968) has the form o f:

Z = A + Bx + Cy + DxZ + Exy + FyZ + GxZ + IxZy + JyZ. The residuals

were minimized using the least squares method.

It is important to determine if the trend surfaces are statis

tically significant. Goodness of fit and analysis of variance (Dixon

& Massey, 1969) were implemented to determine the surfaces of best

f i t .

Goodness o f f i t is calculated by summing the squares of devia

tions from the mean.

S+1 = ^ ^ o b s ' Zobs^

where: S+^ = to ta l corrected sum o f squares o f deviations from the

mean,

n = degrees o f freedom,

Zobs = observed value o f variables Z a t data points,

Zobs = arithm etic mean o f observed values of Z.

This is expressed as a percentage because th is permits comparison of

trend functions fitted to different sets of data. A perfect trend

surface has a value of 100%, meaning no deviations of data point from

the trend.

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Analysis of variance involves freedom for variation to occur.

This is determined by correlation between degrees of freedom with

other statistical properties essential in analysis of variance

(Harbaugh & Merriam, 1968). These statistical properties include:

sums of squares apportioned among linear, quadratic, and cubic regres

sion components; sums of squares associated with deviations from

cumulative components; number o f degrees of freedom associated with

regression components and with deviations.

This data is used.to calculate the mean square of components

and the F factor as outlined in Krumbein and Graybill (1965), and

Davis (1973). F factor values fo r f ir s t , second, th ird , and fourth

degree surfaces are compared to determine which degree surface has

the highest statistical probability of being the most accurate.

Maps of the trend surfaces of best f it for modal analysis of

quartz, plagioclase, potassium feldspar, and color index of the

Summit Creek stock are shown in Figures 16, 17, 18, and 19. The

surfaces of best fit for this analysis are all third degree surfaces.

The percentage o f quartz (Figure 16) and plagioclase (Figure 17)

decrease with depth in the pluton. The potassium feldspar percentage

increases with depth, as noted by the gradation from the upper east

side of the pluton to the lower west section of the exposed stock.

Bateman and Nokleberge (1978) explain the increased percentage of

quartz and potassium feldspar in the in te rio r o f the sim ila r Mount

Givens stock by increasing rates of nucleation and crystal growth by

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47

2 .9 2 .4

102,000 E 98,000 E E W 1 KILOMETER

Figure 16. Two-dimensional trend surface analysis fo r quartz. Cross section of stock, vertical exaggeration 4x. The dashed line represents the contact between the Phi Kappa formation and the Summit Creek stock. The * represents the location of sample 29 (Plate 1).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48

102,000 E 98,000 E

1 KILOMETER I

Figure 17. Two-dimensional trend surface analysis for plagioclase. Cross section of stock, vertical exaggeration 4x. The dashed line represents the contact between the Phi Kappa formation and the Summit Creek stock. The * represents the location of sample 29 (Plate 1).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49

'3.7

3 3 .3

102,000 E 9S,000 E

I 1 KILOMETER I

Figure 18. Two-dimensional trend surface analysis fo r potassium feldspar. Cross section of stock, vertical exaggeration 4x. The dashed line represents the contact between the Phi Kappa formation and the Summit Creek stock. The * represents the location of sample 29 (Plate 1).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50

3 0.4

38.4

102,000 E 98,000 E

« 1 KILOMETER

Figure 19. Two-dimensional trend surface analysis fo r color index. Cross section of stock, vertical exaggeration 4x. The dashed line represents the contact between the Phi Kappa formation and the Summit Creek stock. The * represents the location of sample 29 (Plate 1).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51

moderate undercooling below the temperature of in itia l crystalliza

tion toward the center of the stock.

The color index appears to increase near the contact with the

country rock. Also there is an increase in color index in the lower

west section of the stock. Many plutons are lower in mafics and free

of mafic inclusions in their interiors. Some compositional zoning

may be related to contamination by wall rocks (Hyndman,1972). Also,

assimilation of some of the country rock may have produced the

increase in color index in the lower west side of the exposed stock.

All of the trend surfaces show a similar pattern in the trend.

There are small variations within the trends such as the double model

pattern observed in Figures 17 and 18 compared to the single mode

arrangement noted in Figures 16 and 19. These modes in the center

are minor compared to the steep gradations near the edges o f the

trends and in light of the statistical nature of the analysis are

probably not significant.

Of most importance, the patterns all display a smooth gradation

that appears to be a function of distance from the contact. The

change in gradient in the middle of the trend surfaces may possibly

be related to changing temperature and pressure parameters w ithin

the cooling stock. This evidence indicates that the Summit Creek

stock is the product of a single intrusion and is not a composite

stock.

Also, the symmetrical homogeneity of the Summit Creek stock

indicate that it is in its original orientation and has not been

rotated by tectonic stress.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER VI

PETROGENESIS

Petrogenesis of the Summit Creek Stock

As previously mentioned in the introduction, i t was shown that

the depth at which a melt originated cna be determined from KgQ index

fo r potassium at 60% SiOg (Dickinson, 1975). From the parameters

outlined in Table 5 and Figure 4, the Summit Creek stock compares

favorably with high K ca lc-a lka lin e stocks (Keith, 1978). The depth

to the subducting slab and in itia l melt that formed the Summit Creek

stock is approximately 220-260 km based on its chemical composition

of a high K calc-alkaline intrusion.

Emplacement

Depth o f the emplacement o f the Summit Creek stock is estimated

to be 6-7 km. My evidence fo r th is is :

1) Buddington's (1959) experiments of two feldspar granites

indicate that the Summit Creek stock crysta llize d under

similar pressure and temperature conditions of mesozonal

plutons (6.4-13 km depth).

2) Although baking o f the. Phi Kappa formation was noted through

out the study area, along the contact between the intrusion

and the Phi Kappa formation there was found only minor

silicification in a zone .5-1 m wide.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. cn co pyroxene; b io tite b io tite Selected minerals

3 O 2 15.2 15-19% High AI AI content 0+K20 2 Na -.9 4.5-7.5 6 0/Na20 2 K T a b le 5 3.1 .81 7.3 S S index^ 3 38-44 2.3-3.2 . 9 9 index 0) 0) 9 4.24 39.3 K60.5 ) 2 (Na90 (Na90 + K 43 _ and and Depth to Subducting Slab, with Comparison to the Summit Creek Stock 2 slab 220-260 2.4-3 inclined Depth Depth to Chemical Chemical and Mineralogical Parameters fo r C la ssifica tio n of High K C alc-alkaline Rocks index = SiO, - 47 - - S S index = SiQ 9 . . (Na?0 + K?0 Note. A fte r Westra and Keith (1981). Creek alkaline Summit Stock High High K Calc-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54

3) No apparent reaction rims were found in the xenoliths

indicating little assimilation with the intrusion, suggest

ing that the magma was intruded at a relatively low

temperature for a mesozonal pluton.

I believe emplacement to have been forceful as evidenced by the

doming of the sediments and the dike swarms intruding up in to the

planes of weakness of the country rock. This is especially noticeable

on the north side o f Summit Creek.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V II

CHEMICAL ANALYSIS

Whole Rock

Listed in Table 6 is the average geochemical composition o f the

Summit Creek stock. Also included in the table are geochemical compo

sitions of selected producing porphyry molybdenum deposits. Henderson,

Mount Emmons, and Questa are cla ssifie d as "granite molybdenite"

systems (Mutschler et a l., 1981). White Cloud, Thompson Creek, and

Cannivan deposits are termed "granodiorite molybdenite" systems

(Mutschler et a l., 1981). Whole rock chemical analysis o f the Summit

Creek stock indicates that it best fits a granodiorite molybdenite

classification (Table 6 ).

Granodiorite molybdenite type deposits are all mesozonal and have

a composition ranging between calc-alkaline granodiorite and quartz

monzonite. Compared to the granite molybdenite type deposits,

granodiorite molybdenite systems have lower percentages of Si and K,

and higher percentages o f Al and Ca (Table 6 ). The granodiorite

molybdenite deposits in Montana and Idaho are believed to be related

to emplacement of calc-alkaline Mesozoic batholiths (Westra & Keith,

1981). In comparison to source rock chemistry, ore body geometry, and

trace element concentrations, the granodiorite molybdenite systems

have closer sim ilarities to the Cordilleran porphyry copper deposits

than granite molybdenite systems (Lowell & G uilbert, 1970).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CJ1 C T i quartz monzonite granite porphyry granite porphyry quartz monzonite granodiorite Summit Creek Summit stock .06 .05 .03 .04 .13 .02 .01 .1 .05 .28 .03 .25 .01 .21 .52 .05 .26 .14 Ti Mn .9 .07 .55 .14 .03 Ca 2.9 2.24 2.5 1.45 3.1 2.9 4.0 4.1 4.04 3.88 1.253.5 .25 .03 4.1 2 .6 3.25 2.65 5.0 6.4 T a b le 6 .8 1.25 2 .1 2.96 5.5 2 .2 2 .1 6.3 5.95 Al Fe K Na 12.3 16.1 16.1 11.9 15.28 4.2 4.24 3.9 1.04 16.7 17.01 15.3 15.4 4.6 3.95 and and Compositions from Various Deposits Molybdenum Si 76.0 72.0 13.9 2.53 5.46 3.8 1.33 .37 .06 76.0 69.3 67.1 64.3 64.06 6 15.5 .8

3 Geochemical Geochemical Composition of the Major Oxides of the Creek Summit Stock *1

3 3

3 aData aData from Mutschler et a^Data l. from Nockolds (1981). (1954). Calc-alkaline Average Granite Henderson Cannivan Mount Mount Emmons L ittle Boulder Creek 21 6 8 .8 Thompson Creek Thompson 7 5c 72.6 15.0 4.9 5.55 3.8 .67 Sample 24 T - ll 67.2 15.2 3.0 4.0 4.32 1.3 Average 67.88

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Trace Element

Although differentiates of different porphyry molybdenite series

may have sim ila r major element chemistry, the minor and trace element

contents of these magma series may have distinct differences. Concen

tra tions of the trace elements niobium, rubidium, and strontium have

been determined to indicate a possible a ffilia tio n with molybdenum

mineralization associated with the different granite to granodiorite

host intrusions (Westra & Keith, 1981).

The concentrations of rubidium and niobium in the Summit Creek

stock are compared to molybdenum bearing stocks o f d iffe re n t magma

series (Figures 20 and 21). The whole rock chemistry of the Summit

Creek stock closely relates to that of high K calc-alkaline series

(Keith, 1978) (Table 6 ). Figure 20 shows that rubidium concentration

falls on the edge of the field of concentrations generally associated

with molybdenum deposits. Figure 21 shows that the concentrations

of niobium are within the same range as those of high K calc-alkaline

molybdenum deposits.

Also using Rb/Sr ra tios (Westra & Keith, 1981) and p lo ttin g

these values for different molybdenum deposits has shown a relation

between Rb/Sr ratios o f d iffe re n t magma series and molybdenum occur

rences. The Rb/Sr ra tio o f the Summit Creek stock plots with the

Transitional Molybdenum deposits (Figure 22). Transitional Molyb

denum deposits are analogous to deposits that have a high K calc-

alkaline stock (Figure 4).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 00 1200 iOOO 8 0 0 ppm Rb 6 0 0 4 0 0 200 0

of porphyry stocks. molybdenum After Westra .Keith and (1981). 2.5

2.5

CALCIC STOCK HIGH K 57.5 57-5 HIGH K K K K K Figure 20. Comparison of concentrations Rb associated with different whole rock compositions CALC-ALKALIC CALC-ALKALIC CALC-ALKALIC SUMMIT SUMMIT CREEK ALKALI-CALCIC

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. cn <0 ppm Nb 100 100 120 140 160 180 2 0 0 220 240 260 80 60 40 20 0

2.5

CALCIC

porphyry molybdenum porphyry stocks. molybdenum A fte r Westra and Keith (1981).

STOCK 57-5 HIGH K HIGH K 57-5 K K CALC-ALKALIC K K SUMMIT SUMMIT CREEK ALKALI-CALCIC CALC-ALKALIC CALC-ALKALIC Figure 21. Comparison of concentrations Nb associated with different whole rock compositions of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I a> o O Summit Creek • Climax type molybdenum deposits A Calc-alkaline molybdenum deposits □ Transitional molybdenum deposits ppm S r K K 5 7 .5 ^ 2 .5 K 57.5>2.5 After Westra and Keith (1981). 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 - - 100 7 0 0 -* 200 4 0 0 - 9 0 0 - 6 0 0 - 3 0 0 - 8 0 0 - Figure 22. Rb/Sr ratio for granite differentiates of porphyry molybdenite deposits. I. f t & PS 5 0 0 -

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Geochemistry of Altered Rocks

The geochemistry of the altered rock from the Summit Creek stock

is presented in Table 7, along with mean compositions of other

altered a r g illic assemblages from Henderson and other porphyry molyb

denum systems.

Table 7 Composition of A r g illic Altered Rocks from Summit Creek Stock, Henderson, and an Average o f Porphyry Molybdenum Systems

Sample Si Al Fe K Na Ca • Ti Mn

834 67.8 15.4 2.4 1 2 .0 1.95 1.5 .42 0

830 64.0 15.5 5.8 8.9 1.72 .65 .3 0

19 67.4 15.3 4.1 7.2 1.35 .72 .39 0

20 68.4 15.3 3.3 8 .1 1 .50 .78 .33 0

18 69.6 15.15 4.2 6.67 1.35 .65 .42 0

Average Summit Creek 67.7 15.3 3.9 8.574 1.57 .86

Henderson 9 74.6 12.3 .53 6.13 .34 .65 .14 •3

A rg illic . Average3 76.84 14.01 1.39 3.09 .24 .29 .23 .03

aData from Mutschler et a l. (1981).

Alteration associated with porphyry molybdenum deposits is

sim ila r to that of porphyry copper (Barnes, 1979). Lowell and

Guilbert (1970) state that alteration associated with porphyry sys

tems occurs when v o la tile s released by the p a rtia l quenching o f the

stock migrate outward through fractures and brecciated zones in the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 62

cooler margins where alteration and mineralization occur. This is

a response to gradients from near magmatic temperatures at the center

of the stock to relatively cool temperature in the wall rocks. This

w ill form a halo around the ore body consisting of a potassic inner

core with an a r g illic zone and an outer p ro p y litic zone as you move

fa rth e r away from the source of flu id s (Figure 6 ).

The alteration found in the Summit Creek stock is a rg illic and

propylitic. Table 8 compares the change in major oxide composition

of the unaltered rock of the Summit Creek stock (Table 6 ) to the

average altered rock of the stock (Table 7). Aluminum and silica

remained constant in the a rg illite assemblage (Table 8 ). Calcium

and sodium have been depleted relative to the source pluton.

Table 8 Chemical Change in Unaltered Stock (Table 6 ) Compared to A r g illic Altered Assemblages (Table 7) and Change of A r g illic Assemblages o f Various Porphyry Molybdenum Systems

Argillic Alteration of Various Alteration Argillic Alteration Assemblages of Porphyry fo r Summit Creek Stock Molybdenum Systems a

Si Og increased 2%, S.D. 4% remained constant, S.D. 2.5%

increased 1.5%, S.D. 1.5% remained constant, S.D. .15% Al 2 ° 3 Total Fe remained constant, S.D. 1% decreased 15%, S.D. 2% O

CM decreased 2%, S.D. 1.5% increased 4%, S.D. 4%

Na20 decreased 3%, S.D. .5% decreased 2.4%, S.D. .4%

CaO decreased 4%, S.D. 2% decreased .2%, S.D. . 6 %

aData from Mutschler et a l. (1981).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63

These results f it alteration chemistry associated with other typical

porphyry molybdenum deposits (Mutschler et a l., 1981; Lowell &

G uilbert, 1970; Greasy, 1959).

Potassium content on the other hand shows an increase in the

a rg illic assemblage in the Summit Creek stock (Table 8). Although

this also occurs at Henderson, it is not common in most a rg illic

alteration assemblages. This can be explained by an increase in

s e ric ite clays as compared to most a r g illic assemblages.

As discussed in previous chapters, pervasive alteration usually

associated with porphyry molybdenum deposits is absent. Limited

alteration may be explained by: 1) low HgO content of magmatic and/or

hydrothermal fluids, 2) a high degree of fracture control of fluid

movement, and 3) low temperature of hydrothermal flu id s .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V I I I

CONCLUSION

The Summit Creek stock that intruded into the Ordovician Phi

Kappa formation has been determined to be a mesozonal stock, by

comparision to similar two feldspar granites (Buddington, 1959).

The magma was probably the product of melting along a subducting

slab near the boundary between the top of the slab and the mantle

rocks of the overriding lithosphere (Dickinson, 1975). The melt is

estimated to have originated at a depth of 220 to 260 km (Figure 4).

The melt d iffe re n tia te d upon ascent to a quartz monzonite composi

tio n .

Trend surface analysis indicates that the Summit Creek stock

cooled with a border enriched in plagioclase, and became quartz and

potassium feldspar rich toward the center and western side o f the

exposed stock. The piuton 'followed a normal c ry s ta lliz a tio n sequence

as explained by Tuttle and Bowen (1958). The stock is the product of

a single intrusion and appears to be in the same orientation in which

i t was emplaced.

The Summit Creek stock was probably forcefully intruded into the

country rock. This is evidenced by the number of fingerlike dikes of

quartz monzonite along the contact. The sediments were domed up and

presently dip away from the center of the stock.

Whole rock chemical analysis indicates tha t the Summit Creek

stock is chemically equivalent to a high K calc-alkaline system.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65

This is synonymous with granodiorite molybdenite prospects (Mutschler

et a l., 1981). These intrusions are associated with steeper dipping

subducting slabs as compared to the granite molybdenite composition

of the Urad-Henderson (Figure 6) and Climax type deposits.

Trace elements Nb, Rb, and Sr are used to "fin g e rp rin t" stocks

with sim ila r whole rock chemistry and determine th e ir economic molyb

denum p o ten tial. In the Summit Creek stock, the Rb and Nb concentra

tions in ppm fe ll w ithin the range expected fo r high K calc-alkaline

composition field for stocks associated with molybdenum mineralization.

Moreover the Rb/Sr ra tio compared favorably with Transitional Molyb

denum deposits which generally contain economic molybdenum m ineraliza

tio n .

The intense alteration in the Summit Creek stock is a rg illic.

However, this alteration is Vocalized and confined to faults and

fractures, and grades into the widespread but weak p ro p y litic a lte ra

tion. The alteration geochemistry of the a rg illic assemblages was

similar to that of argillic alteration assemblages of most other

porphyry molybdenum systems.

Exposed mineralization is concentrated in quartz veining near

the contact between the intrusion and the impermeable country rock.

Increased fracturing near the contact between the intrusion and

country rock acted as a reservoir for the fluids that moved up to

the contact.

The Summit Creek stock has many s im ila ritie s both geochemically

and geologically to porphyry granodiorite molybdenite deposits. But

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the lack o f pervasive deuteric alte ra tio n is , however, an interesting

problem. This may indicate that the system did not contain large

enough quantities of hydrothermal fluids, or that the system did not

contain large enough quantities of mineralizing fluids. Also, it

may indicate that the ore body is quite deep and the pervasive

a lte ra tio n is not exposed.

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