GEOLOGICAL SURVEY CIRCULAR 704 

Molybdenite in the 

Montezuma District of 

Central Colorado 

Molybdenite in the 

Montezuma District of 

Central Colorado 

By George J. Neuerburg, Theodore Botinelly, 

and John R. Watterson 

GEOLOGICAL SURVEY CIRCULAR 704 

1974 
United States Department of the Interior 

ROGERS C. B. MORTON, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Free on application to the U.S. Geological Survey, National Center, Reston, Va. 22092 
CONTENTS 

Page Page 

Abstract·------------------------------------------------------------------------- 1 Mineralogy and distribution - Continued 

Introduction·------------------------------------------------------------------· 1 MineralogY------------------------------------··--··----------····------ 11 

Acknowledgments ...... ----··----------··----·--·····-··-----······ 1 Distribution of molybdenite in the district....... 13 

GeologY----------------------------------··--------------------··--------------···· 1 SummarY-----·------·--·-----------------------·--------------··-----------···--· 18 

Mineralogy and distribution of molybdenite............ 11 References cited·------------------------··---------------------····--------· 20 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of central and western Colorado, showing the Montezuma district in relation to the 

Colorado mineral belt·-··----···----··-···-----------------------------·-----------------------··-·------·····---------···--·--·---····--··--------· 2 

2. Map showing molybdenum deposits in relation to Tertiary stocks and gravity contours................ 2 

3. Map showing principal geologic elements of the Montezuma district ... ---------····----···--·······--··--······-·--·· 4 

4. Maps showing petrography of the Montezuma stock .... ---·-------------------------------------···---------··-·---·----···-·--··-· 6 

5. Photograph showing dilatancy differentiate of fine-grained aplite ....... ---··-----···---·-····-··--------·--·-·········· 7 

6. Map showing concentration of Precambrian Silver Plume Granite intrusives along the Montezuma shear zone ...... -----------------·-------------·--------------------·-··----····------------···---------------------····------------------------· 8 

7. Diagrams showing alteration zoning of porphyry-metal deposits ........ ---------···----····-------·········-----------·· 9 

8. Maps showing pervasive alteration of the Montezuma stock. ............... ---··-····-·········-·--·······---··············· 10 

9. Map showing areas of known silver-lead-zinc veins·---------------·-----·-·--····--·····-·----------··-----------------------···---- 12 

10. Diagram illustrating the independence of pyrite and molybdenite abundances among 25 olocks 

of unaltered aplite scattered over a small mine dumP------------------------··-·---··---··-····------------------------- 13 

11. Graphs showing molybdenite and pyrite abundance distributions among the three principal 

porphyritic varieties of quartz monzonite composing the Montezuma stock .... ---------···------------··· 14 

12. Maps showing the distribution of molybdenite and pyrite in the Montezuma stock as blip surfaces ....... -----------------------------··---------------···-----···-·--------------··------·--·-------------···---------------------···-------------------·-···· 15 

13. Diagrams showing molybdenite, pyrite, and bismuth blip curves for drill core from the line of 

the Roberts Tu:p.nel access shaft---------------------------------------···------------····------···--·----··---···-------------------······ 16 

14. Diagram showing molybdenite, pyrite, and bismuth blip curves for samples along the Roberts 

Tunnel in the western third of the Montezuma stock. ............... ---··-·········-·-··------·------···-·-·····--------- 16 

15. Map showing the distribution of veins containing bismuth minerals .. ------··-·----·-··-------------------------·-··· 17 

16. Map showing the bismuth blip surface for the Montezuma stock ....... ---··-···--·····-····---·-···----····----····---· 18 

17. Map showing the distribution of veins containing specularite·---------------·-··-···------------------------····---·····--· 19 

TABLES 

Page 

TABLE 1. Principal rock types of the Montezuma stock .... ---··----------·-·--····------------····---------------------------------------------·-·· 3 

2. Molybdenite and pyrite variances in eight igneous rocks according to sample area, rock type, 

alteration, and weight of analyzed fragments·---------------------------····----····--------------------····----····---------- 14 

III 

Molybdenite in the Montezuma District of Central Colorado 

By George J. Neuerburg, Theodore Botinelly, and John R. WaHerson 

ABSTRACT 

The Montezuma mining district, in the Colorado mineral belt, is defined by an assemblage of porphyry, ore, 

and altered rocks that originated in the venting of a 

Tertiary batholith through weak structures in Precambrian rocks. The ore consists of silver-lead-zinc 

veins clustered on the propylitic fringe of a geometrically complex system of altered rocks, which is centered 

on the intersection of the Oligocene Montezuma stock 

with the Montezuma shear zone of Precambrian ancestry. Alteration chemistry conforms to the standard 

porphyry-metal model but is developed around several 

small intrusives strung out along the shear zone and 

is expressed as a mottled pattern, rather than as the 

usual thick concentric zones centered on one large plug. 

The distribution of trace amounts of molybdenite is 

consistent with the postulate of molybdenite deposits 

in the district, but the mottled alteration pattern may 

signify small and scattered, possibly very deep, deposits. Disseminated molybdenite is essentially coextensive 

with altered rock and increases slightly in quantity 

toward the inner alteration zones. Two groups of molybdenite veins, associated with phyllic and potassic alteration, represent possible diffuse halos of molybdenite 

deposits. One group of veins resembles the Climax and 

Henderson deposits but was seen only in a small and 

isolated area of outcrops. The second group of molybdenite veins is in a bismuth-rich part of the Montezuma 

stock and underlies an area of bismuth veins; this 

group records the passage of contact metasomatic ore 

fluids. Another bismuth-rich area is in the southeast 

corner of the stock in a region of bismuth veins and 

may indicate a third group of molybdenite veins. 

INTRODUCTION 

Molybdenite occurs in trace amounts in the 

rocks and ores of the Montezuma district. Molybdenite mineralization is assigned to the 

Oligocene porphyry-ore association of the Colorado mineral belt (Tweto, 1968). This Colorado 

porphyry-ore association belongs to the remarkably consistent and worldwide porphyry-metal 

class of ore deposits. The pattern of molybdenite distribution in the Montezuma district 

1 

provides evidence to help locate probable porphyry-molybdenum deposits in the district. 

ACKNOWLEDGMENTS 

Information regarding the character of rocks 

at depth in the Montezuma district was obtained largely through the kindness of other 

people, beginning with the many who prospected and mined the district from 1864 to 

date. John A. Thomas, of AMAX Exploration, 

Inc., permitted us to examine much drill core 

from the Geneva Creek cirque. Douglas N. 

Stevens, of Earth Sciences, Inc., permitted us 

to sample core from near Webster Pass. These 

cores and parts of the Roberts Tunnel collection, obtained for the U.S. Geological Survey 

by C. S. Robinson, were of exceptional value in 

characterizing the pervasive alteration of the 

district. Molybdenite distribution at depth in 

the western third of the stock was determined 

from the Roberts Tunnel collection, Denver 

Water Board drill core in the U.S. Geological 

Survey core library, and from data obtained by 

P. K. Theobald in his visits to the Roberts Tunnel access shaft. The bismuth analyses summarized herein were made by Eric P. Welsch 

of the U.S. Geological Survey. 

GEOLOGY 

The Montezuma district, situated along the 

Continental Divide just south of Loveland Pass, 

includes. one of the clusters of porphyry, ore, 

and altered rock composing the Colorado mineral belt (fig. 1). The clusters that have stockwork molybdenite deposits (King and others, 

1973) are virtually contained by the -300 

milligal gravity contour of the central Colorado 

Rocky Mountains (fig. 2). The gravity low 

probably closely outlines the long-hypothesized 
109° 105° 104" 41° 

-o 

'V ODENVER 

Ul~ lJ -pGrand /~ ~ / Montezuma 

0 Junction ELKf q ~I district 

MTlj.l :11 ../ 

WES~( i ( Area of 

ELK "'-) f'\ \ figure 2 

MTS 

/ /--

~"'/ (~~ /~~ I I SAN JUAN 

I; MOUNTAINS 

I f 

v 

37° 

0 50 100 MILES 

I I I 

I 

I I 

I 

0 50 100 150 KILOMETERS 

FIGURE 1. -Index map of central and western Colorado, showing Montezuma district in relation to the 

Colorado mineral belt. 

mineral-belt batholith (Crawford, 1924; Tweto 

and Case, 1972), and coincides with an element 

of structural weakness of Precambrian ancestry, expressed as zones of recurrent shearing 

(Tweto and Sims, 1963). 

The porphyry stocks, ore deposits, and altered rocks are cogenetic products of the underlying batholith, and they locate sites of venting 

of the batholith. The Colorado porphyry-ore 

assemblage is evidently a normal product of 

calc-alkalic batholiths that have vented (Hulin, 

1944; Kennedy, 1955, p. 496-497; Sillitoe, 1972, 

p. 189-191) 0 

The geology of the Montezuma district 

(fig. 3) is dominated by the Montezuma stock, 

which underlies most of the district, by the 

Montezuma shear zone, and by a large field of 

altered rock centered on the intersection of the 

stock and the shear zone (Neuerburg and Botinelly, 1972) . Other structures control the distribution of small porphyry intrusives, ore 

deposits, and related altered rock within the 

district. Precambrian gneisses, amphibolites, 

granites, and pegmatites, and Cretaceous black 

shales are the host rocks for the assemblage of 

porphyry, ore, and altered rock. 

Glacial scour and debris are prominent. Talus 

40° 

0 10 20 MILES 

0 10 20 30 KILOMETERS 

EXPLANATION 

Molybdenum deposits--From King(l970), Bryant(l971), 

and this study 

• Disseminated deposits of molybdenum 

~ 

. 

• Veins and other types of deposits of molybdenum 

Outcrop of Tertiary stocks--From Tweto and Sims(l963) 

. 

--250- Gravity contours, in milligals-- From Behrendt and Bajwa (1972) 

2 

FIGURE 2.- Map showing molybdenum deposits in relation to Tertiary stocks and gravity contours. 

and rockslides cover large segments of the 

lower valley walls. Ocher-cemented gravels and 

bog-iron deposits, both preglacial and postglacial, cover large areas of the lower slopes and 

valleys and some of the preglacial upland surfaces in the large field of altered rock along the 

Montezuma shear zone. The bedrock outcrop 

varies greatly in amount but has been estimated 

to be on the order of 5 percent (Wahlstrom, 

1964). 

The Montezuma stock consists of several varieties of quartz monzonite, granite aplite, and 

rhyolite. Variation is largely exhibited by different intergradational porphyritic textures ; the 

principal varieties are listed in table 1 and are 

plotted in figure 4. The composite nature of the 

stock resulted from repeated intrusions, dilatancy differentiation (Mead, 1925; Emmons, 

1940; Nielsen, 1968; fig. 5), and pressure 
Order 

No.1 

TABLE 1.-Principal rock types of the Montezuma stock 

[All rocks occur in the Roberts Tunnel as well as on the surface] 

Rock type, description, and remarks 

0 Porphyritic biotite 

quartz monzonite. 

Medium-grained hypautomorphic; 0-20 percent ragged potassium feldspar porphyroblasts. Makes up most of the western third of the stock; occurs among 

apophyses in the Williams Fork thrust fault. Some volatile loss at an early stage 

of crystallization, producing contact metasomatism but leaving no record in the 

1 Porphyritic-seriate 

biotite quartz 

monzonite. 

2 Seriate biotite 

quartz monzonite. 

3 Biotite quartz 

monzonite porphyrybiporphyry. 

igneous texture. 

Medium-grained hypautomorphic in an interstitial web of fine-grained xenomorphic-graphic. Sparsely miarolitic; 0-25 percent ragged potassium feldspar 

porphyroblasts. Occurs independently as ill-defined masses in the porphyritic 

quartz monzonite and as a transitional zone between the porphyritic quartz 

monzonite and the quartz monzonite biporphyry; occurs among apophyses in 

the Williams Fork thrust fault. Volatiles were lost late in crystallization but 

incompletely, as shown by miaroles and porphyroblasts. 

Medium- to fine-grained hypautomorphic to xenomorphic. Very sparse plagioclase 

phenocrysts and ragged potassium feldspar porphyroblasts. Mode of occurrence 

undetermined; seen only in isolated outcrops. A slow, steadily accelerating and 

complete loss of volatiles beginning midway in crystallization. 

Medium-grained hypautomorphic biotite-quartz-plagioclase aggregates, plagioclase 

crystals, and corroded quartz bipyramids in an aplitic to aphanitic groundmass, 

thereby grading into quartz latite. Sparsely miarolitic; 0-35 percent ragged to 

euhedral potassium feldspar porphyroblasts. Occurs mostly as large irregular 

masses in much of the. eastern half of the stock, but also as sparse small intrusives and as the mesostasis for breccias (fig. 5). Makes up most of the dikes and 

plugs outside the stock. Volatiles exsolved quietly and rapidly early in crystallization, but the loss was incomplete so as to produce miaroles and porphyroblasts. 

The materials removed with the volatiles left a slightly more basic rock than 

would have otherwise crystallized. 

4 Biotite rhyolite Medium-grained phenocrysts, variously euhedral, broken, or corroded; include 

porphyry. pptassium feldspar as well as biotite, plagioclase, and quartz; the groundmass 

is very fine grained aplitic to aphanitic. Euhedral potassium feldspar porphyroblasts are uncommon. Occurs as small intrusives, as transitional zones on aplite 

bodies, as joint selvedges, and as the mesostasis of breccias (fig. 5), especially 

near the upper contacts of the stock. Occurs as dikes and sills in Precambrian 

rocks, mostly off the southeast edge of the stock in the Geneva Creek cirque. 

Volatiles were exsolved early and suddenly, in part violently, during crystallization; the surging exsolution of volatiles was inskumental in the in situ differentiation of rhyolite. 

5 Granite aplite ............ ___________ Very fine to fine-grained, xenomorphic to graphic. Essentially the same chemical 

composition as the rhyolite, although very little biotite is present. Sparse euhedral potassium feldspar porphyroblasts and pegmatite patches with pyrite, 

molybdenite, and (or) hyalophane. Occurs as tabular bodies, ranging in thickness from a fraction of an inch to several hundred feet, and as dilatancy differentiates (fig. 5); not known outside the stock. Volatile loss was essentially 

complete and was sudden, beginning before the onset of crystallization, and was 

instrumental in the differentiation and intrusion of the aplite magma. 

lRock types are arranged in order of suggested extent of volatile loss during crystallization (fig. 4). 

quenching through "sudden" devolatilization 

(Neuerburg, 1971). 

The fine-grained groundmass and partly resorbed quartz phenocrysts of the porphyries 

resulted from pressure quenching (Jahns and 

Tuttle, 1963) and accompanying changes in 

chemical equilibrium. Subsequently, after hermetic sealing of residual fluids in the near-solid 

3 

rock (N euerburg, 1958), large potassiumfeldspar "phenocrysts," properly identified as 

porphyroblasts, formed to give rise to the 

pegmatitic crystallization-recrystallization described by Jahns and Burnham (1969). These 

large potassium feldspar porphyroblasts in a 

porphyry with smaller quartz and feldspar 

phenocrysts are the essential element of the 
35' 

321 

30" 

52'30" 

XWise Mountain / 

0 

I 

0 

50' 

XGroys Peak 

I 

2 

Cone 

I 

2 MILES 

I 

tunnel 

3 KILOMETERS 
EXPLANATION 

~ Oligocene porphyry of the Montezuma stock 

Cretaceous hornfels 

D Precambrian ~neiss and ~ranite 

~=------=-

~~~ Shear zone 

--- Trace of preintrusive fault or shear zone 

0 

Fault 

Rusty altered rock field 

Ore-fluid conduit--Area of vu~~y 

quartz-sericite rock 

FIGURE 3 (facing page and above).- Principal geologic 

elements of the Montezuma district, emphasizing the 

distribution of porphyries and altered-rock fields and 

showing ore-fluid conduits. 

Lincoln Porphyry type of texture (Emmons, 

1886, p. 328), or biporphyry (Neuerburg, 

1971), so common in the porphyry-metal model 

(Fournier, 1967). 

The chemical and textural changes in the 

Montezuma stock that relate to pressure gradients are a record of the selective loss of water, 

other volatiles, and dissolved metals. The 

changes thus identify source rocks for ore fluids. The volatile loss is interpreted to be greater 

as the groundmass grain size is finer and the 

proportion of phenocrysts is less. The distribution of the different rocks within the stock (fig. 

4), ranked according ,to the suggested degree 

of volatile yield, conforms to the geometry of 

the hydrothermal plumbing shown by vuggy 

quartz-sericite rock (fig. 3), except that the 

aplites plugged the vent structures they now 

occupy and, thus, were not appreciably involved 

with the later passage of hydrothermal fluids. 

Quartz monzonite, quartz latite, rhyolite, and 

sparse lamprophyre form dikes, sills, and plugs, 

mostly in and along the Montezuma shear zone ; 

a very few occur in the stock. Small bodies of 

Silver Plume Granite also occur mostly in and 

along this shear zone (fig. 6) and show that 

this shear zone localized intrusion as early as 

the Precambrian. Repeated venting in any one 

5 

area is common on a short-term basis, as in the 

Montezuma district (Neuerburg and Botinelly, 

1972), at Climax (Wallace and others, 1968), 

and at Urad-Henderson (MacKenzie, 1970). 

Rusty to bleached altered rocks form brightly 

colored fields of outcrop and talus above timberline in the Montezuma district. The largest of 

these fields, whose extent is sketched in figure 

3, identifies the strong relation or altered rock 

to the Montezuma shear zone and associated 

porphyries above the roof of the Montezuma 

stock. Other, much smaller fields of altered 

rock occur scattered in the district; some of 

these are shown in figure 3. 

Chemically and mineralogically, pervasive 

alteration in the Montezuma district accords 

well with the standard porphyry-metal model 

(Lowell and Guilbert, 1970; Sillitoe, 1973), 

first sketched by W. H. Emmons (1927). Geometrically, it differs appreciably from the model 

of nested cylinders (fig. 7). The Montezuma 

shear zone has profoundly modified the usual 

pattern of porphyry-ore venting from the single 

plug-cylindrical strain pattern exemplified by 

the Climax and Urad-Henderson deposits, to a 

linear swarm of plugs and dikes stemming from 

a relatively flat-roofed stock. The different 

types or zones of alteration are distributed 

among numerous permeable structures to yield 

a mottled pattern (fig. 8A). All parameters of 

mineralization have this mottled distribution 

pattern and at all scales, from a single hand 

specimen to the entire district. 

The innermost potassic alteration zone of the 

porphyry-metal model is recognized here only 

in two neighboring outcrops of quartz latite 

porphyry in the Snake River cirque (fig. 3). 

The adjacent rocks are argillized gneiss and 

sericitized porphyry. Alteration in these two 

outcrops consists of recrystallization of the 

groundmass to a granoblastic mosaic of quartz 

and potassium_ feldspar, with unaltered plagioclase and biotite phenocrysts, and of veining 

by a stockwork of quartz-magnetite-pyrite-molybdenite veinlets. Corundum, otherwise very 

rare in porphyries of the Montezuma district, 

is relatively abundant in these ·two zones and 

amounts to one-tenth of one percent by weight. 

Corundum, so easily overlooked except in HFseparates, is mentioned here because it may be 
A 

I-Cil

4

z..,3o 011 . ..,_, . uo..20 : a: ::E • 

~~ 10 : 0 . ·. 

B 

0 

0 

A.--AREA IN WHICH SAMPLES ARE--

Aplite 

Rhyolite porphyry 

Quartz monzonite biporphyry 

Seriate quartz monzonite 

Porphyritic seriate quartz monzonite 

Porphyritic quartz monzonite 

2 

2 MILES 

XGroyo 

Pook 

3 KILOMETERS 

5 

4 

3 

2 

0 

8.-- DEVOLATILIZATION 

RANKING (TABLE I) 

FIGURE 4.- Petrography of the Montezuma stock. A, Map and histogram showing the distribution of rock 

types among samples. B, Contours showing ranking of volatile loss during crystallization as represented 

by textures. (See text.) Based on maximum rank per cell in a 2,000-foot (610-m) grid. 

6 
FIGURE 5. - Dilatancy differentiate of fine-grained granite aplite, leaving as a residue quartz monzonite 

porphyry. Stained slab: dark-gray crystals are plagioclase; medium-gray, quartz and potassium feldspar. 

an index mineral of the potassic zone of alteration. 

Irregular cylindrical or tabular bodies of 

phyllic rock occur in the altered rock field scattered along the Montezuma shear zone. The 

phyllic rock is commonly in abrupt contact with 

rock of varying and commonly much lesser 

alteration. Phyllic alteration is concentrated 

principally within the satellitic porphyry intrusives. It also occurs in the Precambrian rocks, 

particularly the Silver Plume Granite, and is 

localized by faults and other planar structures. 

Phyllic zone alteration produced a pyritic 

quartz-sericite rock, whose original texture was 

commonly destroyed. Quartz and quartz-pyrite 

veinlets, locally grading into silicified rock, are 

common in random distribution. Although pyrite is conspicuous and abundant, the amount 

of iron in the pyrite is less than the original 

iron content of the rock. Other sulfides, which 

include bismuthinite, chalcopyrite, molybdenite, 

and sphalerite, are relatively rare. Chemical 

analyses show very little added ore metals in 

phyllic zone rocks as compared to the rocks of 

the propylitic zone. Andalusite, diaspore, pyrophyllite, and (or) talc uncommonly take the 

place of some of the sericite. Barite occurs. erratically but is usually present in the larger 

quartz-pyrite veins of the phyllic zone. Lavender anhydrite and colorless gypsum veinlets 

7 

occur sparingly but may increase in abundance 

with depth. 

Some of the phyllic rock is very vuggy and 

highly permeable, a structure which has been 

called bug-hole porphyry at Urad (MacKenzie, 

1970, p. 41) and elsewhere (Taylor and King, 

1967, p. 12) in the mineral belt. This structure 

is not restricted to porphyry at Montezuma; 

several areas containing abundant examples of 

the bughole structure are shown in figure 3. 

The vugs are concentrated in but not restricted 

to phenocrysts and are lined with euhedral terminations of muscovite and quartz upon which 

are perched euhedral crystals of pyrite, rutile, 

and rarely sphalerite. This vuggy phyllic rock 

is commonly accompanied by quartz-pyrite 

veins and identifies major conduits in the hydrothermal plumbing. 

The argillic zone is volumetrically unimportant in the district. Mineralogically, it is only 

rarely argillic; the usual rock of this zone is 

one in which biotite and plagioclase are sericitized and epidotized- that is, saussuritized; 

about 1 percent pyrite was formed, and the potassium-feldspar was left unchanged. Like the 

phyllic rocks, rocks of the argillic zone show 

a structurally controlled "mottled" distribution. 

Locally, these rocks form incomplete shells on 

bodies of phyllic rock. 

Propylitic alteration is present in most of 

the district. Although it is most apparent in 

the fields of rusty altered rock, it extends far 

beyond their limits. This alteration consists of 

chloritized biotite and moderately sericitized 

plagioclase, together with added pyrite, carbonate, and, locally, minor sphalerite. The 

propylitized rocks are generally enriched in ore 

elements, especially zinc. Precipitation of pyrite, carbonate, and sphalerite is apparently 

catalyzed by the biotite structure (Schwartz, 

1958, p. 17 4; Al-Hashimi and Brownlow, 1970, 

p. 991), with or without concomitant alteration 

to chlorite. The amount of pyrite, however, is 

unrelated to the amount of chloritized biotite, 

and the iron content of biotite is the same as 

the amount of iron in chlorite. The added pyrite 

apparently represents iron removed from the 

phyllic • zone; the propylitic alteration results 

from the fluids modified by the reactions of the 

phyllic zone (Spurr, 1905). Mineralogically, 

propylitie alteration grades out through a re-

. gion in which only biotite is altered (fig. 8). 
35' 

32' 

30" 

52'30" 

Outline of 

Montezuma district 

XWfse Mountain ·.; 

- 0 

0 

50' 

XGroys Peak 

Cone 

EXPLANATION 

• Silver Plume Granite 

2 MILES 

2 3 KILOMETERS 

FIGURE 6.-Concentration of Precambrian Silver Plume Granite intrusives along the Montezuma shear zone. 

8 
A 

WILLIAMS FORK 

~UST FAULT 

8 

Pb-Zn precious 

metal veins 

Disseminated 

Silicification and advanced 

argillic alteration 

Propylitic alteration 

Sericitic (phyllic) alteration 

alteration 

GRANODIORITE PLUTON 

MONTEZUMA STOCK 

I 

MONTEZUMA STOCK 

~MON SHEAR TEZUMA ZONE 

alteration 

Precambrian 

rocks 

FIGURE 7.- Diagrams showing alteration zoning of porphyry-metal deposits. A, The usual model, centered on a 

plug like intrusive (modified from Sillitoe, 1973). B, The Montezuma pattern. 

9 
f!l60l ~50 

~40 1/) 

1-30 

~20 0 

a: 10 

"' "- 0 

8 

0 

0 

A.--AREA IN WHICH SAMPLES ARE--

Vuggy sericitic-- "bughole porphyry" 

Phyllic zone--sericitized 

Argillic zone-- saussuritized 

Propylitic zone-- propylitized 

Propylitic zone-- chloritized 

Unaltered 

2 

2 MILES 

XGroyo 

Peak 

3 KILOMETERS 

5 

4 

3 

2 

0 

B.--SEQUENCE OF ALTERATION ZONES 

FIGURE 8.- Pervasive alteration of the Montezuma stock. A, Map and histogram showing the distribution of 

alteration types among samples. B, Contours showing distribution of maximum alteration among cells in a 

2,000-foot ( 610-m) grid. 

10 
Physically, it grades into unaltered rock through 

restriction of alteration to widely spaced joints 

and foliation planes. 

The known ore deposits of the Montezuma 

district are silver-lead-zinc veins, concentrated 

in and on the edges of the altered rock fields 

(fig. 9) and extending into regions of unaltered 

rock. Episodes of repeated venting have produced telescoping and the emplacement of a few 

veins deep within the phyllic and argillic zones. 

The vein deposits were precipitated from large 

volumes of solutions channeled through restricted structures. In contrast, metal-enriched 

pervasive propylitic alteration zones formed 

where similarly large volumes of solutions permeated large masses of rock. 

The vein deposits are encased in envelopes of 

altered rock, ranging from phyllic, next to the 

vein, to the zone that characterizes the host 

rock. The argillic zone of alteration envelopes, 

where present, is mostly represented by a saussuritized rock like that of the pervasive alteration. In contrast to the pervasively altered 

phyllic and argillic rocks, the corresponding 

altered rocks flanking vein deposits are notably 

enriched in ore metals and contain disseminated 

ore and gangue minerals. No distinction is apparent between pervasive propylitic rock and 

the propylitic rock flanking veins. 

Alteration associated with veins in the western third of the stock differs from the pattern 

just described. In this region, quartz monzonite is replaced by coarse-grained sericite and 

quartz, in places with clots of pyrite and (or) 

molybdenite, and the altered rock resembles 

greisen. Sericitization is first evident in biotite 

and progresses through plagioclase to potassium feldspar; it is localized by joints. The 

change from unaltered to altered rock is abrupt 

in most places. This alteration is related to contact metasomatism on the southwest contact of 

the stock (Neuerburg and others, 1971). 

MINERALOGY AND DISTRIBUTION 

OF MOLYBDENITE 

This study is based upon dissimilar groups of 

samples, each of which has its own bias or error 

(Miesch, 1967). Extensive sampling of outcrop 

has been almost confined to the igneous rocks. 

Unaltered quartz monzonite is poorly exposed 

and much is weathered to a friable state. Propy11 

litic zone rocks are well represented in outcrop, 

as shown in the sample distribution by alteration type in the stock (fig. SA, histogram), 

whereas phyllic · and argillic zone rocks are 

rarely found in outcrop. Samples of argillic and 

phyllic rocks have come largely from mines and 

prospects and probably compose a reasonably 

homogeneous set. Samples of vein material are 

extremely variable intrinsically, as well as being the "capricious" discards of the miner. 

Sampling was designed to test for evidence 

of contact with ore fluids shown by the presence 

of altered minerals and the addition of ore elements, and the chemical composition of the 

hydrothermal fluid indicated by the amount and 

variety of added elements and by the zonal 

sequence of alteration and mineralization. To 

evaluate these hydrothermal effects, the maps 

were gridded on a 2,000-foot (610-m) interval, 

and the highest value of the chosen parameter 

among the samples in each square was plotted 

at the center of that square. A similar~ maximizing procedure has been followed for linear 

traverses, as in the Roberts Tunnel and in diamond-drill holes, except that for the linear 

traverses the interval is reduced to 1,000 and 

100 feet (300 and 30 m), respectively. The 

maps and curves so generated will be called the 

blip surface or blip curve and serve to make 

more legible both general trends and relations 

of local maximums to one another and to geologic structures. 

MINERALOGY 

The only molybdenum mineral found in the 

Montezuma district is molybdenite. The possibility that molybdenum substitutes in a major 

way in rock mineral structures of the Montezuma stock seems ruled out by the rarity of 

molybdenum in biotite and chlorite; only 21 out 

of 500 spectrographic analyses of biotite and 

chlorite showed any molybdenum (lower limit 

of determination: 5 ppm Mo). Molybdenite has 

the 2H polytype (Traill, 1963; Fronde! and 

Wickman, 1970) in all occurrences but one, in 

which a molybdenite grain from a stream sediment is a mixture of the 2H +3R polytypes. The 

crystals are generally thin and tabular hexagonat euhedrons to irregular space-filling 

anhedrons. Most disseminated crystals of molybdenite are 2 mm (millimetres) or less in 

maximum dimension. Molybdenite in fracture 
35' 

32' 

30" 

52'30" 

Outline of 

Montezuma district 

XWise Mountain 

FIGURE 9 A . - reas of known silver-lead -zmc · veins 

12 

50' 

0 

0 

Cone 

EXPLANATION 

~Areas of Ag- Pb- Zn veins 

,J'\ Ore-fluid conduit--

'-"""' vuggy quartz- . A_rea of senclte rock 

3 KILOMETERS 

in th e Montezuma district. 
fillings, in veins, and in the rare pegmatitic 

pockets in aplites ranges from 1 J-tm (micrometre) to 2 em (centimetres) in maximum 

dimension. Supergene oxidation of molybdenite 

is indicated by the presence of molybdenum in 

ammonium carbonate leaches of some altered 

rocks, but no secondary molybdenum minerals 

have been identified. 

Molybdenite is common in HF separates 

(Neuerburg, 1961) from 100-g (gram) samples 

of rocks from the district. Molybdenum is seldom reported in chemical or spectrographic 

analyses of matching samples because molybdenite, a flexible mineral concentrated into a relatively few crystals, is very difficult to disperse 

evenly enough to assure the detection of molybdenum in the small samples used for chemical 

(0.1 g) or spectrographic (0.01 g) analysisthat is, the same particle sparsity effect encountered in gold analyses (Clifton and others, 

1967, 1969). For example, 28 of 86 samples of 

Silver Plume Granite showed molybdenite in 

HF separates ; only 2 of these showed molybdenum by spectrographic analysis. 

DISTRffiUTION OF MOLYBDENITE IN THE 

DISTRICT 

Molybdenite is disseminated in trace amounts 

in many of the rocks of the Montezuma district 

and, uncommonly, in vein and fracture fillings. 

Disseminated molybdenite shows no obvious 

preferential fabric site, or mineral association, 

whereas vein molybdenite is commonly concentrated along the edges of veins. Molybdenite 

conforms to the general distribution pattern of 

ore elements and minerals in the district, except that it has not been found in any of the 

silver-lead-zinc veins. The general pattern of 

distribution is that ore elements vary independently from each other and are independent of 

host-rock type, in accord with the mottled pattern of distribution of alteration zones. Smallscale variance differs among rock types and 

between outcrops of the same rock type, as 

exemplified by molybdenite and pyrite (table 2, 

fig. 10). Detected regularities of element distribution relate to types of alteration, to permeable structures, and to the relative quantities of 

fluid that passed through these structures, as 

indicated by the extent of hydrothermal-related 

change in the structure. Overall, the addition 

12 • 

II 

• • ffi 9 Q. 

(/) 8 

~ 

<[ 7 0::: 

(!)6 

• 

•+ • + ++ 

• 

• • 

• • • ~ 

uj5 

..... 

z 4 lLI 

0 

m 3 

+ 

+ 

• 

+ 

+ 

+ 

+ 

••• 

• 

• 

• 

+.+ 

+ 

• 27 

-· 75 ~ + 

+ + ++ + ~ 2 t -· 

+ + 

13 

• 

0

o 2 3 4 5 6 7 8 9 10 II 

PYRITE, IN GRAMS PER TON 

FIGURE 10.- Diagram illustrating the independence of 

pyrite and molybdenite abundances within one block 

( +), and among 24 additional blocks ( •) of unaltered aplite scattered over a small mine dump. 

of elements (Neuerburg, 1971) from heterogeneous fluids of time-dependent composition to 

rock where access of the fluids was channeled 

and constantly shifting at all scales is superimposed on an earlier igneous pattern of oreelement removal (table 1, fig. 4). 

Patterns of molybdenite distribution in the 

different major rock units of the Montezuma 

district, apart from the Montezuma stock, remain undetermined for the most part. Molybdenite is known to occur in trace amounts in 

some of the Precambrian gneisses. It is a minor 

component of the Silver Plume Granite, to 

which it has probably been added from the 

district ore fluids. Molybdenite has been found 

in the tactites developed from the Cretaceous 

shales, and molybdenum has been found spectrographically in the Cretaceous shale-hornfels, 

but only in samples collected from mineralized 

structures. 

Statistically, molybdenite in the Montezuma 

stock shows a slight increase in abundance with 

decrease in grain size of the groundmass (fig. 

11) ; this trend is like that exhibited by pyrite, 

but ·less pronounced. The degree of propylitic 

alteration also increases in rocks with finer 

grained groundmass. However, the decrease in 

groundmass grain size (fig. 4), along with the 

increase in pyrite and molybdenite abundance 

(fig. 12) and the degree of propylitization, is 
TABLE 2.- Molybdenite and pyrite variances in eight igneous rocks according to sample area, rock type, 

alte'ration, and weight of analyzed fragments 

Sample1 Molybdenite (g/ton) Pyrite (g/ton) 

Rock type and alteration Weight Nature (grams) Number2 Range Avg. Range Avg. 

Silver Plume Granite, Sawed block. ......... 35±3 24 

unaltered. 

0 0 735 -1,930 1,200 

Porphyritic quartz monzonite, ...... do ..................... 40±5 7 0.8-109 22 95 -1,850 355 

unaltered.3 120±15 5 2.3-85 19 75 -7,200 510 

110±8 10 1.7-75 14 45 -1,190 255 

Granite aplite, unaltered' .......... Broken block ........ 65±15 1.7-6.6 3.6 0.8-12 3.5 

Mine dump ............ 56±6 1 .-12 5.4 2.1-75 9.5 

Porphyritic quartz monzonite, Sawed block .......... 35±5 4.8-130 35 14 -750 115 

chloritized. 

Porphoritic-seriate quartz ...... do ..................... 35±3 3 .7-30 7.6 15 -600 175 

monzonite, chilled. 

Quartz latite biporphyry, 

propylitized, pervasive. ...... do ..................... 33±5 24 0 0 11 -975 310 

Porphyritic quartz monzonite, ...... do ..................... 40±4 15 20 -93 49 1,220 -14,600 4,900 

phyllic, vein envelope. 

Quartz latite biporphyry, ...... do ..................... , 34±4 24 0 0 38,500 -61,700 51,800 

vuggy -sericitic, pervasive. 

1Blocks of rock weighing 5-10 kg were sawed or broken into many fragments ; from each group of fragments, 24 samples were taken for analysis. 

"-

0 

2Entry is number of samples, out of the total of 24, with no molybdenite. 3Three sample groups from the same roadcut, 300 ft long, near the western edge of the stock. iThese two sample groups are from the same mine dump, on the north edge of Montezuma. See also fig. 10. 

(nd) 

-$- Median -$-Median and no estimate of abundance is hazarded. Disj ... : seminated molybdenite is relatively uncommon i ~ f and haphazardly distributed. Veinlet- and fraco

i ! co ture-filling molybdenite is likewise randomly 

_ _ ~ distributed but appears to increase with inL-...L......::::=-----...1.....- creasing silicification. 

• Median 

0 2 3 

MOLYBDENITE, LOG OF 

GRAMS PER TON 

4 5 

N 

0., 

:.: 

fTC 0 U N 

a a ·;::: E 

: 

-$-Median 

-$-Median 

(nd) -1 0 2 

PYRITE, LOG OF 

GRAMS PER TON 

Two somewhat contrasting groups of molybdenite veins are known in the district; neither . t constitutes ore deposits. The stockwork of 

~ quartz-magnetite-pyrite-molybdenite veinlets 

~ in the potassic alteration zone in the Snake 

River cirque (fig. 3) is flanked several hundred 

feet to the west by sericitized porphyry containing molybdenite-faced fractures. These outcrops 

: resemble parts of the Climax and Henderson 

~ deposits. Satisfactory characterization is not 0 

• possible because weathering is intense, and 

these small outcrops are isolated in a great field 

3 of bog-iron deposits and ocher-cemented gravels. 

FIGURE 11. - Molybdenite and pyrite abundance distributions among the three principal porphyritic varieties of quartz monzonite composing the Montezuma 

stock; nd, not detected. 

more meaningfully correlated with the position 

of major venting regions, as identified by vuggy 

quartz-sericite rock, than with each other. 

The molybdenite veins on the north flank of 

Independence Mountain (fig. 3) occur over a 

large area. This group is known from three 

caved mines, from the Roberts Tunnel access 

shaft, and from the Roberts Tunnel. The occurrence is of scattered quartz veinlets with coarse 

molybdenite in thin envelopes of coarsely sericitized quartz monzonite. The occurrence was 

early noted in a geographically misleading 

entry under Summit County (Horton, 1916, 

table, p. 64): "Lenawee mine, between KingsMolybdenite distribution in the argillic and 

phyllic zones, both in the porphyries and in the 

Precambrian rocks, appears to be very uneven, 

14 
A 

I&J...J 

~f!l3o 40tt ~~ 20 

~~ 10 

0 HAROLD 0 ROBERTS 

TUNNEL 

~f!l3o 40u : I&J...J • 

~~20 : 

"'"' 10 :., Cl.(l) ·:· 

0 . ·= 

0 

0 

A.--MOLYBDENITE, 

IN GRAMS PER TON 

I, 000- 9, 999 

100-999 

10-99 

1-9 

0.1-0.9 

Not detected 

2 

2 MILES 

3 KILOMETERS 

8.--PYRITE,IN GRAMS 

PER TON 

10,000-99, 999 

1,000-9,999 

100-999 

10-99 

1-9 

FIGURE 12.- The distribution of molybdenite (A) and pyrite (B) in the Montezuma stock as blip surfaces. 

(See text.) 

15 
ton and Montezuma"; to correct, substitute 

the word "Keystone" for "Kingston." Kingston 

was in Clear Creek County (Eberhardt, 1968, 

p. 65). 

Molybdenite veins in the access shaft are 

concentrated in and below a body of aplite 

about 800 feet (250 m) below the collar of the 

shaft; veins above the aplite contain silverlead-zinc minerals, except for one composed of 

a bismuth mineral. Molybdenite veins in the 

Roberts Tunnel are near the foot of the shaft. 

The region of molybdenite veining in the access 

shaft and the tunnel is clearly indicated by the 

blip curves for disseminated molybdenite, pyrite, and bismuth (figs. 13, 14). Bismuth veins 

overlie and partly overlap the molybdenite-vein 

region (fig. 15), which is in a bismuth-rich segment of the Montezuma stock (fig. 16). Apart 

from the three widely separated mines, noted 

above, this molybdenite-vein region is not reflected in surface exposures. It is clearly shown 

by the district plot of bismuth veins (fig. 15). 

The high bismuth content of rocks in Warden 

Gulch, where the field of rusty altered rock 

transgresses the stock (fig. 16), together with 

the nearby areas of bismuth veins (fig. 15) may 

outline another unexposed group of molybdenite 

veins. Areas of specularite veins (fig. 17) are 

related to bismuth mineralization (Neuerburg 

and others, 1971) and are larger and more easily located targets than are areas of bismuth 

veins. 

The molybdenite veins of the Snake River 

cirque have characteristics typical of porphyrymetal deposits, paramount among which is the 

stockwork veining on pyroclastic structures in 

pressure-quenched porphyries. The molybdenite 

veins of Independence Mountain do not have 

these characteristics, differing particularly in 

the absence of pervasive alteration. Further, 

deposition was controlled by flow-restrictive 

major structures, such as the relatively impermeable aplite of the Roberts Tunnel access 

shaft. The ore minerals in both groups of veins 

were precipitated from metal-bearing fluids secreted from the subjacent batholith in response 

to pressure gradients; those of Snake River in 

a temporarily closed system, those of Independe~ce Mountain in a continuously open system. 

The Snake River veins derive from metalbearing volatiles concentrated in a cupola by 

pressure-solubility equilibration in the gravita16 

1-

w 

w 

LL. 300 

w 400 u 

it 

~ 500 (/) 

:::!E 

0 600 ct:: 

LL. 

1000 L_____L_~_ 

00 1.0 2.0 3.0 

MOLYBDENITE, LOG 

OF GRAMS PER TON 

2.0 3.0 4.0 5.0 

PYRITE, LOG OF 

GRAMS PER TON 

Porphyritic 

quartz 

monzonite 

Breccia 

Aplite 

Porphyritic 

quartz 

monzonite 

0 I 2 

BISMUTH, IN 

PARTS PER 

MILLION 

FIGURE 13. - Molybdenite, pyrite, and bismuth blip 

curves for 76 samples of drill core from line of Roberts Tunnel access shaft from depths of 32 to 942 feet 

(9.8 to 287 m). 

Harold D. Roberts Tunnel 

ffiz" 

~~''b~·~·L7ppm ~ ~ 1.0 

~' 

. :: :::: I 

-t.oo~.L_..J._....l.._....J._5;;;;:ol;;-;:oo:--l-__j____J_--J.-:,o,-::-co~oo~---ll___L--J._'5,...J,oo-o::-'--__J 

DISTANCE FROM WESTERN EDGE OF STOCK (IN TUNNEL), IN FEET 

FIGURE 14. - Molybdenite, pyrite, and bismuth blip 

curves for 83 samples along Roberts Tunnel in western third of Montezuma stock (area I of Neuerburg, 

1971). 

tional field (Kennedy, 1955). The solubility of 

potassium, copper, and molybdenum in water 

varies directly with pressure (Kennedy, 1955, 

p. 498) . Potassic alteration and precipitation 

of copper and molybdenum in shattered rocks 

resulted from explosive decompression of the 

cupola (Sillitoe, 1973, p. 812). 

The Independence Mountain molybdenite 

veins are related to contact metasomatism, a 
39° 

37 1 

30" 

35' 

32' 

30" 

52'30" 

Outline of 

Montezuma district 

Wise Mountain 

X 

• 

• 

0 

I 

0 

50' 

XGroys Peak 

EXPLANATION 

• 

Areas of veins with 

bismuth minerals 

I 

2 

I 

2 MILES 

I 

3 KILOMETERS 

FIGURE 15. - The distribution of veins containing bismuth minerals. 

17 
0 

0 2 

2 MILES 

3 KILOMETERS 

X Grays 

Peak 

BISMUTH, IN PARTS PER MILLION 

>2.5 

2.0-2.4 

1.5-1.9 

1.0-1.4 

<1.0 

• Molybdenite vein occurrence 

FIGURE 16.- The bismuth blip surface for the Montezuma stock. (See text.) 

normal if apparently erratic companion of porphyry-metal deposits (Sillitoe, 1973, fig. 1). 

The metal-bearing volatiles for this skarn class 

of deposits are the same as for the porphyrymetal class, but here they move in response to 

an open-ended pressure gradient along an open 

structure, probably the Williams Fork thrust 

fault. Ore precipitation in this system results 

from th~ relative stagnation of structurally restricted flow or from contact with reactive rocks 

-at the edge of the intrusive or even at some 

distance away (Muilenburg, 1925, p. 54). 

The two petrographically contrasting groups 

of molybdenite veins in the Montezuma district 

have equally contrasting signifi·cance for mineral deposit potential. The Snake River veins 

18 

are the tip, or vanguard, of a porphyry-molybdenum deposit. The Independence Mountain 

veins are the roots, or spoor, of a skarn-molybdenum deposit, a deposit long since removed by 

erosion. If the high bismuth content of rocks 

in the Warden Gulch area is properly interpreted as locating another region of molybdenite veins, these veins must overlie another 

porphyry-molybdenum deposit because they 

are accompanied by porphyries and pervasive 

hydrothermal alteration. 

SUMMARY 

The Montezuma district conforms to the 

outer parts of a porphyry-metal model, extending from the outer tip of a potassic zone 
351 

32' 

3011 

52'3011 

Outline of 

Montezuma district 

XWise Mountain 

• 

' 

• / 

0 

0 

50' 

XGroys Peak 

EXPLANATION 

• Areas of speculorite veins 

2 MILES 

2 3 KILOMETERS 

FIGURE 17.- The distribution of veins containing specularite. 

19 
through the propylitic zone with its fringe of 

silver-lead-zinc deposits. Owing to the linear 

influence of the Montezuma shear zone, the 

geometry of the alteration zones deviates from 

the standard pattern of concentric shells centered on an igneous plug. Intrusives and alteration zones are repetitive along the trend of the 

shear zone above the relatively flat roof of the 

Montezuma stock. As a result the alteration 

zones have a highly irregular, mottled distribution. 

The worldwide consistency in the zonal composition of the porphyry-metal class of ore 

deposits suggests one or more disseminated deposits of molybdenum below present exposures. 

The exposed tip of a potassic alteration zone 

with molybdenite mineralization is consistent 

with this model-based postulate. Molybdenite 

mineralization in otherwise unaltered rock of 

the Montezuma stock records only the passage 

of contact-metasomatic ore fluids. If the abundant silver-lead-zinc veins are accompanied by 

a proportionately large amount of molydenum, 

the postulated molybdenite deposits of the Montezuma district may be a very large resource. 

However, the dispersed pattern of multiple 

small tendrils of phyllic alteration rather than 

a thick single blanket may mean that most of 

the molybdenite deposits are small and dispersed. In addition, molybdenite deposits at the 

bottom of this digitated phyllic zone might be 

too deep to be economically minable. 

As a prospecting guide, regions defined by 

high rock-bismuth or by bismuth veins, respectively, contain or overlie regions of molybdenite 

veins. Some of these regions of molybdenite 

veins are probably halos of unknown thickness 

around molybdenite ore deposits. Areas of specularite veins are generally associated with bismuth veins, are larger targets that ~re more 

easily detected, and may serve to focus a search 

for areas enriched in bismuth. 

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20 

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21 

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