<P> Tin mining began early in the Bronze Age, as bronze is a copper - tin alloy . Tin is a relatively rare element in the Earth's crust, with approximately 2 ppm (parts per million), compared to iron with 50,000 ppm . </P> <P> Tin extraction and use can be dated to the beginnings of the Bronze Age around 3000 BCE, when it was observed that copper objects formed of polymetallic ores with different metal contents had different physical properties . The earliest bronze objects had tin or arsenic content of less than 2% and are therefore believed to be the result of unintentional alloying due to trace metal content in the copper ore It was soon discovered that the addition of tin or arsenic to copper increased its hardness and made casting much easier, which revolutionized metal working techniques and brought humanity from the Copper Age or Chalcolithic to the Bronze Age around 3000 BC . Early tin exploitation appears to have been centered on placer deposits of cassiterite . </P>

Where is tin mined in the united states