GEOLOGICAL SURVEY CIRCULAR 562 

Utilization of Humus-Rich 

Forest Soil (Mull) in 

Geochemical Exploration for Gold 

Utilization of Humus-Rich 

Forest Soil (Mull) in 

Geochemical Exploration for Gold 

By Gary C. Curtin, Hubert W. Lakin, 

George J. Neuerburg, and Arthur E. Hubert 

G E 0 l 0 G I C A l 5 U R V E Y C I R C U l A R 562 

Washington 1968 
United States Department of the Interior 

STEWART L. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing 1968 

Second printing 1968 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page 

Abstract -----------------------------------

Introduction -------------------------------

Page 

1 

1 

2 

2 

Materials sampled ---------------------------

Results of investigation ---------------------· 

3 

3 

4 

4 

Acknowledgments _____________________ _ Summary ----------------------------------

Geologic setting ---------------------------- References cited ---------------------------· 

FIGURE 1. 

2. 

3-6. 

ILLUSTRATIONS 

Page 

Index map of Colorado showing area of this report ----------------------------------------- 2 

Geologic map of the north half of the Empire district ------------------------------------- 6 

Map showingS. Sampled localities and area shown in figures 4, 5, and 6 ----------------------------- 8 

4. Distribution of gold in the ash of mull -------------------------------------------- 9 

5. Distribution of gold in soil ---------------------------------------------- _________ 10 

6. Distribution of gold in pebbles and cobbles -·------------------------------- --------- 11 

III 
',' 
UTILIZATION OF HUMUS-RICH FOREST SOIL (MULL) IN 

GEOCHEMICAL EXPLORATION FOR GOLD 

By GARY C. CURTIN, HUBERT W. LAKIN, GEORGE J. 

NEUERBURG, and ARTHUR E. HUBERT 

Abstract 

Distribution of gold in humus-rich forest soil (mull) 

reflects the known distribution of gold deposits in bedrock in the Empire district, Colorado. Gold from the 

bedrock is accumulated by pine and aspen trees and is 

concentrated in the mull by the decay of organic litter 

from the trees. Anomalies in mull which do not coincide 

with known gold deposits merit further exploration. 

The gold anomalies in soil (6- to 12-inch depth) and 

in float pebbles and cobbles poorly reflect the known 

distribution of gold deposits in bedrock beneath the 

extensive cover of colluvium and glacial drift. 

INTRODUCTION 

This report presents preliminary information on the distribution of gold within the zone 

of weathering in the Empire district, Colorado. 

The Empire district is a compact, moderately 

productive gold dis,trict in the Front Range 

mineral belt that is being used as a site for 

studies of ore-weathering processes being conducted under the Geological Survey's Heavy 

Metals program. The investigation reported 

here consisted of the collection and analysis. of 

humus-rich forest soil (mull), of soil below the 

mull at 962 localities, and of float pebbles and 

cobbles from the colluvial and morainal cover 

at 847 localities. The results indicate that 

determination of the gold content of mull may 

be useful to delineate gold deposits in bedTock 

in areas covered by colluvium and glacial drift. 

The Empire district is in the southern part 

of the Empire 71h-minute quadrangle, Clear 

Creek County, Colo., about 37 miles, west of 

Denver, in the Front Range of the Rocky 

Mountains (fig. 1). Altitude ranges f:r.om 9,000 

to 11,000 feet in the district. 

1 

Mining began in the district in 1862 (Spurr 

and Garrey, 1908, p. 401) with the discovery 

of gold in the oxidized and decompc sed rock of 

the Silver Mountain ore zone (fig. 2) . This roclk 

was washed in sluices and treated in the same 

way as placer gravels and yieldfd approximately $2 million in gold durirg 1862-65 

(Harrison, 1964, p. 93). These placer operations decreased in economic importance in the 

1870's, but they continued on a small scale for 

many years. 

Lode mining was conducted inte.rmittently 

from the 1860's to 1943. Some of the more 

important mines- in the north half of the district are the Minnesota, the Silver Moullltain, 

and the Conqueror. During 1933-4f more than 

90,000 ounces of gold was produced from lode 

deposits in the district, mostly fro'"Tl the Minnesota mine. 

The geology of the Empire district has been 

described by Ball (in Spurr and Garrey, 1908), 

Lovering and Goddard ( 1950), anrl Braddock 

( 1967). The ore deposits have been described 

by Spurr and Garrey (1908) and Lovering and 

Goddard (1950). 

In the present investigation, golc was. measured by atomic absorption (Thompson and 

others, 1968). Most of the analyses were made 

in the field in a mobile laboratory. Rock and 

soil samples were ground and pulverized 

before they were analyzed. Mull sr.mples were 

sieved and the minus 2-millimeter fraction was 

ashed and analyzed. The gold cortent of the 

mull samples is reported on an ashed-weight 

basis. 
109° 108° 107" 1060 1050 1040 1030 1020 

4P1T----~---l----------~--------~--------~~------~~--------~--~~~+41° 

0 Colorado Springs 

Alamosa 

0 Durango I 

Burlinrton 0 

I 

I 

I 37" ~:----:t=-----+---~+-..L~---l~~--+------+----~'* 1 37" 

1090 1080 1070 1060 1050 1040 1030 1020 

0 50 100 150 MILES 

FIGURE 1.-Index map of Colorado showing location of Empire district. 

ACKNOWLEDGMENTS 

We tb.ank Mr. H. C. Nelson and Mr. C. P. 

Clifford .of the Minnesorta Mines, Inc., Mrs. H. 

M. Stoner, Mr. Harrison Bristol, and Mr. E. J. 

Woodriff for permission to publish data on 

samples collected on their properties. Mr. H. 

C. Nelson also supplied information not available in published reports. The following members of the U.S. Geological Survey assisted the 

authors in both the field investigations and the 

analytical work: E. L. Mosier, M. A. Chaffee, 

H. D. King, C. W. Gale, K. C. Wa:bts, J. G. Frisken, K. E. Espelie, R. C. Humphrey, K. R. 

Murphy, and 0. J. Roman. 

GEOLOGIC SETTING 

The Empire district is underlain by igneous -

2 

and metamorphic rocks of Precambrian and 

Tertiary age (fig. 2). The Precan1brian rocks 

are bioUte gneiss and microcline gneiss that 

have been intruded by Boulder Creek Granite 

and Silver Plume Granite. A small stock of 

hornblende granodiorite porphyiT and dikes 

of granodiorite porphyry, hornblende granodiorite porphyry, biotite granoiiorite porphyry, biotite quartz monzonite prophyry, and 

bostonite porphyry, all of Tertia:ry age, intrude 

the Precambrian rocks. The bedr')ck in mosrt 

of the area is covered either by Quaternary 

colluvium or by glacial drift which ranges from 

about 3 feet to at least 40 feet in thic[kness. 

The ore deposits of the north half of the 

gmpire district are mostly fissure fillings and 

disseminated deposits of the pyritic gold type 
(Lovering and Goddard, 1950). The principal 

vein minerals are pyrite, chalcopyrite, and 

quartz. The gold tenor increases with chalcopyrite. The veins form an encircling pattern 

around the small s~tock of hornblende granodiorite porphyry. Lovering and Goddard 

(1950, p. 156) suggested that this encircling 

zone was formed by the stresses related to the 

intrusion of the stock and subsequent collapse 

of the roof when some of the underlying 

magma was withdrawn after late-stage consolidation of the stock. The ore bodies occur a.s 

irregular shoots along the veins and range 

from a few feet to at lea.st 100 feet in length 

and height and from less than 1 foot to at least 

20 feet in width. One vein in the Minnesota 

mine, however, has been stoped for a distance 

of 1,800 feet along its strike. The vein system 

shown on the geologic rna p and geochemical 

maps (figs. 2, 4, 5, 6) was derived from field 

mapping during the summer of 1967 and from 

maps by Braddock (1967), by Lovering and 

Goddard (1950, fig. 59, p. 157, pl. 13), and by 

Spurr and Garrey (1908, pl. 81). 

MATERIALS SAMPLED 

Mull and underlying soil (6- to 12-inch 

depth) were collected at 962 localities (fig. 3). 

Float pebbles and cobbles were collected in .the 

colluvial and morainal cover at 847 localities. 

At many localities two or more pebbles or cobbles were collected, each representing a different rock type. 'Vhere more than one rock type 

was collected a weighted average gold value 

was calculated. 

The forest cover of the Empire district is 

chiefly lodgepole pine (Pinus contorta), but 

limber pine (Pinus flexitis') and groves of 

a.spen (Populus tremuloides) are present lo-

~ally. Mull (fores,t humus layer) is defined by 

the U.S. Department of Agriculture (1951, p. 

245) as "a humus-rich layer consisting of 

mixed organic and mineral matter, generally 

with a gradational bounda,ry to the underlying 

mineral horizon." Pine mull is present as pads, 

1-3 inches thick, beneath individual trees. 

Aspen mull is present as a diffuse layer rarely 

more than 1 inch thick. Mull samples were 

collected beneath pine trees where possible, but 

about 100 samples of aspen mull were collected 

at localities where pine trees were absent. 

3 

Beneath the mull layer is. a layer 2-6 inches 

thick of gray, asih ... textured material that contains abundant small roots. Below this layer is 

a mixture of yellow to yellow-brown sand or a 

mixture of fine sand and cobbles, o~ colluvial 

or morainal origin, that does not change noticeably in composition or texture downward to 

bedrock. Soil samples were collect~ in the 

colluvium or glacial drift below the layer of 

ash-textured material at a depth of 6-12 

inches. 

RESULTS OF INVESTIGATIOJIT 

Several areas that contain anomalous 

amounts of gold (0.6 part per million. or more.) 

in the ash of mull were detected in tris inves.tigation (fig. 4). The distribution of these gold 

anomalies appears to delineate the k1own gold 

deposits in the Silver Mountain ar·~a and to 

point out additional areas that merit exploration. The gold anomalies west of Lion Creek 

(fig. 4) suggest that the Silver Mountain and 

Minnesota mines vein systems extend westward under the glacial drift. 

In most places in the north h2lf of the 

Empire district the roots of tr~ extend 

through the cover of colluvium or glacial drift 

into the underlying bedrock. Tree roots were 

observed in bedrock at several localities, and 

in one place, at least 50 feet below tr~ surface, 

in one of the mine workings. The gold, which 

may be present in ground water e~ther in a 

complex ion or in the colloidal state, is taken 

into the trees and then in part deposited in the 

leaves and needles; it is finally conce,.trated in 

the mull as the leaves and needles decay. That 

this is an effective process was den1onstrated 

by the identification of gold in vege~ation --.collected at numerous localities. in the area studied. The ash of wood from the interior of large 

roots (as much as 6 inches in diam ~ter) contains as much as 3 ppm gold, the aslt of wood 

from the interior of tree branches 0'>ntains. a.s 

much as 2 ppm gold, the ash of pine needles 

contains as much as 0.7 ppm gold, and the a.sh 

of aspen leaves contains at least 0.1 pJ:m gold. 

The organic fractions of all the mu II samples 

studied in detail contained most of the total 

gold. Locally, gold may be contributed to the 

mull by windblown auriferous pyrite from 

mine dumps. For example, of the total gold 
in a mull sample collected below the dump of 

the Conqueror mine, 52 percent was in the 

heavy mineral fraction and could have been 

contributed principally by windblown auriferous pyrite. 

With few exceptions, the gold content of soil 

(6- to 12-inch depth) is lower than the gold 

conte:H.t of the ash of the overlying mull. Of the 

mull ash samples, 62 percent contained 0.2 ppm 

or more gold, but only 22 percent of the soil 

samples contained 0.2 ppm or more gold. 

The relatively small, isolated gold anomalies 

in soil (fig. 5) are probably due chiefly to the 

inclusion of gold-bearing vein material in 

otherwise barren colluvial or morainal cover. 

However, most of the 23 scattered high gold 

values in soil coincide wi,th known gold deposits 

in bedrock. These high values may reflect, in 

part, gold enrichment in the soil by the biogeochemical cycle. 

The irregular distribution of floa~t pebbles 

and cobbles that contain anomalous amounts 

of gold (fig. 6) reflects mechanical dispersion 

of vein material in the colluvial or morainal 

cover and shows little relation to known ore 

deposits. However, the high gold content of 

some of the float (21 samples contain 10 ppm 

or more gold) suggests that undetected gold 

deposits concealed by the colluvial or morainal 

cover may exis1t in the Silver Mountain area. 

The scattered anomalous gold values in the 

periphery of the area (fig. 3) reflect the pres .. 

ence of small, individual veins in unaltered 

country rock. 

SUMMARY 

In the Empire district, gold deposits in bed4 

rock beneath the colluvial and glacial cover 

are delineated much better by tl'e gold anomalies in mull than by the gold ano~alies in float 

pebbles and cobbles and in the soil below the 

mull. Gold anomalies in mull which do not coincide with known gold deposits merit further 

exploration. 

The accumulation of gold by trqes is demonstrated by the presence of gold in the interior 

of roots and branches and in pine needles and 

aspen leaves; this process accounts for most of 

the gold found in the mull. 

Mull may be an effective geoc.hemical sampling medium in forested a.reas blanketed by 

colluvium or glacial drift wher~ the transported material offers no clue to the nature of 

the underlying bedrock. 

REFERENCES CITED 

Braddock, W. A., 1967, Geology of the Empire quadrangle, Grand, Gilpin, and Clear Creek Counties, 

Colorado : U.S. Geol. Survey open-file rept. 

Harrison, Louise C., 1964, Empire and the Berthoud 

Pass: Denver, Big Mountain Pres", 482 p. 

Lovering, T. S., and Goddard, E. N., 1910, Geology and 

ore deposits of the F:vont Range, Colorado: U.S. 

Geol. Survey Prof. Paper 223, 319 p. 

Spurr, J. E., and Garrey, G. H., 1908, Economic geology 

of the Georgetown quadrangle (together with the 

Empire district), Colorado, with General geology, 

by S. H. Ball: U.S. Geol. Survey Prof. Paper 63, 

422 p. 

Thompson, C. E'., Nakagawa, H. M., and VanSickle, 

G. H., 1968, Rapid analysis for gold in geologic materials, in Geological Survey research 1968: U.S. 

Geol. Survey Prof. Paper 60Q-.B, p. B13<J-.B132. 

U.S. Department of Agriculture, 1951, Soil survey 

manual: U.S. Dept. Agriculture, Agriculture 

Handb. 18, 503 p. 
FIGURES~ 
47' 

.......-( 

/ / 

( 

I 

I spg J 

\ 

".__/? 

'-'\ beg 

Tim 

spg 

I 

l 

l" 

) 

v-v--

0 1000 2000 3000 FEET 

FIGURE 2.-Geologic map of the north half of the Empire district. Geology modified from B:':"addock (1967). 

Area of figures 4, 5, and 6 shown by rectangle. 

6 
EXPLANATION 

Solifluction debris and morainal deposits 

E~-l Bostonite porphyry 

t~bqm, Biotite quartz monzonite porphyry 

Leucocratic monzonite 

§:=1 Thg E~~i Granodiorite group 1 

Tg, granodiorite porphyry, undivided 

Thg, hornblende granodiorite porphyry 

Tbg, biotite granodiorite porphyry 

Silver Plume Granite 

Boulder Creek Granite 

Microcline gneiss 

Biotite gneiss 

------------- Contact 

Dashed where approximately located 

Vein or mineralized fault 

Dashed where approximately located 

___..£5 -+- Inclined Vertical 

Strike and dip of foliation 

>-

Ad it 

~ 

Shaft 

FIGURE 2.-Continued. 

7 

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0 1000 2000 3000 FEET 

FIGURE 3.-Topographic map showing sampled localities (small dots) and area of figures 4, 5, and 6 (rectangle). 

Gold concentration of 0.2 ppm (part per million) or more in colluvial pebbles and cobbl,~s and bedrock is 

shown by large dots in localities outside the area covered by figures 4, 5, and 6. Value shown if greater 

than 10 ppm. 

8 
39 

46 

30 0 

105°41'30" 

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EXPLANATION 

Parts per million 

0 <0.04 

• 0.04-0.19 

• 0.20-0.59 

• ~0.60 

® 

Area within which the 

concentration of 

gold is 0.6 ppm or 

more 

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2000 FEET 

FIGURE 4.~Distribution of gold in the ash of mull. Heavy lines are veins. 

9 

• 0 

• 

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105°41' 30" 

0 0 • 0 0 0 

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• 00 0 0 0 MINES 

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. .. ,o, 0 @ •• 0 0 •• • • I' 

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~ • • ~ 

0 0~ • •• • 0 ~ • • i • • •• ~ • 0 • • 0 ~ 0 • • • • 0 0 \\ 0 • o• • • 0 \\ • • • 0 0 

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'if' • • • • 0 ~ 0 0 Q ~~. 

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39° 0 0 

46' • 0 30" 0 0 0 

EXPLANATION 0 

0 0 

Parts per million (h • 0 

0 @o 0 <0.04 0 

• 0.04-0.19 • • 0 • • 0.20-0.59 0 0 

• 0 0 

• ~0.60 0 0 

0 0 0 

0 

Area within which the 0 0 

concentration of 

gold in samples is 0 

0.6 ppm or more 0 • 0 0 

0 1000 2000 FEET 

FIGURE 5.-Distribution of gold in soil (6- to 12-inch depth). Heavy lines are ve'ns. 

10 
105°41'30" 

0 0 0 • 0 • 0 • 0 • • 0 0 0 

0 

• o CONQUEROR 

0 0 MINES 0 0 0 0 0 0 0 • 

0 0 

39° 0 0 

47' 

00' 0 • • • 0 0 .13.2 • 0 

00. 

0 0 

0 

0 

0 •• 0 0 24.2 • 0 • • • 0 0 0 • 0 0 0 • 0 0 0 0 • •o 0 0 0 8• 0 ._o 0 

o1l.Oo •oo 0 0 MINNESOTA 0 • 33.0 SILVER 

oiftN 0 \MINES 0 •• 0 o o 0 

~ • 0 • • 0 12.0 • 0 0 ~ MOUNTAIN oo 95.0 ..... ~ 

~ • • 0 0 0 ~ 00 • 0 0 8 0 • 0 

0 ~ •• 

• ~ • 0 \\ 0 0 0 0 

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EXPLANATION 0 

0 0 

Parts per million 0 0 0 

0 • 0 0 <0.20 0 

0.20-1.49 0 0 • 0 

• 1.50-4.90 0 0 

0 0 

• ~ 5.0 0 0 0 

0 0 0 

0 

Value shown if 0 0 

>10 ppm 0 

0 • 0 0 

0 1000 2000 FEET 

FIGURE 6.-Distribution of gold in pebbles and cobbles collected in colluvium or glacial drift. Heavy lines ar<G. veins. 

11 

1< U. S. GOVERNMENT PRINTING OFFICE : 1968 0 - 324-015