Geology, Alteration, and Mineralization of the Kougarok Sn

Economic Geology Vol. 81, 1986, pp. 1775-1794

Geology, Alteration, and Mineralization of the Kougarok Sn Deposit, Seward Peninsula, Alaska

CHRISTOPHER C. PUCHNER*

Anaconda Minerals Company, 2000 West International Airport Road, Anchorage, Alaska 99502

Abstract

At the Kougarok deposit tin mineralization occurs in association with a Cretaceous granite complex. The tin mineralization is hosted in both granite and pelitic schist country rock.

The most significant concentrations of cassiterite occur in a late zinnwaldite granite body which cuts older granite phases of the intrusive complex. The body has a subhorizontal lower contact and an irregular upper contact which merges into higher level dikes and plugs. The intensity of hydrothermal alteration increases upward in the body; successive alteration assemblages, in order of increasing destruction of igneous texture, consist of sericite-tourmaline, brown zinnwaldite-sericite, white zinnwaldite-sericite, and quartz +_ tourmaline greisen. In general, the grade of tin mineralization parallels the intensity of the alteration, ranging from less than 0.10 percent in the sericite-tourmaline zone to over 1.0 percent in the quartz +_ tourmaline greisen. Highly anomalous arsenic (greater than 1,000 ppm), anomalous lead (up to 1,500 ppm), and silver ( up to 30 ppm) are associated with tin grades greater than 1.0 percent. Tantalum and niobium, in tantalite-columbite, are also enriched (up to 900 ppm Ta and approximately 600 ppm Nb) upward in the zinnwaldite granite but vary independently of tin.

Schist adjacent to and above the zinnwaldite granite contains veinlets and fractures with hydrothermal alteration selvages. Alteration zones are distinguished by the alteration selvages on quartz and/or tourmaline veinlets and hairline fractures in schist, which change from tourmaline enveloped by chlorite in a zone proximal to altered granite, to axinite enveloped by chlorite in an intermediate zone, and to chlorite selvages in a distal zone. Tin grades in quartz and/or tourmaline veinlets in the proximal zone commonly exceed 0.10 percent and give way outward to widespread halos of tin grades above 0.01 percent; tin mineralization is weaker or absent in the intermediate and distal alteration zones in schist.

Other granites and schist beneath the zinnwaldite granite body are altered and locally contain anomalous tin. Evidence that the zinnwaldite granite body cuts altered and mineralized older granites implies that hydrothermal activity and tin introduction commenced prior to the intrusion of the late zinnwaldite granite body.

Introduction

THE Kougarok tin deposit, located approximately 130 km north of Nome, is one of seven areas of known tin mineralization on the north-central Seward Peninsula, Alaska (Fig. 1). The discoveries of placer gold and tin at the beginning of this century prompted the first geologic investigations of the region. Collier (1902) conducted the initial regional geologic mapping pro- gram covering the northern Seward Peninsula. Other regional work includes geologic mapping by Sainsbury (1972, 1974) and a study of the granites associated with tin mineralization by Hudson and Arth (1983). Collier (1903), Knopf (1908), and Steidtmann and Cathcart (1922) made the initial investigations of the tin deposits in the Lost River, Cape Mountain, and Ear Mountain areas. Integrated geologic studies treating certain aspects of the geology, alteration, and mineralization have been completed in the Lost River district (Sainsbury, 1964, 1969; Dobson, 1982,

* Present address: Resource Associates of Alaska, 122 First Av- enue, Fairbanks, Alaska 99701.

1984), the Serpentine Hot Springs area (Hudson, 1977, 1979), at Ear Mountain (Bond, 1983), and at the Kougarok deposit (Apel, 1984). Investigations that primarily focused on the delineation of tin mineralization have been completed in the Cape Mountain (Mulligan, 1966), Potato Mountain (Mulligan, 1965), Lost River (Heide, 1946), Black Mountain (Sainsbury and Hamilton, 1967), Ear Mountain (Mulligan, 1959), and Serpentine Hot Springs areas (Sainsbury et al., 1970).

The Kougarok deposit, named for the mountain where it occurs, represents the most recent discovery of tin mineralization on the Seward Peninsula. Gra- nitie rocks were first mapped in the area by Collier (1902), but the presence of associated tin mineralization was not recognized until much later. The dis- eovery of pebbles of eassiterite-bearing quartz-tourmaline rock in a tributary of the Serpentine River draining the Kougarok area by U.S. Geological Survey geologists led to a reconnaissance stream sediment survey by Marsh et al. (1972). This survey showed that tin contents as high as 20,000 ppm were present

0361-0128/86/615/1775-20$2.50 1775

1776 CHRISTOPHER C. PUCHNER

FIG. 1. Location of the Kougarok tin deposit and other areas of tin mineralization associated with granitic rocks on the Seward Peninsula, Alaska.

in pan concentrates from streams draining the west flank of Kougarok Mountain. The anomalous samples eventually caused exploration of the area for tin mineralization. In 1978 Texasgulf, Inc., discovered granite dikes containing anomalous tin contents in the area. In 1979 geologists of the Anaconda Company discovered cassiterite-bearing greisenized granite plugs and dikes on the west side of Kougarok Mountain. Ana- conda subsequently acquired Texasgulf's holdings and explored the Kougarok deposit from 1979 through 1983.

The information presented in this paper is based on data gathered by Anaconda and Atlantic Richfield Company geologists between 1979 and 1983. During this period roughly 9,200 m of diamond drilling, 1,100 m of bulldozer trenching, detailed and regional gravity surveys, regional stream and detailed rock chip geochemical surveys, and geologic maps at scales ranging from 1:100 to 1:63,360 were completed. The author and six other geologists spent 36 working- months in 1983 evaluating the Kougarok deposit. Ap- proximately 9,200 m of diamond drill core were logged at a scale of 1:100 or 1:500, all surface trenching was mapped at a 1:100 scale, and the prospect area of nearly 3 km 2 was mapped at a scale of 1:1,000. Areas of significant geologic relationships were mapped at a 1:500 scale. In addition, a 1:63,360-scale geologic map of the 15-min quadrangle encompassing the Kougarok deposit was completed.

This paper describes the field relationships between the major altered and unaltered rock types, the megascopic petrology of these rock types, and

the general distribution of tin and associated mineralization; these data are used to present a model for the formation of the deposit.

Regional Geologic Setting Granites associated with the tin mineralization at

the Kougarok deposit intrude a metamorphic rock sequence which includes from bottom to top: a quartz- mica schist unit; a unit dominated by micaceous marble but including micaceous quartzite, calc-schist, greenstone, and carbonaceous quartzite; and a unit of massive marble (Fig. 2). The metamorphic rocks are polydeformed; the absence of cleavage or schistosity in the granites suggests that the most recent penetrative deformation predated intrusion of the granites.

Based on work primarily on the Solomon quadrangle 100 km to the southeast of the Kougarok deposit, Till and Dumoulin (in press) have redefined the Nome Group to include all rocks on the Seward Peninsula which have undergone blueschist facies metamorphism. Similarities of the Kougarok area metamorphic sequence to a portion of the Nome Group succession (Table 1) and blueschist localities less than 25 km southeast and southwest of the Kougarok deposit (Forbes et al., 1984) suggest that the Kougarok sequence is part of the Nome Group. The age of the Nome Group protoliths is poorly constrained, but intercalated marbles in the mixed unit (Table 1) contain conodonts of Ordovician age, the chlorite marble unit has yielded late Early to Middle Ordovician conodonts, and the carbonate rocks with uncertain relationships to the sequence contain micro- and mega- fossils of Cambrian to Devonian age (Till and Du- moulin, in press). Radiometric dating suggests that synkinematic regional blueschist and late to postkinematic greenschist metamorphism of the Nome Group occurred during the Jurassic (Forbes et al., 1984).

The description that follows of the geology and structure of the region surrounding the Kougarok deposit is taken in part from the unpublished geologic data of M. C. Gardner (1983) and R. A. Cavalero (1980-1983).

Quartz-mica schist unit Quartz-mica schist, informally designated the

Kougarok schist, is exposed in the cores of antiforms in the Kougarok region (Fig. 2). The dominant min- erMs in the schist are quartz, muscovite, and gray- green chlorite. Garnet porphyroblasts partially retro- graded to chlorite are present but uncommon (J. C. Reid and S.C. Bergman, unpub. data, 1982). Char- acteristic folded and boudinaged metamorphic quartz segregations vary in thickness from less than one to approximately 15 cm and comprise from 5 to nearly 20 percent of the schist. Adjacent to the granitic in-

TABLE 1.

KOUGAROK Sn DEPOSIT

Nome Group (Till and Dumoulin, in press) and Kougarok Area Metamorphic Sequences

1777

Nome Group Kougarok area Comments

Carbonate unit: carbonates with uncertain contact relations to

the sequence below Chlorite marble unit: orange-

weathering micaceous marble with lenses of chlorite-albite schist

Mafic schist unit: chlorite-albite- actinolite-white mica schist

with quartzite and marble lenses, and boudins of glauco- phane, garnet, and epidote- bearing metabasites

Mixed unit: interlayered marble and quartz-graphite schist with minor pelitic schist, mafic schist, and calc-schist

Pelitic schist unit: quartz-muscovite-chlorite schist with

metamorphic quartz segregations

Massive marble unit: massive dolomitic

marble and marble; quartzite locally at base

Micaceous marble unit: micaceous schistose

marble with intercalated greenstone and carbonaceous quartzite; quartz; micaceous quartzite and calc-schist at base

Quartz-mica schist unit: quartz-muscovite- chlorite schist with folded and boudi-

naged metamorphic quartz segregations

Correlation uncertain between

the Nome Group sequence and rocks of the Kougarok area

These units of the Nome Group and the Kougarok area are tentatively correlated on the basis of lithologic similarity

trusions the schist is hornfelsic. It displays a grano- biastic fabric and contains randomly oriented biotite developed at the expense of the other phyllosilicates and minor fine-grained, magnetic pyrrhotite.

Pelitic schist similar to that in the Kougarok area is present at the base of the Nome Group throughout the southern Seward Peninsula and occurs below the

mixed unit marbles in the Nome Group (Table 1) which contain Ordovician conodonts (Till and Du- moulin, in press).

Micaceous marble unit

The Kougarok schist is overlain by a unit composed primarily of micaceous schistose marble with lesser micaceous quartzite, carbonaceous quartzite, calc- schist, and greenstone. The micaceous quartzite occurs as discontinuous thin lenses at the base of the

unit. Calc-schist (muscovite-chlorite-calcite-quartz schist) overlies the micaceous quartzite, or the Kou- garok schist where the micaceous quartzite is absent, and is locally intercalated with the quartzite. The thickness of the calc-schist ranges from 3 to 7 m. The calc-schist grades upward into micaceous schistose marble. The thickness of the marble is not known ex-

actly but is estimated at greater than 300 m. Discon- tinuous lenses of carbonaceous quartzite and greenstone are present within the schistose marble but have not been examined in detail. This unit is tentatively correlated with the mixed unit of the Nome Group (Table 1) defined by Till and Dumoulin (in press) which contains similar lithologies 20 km to the east of the Kougarok deposit (A. B. Till, unpub. data, 1984).

Massive marble unit

The micaceous marble is overlain by a unit of massive marble. This unit consists of a discontinuous basal

quartzite which is successively overlain by massive dolomitic marble and massive marble. The thickness

of the marble unit is unknown; bedding is not everywhere apparent in the massive marble lithologies and the upper contact is not exposed in the area mapped. The unit is assigned a probable Devonian age on the basis of a tentative identification of the rugose coral, Thamnopera sp. (R. A. Cavalero, unpub. data, 1983).

Stratigraphic relations

The occurrence of the massive marble unit resting on both the Kougarok schist and the micaceous marble unit (Fig. 2) suggests that an unconformity exists at the base of the massive marble unit.

The Kougarok schist exhibits an early planar fabric deformed by recumbent isoclinal folds with east-west- plunging axes and an attendant axial plane schistosity which are absent in the overlying rocks (Gardner and Hudson, 1984; R. A. Cavalero, unpub. data, 1980- 1983). No evidence of faulting along the contact between the Kougarok schist and the overlying rocks has been documented in the Kougarok area. The local presence of a basal quartzite in the overlying sequence suggests that the contact is an earlier unconformity.

Granites

Drilling and a detailed regional gravity survey (An- aconda Co., unpub. data, 1981-1982) indicate that the granite dikes and plugs at the surface in the Kou-

Alluvium Granite dike

0 2Km

0 IMi

zg '"• Zinnwaldite granite

Massive marble, minor quartzite at base

29

60

15

Fault; D, downthrown side; U, upthrown side

Strike and dip of bedding

Strike and dip of overturned bedding

Strike and dip of foliation

Micaceous marble, carbonaceous quartzite, calc-schist, and greenstone, local mica-

ceous quartzite at base Quartz-mica schist

Stream

Contact, dashed where approximately located

Syncline, showing trace of axial plane and plunge of axis

Anticline, showing trace of axial plane and plunge of axis

Overturned syncline showing plunge of axis

Overturned antiform showing plunge of axis

Inferred limit of granite intrusive complex at depths of less than 3 kin., based on gravity data.

FIG. 2. Geologic map of the Kougarok region modified from an unpublished map by M. C. Gardner (1983).

1778

KOUGAROK Sn DEPOSIT 17 79

7,287,000 N

:;;¾.¾................. ß :.:,:.:.;.;.;.;.;.F:.: .....

.....

i:i:?i:i'"'"'"'"'"'"'""'"'"'""" .... i:i:i:i:i:.:.:

:::::::::::::::::::::::::::::::::::::::::::::::

:-:-;-:.;-:-:-;-;.;.:.:.5:•/•..F ;.' •.0.

::::::::::::::::::::: F:'X-:-FX-F:,

..... :.: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

::::::::::::::::::::::::::::::::::::::

:::::::::::::::::::::::::::::::::::::::

0 200m

'• Zinnwaldite granite dike, Fault; showing showing dip •0 downthrown side (D) D•'*.. and upthrown side

(U), dashed where ß Zinnwaldite granite plug approximately located

':..':':'-....'::......:.......'• Guarfz-mica schist ..•2 Strike ,rid dip of foliation

FIG. 3. Geologic map of the Kougarok deposit based on surface mapping of rubble and drilling.

garok region (Figs. 2 and 3) are part of a large, buried granitic intrusive complex. The gravity data suggest that the shape of the intrusive complex at depths less than 3 km has the general form, in plan view, of a north-south elongate ellipse with dimensions of 7 X 15 km and a subhorizontal upper contact (Fig. 2). A biotite hornfels aureole is ubiquitous in the Kou- garok schist up to 200 m above the complex. Above zones of pervasive hydrothermal alteration in granite, replacement of metamorphic phyllosilicates by tourmaline, axinite, and chlorite is common both in the hornfels aureole and above it (Figs. 4 and 7, later in the text).

Where intersected by drilling, the intrusive complex is comprised of two distinct granitic rock types: quartz-feldspar porphyritic biotite granite, and a suite of leucocratic equigranular granite types which are spatially associated with significant tin mineralization. The porphyritic biotite granite is known from a single area of frost-riven rubble (too small to be shown in Figs. 2 and 3) and drilling in the area of Figure 3. The age of the porphyritic biotite granite is 72 _+ 2 m.y. based on an Rb/Sr isochron with an initial 87Sr/86Sr ratio of 0.7134 determined on samples taken from drill core (J. C. Reid and S.C. Bergman, unpub. data, 1982).


7• 287,000 N :::::::::::::::::::::::

i !ii ...... iii::: .........

O 200m

:;',":"• TOURMALINE ZONE- Tourmoline-chlorite, axinite rare or absent

:;:• AXlNITE ZONE- Axinite-chtorite, tourmaline rare or absent

•'[ CHLORITE ZONE- Chlorite• tourmaline or oxinite • )lOOppro Sn in schist

FIG. 4. Map of alteration and mineralization of schist. Mineralization shown is based on chip samples collected on a 50 X 50-m grid over the prospect area. The prospect lies on a steep west-facing slope; due to downslope transport of rubble the westernmost contacts of alteration and mineralization shown are suspect. Alteration of schist of the types shown is structurally controlled and affects from 5 to 20 percent of the volume of rock within the indicated area. The area of this figure is the same as Figures 3 and 8.

A variety of leucocratic equigranular granites crop out as dikes and plugs and, based on drilling, occur as extensive intrusive bodies at depth. A sample of equigranular zinnwaldite granite from the westernmost of the two outcropping plugs (Fig. 3) has a K/Ar age (zinnwaldite) of 70.2 _ 2.6 m.y. (J. Slivkoff, unpub. data, 1982). Inclusions of porphyritic biotite

granite in equigranular granites observed in drill core indicate that at least some of the equigranular granites postdate the porphyritic biotite granite.

In a regional study of other granite complexes associated with tin mineralization on the Seward Pen-

insula, Hudson and Arth (1983) describe three prin- cipal textural types of granite emplaced in the se-

KOUGAROK Sn DEPOSIT 1781

quence: seriate, porphyritic, and equigranular. Rb/Sr and K/Ar ages of granite from six of the complexes shown in Figure i range between 70 and 80 m.y.; initial 87Sr/S6Sr ratios for the granites are between 0.7084 and 0.7291 (Hudson and Arth, 1983). The porphyritic biotite granite and the suite of equigranular granites at Kougarok, described in detail below, are megascopically similar to the porphyritic and equigranular varieties described by Hudson and Arth (1983) in other complexes and were intruded in the same relative order. The K/Ar and Rb/Sr ages at Kou- garok, 70.2 _ 2.6 and 72 _ 2 m.y., respectively, and an initial S?Sr/S6Sr ratio of 0.7134 obtained in the porphyritic granite are within the ranges determined for the other tin-bearing granite complexes.

The Late Cretaceous granites clearly cut the two penetrative deformation fabrics, but the data are not sufficient to determine whether the open folding predated or postdated the emplacement of the granites. Northeast-, northwest-, and north-striking, steeply dipping, normal faults offset the Late Cretaceous granite dikes and late open folds. Displacement along these faults in the prospect area (Fig. 3) are relatively minor. In general, the northeast-striking faults are offset by the northwest-striking faults. The north- south faults span the period of normal faulting; they offset, and are offset by, the northeast- and northwest- striking normal faults.

Granitic Rocks

Structure

Early intense, polyphase deformation of the metamorphic rocks in the Kougarok area was followed by granite emplacement, open folding, and normal faulting. Recumbent isoclinal folds in an early planar fabric (bedding?) occur in the Kougarok schist; the fold axes are presently oriented east-west. The early planar fabric is cut by a strongly developed axial plane schistosity. The isoclinal folds and schistosity were subsequently refolded about north-south axes. These later folds are tight and asymmetrical to overturned with north-south-plunging axes and generally westward- dipping axial planes. Accompanying axial plane schistosity is well developed.

The early recumbent isoclinal folds occur only in the Kougarok schist; the second generation of folds is present in all the metamorphic rock types in the Kougarok area. The style and orientation of folds of this second deformational event are widespread on the Seward Peninsula and have been linked to a synkinematic regional blueschist and late kinematic to postkinematic greenschist metamorphic event of probable Jurassic age (Forbes et al., 1984).

Late open folding about both east-west and north- south axes created an egg crate interference pattern.

Tin mineralization and hydrothermal alteration at the Kougarok deposit occur within and adjacent to granite intrusions. Five distinct types of granite have been distinguished at the Kougarok deposit on the basis of texture and hand specimen mineralogy. These include: (1) porphyritic biotite granite, (2) fine- grained aplitic granite, (3) feldspar-quartz porphyry, (4) equigranular granite, and (5) equigranular zinnwaldite granite (Table 2).

Porphyritic biotite granite

Porphyritic biotite granite, known primarily from drill core (Figs. 5 and 6), occurs as a large pluton whose upper contact is nearly planar and subhorizontal at depths of 400 to 600 m below the surface. A float train of rubble too small to be shown in Figure 3 suggests that porphyritic biotite granite occurs as dikes or small plugs at the surface as well.

In hand specimens, the porphyritic biotite granite is composed of medium- to coarse-grained quartz, K- feldspar, plagioclase, and biotite in a fine- to medium- grained groundmass of similar composition. Anhedral to subhedral bipyramidal smoky quartz is the most abundant mineral of the phenocryst assemblage, followed by white subhedral to euhedral K-feldspar,

TABLE 2. Summary of Granite Phases and Field Relations Indicative of Relative Ages of the Phases

Granite phase Mineralogy Field relations

Porphyritic biotite granite

Fine-grained aplitic granite

Feldspar-quartz porphyry

Equigranular granite (types 1, 2, and 3)

Zinnwaldite granite

qtz-Ksp-plag-bio

qtz-feld

feld-qtz (phenocrysts)

qtz-Ksp-plag-bio

qtz-Ksp-plag-zinn

Cut by dikes of and occurs as inclusions in equigranular granite

Xenoliths of schist cut by fine-grained aplitic granite occur in feldspar-quartz porphyry

Cuts fine-grained aplitic granite, occurs as inclusions in equigranular granite and zinnwaldite granite

Cuts porphyritic biotite granite as dikes, contains inclusions of porphyritic biotite granite and feldspar-quartz porphyry, cut by dikes of zinnwaldite granite

Cuts equigranular granite as dikes, contains inclusions of equigranular granite and feldspar-quartz porphyry

Abbreviations: bio = biotite; feld: feldspar; Ksp -- k-feldspar; plag = plagioclase; qtz = quartz; zinn = zinnwaldite


Plugs

6

Di•<e.

4- 4-

+ + + + + + + + + + + + + + + + + + + + + + + -• •- + + + + + + + + + +

9200E 9400E 9600E

0 lOOm

200

Equigranular granite (granite- •:..:..•.: Quartz mica schist, stippled where hornfelsed

•',/• nx) with hornfelsed schist xenoliths (granite-xs) or feldspar- quartz porphyry • Fault inclusions (granite-xp) as shown

Feldspar-quartz porphyry FIC. 5. Generalized geologic cross section A-A' (Fig. 3) of the Kougarok deposit based on surface

mapping and diamond drilling.

white to greenish-white anhedral to euhedral plagioclase, and books of dark brown biotite.

Fine-grained aplitic granite

Fine- to very fine grained aplitic granite occurs as millimeter to 5-cm dikelets which form a stockwork in schist at the surface and in the shallow subsurface

between the two zinnwaldite granite plugs shown in Figure 5 (the dikelets are too small to illustrate at the scale of the figure). Where the density of dikelets is 10 percent or greater, intervening schist fragments are commonly rotated.

The aplitic granite is composed of quartz and feldspar and is aphyric. Hydrothermal alteration of feld-

spar to sericite is pervasive; as a result, the original proportions of K-feldspar and plagioclase are unknown.

Feldspar-quartz porphyry Generally northwest-striking, nearly vertical dikes

of feldspar-quartz porphyry from a few centimeters to 5 m in width occur between the two zinnwaldite

granite plugs (Fig. 5). The porphyry is characterized by abundant anhedral to subhedral medium-grained feldspar phenocrysts and rare quartz eyes in an aphanitic dark brown groundmass. Hydrothermal alteration of the porphyry is ubiquitous; the groundmass is commonly extensively replaced by black tourmaline


70Ore-

600

5OO

4OO

...

C, • ;• - 300

2OO

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 4- + + +

• + + + + + + + + + + + + + 4- + + 4- -i- -i- + + + +

9200E + h + . + 9400E 9600E 9800E I I I

0 lOOm

FIG. 6. Generalized geologic cross section B-B' (Fig. 3) of the Kougarok deposit based on surface mapping and diamond drilling. Symbols are the same as in Figure 5.

and the feldspar is pervasively sericitized or replaced by tourmaline. In drill core, feldspar-quartz porphyry contains xenoliths of schist with fine-grained aplitic granite dikelets cut off at the xenolith margins.

Equigranular granite

Medium- to fine-grained equigranular granite occurs as sheetlike bodies at depth and as a plug near the surface (Figs. 5 and 6). The least altered spedmens of equigranular granite contain anhedral quartz, euhedral to subhedral K-feldspar, and subhedral to eu- hedrai plagioelase in approximately equal proportions plus a trace to approximately 5 percent dark brown, unaligned biotite. Three types of equigranular granite are distinguished on the basis of the presence or absence of inclusions and the type of inclusion present; these are: type 1, equigranular granite with rounded to subrounded feldspar-quartz porphyry elasts (granite-xp, Fig. 5); type 2, equigranular granite containing angular hornfelsie schist elasts (granite-xs, Fig. 5); and type 3, nonxenolithie equigranular granite

(granite-nx, Fig. 5). Inclusions of porphyritie biotite granite also occur but are rare. Significant mixing of inclusion types does not occur.

Type 1 equigranular granite occurs at depth and in the shallow subsurface between the two zinnwaldite granite plugs (Fig. 5). Drilling indicates that the shallow body is a northwest-southeast elongate, steeply plunging plug with plan dimensions of roughly 10 by 40 m, and that the deeper body has a subhorizontal sheetlike geometry. The extent of the sheet in a north-south direction out of the section (Fig. 5) is poorly constrained but must be less than 200 m to the north and less than 800 m to the south of drill hole 23; in both these directions the sheet is absent in deep drill holes.

Inclusions of feldspar-quartz porphyry in this sheet range from less than a centimeter to several meters in diameter and locally comprise up to 30 percent of the sheet. The size of the inclusions varies system- atieally, fining both upward in individual drill hole intercepts and to the west as shown in Figure 5. Unless the intrusion entirely incorporated a prior feldspar-


quartz porphyry sheet in the same location, the inclusions must have been transported in the magma from elsewhere. If gravity sorting of cooler and, therefore, more dense inclusions is responsible for the size grading in the clasts, then this type 1 sheet was intruded from east to west from a feeder in the

area beneath the zinnwaldite granite plugs exposed at the surface.

Type 2 equigranular granite with 10 to approximately 70 percent subangular to angular hornfelsic schist clasts, occurs beneath the surface exposure of the type 1 granite plug (Fig. 5). This body appears to merge into the subhorizontal sheet of type 3 equigranular granite to the west; the extent of the body to north, south, and east is not constrained. Sheets of type 3 equigranular granite are present in drill holes at similar elevations approximately 800 m to the south (Fig. 6) and in a drill hole 200 m to the north of drill hole 23 (Fig. 5).

Contacts between the three types of equigranular granite occur as a generally abrupt change in the inclusion type or as sharp transition from the presence to the absence of inclusions. In three of five drill hole

penetrations of these contacts no variation is seen in the granite at these contacts. In the other two penetrations, the biotite content of type 1 granite increases gradually over 3 to 5 cm from less than 5 to nearly 30 percent at the subhorizontal contact with type 3 granite. Adjacent to the contact the biotite in type 1 granite is strongly aligned parallel to the contact. There is no corresponding change in the biotite content of type 3 granite approaching the contact. Sim- ilarly high concentrations of biotite occur in equigranular granite as halos around schist, feldspar- quartz porphyry, and rare porphyritic biotite granite inclusions. Around the inclusions the biotite is ran-

domly oriented. The lack of mixing of inclusion types in the equi-

granular granite and the two examples ofbiotitic contacts between the different types of granite indicate that the equigranular granite is composed of multiple intrusive phases. If this is the case, the time interval must be short, however, because the contacts between the three varieties of equigranular granite generally are not sharp in terms of matrix characteristics and no chilled border phases have been observed.

The contacts of the equigranular granite bodies with schist and with the porphyritic biotite granite are commonly characterized by a gradual decrease in grain size in the granite toward the contact and by narrow (2 to 5 cm) zones of high biotite concentration in the granite against the contact. Biotite grains are aligned parallel to the contact. Rare inclusions ofpor- phyritic biotite granite in the granite combined with the generally observed decrease in grain size of the granite toward contacts with porphyritic biotite gran-

ite indicate that the granite postdates the porphyi'itic biotite granite.

Zinnwaldite granite

Zinnwaldite granite occurs as a large intrusive body with a subhorizontal base which merges upward into the dikes and plugs (Figs. 5 and 6) which crop out at the surface. Deep intercepts of zinnwaldite granite in drill hole 6 and numerous steep thin zinnwaldite granite dikes near the bottom of hole 26 (drill holes are shown in Fig. 5) suggest that a feeder for the zinnwaldite granite body lies almost directly beneath the surface outcrop of the two zinnwaldite granite plugs. Tourmalinization of schist is widespread above hydrothermal alteration in the zinnwaldite granite body where it has been drilled along cross sections A-A' and B-B' (Figs. 5 and 6). The surface extent of tour- malinized schist (Fig. 4) suggests that the subsurface limit of altered zinnwaldite granite is circular in plan view with a diameter of roughly 1,100 m.

The main mass of zinnwaldite granite at Kougarok is predominantly a fine- to medium-grained equigranular rock which contains anhedral to subhedral quartz, subhedral K-feldspar, subhedral to euhedral plagioclase, and abundant (to 15%) very pale brown mica. Petrographic and microprobe analyses (Table 3) of the mica from the zinnwaldite granite suggest that the mica is zinnwaldite. This is compatible with a whole-rock lithium content of 920 ppm for the zinnwaldite granite (average of 19 least altered sam- pies, J. C. Reid and S.C. Bergman, unpub. data, 1982). Whether the zinnwaldite is primary or not is unknown; no clearly unaltered zinnwaldite granite is present at the Kougarok deposit.

Millimeter- to centimeter-scale subparallel anas- tomosing bands of alternating medium-grained quartz-zinnwaldite-rich and medium- to coarse- grained feldspar-rich granite are locally present at the margin of the zinnwaldite granite body adjacent to schist or equigranular granite and, less commonly, in- ternally in the zinnwaldite granite body. The banding is subparallel to the contact with wall rocks and per- sists up to 1.5 m from it. This textural variety of zinnwaldite granite resembles the "crenulate quartz-lay- ered rock" (White et al., 1981) or "brain rock" (Shannon et al., 1982) at the Henderson molybdenum deposit, where layers of quartz or feldspar alternate with granite porphyry or aplite. Unidirectional growth of quartz in the quartz-zinnwaldite-rich layers at Kougarok analogous to the unidirectional solidification textures in quartz crenulate layers at Henderson (Shannon et al., 1982) has not been documented. Other textural varieties of zinnwaldite granite which occur primarily on the margins of the zinnwaldite granite body include coarse-grained (up to 3 cm) pegmatite and quartz or quartz-zinnwaldite porphyry.


T^BLE 3. Microprobe Analyses (wt %) of Individual Mica Grains from the White Zinnwaldite-Sericite (K6-164), Brown Zinnwaldite-Sericite (K6-210), and Sericite-Tourmaline (K6-277.5) Alteration Zones in the Zinnwaldite Granite Body

K6-164 K6-210 K6-277.5

Sample no. Analyses 1-3 t 7 8 9 1-3 4-9 1-3 4-6

SiO2 52.16 51.98 51.07 51.53 52.46 52.07 52.03 51.16 TiO2 0.07 0.09 0.06 0.07 0.15 0.24 0.08 0.09 AlcOa 17.92 17.99 18.57 18.43 18.74 19.16 18.15 17.91 FeO • 5.11 5.17 5.41 5.18 5.06 4.56 6.31 7.37 MnO 0.87 0.89 1.03 0.93 0.42 0.43 0.32 0.25 MgO bd a bd bd 0.02 0.02 0.02 0.17 0.20 CaO bd 0.04 0.05 0.03 0.04 0.03 0.03 0.03 ZnO 0.07 0.05 0.06 bd 0.05 bd bd bd Na•O 0.16 0.15 0.14 0.12 0.14 0.19 0.04 0.04 K•O 9.22 9.31 8.93 8.95 9.50 9.30 9.98 9.93 F 6.15 5.72 5.64 6.24 5.87 5.68 5.41 5.91

91.73 91.39 90.96 91.50 92.45 91.68 92.52 92.89

-0 -- 2F 2.59 2.41 2.37 2.63 2.47 2.39 2.28 2.49

Total 89.14 88.98 88.59 88.87 89.98 89.29 90.24 90.40

t The mean is given for multiple analyses of the same mica grain 2 Total Fe as FeO

a bd -- below detection; detection limits for these oxides are approximately: MgO, 0.01%; CaO, 0.01%; and ZnO, 0.04% Analyses performed on a Cameca Camebax-MBX electron microprobe at the ARCO Exploration and Production Research facility

at Plano, Texas

Pegmatite occurs along the upper contact of the large body of the zinnwaldite granite, more rarely along one or both contacts of zinnwaldite 'granite dikes, and as lenses within the zinnwaldite granite body. The pegmatites are generally less than a meter thick and grade into equigranular zinnwaldite granite. The pegmatites are typically composed of euhedral feldspar, anhedral quartz, and books of light brown zinnwaldite; topaz, beryl, and tourmaline occur spo- radically as accessories. Marginal pegmatites have also been described in granites associated with tin mineralization in the Soviet Union (Shcherba, 1970) and the Erzgebirge tin district of eastern Europe (Bau- mann, 1970).

Quartz porphyry or quartz-zinnwaldite porphyry occurs at contacts between zinnwaldite granite dikes and schist or equigranular granite wall rocks and along the borders of larger zinnwaldite granite bodies where they locally contain inclusions of feldspar-quartz porphyry, schist, and equigranular granite. Steep zinnwaldite porphyry dikes less than a meter thick are also present within the large body of zinnwaldite granite. A few equigranular zinnwaldite granite dikes grade into porphyry on one margin and pegmatite on the other. The occurrence of pegmatite or porphyry is not systematic with regard to the attitude of a dike or the type of country rock.

The overall distribution and lateral relationships between the three varieties of granite commonly occurring at the margins of zinnwaldite granite bodies is poorly defined. The occurrence of porphyry is most

common on the margins of steep dikes and the contacts of the zinnwaldite granite plugs (Fig. 5); peg- matitc and feldspar-quartz-zinnwaldite-banded granite are most common where the contacts of zinnwal-

dite granite bodies have relatively shallow dips.

Summary of age relationships Field observations discussed above and summa-

rized in Table 2 can be used to determine the se-

quence of intrusion for most of the granite types at Kougarok. Xenoliths of schist cut by fine-grained aplitic granite enclosed in feldspar-quartz porphyry and clasts of feldspar-quartz porphyry in the equigranular granite demonstrate that emplacement of the fine-grained aplitic granite was followed by intrusion of feldspar-quartz porphyry, which was followed, in turn, by intrusion of the equigranular granite. Equi- granular granite cut by dikes of zinnwaldite granite with chilled porphyry margins and inclusions of equigranular granite in zinnwaldite granite show that the zinnwaldite granite is younger than the equigranular granite. Equigranular granite contains rare inclusions of porphyritic biotite granite and displays chilled contacts against porphyritic biotite granite; both relations indicate a younger age for the equigranular granite.

In summary, the granitic types were intruded in the following order: (1) fine-grained aplitic granite, (2) feldspar-quartz porphyry, (3) equigranular granite, and (4) zinnwaldite granite. The porphyritic biotite granite is older than the equigranular granite, but


its age relative to the fine-grained aplitic granite and the feldspar-quartz porphyry is unknown.

Alteration

Both Kougarok schist and granite have undergone hydrothermal alteration. Four distinct alteration assemblages are recognized in the granite; these assemblages are, in order of increasing destruction of primary igneous texture: (1) sericite-tourmaline, (2) brown zinnwaldite-sericite, (3) white zinnwaldite- sericite, and (4) quartz (_ tourmaline and topaz) grei-

sen. Alteration of the schist occurs as replacement of the metamorphic phyllosilicates by tourmaline, axinite, or chlorite. The distribution of these alteration assemblages in the granite and in the schist is discussed in detail below.

Sericite-tourmaline

The least altered granitic rocks at Kougarok are characterized by a ubiquitous alteration mineral assemblage of sericite and tourmaline (Figs. 7 and 8). White, very fine grained sericite occurs at the expense of feldspar and is present in amounts from a trace to

W-21

29

W-2

! i

I

9200E 9400E

0 lOOm

I GREISEN: Quartz tourmaline +_ topaz

WHITE ZINNWALDITE- SERICITE: Caarse white

zinnwaldlte, pervasive sericitizatian, tourmaline rare or absent

9600E I

BROWN ZINNWALDITE-

SERICITE: Coarse pale brown zinnwaldite, strong sericitization, trace tourmaline

SERICITE- TOURMALINE:

Weak to strong sericltization, trace rolO% tourmaline

Tourmaline t chlorite replacing metamorphic

phyllosilicates adjacent to quartz and/or tourmaline stringer zones

FIC. 7. Generalized cross section A-A' (Fig. 3) showing distribution of alteration assemblages based on diamond drilling and surface mapping.


7,287,000 N

l

0 200m

GREISEN: Quartz •_ tourmaline _* topaz

WHITE ZINNWALDITE- SERICITE: Coarse white

zinnwaldite, pervasive serlcitizatian• tourmaline absent or rare

BROWN ZINNWALDITE-

SERICITœ: Coarse pole brown zinnwoldite, strong sericifizofion, trace Iourmollne

SERICITE-TOURMALINE: weak I'o strong sericitizotion• trace rolO% tourmaline

FIG. 8. Map of alteration in granite dikes and plugs. Area shown is the same as Figures 3 and 4.

approximately 30 percent by volume of the feldspar. Primary dark brown biotite in the porphyritic biotite granite and the equigranular granite is replaced by white mica as sericitization of feldspar approaches 30 percent. Black tourmaline is present (generally between 1 and 10%) as variable combinations of disseminated fine-grained anhedral grains, clots of sub-

hedral to euhedral acicular crystals, very fine grained partial to complete replacement of feldspar, and veinlets. Zinnwaldite is present in sericite-tourmaline- altered zinnwaldite granite. As discussed above, the zinnwaldite may or may not be primary; in any case, it is apparently stable in the sericite-tourmaline assemblage. Sparse disseminated fine-grained arseno-


pyrite, cassiterite, and less commonly chalcopyrite and pyrite occur within this alteration assemblage.

The sericite-tourmaline alteration assemblage is the dominant alteration type in the porphyritic biotite granite, equigranular granite, and the lower portion of the zinnwaldite granite body (Fig. 7), but it also occurs in zinnwaldite granite dikes at the surface (Fig. 8). The abundance of sericite and tourmaline in the sericite-tourmaline-altered zone is variable but tends

to increase very gradually upward in granite bodies. Abrupt changes in their abundance is rare except at contacts between the porphyritic biotite granite and the equigranular granite. In drill hole 6 (Figs. 5 and 7), porphyritic biotite granite at the contact contains between 20 and 30 percent tourmaline in clots and replacing feldspar, together with several percent sericite. In the adjacent equigranular granite, immedi- ately above an 8-cm-thick zone of abundant biotite, the granite contains less than 5 percent tourmaline as disseminated grains and clots and only a trace of sericite. In two other drill hole penetrations (17 and 18, Figs. 5 and 7), this contact displays a similar, but less pronounced, contrast in the degree of sericite- tourmaline alteration across the contact; in both cases the porphyritic biotite granite is more altered. Two additional examples, in drill core, of the contact ex- hibit no contrast in the degree of sericite-tourmaline alteration across the contact.

Brown zinnwaldite~sericite

In the zinnwaldite granite body the sericite-tourmaline alteration is generally succeeded upward by an alteration assemblage dominated by coarse-grained (greater than 5 ram) pale brown zinnwaldite (to 15%) and sericite (Fig. 7). From 10 to 50 percent of the feldspar is sericitized. Tourmaline is present in trace amounts as very fine grained disseminated grains and veinlets. Quartz is coarser grained (to 5 mrn) than in the sericite-tourmaline zone and is generally subhedral as opposed to dominantly anhedral. Trace amounts of arsenopyrite and cassiterite are visible in hand specimens in brown zinnwaldite-sericite-altered granite.

The transition from the sericite-tourmaline zone

upward to the brown zinnwaldite-sericite zone is generally gradational over several to approximately 15 m although abrupt transitions over a centimeter have been observed. This transition is characterized

by: (1) gradual increases in the grain size of zinnwaldite and quartz from roughly 2 mm to around 5 mm, (2) a general increase in the degree of sericitization of feldspar, and (3) a gradual decrease in the abundance of tourmaline.

White zinnwaldite-sericite

Brown zinnwaldite-sericite alteration of zinnwal-

dite granite is commonly replaced upward by more

intense alteration characterized by coarse-grained white zinnwaldite (to 15%) and pervasive sericitization of feldspar (Fig. 7). Rare tourmaline occurs as disseminated grains, and traces of disseminated medium-grained arsenopyrite and fine-grained cassiterite are visible in hand specimens. Traces of disseminated tantalite-columbite are visible in thin sections (J. C. Reid and S.C. Bergman, unpub. data, 1982). The alteration creates a pseudoporphyritic texture in the granite, with subhedral to euhedral secondary quartz and zinnwaldite in a sericite groundmass. In some cases white zinnwaldite-sericite alteration occurs in the brown zinnwaldite-sericite zone as steep, narrow (up to 3 cm) bands with indistinct gradational margins. The transition from brown zinnwaldite-sericite alter-

ation upward to white zinnwaldite-sericite (Fig. 7) commonly occurs over less than 1 to 5 rn by a change in the color of zinnwaldite and a gradational but pronounced increase in the degree of feldspar replacement by sericite.

Analyses of zinnwaldite from each of the three alteration zones described above indicate a minor but systematic change in the composition of the mica (Ta- ble 3); manganese content increases and magnesium decreases upward in the zinnwaldite granite body. These effects parallel an overall increase in alteration intensity.

Greisen

The term greisen is used for altered granite in which the original texture and mineralogy have been entirely destroyed. Minerals which characterize greisen at Kougarok are subhedral to euhedral quartz (50- '-•100%) with subhedral to euhedral blue-green to black tourmaline (0-50%), topaz (trace-5%), and fluorite (0-2%). Medium-grained cassiterite and arsenopyrite, generally as disseminated grains, are commonly present in amounts up to several percent. Over narrow widths, up to 30 percent of cassiterite may be present. The greisen is, in most cases, a massive fairly homogeneous medium- to coarse-grained rock; changes in the relative abundance of minerals are generally gradational.

Quartz or quartz-tourmaline greisen occurs in the zinnwaldite granite as discontinuous lenses at or near, and parallel to, the upper contact of the body, in two near-vertical pipes (Fig. 7) and in thin generally steep shoots which are most common in the area around the greisen pipes. Contacts of the greisen bodies are seldom sharp but occur as a several-meter transition zone in which zinnwaldite and sericitized feldspar give way to quartz or quartz and tourmaline.

The two pipelike bodies are steep sided and nearly cylindrical. They have diameters of approximately 30 rn at the surface and appear to merge at depth (Fig. 7). The bodies are comprised of generally massive quartz and quartz-tourmaline greisen. The greisen


pipes appear to be surrounded by a zone of sericitization. In drill hole 6 (Fig. 7) the degree of sericitization of feldspar in the zinnwaldite granite increases dramatically toward the greisen pipe. At 2 m from greisen, feldspar is pervasively sericitized and white zinnwaldite is abundant. This alteration sequence is very similar to, although much narrower than, the transition from brown zinnwaldite-sericite through white zinnwaldite-sericite to greisen alteration upward in the zinnwaldite granite body. However, white zinnwaldite peripheral to the greisen pipe is not as coarse grained as in the white zinnwaldite-sericite zone. Similar alteration halos occur over less than a

meter adjacent to narrow, steep greisen shoots peripheral to the larger greisen pipes. Late sericite

In hand specimens, partial replacement of subhedral to euhedral tourmaline by sericite is evident in quartz-tourmaline greisen where tourmaline is abundant and relatively coarse grained. This late sericitization of tourmaline is probably widespread, but the extent is not known at present due to the difficulty of recognizing this type of alteration in hand specimens with abundant early sericite and much less (and significantly finer grained) tourmaline.

Alteration of schist

Kougarok schist adjacent to and above the various granite phases of the intrusive complex is cut be veinlets and fractures with hydrothermal alteration selvages. The alteration selvages comprise between 5 and 20 percent of the schist and contain the minerals tourmaline, axinite, and chlorite. Mineralogic zoning is common around individual fractures; the dominant alteration mineral occurring adjacent to a fracture or veinlet varies systematically with distance from the granite complex. These variations in alteration mineralogy define a zonal pattern with a diameter of at least 2 km at the surface, consisting of a proximal tourmaline zone, an intermediate axinite zone, and a distal chlorite zone (Fig. 4). Drilling from the surface within the tourmaline zone along sections A-A • and B-B • shows that stringer zones and schist alteration are distinctly more abundant above the zinnwaldite granite than above other granites (Fig. 7).

Drill intercepts of the tourmaline zone contain quartz, quartz-tourmaline, and tourmaline stringers with subordinate cassiterite and sulfides (dominantly pyrrhotite but including arsenopyrite and chalcopyrite). The stringers are a few millimeters across with selvages 5 to 10 cm wide in which tourmaline has completely replaced muscovite, chlorite, and biotite. Tourmaline replacement of mica is succeeded outward by several centimeters of mixed tourmaline and green hydrothermal chlorite, which grades outward to partial replacement of metamorphic phyllosilicates

by chlorite for up to a meter away from the stringer. Where the stringers are closely spaced, massive tourmaline replacement of mica occurs over larger intervals.

In the axinite zone, axinite replaces micaceous folia in the schist for up to several centimeters away from fractures without an identifiable vein mineralogy in hand specimens. Where axinite replacement of phyllosilicates is incomplete, hydrothermal chlorite is commonly present and generally becomes the dominant mineral away from a given fracture. Although scattered boulders containing axinite-chlorite fracture selvages have been mapped in the tourmaline zone, occurrences of tourmaline and axinite together are very uncommon in selvages in either the axinite zone or the tourmaline zone.

Hydrothermal chlorite of the chlorite zone replaces metamorphic micas in the schist adjacent to hairline quartz or tourmaline veinlets. Subordinate axinite or tourmaline occurs locally in selvages in the chlorite zone, but they seldom occur together. A schist xenolith in zinnwaldite granite near the base of the body exhibits alteration similar to that in the chlorite zone

at the surface; chlorite selvages occur around tourmaline veinlets which terminate against the granite.

Crosscutting relations between veinlets with sel- vage mineral assemblages of the tourmaline, axinite, and chlorite zones are very rare; where they occur, the relative ages indicated by them are not consistent. This suggests that development of the alteration zones was probably synchronous. The nearly mutually ex- clusive occurrence of tourmaline and axinite is poorly understood. That the axinite zone is not everywhere present between the tourmaline and chlorite zones suggests that deposit-scale hydrothermal zonation alone is probably not responsible. Another controlling factor, perhaps variation in composition of the schist, is likely.

Mineralization

Altered granite and schist at Kougarok locally contain 0.10 to greater than 5.0 percent tin. Arsenic and fluorine occur at very high levels, and anomalous tantalum, niobium, copper, lead, zinc, and silver are also present. The general distribution of these elements and their association with the various altered rock

types is described below.

Mineralization in the zinnwaldite granite body

In the zinnwaldite granite body significant tin mineralization is concentrated locally along the roof zone of the body and in and around the greisen pipes (Fig. 9). The general pattern of tin distribution where the zinnwaldite granite is mineralized away from the greisen pipes is illustrated by drill hole 23 (Figs. 9 and 10). The tin content increases somewhat errati- cally upward in the body, paralleling the overall in-


W-21

29 W-18

15 18 W-2

300

20(

9200E 9400E 9600E I I I

I >_1.oo% Sn ß

• _) 0.10% Sn, ß\ _)0.10ø/o in stringer zones [:.:'f.':'".• •_0.01% Sn

0 lOOm

FIG. 9. Generalized cross section A-A' (Fig. 3) showing tin distribution based on diamond drill core assays.

crease in alteration intensity. Tin grades in this drill hole and elsewhere in sericite-tourmaline-altered

zinnwaldite granite rarely exceed 0.30 percent and generally fall between a trace and 0.10 percent (Fig. 9). Tin grades between 0.10 and 1.00 percent commonly occur in the brown zinnwaldite-sericite, white zinnwaldite-sericite, and greisen alteration zones at the roof of the granite body and in and adjacent to the greisen pipes. Tin grades greater than 1 percent are confined to the white zinnwaldite and greisen alteration assemblages. Significantly higher grades are restricted to greisen. Generally, quartz greisen carries higher grades than quartz-tourmaline greisen in both the roof zone and in the pipes (Figs. 10 and 11). Anomalous values of arsenic (to greater than 1,000 ppm), lead (to 1,340 ppm), and silver (to 17 ppm) in drill holes 6 and 23 show a general correlation of these elements with high-grade (greater than 1%) tin mineralization. Anomalous copper (to 1,145 ppm) and

zinc (to 500 ppm) occur with high-grade tin mineralization but are not significantly and systematically enriched in altered zinnwaldite granite. The locally high values for the base metals suggest base metal sulfides may be present, but except for rare chalcopyrite they have not been identified in hand specimens in the higher grade tin zones.

Tantalum and niobium, occurring together in tantalite-columbite, are enriched upward in the zinnwaldite granite body. Tantalum gradually increases upward from approximately 20 ppm near the base of the body to locally as high as 845 ppm near the roof of the granite. This upward enrichment is present in- dependent of the tin mineralization and occurs to some degree in every drill intercept of the zinnwaldite granite. Tantalum is most strongly enriched where tin is absent (Fig. 11) in white zinnwaldite-sericite- altered granite at the top of the granite body. Niobium varies ahnost directly with tantalum and is generally


• To (pprn) Ag (ppm) Pb (ppm) Zn (ppm) Cu (ppm) .. •'• '*•o s• (%) o,• i i I i _ o , , , ,',øø o , • o .... •,o o • o •

•00 -I•• • 114•

]00 • i , I I i i • i i i i I i I i

FIG. 10. Geochemical profiles for diamond drill hole 23 (Figs. 5, 7, and 9). Drill hole sample interval is 1.5 m or less. Detection limits for the elements shown are: Sn, 5 ppm; Ta, 3 ppm; Ag, 0.1 ppm; Pb, 2 ppm; Zn, i ppm; Cu, 1 ppm; As, 2 ppm; and F, 20 ppm. Upper limits for the analytical methods used for As and F are 1,000 and 20,000 ppm, respectively. Lithology and alteration symbols are the same as in Figures 5 and 7.

two-thirds as abundant. Anomalous silver (less than 10 ppm) and lead (up to several tenths of a percent but generally less than 500 ppm) locally occur together with tantalum and niobium but do not vary systematically with them (Fig. 11). Fluorine levels are very high throughout the zinnwaldite granite body, often exceeding the upper detection limit of 2 percent in the upper portion of the body. Where values are lower, fluorine does not show systematic direct variation with any other element but probably reflects the abundance of zinnwaldite (to nearly 6% F by weight; Table 3), sericite, and tourmaline in the micaceous alteration zones and fluorite, topaz, and tourmaline in greisen. A similar variation in fluorine is observed in the other granitic phases.

Mineralization in other granites

Anomalous tin occurs as a very low grade (100- 600 ppm) zone between 5 and 40 m thick in sericite- tourmaline-altered porphyritic biotite granite along

the upper contact of the body (Fig. 9). Tin is accom- panied h¾ elevated values of silver (less than 10 ppm), lead (to nearly 0.2% but usually much lower), and copper (to 500 ppm). Disseminated cassiterite in the altered granite and in tourmaline veinlets is visible in hand specimens; other tin-bearing minerals may be present but have not been identified. Arsenic values derived primarily from arsenopyrite are high (to greater than 1,000 ppm) and correlate to tin as well, except in drill hole 6 (Fig. 11) where it is significantly more anomalous beneath the tin zone. Fluorine values

are elevated (1.0-1.5%) reflecting the abundance of sericite and tourmaline; fluorite is present only in trace amounts. Where tin anomalous porphyritic biotite granite is in contact with younger equigranular granite (drill holes 18 and 23, Figs. 5 and 9), the anomalous tin content does not extend into the younger granite.

Anomalous tin (averaging approximately 0.06%) occurs in equigranular granite with feldspar-quartz

17 9 2 CHRISTOPHER C. PUCHNER

•'%•'• 0 Sn 5O0 0 m o 5 o

L ioo

i•o Tl•

77o • moo

200

? • • ?45 )•o

250

•.o

400

450 7.9 • e•o

o i o o • o 5 o 5• o

Zn lppm} Cu (ppm) AS (ppra} F (%) METERS o iooo o i 2 DOWNHOLE , ,, ,.•o, o ....

>

J "oo1 >> i I > • 50 --

>• ioo -

>1ooo z >=

>• 15o-

200 - >•

1440

300 --

o .... >•

•>• • •oo-

>1ooo >1ooo

4•-

> •oo

>•ooo

e e i m m i m e $ i m m m • , , • m • 0 •0 0 I000 0 I

FIG. 11. Geochemical profile for diamond drill hole 6 (Figs. 5, 7, and 9). Lithology and alteration symbols are the same as in Figures 5 and 7. The significant tin intercepts occur in the greisen pipes shown in Figure 7.

porphyry inclusions (type 1) in drill holes 17 and 26 (Fig. 10). The alteration of granite, mode of occurrence of the mineralization, and the suite of compa- nion elements are similar to those described in asso-

ciation with the porphyritic biotite granite. In drill hole 26 (Figs. 5 and 9) the tin mineralization in the equigranular granite is apparently cut off by the younger zinnwaldite granite above, which although altered to sericite-tourmaline assemblage, is not anomalous in tin.

Mineralization in schist

Quartz, quartz-tourmaline, and tourmaline stringers cutting schist above granites of the intrusive complex (Figs. 7 and 9) locally bear cassiterite and pyrrhotite, and less commonly arsenopyrite and chalcopyrite. Rare high zinc assays (to several tenths of a percent) suggest that sphalerite may be present;

however, it has not been identified in hand specimens. Within narrow zones of multiple stringers, tin grades are locally as high as 1 percent and may average 0.10 to 0.20 percent over wider intervals (i.e., drill hole 23 at 25-m depth; Fig. 10). These stringer zones are enclosed by significant halos of 0.01 to 0.10 percent tin mineralization (Fig. 9). This lower grade mineralization is hosted in part in tourmaline-chlorite-altered schist but also occurs in schist which is not vis- ibly altered in hand specimens above the zinnwaldite and porphyritic biotite granites (compare Figs. 7, 9, and 11).

At the surface (Fig. 4) the correlation of the tourmaline zone with a zone of greater than 0.01 percent tin is pronounced. Alteration and mineralization of schist is more widespread above the zinnwaldite granite body than in schist above the porphyritic biotite granite (Figs. 7 and 9).


Summary and Discussion

Three major distinct granite types have been intruded at the Kougarok tin deposit. These are, from oldest to youngest, porphyritic biotite granite, equigranular granite, and zinnwaldite granite. Distinct decreases in both tin grade and the intensity of sericite-tourmaline alteration observed in crossing from the older porphyritic biotite granite to the younger equigranular granite suggest that alteration and mineralization began prior to emplacement of the equigranular granite.

Similar relations occurring between the equigranular granite and the younger zinnwaldite granite im- ply that hydrothermal activity continued, or was ep- isodic, and led to alteration and tin deposition following intrusion of the equigranular granite but before emplacement of the zinnwaldite granite. However, this early hydrothermal activity did not generate high- grade tin mineralization. Most of the significant tin mineralization at the Kougarok deposit occurs in the zinnwaldite granite body. Successive alteration zones are arranged in a roughly subhorizontal succession in the body, with the least altered granite at the base of the body. More intense alteration characterized by brown zinnwaldite and sericite occurs above the ser-

icite-tourmaline zone and is succeeded, in turn, by a zone characterized by white zinnwaldite and sericite. Quartz or quartz-tourmaline greisen without any remnant igneous texture is present locally beneath the upper contact of the zinnwaldite granite and in steep pipelike bodies.

Both tin and tantalum-niobium, occurring in the minerals cassiterite and columbite-tantalite, respectively, are concentrated upward in the zinnwaldite granite. The highest tin grades (greater than 2%) are associated with greisen lenses at the top of the body and in the greisen pipes. Tantalum-niobium concentration is, in virtually all cases, present at the roof of the zinnwaldite granite and is strongest where greisen and high-grade tin are absent. Anomalous concentrations of arsenic, lead, and silver occur with high-grade tin mineralization. Tantalum and niobium are co-

anomalous but are not systematically associated with elevated concentrations of other elements.

Based on the major and minor element content of granites associated with tin mineralization elsewhere on the Seward Peninsula and the timing of their emplacement, Hudson and Arth (1983) proposed that the granites originated by: (1) widespread melting of sialic crust in response to Mesozoic crustal imbrica- tion, (2) crystallization of batholiths and separation of residual magma, (3) crystallization of this residual magma to form seriate and porphyritic biotite granite, (4) separation of a second-order residual magma, and (5) crystallization of equigranular granite from the second-order residual magma. Hudson and Arth

(1983) suggested that tin mineralization was depos- ited from a volatile phase derived from late equigranular granite in the intrusive complexes.

Juxtaposition of early porphyritic biotite granite with late equigranular granites and association of significant tin mineralization with the late granite phase at Kougarok suggests that the general history of the granite complex was similar to that described by Hudson and Arth (1983). However, in detail, many aspects of the Kougarok deposit differ from the other granite-associated tin deposits on the Seward Penin- sula, where tin occurs primarily in skarn or replacement bodies in limestone above or adjacent to the granite complexes and in quartz and quartz-tourmaline veinlets cutting both granite and wall rock (Steidtmann and Cathcart, 1922; Hudson and Arth,

At the Lost River deposit (Fig. 1) tin mineralization occurs in a greisenized granite cupola, in breccia and skarn veins above the cupola, and in altered rhyolite porphyry dikes (Sainsbury, 1964; Dobson, 1982). D.C. Dobson (1982) studied the Lost River deposit and described quartz-rich greisen which is mineral- ogically similar to quartz greisen at Kougarok. Mica- rich greisen (massive white mica-quartz-topaz-topaz-tourmaline rock) in granite at Lost River occurs above the quartz-rich greisen as opposed to beneath it, as is common at Kougarok and in tin deposits of the Soviet Union (Shcherba, 1970).

Tin deposits with alteration, mineralization, or ge- ometries similar to that at Kougarok have been described in the Soviet Union (Shcherba, 1970), the Erzgebirge district of East Germany and Czechoslo- vakia (Baumann, 1970), and the Blue Tier batholith of Tasmania (Groves, 1978).

Acknowledgments

This study was carried out for the Anaconda Min- erals Company, Alaska District. D. A. Heatwole, Alaska District Manager, originally suggested pub- lishing the work and secured permission from Ana- conda to do so. This paper would not have been pos- sible without the work of many Anaconda geologists throughout the exploration of the property, especially that of R. A. Apel, T. D. Bowden, G. Dimo, C. L. Edmond, E. L. Erler, and T. L. Hudson during 1983. Their contributions are gratefully acknowledged. Special thanks are due G. Dimo and T. L. Hudson whose extensive prior experience with porphyry systems and tin systems, respectively, was invaluable in unraveling the history of the mineralization at Kou- garok. The present understanding of the regional geology stems primarily from the unpublished work of R. A. Cavalero and M. C. Gardner.

D. Henry at the ARCO Exploration and Production research facility in Plano, Texas, carried out the microprobe analyses. I appreciate the help of R. L. Bis-


sonette who typed the manuscript. Thoughtful re- views by Economic Geology reviewers have improved the manuscript.

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Documents

Geology, Alteration, and Mineralization of the Kougarok Sn