Trace elements in magnetite from porphyry Cu–Mo–Au deposits in British Columbia, Canada

Abstract

27This study examines trace elements in hydrothermal magnetite from five porphyry Cu-Mo-28Au deposits and two skarns in British Columbia, Canada. Trace element concentrations 29 vary several orders of magnitude both within and between magnetite from skarn and 30 porphyry deposit settings. The heterogeneous composition of hydrothermal magnetite may 31 in part be due to the short duration, low temperature and multiple-fluid events that attend 32 the formation of porphyry ore deposits. Principal component analysis shows two dominant 33 patterns of trace element abundances in hydrothermal magnetite. Firstly, positive 34 correlations of Ti, Al and V, which account for nearly 40% of the total variation in 35 magnetite, are inferred to depend on temperature and oxygen fugacity. Secondly, antithetic 36 abundances of lower valence cations (Co, Mn) with higher valence cations (Sn and Mo) 37 may reflect variations in the redox potential, acidity and metal speciation of hydrothermal 38 fluids. The Cu/Fe and Mn/Fe ratios calculated for fluids in equilibrium with the 39 hydrothermal magnetite using experimental partitioning data are similar to those measured 40 directly in brines trapped in quartz-hosted fluid inclusions from porphyry Cu-Mo-Au 41 deposits. 42 43 44 45 46 3

Magnetite as an Indicator Mineral in the Exploration of Porphyry Deposits: A Case Study in Till near the Mount Polley Cu-Au Deposit, British Columbia, Canada

“…In five porphyry and skarn deposits of British Columbia, sphenitization was observed petrographically in very few samples. The chemistry of magnetite from Mount Polley (< 0.3 wt.% Si, < 0.05 wt.% Ca) confirms that this is not a characteristic of the deposit (Canil et al, 2016). In fact, greater degrees of sphenitization in magnetite in till samples were observed at greater distances down-ice or away from the Mount Polley deposit.…”

“…In contrast, hydrothermal magnetite typically contains substantially lower Ti and Al than magnetite in igneous rocks ( Fig. 8A - Dupuis & Beaudoin, 2011;Dare et al, 2014;Nadoll et al, 2014;Canil et al, 2016).…”

“…Approximately 100-125 grains per till sample were randomly selected, mounted in epoxy and polished for further study . The composition of magnetite from both mineralized and barren rocks in the Mount Polley deposit were previously investigated by Canil et al (2016).…”

“…Magnetite (Fe 3 O 4 ) is a member of the spinel group of minerals that are recognized for their systematic variations in composition as a function of temperature, cooling rate, fO 2 , or silica activity (Buddington & Lindsley, 1964;Irvine, 1965;Sack & Ghiorso, 1991;Roeder, 1994;Barnes & Roeder, 2001;Dare et al, 2014). Several studies suggest that variation in minor and trace element contents in magnetite is systematically related to the type of mineralizing environment, including volcanogenic massive sulphide (VMS), skarn, Fe-oxide-Cu-Au (IOCG) and porphyry Cu(-Mo-Au) (McQueen & Cross, 1998;Kamvong et al, 2007;Dupuis & Beaudoin, 2011;Nadoll et al, 2012Nadoll et al, , 2014Nadoll et al, , 2015Huang et al, 2014;Dare et al, 2012;Chen et al, 2015;Makvandi et al, 2015;Canil et al, 2016).…”

“…used data from a variety of hydrothermal ore deposits and igneous rocks to broadly distinguish hydrothermal from igneous magnetite using Ti, Ni and Cr. Canil et al (2016) used principal components analysis on hydrothermal magnetite from porphyry Cu(-Au-Mo) and skarn deposits to recognize positive correlations of Al, Ti and V related to temperature, and negative correlations of Sn and Mo, with Mn and Co, governed by fluid chemistry. Few studies have examined magnetite as an indicator in till proximal to an ore deposit (McMartin et al, 2011;Sappin et al, 2014).…”