Ruby Creek, Bornite

Alaska, USA

Main commodities: Cu
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The Ruby Creek sediment (carbonate) hosted copper deposit is located in the Ambler District of the south-western Brooks Range, northern Alaska, USA, ~260 km east of Kotzebue, ~460 km NW of Fairbanks and ~17 km north of the settlement Kobuk (#Location: 67° 3' 59"N, 156° 56' 22"W).

The deposit is part of the Bornite Project, which comprises the Ruby Creek and down plunge South Reef Zone.

Although copper had been known in the district since the 1900's, the deposit was not discovered until the late 1950's.   It has not as yet been developed for reasons of location and access.

The Bornite/Ruby Creek deposit is located within the Arctic Alaska Terrane of far northern Alaska, a sequence of mostly Palaeozoic continental margin rocks that make up the Brooks Range and North Slope of Alaska (Moore, 1992). The deposit lies within the Phyllite Belt geologic subdivision, which together with the higher-grade Schist Belt, stretches almost the entire length of the Brooks Range, and is interpreted to represent the hinterland of the Jurassic Brooks Range orogeny. The southern limit of the Phyllite Belt is represented by mélange and low angle faults associated with the Kobuk River fault zone, whilst the northern boundary is believed to be gradational with the higher-grade metamorphic rocks of the Schist Belt (Till et al., 2008).

The tectonic setting of the area during the early Devonian has been masked by subsequent deformation. Hitzman et al. (1986) suggest the bimodal volcanic rocks and abrupt sedimentary facies transitions within the region indicate an extensional tectonic setting. The deposit area underwent regional deformation and metamorphism during the Middle Jurassic to Early Cretaceous Brooks Range orogeny. The collision of the Koyukuk Arc Terrane from the south caused north-directed intense imbrication and over-riding of the Arctic Alaska passive margin sedimentary sequence. Rocks in the Schist Belt were metamorphosed to blueschist facies but were partially exhumed by north-directed faulting prior to full thermal equilibration. Both the Schist Belt and the Phyllite Belt cooled from greenschist conditions during a period of rapid extension and erosion beginning around 103 Ma (Moore et al., 1994, Vogl et al., 2003).

The host Bornite carbonate sequence is composed of middle to late Devonian carbonates, comprising 200 to 1000 m of lower greenschist facies alternating carbonate rocks and calcareous phyllite, including argillaceous marbles, lesser dolostones, limestones and lenses of graphitic marble. Limestone transitions laterally into dolostone, which hosts the majority of the mineralization and is considered to be hydrothermal in origin. This unit is unconformably underlain by ~3000 m of lower Devonian pelitic schist, quartzite, marble and minor metabasalts of the Anirak schist, and then below a major thrust zone by the Proterozoic to Devonian Kogoluktuk schist, comprising >1000 m of epidote-amphibolite facies pelitic schist, quartzite, metagabbro, calcareous schists and minor marble (Hitzman, 1986).

In more detail the Bornite carbonate sequence is divided into the following lithological units, above the quartz phyllite of Anirak Schist, which comprises moderately graphitic quartz rich phyllite, that is locally moderately calcareous:
Bleached calcareous phyllite - interpreted to be an altered equivalent of the carbonaceous calcareous phyllite described below, with which it is texturally similar. It is often characterised by a strong white mica component, historically misidentified as talc.
Carbonaceous calcareous phyllite - weakly to moderately carbonaceous calcareous phyllite defined by the presence of a significant (5 to 95%) shale-rich component in the carbonate section. These phyllites often act as limits that bound mineralisation.
Dolostone - dolomitised carbonate sedimentary breccia consisting of abundant (±90%), polylithic clasts (0.5 to 50 cm in diameter), which is the host to mineralisation at Bornite.
Limestone - carbonate sedimentary breccia consisting of 10 to 90% polylithic carbonate clasts supported in a calcareous matrix. The clast lithologies include limestone, dolostone, ferroan dolostone and locally massive pyrite.

Detailed re-logging and interpretation of core from the host carbonate sequence, as reported in Davis et al. (2016), indicates stacked debris flows composed of basal non-argillaceous channelised debris flows breccias, with a fining upward sequence of increasingly argillaceous-rich breccias, capped by high Ca phyllites, and laterally confined to channels between either thin-bedded or massive platform carbonates. Davis et al. (2016) reports two apparent stacked debris flow sequences, the Lower and Upper reefs. The Upper reef passes upward into capping argillaceous limestones rather than discrete high Ca phyllites, indicating an upward shallowing or filling of the debris flow channels. A series of individual debris flow cycles have been interpreted.
Low Ca phyllites, such as those of the unconformably underlying Anirak schist and the structurally capping Beaver Creek phyllite most likely had a different provenance to the locally derived high Ca phyllites of the debris flow dominated Bornite Carbonate sequence stratigraphy. In addition to the stacked sedimentary stratigraphy, a crosscutting breccia known as the P-Breccia has been identified in and around the South Reef mineralisation which contains high grade copper. It remains unclear whether the P Breccia is a post-depositional structural, hydrothermal or solution-collapse induced breccia (Davis et al., 2016).

The mineralisation at Bornite occurs as tabular zones that coalesce into crudely stratabound bodies hosted in secondary dolomite. Two significant dolomitic host units have been mapped (after Davis et al., 2016), namely the:
i). Lower Reef, a 100 to 300 m thick dolomitised zone lying immediately above the basal quartz phyllite of the unit of the Anirak Schist. This dolomite outcrops along the southern margin of the Ruby Creek zone and is extensively developed throughout the deposit area, hosting a significant portion of the shallow resources in the Ruby Creek zone, as well as higher grade resources down dip and to the northeast in the South Reef.
ii). Upper Reef, a 100 to 150 m thick dolomite band roughly 300 m higher in the section. This zone hosts relatively high-grade resources to the north in the Ruby Creek zone, and appears to lie at an important NE- trending facies transition to the NW of the main drilled area, locally appearring to be at least partially thrust over the Lower Reef stratigraphy to the SE.

Drilling has shown dolomitisation and copper mineralisation in the Upper and Lower Reefs coalescing into a single horizon to the north. The NE- trending Ruby Creek and South Reef zones also coalesce into a roughly 1000 m wide zone of >200 m thick dolomite containing significant copper mineralisation dipping at roughly 5 to 10°N (Davis et al., 2016).

Copper mineralization comprises chalcopyrite, bornite and chalcocite distributed in stacked, broadly concordant and tabular with thicknesses of up to 25 m which follow favourable stratigraphy, within the dolomitised limestone package. It occurs, in of increasing grade order, as disseminations, irregular and discontinuous stringer-style veining, breccia matrix replacement, and stratabound massive sulphides. The distribution of copper mineral species is zoned around the lower and central part of the individual lenses, with bornite-chalcocite-chalcopyrite at the core, progressing outward to chalcopyrite-pyrite. Additional minor Cu(-Co) minerals include carrollite, digenite, tennantite-tetrahedrite and covellite (Bernstein and Cox, 1986). Stringer pyrite and locally significant sphalerite occur above and around the copper zones, while locally massive pyrite and sparse pyrrhotite occur in association with siderite alteration below the Cu mineralisation in the Lower Reef (Davis et al., 2016).

In addition to Cu, significant Co mineralisation accompanies the bornite-chalcocite assemblage. Co occurs with high-grade Cu as both carrollite (Co2CuS4) and as cobaltiferous rims on recrystallised pyrite grains (Bernstein and Cox, 1986). Appreciable Ag values of as much as 30 g/t Ag are also found with bornite-rich mineralisation in the South Reef and Ruby Creek zones (Davis et al., 2016).

The dominant hydrothermal alteration mineral is dolomite, which is particularly pronounced within: i). certain massive carbonate units; ii). the Lower and Upper reef debris flow breccias; and iii). the P Breccia. More intense and complete dolomitisation occurs at the base of both of the Lower and Upper Reefs (Davis et al., 2016).

Mineralisation is associated with zones of intense dolomitisation, the distribution of which is influenced by the host stratigraphy. On most scales however this dolomitisation is transgressive. There are three main phases of dolomitisation grading upwards and laterally outwards from an elongate core. The outermost is a low Fe dolomite, followed by a ferroan dolomite grading to a siderite core. The dolomitised rocks in the low Fe zone appear homogenous to mottled to brecciated with no remaining sedimentary textures. The breccia 'clasts' are not exotic or rotated, have sharp margins and appear to be due to selective dissolution and introduction of a matrix. The ferroan dolomite and siderite stages cut each of the preceding phases with increasing veining and brecciation textures and dolomite/siderite grain sizes up to 1 mm in a microscopic ground mass. The alteration culminates in a complex late stage crackle breccia vein system of white low ferroan dolomite that cuts all three zones. In large sections of the alteration system these veins comprise more than 15% of the rock (Hitzman, 1986).

Copper grade generally correlates with the intensity of dolomite alteration expressed as Mg/Ca ratios of 0.4 to 0.67. The Fe composition of the carbonates also significantly influences grade. High Fe carbonate species such as siderite and ankerite are almost barren while low Fe dolomites are more strongly mineralised with Cu (Davis et al., 2016).

The outer low Fe dolomites have very little sulphide or Cu. The ferroan dolomite has increasing interstitial pyrite inwards until a band of semi massive to massive pyrite separates the ferroan dolomite from the sideritic core. Minor sphalerite and barite accompanies the pyritic sections of the ferroan dolomite zone.   The copper mineralisation is largely associated with the last veining phase, with the best grades being found where these veins cut the massive to semi massive sulphides at the sideritic to ferroan dolomite boundary (Hitzman, 1986).

This zoning produced Fe-rich dolomites zoned around high Fe siderite and ankerite localised down plunge of the lowermost debris flows in the Lower Reef. Low Fe dolomites, zoned around the basal core of high Fe dolomites, are well mineralised, forming an annulus or horseshoe around the core of un-mineralized high Fe siderite and ankerite lying between the Ruby Creek area and the South Reefs (Davis et al., 2016).

The overall dolomite alteration pattern suggests a mineralising fluid sourced from the south, with transport to the north down the principal axis of debris flow emplacement (Davis et al., 2016).

Alteration within the high Ca phyllites capping successive debris flows occurs as albitisation of pre-existing K feldspar and the development of Mg-phengite at the expense of early detrital muscovite, biotite and chlorite. Increased albite and Mg-phengites are characteristically seen as bleaching of the high Ca phyllites with the highest intensities of alteration immediately below strong copper mineralisation in the debris flow breccias (Davis et al., 2016).

Sulphur isotope and fluid inclusion studies indicate temperatures of up to 300° C in the early stages of dolomitisation and 220 to 100° C for the Cu minerals from the core of the 'ore' lenses to their margins. The dolomitised zone carries interstitial crusts of graphite and hydrocarbons suggesting that the fluids permeating the system included hydrocarbons (Hitzman, 1986).

Re-Os dating of sulphides from main stage copper mineralisation (chalcopyrite, pyrite and bornite) from Bornite/Ruby Creek (Selby et al., 2009), produced a Middle Devonian age of 384±4.2 Ma. However, Connor (2015) suggests a post Juro-Cretaceous age based on two lines of evidence: i). albite alteration associated with the mineralising event cross cuts the pronounced Juro-Cretaceous penetrative fabric at Bornite, and ii). the presence of cymrite, a barium-rich blueschist-stable metamorphic mineral related to the Juro-Cretaceous deformation is common within all the various mineralised assemblages (Davis et al., 2016). These suggest significant subsequent remobilisation.

The resource quoted in 1986 was 90 Mt @ 1.2% Cu, including 36 Mt @ 2% Cu (Hitzman, 1986).

Published NI 43-101 compliant resources in May 2016 were (Nova Copper, Inc., 2016):
  Open pit - at a 0.5% Cu cut-off
      Indicated mineral resource - 40.5 Mt @ 1.02% Cu,
      Inferred mineral resource - 84.1 Mt @ 0.95% Cu.
  Below pit - at a 1.5% Cu cut-off
      Inferred mineral resource - 57.8 Mt @ 2.89% Cu.

This summary is drawn and paraphrased from Hitzman, 1986 - see below, and "Davis, B., Sim, R. and Austin, J., 2016 - Technical Report on the Bornite Project, Northwest Alaska, USA; prepared by BD Resource Consulting, Inc., SIM Geological Inc. and International Metallurgical & Environmental Inc. for NovaCopper Inc., 169p.

The most recent source geological information used to prepare this summary was dated: 2016.     Record last updated: 1/7/2016
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.

Ruby Creek

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Bernstein L R, Cox D P  1986 - Geology and Sulfide mineralogy of the Number One orebody, Ruby Creek Copper deposit, Alaska: in    Econ. Geol.   v81 pp 1675-1689
Hitzman M W  1986 - Geology of the Ruby Creek Copper deposit, Southwestern Brooks Range, Alaska: in    Econ. Geol.   v81 pp 1644-1674
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
Selby D, Kelley K D, Hitzman M W and Zieg J,  2009 - Re-Os sulfide (bornite, chalcopyrite, and pyrite) systematics of the carbonate-hosted copper deposits at Ruby Creek, Southern Brooks Range, Alaska: in    Econ. Geol.   v104 pp 437-444

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