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1 EXPLORATION MODELS FOR MID AND LATE CRETACEOUS INTRUSION-RELATED GOLD DEPOSITS IN ALASKA AND THE YUKON TERRITORY, CANADA Brian Flanigan , Midas Touch Consulting Corp. Curt Freeman, Avalon Development Corp. Rainer Newberry, University of Alaska – Fairbanks Dan McCoy, Placer Dome Exploration Craig Hart, Yukon Geology Program Abstract Introduction Several major plutonic-related gold deposits are Several major plutonic-related gold deposits are among a band of mineral occurrences extending from among a band of mineral occurrences extending from southwestern Alaska into the Yukon Territory, Canada, southwestern Alaska into the Yukon Territory, Canada (Fig. called the Tintina Gold Province (TGP). Current gold 1). The outline of the mineral province generally conforms resources in Alaska and the Yukon Territory have increased to the trace of the Ti ntina-Kaltag fault system to the north from approximately 1 million ounces in 1985 to over 70 and the Denali-Farewell fault system to the south and has million ounces in 1999, with more than half of these been informally called the “Tintina gold belt.” However, resources occurring in the TGP. because it is comprised of several separate mineral belts, we Rb-Y-Nb concentrations from plutonic rock refer to it as the “Tintina gold province” (TGP) to avoid samples indicate most of this gold mineralization is due to confusion (after Bundtzen et al., 2000). arc-related magmatic belts of two separate age suites: ~88 to The scope of this paper is restricted to the 110 Ma and ~65 to 70 Ma. Because the mid Cretaceous metallogeny of some of the larger plutonic-gold systems in plutons are related to a subduction event along the Denali the TGP; primarily of mid and late Cretaceous ages. fault, Fort Knox, Pogo, Ryan Lode or True North direct Although metallogeny has previously been considered in analogues are not likely to be found south of the Denali various places in Alaska and the Yukon Territory, these fault. Conversely, the 65-70 Ma subduction-related plutons models predate the discovery of many gold deposits in the and related deposits can be found on either side of the Denali TGP. Newly acquired isotopic, trace element and multi- fault due to a more southerly subduction zone. Paleo- element analyses for this study and other recent studies have placement of the magmatic suites relates deposits now been applied to previous metallogenic models. Radiometric separated up to 450 kilometers from one another, and shows ages and trace element data of mid and late Cretaceous areas of favorable gold expl oration, particularly where magmatic suites were used in conjunction with tectonic pluton density is high and where the mid and Late models to rationalize the formation and placement of Cretaceous regions overlap. magmatic suites and deposits through time. Extensive multi-element assay data have been Over the past 15 years most of the gold exploration compiled from more than 10 of the TGP deposits for in the Yukon and Alaska has been directed toward plutonic- systematic comparison with respect to mineralization age, related deposits of two age groups, ~88 to 110 Ma (e.g. depth of emplacement, and proximity to causative pluton. Illinois Creek, Fort Knox, Pogo, Ryan Lode, Dublin Gulch, Bismuth, gold and arsenic are the three most closely linked Brewery Creek, etc., Fig. 1) and ~70 to 65 Ma (e.g. Donlin metals of these deposits, and variations in their statistical Creek, Golden Zone, Mount Nansen, Casino, etc., Fig. 1). correlations and ratios change systematically with respect to Our objective is to provide an explanation for the origin of depth of emplacement and proximity to a causative pluton. these deposits, demonstrate th eir probable placement at the We show that the correlation between Bi and Au (as well as time of their origin, and to tie metal signatures to the Bi:Au ratio) systematically increases with depth of emplacement depth and distance from a causative pluton, emplacement and proximity to a causative pluton. Average both of which are expressed by pressure estimates and host Bi:Au ratios range from 31:1 for deeper and more proximal rocks. The placement of deposits at the time of their origin deposits to 0.36:1 for shallower and more distal deposits. Bi illustrates regions possible for direct analogous types of gold vs. Au correlations range from r = 0.89 for deeper and/or deposits. The comparison of geochemistry from known more proximal deposits to r = 0.12 for shallower and/or deposits provides a model for identifying district-scale more distal deposits. Conversely, the correlation between favorability as well as fluid source and path modeling for an As and Au increases from r = 0.23 with deeper and/or individual deposit. intrusion-hosted deposits to r = 0.86 for shallower and/or With few exceptions (e.g. Illinois Creek), the mid more distal deposits. The change in As:Au ratios are much and late Cretaceous deposits dealt with in this paper have less predictable than Bi:Au ratios. Such variations in metal 8Sr/ Sr isotopic and/or Rb-Y-Nb trace element signatures correlations and Bi:Au ratios may be used on a regional consistent with subduction arc generated magmas (Fig. 2). scale to target areas for exploration. Further, the Bi:Au ratio The Illinois Creek deposit, is presented as the only known varies so systematically with respect to proximity to example of a collisional plutonic-related gold deposit of mid probable fluid sources that it may be possible to use it on a Cretaceous age in the TGP (Flani gan, 1998). To illustrate deposit scale to model the direction and path of fluid regional favorability, we have chosen to describe these transport, and hence, target new areas for drilling. deposits in relation to their magmatic suites for which they 2 in Alaska derived from Beikman, (1980). Major faults in the Yukon derived from Wheeler and McFeely (1991).old Province boundary (dashed lines). Major faults described in some detail relating to smaller mineral belts, such as the Kuskokwim and east-central Alaska belts. For a comprehensive description of these belts and their deposits we refer the readers to Nokleberg et al. (1994, 1995 and 1996), Bu ndtzen et al. ( 1997, 20 00), Go ldfarb (1 997), Young et al. (1997) and McCoy et al. (1997). Because the major gold deposits we describe in this paper are inferred to be arc pluton-related it is no surprise that the location of these deposits are largely irrespective of sedimentary and m etamorphic t erranes and greatly influenced by paleo-subduction boundaries and offsets along major lateral faults. Two co ntinental scale structures, the Tintina an d Denali fault syste ms, reflect major tecton ic activity indirectly associated with most of the mid and late Cretaceous gold mineralization in the Yukon Te rritory and adjacent Alas ka. Although these fau lt syste ms are no t directly resp onsible for t he go ld d eposits, lateral m otion along these faults has translated the position of m any gold deposits 100’s of kilom eters from their place of origina l formation. Figure 2: Rb- (Y + Nb) discriminant diagram showing fields for different Large-scale regional faults are numerous in Yukon Szumigala (1993, 1996), Gordey and Anderson (1993), Newberry and Solie Alaska and are i mportant in both lateral and vertical (1994), Newber ry and M cCoy ( 1997), Duncan ( 1999), and Baker et al.tapositioning of blocks (Solie et al., 1995 and Newberry (1998). Diagram after Pearce et al. (1984). et al ., 199 8). These faul ts may or may not host gold, depending on their age and the presence of a m ineralizing owe t heir ge nesis rat her t han p reviously defi ned m ineral fluid source. Plutonic-related gold deposits in the Yukon belts, which are more geographical. The re are num erous and Alaska represent many different levels of exposure due deposits not di scussed in t his paper w hich have be en to block faulting and subsequent erosion (McCoy et al., 1997 3 and O’Dea, 2000). Subtle differences in metal zoning of and inversely with Ca and Mg (Flanigan, 1998). Bismuth these deposits have been documented and reflect depth of concentrations typically range from 10 ppm to over 10,000 deposit formation and/or distance from a causative pluton ppm in a few drill intercepts. Based on data from Flanigan (this study). In Alaska, large-scale northeast trending high- (1998), Bi concentrations in the hundreds and thousands of angle normal block faults play a primary role in the depth of ppm occur within or very near the highest gold grade ore bedrock exposure and hence the type of gold deposit that can zones. Average ratios are Bi:Au = 87:1, Ag:Au = 23:1, be exposed at the current erosional surface. In Yukon, As:Au =3745:1, Pb:Au = 870:1, and Sb:Au = 443:1 (this analogous large-scale northeast trending faults generally are study). not recognized. However, in the Scheelite Dome area Arsenopyrite thermometry and fluid inclusion (Yukon) north-south normal faults have been documented in thermometry/barometry indicates ore mineralization recent mapping, showing evidence for block faulting temperatures of 300 ±25 oC and pressures of 1.3 ±0.4 kb (O’Dea, 2000). In both Alaska and Yukon, voluminous (Flanigan, 1998). volcanic rocks, small intrusive stocks, and dikes typically exemplify shallow exposures (<2 km). Large intrusion Mid Cretaceous Arc Related Deposits (88 to 110 Ma) exposures of batholithic proportions and a paucity of volcanics is indicative of much deeper exposures (>4 km). Fort Knox (7.2 Moz): Fort Knox is a deep level, However, significant uplift between magmatic events, scale intrusion-hosted Au deposit in the Fairbanks mining district, of magmatism, or unresolved structure can complicate this Alaska (Figs. 1, 3 and 4). Mineralization at Fort Knox is simplified interpretation. almost completely confined to the 92 Ma Fort Knox pluton, In this paper we present selected summaries of gold a composite granitic body comprised of equigranular deposits related to the mid and Late Cretaceous magmatic hornblende-biotite granite, medium grained porphyritic suites followed by a simplified tectonic reconstruction granite, and coarse-grained, seriate, hornblende-biotite showing probable displacements of these magmatic suites granite/granodiorite (Bakke, 1995, Bakke et al., 2000 and and their deposits. The paper concludes with a metal zoning Newberry et al., 1995). The majority of the gold occurs model from data acquired from 13 of the selected deposits within the main granitic body where it is dissected by west- and implications for future exploration. northwest trending, high angle stockwork veins, shears, pegmatite/aplite dikes, and lower angle sheeted, shear- Descriptive Summaries of Selected Tintina Gold hosted quartz veins. Province Deposits Gold mineralization is relatively low in sulfides (<1%), but generally associated with pyrite, arsenopyrite, Ruby Batholith Related Deposits (112-100 Ma) bismuthinite, tellurobismuthite, scheelite, and molybdenite. While propylitic alteration is pervasive throughout the Illinois Creek (0.6 Moz): Illinois Creek is a distal intrusion, potassic, albitic, and sericitic alteration appear to intermediate depth intrusion-related Au-Ag deposit in west be restricted to mineralized zones (Bakke, 1995 and Bakke central Alaska (Fig. 1). Mineralization at Illinois Creek is et al., 2000). Gold strongly correlates with Bi and Te hosted in a deeply weathered and supergene oxidized shear (Bakke, 1995, this study, Fig. 5). Although W, Mo, Sb, and (Flanigan, 1998). Toe shear strikes east-west and dips to the As minerals are common in ore zones, their correlations with south at about 60 . Hydrothermal fluids derived from the Au are weak (this study). Fo rt Knox lacks appreciable Hg, nearby 111-112 Ma collisional Khotol granite/granodiorite Cu, Pb, and Zn. mineralized the quartz-carbonate metasediments fractured Fluid inclusion thermometry/barometry indicate within the shear (Flanigan, 1998). The Khotol pluton marks mineralization temperatures of 305 ±25 oC and pressures of the southernmost exposure of the 8000 km Ruby batholith, 1.25 to 1.5 kb (McCoy et al., 1997). the rest of which outcrops north of the Kaltag fault to just south of the Brooks Range. Subsequent to primary Pogo (5.2 Moz): Pogo is a deep-level, proximal mineralization, percolation of meteoric water through the intrusion-related deposit in eas t-central Alaska (Figs. 1 and shear oxidized the sulfides almost entirely, leaving a massive 4). Mineralization at Pogo formed under ductile conditions iron and manganese gossan with gold and other precious and occurs in gently dipping, subparallel quartz bodies along metals. The country rock surrounding the shear is favorable permeable horizons. Brittle structures are found essentially non-mineralized, except for a short distance only as stockwork veining where the replacement bodies along a few permeable calcareous layers on the hanging wall come in contact with dikes (Smith et al., 1999, 2000). The side of the shear. replacement bodies are mostly quartz primarily hosted in Although most of the ore minerals are secondary amphibolite facies gneiss approximately 1.5 miles south of due to weathering, some primary sulfides have been the mid-Cretaceous (93.7 Ma) Goodpaster Batholith (Smith identified in thin-section including pyrite, arsenopyrite, et al., 1999, 2000). The mineralized zones contain about 3% chalcopyrite, stannite, stibnite, boulangerite, pyrrhotite, ore minerals, including pyrite, pyrrhotite, loellingite, galena, tetrahedrite, native Bi, bismuthinite, (Bi, Sb)2 3, and arsenopyrite, chalcopyrite, bismuthinite, various Ag-Pb- electrum. Due to supergene en richment and remobilization, Bi±S minerals, maldonite, native bismuth, and native Au simple bivariate plots reveal few correlations (Flanigan, (Smith et al., 1999, 2000). 1998). However, from a deposit-wide view of elemental Alteration in the mineralized zones occurs as early contours, Au correlates well with As, Ag, Bi, Sb, Pb, and biotite and quartz-sericite-stockwork, and sericite-dolomite. Cu; somewhat well with Fe and Mn; very poorly with Zn; This alteration assemblage is characteristic of both vein and 4 Figure 3: Plan map of the Fairbanks mining district showing deposit locations and high angle northeast trending faults responsible for different exposure levels. Map after Newberry et al. (1996) at R yan Lo de i s shear -hosted an d occurs i n both t he intrusion and th e su rrounding am phibolite g rade pelitic schist. The 90 M a pl uton is com prised of e quigranular, hornblende-biotite g ranodiorite, porphyritic b iotite g ranite, and intrusion breccia of fine-grained granodiorite with clasts of schist (Newberry et al., 1995 and Bakke et al., 2000). Gold m ineralization is primarily asso ciated with variably granulated quartz veins, gouge, and tectonic breccia within the shear zones. Gold mineralization is high sulfide (>1%) including arsenopyrite, stibnite, sphalerite, chalcopyrite, molybdenite, boulangerite, and Pb-Bi sulfosalts (Newberry et al., 1995). Scheelite also is co mmon within th e m ineralized zon es. Figure 4: Cr oss section schematic of modeled depth of em placementArsenopyrite is the predominant sulfide and occurs in higher many mid and L ate Cretaceous plutonic-related deposits in Alaska. Afte r McCoy et al. (1998). percentages with the quartz-calcite veins (1-10%). Stibnite is l ess abunda nt t han arsenopyrite ( ≤1%) a nd is replacement-type m ineralization. Gold s trongly paragenetically later (Newb erry et al., 199 5). Molybdenite and sch eelite o ccur alm ost ex clusively in th e in trusion- correlates with Bi and shows a lesser correl ation with other metals (Smith et al., 1999, Fig. 5). hosted shea r in trace am ounts. Propylitic alteration is Fluid i nclusion st udies a nd arse nopyrite t hermometry pervasive, but weak in the intrusion. Albitic alteration is (McCoy et al., 1998) i ndicates high mineralization restricted to small cen timeter sized envelopes about quartz temperatures (310 to 640 o C) and press ures indicative of veins in the albite flooded areas. Sericite alteratio n occurs deep emplacement (~6-7 km). throughout the porphyritic intrusion, but is most intense in the gold-bearing shear zones. Ryan Lode (2.4 Moz): Ryan Lode is a proximal Gold mineralized zones contain high concentrations and i ntrusion-hosted in termediate lev el Au d eposit in th e of Bi, Te, As, and Sb , and low Cu, P b, Zn. Bi and As Fairbanks district, Alaska (Figs. 1, 3 and 4). Mineralization strongly correlates with Au (this study, Figs. 5 and 6). 5 hydrothermal sericite give a plateau age of 88 Ma c oeval with late stage mineralization of the nearby Dolphin deposit (McCoy et al., 1998). Gold associated minerals include arsenopyrite, stib nite, j amesonite, and lesser tetrahedrite, pyrite, and galena. Although the deposit is recognized for it past p roduction o f t he high-grade ore, co nsiderable disseminated gold typically envelopes the high grade veins. Mineralization is generally comprised of sericite altered and highly silicified quartz mica schist that has been fractured and contains millimeter scale quartz vei nlets with sulfides. The geometry of the deposit is a ge nerally east-west shear that dips about 60 degrees to the south. Temperature and press ure estim ates from fluid inclusion analyses indicate a temperature of about 300 oC and a pr essure of 0.9 kb (Metz, 1991). Bi and As show a moderate to strong correlation with Au (this study, Figs. 5 and 6). Figure 5: Bi vs. Au scatter plots fo r mid Cretaceous arc plutonic-related deposits in interior Alaska, and the Yukon Territory, Canada. Plots show Bi vs. Au cor relations increasing with depth of de posit emplacement and/or proximity a causative pluton. Data sources: Arne Bakke and John Odden of Kinross, wr itten co mmunication ( 1999); Sm ith et ( 1999); John Royall/International Freegold, writte n communication (1999); Marsh et al. (1999); Joh n M air, wr itten co mmunication ( 1999); and M ark L indsay, written communication (1999). Dolphin (1.5 Moz): Dolphin is an intrusion hosted, intermediate-level Au d eposit in th e Fairb anks d istrict, Alaska (Figs. 1, 3 and 4). Mineralization at Dolphin occurs within a small 91 Ma granodiorite/tonalite stock (McCoy et al., 199 7). The m ineralization is stru cturally con trolled along fractures and a hi gh angle shear of s peculative strike (east-west) as millimeter to ce ntimeter thick quartz±carbonate±albite veins (Adams et al., 2000). Typical gold mineralization i ncludes arsen opyrite, py rite, st ibnite, boulangerite, m aldonite (A u B2), tetrad ymite, tetrah edrite, galena, and sphalerite. Sericite alteration occurs throughout the stock, but is most intense in highly fractured areas where Figure 6: As vs. Au scatter plots fo r mid Cretaceous arc plutonic-related deposits in inter ior Alaska, and the Yukon Territory, Canada. Plots show veining do minates. Th e sericite g enerally o ccurs as As vs. Au cor relations decreasing with depth deposit emplacement and/or centimeter-scale h alos ar ound h airline to centimeter th ick proximity to a causative pluton. Data sources: Arne Bakke and John Odden quartz±carbonate±albite veins. of Kinr oss, wr itten co mmunication ( 1999); Sm ith et al. ( 1999); John Dolphin i s highly a nomalous i n Au, m oderately Royall/International Freegold, writte n communication (1999); Marsh et al. (1999); Joh n M air, wr itten co mmunication ( 1999); and M ark L indsay, anomalous in As, Sb, Ag, and Pb, and weakly anomalous in written communication (1999). Bi and Te. Gold carries a st rong correlation with Bi, As (this study, Figs. 5 and 6). Silver, Pb and Zn are common True No rth (1.3 Mo z): True North is an throughout the ore zones but have a poor overall correlation intermediate-level, distal, p lutonic-related d eposit i n t he with Au and are attributed to a hydrothermal event later than Fairbanks district, Alaska (Figs. 1, 3 and 4). Mineralization the 90 Ma main-stage mineralization (McCoy et al., 1998). is structurally and chemically controlled and primarily flat- Gold occurs as free grains in quartz and as inclusions in high lying (Masterman et al., 19 95). Th e bu lk of th e gold temperature arsenopyrite (Adams et al., 2000). mineralization is confined to a s hallow dipping horizon of fractured calcareous and carbonaceous schist bounded on the Cleary Hill: Cleary Hill is an intermediate level, top by Chatanika terrane eclogite. The horizon is intersected distal, plutonic-related gold deposit in the Fairbanks mining by several northeast-trending, northwest dipping faults and district, Alaska (Figs. 3 and 4). Hi gh-grade, mesothermal shears, which also are mineralized and pre sumed to ha ve gold-quartz ve ins h osted i n Fai rban40 sc39ist charact erize acted as feeders to the flat lying horizon. The mineralizing mineralization (Hill, 19 33). Ar/ Ar s pectra of 6 fluids are believed to have been derived from an underlying associated with these stocks (excluding Big Creek) as ~90 Ma Pedro Dome, Fort Knox, or Dolphin granodiorite sheeted veins within the margin s of the stocks and adjacent equivalent (Bakke et al., 2000). hornfels, irregularly spaced quartz veins in hornfels, and The major ore minerals include pyrite, arsenopyrite, tungsten skarn (Marsh et al., 1999). The stocks and hornfels and stibnite. Anomalous metals include Au, As, Sb, Ag, Hg, are cut by the dikes, which ar e in turn cut by the sheeted and Bi. Gold has a strong correlation with As, a moderate veins (Marsh et al., 1999). Thus, it is presumed that correlation with Sb, and a weak correlation with Ag, Hg, and mineralizing fluids are associated with the latter phases of Bi (this study, Figs. 5 and 6). magma crystallization. Overall, Au correlates well with Bi, As, and Te and Ag shows a mode rate to strong correlation Brewery Creek (0.6 Moz): Brewery Creek is a with Bi, Te, Pb, and Sb (this study, Figs. 5 and 6). shallow-level intrusion and siliciclastic-hosted deposit situated on the north side of the Tintina fault in the Yukon Scheelite Dome: The Scheelite Dome area is Territory (Fig. 1). The deposit is hosted in Cretaceous located northwest of Mayo, Yukon Territory (Fig. 1). Gold monzonite sills (91.5 Ma) in Devonian aged Earn Group mineralization is hosted within the 91 Ma Scheelite dome siliciclastic rocks of the Selwyn Basin (Diment and Craig, stock (biotite and hornblende-bearing granite) and adjacent 1999 and Lindsay et al., 2000). Mineralization is controlled upper Proterozoic-lower Cambrian metasedimentary rocks by northeast and northwest trending high angle shears and of the Hyland Group as structurally controlled south dipping listric normal faults. Sills and listric structures metasediment-hosted quartz-sulfide veins, skarn, granite are controlled by pre-existing thrust faults. Gold occurs in hosted low sulfide veins, and replacement-type occurrences submicron form in solid solution with pyrite and (Hulstein and Zuran, 1999 and Mair et al., 2000). The arsenopyrite and as growth bands around larger sulfide metamorphic rocks, where most of the mineralization grains (Diment and Craig, 1999). Mineralization is hosted in occurs, include muscovite-chlorite phyllites, thin quartz veinlets and is associated with district scale quartzofeldspathic and micaceo us quartzites (Hulstein and antimony, arsenic and mercury anomalies. Gold strongly Zuran, 1999 and Mair et al., 2000). The regional foliation is correlates with As (this study, Fig. 6). cut by three sets of fault st ructures oriented east-west, northwest-southeast, and north-south. The north-south faults Dublin Gulch (1.5 Moz): The Dublin Gulch area are rarely mineralized, have normal down-to-the-west encompasses about two dozen st ocks located about 50 km displacement, truncate and offs et east-west structures, and north of Mayo, Yukon (Fig. 1). The Dublin Gulch intrusion are presumed to be post-mineral (Hulstein and Zuran, 1999 is the most economically important stock, comprised and Mair et al., 2000). The east-west and northwest- dominantly of equigranular granodiorite to quartz monzonite southeast structures are commonly mineralized with (Hitchins and Orssich, 1995). Mineralization of the Dublin auriferous quartz veins containing arsenopyrite, +/-stibnite, Gulch stock bares a strong resemblance to the Fort Knox +/-galena, +/-pyrite. The veins occur as breccia veins up to deposit in Alaska. Mineralization occurs in sheeted quartz several meters thick, to thin quartz veinlets filling joint sets, veins, quartz-sulfide fissure veins, scheelite skarn, and tin- and as sheeted veins (closely spaced quartz veinlets). tourmaline breccias (Hitchins and Orssich, 1995). Analyses of fluid inclusions iodicate mineralization At the Eagle Zone, subparallel, sheeted quartz veins temperatures of 240 to 350 C and pressures up to 2.5 kb occur within the intrusion. The veins are milky white or (Hulstein et al., 1999). Au strongly correlates with As, Bi, clear gray and typically 1 to 2 cm in width (Hitchins and and Te (this study, Figs. 5 and 6). Orssich, 1995). Ore minerals within the veins include arsenopyrite, pyrrhotite, chalcopyrite, bismuthinite, Late Cretaceous Magmatic Belt Deposits (70-65 Ma) tetradymite, tellurobismuthite, native bismuth, and rare molybdenite (Hitchins and Orssich, 1995). Gold grains Donlin Creek (10.1 Moz): Donlin Creek is a occur as complex intergrowths with native bismuth and a shallow level, intrusion-hosted Au deposit in southwestern strong correlation occurs between Au, Bi, and Te (Hitchins Alaska (Fig. 1). Mineralization at Donlin Creek is hosted in and Orssich, 1995). a 6-km-long 70 Ma rhyodacite porphyry sill complex Fluid inclusion thermometry/barometry indicates intruding upper Cretaceous gray wacke and shale (Ebert et o mineralization temperatures of 200 to 350 C and pressures al., 2000). Gold mineralization is structurally controlled >1.5 kb (Smit et al., 1996 and Lang et al., 2000). within the dikes and sills (and directly adjacent country rock) occurring in quartz-carbonate veinlets, and Clear Creek: The Clear Creek area is located about disseminated zones in sericitized intrusions. 50 kilometers north-northwest of Stewart Crossing, Yukon Ore minerals include early pyrite, arsenopyrite, Territory (Fig. 1), and contains six plutons of the Tombstone chalcopyrite, and later native As, realgar, and orpiment suite, Rhosgobel, Big Creek, Pukelman, Saddle, Eiger, and accompanied by supergene As-Sb enrichment. Gold is most Josephine stocks. The stocks are comprised of granite, commonly associated with arsenopyrite, correlates very well rhyolite, quartz monzonite, granodiorite, and diorite (Marsh with As (Ebert et al., 2000, this study, Fig. 7), and is lattice et al., 1999). U-Pb ages of these intrusions average ~92 Ma bound (refractory). Alteration includes pervasive sericite (Murphy, 1997). Dikes are also common to the Clear Creek and carbonate with thin, wispy graphite. Silicification is area and they are typically comprised of rhyolite, granite rare. quartz-feldspar porphyry, pegmatite, aplite, and lamprophyre Interpretation of fluid inclusion data (McCoy et al., (Marsh et al., 1999). Gold mineralization is intimately 1999) suggest that early saline magmatic fluids (>550 oC) 7 cooled and separated into gas and brine rich end members Gold mineralization occurs with pyrrhotite, pyrite, (<550 C and <0.5 k b) l eading t o ea rly metal deposi tion. chalcopyrite, a rsenopyrite an d s ome josei te, bi smuthenite, Subsequent i nflux of m eteoric wat er decrease d t he and bismuth identified by electron microprobe. Gold has a temperature a nd/or i ncreased fS 2/fO 2 lead ing to further strong correlation with Bi and essentially no correlation with metal deposition as well as late p aragenesis to higher fS 2 As ( this study, Fig s. 7 an d 8). Sph alerite b arometry mineral stability (native As, orpiment, realgar). performed by C utler (19 94) i ndicated a mineralization pressure of 1 kb. Figure 7: As vs. Au scatter plots for late Cr etaceous arc plutonic-related deposits of south and southwest Alaska. Plots show As vs. Au correlaFigure 8: Bi vs. Au scatter plots for late Cr etaceous arc plutonic-related decreasing withdepth of deposit em placement an d/or pr oximity to adeposits of south and southwest Alaska. Plots show Bi vs. Au correlations causative pluton.Data sources: Rainer Newber ry, unpublis hed data;increasing with d epth o f dep osit emplacement and/or pr oximity to a Cameron Ro mbach and Gr eg J ohnson/Novagold Resour ces, wr itten causative plutonData sour ces: Ra iner Newber ry, unpublished d ata; communication (1999); Ben Gage, wr itten co mmunication (1999); a ndCameron Ro mbach and Gr eg J ohnson/Novagold Resour ces, wr itten Lance Miller/Placer Dome Exploration, written communication (1999). communication (1999); BenGage, wr itten co mmunication (1999); a nd Lance Miller/Placer Dome Exploration, written communication (1999). Nixon Fork (0.1 Moz): The Nixon Fork deposit is an intermediate level gold skarn deposit located in southwest Shotgun (1. 0 Moz ): Shotgun i s a proximal Alaska (Figs. 1 and 4). The deposit is hosted by Ordovician intrusion-hosted Au deposit in southwestern Alaska (Fig. 1). limestone and dolomite (Herreid, 1966) near a 69 M a, very Shotgun is classified as a gold porphyry deposit with Au-Cu- reduced, quartz monzonite-quartz monzodiorite stock. T he As quartz stockwork in a 70 Ma rhyolite (granite porphyry) stock is cu t by 6 9 Ma, quartz-sericite altered, gold-bearing stock and adjacent hornfels (Rombach, 2000). Ore zones are porphyry dikes (Cutler, 1994); skarns seem to be focused primarily hi gh-density st ockwork quartz vei ning a nd hydrothermal breccias within rhyolite. Unlike typical Cu- near these dikes and are not uniformly distributed around the stock (Newberry et al., 1 997). D etailed mapping and core Mo po rphyry d eposits, which sh are m any similariti es, logging has shown the presence of vertical skarn "pipes", Shotgun is highly anomalous in Au an d As and esse ntially with garnet-rich co res, garnet-pyroxene m argins, an d lacks potassic alteration. At Shotgun the dominant alteration is albite-sericite-quartz±carbonate. wollastonite-idocrase-scapolite rims, near the pluton margin (Newberry et al., 1997). Zoning is complicated by extensive Ore minerals include hypogene native Au and Bi, dolomite-rich layers within lim estone, wh ich g enerate Bi-Te m inerals, arse nopyrite, ch alcopyrite, lo ellingite, extensive sulfide-poor serpentine (after f orsterite)-diopside- pyrrhotite, p yrite, sch eelite, sph alerite, and sup ergene phlogopite-actinolite sk arns, and b y post-ore fau lts covellite, ch alcocite, n ative Cu , and m arcasite (Ro mbach, (Newberry et al., 1997). Historic production at Nixon Fork 2000). Gold at Shotgun occurs as free Au. Based on data was from supergene enriched, clay-quartz rocks containing from Rombach (1999, written communication), Shotgun is multiple o z/ton go ld and sev eral percent co pper highly anomalous in Au, As, Ag, and Cu, and moderately oxide/carbonate conce ntrations ( Herreid, 1966). Prese nt anomalous in Bi and M o. Ove rall, gold exhibits a st rong efforts are directed towards primary ores in sulfide-salite- correlation with Ag and a weak to moderate correlation with rich s karn a nd l esser s ulfide re placement bo dies i n As, C u, a nd Bi (t his st udy, Fi gs. 7 a nd 8) . H owever, limestone. Rombach (2000) found that in discrete drill h oles Bi can exhibit a strong correlation with gold. 8 The high degree of brittle fracturing and stockwork Gold Producing Magmatic Events veining suggest a low pressure, near surface emplacement. Rombach (2000) estimated temperatures from arsenopyrite Ruby Batholith (112 to 100 Ma): Based on Sr-Nd- and fluid inclusion thermometry between 450 o to 600 C. Pb isotope and trace element studies, granitic rocks of the 40 39 Highly saline fluids in inclusions, identical Ar/ Ar Ruby batholith (~111 Ma) are interpreted to be derived from intrusive and mineralization ~70 Ma ages were also the melting of lower crustal rocks during crustal thickening observed. (Arth et al., 1989 and Miller, 1989). Data from these plutons plotted on a Rb-(Y + Nb) discriminant diagram (Fig. 2) Golden Zone (1.2 Moz): The Golden Zone deposit indicates a collisional tectonic setting consistent with is shallow-level intrusion-hosted deposit located just south Miller’s interpretation (Flanigan, 1998). This collisional of the Denali fault in south central Alaska (Fig. 1). Ben event is apparently related to the opening of the Canada Gage (written communication, 2000) describes the deposit basin (Fig. 9). The Ruby batholith is comprised of highly as being hosted in a 69 Ma, sub-vertical, monzodiorite evolved tin-granites and several tin placers are found in porphyry stock that intrudes a shaley-chert and pebble streams draining the plutons on the north side of the Kaltag conglomerate metasedimentary unit. The phenocryst fault. Additionally, many of these streams also show assemblage is plagioclase, biotite and hornblende. Most of elevated concentrations of W, Au, Ag, Sb, Bi, and Te (Solie the mineralization occurs in a steeply dipping breccia-pipe, et al., 1993a, b). Ruby batholith plutons north of the Kaltag contained in the stock. The clasts are angular, sericite fault exhibit K-Ar ages from about 98 to 111 Ma (Miller, altered monzodiorite, and the matrix consists of quartz, 1989). The Ray Mountains pluton yields zircon U-Pb dates carbonate and sulfides. Th e major ore minerals are from 109 to 112 Ma (Patton et al., 1987). The Jim River and arsenopyrite, pyrite, chalcopyrite, sphalerite and pyrrhotite Hodzana plutons give a Rb-Sr whole rock isochron age of and significant amounts of galena, Freibergite (Ag-rich 112 Ma (Blum et al., 1987). South of the Kaltag fault, in the tetrahedrite). Gold, Bi-minerals and Pb-sulfosalts have been Illinois Creek area, the Khotol pluton exhibits a K-Ar age of identified in thin section and electron microprobe. Gold has 112 Ma (Patton et al., 1984) and an 40Ar/ Ar plateau age of a moderate to strong correlation with Bi and As (this study, 111 Ma (Flanigan, 1998). The younger K-Ar ages as 40 39 Figs. 7 and 8). Alteration consists of minor potassic (biotite compared to U-Pb zircon and Ar/ Ar ages for the Ruby and K-spar) and quartz/sericite /carbonate, which is much batholith may be indicative of thermal resetting during more extensive. The deposit is diced up with pre- and post- probable slow cooling of the magmas. Thus, many of the K- mineralization high angle shears and joints sets. Ar ages for the Ruby batholith may be as much as 10 Ma younger than the age of emplacement. Casino (2.7 Moz): The Casino deposit is located in The southern extension of the Ruby batholith the Dawson Range mountains in the west-central Yukon (Khotol pluton) found south of the Kaltag fault is displaced Territory (Fig. 1). Gold and copper mineralization is related about 150 km west of correlative granites north of the fault. to the breccia and microbreccia pipe of the 72 to 74 Ma Illinois Creek and other Au-Ag prospects are located at the Casino Intrusive Complex (Bower et al., 1995). The deposit southernmost end of the Ruby batholith (Fig. 9). contains a 300-meter thick supergene Cu-enriched cap overlying a hypogene sulfide zone which contains most of Mid Cretaceous Magmatism: Possibly the most the gold. The Intrusive Casino Complex is comprise of latite significant group of mid Cretaceous intrusions with respect and rhyodacite porphyry , quartz monzonite, intrusion to gold deposits is the 88-110 Ma Tombstone-Tungsten breccia, and microbreccia (Bow er et al., 1995). Ore suite. The Tombstone-Tungsten suite is a composite minerals include chalcopyrite, pyrite, sphalerite, galena, comprised of plutons from the Tombstone and the Tungsten tetrahedrite and some Bi minerals. Anomalous metals suites. The Tombstone suite is a group of metaluminous, include Ag, As, Au, Bi, Cu, Cd, Mn, Pb, Sb, Zn, and W subalkaline to alkaline, mainly intermediate to felsic (Bower et al., 1995). Gold o ccurs as 50 to 70 micron sized intrusions (Mortensen et al., 2000) dominantly located north grains in quartz and as 1 to 15 micron sized inclusions in of the Tintina fault in the Yukon and south of the Tintina fractures in pyrite and chalcopyrite (Bower et al., 1995). fault in Alaska. The Tungsten suite intrusions are peraluminous and located up to 200 kilometers north of the Mt Nansen (0.15 Moz): The Mount Nansen Tintina fault in eastern Yukon. The Tungsten suite property is a sub-volcanic feldspar porphyry intrusion of the intrusions are of equivalent age to the Tombstone intrusions late Cretaceous Mount Nansen volcanic complex (Stroshein, and likely represent more evolved magmas of the 1999). The property is located about 50 km west of Tombstone suite. Other mid Cretaceous plutonic suites Carmacks, Yukon (Fig. 1). include the Selwyn, Cassiar, and Whitehorse suites (Wheeler Precious metal mineralization consists of and McFeely, 1991). K-Ar ages of these plutons range from structurally controlled planer veins and pipe-like breccias 88 to 110 Ma in the Yukon (Mortensen, 1999), and 88 to peripheral to the central porphyry complex (Stroshein, 108 Ma 40Ar/ Ar ages in Alaska (Newberry et al., 1998). 1999). Gold and silver-rich sulfide consists of pyrite, The plutons associated with known gold deposits generally 87 86 arsenopyrite, sphalerite, galena, sulfosalts, bornite, stibnite exhibit Rb-Y-Nb concentrations (Fig. 2) and Sr/ Sr and chalcopyrite. Gold is apparently related to the early isotopic signatures (0.707-0.714) most consistent with arc- pyrite phase and occurs as 5 to 40 micron sized inclusions in generated magmas that have assimilated a crustal component the pyrite (Stroshein, 1999). during their ascent (Sinclair, 1995, Newberry et al., 1998, Mortensen et al., 2000, and Newberry, 2000). Subduction of 9 Figure 9: L ocation of TGP gold occurrences with r espect to magmatic belts in their paleo-positions (112-85 Ma). Form ation of the Ruby batholith and rotation of the Ruby terrane as a result of the opening of the Canada basin. Tombstone suite magmatism as a result of the subduction of the Farallon plate. Dotted lines represent future approximate major fault locations for reference. Timing and direction of subduction plates and rotation of the Ruby terrane are derived from Plafker and Berg (1994) and Plafker et al. (1994). the Faral lon plate i s cont emporaneous with t hese plutons same subduction eve nt ass ociated with t he southwestern (Plafker a nd Berg, 1994). 450 km of displacem ent has Kuskokwim g roup d eposits ( Donlin Cr eek, Nixon For k, occurred along the Tintina fault since early Tertiary time. In Vinasale Mountain, etc., Fig. 10). Tectoni c reconstruction Yukon, known Tombstone-Tungsten plutons are dominantly of th is m agmatic su ite sh ows a cen tral area withou t late north of th e Tintina Fau lt, wh ereas in Alaska correlativ e Cretaceous deposits. The reason for this blank area may plutons a re south of t he Ti ntina fa ult. A t ectonic reflect lesser exploration activity due to lack of easy access. reconstruction of the Tombstone-Tungsten suite shows more However, the potential for t his region appears to be hi gh clearly th e sp atial relat ionship of in terior Alaska p lutonic considering t his magmatic s uite o verlaps the To mbstone- rocks and their associated deposits with those in Yukon (Fig. Tungsten plutonic suite (Figs. 10, 11 and 12). 9). Eocene Volc anics (~55 M a): B imodal Eocene Late C retaceous Ma gmatism: Late C retaceous, volcanics shown on figure 11 is a small part of this disperse ~70-65 Ma plutons and intrusive dikes are associated with suite app arently related to so me Au-Ag ep ithermal several gold deposits in Alaska and the Yukon. These gold occurrences in Alaska (Ptarm igan Hill) and the Yukon associated igneous rocks include porphyritic rhyodacite and (Grew Creek and Pluto). K- Ar ages of secondary biotite rhyolite, q uartz monzonite, an d gran ite p orphyry. In (57±2 Ma) from th e Taur us d eposit ho sted in a ~ 70 Ma southwest Alaska, conce87rat86ns of Rb, Y, and Nb (Fig. 2), porphyry (Nokleberg et al., 1995) and the nearby 56±2 Ma major oxide analyses, Sr/ Sr isotopic ratios (0.704-0.709) Push-Bush prospect (Sinclair, 1986) indicate a broad region from Kuskokwim group plutonic rocks indicate that they are of thermal resettin g and m ineralization as a resu lt of th e principally I-t ype m etaluminous t o sl ightly peral uminous Eocene volcanics. Intrusions of this suite are granite cut by and probably arc-related (Szumigala, 1993, 1996, Sinclair, diabase dikes. The bimodal character a nd trace elem ent 1995 and Newberry et al., 200 0). The interpretation of arc signature of this widespread suite indicates a broa d, short related plutonism is co nsistent with the subduction of the lived, extensional setting. Although this igneous event is not Kula pl ate as modeled by Plafker an d B erg ( 1994) an d currently considered as a major gold producer, displacement Plafker et al. (1994). Age relations of mineralization suggest of the associated ~55 Ma mineral occurrences is believed to that co eval ign eous bod ies and m ineral dep osits in east mark the earliest onset of major motion along the Tintina central Alaska (Mosquito and the Taurus porphyry, Leriche, and Denali faul t sy stems (Fi gs. 11 and 12). Other 1995), so uthern Alaska (Golden Zo ne), and t he cent ral occurrences of this suite are mostly Ag-Sn systems. Yukon (Casino and Mount Nansen) may be related to the 10 thin dashed lines. Shaded regi ons represent magmatic belts based on deposit and intrusion age dates and may not be all- inclusive. Plate motions and as subduction zones are derived from Plafker and Berg (1994) and Plafker et al. (1994). Displacement Along Major Faults hinge splays and crustal shortening where the Denali and Farewell faults meet. Because of these c omplexities our The timing and motion along the major faults has model i s rest ricted t o m ajor m ovement al ong faults of been studied previously by several workers. Although there reasonable certainty. are numerous ch ronological an d geometric di screpancies The tim ing of in itial m ovement o n th ese fau lt that have y et t o be rec onciled, o ur reconstruction relies systems also is somewhat dubious. The initiation of motion primarily on the model provided by Plafker and Berg (1994) on the Tintina fault is unkno wn but major displacement is and Plafker et al. (19 94) to estimate the positions of gold modeled to be post late Cretaceous in age (Plafker and Berg, deposits in the TGP (Figs. 9, 10, 11 and 12). 1994, Pla fker et al., 1994). Displacement of ~55 Ma Motion along th e Tin tina-Kaltag f ault syste m volcanic-related epi thermal Au-Ag sy stems nea r C hicken, (including the Iditarod fault) is considered to be a dextral Alaska (Ptarmigan Hill) and Ross Riv er, Yu kon (Grew displacement of 450 km on the Tintina fault (on the eastern Creek) indicate that m ajor displacement bega n no s ooner end), and 150 and 90 km on the Kaltag and Iditarod faults, than 55 Ma (Figs. 11 and 12). A ge data from mineralized respectively (on th e w estern end) . While a 450 k m structures (~90 Ma) within the Tombstone Suite intrusions displacement on the eastern side of the Tintina fault system (~90 Ma) in Alask a indicate syn-plutonic faulting (e.g. Fort implies the same displacem ent on the we stern end, current Knox, Ryan Lode, and D olphin). These mineralized information does not easil y account for 210 km of structures m ay have f ormed during st rike-slip m ovement displacement on the Kaltag side of the syste m. It has been initiated by oblique subduction of the Farallon plate and may suggested that the remaining 210 km lies p rimarily in the mark the earl y inception of a st ructural regime that later hinge splays and cr ustal shortening where the Tintina and became the Tintina fault. Kaltag faul ts meet (Dove r, 1994, C hapman et al ., 198 5). The early formatio n o f th e Denali fault structure Poor exposure and multiple locations of mapped high angle likely bega n as a t rench during t he s ubduction of t he faults make structural interpretation difficult in this region. Farallon p late (Fig . 9). Ti ming of in itial d extral m otion Motion along the Denali-Farewell fault system is along t he D enali faul t sy stem has y et t o be c onstrained. analogous to the Tin tina-Kaltag system . Dex tral m otion Assuming movement along the Tintina fault system is post along the Denali fau lt is app roximated at 35 0 km (to the ~55 Ma, and that southern Alaska closed the trench along east) and 140 km on the Farewell fault (to the west). Again the n ow reco gnized Denali fau lt at ab out 8 5 Ma, there is a discrepancy of 210 km that is accounted for in the simultaneous dextral movement of t he Tintina and Denali 11 Figure 11: Introduction of Early Tertiary bimodal volcanics. The relatively small region selected relates deposits on either side of the Tintina faul t. Note that mid and Late Cretaceous regions overlap north of the Denali fault. Plate motions, subduction zones, and the rotation of western Alaska are derived from Plafker and Berg (1994) and Plafker et al. (1994). fault systems is likely. It als o is reasonable to assume that, Metal Characteristics and Zoning as with th e Tintina fault, oblique su bduction of th e Kula plate initiated a strike slip structural regime along the Denali Multi-element assay d ata from d rill an d ro ck trench that later became the Denali-Farewell fault system. samples were acqui red f or sy stematic eval uation and comparison of va rious Mid and Late Cretaceous Gold deposits in the TGP. E xcept for the Dolphin and Cleary Rotation of Western Alaska Hill, data sets rep resent ICP multi-element assay d ata. Bi Based o n geologic an d pal eomagnetic dat a, values for Do lphin and Cleary Hill r epresent ato mic beginning ap proximately 65-5 0 M a western Alaska w as absorption assays. Fo r statistical evaluation, values below rotated about 45 to 60 o to its present position (Plafker and the det ection l imits were assigned a va lue hal f of t he Berg, 1994). This bend is attributed to the convergence of detection val ue. C orrelation coefficients were calculated the North American and Eurasian plateso(Fig. 11). The axis from log transformed data to reduce any bias from possible of the bend is at approximately 147 longitude (roughly a outliers. Av erage ratios were calcu lated fro m raw d ata. vertical lin e th ough Fai rbanks an d Anchorage) a nd With the exception of the Po go de posit no cutoffs we re corresponds to the hinges in the Tintina-Kaltag and Denali- invoked and all calculations were performed identically. For Farewell fault systems. Th e presence of the hinge spla ys the Pogo deposit the data set was tri mmed (Smith et al., and the relatively small amount of movement on the Kaltag 1999). Because the Pogo data was treated in a manner and Farewell fau lts in dicate th at th e Tin tina an d Denali inconsistent with th e other deposits, it is lik ely th at th e faults were active during rotation of western Alaska. overall Bi vs. Au correlation is slightly less and the As vs. Au correlation is slightly more than reported. However, on the basis of t he scatter plots for P ogo, conclusions a re unaffected. Trend lines through the data points on scatter plots are reduced m ajor axes calcul ated from log transformed data from the following equation (Davis, 1986): 12 Figure 12: Present day configuration of magmatic belts and deposits. Note that the west end of the Late Cret aceous magmatic belt likely continues to th e west but is not shown due to lack of data. Plate motions and subduction zones are derived from Plafker and Berg (1994) and Plafker et al. (1994). y = Y – (σ yσ xX Au. Additionally, in rock samples, Au and Bi are cl osely linked i n i ntrusion hosted (e.g. Fort K nox) a nd deeply Where Y is the mean log Bi value, X is the mean log emplaced plutonic-related deposits (e.g. Pogo) such that Au Au value, σyis the standard deviation of the log Bi and Bi s how a statistical correlation r > 0.8 (Fig. 5). This close association of Bi and Au alm ost c ertainly reflects values, and σxis the standard deviation of the log Au values. initial precipitation of the mineral maldonite (Au Bi2, or a similar alloy or alloy melt. Conversely, Au and As show a Arc plutonic-related ~ 90 M a de posits of interior statistical correlation r > 0.8 for th e shallower and/or more Alaska (e.g. Pogo, Forth Knox, Dolphin, Ryan Lode, Cleary distal deposits (e.g. Ryan Lode, Cleary Hill, True North, Fig. 6). T he strong correlation of Au and As in shallow/distal Hill, True North) represent different deposit emplacement depths due t o ve rtical di splacement along high angle environments is likely due to Au inclusions in arsenopyrite northeast t rending n ormal faul ts (Fi gs. 3 and 4). Th ese and/or as Au in solid solution in arsenopyrite (McCoy et al., deposits display distinctive relative elemental variations that 1998). In addition to the systematic variations in Bi vs. Au can be related t o dept h o f host pluton e mplacement and correlations, best fit trend lines (reduced major axes) through the data clouds decrease in slope in both shallower and more distance from a causative pluton (Table 1). Further, deposits in the Yukon Territory believed to be related to the same distal environments (Fig. 13). plutonic su ite ex hibit id entical zo ning patterns, an d Late With the exception of the Shotgun deposit, the Late Cretaceous (~70 Ma) arc plutonic-related deposits in south Cretaceous south and southwest arc-plutonic related deposit s and southwest Alaska show an analogous pattern. behave sim ilarly to the m id Cretaceous deposits in all respects concerning metal variations. S hotgun is the only For the more proximal and deeper varieties of these deposits, Au conce ntrations are t ypically an or der of deposit that does not fall into a consistent order based on Bi magnitude higher than Te and an order of magnitude lower vs. Au and As vs. Au correlations (Figs. 7 and 8). This than Bi. Th e more distal and shallower deposit varieties inconsistency at Sh otgun is ratio nalized as multiple fl uid contain lower concentrations of Bi and Te with respect to pulses redistributing earlier mineralization events and metal tio tio tio ra ra ra 1.8 148112 173190 4.9 97 20 7.840 48 , s b:Au b:Au 1614 b:Au 13 S S S 1998 , bred assay tio tio tio m e ra ra ra et al. 15 28817 86 5.3 20 2.6 12 615 21 b:Au b:Au b:Au Coy encu P P P c trsform tio tio tio ra ra ra 2=M 2.01.3 2.3 1.0 5.4 7.2 1.0 mlog- 0.140.16 0.750.21 0.74 o A:Au
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