Porphyry indicator minerals (PIMS) and porphyry vectoring and fertility tools (PVFTS) Pete Hollings, David R Cooke, Paul Agnew, Michael Baker, Zhaoshan Chang, Jamie J. Wilkinson, Noel C. White, Lejun Zhang, Jennifer Thompson, Ayesha Ahmed, J. Bruce Gemmell, Nathan Fox, Huayong Chen, Clara Wilkinson – Indicators of mineralization styles and recorders of hypogene geochemical dispersion halos Epidote – albite alteration in McLeod Hill quartz monzodiorite, Yerington, Nevada
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Porphyry indicator minerals (PIMS) and porphyry vectoring and fertility tools (PVFTS)
Pete Hollings, David R Cooke, Paul Agnew, Michael Baker, Zhaoshan Chang, Jamie J. Wilkinson, Noel C. White, Lejun Zhang, Jennifer Thompson,
Ayesha Ahmed, J. Bruce Gemmell, Nathan Fox, Huayong Chen, Clara Wilkinson
– Indicators of mineralization styles and recorders of hypogene geochemical dispersion halos
Epidote – albite alteration in McLeod Hill quartz monzodiorite,
Zircon – A porphyry indicator mineralMagmatic oxidation state, water content, degree of fractionation
• Barren Paleozoic granitoids in the Lachlan Fold belt, Australia, have low Ce4+/Ce3+ ratios (Belousova et al., 2006)
Shen et al. (2015)
Ce4+/Ce3
+
Cu (Mt)
300
250
150
100
0
200
50
0 2 4 6 8 10 12 14
Zircon Eu/Eu* and Ce4+/Ce3+ anomalies – a product of titanite fractionation? (Loader et al., 2017)
• Loader et al. (2017) showed that small amounts of titanitecrystallisation can produce zircon Eu/Eu* and Ce4+/Ce3+ anomalies
• Titanite fractionation will deplete REE from the melt
• MREE are more depleted than LREE or HREE during this process
• Sm and Gd more depleted than Eu
• This process could produce false positives for porphyry explorers applying zircon as a PIM in regional exploration
0,1
1
10
100
1000
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
mel
t/cho
ndrit
e
% titanite crystallisation
Loader et al. (2017)
Hornblende geochemistry – petrogenesis and fertility
• high Si, low Al, Ca and alkalis group in intermediate‐felsic rocks ‐shallow crystallization
• high alkali, Al and Ca, low Si group in intermediate‐mafic rocks ‐ crystallising at deeper crustal levels.
Hollings et al. (2013)
Cao et al. (2018)
• Zoning suggests mixing of the two magmas
• Presence of both groups in a single sample indicates interaction and phenocryst exchange between the two parental magmas
Porphyry indicator minerals – Plagioclase (Williamson et al. 2016)
Cu in plagioclase (ppm)
Sr/Y in plagioclase
Excess Al
Anorthite %
An % Excess Al
Cu (ppm)Sr/Y
Excess AlAn %
Position along the LA‐ICP‐MS point traverseA B
A
B
A
B
Williamson et al. (2016)
• Plagioclase from fertile porphyry systems contains ‘excess’ Al related to high melt water contents
• It may record injections of hydrous fluid or fluid‐rich melts into the sub‐porphyry magma chamber
• Excess Al may exclude copper from plagioclase, enriching the remaining melts in Cu
Porphyry indicator minerals – Magnetite
• Magnetite is resistive and easily separated, making it an attractive PIM candidate
• Two decades of research have shown that major and trace element ratios can effectively discriminate magmatic and hydrothermal magnetite from a diversity of ore deposit types
• Fine exsolution lamellae can affect magnetite LA‐ICP‐MS analyses• Magnetite is prone to diffusional resetting by post‐crystallization
hydrothermal fluids – care must be taken in data interpretation
Al/Ti
0.01
10
1.0
0.1
100
0.001 0.01 0.1 1.0 10 100
MagmaticHydrothermal
V/Ti
Cross (2000)
Dare et al. (2014)
Ilmenite exsolution lamellae in magnetite, Grasberg50 µm
Discrimination of porphyry magnetite
• Existing deposit type discrimination does not work for porphyries with new data
• Porphyry results plot from Fe‐Ti‐V deposits across into Kiruna and IOCG fields
Diagram: Dupuis and Beaudoin (2011); Data from Sievwright (2017)Recrystallised hydrothermal
magnetite vein
Sievwright (2017)
Magnetite alteration association
• Hydrothermal magnetite derived from different porphyry alteration domains can be discriminated
• DP1 is mainly controlled by Co+ Mg‐ and Al‐• DP2 is mainly controlled by V+ Co‐ and Mg‐
Colour
Weak Propylitic
Unaltered
Propylitic
Potassic-Propylitic
Potassic-Phyllic
Potassic
Phyllic
Argillic-intermediateAlteration
Sievwright (2017) Magnetite in chalcopyrite
Porphyry indicator minerals – Apatite
• Apatite chemistry and luminescence discriminates magmatic and hydrothermal apatites from different porphyry alteration zones (Bouzari et al., 2016; Loader, 2017)
• Discriminant projection analyses can distinguish apatite from magmatic and a variety of hydrothermal environments, including porphyries (Mao et al., 2016)
• Porphyry apatite • Low Mg, Dy, Pb, U • High Mn, Y, Ce, Eu, Yb, Th
Igneous apatite potential
• Redox sensitivity of apatite chemistry (Mn & V)• No fertility discrimination but broad separation of porphyry types• Does not take into account complex apatite paragenesis
AlkalicCu-Au
OxidisedCu-Mo(-Au)
Low fO2Cu-Mo/Au
Rukhlov et al. (2017)
Miles et al. (2012)
Geochemical footprints of porphyry depositsPorphyry fertility and vectoring tools (PFVTS)
• Subtle, low‐level hypogene geochemical signals are preserved in hydrothermal alteration minerals distal to porphyry deposits
• Analysis of these alteration minerals can potentially provide explorers with both fertility and vectoring information
• They allow the presence, location and significance of porphyry and epithermal deposits to be assessed during the early stages of exploration
• This can potentially be achieved with remarkably low‐density sampling and very low cost relative to most other available search technologies
Propylitic alteration: a distal indicator of porphyry Cu deposits
2-6km
Slide courtesy of Paul Agnew
Modified after Holliday and Cooke (2007)
2-6 km
AMIRA International’s footprints research program (2004 – 2018+)P765 (2004 – 2006)
Transitions and zoning in porphyry ‐ epithermal districts:Indicators, Discriminators and Vectors
P765A (2008 – 2010)Geochemical and geological halos in green rocks and lithocaps:The explorer’s toolbox for porphyry and epithermal districts
P1060 (2011 – 2014)Enhanced geochemical targeting in magmatic‐hydrothermal systems
P1153 (2015 – 2018)Applying the explorers’ toolbox to discover porphyry and epithermal
Cu, Au and Mo deposits
Three major questions being addressed:
1. Fertility: Can we detect the presence of well‐endowed systems – how large?
2. Vectoring: • How far to the ore zone?
3. Vectoring: • In what direction?
P1202 (2018 – 2021)Far‐field and near‐mine footprints: finding and defining the next
generation of Tier 1 ore deposits
Porphyry footprints – Arsenic in epidoteFertility indicator – Baguio district, Philippines (Cooke et al., 2014)
loreplacement epidote vein epidote skarn epidote whole rock
Mexico skarn prospectGeochemical anomaly
Black MtSmall porphyry Cu‐Au
Nugget HillLarge porphyry Cu‐Au
A
B
B’
C
Size of symbols proportional to ppm; Maximum symbol size = 84 ppm
No Sr depletion
1000 mBase map modified from Garwin (2000)
Sr in chlorite (sample mean values)
0.1
1.0
10.0
100.0
1000.0
10000.0
500 1500 2500 3500 4500 5500
W traverseSW traverse (original)SW traverse 2010
Bambu
2009 traverse north
2009 traverse southWhole rock
Ti/Sr
Porphyry footprints – Ti/Sr in chloriteVectoring tool – Batu Hijau, Indonesia (Wilkinson et al., 2015)
• Chlorite trace element ratios provide vectors to the mineralized centre of Batu Hijauwithin 2.5 km (potentially up to 5 km for some trace elements) • Trace element substitution into the chlorite crystal
lattice is strongly controlled by temperature
Distance to centre (m)Distance = ln {[Ti/Sr]/3x10‐6 } ‐0.0088Batu HijauBatu Hijau
Green rock vectoring – Example from Resolution, Arizona, USA
• Provided as green rock blind test site to P765A by Rio Tinto
• Porphyry Cu‐Mo deposit with total inferred resource of 1.624 Gt at 1.47% Cu and 0.037% Mo
Data from Resolution Copper and Rio Tinto websiteshttp://www.resolutioncopper.com; http://www.riotinto.com
Reproduced from Hehnke et al. (2012)
Rio Tinto blind site – plan viewRio Tinto blind site – plan view10036354 10036354
• Grid sampling of 4 x 1.5 km area (27 samples)• One outlier collected 4 km away with distinctive features
TGR‐25‐ this sample does not contain pyrite‐ the epidote has anomalous Pb, low As and Sb‐ Metamorphic epidote in Devonian cover –unrelated to Ordovician porphyry deposit
1 km
27 samples with porphyry‐related epidote – chlorite alteration
Conclusions• There are several magmatic and hydrothermal minerals that show considerable potential as PIMS and/or PVFTS
• Access to LA‐ICP‐MS technology is mostly through university laboratories – this needs to change for widespread uptake
• Contribution to a major porphyry discovery would help to validate these approaches and to facilitate their widespread acceptance as geochemical exploration techniques
• We predict that some of these are likely to become routine tools used by explorers over the next decade
• New and emerging technologies need to be embraced by industry if geochemistry is to maintain a critical role in the discovery of new resources over the next decade