Quartz Veins & Lodes

Gold-silver quartz vein at the Commonwealth mine, Pearce, Arizona. The vein is 12 to 15 feet wide
can has significant gold values. The quartz was formed from high-temperature fluids similar to
those found in hot springs.

Quartz is considered to be second most common mineral in the earth's crust, after feldspar.  To be honest, I'm not sure who actually measured all of the quartz deposits and compared them to feldspar occurrences to come up with this conclusion. But, I suspect minerals such as calcite, gypsum, mica are not far behind feldspar and quartz in the most voluminous minerals in the crust.

If you have ever walked along a beach, drove over sand dunes in a desert, or spent time hiking through basins filled with sandstones, greywackes, and quartzites, you will likely agree that quartz is common. Quartz is silicon-dioxide and comes in a variety of habits including crystalline quartz, chalcedony, agate, jasper, amethyst, citrine, smoky quartz, and rose quartz. 

Quartz stockworks in silicified rhyolite. Many narrow quartz
veinlets fill fractures in the rhyolite host rock at the
Commonwealth mine, Arizona.

During mountain uplift, many volcanic eruptions, igneous intrusions, and hot springs percolating through the hills, you likely have seen some crosscutting to parallel, white to gray quartz veins in hill sides and long road cuts. Some veins are tiny, some pinch and swell, others many be wide, filling fractures and faults in the earth's crust, or they may just be a silica-rich goo that forms layers in flat-lying sediments and volcanic rocks. 

Quartz comes in a variety of colors and some may contain vugs filled with hexagonal, quartz crystals, other may contain drusy quartz (tiny, fine-grained, quartz crystals), or occur as massive layers without any distinct crystals. And periodically, one may come across crystals of quartz that replaced other minerals such as jade, or even jade that replaced quartz.

Prismatic quartz (tiny crystals) with banded quartz,
Commonwealth mine, Arizona.

For the prospector, quartz veins are eyed for their gold content. But, just because you find a quartz vein, sometimes referred to as a seam, lode, or dike, this does not mean that there is any gold associated with the vein. In fact, you will find many, many more barren quartz veins than those with gold and or silver. Over the years, I found veins containing visible gold filling small fractures in the quartz vein, or found pyrite (fool's gold), chalcopyrite (copper-fool's gold), arsenopyrite (arsenic-pyrite), limonite, goethite, berthierite, malachite, azurite, galena containing no-see-um gold either as tiny flakes within the various minerals, or found exchanging localities with various atoms within crystal structures. To find this gold, one needs to crush the sulfide minerals and pan them, or if the gold occurs as an exchange of atoms, one must roast the ore and leach it with cyanite to try to get the gold.

Ginguro bands in banded quartz vein, Commonwealth mine, Arizona.

A ribbon vein in mine back (ceiling) cutting other near-horizontal veins, Giant King, California.

Large milky quartz vein in mine back, California.

Shear zone filled with gold-bearing 
quartz veins, Carissa mine, South Pass.
Wyoming.

Narrow off-set, gold-bearing quartz vein in tonalite, Mary Ellen mine, South Pass, Wyoming.

45-degree, dipping, iron-stained quartz vein, Vulture mine,
Arizona.

Silver-rich quartz vein grab sample, Oregon vein, Kirwin, Wyoming.