Bonnifield District - Dry Creek, WTF, Anderson Mountain
Zn Pb Cu Ag Au
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The Bonnifield VHMS Zn-Pb-Cu-Ag-Au deposits, the largest of which are Dry Creek, WTF and Anderson Mountain are located ~90 km south of Fairbanks in Alaska, USA, in the foothills of the Alaska Range. More than 25 VHMS occurrences are known in the district, many are small and have not been tested.
The Bonnifield group of deposits are to the north of the Hines Creek strand of the Denali fault system. They are within the continental margin assemblages of east-central Alaska, interpreted to be parautochthonous North American strata (deposited on, or in close proximity to a craton) and part of the Yukon-Tanana terrane (Dusel-Bacon et al., 2004; 2006). These assemblages were intruded by bimodal Late Devonian to earliest Lower Carboniferous (Mississippian) igneous rocks during NE-directed subduction below the continental margin. Attenuation and the development of a restricted marine basin or submerged continental margin resulted from slab rollback. Subduction also produced Early Lower Carboniferous (~360 to 341 Ma) arc and back-arc magmatism in the outboard part of the continental margin (Dusel-Bacon et al., 2004: 2006). Subsequent backarc spreading led to rifting of the outer continental margin and development of the intervening Slide Mountain-Seventymile ocean basin, preserved as klippe or imbricated fault panels of ultramafic, mafic and sedimentary rocks (locally ophiolitic) in the northern Cordillera. SW-dipping, oblique-dextral subduction during the mid-Permian to earliest Triassic resulted in the subsequent closure of the ocean basin and juxtaposition of the outboard, rifted fragment of continental substrate with the superimposed arc and intervening ocean-basin rocks against the inboard, parautochthonous continental margin assemblages (e.g., Hansen and Dusel-Bacon, 1998; Nelson et al., 2006).
Greenschist-facies meta-igneous and meta-sedimentary rocks of the district occur as E-W-trending belts along the northern flank of the Alaska Range (Wahrhaftig, 1968; Gilbert and Bundtzen, 1979; Newberry et al., 1997), and represent protoliths of bimodal volcanic and shallow intrusive rocks, interlayered with submarine, basinal, graphitic and siliciclastic meta-sedimentary rocks. The felsic meta-volcanic rocks and subvolcanic intrusions are dated at Middle Devonian to Early Lower Carboniferous 373 to 356 Ma (Dusel-Bacon et al., 2004; 2006; 2010).
Quartz-rich schist and subordinate graphitic schist and marble (the Healy schist) form the core of an E-W-trending anticline (Wahrhaftig, 1968; Newberry et al., 1997). Graphitic meta-sedimentary rocks and minor meta-conglomerate and meta-rhyolite (the Keevy Peak Formation) stratigraphically overlie the Healy schist. The strata that overlie the Keevy Peak Formation on the north flank of the anticline constitute the Totatlanika Schist, which have been divided into (from bottom to top) the: i). Moose Creek Member - felsic and minor mafic schist; ii). California Creek Member - augen gneiss, grading to meta-rhyolite porphyry; iii). Chute Creek Member - meta-basic rocks, which interfingers with both the underlying California Creek and the overlying Mystic Creek Members; iv). Mystic Creek Member - meta-rhyolite; and v). Sheep Creek Member - quartzo-feldspathic semi-schist derived from near-source, reworked volcanic rocks, meta-siltstone, meta-tuff and marble).
Dark grey graphitic phyllite is found within all members of the Totatlanika Schist.
The Totatlanika Schist is exposed in a synclinal structure, the core of which is occupied by the Sheep Creek Member on the northern limb of the anticline cored by the Healy schist, as detailed above, while the lithologically equivalent rocks on the southern limb of the same anticline are known as the the Wood River assemblage, comprising meta-volcanic and meta-sedimentary rocks. This assemblage, while lithologically equivalent to the Totatlanika Schist is more compositionally continuous than those of the bimodal Totatlanika Schist.
Significant VHMS deposits in the district occur within the Mystic Creek Member of the Totatlanika Schist Formation and in the Wood River assemblage.
This deposit is hosted in the Mystic Creek Member of the Totatlanika Schist at Red Mountain, named for its distinctively coloured, 1800 m thick quartz-sericite-pyrite footwall alteration zone. The Mystic Creek Member occurs as a steeply north dipping submarine sequence of peralkaline meta-rhyolite and synvolcanic quartz porphyry, meta-rhyodacite, with subordinate graphitic meta-sedimentary rocks. Sulphide concentrations occur near its contact with overlying, predominantly meta-sedimentary rocks of the Sheep Creek Member (Newberry et al., 1997; Dusel-Bacon et al., 2006, 2007). Most of the Mystic Creek aphyric and quartz-phyric meta-rhyolite layers formed as coherent facies (i.e., lava flows or sills), although some layers exhibit relict textures suggesting deposition as volcaniclastic felsic tuff. Some small quartz±K feldspar-phyric bodies mapped in the area, likely originated as rhyolite plugs or domes. Alkali meta-basalt and meta-gabbro of the Chute Creek Member which underlie the felsic Mystic Creek Member are unmineralised.
Diamond drilling north and south of Red Mountain has identified the DC North and DC South massive sulphide zones, respectively. The DC North zone consists of two en echelon and partially overlapping mineralised lenses, the Discovery and Fosters Creek zones (representing two different styles of mineralisation), that have been traced for 4.5 km on the basis of geophysical and geochemical anomalies. The Lago Creek zone occurs in the area of overlap between these two zones.
The Discovery zone consists of massive to semi-massive Zn-Pb-Ag sulphides that occur within, and at the base of, an aphanitic, intensely altered quartz-sericite-pyrite rock, the "mottled meta-rhyolite", the protolith of which is uncertain. The "mottled meta-rhyolite is a pale to dark grey, strongly foliated rock, largely composed of heterogeneous patches of fine-grained quartz interspersed with discontinuous thin, dark folia of fine-grained sericite. Quartz augen, up to 1.5 cm in length, may represent boudinaged remnants of quartz veins. Massive sulphides are generally associated with disseminated and stringer veins of chalcopyrite and pyrite. The Discovery zone is characterised by a vertical metal zonation from pyrite-chalcopyrite to pyrite-sphalerite to sphalerite-galena, each separated by sheared rock, making interpretations difficult, although the metal zonation suggests the deposit is overturned.
The Fosters Creek zone is hosted by a brown pyritic mudstone in the hanging wall of, and along strike from, the mottled meta-rhyolite. Mineralisation occurs as disseminations, wispy laminae, and lenses of semi-massive to massive sulphide. Pyrite, sphalerite, galena and chalcopyrite are hosted by weakly graphitic and sericitic phyllitic mudstone, with sphalerite locally forming oval to round clots up to 1 cm in diameter. The massive sulphides are concentrated in two stratabound lenses, the upper of which has moderate to abundant sphalerite with lesser galena, minor chalcopyrite, and variable amounts of pyrite. Precious metal contents are typically high in the footwall section of the upper lens. The lower lens is predominantly pyrite. Drilling suggests an apparent spatial relationship between the thinning of the mottled meta-rhyolite and the thickening of the brown mudstone unit, with the mudstone appearing to thicken within inferred palaeo-basins or grabens. The thickest mass of sulphide occurs at the margin of a steeply W dipping, north-south trending structure, that is interpreted as a growth fault.
The DC North massive sulphide zone is directly underlain by quartz-sericite-pyrite schist (altered rhyolitic meta-siltstone?), graphitic argillite, aphyric and quartz-phyric meta-rhyolite and possible felsic tuff. The central footwall area where the Discovery and Fosters Creek zones overlap, Lago Creek zone, is occupied by a peralkaline, sub-volcanic, quartz-porphyritic meta-rhyolite intrusion (dated at 356±3 Ma, 4 km to the north). This intrusion is cut by thin (<1 cm) veins of fluorite, sphalerite, pyrite and quartz. Some fluorite veins are cut by veins composed of fine-grained beige sphalerite with minor pyrite and trace galena. Hanging-wall strata to the DC North massive sulphide zone include, from bottom to top, fine-grained meta-rhyolite, green and maroon meta-rhyodacite, black meta-rhyolite, graphitic argillite and quartz-phyric meta-rhyolite (dated at 357±4 Ma).
Total inferred resource (not NI 43-101 compliant) for the DC North zone is 2.9 Mt @ 4.4% Zn, 1.9% Pb, 0.2% Cu, 103.5 g/t Ag, 0.62 g/t Au (Szumigala and Swainbank, 1998).
DC South massive sulphide zone has been traced for ~2400 m along strike, and is hosted by the lower Mystic Creek Member. It contains massive sulphide lenses within a 15 m thick siliceous (volcanic?) interval with disseminated sphalerite and pyrite. One of the massive sulphide lenses is represented by the drill intersection ~1 m @ 14.5% Zn, 8.3% Pb, 0.5% Cu, and 137 g/t Ag.
The WTF massive sulphide deposit occurs at the contact between the Mystic Creek and overlying Sheep Creek members on the shallowly south dipping, northern limb of an east-west trending asymmetric syncline, 3 km northeast of the Dry Creek deposit. Tertiary sedimentary rocks occupy the core of the syncline. Drilling has defined a mineralised zone that dips at ~15°S over a distance of 850 m, and extends 850 m east-west along strike.
Stratabound massive sulphides occur within an up to 20 m thick siliceous sequence at the contact between black, graphitic argillite at the base of the Sheep Creek Member, and underlying peraluminous meta-rhyolite of the Mystic Creek Member. The massive sulphide lenses are 0.3 to 2 m thick and contain fine-grained sphalerite, galena, pyrite, and chalcopyrite. Whole-rock major and trace element analyses from the mineralised zone suggest the silicified zone represents a protolith of peralkaline rhyolite with superimposed silicification. Textures in the meta-rhyolite range from aphyric to quartzfeldspar-phyric. Stratigraphic relations between the WTF and Dry Creek North massive sulphide zones are uncertain, although it has been speculated that these zones are at the same stratigraphic horizon but on opposite limbs of the syncline, or that WTF is ~250 m higher stratigraphically than DC North (see Dusel-Bacon et al., 2012). SHRIMP U-Pb zircon geochronology of meta-rhyolite at the WTF deposit yields an age of 363±2 Ma (Dusel-Bacon et al., 2010).
The WTF deposit contains an inferred resource (not NI 43-101 compliant) of 2.8 Mt @ 6.0% Zn, 2.5% Pb, 0.1% Cu, 196.5 g/t Ag, and 0.99 g/t Au (Szumigala and Swainbank, 1998).
The Anderson Mountain VHMS deposit is ~32 km SW of Dry Creek and WTF. Massive sulphide zones occur in Late Devonian (362±3 Ma) meta-volcanic and meta-sedimentary rocks of the Wood River assemblage, on the south limb of the anticline cored by siliceous meta-sedimentary rocks of the Healy schist as detailed above. The stratigraphic section hosting the deposit is dominated by quartz-sericite±feldspar±chlorite schist, quartz±sericite±chlorite phyllite, and black graphitic phyllite.
A 1200 m long zone of in situ of massive sulphide lenses has been outlined by drilling, associated with black graphitic phyllites. One of these massive sulphide lenses has been drilled over a strike length of 275 m, containing intersections of e.g., 0.9 m @ 16.0% Zn, 5.0% Pb, 0.4% Cu, 102 g/t Ag, 0.79 g/t Au. Massive sulphide layers are hosted by black graphitic argillite near the original Anderson Mountain discovery site, while holes farther east intersected felsic schist. Correlation of sulphide lenses is difficult because they are hosted by more than one rock type and deformation has complexly folded the strata. In addition thrust and high-angle faults cut the mineralisation and juxtapose massive sulphides against unrelated rock units, while late (Tertiary?) mafic to felsic dikes locally obliterate, dilate, or offset mineralised zones (Drechsler et al., 1998). The intense deformation may be due, in part, to proximity of the Denali fault zone, the northern splay of which (Hines Creek Fault) is 5 km south of the deposit. Mapping suggest she massive sulphides are overturned to the north. Assuming an overturned stratigraphy, Drechsler et al. (1998) proposed the following succession, from youngest to oldest: i). grey/green rhyolite (tuffs and possible lava flows); ii). argillite (graphitic and variably tuffaceous); iii). massive sulphides; iv). mafic ash tuff; v). mafic lava flow, dike, or sill(?); vi). volcaniclastic sediments; vii). disseminated base metal sulphides; viii). massive sulphides; ix). footwall rhyolite; and x). undifferentiated rhyodacite and dacite.
The Dry Creek and WTF deposits have a combined resource of 5.7 Mt @ 10% Zn, 4% Pb, 0.3% Cu, 300 g/t Ag, 1.6 g/t Au (Dusel-Bacon et al., 2012).
This summary is paraphrased from Dusel-Bacon et al., 2012
The most recent source geological information used to prepare this summary was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Dusel-Bacon C, Foley N K, Slack J F, Koenig A E and Oscarson R L, 2012 - Peralkaline- and Calc-Alkaline-Hosted Volcanogenic Massive Sulfide Deposits of the Bonnifield District, East-Central Alaska: in Econ. Geol. v.107 pp. 1403-1432|
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