Sedimentary pyrite as a proxy for gold fertility in the Selwyn basin area, Yukon: A greenfields
pilot study
1
2
2
2
2
Patrick Sack , Ross Large , Leonid Danyushevsky and, Sarah Gilbert , Daniel Gregory
10 1
Co (ppm)
Tl (ppm)
10 2
10 3
Ni (ppm)
10 4
tin
a
fa
Bi (ppm)
10 2
t
10 -1
10 -3
10 -1
• 3Ace pyrite trace element enriched
compared to pyrite from Conrad.
Selwyn basin
10
0
10 -2
Conrad
ul
10 1
10 0
10 1 10 2
Se (ppm)
10 3
10 0
10 1 10 2
Se (ppm)
10 3
Basinal facies
3Ace
Road
Fault
0
100
• Conrad diagenetic pyrite is elevated in trace elements associated with synsedimentary
hyrothermal mineralization (Tl, Pb, Cu), all generations of pyrite are consistent with
low temperature fluids.
Ma
Period
Kilometres
325
350
450
NO19A051
• Analyses done at the University
of Tasmania (CODES).
• Forty-two major and trace
element data from 22 samples (249
spots and 12 maps).
(from Large et al., 2011)
• If sedimentary pyrite is a significant source of gold in sediment-hosted deposits, a
regional study of gold in sedimentary pyrite may allow a first-order assessment of the
gold fertility within the basin.
• Four generations of pyrite
recognized:
- early diagenetic;
- late diagenetic;
- hydrothermal; and
- vein.
500µm
Analysis No.
Pyrite type
Co (ppm)
Ni (ppm)
Cu (ppm)
Zn (ppm)
As (ppm)
Ag (ppm)
Sb (ppm)
Te (ppm)
Au (ppm)
Tl (ppm)
Pb206 (ppm)
NO19A051
NO19A050
Fuzzy pyrite
overgrowth
Sooty pyrite
core
176
164
180
143
145
0.89
3
0.93
0.021
11
125
688
322
381
40
708
1.57
9
3.32
0.033
57
181
10
10
Au = 200 ppb
-1
10
As = 2000 ppm
10 3
0
10
10
Se (ppm)
1
10
2
10 1
-1
10
Legend
Road River
Group
!
Ordovician
2
475
Te = 2.5 ppm
1
mid-Cretaceous plutons (mKS)
60°N
500
Cambrian
0
10
10
Se (ppm)
• Our preliminary conclusion is that Conrad
and 3Ace sedimentary pyrite have
comparable levels of Au, As, and associated
trace elements to analogue deposits
elsewhere in the world.
• Thus, applying the Large et al. (2011)
model, the Selwyn basin area looks to be
prospective for sediment-hosted gold.
1
10
2
10 0
10
-1
post-Mississippian stratigraphy
10
-2
10 1
10 0
575
0
10
10
Se (ppm)
1
10
2
productive
nonproductive
Au = 200 ppb
10 -2
-3
10 -1
10 0
125°W
10 1
For further information
3Ace
10 -1
10
Hyland
Group
= sample stratigraphic location
Conrad
10 2
Road River Group (ODR)
Ediacaran
10 -3
-1
10
10 3
Earn Group (DME)
550
?
10 2 10 3
As (ppm)
10 4
10 5
Trace element thresholds and productive, non-productive fields from Large et al. (2011).
2013 sample
Road
Occurrence studied in 2012
Regional geology
525
10 2
-2
10 -3
-1
10
4
10
Te (ppm)
1
10 0
10
10
3Ace
As (ppm)
NO19A050
Conrad
Au (ppm)
• Gold, arsenic, and sulphur are
released from the sedimentary
rocks during metamorphism
deeper in the basin (mid- to upper
greenschist facies).
Au (ppm)
10
• Chemistry of pyrite determined
using laser ablation-inductively
coupled plasma-mass spectrometry
(LA-ICP-MS) spots and element
maps.
New opportunity to assess regional gold potential
OS11-70-57.0 m
500µm
(from Large
et al., 2011)
2 cm
10
5
_
^
!
(
!
(
Silurian
3A
ce
3Ace
!
(
!
(
400
Preliminary conclusion
Nahanni Range
Road
Earn
Group
Devonian
425
North Canol
Road
Nahanni
Range
Road
Carboniferous
375
2
62°N
!
!(
(
Analytical methods
North
Canol
Road
Conrad
130°W
• Elevated Bi, low Tl pyrite chemistry at 3Ace is consistent with orogenic gold style
mineralization.
• This sedimentary pyrite is a
potential source of gold for
sediment-hosted gold deposits.
• Gold and arsenic are deposited in
the upper stratigraphy, associated
with fluids focussed along faults,
in anticlinal zones or shears, and
along favourable rock contacts.
Regional Dempster
Highway
Unit
135°W
s
rie
ito
10 2
10 3
Ni (ppm)
rr
Te
10 1
st
10 -4
• Goals:
- characterize sedimentary pyrite in each
regional map unit;
- document background values of gold
and associated trace elements for
regional units;
- evaluate secular variations in
sedimentary pyrite composition that
may affect gold fertility; and
- begin compiling a chemostratigraphy
using whole-rock and pyrite chemistry.
we
10
-3
rth
10 -2
• Early diagenetic pyrite is most trace
element enriched. Successive generations
of pyrite are increasingly depleted.
Study location
B
N
/
o
C
-1
300 km
N
_
^
• Large et al. (2011) suggested
gold and other trace elements
are enriched in diagenetic
pyrite.
i=
0.1
10
Selwyn
basin
!(
!
!(
(
!
(
New gold-source model
C
10
0
64°N
!
(
!
(
• Diagenetic and hydrothermal
pyrite characterized visually and
chemically.
10 -1
10 0
=
i
N
2
• Analyses in progress for 40 samples from
regional stratigraphic units to be analyzed
using LA-ICP-MS at CODES.
E. diagenetic
L. diagenetic
hydrothermal
10 1
o/
/
o
C
/
o
C
10 0
Richardson
Trough
Ti
n
=
Ni
0.1
3Ace
!
(
B) Visible gold from the Main
(photo: ATAC Resources)
Vein at Northern Tiger Resources’ recently discovered 3Ace occurrence.
1
=
Ni
2
10
2
Conrad
Dempster
Highway
(
!
(!
B
10
2
10
3
Yukon
!
(
A
• Samples are of mudstone,
siltstone, and phyllite from two
diamond drill holes at each
occurrence.
CANADA
10
3Ace
E. diagenetic
L. diagenetic
hydrothermal
10 3
• Two sediment-hosted gold
deposit-types studied:
1. Carlin-type - Conrad
2. Orogenic gold - 3Ace
A) Massive realgar from the
recent Carlin-type gold discovery
by ATAC Resources.
Conrad
4
140°W
!
(
Preliminary results
10
New exploration interest
Regional study:
2013 fieldwork
No
Deposit study: pyrite characterization
and comparison
Why study sedimentary pyrite from
Selwyn basin, Yukon?
Alaska
1Yukon Geological Survey, 2CODES, ARC Centre of Excellence in Ore Deposits, University of Tasmania
Patrick Sack
Yukon Geological Survey
Telephone: (867) 667-3203
[email protected]
Sack et al. (2014) available for
download from:
http://data.geology.gov.yk.ca
Hyland Group (PCH) and
Vampire Formation (uPCV)