What does this mining map symbol indicate?(circle, dot, and tail)

Question

This symbol is on a Canadian (Ontario) prospecting map from 1984:

What does the symbol indicate? There is no legend.

• Outer circle
• Center dot
• Tail

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A colleague, said this:

It likely indicates a diamond drill hole. My assumption is that the hole starts at the circle and terminates at the perpendicular line. Since the path of the drill hole is often not vertical, you end up with a line since the map is in plan view. Some of the drill holes are drilled at 45° to vertical, therefore, travelling in plan view quite a ways.

Answer 1

A circle with a directional line is a traditional symbol for an inclined drill hole. For examples, see

• The Preparation of Illustrations for Reports of the United States Geological Survey (Plate II) has an arrow at the end of the directional line.
• Conventional Symbols for Mine Maps (Plate 2) has no endcap arrow or perpendicular line, but is anotated with the length of the hole and inclination angle.
• Rite in the Rain Geological Notebook (Page 151) doesn't have the central dot, but does have the endcap perpendicular line and (I think) the inclination.

So there is some variation in whether there is a central dot, in how the directional line ends, and in how the symbol is annotated.