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Ministry of Energy Mines and Responsible for Core Review

Preliminary Map No. 52:  Geology of the Ruby Creek - Boulder Creek Area (104N/11W)

by P.A. Christopher, R.H. Pinsent

 

View Map (PDF, 5.1 MB), (1:12 000)

Notes to accompany Map

 

Preliminary Map 52 presents the geology of the Ruby Creek-Boulder Creek area (104N/11W) near Atlin in northern British Columbia.  The area covered includes the Adanac molybdenum deposit.  The 1:12 000 scale geology map includes a legend and a generalized geological plan of the Adanac underground workings.  Ten pages of notes accompany the map.

 

The bedrock geology of the Ruby Creek-Boulder Creek area is described in terms of 11 map units. The oldest rocks in the area are believed to be Permo-Pennsylvanian in age.  They consist of crystalline limestone or skarn (unit 1) and undifferentiated metavolcanic or metasedimentary rocks (unit 2) of the Cache Creek Group, and serpentine or metaperidotite (unit 3) of the Atlin Intrusions.  Metamorphic rocks generally strike northeasterly and dip westerly to the north of the Mount Leonard boss but have variable orientations southwest of it.

 

The Fourth of July batholith (unit 4) is a fairly uniform medium-grained biotite-hornblende diorite to granodiorite. This unit is generally fresh or weakly altered but contains minor quartz veining and aplitic dykelets near its contact with the Mount Leonard boss.  The Atlin Ruffner silver-lead-zinc deposit, which is northwest of the map-area, is hosted by this unit.

 

The multiphase Mount Leonard boss is divided into six main units that host the Adanac molybdenum deposit. A post-mineral equigranular granite phase encountered at a depth of 412 metres in drill hole 1W-1N has not been seen in outcrop.

 

The various units generally represent textural and mineralogical varieties of chemically similar units. K/Ar ages of the various units also suggest a nearly coeval emplacement of the phases.  Field relationships indicate that either the mafic quartz monzonite or the coarse-grained quartz monzonite is the oldest unit and the aplitic phases or the equigranular granite is the youngest phase.

 

Unit 5 (Mafic Sparse Aplite) is distinguished from other fine-grained quartz monzonite by higher biotite content and a mottled green colour.  A sharp contact with unit 6 occurs just west of Molly Lake.  Biotite appears to increase in this fine-grained phase near contacts with older metamorphic rocks.

 

Unit 6 (Fine-grained Quartz Monzonite) includes a variety of grey, aplitic, fine-grained equigranular and/or subporphyritic rocks.  A gradation of equigranular to porphyritic varieties appears to occur. Crystallization of this rock type may have been controlled ~ fluid loss.

 

Unit 7 (Crowded Quartz Monzonite Porphyry) has an average of 60 per cent subhedral to euhedral plagioclase, orthoclase (perthite), quartz, and biotite phenocrysts.  The matrix is commonly aphanitic or aplitic, but locally becomes medium grained and the unit would be mapped as a hybrid coarse-grained phase.

 

Unit 8 (Sparse Quartz Monzonite Porphyry) has similar mineralogy to crowded porphyry but carries between 10 to 30 per cent phenocrysts (2.0 to 6.0 millimetres).  The matrix is brown aphanitic to fine grained or chilled.

 

Unit 9 (Mafic Quartz Monzonite Porphyry) is a distinctive grey rock characterized by disseminated fine biotite, chalky white plagioclase phenocrysts, and a seriate texture. The ragged feldspar and rare quartz phenocrysts (6.0 to 10.0 millimetres) lie in a finer (1.0 to 4.0-millimetre) quartz monzonite matrix.  Nearly 5 per cent biotite is dusted through the matrix.  The rock is significantly more biotite-rich than any of the other phases.   It has been variously called granodiorite or quartz monzonite porphyry.

 

Unit 10 (Coarse-grained Quartz Monzonite), previously mapped as coarse alaskite or coarse granite, is the most common rock type in both the Surprise Lake batholith and the Mount Leonard boss.  It is weakly to moderately deformed and granulated and consists of pink to grey, equigranular, coarse-grained (0.5 to 3.0 centimetres) quartz monzonite.  The rock consists of approximately equal amounts of perthitic potassium feldspar, intermediate plagioclase, and grey quartz.  Much of the feldspar is seriate and there is commonly a small amount of fine-grained  (2 to 4-millimetre) matrix.  With increased abundance of matrix, the unit has been called 'transition' or 'hybrid.'

 

Olivine basalt (unit 11) of at least two ages occurs.  The stratified mass that forms Ruby Mountain is considered to be older than columnar lava flows that wall Ruby Creek.  The younger basalt may be related to the conelet mapped at the head of Cracker Creek.  Altered late-stage mafic dykes exposed in the Adanac adit may also be related to this unit.

 

Structure is important in the area.  The Adera fault zone is bounded by the Molly Lake fault on the west and the Adera fault on the east.  It has many connecting splays and forms the northern boundary of the Adanac deposit. Between Molly Lake and Ruby Creek the zone strikes north 65 degrees east and dips 80 degrees to the northwest.  Geophysical surveys (EM and magnetic), diamond drilling for the proposed dam site, and mapping indicate a strike of north 45 degrees east for the eastern extension of the Adera fault.  A dyke of crowded porphyry and the Black Diamond vein occupy structures that are subparallel to the Adera fault zone.

 

Four principal trends occur: 36 degrees/82 degrees southeast, 330 degrees to 345 degrees/70 degrees to 80 degrees southwest, 83 degrees/77 degrees northwest, and nearly horizontal. Molybdenum veins occur in the nearly horizontal and 36-degree structures.  Trends of 265 veins measured in the underground workings confirm that many mineralized veins are nearly horizontal and that a lesser number strike northeastward. However, the workings also cut mineralized east-west structures with nearly vertical dips. Similarly, analysis of 171 barren fractures showed strong 25 degrees/75 degrees northwest, 355 degrees/nearly vertical, and east-west/near vertical trends.  Both studies illuminate the key role that nearly horizontal fractures play in producing ore grade molybdenite mineralization.

 

Molybdenum mineralization occurs in all phases of the Mount Leonard boss, except the post-mineral equigranular granite.  The 1 448-metre bench outline shows a doughnut-shaped mineralized body of greater than 0.06 per cent molybdenum that has been truncated by the Adera fault.  The mineralized zone occurs mainly in coarse-grained quartz monzonite and fine-grained quartz monzonite that form a trough-like structure around a weakly mineralized sparse quartz monzonite porphyry core.  An intrusive breccia body in the axis of the mineralized trough is well exposed in the underground workings.

 

Molybdenite occurs mainly as fracture coatings and in quartz veins.  The modes of occurrence are as: rosettes in smoky quartz veins; fine-grained fracture coatings with quartz envelopes; quartz-pyrite-molybdenite-carbonate veins; molybdenite gouge on fault surfaces; molybdenite-bearing quartz veins with potassium feldspar and/or biotite envelopes; and quartz-molybdenite-fluorite-potassium feldspar-biotite veins.

 

Other vein and fracture types include: quartz veins; fracture coatings of pyrite with rare chalcopyrite and copper carbonates; pyrite-bearing quartz veins with rare chalcopyrite and copper carbonates; quartz, calcite, and chalcopyrite veins; potassium feldspar and/or biotite veins; quartz-wolframite, arsenopyrite, scheelite, and fluorite veins; fluorite, quartz, and pyrite veins; and carbonate veins.

 

Chalcopyrite is rare within the deposit; pyrite is less than 1 per cent and is restricted to veins and fractures. Examination of the adit area east of the deposit confirms the existence of pyrite.  However, pyrite is equally abundant within the molybdenum deposit and no obvious pyrite halo was identified. The density of fluorite-bearing veins appears to increase in the fine-grained 'core' in the adit but regional rock geochemical studies show a fluorine halo around the deposit.  Only one quartz-wolframite vein was identified in drill core from the northern margin of the deposit.  However, quartz-wolframite-arsenopyrite veins and breccia zones are more common in the peripheral areas.

 

Information suggests a deposit with open pit mineable reserves of 151 971 000 tonnes of 0.063 per cent molybdenum with a 1.47 to 1 waste to ore stripping ratio.

 

The Adanac deposit lacks strong zonal alteration.  The principal pervasive alteration consists of late chloritization of both biotite and feldspar, accompanied by minor sericite, clay, and carbonate alteration of feldspar.  Locally, orthoclase develops peripheral to fractures, and silicification may extend a few centimetres beyond vein boundaries.

 

The YKR tungsten property is situated south and west of Adanac.  It covers parts of the southerly contact between the Mount Leonard boss and Cache Creek metasedimentary rocks.  The vein tungsten, contact skarn and porphyry potential of the property is presently being examined.  Vein mineralization occurs mainly in the granitic rocks but has also been found in the contact zone.  

 

The Black Diamond vein can be traced about 4 kilometres from near the Ruby Creek valley, where it strikes north 70 degrees east, to Boulder Creek where the strike is about north 50 degrees east.  Veins consist of quartz, muscovite, arsenopyrite, and wolframite, with minor amounts of chalcopyrite, scheelite, molybdenite, cassiterite, fluorite, and gold.  Diopside-tremolite-garnet skarn occurs in pendants in the stock and in contact metamorphosed Cache Creek rocks.  The skarn carries pyrrhotite, pyrite, chalcopyrite, scheelite, wolframite, tetrahedrite, sphalerite, and fluorite.  Bismuthinite (PbBi2S4) and possible tetradymite (BiTe2S) have been reported.  Assays of up to 0.2 per cent tin have also been reported from the skarn occurrences.

 

Secondary uranium minerals, generally kasolite or zeunerite, occur in quartz veins or breccia zones with arsenopyrite, fluorite, tetrahedrite, chalcopyrite, and wolframite.  Zeunerite has also been reported from the Black Diamond vein and the Purple Rose prospect.

 

More than 3 000 geochemical analyses of rocks from the Adanac deposit area indicate an average value of between 18 ppm and 20 ppm uranium.  Values increased from about 13 ppm to 24 ppm as the phases evolved. No significant concentrations of uranium occur in the area of the Adanac deposit.

 

Eight holes were drilled by Cominco Ltd. to test for secondary uranium mineralization in paleostream sediments capped by olivine basalt flows on the Vol claim group between Boulder and Ruby Creeks. No significant uranium concentration was detected.

 

The Adanac molybdenum deposit occurs in the Mount Leonard boss, a composite quartz monzonite to granite stock, which is peripheral and probably related to the main body of the Surprise Lake batholith. The Mount Leonard boss and the Adanac deposit formed during a protracted Late Cretaceous (about 71 Ma by biotite K/Ar dating) intrusive and mineralizing event. Mineralization was localized in a trough-like structure that formed around the cupola of a quartz monzonite porphyry intrusive.  Intersecting north-south, east-west, and northeast structures controlled the location.

 

The Adanac area differs from other parts of the Surprise Lake batholith in that there are finer grained phases that probably reflect rapid crystallization due to fluid loss.  Release of pressure in the magma chamber resulted in reduced vertical pressure and horizontal fractures formed in the cupola area.  Ore grade mineralization occurs where mineralized horizontal fractures coincide with mineralized north-south and east-west structures.  Late movement along the Adera fault zone truncated the deposit on the northwest.

 

All publications of the BC Geological Survey are available digitally, free of charge, from this website.

 

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