Electron probe major element analyses are presented for amphiboles and biotites from porphyry copper-generating and barren intrusions in the Western Highlands, Papua New Guinea.Amphibole crystals from mineralized intrusions are characteristically zoned from magnesio-hornblende cores, through actinolitic hornblende, to actinolite rims (Mg enrichment toward rims). The frequent lack of intermediate compositions might record a sudden change in crystallization conditions. Coexisting biotites display less well-marked but qualitatively similar compositional variations (increasing Mg/Mg + Sigma Fe and decreasing Ti toward rims).In contrast, amphiboles and biotites from barren intrusions display variations ranging from common Fe enrichment toward rims (a trend previously considered typical of calcalkaline rock suites) to rare Mg enrichment toward rims (a trend observed only in very felsic differentiates of barren suites).The range of ferromagnesian mineral compositional trends observed in mineralized and barren intrusions is attributed to a range of likely f (sub O 2 ) conditions: in barren intrusions, f (sub O 2 ) was initially low and possibly increased in final stages of crystallization, whereas, in intrusions yielding mineralization, f (sub O 2 ) was initially high and became increasingly higher during crystallization.

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