Which one of the following pairs of geophysical signatures most commonly indicates volcanogenic massive sulphide deposits?
Show Hint
- High gravity and low resistivity
- Low gravity and high resistivity
- Low gravity and low resistivity
- High gravity and high resistivity
The Correct Option is A
Solution and Explanation
Volcanogenic massive sulphide (VMS) deposits form on or near the ancient sea floor, where hot hydrothermal fluids linked to submarine volcanic activity deposit thick, massive layers of sulphide minerals such as pyrite, chalcopyrite, sphalerite and galena.
Step 2: Think about the density of these minerals.
Sulphide minerals are much denser than the surrounding volcanic and sedimentary host rocks. A dense mass of ore in the subsurface pulls the local gravitational field up slightly, so a gravity survey over a VMS body shows a positive gravity anomaly, that is, high gravity.
Step 3: Think about the electrical behaviour of these minerals.
Massive sulphides, especially pyrite and chalcopyrite, are excellent electrical conductors compared with the normal silicate host rocks. A good conductor lets current pass through it easily, which shows up as low resistivity, or equivalently high conductivity, in resistivity or electromagnetic surveys.
Step 4: Combine both signatures.
So a VMS deposit is expected to show high gravity together with low resistivity, since it is both denser and more conductive than its surroundings.
Step 5: Rule out the other options.
Low gravity would mean a low density body, which does not fit a massive sulphide accumulation. High resistivity would mean poor conductivity, which does not fit sulphide minerals either. So options (B), (C) and (D) all go against the physical properties of massive sulphides.
Step 6: Final answer.
\[ \boxed{\text{High gravity and low resistivity}} \]
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