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Ambler District - Arctic, Smucker, Sun/Picnic Creek |
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Alaska, USA |
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Cu Zn Pb Ag Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Ambler District is located along the southern margin of the south-western Brooks Range, northern Alaska, USA, ~260 km east of Kotzebue. It contains a number of significant deposits and prospects of volcanic hosted massive sulphide deposits, distributed over a WNW-ESE trending interval of ~90 km, including Arctic, Smucker and Sun-Picnic Creek.
The host Ambler Sequence consists of Devonian and older schistose meta-sedimentary rocks overlain by Middle Devonian to Lower Carboniferous phyllitic shales and siltstones. The sequence averages 1.5 km in thickness and is composed of volcanic and volcaniclastic rocks, 25% carbonate sedimentary rocks and 15% pelitic sedimentary rocks. The volcanic rocks are bimodal, but predominantly of rhyolitic composition, much of which was deposited as a submarine ash-flow. The remainder is basaltic. The volcanism is interpreted to be rift related. For a more detailed descriotion of the host sequence and its regional setting, see the Ruby Creek record. Ruby Creek/Bornite is a sediment hosted Cu deposit within the adjacent Bornite Sequence, ~20 km to the SW.
Arctic
The Arctic volcanic hosted massive sulphide copper-zinc-lead-silver-gold deposit was discovered in 1965, but has not as yet been developed for reasons of location and access.
The host rocks comprise the lower greenschist facies, Devonian-lower Carboniferous (Mississippian) Ambler Sequence, which are up to 1830 m thick at Arctic and comprise bimodal basaltic and rhyolitic volcanic and volcanoclastic rocks, pelitic, graphitic and calcareous meta-sediments and marble.
The Ambler Sequence lies with apparent conformity within the 2100 to 6000 m thick pre-middle Devonian Anirak schist, a fine grained meta-pelite with minor marble and meta-basalt. The main lithologies of the Anirak Schist are brownish-grey, variably graphitic, medium grained quartz-white mica (-pyrrhotite-pyrite) meta-pelite with minor amounts of chlorite , albite, graphite and epidote and has a 1 to 10 mm wavy discontinuous foliation.
Mineralisation at Arctic, within the Ambler Sequence, is interlayered with graphitic schists between two meta-rhyolite porphyries. There are four different meta-rhyolites porphyries at different stratigraphic levels and with different characteristics. They have quartz and feldspar phenocrysts and include tuffs, lavas and breccias and have been altered. Ore is developed within graphitic schists which are dark grey to black, fissile, fine grained white mica-(graphite) schists. Pyrrhotite, pyrite and calcite are ubiquitous accessories, and may constitute 1 to 10%. These schists are interlayered with most of the other lithologies of the Ambler Sequence.
Mineralisation occurs at three separate stratigraphic levels, although the Main horizon contains 90 to 95% of the ore. This main horizon in turn is composed of up to 14 semi-massive sulphide lenses within a well developed horizontally stratified sequence of alteration types, including pyritic (pyrite-phengite-calcite), and chloritic (chlorite-quartz) alteration, dolomite, phlogopite, albite, talc and white mica.
The Arctic deposit is confined to a narrow stratigraphic interval within the upper third of the Ambler Sequence. The Main horizon is a conformable tabular sheet 6 to 80 m thick and elongate to oval in shape (915 m NW-SE by 1220 m SW-NE) dipping at 10 to 30° SSW. The sulphide bodies contain 10 to 55% economic sulphides with abundant non-sulphide gangue and iron sulphides varying from 0 to 55%. Chalcopyrite is the most abundant sulphide, although there may be up to 60% sphalerite locally. Galena, which is argentiferous, is widespread and ranges from 0 to 10%. Texturally the sulphides and gangue are variable, including homogeneous, banded, streaked, fragmental, clotty and stringer-like.
Published Mineral Resource estimates at 9 November 2017 were (Trilogy Metals Inc. website, viewed October, 2018):
Indicated Resource - 36 Mt @ 3.07% Cu, 4.23% Zn, 0.73% Pb, 47.6 g/t Ag, 0.63 g/t Au.
Inferred Resource - 3.5 Mt @ 1.71% Cu, 2.72% Zn, 0.60% Pb, 28.7 g/t Ag, 0.36 g/t Au.
Smucker
The Smucker deposit is located ~40 km WNW of Arctic. Mineralisation, which is also hosted by the Devonian to Carboniferous Ambler Sequence, comprises stratabound, banded, disseminated fine- to medium-grained pyrite, sphalerite, galena, chalcopyrite, and owyheeite (Ag3+xPb10-2xSb11+xS28, where x is between -0.13 and +0.20) in a quartz-calcite-pyrite matrix. Mineralisation averages 1 to 5% Pb, 5 to 10% Zn, 103 to 343 g/t Ag and minor Au, and is distributed over a strike length of 1000 m and lateral width of up to 60 m. The sulphide zone ranges from 3 to 9 m in thickness.
The deposit is located on one limb of a recumbent, asymmetric antiform, hosted by a mafic and felsic metavolcanic sequence that is composed of quartz-muscovite-feldspar schist, quartz-chlorite-calcite phyllite, and porphyroclastic quartz-feldspar-muscovite schist, with an interlayered metasedimentary sequence of quartz-muscovite-chlorite phyllite, calc-schist and marble. The protoliths are interpreted to have been bimodal calcic and calcalkaline volcanic rocks and impure clastic and calcareous sedimentary rocks. Greenschist-facies metamorphism of the altered host rocks have produced the schists observed. The host rocks strike WNW and dip moderately to the south, but contain abundant south-dipping, tight to isoclinal folds.
Hitzman et al. (1986) report a resource of >8 Mt @ 0.8% Cu, 6.8% Zn, 2.3% Pb, 200 g/ton Ag.
Summary after Nokleberg et al., 1987.
Sun - Picnic Creek
The Sun and Picnic Creek cluster of occurrences comprise stratabound disseminated to massive sulphide mineralisation composed of sphalerite, chalcopyrite, galena and argentiferous tetrahedrite with a gangue assemblage pyrite, arsenopyrite and barite. The deposit occurs as a series of elongate, SW-plunging, lenticular bodies within at least three distinct, zoned mineral horizons, an upper Ag-Pb-Zn rich, a middle Cu-rich, and lower Cu-Zn rich layer.
The mineralisation is also hosted within the Ambler Sequence which at Sun is ~1500 m thick and comprises Devonian felsic to andesitic to basaltic volcanic, volcaniclastics, and intercalated muscovite-quartz-feldspar schist, micaceous calc-schist and marble. The sequence is divided into an upper and lower unit by calcareous metabasaltic beds of variable thickness. The upper unit which is predominantly metarhyolite and related rocks contains all of the identified massive sulphides, with the bulk of sulphides in felsic schists, although thin concordant bands of sulphides occur within metarhyolite. The lower unit is dominantly pelitic schist and metarhyolite. The host rocks generally strike NE-SW and dip moderately to the SE. Small- and large-scale isoclinal folds are evident in both the host rocks and sulphide layers.
The sulphide bodies tested have average grades of 1 to 4% Pb, 6 to 12% Zn, 0.5 to 7% Cu, 103 to 343 g/t Ag.
Individual quartz-barite beds contain 685 to 1,029 g/t Ag.
Summary after Nokleberg et al., 1987.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2009.
Record last updated: 1/11/2018
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Smucker Arctic Sun
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Hitzman M W, Proffett J M, Schmidt J M, Smith T E 1986 - Geology and mineralization of the Ambler district, Northwestern Alaska: in Econ. Geol. v81 pp 1592-1618
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Schmidt J M 1988 - Mineral and whole-rock compositions of seawater-dominated hydrothermal alteration at the Arctic volcanogenic massive Sulfide prospect, Alaska: in Econ. Geol. v83 pp 822-842
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Schmidt J M 1986 - Stratigraphic setting and mineralogy of the Arctic volcanogenic massive Sulfide prospect, Ambler district, Alaska: in Econ. Geol. v81 pp 1619-1643
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