Monday, June 28, 2010

Enargite from Goldfield

I've got a couple final mineral pictures from Goldfield, NV. The first two are of enargite and pyrite in a quartz matrix, with a fair amount of iron-oxide and possible jarosite.
Enargite is metallic, often dark gray, and it's orthorhombic — although the crystal form doesn't show well in these examples, and instead the enargite masses look almost globular. A key feature with these enargite specimens are tiny little pyrite crystals, the pyrite being in a bipyramidal (octahedral) form. Our field trip guide said that the occurrence of bipyramidal pyrite at Goldfield was often related to enargite, and that when one sees bipyramidal pyrite, one should start looking for enargite and other ore-related minerals (and gold).
Here's another example of what the enargite looks like, this time without much (or any) pyrite.
Enargite is a copper arsenic sulfosalt sometimes found at porphyry copper districts and at high-sulfidation epithermal gold districts like Goldfield (also see this report on a high-sulfidation system in Mexico).

High-sulfidation epithermal gold deposits used to be called quartz-alunite deposits, because of their dominant alteration minerals. "High sulfidation," which is really an adjective but is sometimes used as a noun, isn't something you can necessarily see in the field (it doesn't always indicate the existence of a high percentage of sulfide minerals, though abundant sulfides are common in high-sulfidation systems), so I prefer the old term, quartz-alunite. That preference definitely dates me as an oldtimer.

The opposite of quartz-alunite is, generally speaking, quartz-adularia (or quartz-sericite); the opposite of high-sulfidation epithermal is low-sulfidation epithermal. The former classification system is based on descriptive field terms; the latter is ultimately chemical or geochemical.

No comments: