Dunn Colin E. Biogeochemistry in Mineral Exploration
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Chapter 11
CASE HISTORIES
The following pages contain a selection of case history studies of biogeo-
chemical surveys that have been conducted over known and/or potential zones of
mineralization. There are many case history studies in the literature – enough to
warrant a separate volume to do them all justice. Some of the richest sources
of such studies can be found in the Journal of Geochemical Exploration,andfor
Russian studies that have been translated into English there is a book by Kovalevsky
(1987).
The format adopted in the following pages is similar for each case history,
with basic information on geology, mineralogy, climatic and botanical environ-
ment, the scope of each study, analytical results and data interpretation. Where
available, a summ ary of follow-up exploration results are provided. Except for
one contribution from the University of Barcelona, each account is a synopsis of
studies that have had personal involvement. By s o doing, this permits a full and
detailed appraisal of the full procedures from planning a survey through to data
interpretation.
These case histories focus on Au, PGEs, Ni, U and, because of their diamond-
bearing potential, kimberlites. Certain aspects of case history results from surveys
for other commodities are summarized as examples throughout this book. On the
accompanying CD, the publica tion on halogens inc lude s full a ccoun ts of addi-
tional surveys over several zones of Au and porphyry Cu mineralization in the
forests of British Columbia. Additional case histor ies can be viewed by referring
to the many complete papers for which abstracts are provided on the CD with
this book.
The intention here is not to provide a comprehensive account of each study, but
to put results in context of geological substrate and concealed mineralization, and
in some examples provide consideration of potential processes that might have
contributed to a positive biogeochemical signature. As such, they are aimed at dem-
onstrating the role of biogeochemical surveys in mineral exploration programmes.
The scope of the surveys for which examples are presen ted cover the spectrum from a
few samples representing an orientation survey, to surveys involving a thousand or
more samples collected with expensive airborne support.
GOLD
Canada: La Ronge and Glennie Domains, Northern Saskatchewan
Gold occurs mainly within volcanic-derived supracrustal and felsic intrusive rocks
of the La Ronge and Glennie Domains. Native gold may be associated with pyrite,
pyrrhotite, chalcopyrite, galena, sphalerite and tourmaline. Molybdenite, stibnite,
marcasite, hematite, magnetite and arsenopyrite may be present locally (Coombe,
1984; Delaney, 1995). Bedrock has a veneer of glacial deposits (mostly tills) upon
which soil development is primarily podzol. Many of the Au deposits in this region
are associated with quartz veins and/or shear zones. In the following, the descriptions
of the geology and production statistics are summarized from the Mineral Deposit
Index on the website of the Saskatchewan Department of Industry and Resources.
Further details are provided in the description of each individual deposit.
The area experiences a cold continental climate wi th temperatures typically in
the range of 40 1C–+35 1C with 40–60 cm of precipitation occurring mainly in the
summer months. These conditions sustain a boreal forest (see Chapter 3 for details).
Principal biogeochemical sample media are the dominant black spruce (Picea mariana),
with jack pine in drier areas and the understory shrubs mountain alder (Alnus crispa),
river alder (Alnus rugosa), paper birch (Betula papyrifera) and Labrador tea (Ledum
groenlandicum).
Rod Zone – Jolu Mine – La Ronge Domain
Exploration of this deposit in the 1980s was conducted under several names,
mostly Mahogany Minerals Resources, Royex and Corona Corp.
Geology and mineralization
Located in a northeast-trending linear belt of greenstones of the La Ronge Do-
main (Central Metavolcanic Belt) in the Star Lake area 120 km north of the town of
La Ronge, Au deposits are underlain by an Aphebian (1875 Ma) metasedimentary-
volcanic assemblage intruded by the Hudsonian Star Lake Plu ton. Metamorphism is
lower amphibol ite facies resulting in chlorite-biotite-hornblende schists, locally rich
in tourmaline. Gold occurrences are located within a system of regional shear zones
in the metavolcanic-sedimentary succession or with shear zones in the diorite-mon-
zonite members of the Star Lake Pluton. Shears form mylonitic zones trending 0601.
The surrounding metasediments have been fractured to form a system of intersecting
fractures trending 0601 and 0351 into which quartz veins have been emplaced. Tour-
malinization and chloritization of the wall rocks to the veins are well developed.
In 1985, the Rod Zone was a trenched occurrence of a quartz vein containing
visible Au and molybdenite. Field observations indicated that the quartz veining
was likely to follow the regional trend of the shears, extending north-eastward be-
neath glacial mate rial covered by black spruce forest rooted in moist to wet boggy
344
Case Histories
ground. The challenge at the time for Geoff Burrill, working for International
Mahogany Corp., was to discern wher e along this trend the quartz might be Au-
bearing, without going to the expense and environmental disruption of extensive
stripping of the ground and trenching of the bedrock. It had been ascertained that
the glacial till comprising the overburden was variable in thickness up to a maximum
of 3m.
Biogeochemical survey
Samples of outer bark scales (about 50 g) were scraped from black spruce at 20
sites spaced at 50 m intervals along the projected strike of the quartz vein. This took
the field crew of three a little more than an hour to accomplish. Samples were dried,
reduced to ash by controlled ignition at 475 1C and 0.5 g portions analysed by INAA.
In the following account concentrations are shown in ash with dry weight (DW)
equivalents in parentheses.
In the boreal forests of Canada background levels of Au in spruce bark are
typically 5–10 ppb in ash (0.1–0.2 ppb Au DW). In sharp contrast, at the trench the
bark ash yielded 230 ppb Au (3.7 ppb Au DW), and levels greater than 100 ppb Au
(2 ppb Au DW) were recorded in bark from four addition al sites reaching a max-
imum of 690 ppb Au (20 ppb Au DW). Alder twigs from selected sites contained 25
times usual background levels of Mo (maximum of 61 ppm Mo in ash [2.5 ppm Mo
DW]) and more than 5 times background levels of Co (maximum of 28 ppm Co in
ash [0.6 ppm Co DW]).
Post survey exploration
Subsequent drilling at these sites yielded Au in each drill hole (Table 11-I).
In a letter from Geoff Burrill advising of the results summarized in this table,
he also stated that another ‘anomalous result [200 ppb Au in bark ash] that you
obtained in the muskeg y also appears to correlate with a new zone which has been
found y values over 1 oz [per tonne] were intersected in shallow drill holes very close
to your sample location. Overburden is about 2.5 to 3 m’.
After a decline was excavated at the Rod Zone, Corona Corp. identified sufficient
reserves to establish the underground Jolu Au mine that went into production
in 1988. Reserves were exhausted by 1991 at which time the mill had processed 520,000
tons of ore and recovered 205,000 oz of Au at an average grade of 0.42 oz/t (14.4 g/t).
Jasper pond
Geology and mineralization
Located in the La Ronge Gold Belt, the Jasper North Showing was discovered in
the summer of 1987 during prospecting by Rick Kusmirski of the Saskatchewan
Mining and Development Corporation (later renamed Cameco Corp.). The area is
345
Biogeochemistry in Mineral Exploration
underlain by an intercalated series of intermediate to felsic tuffs and andesitic flows,
and minor turbidites intruded by (a) gabbros (Fork Lake gabbro) and ultramafic
rocks; followed by (b) empla cement of the felsic Island Lake pluton; then (c) shear-
hosted mafic dykes and quartz veins. The rocks in the area have been metamor-
phosed to upper greenschist facies.
Mineralization is pyrite, minor marcasite, sphalerite, galena, chalcopyrite and
visible gold. The gold occurs predominantly in silicate aggregates, but also occurs
within the pyrite. Quartz flooding and mineralization were accompanied by hydro-
thermal alteration of the wall rock. A narrow sericite7tourmaline halo encloses the
mineralized zone.
Biogeochemical surveys
Prior to development of the showing, geochemical surveys were undertaken by
Cameco that included sampling of alder twigs and Labrador tea stems. Their data
showed that northward from the showing into a boggy area Labrador tea yielded up
to 1000 ppb Au in ash (20 ppb Au DW) and alder twigs contained up to 750 ppb Au
in ash (12 ppb Au DW). The following year soon after co mmencement of a drilling
programme, the Geological Survey of Canada conducted a more extensive biogeo-
chemical sampling programme through the area, using the same sample media, and
recorded a maximum of 779 ppb Au in ash of Labrador tea stems (15 ppb Au DW)
and 289 ppb Au in ash of alder twigs (5.6 ppb Au DW). Multi-element INAA of the
TABLE 11-I
Relationship between gold concentrations in the outer bark of black spruce (Picea mariana)
and results of a subsequent drilling programme, La Ronge Gold Belt, Saskatchewan. Drill
results courtesy of G. Burrill. Modified after Dunn (1986a) and Dunn et al. (1990)
Gold (ppb) in outer
bark from black spruce
Dry weight Overburden
thickness (m)
Drill results
in ash
230 3.7 1 Near main trench
120 2.4 1 Erratic mineralization: 1 m of
0.85 oz/t (29 ppm) Au at depth of
50 m
690 14 2.5 Subcropping mineralization:
0.11 oz/t (3.8 ppm) Au over 60 cm
450 11.3 1 Well mineralized shear zone:
0.3–0.7 oz/t (10-24 ppm) Au over
4m
200 3.5 3 Mineralization, locally over 1 oz/t
(35 ppm), at shallow depth
346 Case Histories
ashed Labrador tea stems provided information on the relative concentrations of
several elements – notably Au, As, Co, Cr, Th, U and W (Table 11-II).
Post survey development
During 1990 and 1991 when the mine was in production, the Jasper Mine pro-
duced 83,700 oz of Au from 140,000 tons of ore at an average grade of 0.55 oz/ton,
including some very high grade sections.
Seabee Mine, Laonil Lake
Laonil Lake is located within the Glennie Domain 125 km northeast of the town
of La Ronge. Gold mineralization was discovered in 1947 and drilled by Cominco in
1949, but exploration languished until 1985 when Placer Development Ltd. entered
into a joint venture agreement with Claude Resources Inc.
Geology and mineralization
The area underlying the Seabee deposit consists of hornblende gabbro intruded
into metasediments and metavolcanic rocks. Mineralization occurs in a series of
0701-trending narrow shear zones which parallel the main foliation in the gabbro.
A conjugate set of shear zones trend at 1401–150 1. Both sets of shear zones host the
auriferous quartz-tourmaline that forms the deposit. The pyrite, pyrrhotite and
associated Au mineralization appear to postdate deformation and may be associated
with high-grade metamorphism. Ultramafic-layered lenses lie at the base of some of
the gabbroic sheets that host the deposit.
The vein mineralization consists of fine-grained Au-bearing pyrite and chalco-
pyrite and minor visible Au. Other minerals present are sphalerite, pyrrhotite, minor
arsenopyrite, pentlandite, tellurides, magnetite, barite and molybdenite in a matrix
dominated by sericite and actinolite-tremolite.
TABLE 11-II
Jasper Pond. Concentrations in ash of Labrador tea twigs from nine sites in a bog adjacent to
the discovery outcrop of Au mineralization at Jasper Pond (Dunn et al., 1990)
Range Average Background Concentration factor
(average:background)
Au (ppb) 38–389 129 20 6.5
As (ppm) 2–4 3 2 1.5
Co (ppm) 5–65 25 5 5
Cr (ppm) 12–37 25 10 2.5
Th (ppm) 1–3.6 1.8 0.2 9
U (ppm) o0.1–3.3 1.4 o0.1 >15
W (ppm) o1–22 7.3 o1>7
347Biogeochemistry in Mineral Exploration
Biogeochemical survey
In 1986, an orientation survey was undertaken along a shear zone with the
purpose of assessing any biogeoch emical response. Thirty-eight samples were col-
lected from 18 sites across and along the V2 shear zone, which forms a well-drained
northwesterly facing ridge. The most common species in the area is black spruce.
Outer bark scales were collected a t all sample stations. Spruce twigs (most recent
10 years of growth) were obtained from the 12 sites where branches could be reached.
In addition a few samples of jack pine outer bark and 3-year growth of alder and
birch were collected.
Figure 11-1 shows strongly anomalous Au concentrations in both spruce bark
and twigs (330 ppb and 750 ppb Au in ash [6.5 and 15 ppb Au DW]) at Zone 14, and
elevated levels at several additional sites. Of note is that several elements concen-
trated in vegetation at other Au de posits in the La Ronge Belt are also concentrated
at Seabee. Examples are, with dry weight equivalent in parentheses, 450 (9) ppm Mo
in alder twigs, 6 6 (1.3) ppm Co in birch twigs, 3 (0.06) ppm W in spruce bark and a
feature noted in Chapter 9 in the section on Nd is that high Nd values flank the Au
anomaly.
The biogeochemical signatures indicated that near the mineralized veins in the
Laonil Lake area there are enrichments of Au, Co, Mo and W with what appears
to be some chemical differentiati on of the REE giving rise to Nd enrichment flanking
elevated Au concentrations. The brief study concluded that multi-element biogeo-
chemical surveys in this area may assist in outlining mineralized zones, and could
help in differentiating between auriferous and barren shear zones (Dunn, 1986c).
Post survey developments
Subsequent to this study, exploration programmes that included extensive drilling
to outline resources resulted in the delineation of substantial Au reserves. Claude
Laonil Lake
99
110
12
26
7
<5
22
330 /
750
45
71/
17
22
6
<5
ZONE14
ZONE 2
200 m
Metavolcanics
GOLD ppb in ash
Spruce Bark / Twigs
Fig. 11-1. Gold in ash of black spruce bark and twigs. Seabee Mine, northern Saskatchewan.
348 Case Histories
Resources took over development of the site in the late 1980s and the Seabee mine
that opened in 1991 is still in operation. Early in 2005 reported reserves were 730,000
tons grading 0.20 oz/t (6.9 g/t) and a resource of 406,200 tons grading 0.24 oz/t
(8.16 g/t) Au.
Canada: Temperate Forest of British Columbia
QR (Quesnel River) Deposit
Geology and mineralization
Propylitic gold skarns are contained within a 300 m-wide hornfelsed aureole
around a diorite stock. Gold occurs in and along the contact between a calcareous
basalt and overlying siltstones and argillites. Gold occurs in five known zones in
association with pyrite and pyrrhotite. There is minor chalcopyrite, sphalerite and
galena. A detailed account of the geology and soil geochemistry of the area is given
by Fox et al., 1987.
Environment
The QR deposit is locate d in central British Columbia 140 km southeast of Prince
George, adjacent to the steep banks of the Quesnel River. Outcrop is mostly masked
by glacial tills of variable thickness, and the extensive forest cover is dominated by
interior Douglas-fir and lodgepole pine.
Scope of survey and analysis
In 1988, a helicopter-borne treetop-sampling programme was undertaken by the
Geological Survey of Canada over the QR Au deposit. At the time of sampling there
was little ground disturbance. Douglas-fir tops were collected during a 2-h period in
late April, at intervals of approximately 200 m (100 m on one line) along 12 lines
spaced 200 m apart. The length of line varied from 500 to 2300 m. In total 103
samples were collected, including a few from distant areas to determine background
levels of elements. After drying and separation of needles from stems, the stems were
reduced to ash and the ash analysed by INA.
Results
A substa ntial and intense Au anomaly in the treetops was outlined (Dunn and
Scagel, 1989). Subsequen tly, the Main and West Zones were mined by Kinross Gold
Corporation between 1994 and 1998. During that period the mine produced
118,000 oz of Au (approximately 30,000 ounces per year) from 1,060,000 tonnes of
ore milled at an average grade of 4.1 g/tonne Au.
In 2005, Douglas-fir needles from the samples that were co llected in 1988 were
retrieved from archive storage. Dry needles were milled to a powder, 1 g portions
349
Biogeochemistry in Mineral Exploration
dissolved in nitr ic acid followed by aqua regia and analysed by ICP-MS (Dun n et al.,
2006a). Results confirmed and refined the original anomalies identified from analysis
of stems validating the robustness of the geochemical signature (Figure 11-2).
In 2006, Cross Lake Minerals announced that they were restarting the mine after
significant new reserves were discovered at the North Zone (possibly 200,000 oz Au).
The North Zone is a broad area of propylitically altered basalt located below
the Main Zone in the footwall of Wally’s Fault. This is the largest zone of gold
mineralization on the property with a drill indicated strike length of at least 1 km.
Average grade for this deposit is estimated at approximately 6 g/tonne Au.
South America: Peru-Ecuador border
Cordillera del Co
´
ndor
Geology and mineralization
Host rocks comprise
Triassic to Jurassic marine clastic and carbonate rocks, continental red-beds and
volcanic units of the Piuntza and Chapiza Format ions.
0
1
2
3
4
5
Au ppb
Max 10.9
QR - Au Mineralization
GOLD in Dry Douglas-fir Needles (Treetops)
0 500 1000
metres
Mineralization
Sample Site
West Zone
Main Zone
North Zo
ne
Quesnel River
Fault
Wally's Fault
Fig. 11-2. Gold in dry Douglas-fir needles from QR Deposit, central British Columbia.
350 Case Histories
Felsic to intermediate intrusions (Jurassic).
Sub-aerial volcanics of the Chinapintza Fm (Cretaceous and Tertiary) with coeval
felsic to intermediate stocks. The lowermost strata along the Nangaritza River are
Cretaceous shales with abundant fossil ammonites, overlain by karst limestone.
Sandstones form flat-topped ridges an d table -mountains.
Known styles of mineralizatio n include the following:
Porphyry Cu+/Mo+/Au associated with the Zamora batholith.
Garnet-scapolite skarn.
Epithermal carbonate, base metal-sulphide veins and breccias.
Epithermal Au, Hg, As and Bi mineralization at Chinapintza.
Low sulphide, low sulphidation epithermal veins north of Chinapintza.
Environment
The Cordillera del Co
´
ndor forms the border between Ecuador and Peru and
extends from 31 to 51 south of the equator. Located in the eastern foothills of the
Andes at elevations from 800 to 2100 m, it comprises the upper reaches of the rain-
forested Amazon basin and experiences wet, cool, tropical weather. Dense sub-trop-
ical vegetation grows in the rugged terrain that is deeply incised by streams. Except
for access from the Ecuadorian side of the border, roads are virtually non-existent.
Vegetation cover has been described as ‘perhaps the richest flora of any similar-sized
area anywhere in the New World’ (Schulenberg and Awbrey, 1997) and it has one of
the highest concentrations of vascular plant species of any place on earth.
Based on results obtained from orientation surveys in a similar environment, the
selected sample media were foliage from the commonly occurring tree fern Cyathea
spp. (local name ‘Chonta con Espinas’) and foliage of the evergreen liana Clusia
cf. hammeliana (local name ‘Churgun’).
Scope of survey and analysis
At each of 366 sample stations, samples of fern fronds and liana leaves were
collected at intervals of 500 m on an evenly spaced square grid covering approximately
100 km
2
. Samples were oven-dried in Lima and shipped to Canada for milling. Coarse
ribbed veins from both ferns and leaves were removed, and the leafy tissues milled to a
fine powder. Analysis was by ICP-MS for 37 elements after a nitric acid/aqua regia
digestion. Appropriate QC samples were inserted that included standards, duplicates
of field samples, splits of prepared samples and analytical solutions. Data were sta-
tistically evaluated and kriged for plotting as gradational coloured contour maps. The
98th percentile was selected as the maximum value for plotting, so that outliers did not
distort the data distribution patterns, and the structure of the bulk of the dataset could
be observed.
With few exceptions, the ferns yielded higher concentrations of elements than the
liana foliage. Table 11-III provides a comparison of the mean, median and maximum
351
Biogeochemistry in Mineral Exploration