Cretaceous reduced granitoids in the Goodpaster Mining District, east central Alaska

Abstract

The >5 Moz (1 oz (troy) = 31.103 g) gold veins of the Liese Zone and nearby prospects in the Goodpaster Mining District of east central Alaska are spatially and temporally associated with late-Early to Late Cretaceous reduced granitoids that are divided into a granite suite, tonalite suite, and diorite suite in decreasing age. Synkinematic to postkinematic biotite ± hornblende granite, granodiorite, pegmatite, and two-mica granite with accessory garnet compose the granite suite (109-107 Ma). The tonalite suite (107-103 Ma) forms small to large bodies of postkinematic hornblende-biotite granodiorite to tonalite with rare granite. Intruding the granite and parts of the tonalite suites are aplite and pegmatite that grade through sugary pegmatoidal or aplitic quartz veins and finally into quartz veins along strike. The diorite suite (95.4-93.7 Ma) consists of small stocks of diorite to tonalite that intrude the older suites and are inferred to intrude the shallowly dipping auriferous quartz veins. Limited data indicate granitoid emplacement at 5-9 km depths, consistent with formation of the auriferous quartz veins based upon published fluid inclusion data. The weakly peraluminous granite and tonalite suites are distinguished by variable amounts of monazite and zircon whereas the diorite suite is metaluminous, contains <5% magnetite, and lacks monazite. All suites are subalkalic, calc-alkaline and have low magnetic susceptibilities, high large-ion lithophile element/high field-strength element (LILE/HFSE), and depleted Nb and Ti. The granite suite has higher 206 Pb/ 204 Pb values (19.4-19.6) than the diorite (19.1). Overall the granite and tonalite suites likely represent melts generated late during crustal thickening that intruded along shallowly dipping faults during exhumation, whereas the diorite suite represents postdeformation melts that underwent less interaction with the old silicic crust.Résumé : Les filons aurifères de la zone Liese (>5 Moz) (1 oz (troy) = 31.103 g) et les prospects avoisinants dans le district minier de Goodpaster du centre-est de l'Alaska sont associés dans le temps et l'espace à des granitoïdes du Crétacé précoce à tardif qui ont subi une réduction et qui sont regroupés en une suite granitique, une suite tonalitique et une suite dioritique, selon un âge décroissant. La suite granitique (109 à 107 Ma) est composée de granite syn-à post-cinématique à biotite ± hornblende, de granodiorite, de pegmatite et d'un granite à deux micas avec du grenat accessoire. La suite tonalitique (107 à 103 Ma) forme de petits à de grands amas post-cinématique de granodiorite à hornblend-ebiotite à une tonalite avec, rarement, du granite. Les suites granitiques et certaines parties des suites tonalitiques ont subi l'intrusion d'une aplite et d'une pegmatite qui passent de filons de quartz pegmatoïde finement granulaire ou aplitique à des filons de quartz longeant la direction. La suite dioritique (95,4 à 93,7 Ma) comprend de petits massifs intrusifs, de diorite à tonalite, qui pénètrent les suites plus ancienn...

Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes

“…2) west of our study area (Rhys et al, 2003;Dilworth et al, 2007). In southern Yukon, south of the area shown in Figure 2, metallogenic episodes, including Au-bearing breccia complexes, skarns, and polymetallic veins, are associated with 115-98 Ma magnetite-series arc magmas in the Dawson Range (Allan et al, 2013, and references therein).…”

“…Figure 12A shows the Pb isotopic ratios of the 15 igneous feldspars determined in our study. Figure 12B presents the isotopic compositions of 30 previously published analyses of igneous feldspars collected in east-central Alaska (Aleinikoff et al, 1987Dilworth et al, 2007) in relation to the fields defined by the igneous feldspar compositions determined in our study. The feldspar Pb isotopic compositions are divided into the following age groups, based on the U-Pb zircon or titanite crystallization age of the sample: Late Triassic to earliest Jurassic (pluton of Diamond Mountain) , Early Jurassic (187-181 Ma), mid-Cretaceous (115-95 Ma) and Late Cretaceous (94-78, 73-70, and 68-66 Ma) (Fig.…”

“…Plutons in this age range are present in the Yukon-Tanana Upland west of the study area, as are dikes and small mafic bodies dated as ca. 93 Ma (e.g., Dilworth et al, 2007;Werdon et al, 2004b;Hart et al, 2004). As with the older Cretaceous age group, whole-rock geochemistry of the ca.…”

“…12.) All of the samples in the 94-78 Ma age group and all but 2 of the 18 samples in the 115-95 Ma age group (Fig. 12B) are from outside the study area, in other parts of the Yukon-Tanana Upland or the foothills to the Alaska Range south of the Tanana River Dilworth et al, 2007). The shale curve of Godwin and Sinclair (1982), which closely approximates the Pb isotopic evolution of upper crustal rocks in the northern Cordillera (including both the parautochthonous YTa and the allochthonous YTT; , the average crustal growth curve of Stacey and Kramers (1975), and the estimated average mantle curve of Doe and Zartman (1979) are shown for reference in Figure 12, as is the field of feldspar data derived from Pb isotopic compositions of Late Devonian and Early Mississippian metaigneous rocks of the parautochthonous YTa (Aleinikoff et al, 1987 (Figs.…”

“…The Mount Harper batholith is probably composite in nature (it has only been mapped in a reconnaissance manner) and it is therefore not possible to know whether different ages reflect different pulses of magmatism or differences in mineral cooling ages or other aspects of the geochronology. Granitoids of this age make up the Walker Fork pluton in the southeastern Eagle quadrangle and Ruby Creek granite in the central Eagle quadrangle (Table 5) and are present in the southeastern Big Delta quadrangle to the west and adjacent Yukon (Day et al, 2003;Dilworth et al, 2007;Werdon et al, 2004b;Hart et al, 2004, and references therein) (Fig. 2).…”