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Ruby Creek, Adanac |
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British Columbia, Canada |
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Mo
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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 Ruby Creek or Adanac porphyry molybdenum deposit is located 22 km NE of Atlin in northern British Columbia, Canada, at an elevation of 1460 m, in the floor of an alpine cirque at the head of Ruby Creek (#Location: 59° 42' 37"N, 133° 24' 32"W).
Setting
The deposit is a disrupted, dome-shaped occurrence hosted by the composite Mount Leonard quartz monzonite stock (dated at 70.63 ±3.8 Ma from K/Ar dating of 3 biotites; Christopher and Pinsent, 1982?), a satellite of the post-accretionary Cretaceous, Surprise Lake Batholith. The stock crosscuts deformed and weakly metamorphosed ophiolitic rocks of the Carboniferous and Permian-age volcanic and sedimentary rocks of the Cache Creek Group, which include serpentinites and basalts in addition to limestones, cherts and shales, as well as serpentinite of the Atlin Intrusions, and Jurassic diorite of the Fourth of July Batholith. The Mt Leonard stock is partially overlain by Late Tertiary-Quaternary volcanoclastic debris and columnar olivine basalts.
Ruby Mountain, immediately to the south of the deposit, is a Late Tertiary to Quaternary volcano that erupted and filled the lower part of the Ruby Creek drainage with columnar basalt and volcaniclastic debris which unconformably overlie placer, gold-bearing gravels. The gold was probably sourced from quartz-carbonate veins hosted by shears that cut Cache Creek Group strata.
The rocks of the district are locally strongly faulted, with the Ruby Creek deposit being located in the disrupted Mt Leonard stock peripheral to the Surprise Lake Batholith, near the intersection of two major, pre- to post-mineralisation fault systems. The deposit is partially controlled and offset by the Adera Fault system, which trends SW-NE down Ruby Creek, and forms much of the southern boundary of the Fourth of July Batholith. This steeply NE dipping fault locally offsets the northern portion of the deposit, down-dropping the northwestern part of what was originally a dome, or ring-shaped deposit to the north,
after the latest intrusion of the host stock. The deposit is also partially controlled by the Boulder Creek Fault system, which trends due north and cuts across the head of the Ruby Creek drainage. The Boulder Creek Fault apparently influenced the emplacement of the deposit, and is intimately associated with late-stage porphyritic and aplitic plutonic rocks.
Geology
The deposit is located near the northeastern contact of the Mt Leonard stock, which is composed of two separate pulses of plutonic rocks.
The first intrusive pulse, which includes the contact phase between the stock and the Fourth of July Batholith, is composed of a highly variably textured unit that grades from:
• coarse-grained quartz monzonite south of the Adera Fault. It is weakly to moderately deformed, and comprises pink to grey equigranular, coarse grained (0.5 to 3.0 cm) quartz monzonite, with equal amounts of orthoclase, plagioclase and grey quartz (Christopher and Pinsent, 1982). The feldspar is commonly seriate and, locally, includes a small amount of fine-grained (2 to 4 mm) matrix. It grades into a number of texturally transitional phases including,
• transitional and/or hybrid coarse-grained quartz monzonite, and
• crowded quartz feldspar porphyry, to
• sparse quartz feldspar porphyry upward and outward from the deposit. The gradation into this variety from the coarse-grained quartz monzonite occurs as an increase in the matrix content, and increased isolation of constituent phenocrystic crystals, particularly orthoclase and quartz.
• A distinctive mafic quartz monzonite porphyry unit that is also part of the first phase of intrusion, occurs to the east of the deposit, and comprises a distinctive, grey rock type with short intervals of porphyritic texture (1 to 4 mm). It is composed largely of chalky white plagioclase, disseminated biotite and phenocrysts of ragged plagioclase and lesser quartz.
The rocks of the first pulse were fractured and deformed prior to emplacement of the second intrusive pulse, which is composed of three main phases, namely:
• crowded quartz monzonite porphyry, which has an average of 50 percent (2 to 6 mm) subhedral to euhedral plagioclase, orthoclase, quartz and
biotite phenocrysts in an aphanitic matrix;
• sparse quartz monzonite porphyry, that is similar, but has fewer (10% to 30%) phenocrysts.
• fine-grained quartz monzonite, a variably textured aplite that intrudes the first phase coarse-grained quartz monzonite (and also its variants) and the mafic quartz monzonite porphyry, above and around the sparse and crowded porphyry intrusions. This rock-type is not exposed on surface, but forms a
series of 0.05 to 10 m thick, approximately flat-lying, structurally-controlled sills in the higher-grade (northeastern) portion of the deposit.
The rocks of the second pulse are fresher and generally less deformed than the surrounding first phase intrusions, and have a much finer, more chilled matrix. In this sense, the sparse quartz monzonite porphyry and sparse quartz feldspar porphyry in the two phases are distinctly different.
The second pulse porphyries cut the older rock units, and are found in the subsurface, below the valley floor. Near the head of the valley, the first phase coarse-grained quartz monzonite and its variants are intruded by a buried cupola of sparse quartz monzonite porphyry with a shape that has strongly influenced the locus of mineralisation. Mineralisation surrounds the buried cupola and, to a lesser extent, covers it.
Some of the rock immediately north of Ruby Creek can best be described as plutonic breccias.
Megacrystic feldspar porphyry has been intersected on the SW end of the deposit, which appears to be a relatively young phase of the intrusion, consisting of rare to abundant large (>10 mm), euhedral, orthoclase phenocrysts in a chilled, locally glassy, matrix. Coarse-grained quartz-feldspar pegmatite is also found locally within the deposit. It is not abundant, but covers a wide area as small dykes and structurally controlled sills.
Alteration
The host rocks at Ruby Creek are mostly fresh with much of the alteration seen in drill-core being post-mineralisation, related to fluids that circulated during later faulting. The primary alteration is weak, although orthoclase and less commonly sericite are found as selvages to mineralised quartz veins, with local K feldspar flooded areas scattered throughout the deposit. In addition, there are slightly younger sill-like zones of intense silicification intermixed with bodies of aplite in the higher-grade, northeastern section of the deposit, although locally this alteration can be shown to pre-date both intrusion of aplite and emplacement of mineralized quartz veins.
In the vicinity of post-mineral faults, fractured rocks have commonly been subjected to late hydrothermal alteration, and are either weakly or strongly altered to a mixture of sericite, carbonate, clay and locally chlorite, with no addition of secondary quartz. These altered rocks are soft and friable. Major faults commonly contain breccias cemented by a grey gouge of similar mineralogy, with or without smeared molybdenite. Some of the altered rocks contain fluorite veins. Most of the light-coloured clay is montmorillonite, although grey clay in the main Adera Fault zone is largely composed of kaolinite.
Mineralisation
The deposit comprises of a blanket of mineralisation that underlies the floor of the Ruby Creek valley, characterized by a stockwork of quartz-molybdenite and molybdenite-bearing veins and fractures that are found in all the principal rock-types but are best developed in the first pulse coarse-grained quartz monzonite units, which seem to be more deformed and amenable to mineralisation than the intermixed sparse and crowded quartz feldspar porphyry and cross-cutting second pulse sparse and crowded quartz monzonite phases. The feldspars in the coarse-grained varieties are commonly cracked and/or broken and the rocks are locally cut by ductile shears and narrow zones of intense silicification not found in the other units.
This blanket overlies and surrounds the buried sparse quartz monzonite porphyry cupola under the Ruby Creek valley. Mineralisation is rare within this porphyry itself and relatively weak immediately above it. The mineralisation appears to dip away from the porphyry intrusion to the SE, and to be down-dropped slightly by the Adera Fault to the northwest of Ruby Creek. The blanket also dips to the SW, away from the porphyry cupola near the head of the valley. The mineralized veins appear to have formed late in the development of the stock. The molybdenite bearing veins are most commonly without other metallic phases, although pyrite is found locally and chalcopyrite has been observed. The veins locally contain traces of scheelite, orthoclase, fluorite, biotite, sericite and carbonate.
The Ruby Creek stockwork of molybdenite- and quartz-molybdenite-bearing veins comprise three dominant vein orientation sets, namely: Set 1 with a strike of 80 to 105° and dip of 50 to 80°S; Set 2 with a strike of 210 to 220° and dip of 50 to 85°NW; Set 3 with a strike of 260 to 280° and dip of 5 to 15°N. The sub-horizontal set 3 is dominant, followed by the two sub-vertical sets.
The mineralisation generally occurs as sulphide veins, as coatings on quartz-free fractures and as coarse and fine rosettes and blebs in both smoky and lesser clear quartz. It alsofound as streaks and smears in deformed rock and may locally be enriched in fault zones. In the higher-grade central part of the deposit, the bulk of the mineralisation is in horizontal to sub-horizontal Set 3 veins and fractures from 1 to 5 mm thick, interspersed with veins that are considerably wider (up to 20 mm). The Set 3 veins are commonly cut by narrow 1 to 3 mm quartz veins of the other two sets. All vein sets are mineralised, with blebs of molybdenite commonly found at vein intersections.
The Set 3 veins are locally, extremely well mineralised, with coarse-rosettes of molybdenite up to 30 mm in diameter formed in the plane of the vein. The spacing between the rosettes is very variable, causing a pronounced nugget effect in drilling.
The crowded and sparse porphyries below the central higher-grade zone are cut by narrow (1 to 3 mm) mineralised quartz veins and fractures that also occur at both high and low angles to the horizontal, commonly containing fine-grained to powdery molybdenite, but with fewer high-grade
rosettes.
Locally, some of the veins contain orthoclase and biotite, whilst others contain sericite and/or fluorite, and small, visible traces of pyrite, chalcopyrite and wolframite. Some of these must also contain trace amounts of sphalerite and arsenopyrite as geochemically significant amounts of Cu, Pb, Zn, Ag and As, with or without W and Sn. More typically, polymetallic veins are lower in grade and found on the periphery of the deposit. The tungsten content of the main deposit is generally low (<200 ppm W).
The Ruby Creek deposit mineralisation zone has approximate surface dimensions of 1500 x 300 to 900 m (average 600 m) and is known to depths to 500 m (average 200 m depth) excluding the overburden of 4 to 13 m.
Resources
Published resources estimates include:
Resource: 104.234 Mt @ 0.16% MoS2 (0.096% Mo) (Chapman, Wood & Griswold Ltd., 1971)
Geological reserve: 270 Mt @ 0.053% Mo at 0.03% Mo cutoff , or 187 Mt @ 0.061 Mo at 0.04% Mo cutoff, (Placer Dome 1979).
Undiluted mineable mineral reserve: 151.9 Mt @ 0.063% Mo at 0.04% Mo cutoff & stripping ratio of 1.5:1. (Placer Dome, 1981).
NI 43-101 compliant mineral resources as at April 2009 (Palmer, 2009, for Adanac Molybdenum Corporation) at a 0.04% Mo cut-off were:
Measured resource - 43.642 Mt @ 0.078% Mo
Indicated resource - 231.712 Mt @ 0.065% Mo
Measured + indicate resource - 275.354 Mt @ 0.067% Mo
Measured resource - 39.076 Mt @ 0.062% Mo.
This summary is drawn from and paraphrases sections of "Palmer, K., 2009 - 2009 Mineral Resource Update, Ruby Creek Molybdenum Project in Northern British Columbia, Canada; Prepared by Golder Associates, Ltd. for Adanac Molybdenum Corporation, 89p.
The most recent source geological information used to prepare this decription was dated: 2009.
Record last updated: 2/7/2016
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.
Adanac Ruby Creek
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White W H, Stewart D R and Gansier M W, 1976 - Adanac (Ruby Creek): in Sutherland Brown A (Ed.) 1976 Porphyry Deposits of the Canadian Cordillera Canadian Institute of Mining and Metallurgy, Special Volume 15, pp 476-483
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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