Potential for lode deposits in the Livengood gold placer district, east-central Alaska

GEOLOGICAL SURVEY CIRCULAR 590 By R. L. Foster United States Department of the Interior

Geological Survey

William T. Pecora, Director

Fint printing 1968 Abstract - - - - - - - - - - - - - - - - - - - Introduction - - - - - - - - - - - - - - - - - Geologic environment - - - - - - - - - - - - Economic geology - - - - - - - - - - - - - - Lode deposits - - - - - - - - - - - - - - -

2. Semiquantitative spectrographic analyses and gold analyses from lode prospects - - - - -
3. Semiquantitative spectrographic analyses and gold analyses of rocks - - - - - - - - - - -
4. Map showing distribution of gold lode prospects and productive gold placer deposits- - -
5. Map of Livengood district showing location of stream- and spring-sediment samples and anomalous metal concentrations - - - - - - - - - - - - - - - - - - - - - - - - - - -
6. Diagrammatic geo1ogic cross sections of the Ruth Creek area (C-C' in fig. 1), showing alternative structural int.erpretations- - - - - - - - - - - - - - - - - - - - - -

of stream- and spring-sediment samples - - - - - - -- - - - - - - - - - - - - - - - - - - CONTENTS

1 Conclusions and recommendations - - - - -

TABLES Economic geology-Continued Geochemical investjgations- - - - - - - -

References cited- - - - - - - - - - - - - - -

By R. L. Foster

The Livengood district is in the Yukon-Tananaupland, approximately 80 road miles northwest of Fairbanks (fig. 1). It is accessible during the summer and fall via State Highway No. 2, an improved dirt and gravel road, and accessible during the entire year by fixed-wing aircraft.

Gold placers of the Livengood district have produced approximately 380,000 fine ounces of gold from stream and bench placer deposits (Cobb, 1964), distributed approximately as shown in figure 2. Production has come from nonfloat placer operations, dredging, and drift mining. The major production was derived from the Livengood Creek area, Olive Creek, Lillian Creek, Ruth Creek, Amy Creek, and the Gertrude Creek area. There are at present active nonfloat gold placer operations on Livengood Creek and Amy Creek, but only sporadic mining activity on Ruth, Lillian, and Olive Creeks.

Small veins and areas of mineralized rock have been prospected in several places, as shown in figure 2, but mineral production from the bedrock deposits has been negligible.

Field investigations were carried out during the summer of 1967 as part of the Heavy Metals program of the U.S. Geological Survey and included geologic mapping, sampling of geologic materials, and evaluation of prospects. Sediment samples were collected chiefly from flowing streams and springs; some were collected from the dry beds of intermittent streams. The sediment samples were dried and sieved and the minus 80-mesh fraction analyzed by atomic absorption for gold, by spectrograph for about 30 elements, and by a specific atomic-absorption-type instrument for mercury. Bedrock samples were analyzed by the same methods, and a few samples of both rock and stream sediment materials were analyzed for go 1d by fire assay to check the atomic-absorption results.

GEOLOGIC ENVIRONMENT The southern two-thirds of the Livengood district is underlain chiefly by a metamorphosed clastic facies of sedimentary rocks, the northern one-third, by a chert-carbonate facies. These facies are separated by an eastward-trending belt of serpentinites (fig. 1). The serpentinites and associated rocks are considered to have been emplaced or-iginally along major fault zones after or concurrently with diorite intrusion at Amy Dome. The metadiorite at Amy Dome is not believed to be associated with any metallization. Much later, probably in Tertiary time, plugs which range in composition from quartz monzonite to monzonite were intruded, and it is with these rocks that the mineral deposits of the district are associated.

The clastic facies consists chiefly of fossiliferous graywacke and argillite strata of Devonian age and similar lithologies of questionable Devonian age south of the Tolovana River and east of Cleary cr·eek, together with quartzite, chert-pebble conglomerate, and minor carbonate beds. Serpentinite with tectonic inclusions is spatially associated with this facies (Foster, 1967). Rocks commonly associated in turn with the serpentinites are metadiorite, metabasalt, peridotite, and silica-carbonate-talc rocks formed by the hydrothermal alteration of ultramafic rocks. Metadiorite is limited to the Amy Dome area and several places within the serpentinites. The chert-carbonate facies exposed in the northern part of the district consists chiefly of siliceous dolomite, black chert, black and gray banded chert, chert breccia, volcaniclastic and tuffaceous beds, and massive mafic igneous rocks of probable pre-late Mid d 1 e Devonian age. The Tertiary(?) quartz monzonite and monzonite occur chiefly as small discordant intrusives in the westcentral part of the area.

The serpentinites form an eastward- to northeastward-trending complex belt of outcrops which extends beyond the boundaries of the district (Foster, 1967). The serpentinites are believed to have been emplaced originally as subhorizontal sheets associated with regional thrust faults, much as suggested by Irwin (1964) for the Klamath Mountains and Coast Ranges of northwestern California and southwestern Oregon. Their present outcrop pattern however is believed to reflect subsequent upward dragging along high-angle reverse faults; this interpretation is supported by the generally straight-line trend of the outcrops and by the apparent absence in the adjacent rocks of mineral or textural zonation such as reported by Blake, Irwin, and Coleman (1967) below the sole of a major thrust fault in northwestern California along which ultramafic rocks have been emplaced.

The clastic facies is interpreted as overlying the serpentinite complex, and the chert-carbonate facies as underlying the complex. The serpentinite complex has been the locus of repeated fa u 1 tin g and consequently its' contacts in many places are faults, as shown in figure 1.

Monzonitic plugs occur along a northwest-trending lineament that cuts the clastic facies and is thought to cut both the serpentinite complex and the chert-carbonate facies.

ECONOMIC GEOLOGY Lode deposits The small lode gold deposits known in the Livengood district (fig. 2) are in siliceous graywacke ·near Lillian, Olive, and Ruth Creeks, in altered breccias on Ruth Creek, and in silica-carbonate rock on Gertrude Creek and at the Griffin prospect. These deposits are spatially related to .monzonitic stocks and dikes of probable Tertiary age (fig. 1). Brief descriptions of the deposits follow; the n u m be r s refer to locations shown in figure 2.

(2) Ruth Creek prospect.-Pyritized, brecciated, iron-stamed Igneous rock shows replacement by silica and carbonate and contais traces of gold (sample Nos. 20 and 23-29, table 2).

(3) Lillian Creek prospect. -Narrow auriferous arsenopyrite-quartz-scorodite veins occur in and near a limonite-stained dike in altered and contortedgraywacke-argillite country rock. Samples from theprospect contain from 0.5 to 48 ppm gold (sample Nos. 1-11 and 22, table 2 ).

(4} Griffin prospect.-Massive, sulfide-bearing, green-stained silica-carbonate-talc rock veined by quartz contains as much as 3. 9 ppm (parts per million) gold (sample Nos. 16-17, table 2 ).

(5) Old Smoky prospect.-Narrow, northwestwardtrending auriferous arsenopyrite-quartz veins occur in a ferruginous quartzite footwall near the intersection of an altered porphyritic biotite monzonite dike and a potassium feldspar-rich porphyry dike. Samples from the p r o spec t contain from 3 to 13 ppm gold (sample Nos. 12-15, table 2.)

(6) Sunshine No. 2 prospect. -A northwestwardtrending, crumbly, auriferous dike with internal limonite veinlets is in contact with altered argillite. Recent work is a few hundred feet above two caved adits and an arrastre at the Hudson mercury prospect. Cinnabar nuggets were abundant in the heavy mineral concentrates from a cleanup made in August 1967 at the Olive Creek gold placer.

Sketch maps, sample locations, and chemical analyses from the Lillian Creek, Old Smoky, Sunshine No. 2, Ruth Creek, and Griffin prospects, together with data from soil samples collected during pro spec t evaluation, are available in a U.S. Geological Survey open-file report (Foster, 1968). These data and other investigations (Foster and Chapman, 1967) indicate that the known gold lode deposits of the Livengood area are economically unattractive ·owing to their limited tonnages and(or) low grade. They may be significant, nevertheless, in that they suggest the possibility of concealed deposits in this seeminglyfavorable lithic and structural setting. Geochemical Investigations Geologic studies, rock analyses, and reconnaissance stream-sediment investigation have revealed a large polymetallic anomaly near Ruth Creek and several minor anomalies, including a go 1 d anomaly, near Lucille Creek (fig. 3).

The Ruth Creek anomaly (fig. 3, localities 42-47 inclusive) is characterized by above-background concentrations of gold, arsenic, mercury, antimony, molybdenum, silver, and boron in stream sediments (sample Nos. 42-47, table 1). Small sulfide veins, fracture coatings, disseminations, and replacement deposits (field Nos. 2-6, table 2) near small monzonitic stocks and dikes (field Nos. 1-3, table 3) contain anomalous concentrations some of these elements. which account,. at least in part. for their presence in stream sediments down drainage. The anomalous zinc concentrations on table 1 (sample Nos. 4 7, 48) may be real but are suspect because the samples were collected downstream from galvanized culverts.

The presence of sulfidized rocks is also indicated by acid metalliferous water in Ruth Creek. The water is red from suspended oxidized i_ron compounds, although the ground water discharging into Ruth Creek is clear. The pH determined in the field on four samples ranged from 3.0 to 5.0; a laboratory determination on one sample gave apHofabout 2.75. The chemical composition of the stream water (table onpage 3) shows that at least some of the water has been in contact with various sulfides or sulfosalts other than pyrite.

Other anomalous areas inc I u de those in the Olive Creek drainage, Alder Gulch are a, and near Cucille Creek.

A sample from the left t rib uta r y of Alder Gulch (sample No. 40, table 1) contains anomalous amounts of lead and mercury, which are attributed to mineralization along or near the major east-trending fault inferred to underlie the headwater area of this drainage basin (fig. 1).

The anomalously high gold content in stream-sediment samples from Fanny Gulch (sample No. 9, table 1) and Lucille Creek (Sample No. 20, lab. No. AC E-419, table 1) suggests gold metallization in the chert-carbonate terrane, whereas the strongly anomalous gold values in panned concentrates from Lucille Creek (sample No. 20, lab. Nos. ACE-486,-488,-490, table 1) indicate the possible presence of a small gold placer deposit.

The significance of anomalous amounts of manganese and antimony in a sample collected in the Frank 1 in Creek drainage (sample No. 23, table 1) is unassesseu; these anomalies may be related to the presence of mafic igneous rocks and massive chert-carbonate strata.

The combination of minor lode deposits, geochemical anomaly, and structural pattern at Ruth Creek makes this area worthy of further investigation as a potential exploration target. Dip of the faults in the area is a critical factor. If the faults near Livengood (figs. 1, 4£l.) are low-angle thrusts dipping south, a footwall section of chert -carbonate rocks may exist at shallow depth. The siliceous dolomite and contorted or fractured chert in such a section are considered favorable host rocks for gold deposition. If the minor lode deposits in the H. uth Creek area represent leak.;. age over a mineralized zone, the chert-carbonate rocks, as well as some of the rocks in the fault zone would be logical targets for physical exploration. On th--: uther hand, if the faults are high-angle reverse '~~- strike-slip in nature (fig. 4I2_), then the favorable c't~ n-carbonate strata may be buried at too great a L:l'tl~ tu be a practicable target.

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It is ·interesting that the geologic setting and geechemica 1 suite of anomalous metals (arsenic. mercury, and antimony in particular) are similar to the setting and suite known to be favorable for the occurrence of Carlin-type gold deposits in north-central Nevada (Roberts, 1960, 1q66; Hardie, 1966; Erickson and others, 1966). However, the significance of these apparent similarities in not known.

Geophysical investigations and more detailed geologic studies would be a logical first step in determining the dip of the faults, but ultimately d r i 11 in g would be required. If the information gained from an investigation of the Ruth Creek anomaly indicates economic mineralization associated with thrust sheets cut by Tertiary(?) intrusive rocks, then a search for similar. geologic environments along the Livengood trend is suggested. Both the anomalous metal concentrations in the fault-zone metasomatites and the silica-carbonate-talc rocks themselves are indicative of hydrothermal activity within and along the postulated major faults in the vicinity of the monzonitic rocks.

The silica-carbonate-talc rocks qerived from the serpentinites are considered favorable hosts. Their low magnetic susceptibility, due to the oxidation of primary and secondary magnetite during hydrothermal alteration, contrasts sharply with that of the serpentinites. Thus, magnetometer surveys along and on strike with highly magnetic serpentinites might reveal the presence of silica-carbonate-talc rocks which could provide additional exploration targets.

The available data suggest that the Livengood Creek gold placer deposits represent a sizeable gold resource which warrants additional economic a p p r a is a 1. Information from 640 drill holes in the gravels of the Livengood Creek area indicates some 17 million cubic yards of material with an average recoverable gold content of about 70 cents per cubic yard (gold at 535 per ounce) (C. F. Herbert, written commun., 1966). The Lucille Creek gold placer deposit found during this study also should be investigated further, although the thick overburden and lack of water might make this deposit uneconomic.

REFERENCES CITED Blake, M. C., Jr., Irwin, W. P ., and Coleman, R. G., 1967, Upsidedown met amorphi c zonation, blueschist facies, along a regional thrust in California and Oregon: U.S. Geol. Survey Prof. Paper 575-C, 9 p. Cobb, E. H., 1964, Placer gold occurrences in Alaska:

U.S. Geol. Survey Miner a 1 Inv. Resources Map MR-38. Erickson, R. L., VanSickle, G. H., Nakagawa, H. M., McCarthy, J. H., Jr., and Leong, K. W., 1966, Gold g eo chemic a 1 anomaly in the Cortez distri.ct, Nevada: U.S. Geol. Survey Circ. 534. Foster, R. L., 1967, Tectonic inclusions from a serpentinite, east-central Alaska: U.S. Geol. Survey Prof. Paper 57 5-D, p. D120-D122~ Foster, R. L., and Chapman, R. M., 1967, Locations and descriptions of lode prospects in the Livengood area, east-central Alaska: U.S. Geol. Survey open-file rept., 3 p. Foster, R. L., 1968, Descriptions of the Ruth Creek, Lillian Creek, Olive Creek, Griffin, Sunshine No. 2, and Old Smoky prospects, Livengood district, Alaska: U.S. Geol. Survey open-file rept., 10(?) p. Hardie, By:;_on S., 1966, Carlin gold mine, Lynn disrncr, Nevada: Nevada Bur. Mines Rept., 13, p. 73- 83. Irwin,· W. P ., 1964, Late Mesozoic orogenies in the ultramafic belts of northwestern California and southwestern Q_regon: U.S. Geol. Survey Prof. Paper, 501-C, 9 p. Roberts, R. J., 1960, Alinement of mining districts in north-central Nevada, i!l Geological Survey Research 1960: U.S. Geol. Survey Prof. Paper 400-B, p. Bl7-Bl9.

1966, Metallogenic provinces and mineral belts ----rilNevada: Nevada Bur. Mines Rept., 13, p. 47-72. TABLES 1-3 AND FIGURES 1-4

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!/Panned concentrate. Table I.--Semiquantitative spectrographic analyses and other analyses for gold and

Lab. Field No. No. Ag As

A A A A A

7 A A A A Au Spectro- Atomic graphic Absorp- B

A A A A A

A A A Au

<.02 <.02 <.02 Be

150 A 100 150 <1 500 <1 Cr Co

30 1,000 30 Cu

150 30 150 so

300 30 so 30

700 30 150 30 150 15 Hg La

.70 20 .85 so .30 <20 .55 A .20 A

.15 A .18 A .so <20 mercurv of stream- and spring-sediment samples from the Livengood district--Continued

pH Ti Ca Sh Sr Nh Ni Ph Zn Zr

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v

w

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Black and dark-gray to grayish-brown graywacke

Green fine- to coarse-grained metadiorite (dike

Dark- to light-green serpentinite, peridotite, metabasalt, tectonic inclusions, and silica-carbonate-talc metasomatites

}·--~;wo::f-<(

• Monzonitic rocks~

equivalents included in Pzsp)

w

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Dark-green metabasalt (locally sheared)

Gray siliceous dolomite with interbedded gray and

Black and gray banded chert breccia, black marble, volcani-clastic and tuffaceous beds

Thrust or reverse fault

Strike and dip of foliation

Strike and dip of joints

Fault Strike and plunge of lineation

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Figure 3.--Map of Livengood district showing location of stream- and spring-sediment samples and anomalous metal concentrations.

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Figure 4.--Digrammatic geologic cross sections of the Ruth Creek area, (C--C' irt:fig. l), showing alternative structural interpretations. A. Thrust interpretation, witn possible mineralization in: a, silica-carbonate-talc selvage-below faulted unconformity; b, brecciated mafic stock; and-c, chert-carbonate footwall complex. B. High-angle reverse or strike-slip fault interpretation with mineralization in: ~' silica~carbonate-talc eelvage; and b, brecciated mafic stock. -

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