Abstract The Willow Creek mining district is located in the Peninsular terrane in Alaska, on the southwestern margin of the Talkeetna Mountains batholith. The district contains exposures of tonalite (74-73 Ma) and adamellite (67-65 Ma) of the batholith and an older unit of schist that has no nearby correlative units. The tonalite (quartz diorite) and schist both host gold-bearing quartz veins in fractures and shears, whereas the adamellite (quartz monzonite) appears to be barren of gold mineralization. Our data suggest that there is a previously unmapped fault along the contact between the mineralized tonalite and schist. The fault may have provided a conduit for mineralizing fluids. Ceologic relations and K-Ar ages indicate that at least two periods of mineralization occurred at 66 and at 57 to 55 Ma. At 66 Ma, the intruding adamellite provided heat and possibly fluids to the tonalite as suggested by K-Ar ages from a gold-bearing quartz vein and from dikes in the tonalite. At 57 to 55 Ma, the source of heat is uncertain. Both periods of mineralization occurred during right-oblique subduction of the Kula plate beneath the Peninsular terrane and some mineralizing fluids may have originated in zones of metamorphism and partial melting in the descending Kula plate, Fluids of deep origin, magmatic and/or met amorphic, could have moved through a deeply rooted fault system and out into the host rocks along splays and fractures related to a fault separating the schist and tonalite. Oxygen isotope data suggest that the mineralizing fluids were similar in their oxygen isotope composition to the tonalite, and perhaps the adamellite, but were unlike the schist. This similarity could have resulted if the fluids equilibrated with a plutonic body (tonalite or adamellite) at temperatures high enough that fractionation approached zero. The measured values of δ 18 O from quartz in gold-bearing veins are 13.2 to 15.8 per mil, with one low value of 9.2 per mil from a pegmatite, and the calculated fluid values are 6.3 to 8.9 per mil. These values occur in veins with ages of 66 Ma, as well as in undated veins. The Pb isotope compositions of sulfides from two veins in the tonalite resemble each other but differ from those in the schist. These compositions suggest that the Pb in veins in the tonalite had a common source. In the schist, the gold-bearing veins may have farmed from the same mineralizing fluid that was modified in Pb isotope composition by circulation through the schist, or from a fluid that was different from the one that produced gold-bearing quartz veins in the tonalite.

Abstract Approximately 30 million oz (Moz) of gold has been recovered from metamorphosed terranes in Alaska. One-third of the production has come from lodes in rhe southern half of the state, predominantly from mines in the Juneau gold belt, Chichagof district, and Willow Creek district. The majority has been taken from placers in northern and interior Alaska, including those near Nome and Fairbanks, where many of the lode sources have been completely eroded. Host terranes for gold-bearing quartz veins are dominated by pelitic rocks and range in age from Precamblian(?) to Tertiary. Ore formation, however, was reshicted to a 60-m.y. interval between Albian and Eocene time. In northern Alaska, ore-forming hydrothermal systems were active in the southern Brooks Range during Albian time, on the Seward Peninsula at 109 Ma, and in the Yukon-Tanana upland between 92 and 77 Ma. In southern Alaska, seaward of the late Mesozoic to early Cenozoic Alaska Range-Coast Mountains magmatic arc, ore formation was younger; veining occurred at 63 to 57 Ma in the Valdez Creek disttict, 66 Ma in the Willow Creek district, 55 ± 2 Ma along the Juneau gold belt, and 57 to 49 Ma in the Chugach accretionary prism. Calc-alkaline plutonic rocks occur within a few tens of kilometers of most of the gold deposits in Alaska and generally were emplaced coevally with hydrothermal activity. Ore-bearing veins are composed predominantly of quartz and show brittle and/or ductile features depending on the district. Carbonate minerals, chlorite, and white mica are common gangue phases. Sulfide minerals, most commonly pyrite or arsenopyrite, compose less than 3 percent of the vein material. Silicification, carbonization, and sericitization characterize wall rocks within a few meters of most veins. This alteration is best developed adjacent to veins in igneous host rocks. Ore fluids are H 2 O dominant, with approximately 5 to 15 mole percent CO 2 and significant amounts of CH 4 , N 2 , and H 2 S. Relatively high nonaqueous gas contents in parent fluids responsible for the Alaska-Juneau deposit, deposits in the Fairbanks disttict, and many or the veins in the Valdez Creek district resulted in episodic immiscibility accompanying gold getnesis. Fluid salinities are less than 8 wt percent NaCI equiv. Vein formation temperatures rnnged from 225° to 375°C, with emplacement depths of about 3 to 10 km. In the Fairbanks district, however, some ores were more shallowly emplaced. Ore fluid values of δ 18 O (7-12‰) and δD (-15 to -55‰) for most deposits are consistent with a fluid in isotopic: equilibrium with the metamorphic terranes and one that was produced during greeuscltist to amphibolite facies devolatilization reactions. Values of δ Dchlorite between -135 and - 187 per mil for veins from the Nome disttict provide a notable exception and may be the product of postcrystallization recquilibmtion between the micas and circulating metelogic waters. A sedimentary rock origin for sulfur in most veins is favored because δ 34 S sulfide values vary with the age of host terrane according to a pattern approximating that of matine sulfate through time. Lead isotope compositions of sulfide minemls are a function of vein host lithologies and provide little input into the understanding of gold ore genesis. Gold vein formation was related to the internction among the Kula, Farallon, and North America plates. Middle to Late Cretaceous veining in northem and interior Alaska postdated initial terrane collision and obduction of oceanic rocks by 100 m.y. The veining might be a product of rising crustal temperatures associated with continued collision, accompanied by development of a major continental margin trench and the eventual subduction of the Famllon plate. Alternatively, a shift to trench retreat and slab rollback may have caused asthenospheric upwelling, crustal thinning, plutonism, and Cretaceous hydrothermal activity above the subducting Farallon plate. Veining in southern Alaska in the early Tertiaty is hypothesized to be a product of crustal thickening and shear heating in the inner fore arc and Farallon-Kula tidge subduction in the outer fore arc.