Death Valley CA & Magdalena, NM
— April 14, 2021
Many gold and silver mines follow quartz veins that were precipitated from hydrothermal groundwater. The first photo [from Death Valley] shows such a quartz vein (just below the hand), which is about 20 cm wide. In the lower-right of the photo is a smooth rock face (covered in dust) that corresponds to a fault plane (dipping to the left). Just above this fault plane is another quartz vein that is 3-4 cm-wide. Many quartz veins form parallel to faults, like what is shown in the first photo. Fracturing during faulting may make a pathway for groundwater flow. Another way is simple dilation (opening) along a fault. I saw this on a recent trip in the Magdalena Mountains, shown in the second photo. The hand is resting on the fault plane (dipping steeply to the left), and you can see near the center of the photo a 1 cm-wide fracture formed by dilation that occurred right at the fault plane--presumably from the last earthquake event on that fault. So if there was still hot groundwater circulating through the bedrock here, quartz could have been precipitated in this fracture and a quartz vein formed.
— Dan Koning, Sr. Field Geologist, NMBGMR