Why are metals specifically deposited in a specific place?

Q: Why are metals specifically deposited in a specific place?

Krishna: Ultra high precision analyses of some of the oldest rock samples on Earth by researchers provides clear evidence that the planet's accessible reserves of precious metals are the result of a bombardment of meteorites more than 200 million years after Earth was formed (1).

During the formation of Earth, molten iron sank to its centre to make the core. This took with it the vast majority of the planet's precious metals -- such as gold and platinum. The impacting meteorites were stirred into Earth's mantle by gigantic convection processes. Subsequently, geological processes formed the continents and concentrated the precious metals (and tungsten) in ore deposits which are mined today (2).

Most pure metals, like aluminium, silver and copper, come from the Earth's crust. They are found in ores. Iron is one of the most common elements in the universe. Uncombined iron, cobalt, and nickel can be found in meteors.

Ore minerals tend to be concentrated in small, localized rock masses that form as a result of special geologic processes, and such local concentrations are called mineral deposits (3). 

How do these metals get concentrated in certain areas?

There are active geological processes that lead to separation of mineral constituents. A source material that contains specific primary minerals, with some transport mechanism (like convection or liquid flows) to physically separate them and 'trapping' conditions that allow them to accumulate are the conditions that  are required. Processes that result in minerals being separated from the source material include (4):

Magmatic - (within molten and partly molten magmas). Differential melting will allow specific minerals to melt and be squeezed out separately. Differential crystallisation will allow some minerals in a mixture to crystallise within a molten mass and (with changing temperature and/or pressure) to separate and reform as crystals and precipitate separately in more concentrated form in sediments at the base of a molten mass (or to rise where they are lower density than the surrounding material). Some materials can separate in their molten state, because they are immiscible (they don't mix or dissolve within each other), with similar separation because of differing density or pressure squeezing them out and solidify in concentrated forms.

Hydrothermal - minerals dissolving (usually in association with salts), with different temperature and pressures dissolving minerals preferentially. As liquid they can travel and come into contact with other minerals, resulting in chemical reactions, with potential for further differentiation. Similar to differential crystallisation in magmas, specific minerals will crystallise and precipitate differentially out of concentrated water based solutions. Lots of sulphide mineral ore bodies were formed this way.

Physical process like erosion and sedimentation also separate and concentrate existing minerals.

When the Earth was formed and was still in a molten or semi-molten state, the heavy metals, principally iron and nickel, were attracted by gravity to the Earth's deep interior, leaving only traces of them on the surface. However, the bombardment by comets, asteroids and mega-meteorites continued after a crust was formed. These were of varied composition and dumped their cargoes of metals randomly across the globe.

In addition to this, volcanoes spewed metal-rich magma into the upper layers of the crust, again rather randomly, and hydro-thermal seepages brought up metals from the depths and deposited them in the rocks on or near the surface. Magma plumes originating far down in the mantle can carry various elements, including metals, up to the surface.

Footnotes:

1. https://www.sciencedaily.com/releases/2011/09/110907132044.htm

2. Matthias Willbold, Tim Elliott, Stephen Moorbath. The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment. Nature, 2011; 477 (7363): 195 DOI: 10.1038/nature10399

3. https://www.britannica.com/science/mineral-deposit

4. https://en.wikipedia.org/wiki/Ore_genesis#:~:text=Various%20theorie....