Relationship between amorphous silica and precious metal in quartz veins: examples from Brucejack, British Columbia and Dixie Valley, Nevada

Nicolas J Harrichhausena, Christie D Rowea, Warwick S Boardb, and Charles J Greigb

a Department of Earth and Planetary Sciences, McGill University, Montréal, QC, Canada; b Pretium Resources Inc. #2300-1055 Dunsmuir St,Vancouver, BC, Canada

10m

N

76°

85°

80°

83°

faul

t pla

ne: 1

92°/

83° E

Kamb ContoursC.I. = 2.0 Sigma

Equal AreaLower Hemisphere

N = 273

Legend

76°Medium-grained immaturesandstone

Interbedded �ne-grainedsandstone and argillite

Matrix-supported pebbleconglomerate

Undi�erentiated quartz-sericite schist

Stockwork quartz-carbonate veining

Fault (sense indicated where possible)

Quartz-carbonate vein

Bedding measurement

Areas of outcrop shown in darker colour

b)

a)

Equal arealower hemisphere

Kamb contoursC.I. = 2.0 sigma

N = 273

10 nm

075/57 SE°

Motivation

100 µm

Above: TEM images of silica from Dixie Valley, Nevada (sample from Jonathan Caine). Amorphous silica with ~ 3 wt. % Au (A). As amorphous silica recrystallizes gold concentration drops (B) until it is below detection limit in completely recrystallized quartz (B) (Rowe et al., 2012).

Amorphous silica with 3 wt % gold

Intermediate phase of amorphous silica recrystallizing to quartz with 1 wt % gold

Amorphous silica completely recrystallized to quartz with < 0.1 wt% gold

Right: Large quartz crystals from quartz-

carb vein breccia (XPL). Euhedral core de�ned by hexago-nal inclusion arrays

with �amboyant ex-tinction around rims.

Above: Dendritic electrum sample in drillcore from the Brucejack deposit. This sample has gold concentrations above 10 kg/t. Photo taken from Pretium resources website.

I) Fault zone stockwork II) Amorphous silica precipitation

III) Precious metal textures at Brucejack

Above: Schematic showing model for transport of precious metals (e.g. Au) as suspended nanoparticles. Inset shown in box I is indicated along the fault system.

• Fluid from a magmatic source travels down temperature gradient (a & b). With a drop in T, solubility of AuCl2

- drops until ∑Au is dominated by Au(HS)2

- (b). • If ∑S in the system remains constant due to previous alteration, solubility increases with drop in T. If ∑S is reduced via

precipitation of sulphides (e.g. pyrite) in wall ∑Au continues to drop (Williams-Jones et al., 2009).

• Au may precipitate before silica (Saunders, 1995) and form nanoparticles which may be carried along fluid pathways in suspension (Saunders, 1990; Herrington & Wilkinson, 1993). When hydrothermal fluids reach fault systems (b), amorphous silica precipitation can trap suspended particles. Solubility curve modified from Williams-Jones et al., 2009.

Above: Proposed depostional model at normal fault zone.

I) Fault rupture will cause sudden extension and depressurization in right stepping extensional faults.

II) During sudden depressurization, amorphous silica precipitation is favoured over quartz and suspended nanoparticles are trapped.

III) Amorphous silica recrystallizes and forces impurities to grain boundaries (Herrington & Wilkinson, 1993).

Dixie Valley, Nevada

Brucejack, British Columbia

Right: Schematic of fault zone geometry at Dixie Valley. Qal: Quaternary alluvium. Modi�ed from

Power and Tullis, 1989 and Caine, 2010.

• Normal fault in the basin and range with active auriferous hydrothermal system.

• Gold ore mined at Dixie Comstock consisted of fault related electrum bearing quartz breccia (Vikre, 1994).

• Amorphous silica deposited on fault slip surfaces and related veins.

Quaternary alluvium:matrix supportedpoorly sorted boulder conglomerate

Fault core: poorly sorted, clast supported

‘Mirrors’ slip surface 1 m

• High-grade gold-silver epithermal vein deposit.

• Electrum (gold-silver alloy) hosted within quartz-carbonate vein stockwork that is associated with faulting.

Right: a) underground exposure of stockwork vein-ing. b) equal-area stereonet plot of poles to veins

within stockwork shown in (a); c) stockwork vein breccia with clasts of electrum bearing vein frag-

ments circled in yellow.

Below: a) outcrop map of quartz-carbonate vein stockwork; b) equal-area stereonet projection of poles to veins measured within map area.

Dixie Valley, Nevada Brucejack, British Columbia

Conclusions

Acknowledgments

References

Cryptocrystalline quartz veins from damage zone.

Cryptocrystalline quartz vein thin sec-tion taken from sample in photo above (XPL). Wall-rock shown at bottom of photo mi-crograph, vein at top.

Bright-�eld TEM image of cryptocrys-talline quartz vein from above showing relict texture of silica nanoparticles with a diameter be-tween 5-10 nm.

Right: Bright-�eld TEM image of amor-

phous silica inclu-sion (A) within

quartz-carbonate stockwork. Inset:

di�raction pattern of amorphous silica

inclusion.

Right: inverse FFT image of grain

boundary between quartz (left) and

electrum (right). Lattice planes are

parallel to sub-parallel with a small

bend at grain boundary.

1: Electrum (large opaque mass at center) within quartz-carbonate stockwork in XPL. Focused-ion-beam foil location for TEM is shown.

2: Bright-�eld TEM image of foil from 1. Dark material in center is electrum. Locations of 3 & 4 shown.

3: Bright-�eld TEM image showing electrum on left (C) and spherical particles comprised of Ag and silica (A & B).

4: Bright-field TEM image showing electrum mixed with silica (A, B, & D) and electrum (C) at junction between electrum and quartz grains. This may be indicative of expulsion of electrum during quartz recrystallization.

• Observed textures show relict amorphous silica at both Brucejack and Dixie Valley (e.g. Dong, 1995).

Caine, J. S., Bruhn, R. L., & Forster, C. B. (2010). Internal structure, fault rocks, and inferences regarding deformation, fluid flow, and mineralization in the seismogenic Stillwater normal fault, Dixie Valley, Nevada. Journal of Structural Geology, 32(11), 1576-1589.

Dong, G., Morrison, G., & Jaireth, S. (1995). Quartz textures in epithermal veins, Queensland; classification, origin and implication. Economic Geology, 90(6), 1841-1856.

Hardardóttir, V., Brown, K. L., Fridriksson, T., Hedenquist, J. W., Hannington, M. D., & Thorhallsson, S. (2009). Metals in deep liquid of the Reykjanes geothermal system, southwest Iceland: Implications for the composition of seafloor black smoker fluids. Geology, 37(12), 1103-1106.

Herrington, R. J., & Wilkinson, J. J. (1993). Colloidal gold and silica in mesothermal vein systems. Geology, 21(6), 539-542.

Pearce, M. A., White, A. J., Fisher, L. A., Hough, R. M., & Cleverley, J. S. (2015). Gold deposition caused by carbonation of biotite during late-stage fluid flow. Lithos, 239, 114-127.

Power, W. L., & Tullis, T. E. (1989). The relationship between slickenside surfaces in fine-grained quartz and the seismic cycle. Journal of Structural Geology, 11(7), 879-893.

Rowe, C. D., Kirkpatrick, J. D., White, J. C., Faber, C., & Caine, J. S. (2012, December). Gray Areas": Silica gels, amorphous silica and cryptocrystalline silica on fault surfaces. In American Geophysical Union, Fall Meeting 2012, abstract# T13E-2654.

Saunders, J. A. (1990). Colloidal transport of gold and silica in epithermal precious-metal systems: Evidence from the Sleeper deposit, Nevada. Geology, 18(8), 757-760.

Saunders, J. A., & Schoenly, P. A. (1995). Boiling, colloid nucleation and aggregation, and the genesis of bonanza Au-Ag ores of the Sleeper deposit, Nevada. Mineralium Deposita, 30(3-4), 199-21

Simmons, S. F., & Brown, K. L. (2006). Gold in magmatic hydrothermal solutions and the rapid formation of a giant ore deposit. Science, 314(5797), 288-291.

Simmons, S. F., & Brown, K. L. (2007). The flux of gold and related metals through a volcanic arc, Taupo Volcanic Zone, New Zealand. Geology, 35(12), 1099-1102.

Stefánsson, A., & Seward, T. M. (2004). Gold (I) complexing in aqueous sulphide solutions to 500 C at 500 bar. Geochimica et Cosmochimica Acta, 68(20), 4121-4143.

Vikre, P. G. (1994). Gold mineralization and fault evolution at the Dixie Comstock Mine, Churchill County, Nevada. Economic Geology, 89(4), 707-719.

Williams-Jones, A. J., Bowell, R.J., Migdisov, A.A. (2009) Gold in solution. Elements, 5(5), 281-287

• Hydrothermal systems at both Dixie Valley and Brucejack follow normal fault zones and show evidence of vein formation during fault rupture.

• Amorphous silica formed by fault processes is at both locations and is now present as cryptocrystalline quartz or euhedral quartz.

• Some Ag at the Brucejack deposit has a spheroid nanoparticle texture and electrum is associated with relict amorphous silica. and potential recrystallization at grain boundaries.

• There is circumstantial evidence for precious metal colloids, but prevalence within ore system has not yet been established.

Below: Photo of fault system at Dixie Valley. Green traces outline fault traces.

Right: bright-�eld TEM image of grain boudary above be-tween quartz (left)

and electrum (right). Quartz

shows faint relict texture of nanopar-

ticles.

• Electrum displays dendritic texture grain boundaries.

• Electrum contains inclusions of silica.

• Electrum observed at nanoscale at Brucejack is associated with relict nanoparticle textures of both precious metals and silica.

• Aligned crystal lattices may suggest either nucleation on a pre-existing surface or recrystallization.

Kamb ContoursC.I. = 2.0 Sigma

Equal AreaLower Hemisphere

N = 71

105°/53° S

b)

N

N S

S

c)

a)

Equal arealower hemisphere

Kamb contoursC.I. = 2.0 sigma

N = 273

4 m

Location of TEM Foil

2 µm

100 nm10 nm

100 µm

1 2

3 4

4

3

• High-grade, discrete mineralization of Au is tough to explain via transport in solution due to low solubility and low �ux (Pearce et al., 2015).

• Alternatively, precious metals such as Au, could be transported in a suspension.

• Suspended nanoparticles may be precipitated deeper within a hydrothermal system and deposited as trapped impurities in amorphous silica formed at structural traps (e.g. Saunders, 1990; Herrington & Wilkinson, 1993).

• Recrystallization of amorphous silica to quartz may affect mineralization textures.

• We compare two sites with known auriferous hydrothermal silica deposition, one recent at Dixie Valley, Nevada, and one Jurassic at Brucejack, British Columbia.

• By comparing a fresh deposit with a deformed and metamorphosed one, we hope to determine whether amorphous silica can be related to precious metal deposition and if evidence for transport via suspension can be preserved.

Above: Dendritic electrum sample in drillcore from the Brucejack deposit. Photo taken from Pretium resources website.

Above: Measured gold concentrations in geothermal brines. Overlain are the highest theoretical concentrations calculated for hydrothermal �uids below 500°C.

Bright-�eld TEM image of cryptocrys-talline quartz from ‘Mirrors’ fault slip surface at Dixie Valley with relict silica nanoparticle texture.

Left: SEM image of silica inclusions (EDS spot 4, 5 & 6) within electrum (EDS spot 7).

300°C

400°C

500°C

1 kmH4SiO4 (aq)

AuCl2- (aq)

Intra-arc basin with syn-depostionalgrabens

H4SiO4 (aq)

Au(HS)2- (aq)

Au solubility minimum

Lateral �uid �ow

Magmatic�uid source

200°C

Recrystallization of amorphous silica to quartz

Impurities (i.e. Au) pushed to grain boundaries

Chloride Bisulphide ∑Au with ∑S constant due to Fe bearing minerals in wallrock already altered to pyrite

AuCl2-

Au(HS)2-

1000

100

10 1

550 450 350 250 150

Au (ppb)

T (°C)

∑Au with drop in ∑S due to pyrite precipitation

(b)

Quartz-sericite-pyrite alteration

Au (s)

Sudden P drop due to extension in fault system causes amorphous silica precipitation

Inset to right

Fault core

I

II

III

I

(a)

Au nanoparticles

Model

The authors thank Pretium Resources Inc. and the Natural Sciences and Engineering Research Council of Canada for providing �nancial support for this project. The authors also thank Pretium Resources for access to the Brucejack property, including the underground development. This work would not have been possible without the accommodation and support provided by Pretium Resources at their Brucejack camp. Data collection and mapping at Brucejack and Dixie Valley were done with the help of two great �eld assistants, M. Tarling and P. Rakoczy, and their work is greatly appreciated. The �rst author thanks Geoscience BC and the Society of Economic Geologists for their generous financial support.