www.dmirs.wa.gov.au

FEBUARY 2019

Geological Survey ofWestern Australia

Government of Western AustraliaDepartment of Mines, Industry Regulationand Safety

A NEW LOOK AT LAMPROPHYRES AND SANUKITOIDS, AND THEIR RELATIONSHIP TO THEBLACK FLAG GROUP AND GOLD PROSPECTIVITY

EASTERN GOLDFIELDS

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Inset Fig. 1b

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SOUTH WESTTERRANE

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Lamprophyre and Sanukitoid Black Flag Group Regional felsic volcanic rocks

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hydrated and enriched mantle lithosphere

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MenziesMenzies

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MenziesMenzies

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NorsemanNorseman

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KalgoorlieKalgoorlieCoolgardieCoolgardie

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Mount FisherMount Fisher

123°122°121°120°

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WilunaWiluna

MenziesMenzies

LeonoraLeonora

NorsemanNorseman

KambaldaKambalda

KalgoorlieKalgoorlieCoolgardieCoolgardie

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Mount FisherMount Fisher

123°122°121°120°

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The relative contribution that crustal and magmatic sources have made to the Archean gold endowment of the Eastern Goldfields Superterrane (EGST) of the Yilgarn Craton has been debated for several decades without any ensuing, clear, consensus. From an empirical perspective at least, several specific intrusive magma types have been directly linked to gold mineralization. These include lamprophyres and high-Mg dioritic to granodioritic magmas (i.e. sanukitoids) derived from metasomatized lithospheric mantle.

A significant increase in the amount of high-quality lithogeochemical data from volcanic and subvolcanic rocks of the EGST allows a robust assessment of links between calc-alkaline lamprophyric intrusions and sanukitoid intrusions, links between these and felsic volcanic rocks, including the Black Flag Group, and links between all of these and gold prospectivity. To do this, we use an extract of volcanic and subvolcanic rocks, with broadly contemporaneous crystallization ages between 2.69 and 2.64 Ga, from the larger geochemical dataset currently being accumulated as part of the Eastern Goldfields greenstone geochemical barcoding project, an initiative under the Exploration Incentive Scheme (see geochemical barcoding poster).

The Black Flag Group and regional felsic volcanic dataWhen we compare geochemical data for the Black Flag Group with data for regional felsic volcanic rocks (Figs 5,6), including the volcanic equivalents of High-Ca, Low-Ca and HFSE granites, two startling observations are:· the Black Flag Group is compositionally distinct from all other regional

datasets· the Black Flag Group overlaps and extends the geochemical trends

defined by the EGST lamprophyres and sanukitoids

Figure 1. Location of samples within the EGST: a) terrane subdivision of the Yilgarn Craton; b) aeromagnetic image of the EGST showing the locations of samples used for this study. Note that many sites represent the location of a diamond drill core that commonly yielded several samples

Figure 2. Variation of Mg# with MgO (anhydrous) for spatially associated lamprophyre and sanukitoid from two diamond drillcores taken from the Kambalda region, with fields denoting: a) individual drillcore and b) lithology. In both cases, when spatially associated magmas are independently assessed, lamprophyric magmas have more primitive compositions than the associated sanukitoid and clearly permit a direct genetic relationship

Figure 3. Variation in initial Nd isotope ratio with SiO2

(anhydrous) for lamprophyre, sanukitoid, igneous rocks of the Black Flag Group and samples of broadly contemporaneous ‘regional felsic volcanic rocks’. The Black Flag Group has primitive Nd isotopic compositions identical to those of lamprophyre and sanukitoid, consistent with continuous geochemical arrays that together strongly support a genetic relationship

Figure 4. Cognate hornblende-cumulate xenolith in a sanukitoid (CD16068) representing the expected fractionation assemblage

Figure 7. Outline of the EGST showing the locations of: a) lamprophyre and sanukitoid; b) samples of the Black Flag Group and regional compositional equivalents; c) felsic volcanic rocks and sub-volcanic equivalents compositionally dissimilar to the Black Flag Group. Note that many sites represent the location of a diamond drillhole, cores from which commonly produced several samples. Hence, a single point (or symbol) may represent several samples

TimingAu mineralization 2.66 – 2.64 Ga

Lamprophyres, sanukitoids 2.68 – 2.64 Ga

Mafic granites 2.67 – 2.65 Ga

Black Flag Group 2.68 – 2.66 Ga

Figure 8. Block model of an Archean lithospheric section schematically showing melting of hydrated mantle lithosphere yielding dykes or plutons of lamprophyric magma, which fractionate and exsolve voluminous highly reactive volatiles as they ascend along a pre-existing trans-lithospheric fracture. In this long-lived system, earlier exsolved volatiles and renewed magma inputs cyclically interact (i.e. multiple generations) maximizing the upward flux of original ‘magmatic’ and ‘scavenged’ metals

For more information, contact:Hugh Smithies (hugh.smithies@dmirs.wa.gov.au)

Definitions

Lamprophyre: a highly variable group of mafic to ultramafic intrusive rocks enriched in alkalis, incompatible trace elements and volatiles and, at the subvolcanic level where they are typically recognized, biotite and/or amphibole macrocrysts. In many Archean terranes, the close spatial and temporal link between gold mineralization and lamprophyres is specifically with ‘calc-alkaline’ lamprophyres. On mantle-normalized incompatible trace element diagrams, calc-alkaline lamprophyres have highly fractionated rare-earth element (REE) patterns (i.e. strongly elevated light-REE concentrations), strongly elevated large ion lithophile element (LILE; Sr, Ba, Rb, Pb) concentrations but significant relative depletions (i.e. negative anomalies) in high field strength elements (HFSE: Nb, Ta, Zr, Hf, Ti).

# 2+The Mg [i.e. Mg / (Mg + Fe )] ranges to high values reflecting equilibration with mantle peridotite (i.e. >60), and Cr and Ni concentrations are correspondingly high. Many Archean calc-alkaline l a m p r o p h y r e s a r e d o m i n a n t l y h o r n b l e n d e - p o r p h y r i t i c (hornblende>biotite) and strongly resemble the Caledonian ‘appinites’ of western Scotland.

Sanukitoid: a range of typically medium-grained, intrusive, hornblende ±clinpyroxene-bearing monzodioritic, dioritic to granodioritic rocks

#characterized by high Mg and elevated concentrations of Ni, Cr and LILE. An Archean sanukitoid series should include members that, at 60 wt% SiO2, have Mg# of at least 60, Cr and Ni concentration each >100 ppm and Sr and Ba concentrations each >1000 ppm. Volcanic equivalents are high-Mg andesite and low SiO2 adakite.

The paradox with both calc-alkaline lamprophyre and sanukitoids is that compositional features strongly implicating a peridotitic source (i.e. high

#,Mg Cr and Ni) co-exist with strong enrichments in ‘crustal’ components (e.g. LILE, Th, U etc.). Thus, both are generally thought to reflect low-degree partial melts of mantle (peridotite) previously metasomatized through addition of subducted slab components. As with shoshonite, their petrogenesis has been linked to post-subduction destabilization of metasomatized mantle lithosphere. However, sanukitoid is not believed to have any direct genetic relationship to contemporaneous mafic or ultramafic magmatism — including the commonly temporally and spatially associated calc-alkaline lamprophyre. This notion stems from studies from Canadian Archean terranes that showed many sanukitoids to be more primitive than members of regional lamprophyre suites. This potentially gives sanukitoid a unique petrological status as the only felsic magma directly extracted from a mantle source.

Are calc-alkaline lamprophyre and sanukitoid really genetically unrelated?

Despite earlier studies discounting this possibility, Perring and Rock (1991: Precambrian Research v. 52, p. 245–273) described intricate relationships and textural transitions leading them to infer that sanukitoid was indeed produced through hornblende fractionation of

Other occurrences regionally

What little felsic volcanic data we have from other regions identifies other occurrences of Black Flag Group-like rocks. Most Mafic granites, the smallest of the four main Yilgarn Craton granite types identified by Champion and Sheraton (1997: Precambrian Research, v. 83, p. 109–132), also have the same compositional characteristics and represent larger sanukitoid intrusions.

Figure 6. Variation in Mg#, Cr, Sr and Nb with SiO2 (anhydrous) for lamprophyre and sanukitoid, igneous rocks of the Black Flag Group and samples of broadly contemporaneous ‘regional felsic volcanic rocks’. To aid comparisons with the field for rocks of the Black Flag Group, the right hand panel shows only the regional felsic volcanic rocks. In the Mg# and Cr plots, sanukitoid should mainly lie above the solid line and should also be characterized by high Sr concentrations. Importantly, most rocks of the Black Flag Group lie above the line in the plots for Mg# and Cr, whereas most of the regional felsic volcanic rocks lie below

Figure 5. Variation in a) Nb with P2O5 (anhydrous) and b) La/Nb with SiO2 (anhydrous) for lamprophyre, sanukitoid, igneous rocks of the Black Flag Group and samples of broadly contemporaneous ‘regional felsic volcanic rocks’

Gold prospectivity

The observation that gold mineralization and lamprophyre–sanukitoid (Black Flag Group) magmatism are related in time and space does not prove a genetic relationship since both might simply follow the same structural pathways. At the very least, the origins of the lamprophyre–sanukitoid (Black Flag Group) magmas can be traced back to a metasomatized lithospheric source and so the occurrence of such magmas indicates proximity to a translithospheric structure.

The significance of the suggestion that sanukitoid forms from lamprophyre and further evolves to the dacitic compositions of the Black Flag Group is that mineralogical and geochemical evidence shows that this magmatism is also amongst the most volatile-rich and oxidized Archean magmatism known. Intrusion and fractionation of wet hornblende-bearing lamprophyric magma involves exsolving large volumes of oxidized fluids, which are subsequently also channelled along trans-lithospheric pathways.

Even if such magmas and fluids are not initially intrinsically gold rich, they likely scavenge a significant metal cargo as they ascend through the crustal greenstone sequences. Additionally, even if such a process seldom produced primary gold mineralization, it may have represented a critical enrichment process along long-lived fluid pathways. The extraordinary gold endowment of the areas within and peripheral to the Black Flag Group might indicate that these very shallow systems reflect the most favourable crustal level in terms of (magmatic) gold enr i chment . A l te rna t i ve ly, the ex t raord ina ry vo lume o f lamprophyre–sanukitoid magmatism in that region might reflect either an extremely efficient trans-lithospheric fluid pathway or a particularly volatile-rich and fertile lithospheric mantle source, or a combination of all these factors.

lamprophyre magmas. Our new geochemical and isotopic data (Figs 2,3) are in total agreement with this suggestion — when only data that share a clear spatial and temporal relationships are considered, all evidence suggests sanukitoid can be derived from calc-alkaline lamprophyric magmas through hornblende (+apatite) fractionation.

Most samples of the Black Flag Group represent eruptive or near eruptive equivalents of sanukitoid. They are the highest level in a long-lived, translithospheric, lamprophyre–sanukitoid magmatic system.