Preliminary report on the geology and ore deposits of the Willow Creek mining district, southern Alaska

Ray, Richard G. · 1951 · Report

<p><span id="_mce_caret" data-mce-bogus="1" data-mce-type="format-caret">The Willow Creek mining district is a small but important lode gold mining district…

Public-domain full text preserved in the Mountain Man Mining Library. Original source: pubs.usgs.gov.

\c,NL SUR7,,„, no.103,1951

USGS LIBRARY - RESTON II INII 11 11 II I II 3 1818 00082919 0 ; 1,4 (t it ,,, UNITED STATES DaARTAENT OF THE INTERIOR GEOLOGICAL SURVEY W. E. WRATHER, Director PRELIMINARY REPORT ON THE GEOLOGY AND ORE DErOSITS OF THE WILLOW CREEK AINIAG DISTRICT, SOUTHERN ALASKA by Richard G. Ray This report is preliminary and has not been edited or reviewed for conformity with U. S. Geological Survey standards and nomenclature. GIOLOGICAL -Si/ 44iksHING;' .)1,, trie,. c((J3r 1 1951

,1ó 3 Errata clininarj report on tho geology and ore deposits bf the Willow Creek mining district, southern Alaska page 3, last paragraph, line 3.read quartz, hiotite, hornblende etc. (ins erting "and") page 4, last paragraph, line 11--for nyrenekite read rne1dte page 5, quotation line 3--read "areal extent is small, and none etc. (inserting Itnonet?) page 6, first paragraph, line 15--for mush read much page 7, first paragraph, last line--for have read second paragraph, line 1--for read page 9, fourth paragraph, line 7--for parellel read parallel page 10, first paragraph, line 4--for throuhtout read throughout second paragraph, line 3--for locus read c@ third paragraph, line ]4-f or cliplacerients read displacerlents third paragraph, line 22--for i4able read Mabel page 11, first paragraph, line 3--for certainly read certainty page 14, first paragraph, lines 3 & 4--for errdnation read terminations third paragraph, line 6--for mineral read minerals page 15, last line on page—for certainly read certainty page 16, first paragraph, line 3--for J. S. Ray read J. C. Ray last paragraph, line 7--for ,veins read veins last paragraph, line g--f or Lingren read Lindgren page 17, third paragraph, line 7-f or have read has second paragraph, line 21--for read Duplicating Section of t Department of the Interior, Wash., D. C.

COAITILATS Page Abstract.. 0000 01 1000110 .01 Introduction General Geology General Geo1o6ic Features Intrusive Rocks

. ',uartz diorite. Gabbro Granite00000*** .0 Dike Rocks

Lamprophyre Joints Diabase.

Aplite-Pegmatite.woe.. .e Seuimentary rocks Structural Features (10000,60* 8 General structural features and age relations..8 Structural features as related to the quartz diorite Foliation Inclusions and segregations Faults Economic geology Character Placers

Veins. General statement Veins trending with southwest-dipping joints..12 Productive golu quartz veins Distribution and Attitude 1,Iinera1ogy and Paragenesis of tne Ore rock

Ore Shoots Structural Control of Ore Bodies.. ILLUSTRATIOAS Figure 1. Index Lap of Alaska showing location of ­.Mow Creek mining district. 2. Geologic map of tne Willow Creek mining district, Alaska.

AB3TIhCT The pillow Creek mining district is a small but important lode gold mining district along the southern border of the rilaikeetna lvbuntains in southern Alaska. To date the district has produced about 5 percent of Alaska's lode gold output. productive gold quartz veins occupy early shear zones in the southern margin of tue ThLceetna batitolith, largelj quartz diorite in this area. The quartz dio­ rite is flanked on the southwest by mica sohist and on the southeast b sedimen­ tary rocks. DiKes of lamprophyre, diabaso, and aplite-pegmatite cut especially the uartz diorite. La.uprophyre dies particularly fall into a recognizable patt­ ern and may be useful in determining quantitative displacements of post-ore faults Tnese dikes have been raped in considerable detail. The major post-ore faults, whien define a second set of sheLr zones, have displaced productive Veins as much as 600 feet. An older group of veins, commonly containing molybdenite, chalcopyrite, and sometimes stibnite, rarely gold, follow strong southwest-dipping joints. These older veins are nonproductive. Cavity filling was prominent in the formation of productiv.. veins. Continued movement in the planes of the shear zones opened new cavities where additional quartz as deposited. Bodies of quartz several feet thick formed in places, but these do cuot persist for great distances alone the shear zones. uartz bands characteristically sell and rich, and pass into barren parts of shear zones. The sheer zones themselves, however, are notably persistent but are everywhere limited by major post-ore faults. The ore is essentially a free-üllin gold 1uartz ore couitaining small amounts of sulfidésana telli.u'ides. Tao 'golu is husua1ly fine grained. It occurs as small isolated flakes or as blebs strung out in quartz; sometimes it forms as fill­ ings around earlier euliedral quartz crystals; but most commonly it is directly as­ sociated ,iitn sulfids and tellurictes. The gold is especially clOsely associated with the telluride nagyagite, and where nayagite has been observed, the tenor of the era is hi1i. Tellurides have been reported previously but are identified here for ;ie first time. Gold was deposited :here favorable condit:.ons existed, parti­ cu1ar11at tue intersections of smaller veins 1ththe main veins where the quartz host vies more susceptible to fracturing. '.'all rock alteration has been intense adjacent to productive quartz veins but seldom extends more th:n 10 o 12 inches beyond the quartz filling. This hydro­ thermal activity has destroyed hornbiende, biotite, plagioclase, magnetite, and chlorite, and has resulted in the ormatio.i of sericite, carbonate, sulf ides, and quartz('). It is a mind of alteration tyica11y accompanying veins described as mesothenrjal. All mines in the 'Jillow Creec district are small, and most minable veins have been exploited only to shallow depths Yet the veins L this district are similar mineralogical1y.ad structurally to veins of other districts wheremiuing has been Carriect out for several thousand feet in depth. Larger scale operations are needed to exploit tne Willow Creec type of veins ASt successfJ.ly. The, geologic setting of tne aillo. GreeN veins is favorable to successful lode mining, but this is tem­ porarily ovebalauiceu cy unfavorable economic conditions.

INTRODUCTION The Willow Creek gold mining district is an irregularly shaped area of about 50 square miles lying east of the railroad belt in southern Alaska (fig. 1). The ,center of the district is 20 miles by dirt road from the town of Wasilla, on the main line of the Alaska railroad, and 25 miles from Palmer, on a spur of the Alaska railroad. Both highway and rail connections link Palmer with Anchorage, 50 miles to the south. The mining district is within an area that was intensely glaciated, and it now presents features of typical "biscuit board" topography. Steep-walled cirques and hanging valleys separated bz; sharp argues are characteristic. The recorded glaciation is of the alpine type as attested by the jagged, saw-tooth ridges which give most of the district a rugged and impressive appearance. The intense glacia­ tion has sculptured peaks which attain 4p0o feet in places. Relief of 2p00 feet within 1 mile is not uncommon. The steep cirque and valley walls are usually mantled near their bases by wide talus slopes commonly containing blocks of quartz diorite 10 to 1.5 feet across. Valley floors were originally covered with glacial debris, but these have almost everywhere been modified somewhat by postglacial drainage. Postglacial stream gravels have not developed to any extent, however, and stream beds are typically strewn with large boulders. Glaciation has destroyed the possibility of commercial placers for the most part and has made prospecting for lodes somewhat more difficult. The best rock exposures are high on the valley walls, and as a conseueace many mine openings occur in these more ealilyprospected, bLt difficultly accessible areas. In general the slumosrs within the mining district are rather wet and cool. From the end of Aay through September temperatures between 40P and 500 F. are common. The seasons are not predictable. however, The summer of 1948 was cool and wet whereas the summer of 1950 was unusually warm and dry. The normal snow­ fall is reported to be between 4 and 5 feet. Between 16 and 20 feet of snow fell during the winter of 1948-1949, but tnis.was an exceptionally severe season. Al­ though most of the snow falls during the winter months light snowfalls can be ex­ pected at and time throughout the year. Heavy winter snowfalls together with the rued character of the terrain are responsible for numerous snowslides which not only increase the difficulty of maintaining roads during the winter months but are a constant hazard to the mining camps, many of which have suffered extensive dam­ age in the past. The mining district lies entirely above timber line. Some small willows grow in the upper reaches of the.Little Susitna River, but most areas are barren of trees, aau timber for mining purposes must be shipped in. Between the discovery of lode gold in 1906 and the present, a number of invest. igations have been made by the Geological Survey in the Allow Creek district. The more important investigations were by S. R. Capps 1/ in 1913 and.by J. -C. Ray i Capps, S. R. 1915, The Willow Creek district, Alaska: U. S. Geol. Survey Bull. t07, 86 pp. 2/ Ray, J. C., 1933, The Willow Creek gold lode district, Alaska: U. S. Geol. Survey Bull. 849-0, pp. 165-229.

in 1931. Although these workers contaibutoci a great deal to the geologic knowledge of the mining district, Capps hoped the area only in a /general way at a time when the district \Jas just opening up, and J. C. Hay .ppears to have confined his attention largely to underground studies. Between 1948 and 1951 H. G. Ray con­ ducted a more detailed investigation of the area. Ho was assisted during the season of 1950 by B. W. V/ilson. A comprehensive report is now in preparation, based on field and laboratory studies. In order to rke available immediately the pertinent results of this recent study the following short preliminary report and geologic map ar issued. Photographic illustrations, detailed mine maps, and discussions of individual mines have of necessity been omitted here. GNRAL GC LOGY General Geologic Features The Vil1oi Creek mining district is a small marginal segment along the south front of the T1keetna batholith in southern Alaska. The batholit.h is roirghly 1,500 square miles in extent and is believed to have been emplaced in late ikso­ zoic time, Older reconnaissance surveys have revealed several rock types within the Talkeetna batholith, but in the '/i11ow Creek mining district the igneous rock is largely quartz diorite. This intrusive mass is bordered on the southwest by 'ow-grade mica sciiiit. To the southeast, steeply tilted sedimentary rocks roat unconformably upon the batholith front. FThcturing in the batholith margin provided structural channelviays for the introduction of gold quartz veins and associated lai.iprophyre, diabase, aplite, and pegmatite dikes. All these features were disrupted by major faulting Which took )1ace late in the tectonic history of the intrusive mass. Intrusive Hocks Quartz diorite The i1low Creek mining district is underlain predominantly by quartz diorite although small masses of granite and gabbro are present locally. ist of the rocks exhibit prithary flow structures and have a gnèissoid appearance. The quartz diorite ranges from fine to medium gräined; the finer grained phase is generally restricted to the southern border of the igneous mass. Plagioclase, quartz, biotito, hornblende are the chief minerals with xuicrocline, orthoclase, sphene, aatite, zircon, and magiietite comprising the accessory constituents. The plugioclase ranges from about 32 to 48 percent anorthite. In soma areas horn­ blende is more prominent magascopically and appears to be the most abundant mafIC mineral, whereas i1i other areas biotite ismore conspicuous to the naked eye and Seemingly predominates over iiornblende. No uniform distribution of one or the Other of these types could be mapped, aithougn, in a general way, the more notice­ abIybiotitie phase occupies an area nearer the southern border of the intrusive mass, and hornblende is more prominent magascopicaily in the northern part of the area mapped. Vhere hornblende is more prominent, it is generally coarser grained than the associated biotite, and vice versa.

The quartz and plagioclase content of the quartz diorite is rough1 uniform throughout the district, The feldspar tends to become more basic and more stron€ zoned towards the center of the igneous mass, however. fdon trie southern border o the igneous tile quartz diorite is finer grained and more strong' foliated tuaii that which under1ies most of the mining district. Jineralogically, the finer grained border phase is similar to the nor­ mal mediuin-grained quartz diorite to tue north except that microinetric analyses sho' hornblende to be present in slight excess over biotite. So far as can be determined from field observations the finer graineu phase and the normal quartz diorite are parts of tne same intrusive sequence. The finer grain-size reflects tne cooler environment in which these rocks crystallized. Gabbro In tle southeaat part of the mining district the coLntry rock is a greenishblack, medium-grained hornblende g.bbro. This rock appears to grade northward into tue normal quartz diorite although more detailed study of the area between these to rocic types is needed to clarify tneir relationships. The rock is com­ posed e35entia11y of nornblende and plaioclase feldspar. Pyroxene is notably absent in specimens collected by tue writer although in a specimen collected by Capps, presumably from tiil same area, pyroxene is a1undant. Apatite, sphene. and magnetite are the accessory i1nera1s. Calcite is present mainly in a network of seconth.r carbonate veinlets that cut the gabbro. In a few places hornblende has altered to chlorite. Granite Granitic rocks have been described from the western part of the Talkeetna untains 3/, but such types are present only sparingly in the Willow Greek mining 3/ Capps, . 1., and Thek, Ralph, 1935, The Willow Creek-Kashwitna district: U. Geol. 4urve Bull. 864-B, pp. 95-115. district. Granite has been observed only in the western part of the mining dis­ trict ..here it is cleurly intrusive into older 1uartz diorite. One small pluglike body is exposed on the hi&i point of tile ridge 3 miles northeast of the Lucky 3hot mine. It ias not trace westward. Elsewhere along the western part of the area mapped, granite occurs in djrs as much as 10 feet iide ].fl tile quartz diorite' The granite is a light-colored, maf ic-poor, medium-to fine-grained rock composed of essential1agioc1ase ranging irom 7 to i6 percent anorthite, quartz, and p0t88 feldspar, generally microcline. iicoVit and biotite are present in small ainount but hornblende is lacking. In places small quantities of myremekite and inicroclifl m.icropertiiite have developed. t

Dike rocks General Statement:--In Capps's original discussion of dike rocks in the Willow Creek district, it was stated that dikes are ...not abundant...most are only a few feet wide and occur in places where. their longitudinal extent can be traced for only short distances. Their areal extent is .small, and...were large eaough- to justify their represen­ tation on a map of the scale of Plate III...4/ (scale is 1/62,500). TTICaPPS. j. R., op. cit. p. 48. The possible significance of he dikes was overlooked, and only brief descriptions were given by Capps.. J. G. Bay 5/, on the other head, recognized 'that dike rocks lay, J. C., op. cit. p. 181. might be of value in locating faulted vein segments, but he made nO effort to bear out this idea, :and no dikes were shown on his geologic map. The productive veins lie aLlost entirely within the monotonous quartz diorite, and by detailed mapping Of dikes it may be possible to obtain quantitative data regarding late major fault displacements of the gold quartz veins. Detailed surface mapping by the writer has sho,a that dikes are indeed not.too abundant, but several have been observed (see fig. '2). . Dike rocks in the quartz diorite may be subdivided into four general groups: 1) laMprophyrL.. 2) 4iabase, 3) aplite, and 4) pegmatite. For the moSt part only. the lamprophyre dikes fall into a recognizable pattern, and these have been mapped in considerable detail. All dikes are older than the post-ore faults with the ; possible exoeptionof tae diabase which follows the major transverse fault pattern and maybe contemporaneous with the post-ore faulting. No dikes have been traced witn certaintzi across major fault zones, but more detailed mapping in areas where dike segeats are now known may prove fruitful. Age relations of the various dike tyl2les nave been determined from scattered observations throughout the district,. and the se.,juence from oldest to-youngest has been found to be aplite.-pematite, lamprophyre, atd diOase. Observations along the outcrop of the Lucky Shot vein Show that the lamprophyre dikes are older than the vein and have been offset by post-mineral 1Jovement:in the plane of the vein. Whether the movement within the : vein .was normal or reverse could not be deter.died with certainty. At the label mine an aplite dike has been offset about 10 feet by reverse movement in the plane of tne vein. A similar type of moveAent is also believed to have taken place in the plane of the Independence VOiA, but it is unlikely that movement wi.thin the veins would affect the use of dike displacements in marking offSets due to late transverse major faults, for even if dike displacements by vein fissures were large, it %.ould be a unique circumstance which would prevent dike segments on opposite Sides of late major faults from being effective keys to measurement of the major fault offset.

Inprophyre:--The lamprophyre dikes are dense, fine-grained, greenish-black rocks vdhlCh break ith greut difficulty. Aside from their color they generally can be recognized in hand specimens by the presence of hornblende phenocrysts which attain 0.2 inch in length in some specimens. The weathered surface of these dikes ns a dark gray color not unlike that of the host j,uartz diorite. Graycolored lichens I'OW on nst weathered surfaces of the dicu rocks and quartz diorit and aaaresult it is often extremely difficult to distinguish the two unless.fresh specimens are obtained. This undoubtedly explains why re occurrences 01' the dik rocks have not been reported although lamprophyre dike float is conspicuous \here present. The dikes range in viath from a few inches to about 6 feet. Contacts with the quartz diorite are always sharp. The strike ranges from about 090 to 1800, but the st prevalent trend is 120 to 140°. The dip is :oder.te and alway to te southwest. A1st always these dizes follow strong southwest-dipping joints: in places they may be observed to break across from one joint to another. Faulted segments of the lamprophyre dikes are continuous.in places for as mush as 200 feet, and segments belonging to the same dike have been traced for nearly a mile (see fig. 2). Offsets by -minor faults are numerous,and in places these dikes are sveied by the major transverse faults oi the mining district. It is in this regard especially that they may be of considerable geologic- -and hence, economic-­ importance. Diabase:--Diabase dike segments have been observed in only a few localities within the mining district. These dense black rocks reseib1e the lamprophyre auperficially, but ty do not conform to the lamprophyre ditch pattern and should therefore be differentiated. The diabase is always badly sheared and in this respect contrasts sharply with the lamprophyre which is solid and compact. Note­ worthy also is the almost complete lack of phenocrysts in the diabase, whereas hornblende phenocrysts cbaractefize tue lamprophyre, As a result, distinctions in the field can be made easily, and microscopic determinations are not neede The few diabase dike segments mapped range in width from 2 feet to about 20 feet. No seneiits could be traced for more than a few hundred feeto The general strike of tnese dikes is between 095° and 1100; the dip is steep in a northerly direction. In contrast tue lamprQphyre dikes trend slightly more northerly and aLiays dip in an opposite direction, that is, to the southwest. The smaller number of diabase dikes 'ithin the 1lio.w Creek district suggests that these might be of considerably more value than other di1ces as marker horizons for major fault offsets, but the general conformity of the diabase dikes in strike and dip 'itn one or more major post-ore faults of the area lessens their correla tion, value, although they may be useful in determining displacements along northeastward-trending faults. In all probability the shattering in the diabase is due to later shearing along major faults which tne' occupy.

..4plite-Pegmatite:--The aplite-pegmatite dikes are not uncommon, but because of the usual 'weathered, lichen-covered surfaces they are easily overlooked. Along the southei'n a,rgin'of the batholith mush aplite-pegmatite dike float has been . observed, and it' is here that these dikes .are most abundant. They do not .Conform as a rule to any general pattern, although locally they may follow the strong southwest-dipping joint set. Widths range from about 1 inch to 5 feet. These dikes typically's::Ait, pinca, and swell along the strike, and only rarely can they be traced for more . than a hundred feet or so. Strikes and dips vary radically in many places. Locally these dikes have been useful in marking very minor fault - displacements, but it is doubtful'if tney would ever be of value in correlating movement alonL Major faults of the district. Aplite Aay occur independent of Peg­ matite but often the aplite and pegmatite are found together in the sane dike and nave been treated together here. Where aplite and petite do occur in the same dike the aplite may fbna the finer grained border with pegmatite in the cen­ tral band, but aplite flanked 1%. - pegmatite borders 'have also been observed. . Recent work was shown the pegmatite's to be slightly radioactive due to small aunts of uraninite,'thorite, allanite,-and cyrtolite 6/. 140xham, R. Id., and Aelson, A. E., 1951, iladioactive pegmatite minerals in the .4.1low Creek mining district, Alaska: Trace Elements Investigations Report 74-C. Schist:. Where schist borders the quartz diorite in the Willow Creek raining district it is a strongly foliated, silvery-gray rock, with foliation trending northeast* ward and generally dipping moderately to the northwest. The foliation is a flow cleavage formed by the'subparallel alinement of muscovite plates. Plagioclase teldspar pOrphyroblasts - up to 0.2 IAIOII across are genen,lly strongly developed, and occaSionally needles of black tourmaline sup tp. 0.4 inch long are seen. - Throu6hout.'most of the area the schist is highly fissile due to the extreme devel­ opment of Muscovite, but in some places tile rock is .more massive. due to a greater amount of feldspar - and lesser- . development of mica. Small open folds superposed on the regional aortnward-diPping -fOliation are numerous adjacent to the bathOlith contact, but li miles to tae.south isoclinal folds; predomirgate. Original bedding is generally obscUred by'ttie matamorphiSm, but in maay streagbeds and in under­ ground 'workings at the. Thorpe Mine color: banding representing compositional layer­ ing is clearly seen. Aare original bedding has been recognized, it ip parallel to tile foliation of tae scnist. Dikes are not common in the schist, and partic­ ularly are dike types similar to those in the quartz diorite lacking.

sedimentary rocks Sedimentary rocks gamprised largely of congloAerate, arkose, shale, and sand­ stone, clip gently to the south away from the quartz aiorite batholith, Coal beds and lava flows are locally interbedded, and in places the sequence is gently fold­ ed Vi Where the sedimentary sequence is in contact with the batholith the beds 2/ Capps, S. R., 1915, The 'Allow Creek District, Alaska: U. 3. Geol. Survey Bull. 607, pp. 32-33. are steeply tilted. Southwest of the napped area the sedimentary rocks are sep­ arated from the quartz diorite by the older mica schist. Study of the sedimentary rocks is outside the scope of the immediate mapping project of the mining district, and these rocks have only been observed in the southeastern part of the district. Here tae basal conglomerate bed, about 100 feet thick, contains well-rounded bould­ ers of quartz diorite as much as 5 feet in diameter together wita a variety of cobbles and pebbles in an arkosic matrix. Other rock types in the conglomerate include .,uartzite, aafic dike (7) or greenstone (?), aplite, and chert. The arkose is generally fine grained and breaks with great difficulty. It has a striking resemblance to the .ivartz diorite from which it was undoubtedly derived in part. 3hales and sandstones which crop out to the south were not observed by the writer. Structural Features General ptructurLl'features and age relations The southern border of the Talkeetna quartz diorite batholith as exposed in the 1ii11ow Creek mining district is flanked by mica schist and by sedimentary rocks composed chiefly of conglomerate, arkose, sandstone, and shale. Nowhere is the contact between the quartz diorite and mica schist exposed, but comparison.of the strikes and dips of foliation in the mica schist with those in the nearby quartz diorite suggests that the structures of these two rock types are locally uncon. formable. The grade of metamorphism does not increase as the quartz diorite con­ tact is approached. Indeed, in some places away from the contact deformation is stronger in the schist, and the possibility of a fault cont,ct between these two rock types should be considered. Jointing, which is so characteristic of the quart diorite, is only poorly developed in the schist. In the Southeastern part of the mining district the basal conglomerate of the younger sedimentary sequence is in contact with the gabbro phase of the Talkeetna batholith. ilere the bedding is steeply tilted to the south at an angle of 580 to 700. The degree of dip decreases rapidly away from tne contact. luartz diorite boulders in the conglomerate attest to tae younger age of the sedimentary sequences and these boulders together with arkosic beds mineralogically similar to the quartz diorite indicate at least partial derivation of the sedimentary rocks from theAg­ neous roc4 types. The steep dip of the sedimentary sequence at the batholith front is in all probability due to a renewed uplift of the intrusive mass.

Structural features as related to the quartz diorite Foliation:--gascOpic foliation due to primary ,Lineral orientation is gener­ ally moderately develop6d in the quartz diorite. It has a rathei constant trend to tile northeast and moderate dip .to the northwest. It is most easily seen in arcas where hornblende is the predominant mafic mineral. Inclusions and segre6ations:--Inclusions in the quartz diorite are exceedingly common. These are comprised of at least three different rock types. They are co,Apos.ed of similar minerals bgt differ in texture, grain-size, and relative pro­ portions of the various minerals. By far the greatest number of inclusions are fine- to medium-grained, gray or gray-black,-elong,, te bodies generally lying in the foliation planes of"tne quartz diorite. They are characteristically porphy­ ritic arid shoL hornblende aau feldspar phenocrysts 0.25 to 0.5 inch long, but some are entirely even grained. As a result of weathering they often stand out in re.. lief abov,3 tae aost quartz diorite, 14bst are 1 to 2 feet long and are but a few inches wide. A second type of inclusion is a fine-grained,..lightcolored quartz diorite. It is most often found as large blocky masses several feet across. Contacts may ox :aay not be conformable witn the foliation of the host rock. A foliation with­ in Liie included Material is common, but tills is generally not conformable with the cont. 0t hor with the foliation in the host rock. -within these large inclusions of fine-grained quartz dioritic material smaller elongate mafic inclusions of the type described above may occur rarely. A third type of included material in the quartz diorite is confined largely to an area near the south end of the divide separating Craigie and Upper Willow Creeks, although scattered outcrops at higielevations from other parts of the district are known. The roc is Wically dark green to black, very fine grained, and often strongly banded. .The largest block of this material seen was about 100 feet long and had a maximum width of 3i to 4.feet. . It is a tabular body lying essentially horizontal and ierelleI with the foliation in the.surrounding host rock. Alternating.light and dark bands ranging in width from a knife edge to several inches characterize this inclusion.. Fine-grained mafic-rich bands Separated by lightcolored som4what aplitic bands predominate. Segregations, or schli6ren, of mafic minerals are much less coMmon than inclu­ sions in the ,uartz diorit. They are not restricted-areally, although most have been observed in areas where hornblende is. the predominant mAfic mineral. The schlieren are typically black due to the concentration of biotite and particularly hornblende. .They are often coarser grained than the surrounding quartz diorite but are mineralogically similar differing only in the relative abundance Of various . minerals..., Fine-grained schlieren of light colored minerals, predominantly quartz and feldspar, are present to much lesser extent.

Joints:--Three sets of joints characterize the quartz diorite except for a zone ab,t a mile wide )araliel and immediately adjacent to ita southern border where only a few joints in no definite pattern occur. In any one area only two of the three joint sets may be conspicuous, but throughout the quartz diorite the southwest-dip..)ing set of joints is always prominent. The average trend of tais set is 135° and dip is about 400 to 450 southwest. These joints are often develcpeO, over many tens of feet and are characterized in general by remarkably smooth flat faces. They may be further characterized by barren quartz filling 1 t 4 inches wide, and in many places they are the site of chalcopyrite-molybdenite veins, aplite and lamprophyre dikes. The second joint set trends just west of north and dips steeply to the east. A third joint set strikes nearly due eastwest and may dip steeply either to the north or south. This third joint set is poorly developed and is marked by .irregular surfaces. Altnougn tae southwest-dipping joints may very locally appear to have exerted control on the emplacement of certain of the productive gold quartz veins, these joints are not tue locus of the productive veins as described by Gapps V. Where 8/ Capps, 3. R., op. cit., p. 56. quartz veins are parallel to the southwest-dipping joints prospecting nay be ex­ pected to be unprofitable. iauits:--A variety of faults ranging from a few inches to a few hundred feet displacement cut the quartz diorite. The major faults of large displacement divid the quartz dioriteinto a number of segments in which productive gold quartz veins are conlineds These faults establish a well-defined fracture pattern and are most important in a study of tne ore depOsits because they are post-ore in age. In several places they truncate valuable ore shoots. The major post-ore faults appear to define a coajugate set trending to tae northwest and diplAng to the southwest aid northeast. They are believed to be mostly of normal displacement. The faults are wide zones of comminuted, generally strongly altered quartz diorite which exceed 100 feet from hanging*.wall to footwall in Some mines. Horses of little-altered quartz diorite arc3 present in places within the major fault ?cries. The Gold Cord fault on the 200 level of the Gold Cord mine is 120 feet wide, and the ivilartin fault which cuts off the Independence vein on the south is reported to , be 140 feet wide on the 900-level southwest crosscut. Diplacements on these major post-ore faults are all believed to be large, but in only a few places is the mag­ nitude of the offset known. Faulted segments of the rich Gold Bullion vein have never been located. Older maps of t.e Fern.mine show that the horizontal compon­ ent of displacement of the Fern vein where it is cut off on the west end, is appre, imately 300 feet. To the east it is displaced a similar distance by a second post ore fault. If the vein north of the Gold Cord fault is a continuation of the seg­ ment suut of tne fault, the horizontal component of displacement is of the order of 125 feet. Displacement of the vein at the Abel mine along the ikable fault has also been about 12.5 feet, and according to J. C. Ray the horizontal component of i displacement on the fault in the east end of the Lucky Shot mine is. more than 600 feet 2/. Although the total offset on the major post-ore faults has been 2/ Bay, J. C., op. cit., p. 211.

considerable, at any one point within the fault zone the movement may have been small. ':ihere the Martin fault is exposed in a small stream tributary to Craigie Creek from the southeast, for example, foliation within the thoroughly altered quartz dioritJ of the fault zone is well preserved over a width of several feet. In the R4rtin mine a segment of the minable quartz vein was reported to lie in part within the Ahrtin fault. Thus there is some evidence that the greatest amount of movement along post-ore faults may have been confined to a narrow part of the fault zone. Because marker horizons in the quartz diorite are generally lacking, detailed mappThg of lamprophyre dikes was carried out in an attempt to determine the quant­ itative displacement of various faults. iNko, dikes have been traced with certainly across the wide northwestward-trending post-ore faults, but many narrow northeastward-trending faults, with displacements as great as 200 feet. have been mapped and a general pattern of displacement .).11 them established (see fig. 2). Ath few exceptions the displacement of dike segments.north of the northeastward-trending faults has been reLAtively o the east and probably downward. The same pattern holds for the 1:11jor northwestward-trending faults, so far as can be told from underground studies. Other minor faults have displaced productive veins a few feet but generally not enou6A to hinder :dining operations. In the Gold Cord mine, however, so-called "strike faults' created a serious problem in mining. These faults trend nearly due north, the same as the gold quartz rein, but dip to the east, a direction op.,,osite to that of the vein. The mine record shows that these faults are hinge faults of normal displacement. Displacement increases 1:o the north. In contrast , to the wide northwestward-trending post-ore fault zones, strike fLailts in the Gold Cord mine are at most only a few inches dde. Two strong strike faults are also knon in the Independence mine, but these do not offset the vein seriously. There has also been post-minel:L1 movement in the planet of productive veins. Shearing has caused late fracturing, and in the Fern and Snowbird mines the quartz in places has been reduced to lenses and pods of sugary, granular texture. The relation of aplite dike6 to the vein at the la..bel mine indicates that movement in the plane of the vein was reverse. Stoll 12/ reports shearinc, has caused a slight 12/ atoll, W. c., 1944, Relations of structure to mineral deposition at the Inde­ pendence mine, Alaska: U. S. Geol. Survey Bull. 933-c, pp. 201-216. reverse displacement along tne Indepencence vein.

EC0140141IC GEOLOGY Veins General statement Excluding pegmatites, which in places contin minor amounts of chalcopyrite LiE bornite, tae veins may be divided structurally into two 6rous, a younger group occupying early major shear zones id the tectonic pattern of the quartz diorite, i and an older group generally conforming to tae strirce and dip of southwest-dippiN joints. The quartz bodies in the early shear zones comprise the productive veins of tae district. They in turn form two groups, one trending 060° to 080°, dippin€ 300 to 60° to tae north, and one trending nearly cue north, dipping from a few degrees to about 4_5° to the west. Veins along the southwest-dipping joint planes are, in contrast, nOnproductive. . They can in. turn be subdivided into three types on tae b,.sis of mineral content. The more common type is composed of chalcopyrite molybdenite, pyrite,, and quartz; the second type is made up of pyrite, stibnite, and quartz; the third contains coarsely crystalline quartz with sparse pyrite, Sphalerite, possIbly other sulfides, and coarse gold. Veins trending with southwest-dipping joints By far the wost abundant veins in the district are vuggy, glassy quartz veins containing wolybdenite, cnalcopyrite, and pyrite. They standout readily due to t the blue and green colors of azurite and malachite which have resulted from surfac. alteration of chalcopyrite. LimOnite also is conspicuous in some vein outcrops. 14ost of tnese veins are only a few inches in width, and with few exceptions they : pinch out in short distances along the strike. A few veins as much as 1 foot wide have been observed. The strike ranges from130° to 1'00, always in conformity wit the local variations in the striA:e of the joins; the dip is generally between 350 and 500 southwest. The quartz is often somewnat sheeted, and these fractures gen­ erally are coated with a veneer of molybdenite. Oxith.tion of the molybdenite in some veins nas yielded a yellow or orange-yellow. coating, possibly ferrimolybdite( or A second vein type co1formin6 to the structural pattiarn of the southwest-dippr ing joints of the ,ivart:2, diorite contains pyrite, stibnite, and glassy quartz. ';whether it is a distinct vein type deposited under different temperature conditiom or at a different time tnan the chalcopyrite-Alolybdenite veins is not certain, but. it may be pointed out that no specimens have been found showing molybdenite and stibnite associated in the same veia. The pyrite-stibnite veins are of the same magnitude as the chalcopyrite-molybdenite veins but are far less common. In a few places vuggy quartz veins filling he southwest-dipping joints contEW coarse flakes of free gold. Pyrite, sphalerite, aria possibly other sulfides are associated sparsely. A vein of this type in the first valley northeast of the Golc Core mine has been unprofitably exploited. The gold has been deposited as coarse i plates around quartz crystals. Two oher joint quartz veins containing sparsely distributed gold crop out on the ridge top east of the Gold Cord mine upper work- . ing.

l'roductive gold quartz veins Distribution and Attitude:--Productive gold quartz veins are confined almost entirely to an area in the quartz diorite along the southern border of the Talkeet­ na batholith. This area is about 8 miles long in a nearly easterly direction and about 4 miles ide. In general, the productive veins fall into two groups: 1) one trending 0600 to 0300 and dipping 30° to 600 to the north, 2) the second trending approximately north and dipping from a few degrees to about 450 to the west. These trends are average trends only; it is common for both the strike and dip of any one vein to vary, in places considerably, along the strike and down dip. Character:--Introduced vein material consists essentially of quartz but with some carbonate and small amounts of pyrite, arsenopyrite, sphalerite, .chalcopyrite, tetrahedrite, nagyagite, altaite, &Jena, stibnite(?), gold, and rarely scheelite. In only a few places have the sneer zones been entirely filled and replaced with introduced vein matter. Commonly much mechanically ground upand altered country rock is present, and throughout the shear zones ground up diorite now reduced to clay gouge in/ form an essential part of the "vein". Clay gouge also occurs gen­ erally as a selvedge along the vein walls permitting free breaking. The northeastward-trending ore bodies are in strong shear zones which are as much as 25 feet wide in some places. The quartz may have a sugary texture and most often occurs in long lenses, aggregates of small lenses, or branching quartz stringers a few inches ide. Slickensided blocks of partly altered country rock together with stic14 clay gouge in places make up much of the zone. The north-trending ore bodies, on the other hand, often form well-defined veins taich in places occupy the entire. width of the shear zone. Widths of 6 feet are not uncommon, and a few zones much wider th&n this have been reported. Often a strong band of quartz is confined to one wall or the other of the. shear zone, but in maay places tne vein is composite in nature. danging.wall quartz and footwall quartz may be separated by altered and comminuted quartz diorite, or there may be, in addition, a central band of quartz with cross stringers connecting with the footwall or hanging-wall. In many places hanging-wall quartz seems to be more prominent. Generally, minable quartz is 1 to 3 feet wide, although vein segments containing wider zones of minable ,luartz have been exploited, and in places veins Composed of two or more distinct bands of iluartz only 6 to 8 inches wide have been mined. Quartz may pinch out along the strike as well as dip and pass into barren parts of the shear zone composed only of broken up and altered quartz diorite frag­ ments. Cne or .two exceptional arecis have been reported :,here shear zone material was actually productive. Quartz varies from gray throu,,h blue gray to milky white, and may or may not be banded. Coarsely crystalline quartz with open vugs characterize certain veins, but these have almost never been worked successfully. Quartz in productive veins contains few or no vugs but is bdly sheL red.

Banding in the quartz stands out clearly in many thin sections. Bands of vex, fine grained quartz alternate with bands of slightly coarser grain. The coarser bands in places show elongate, clear quartz crystals with well-developed termina­ tion on one end; these are believed to represent open space filling. Late car­ bonate filling. around terminated quartz crystals . has been observed in many specie mans suggesting that open spaces may have existed for some time. Quartz microbreccia is .common throughout the veins, and microbrecoiation has been suggested as a mechanism which opened early 'quartz to the gold bearing solu­ ions. This is perhaps borne out by the occurrences of .quartz microbreccia in rio portions of some veins, but microbrecciated quaitz is also common in veins which are of low tenor or are barren of gold. In one - specimen what appeared to be a wicrobreccia zone contained small quartz crystals A.th well developed. hexagonal outlines. This may represent a group of crystals torn off from an open cavity wall and later cemented together. i4dneralogy and ParaRenesis of the Ore:--The ore of"this district is essentia1'. a free-willing gold quartz ore with small amounts of sulfides, estimated to be about 2 or 3 percent by weight. Small quantities of tellurides are present local J. C. Bay reported tae gold to be about 950 fine 11/. So far As can be determined 11/ Ray, 3.0., op. cit., p. 191. from polished specimens, binocular study of hand Specimens, and heavy mineral sep­ arations, the variety of ore mineral is small. Because of the scarcityof sulfide and tellurides, and because of their small grain-size when present, it was not possible to deter:line completely the paragenetic relationships in the ore. "alartz was probably the first mineral introduced into the shear zones. OrigiD al quartz of tne i.,uartz diorite was supplemented by this introduced quartz vihich filled open spaces and replaced s.-Ae constituents of the host rock. ,Alch of the quartz deposited b, the circulating solutions xobably ce from the breakdown of silicates in the shear zone and also to some extent from the altered wall rock. Polished specimens show even more strikingly - than thin sections that open spade filling is characteristic of the ore. It is common to find elongate, terminated quartz crystals-which formed in open cavitieS, later filled Predominantly by car­ bonate, but also by late quartz and sulfides. Pyrite is tne most common sulfide, occurring .as striated cubes, pyritohedronal and irregular grains. within the veins it is reStricted: for the most part to the earlier quartz,. list pyrite grains are typically fractured,. and the larger grains often are filled With late carbonate, rarely quartz. Aagyagite, chalcopyrite, and gold nave also been observed as fracture fillings in pyrite. Only in a few specimens as sphalerite observed. It occurs as smell isolated masses in quartz, 'out re often it is aSsociated with otae/ sulfides. It gener­ -ally"contains randomly 'oriented blebs of cnalcopyrite and 'further is col4letely, or pearl,, completely, surrounded by a narrow fringe of chalcopyrite. In specimens, showink, a spnalerite core surrounded uy chalcopyrite, the chalcopyrite is generall): in turn enclosed by tetrahedrite which is closely associated with nagyagite and altaite.

Chalcopyrite fills fractures in, surrounds, and replaces grains of pyrite and was probably the next sulfide to be deo5ited after sphalerite. In a few places irregular flakes of gold occur in the chalcopyrite, but this relation alone does not establish the relative ages of the two minerals. Tetrahedrite occurs sparingly, but closely associated with nagyagite and altaite uith which it is probably nearly contemporaneous. Where sphalerite occurs, a surrounding fringe of chalcopyrite, in turn enclosed by tetrahedrite is usually present. Small blebs of tetrahedrite may form directly in the sphalerite and chal­ copyrite, but in general it seems to have followed the deposition of chalcopyrite. In places it forms mutual lobes with nagyagite. Spectrographic analyses of ore samples i:lade in the chemical laboratory of the U. S. Geological Survey showed traces of mercury. The mercury is likely contained in the tetrahedrite. Older reports of cinnabar in the Gold veins thus have support. Cinnabar is occasionally found in similar gold quartz veins in other mining districts. Ierhaps the most important discovery resulting from a study of the 1allow Creek ores is the identification of undoubted tellurides. Tellurides have been reported in the past but never identified. An earlier report of tellurides at the Lucky Shot mine 1?_/as checked by J. C. Ray who reported no tellurium in the sample ant­ alysed 19/. The present study. has shown that in some ore at the Fern mine and at 1..?./ Smith, P. S. 1932, idineral industry of Alaska in 1929: U. S. Geol'. Survey Bull. 824, p. 18. 19/ Rays J. C., opt cit. pp. 191-192. the Schroff-O'Neil mine nagyagite, a sulphotelluride of le<.d and gold, is an import­ ant mineral especially in the richer ore, and free gold always shows a strong pre­ ference for it, even where fractured pyrite, an ideal host, is present in the same Specimen. The gold commonly oocurs as irregular blebs or flakes in the nagyagite, and the paragenetic relationship is difficult to determine. Binocular examination of some ore samples shows plates of gold largely surrounding nagyagite suggesting a younger age for the gold. But from polished section study the possibility of contemporaneous deposition. at least in part cannot be overlooked. In some specimens nagyagite occurs associated with euhedral quartz suggesting some open space filling around e-rlier Livartz. In other places nagyagite fills fractures in earlier pyrite. Some nagyagite forms along the borders of chalcopyrite and may protrude into chal­ copyrite along an otherwise straight contact. Wherever nagyagite was observed it was lound to coh.Liia small irregular grains of the lead telluride, altaite, scat­ tered through it. The nagyagite, altaite, and gold appear to be very closely re­ lated in age and were undoubtedly among the .latest metallic minerals to be deposited. Galena has been identified ia only a fev, specimens where_ it occurs in masses Which exhibit good cubic cleavage. Judging from polished section studies, it is not an important sulfide in the ore. Gold is usually closely associated with the galena cubes either as flakes adjacent to or within the galena. Altaite is also present as still blebs in galena. Galena was deposited later than epaalerite, but its posi­ tion ia the paragenetie sequence is not knolin with certainly.

Arsenopyrite i8 a conspicuous sulfide as large individual, grains or in cruciform twins, but nowhere was gold seen, An direct assoeiatioa with it, although J. S. Ray has reported bOlä replacing arsenopyrite 14/ Rai, J. C., Op,cit., pp.- 190, 192. Stibnite may occur pparinly in the ore.. 41. cluster of long radiating needles of &strongly anisotropic metallic mineral, believed to be stibnite, was, present in one sample examined. Its paragenetic position is unknown. . 'Scheelite is present sparingly ia mOst of the productive veins, but like stib nite, it position in the parageaetic sequence is unknown. . , Gold in the Willow Creek district ores is extremely fine grained. A-few flake. as -luch as 0.03 inch across were seen, but most measured only a few thousandths of an inch across. Gala occurs as isolated flakes:or_blebs strung out within the quartz, aad sometimes. as fil14ngs. around early-euhedral quartz crystals, but com­ monly directly associated with the.sulfides and.tellurides Pyrite is the only metallic. mineral wnich can be shown to be definitely older, in part,. than time g91d, foxy gold in places fills fractures in pyrite grains. Aside:froathis relationship in polished sections gold is seen as irregular blebs in :write, chalcopyrite, and nagya6ite; or 0 stringers in juxtaposition of. one of these minerals; 9r these min. erals may rarely be enclosed by Gold. These relations tell nothing definite,a­ the relative ages. liowever, the close association of eold with, te late:mineral nagyagite and the apparent restriction of gold to nagyagite .in some places where riagyaGite ia present as minute stringers in .other sulfides, suggests. that the gold is younger than moSt metallic minerals except nagyagite with .which it may be con-, temporaneous in part.. The,i:e is little doubt that movement in the...planes of the veins was at . least recurrent if not Continuous during the. period of. ore deposition. The fact that most sulfide's are fractured,., but that Certain ones are definitely younger or older than other's 1Liple . evidence for this conclusion.. -41artz deposition apparently continued for a long period of time for in places it is also found to cut and.re­ place especiallY pyrite and earlier. quartz. Late carbonate, particularly as small veinlets, is conspicuous. . Many of the ./ 8.1a11;er carbonate veinlets exhibit matchinG walls and are..attributed to open space filling, but Some of the larger veinlets have ragged . contacts with the quartz sug­ .gesting that replacement to a certain degree took place. Even where . distinct vein' lets cannot be recognized, the hexagonal outlinesof , quartz crystals surrounded by carbonate--as well assulfides, tellurides, and gold, a2e a good indication of ()pet space filling.' Wall rock alterations--itdrotherAal alteration-of.taewall rock has been intend adjacent to the productive quartz veins, but it seldom! extends more than 10 to, 12 inclies beyond theuuartz filling. ,iiydrothermal. activity. has.resulted notably in the destruotiOn of hornblende, biotite, plagioclase, ,magnetite, and even chlorite, whereat.; sericite, carbonate, sulfides and quartz(?) were formed as a result of th15 activity. This type of wall rock alteration typically accompanies what Lindgren has ter,Jed "sericitic and calcitic gold-silver veins" 1.V. These veins were later classed 4 Lingren as mesothermal 1.Y. la/ Lindgren, W., 1900, 1,1etasomatic processes in fissure-veins: Trans. vol. 30, pp. 664-668. 16/ Lindgren, W., 1933,Aineral Deposits164th. FA.. pp. 544-355.

Adjacent to the vein quartz, the quartz diorite wall rock has been bleached to a cnaiky or chalizy gray-green color due particularly to the alteration of mafic constituents to chlorite and sericite, and due to the sericitization of the plag­ ioclase feldspar. Witn decrease in intensity of alteration the wall rock becomes darker green and finally passes into the gray color of fresh quartz diorite. Carbonates have developed more or less throughout the whole altered zone, where­ as sericite is most strongly developed nearest the vein filling and chlorite farth­ est from the vein filling. Fyrite and arseaopyrite, also new products of hydro­ thermal alteration, are found nearest the vein filling. Secondary quartz is pre­ sent in a few Places, Many of the original minerals in the wall rock are partially or completely destroyed. Plagioclase is almost always co..,pletely altered to patches of carbonate and fine-grained sericite, but in places severL1 inches from the vein the feldspar twibning is sometimes preserved. Some distance from the vein remnants of hornblende in places are associated \,ith chlorite-biotite alteration products, or hornblende crystal outlines may enclose dull brown biotite. Biotite in most parts of the alter­ ed zone have broken down to chlorite, and nearest the veins the chlorite is in turn altered to muscovite and carbonates. aagnetite, a common constituent of fresh quartz diorite, is present only sparingly in the altered all rock, but leucoxene is abundant locally suggesting that original magnetite may have been slightly titan­ iferous. In most places quartz in the altered wall rock is badly strained and often shattered, and where alteration has been intense the quartz has been replaced by sericite. Within some parts of the veins small horses of quartz diorite have been part­ ially preserved. This altered quartz diorite within the vein--known as vein fillin some places co,Jprises most of the shear zone. Generally, the plagioclase is altered to carbonate and sericite just as in the altered wall rock. Quartz Grains and any feldspar grains which survive are typically highly fractured and filled witn small stringers of fine-grained sericite and carbonate. In a few places very fine-grained, recryatallized quartz may fill these fractures. Apatite is always preserved, but the mafic minerals biotite and hornblende generally do not survive, although in some specimens biotite plates with wavy extinction have been seen. As in the altered wall rock, patches of carbonate and muscovite laths probably mark the position of original mafic minerals. In general, alteration of the mafic min­ erals within tile shear zones has gone completely through the chlorite stage and way muscovite and carbonate now remain. Opaque minerals are usually absent or occur only sparingly. Especially iron must have been removed from the partially altered quartz diorite in tne shear zones, and this may now be represented by the iron sulfides in the wall rock or in the vein quartz. Mineralogical changes in wall rock adjacent to productive parts of a vein are markedly similar to alteration adjacent to barren portions of tae same vein. Con­ sequently there appear to be no diagnostic mineralogical criteria of wall rock Changes which may be useful as a guide to ore.

Ore 3hoots:--i4ost of tae mines in the Willow ()reek district have been devel­ oped only:to shallow depths, and the ore shoots are not well delineated on stope sheets. In some wines:ore shoots have been truncated by major transverse faults, and it is not possible to: determine the pattern of the shoot. Stope sheets of tile Independence Aline do indicate a general moderate rake of elongate ore bodies to the north, and this is in agreement with information obtained from a-former mine foreman. But in some places he ore bodies are entirely irregular in shape. In no mine is quartz always ore, :but rather the minable bodies are distributed with­ in larger areas of lower tenor or barren quartz.' It must be remembered, of coarse , taat mining has exceeded the limits of. the riche:: shoots, and no stope map will give a representative picture 0f these shoots unless careful assays have been-math Structural Control of Ore Bodiest--Certain irregularly shaped ore bodies riay be due to late fracturing of vein quartz where no other'structur41 elements are cot earned, but some of the richer ore shoots have formed at intersections' of the it reins smaller "intersecting", veins, erroneously called "feeder' veins local4 In the Independence mine several intersecting veins have-been exposed. These vei are generally narrow--a few inches mostly, but as 1,1uch as 24 inches in the extre most trend northwestward and di..2 to the southwest.. One such vein is reported to have intersected the IndependenceArein on the now inaccessible 700 level. ilere a considerable amount of ore was ..ained. it will be noted that the intersection of tae "feeder" vein and the main vein on the inaccessible 700 level would plunge to the northwest-,-the direction of rake of the ore shoot as described by a former mine foreman. i4Ore. recently a rica orp shoot was mined on the 1400 level. The. occurrence was somewhat unique for not only aid a strong north-trending vein inter sect tne wain Independence vein, but the main vein reversed dip and formed a smell plunging synclinal structure at the intersection. At the Fern mine the Ore wined during 1949-.50 was actually in the intersecting vein but very close to the gain Fern vein.. The small intersecting veins would appear .to be either splits off the. Wain veins, intetseCting veins of the other productive shear .zone,' Or veins trend­ ing with tne southWest-dip,ing'joints. In those areas where intersections have been loci of oie shoots, the tenor of ore decreases in the smaller intersecting ' vein. Consequently it is believed taat the Liain.'veins acted at the Principal channelways for tne 661d-bearing hydrothermal Solutions. In .some' places not re­ lated to other structural elements ore vias deposited in somewhat irregular bodia4 'where quartz was fractured or open spaces existed as.a res4lt of late stress .prob­ ably applied in a direction similar to that which formed the shear zones; in other places adjustment to late stress took plaae at vein intersections linicn were oven more favorable for fracturing of the brittle quartz, 4.iininc; in the "Allow Greek district has not ,been carried far 'enough to more than Suggest that certain features are particularly important. This holds true for Lhe rake of ore shoots. It will'be .noted, tsywever,' that the intersections of , the main productive shear zones, and most of.theintersectione of veins of the southwest-dipping joint set r..ith either of the productive shear zones, \;ill rake V the northwest. Thus if this' fe6ture is significant, One dould expect ore shoots e the nortn-trending6 veins to rake to the right - as. an observer'loc.ks down the diii.of a vein, and to tile left in the case of northeastward-trending veins. An obvious exception is tae occurrence of an ore shoot where two veins, parallel in strike bul differing in dip, intersect.

Placers Prior to the discovery of lode gold in 1906 some placer 6o1d was recovered froLi the western part of the minihg district, but placer mining has never attract­ ed much interest. Original concentrations of sold in stream gravels have been destroyed by glaciation and reconcentrations have been s116ht. The restricted developia!at of post-glacial strea.a gravels and tne occurrence of innumerable bould­ ers dropped by the glaciers have been discouraging to the placer miner. In 1950 two placer properties were prospectea, one near the mouth of Grubstake Gluch and.onenear the mouth of the next stream to the west. Both streams head in the mica schist and drain into Willow Creek. The gold in these placers appears to have come from quartz lenses in the schist. Poorly sorted gravels on a high bench just east of the mouth of Grubstake Gulch yield some free gold on panning. A sli,_nt depression at the to of this bench may represent a former stream channel of Grubstake Gulch, and it is possible that locally reworked gravels exist which could bL: mined successfully on a small scale. Duplicating 3ervice. Department of the Interior, Washington 2.5, D. C. 3303

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