Samples were prepared as polished thin sections for transmitted and reflected light microscopy. Optical microscopy was used for transmitted light studies using a range of objectives from 4x to 63x. Reflected light microscopy was conducted using a range of objectives from 2x to 100x in air. Oil immersion studies were performed using 20x, 50x, and 125x objectives. The ocular lens was 10x in both cases. Hercynite required a special technique to achieve the condition that permits the detection of this mineral in the characteristic assemblage of occurrence. Hercynite is commonly surrounded by magnetite, which renders the assemblage opaque and obscures the appearance of hercynite. Light conditions required to observe hercynite are as follows:
Because of the grain size and the association with opaque oxides, hercynite is not observable if the focal distance is >1 or 2 mm.
Selected samples with high oxide contents were studied by X-ray powder diffraction (XRD). XRD was performed with a Philips PW 1710 diffractometer using a copper tube and operating at 40 kV and 40 mA at the University of Lisbon and the University of Toronto. XRD confirmed the presence of magnetite and Ti magnetite and was used to test for the possible presence of maghemite. Samples were ground using an agate mortar. Details of the XRD results will be presented elsewhere.
Chemical microanalyses of gold, sulfide, and oxide minerals were carried out using two different electron microprobe analyzers: a Cameca Camebax at IGM (Instituto Geológico e Mineiro) Porto, Portugal, and a Cameca SX-50 at University of Toronto, Canada. Analytical conditions used for both electron microprobes are presented in Table T1.