By 1870 what type of organization had taken over mining in the American West?

After the California Gold Rush that started in 1848, the waves of people (Called the forty-niners) looking to get rich quickly arrived, and by 1849, they have extracted most what was known as "easy gold".

After this period, the mineral's extraction was far harder and required more destructive and elaborated methods. This situation benefited wealthier individuals that opted to form massive corporations in order to get the most out of the mines.

The result of the birth of these organizations was that they transformed the mining industry by creating large national networks of production and finance.

So the answer is: Corporations.