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Distinguishing between barren and fertile batholiths using zircon trace element geochemistry

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Author A Ahmed, D R Cooke and A Kobussen

Description

*This is an abstract only. No full paper is available for
this abstract.*


Porphyry-Cu deposits (PCDs) are typically associated with evolved, hydrous, oxidised magmas (Richards, 2009; Dilles et al, 2015). A number of key trace element ratios including Eu/Eu*, Ce/Nd, and (10 000* Eu/Eu*)/Y have been developed as proxies for the oxidation state and water-content of a magma, and can be used to distinguish barren batholiths from fertile batholiths that generate PCDs (Ballard et al, 2002; Dilles et al, 2015; Lu et al, 2016).

In this study, 14 samples from four different igneous units of the Yerington Mining district, Nevada (Figure 1) were analysed for 33 elements using LA-ICPMS. The Yerington mining district occurs in west-central Nevada, United States. The Yerington batholith comprises the McLeod Hill (169.4 ± 0.4 Ma; Dilles and Wright, 1988), Bear (169.6 ± 1.3; Banik, 2015), and Luhr Hill (168.5 ± 0.4 Ma; Dilles and Wright, 1988) intrusions. The Luhr Hill granite is associated with multiple Cu-porphyry and Cu-skarn deposits in the district (Harris and Einaudi, 1982; Dilles, 1987). The Shamrock batholith is a post-mineralisation intrusion (165.8 ± 0.4 Ma) located immediately south of the Yerington batholith (Knopf, 1918; Harris and Einaudi, 1982; Dilles, 1987) and is not associated with any known mineralisation (Harris and Einaudi, 1982).

Figure 2a shows that the average Eu anomaly in zircons from the mineralised Luhr Hill granite is smaller than the Eu anomaly of zircons in either the pre-mineralisation McLeod Hill and Bear intrusions, or the post-mineralisation Shamrock intrusion. This result indicates that the Luhr Hill intrusion was most oxidised compared to unmineralised intrusions. The relatively high oxidation state of the Luhr Hill intrusion is supported by other element proxy data shown in Figure 2b. Zircons from the Shamrock batholith returned (10 000* Eu/Eu*)/Y values less than 2, whereas samples from the Yerington batholith returned values greater than 2. Values for (Ce/Nd)/Y are also highest in zircons from the Luhr Hill intrusion, indicative of a more hydrous magma. Figure 2c shows that Dy/Yb values are highest in zircons from the Shamrock batholith indicating that this intrusion experienced the lowest degree of hornblende fractionation of all the intrusions. High degrees of hornblende fractionation are typically associated with evolved, hydrous magmas and PCD formation (Richards, 2002; Lu et al, 2016).

Results from a 14 variable discriminant projection analysis (a type of statistical analysis that allows the visualisation of data in multivariate space) are shown in Figure 2d. Discriminant projection 1 (DP1) separates zircon populations of the Yerington and Shamrock batholiths. Zircons from the Shamrock batholith contain elevated Ho, La, Pr, Ta, Tb, and Tm values. Zircons from the Yerington batholith contain elevated Er, Eu, Gd, Lu, Sm, and Yb values.

Zircon trace element geochemistry is a valuable, relatively inexpensive tool for early-stage exploration that can help the explorer to focus efforts on fertile search spaces, in areas that contain multiple igneous complexes of similar age with similar whole rock chemistry. In the brownfields exploration environment, this method could prove useful in determining the paragenetic sequence and ore-forming potential of intrusions on a mine-scale.

CITATION:

Ahmed, A, Cooke, D R and Kobussen, A, 2017. Distinguishing
between barren and fertile batholiths using zircon trace element geochemistry,
in Proceedings Tenth International Mining Geology Conference 2017, pp 443–446
(The Australasian Institute of Mining and Metallurgy: Melbourne).