The platinum-group element geochemistry of the Bucknalla Layered Complex, central Queensland
Titel:
The platinum-group element geochemistry of the Bucknalla Layered Complex, central Queensland
Auteur:
Reeves, S. J. Keays, R. R.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 42 (1995) nr. 2 pagina's 187-201
Jaar:
1995-04
Inhoud:
The Bucknalla Complex is a small (10 km2) layered, tholeiitic, mafic-ultramafic intrusion located 50 km southwest of Rockhampton that was emplaced into a regional continental margin accretionary environment in the Late Permian and hosts minor Cu-Pd-Au-Pt mineralization. It is a saucer-shaped lopolith (2200 m × 6 km at maximum stratigraphic intersection) and consists of over 15 laterally discontinuous igneous units ranging in thickness from 1-50 m. Samples from two traverses perpendicular to layering reveal cryptic variation in the primary phases (olivine, plagioclase and clinopyroxene) which is not a simple function of stratigraphic height. The mineralization consists of chalcopyrite and bornite and their alteration products digenite and covellite, electrum (Au-Ag alloy), Pd-As, Pd-Sb, Pd-S, michenerite (PdBiTe2) and sperrylite (PtAs2) and is considered to be primary magmatic because of: (i) the association of the platinum-group elements (PGE) with primary magmatic-textured Cu-sulphides; (ii) the absence of any evidence of pervasive hydrous activity in the rocks that host the mineralization; and (iii) a continuum in mantle normalized PGE patterns from barren to mineralized samples. A hydrothermal overprint may have modified the PGE patterns, which show anomalously high Cu/Ni ratios for magmatic sulphides of this level of fractionation. Background PPGE (Pd and Pt), Au, S and Cu values for the intrusion are high while IPGE (Ir and Ru) are low. A total of 116 analyses gave the range; Pd, 0.3-254 ppb; Pt, 0.3-137 ppb; Au, 0.1-74 ppb; Ir, < 0.01-0.51 ppb; Ru, 0.1-2.4 ppb; S, 23-1000 ppm and Cu, 4-1000 ppm. A significant feature of the data is the systematic decoupling of Pd from Pt-Au-S-Se-Cu throughout the analysed sequence, as in the Munni Munni and Great Dyke intrusions. Major Cu-S-Se peaks generally occur just below whole rock Mg# (Mg2+/Mg2+ + Fe2+) maxima and the precious metal tenor of the sulphides is generally highest between regions of Mg# maxima. A model is proposed in which periodic pulses of primitive (high Mg#), S-undersaturated magma into the Bucknalla magma chamber caused cessation of sulphide formation and subsequent normal progressive fractionation processes led to sulphur saturation. The very first formed sulphides would be relatively enriched in Pt and Pd (high Pd/S and Pt/S). By the time the magma had become fully S-saturated the PGE supply was reduced and, consequently, the sulphides formed (represented by the Cu-S-Se maxima) have low Pd/S and Pt/S. As the PGE-rich sulphides were not precipitated en masse as in the JM reef of the Stillwater Complex,'no discrete, narrow horizon was formed.
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