Bull. Minéral. (1981), 104, 292-297.

The application of the scanning electron microscope

in gossan research

V application du microscope électronique à balayage dans l'étude des chapeaux de fer.

Introduction

A recent development in scanning electron microscopy involves the modification of the instrument so it can operate in the backscattered-electron/low-vacuum mode (Robinson, 1975; Robin¬ son and Nickel, 1979). As compared with the scanning electron microscope (SEM) used in the more normal mode (secondary-electron/high-vacuum), the modified SEM has several advantages for the mineralogist : the sample does not require coating, sample-changing procedures are very rapid, and high-quality atomic-number-contrast images are produced that give valuable compositional information. When equipped with TV display and energy-dispersive X-ray analyzer, the instrument becomes a versatile and powerful mineralogical tool.

The SEM, modified as outlined above, has proven to be very useful in the study of the mineralogy and geochemistry of gossans — the surface products of weathered sulphide bodies. Some examples are given below.

Application to Cu -Zn -Pb Gossans in western Australia

In a landscape of relatively low relief and with a semi-arid climate, such as that which prevails over most of Western Australia, the rocks and any mineral deposits they may contain have generally been weathered to considerable depths. Thus it is not uncommon to find sulphide deposits oxidized to depths of 20-30metres, resulting in gossans now

consisting largely of goethite, hematite and quartz. Much of the goethite and hematite in such gossans has been derived from transported iron, and these frequently obliterate evidence of sulphide mineraliza¬ tion, thereby making the recognition and evaluation of gossans difficult.

Three gossans developed over proven ore bodies in Western Australia have been investigated with the aid of the SEM : Teutonic Bore (lat. 28°24'S, long. 121°09'E), Golden Grove (lat. 28°47'S, long. 116°58'E) and Whim Creek (lat. 20°50'S, long. 117°48'E). All three are stratiform deposits of volcanogenic origin consisting mainly of pyrite, chalcopyrite and sphalerite, with subordinate amounts of pyrrhotite, galena and cassiterite, and trace amounts of a variety of sulphosalt minerals. No sulphides remain in the gossans, except for occasional small inclusions retained in quartz. The only recognizable sulphide pseudomorphs are those of pyrite.

The SEM has been of particular value in readily enabling the detection of residual and secondary ore minerals of high atomic number that are difficult to recognize under the optical microscope.

Residual cassiterite

Cassiterite is the only ore mineral in all three deposits that is resistant to weathering, and therefore retained in the gossan. Cassiterite is not an easy mineral to recognize under the best of circumstances, and in these gossans where it is finely dispersed, it is particularly difficult (Figure la). Under the SEM, however, the cassiterite is strongly contrasted against the gossan minerals (Figure lb). Such cassiterite