Apatite chemistry of orogenic gold mineralised samples from the Fosterville deposit, central Victoria, southeastern Australia
Description
The chemistry of hydrothermal apatite that form during orogenic gold mineralisation can assist in understanding hydrothermal fluid chemistry and mineralisation processes. Mineralisation at the Fosterville deposit and Lockington East prospect, hosted in Ordovician turbidite sequences of the Castlemaine Group, that lie within the gold rich Bendigo Zone, of south-eastern Australia, is investigated using combined geochemistry and U-Pb geochronology of apatite grains. The chemistry of hydrothermal apatite that form during orogenic gold mineralisation, not only can provide understanding hydrothermal fluid chemistry and mineralisation processes, but can provide chemical criteria for exploration. Laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS) geochemical data for 262 apatites from four samples k-means clustered into seven groups. A total of 35 elements (excluding Ca) are presented Na, Mg, Al, Si, P, Cl, K, Ti, V, Mn, As, Sr, Y, Zr, La, Ce , Pr, Nd, Fe, Ba, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Hg, Pb, Th, U. A total of 6 isotopes are presented 204Pb, 206Pb, 207Pb, 208Pb, 232Th, 238U. Additionally QAQC data of reference apatites of 401 and McClure is also presented.
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Apatite geochemistry and U-Pb geochronology data was collected simultaneously using an Agilent Triple Quadropole 8900X Inductively-Coupled Plasma-Mass Spectrometry coupled to a RESOlution 193 nm laser (LA-ICP-MS) at Adelaide Microscopy, University of Adelaide, South Australia. A series of standards were routinely measured throughout the analytical session in a standard bracketing procedure. Iolite (v. 4.0) was used to reduce raw data including corrections for baseline, instrumental drift and down-hole fractionation (Paton et al. 2011). Apatite geochemical data was calibrated and instrumental drift was corrected using the NIST 610 glass primary standard (Chew, Petrus & Kamber 2014). Geochronological data (202Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th and 238U) was calibrated using a Madagascar (MAD) standard (206Pb/238U = 473.46 Ma; Thomson et al. 2012). Geochronological data was reduced by using the ‘VizualAge_UcomPbine’ DRS in Iolite to enable a 207Pb correction from the primary MAD standard (Chew, Petrus & Kamber 2014; Paton et al. 2011). Secondary reference materials used for verification were Mt. McClure (523.5 Ma ± 2.1; Schoene & Bowring 2006) and 401 (530.3 ± 1.5 Ma; Thompson et al. 2016). A weighted mean 207Pb corrected 206Pb/238U age of 522.6 ± 4.6 Ma (MSWD =2.4) was obtained for Mt. McClure, which is within error of the reference age. Apatite from reference 401 had a weighted mean 207Pb corrected 206Pb/238U age of 523.3 ± 4.9 Ma (MSWD =2.4), which is within 2% error of the reference age. The geochronology dataset was filtered to exclude apatite grain ages with large errors, using the procedure set out by O'Sullivan et al. (2018) that removes a grain with a 2 standard deviation error of >25 % or absolute error of >100 Ma. An initial 207Pb/206Pb ratio of 0.87, or initial common lead value, was estimated from the 207Pb/206Pb value at 500 Ma from the two-stage lead evolution model (Stacey & Kramers 1975), which was used to determine the age of each grain.
Institutions
- University of South Australia
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Funders
- Mineral Exploration Cooperative Research Centre