How do geologists find ore deposits?

Question

From my research, there is lots of information on how to extract ore, but I can't find any information on how these ore deposits are found in the first place, apart from 'looking for surface formations'. So, what kind of formations would exist for different types of ore? Are there any other ways of finding ore?

Answer 1

A lot of it does come down to looking for what's cropping out at the surface! However, certain kinds of ore bodies can be found in other ways.

One example would be Mississippi Valley-type metal sulfide (lead, zinc, copper, silver) deposits hosted in carbonate rocks. They form when basinal brines migrating underground are pushed into a different rock layer (which they then react with), usually by some kind of paleo-topographic feature such as a reef which blocks their lateral motion and forces them upwards. The buried topography that causes this can be picked up by large-scale mapping, such as seismic surveys of the area.

Often times, small ore bodies are found at the edges of large, more easily detected features. Looking for a hydrothermal copper/molybdenum porphyry deposit? Well, they're formed by magmatic fluids leaking out of igneous intrusions and reacting with the country rock, so you'd look near large felsic intrusions with porphyritic dikes bursting out of them. Similarly, volcanogenic massive sulfide deposits form from undersea volcanism, so if you want to find them you look for mid-ocean ridges.

Sometimes it's about finding rocks of the right age. A lot of known iron ore reserves come from the Great Oxidation Event about 2.5-1.8 billion years ago, when the appearance of molecular oxygen in the atmosphere caused the oceans to "rust" as oxygen reacted with water-soluble Fe2+ to form Fe3+, which sank to the seafloor and formed banded iron formations (BIFs). If you're looking for economical quantities of iron ore, you want to look for Paleoproterozoic sedimentary rocks.

There's really quite a lot of different types of ore deposits, set apart by where the metals come from (basinal brines? volcanism? leaching?), how they're carried (sulfide complexes? chloride complexes?), and how they're deposited (oxidation? depressurization? pH change?). But if you know what the typical source of a metal is and what kinds of settings it can end up deposited in, you look for where those two factors are found together.