In Eric Cline's 1177 BC, The Year Civilisation Collapsed he notes that one of the Mediterranean civilisation's principal sources of tin was in Afghanistan. Given how important tin was to a bronze age civilisation (Cline compares it to the modern day focus on oil), do we know how this trade actually functioned?
For example, who was mining the Afghani tin and facilitating its transport westward to the Mediterranean? Do we know if there were ever attempts by e.g. Assyria to restrict/bottleneck the supply to put pressure on their more western neighbours?
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This actually appears to be an ongoing controversy in the field, so ongoing it has its own name: the "Tin Problem." Namely, there is so much bronze around the eastern Mediterranean during the Bronze Age, but no one is entirely sure where the bronze tin to make the bronze came from. From my research I'll do my best to shed a little more light on the Tin Problem and address your other questions about how tin was mined and transported.
Some tin — especially later — came from Western Europe, specifically Brittany and Cornwall. Centuries after the period in question, in the 5th Century BCE, Herodotus wrote (3.115):
I cannot speak with certainty, however, about the marginal regions which lie toward the west, in Europe... Nor am I certain of the existence of the Cassiterides Islands, from which we get our tin... Moreover, despite all my efforts to research the matter, I have been unable to find anyone who could say that he actually saw for himself that a sea exists on the far side of Europe. In any case, the far edges of the world are the source of the tin and amber that com to us. (Herodotus, 261)
Editor Robert Strassler comments in an annotation on this passage that "The Cassiterides Islands are thought to be the British Isles, one of the few sources of tin for the ancient Mediterranean world" (Ibid.)
"If Herodotus failed to get beyond the tall stories told by sailors (about the source of tin for well-documented Classical Athens)... we have little chance of doing better for the Bronze Age world," writes James Muhly (276).
While tin could have come to Mediterranean Bronze Age civilizations from Britain, a more likely source early on was modern-day Afghanistan and Iran. Cuénod, Bray and Pollard identify those areas, as well as Oman, Anatolia and the Caucasus Mountains as possible sites for the tin trade. Muhly's analysis of tablets from Mari, a city in modern-day Syria, find they all seem to describe tin "brought up the Euphrates to Mari from undisclosed sources in the east, perhaps from Afghanistan" (Muhly, 282). England and Iberia may have become a more important source for Greece, at least, by the Late Bronze Age in the 1st Millennium B.C.E. (Ibid., 290-1).
Central Asian tin may have been carried overland from Afghanistan to Mesopotamia and Egypt, but one complication is "the absence of tin in eastern Iran, which would have had to be crossed by overland trade routes" (Cuénod et al, 22). That has led some people to propose "a maritime 'by-pass' of the Iranian Plateau and a Persian Gulf route controlled by central authorities in Mesopotamia or the Indus Valley" (Ibid., 22-3).
Tin could be transported over tremendous distances from source to destination, but Muhly argues that it did so through a "series of middlemen" rather than individual merchants going all the way from, say, Cornwall to Greece (288).
Assyrian texts describe tin "brought to Assur and from there shipped overland by donkey caravan to various Assyrian merchant colonies in Anatolia" (Muhly, 282). These donkey caravans could be impressive; just a few excavations combine to show "a total of 90 donkey-loads that brought almost eleven tons of tin into Anatolia" over a hundred-year period (Ibid.).
Cuénod et al conclude based on metallurgical analyses that tin was most likely shipped in ore or ingot form to Mesopotamia, rather than being alloyed with copper near the source and shipped as bronze.
The mining operations could be impressive in scale. A complex at Deh Hosein in western Iran "more than 75 big open depressions, each bordered by waste dumps occur in an area of several square kilometres"; nearby are "two small settlements or workshops presenting evidence of ore-processing" (Cuénod et al, 18) This site produced both copper and tin, and it's unclear how much was traded overland versus being used locally, but the Deh Hosein tin is similar to bronze artifacts found in Mesopotamia (Nezafati, Pernicka & Momenzadeh).
Edited to fix a typo at the top
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