In lode or vein deposits, the gold is mixed with another
mineral, often quartz, in a vein that has filled a split in the surrounding
rocks. Gold is obtained from lode deposits by drilling, blasting, or shoveling
the surrounding rock.
Lode deposits often run deep underground. To mine underground, miners dig
shafts into the ground along the vein. Using picks and small explosives,
they then remove the gold ore from the surrounding rock. The gold ore is
then gathered up and taken to a mill for refinement.
Placer deposits contain large pieces of gold ore (nuggets)
and grains of gold that have been washed downstream from a lode deposit
and that are usually mixed with sand or gravel. The three main methods used
to mine placer deposits are hydraulic mining, dredging, and power shoveling.
All methods of placer deposit mining use gravity as the basic sorting force.
In the first method, a machine called a "hydraulic giant" uses
a high pressure stream of water to knock the gold ore off of banks containing
the ore. The gold ore is then washed down into sluices or troughs that have
grooves to catch the gold.
Dredging and power shoveling involve the same techniques
but work with different size buckets or shovels. In dredging, buckets on
a conveyor line scoop sand, gravel, and gold ore from the bottom of streams.
In power shoveling, huge machines act like shovels and scoop up large quantities
of gold-bearing sand and gravel from stream beds.
Hydraulic mining and dredging are outlawed in many countries
because they are environmentally destructive to both land and streams.