Yerington: Day 1, Pt 3: Sodic-Calcic Et Al

It's hard to believe I'm still on the first day - but I think it's just a matter of being a photophile and not having as much time as I'd like.

Hand points to the part of the Ann-Mason fault block that we started in.

From the Bluestone Mine, we took the vans north to a hill where we were able to examine some intrusive contacts within the Yerington batholith and also some sodic-calcic alteration. The alteration types we spent much of the day looking at were obscure to almost cryptic (or at least it seemed that way to me!).

At Stop 3 of Day 1 [Stop 2 in the guidebook referenced below], we saw the deepest exposure of the Yerington batholith. (The batholith consists of three major intrusive bodies: the oldest, the McLeod Hill quartz monzodiorite; the second, the Bear quartz monzonite, and the youngest, the Luhr Hill granite and related granite porphyry dikes.) The most noticeable and impressive thing about the quartz monzodiorite at this exposure were the large epidote "splotches" - replacements following tiny fractures. The epidote splotches have bleached selvages where the original K-feldspar, biotite, and magnetite are gone, replaced by or altered to plagioclase (albite), amphibole, epidote, and sphene.

Epidote splotches in quartz monzodiorite.

Some relative age relationships between different kinds of alteration features.

The age relationships between the different alteration features, including early endoskarn and later sodic-calcic alteration were a little hard to keep track of, and some of the key veinlets were narrow to cryptic. In the above photo, an early greenish amphibole-diopside veinlet (endoskarn) is cut by a dike. The dike also cuts epidote splotches and associated bleaching (sodic-calcic alteration), so the dike is probably related to the late granite intrusions.

Sodic-calcic bleaching of quartz monzodiorite.

The sodic-calcic alteration, above, has gone outward from narrow fractures and veinlets. This type of alteration is thought to be related to non-magmatic brine fluids, which were drawn into the edges of the magmatic-hydrothermal system. Sodic-calcic alteration is often related to IOCG deposits, examples of which are some magnetite skarns found around the edges of the large Yerington porphyry copper system.

Granite porphyry on the right, with chilled margin, intruding quartz monzodiorite.

Luhr Hill granite on the left, with chilled and foliated margin, intrudes quartz monzodiorite.

Ann-Mason deposit in the wind.

Onward we went, through the day here and there, as it got windier and windier, finally coming to the not-in-production (too deep) Ann-Mason porphyry copper deposit, to the west of the Luhr Hill cupola and main mass of granite porphyry dikes, upward in the system with west being "Jurassic Up." And we found: more sodic-calcic alteration and some tourmaline breccia.

Finally, we reached the culmination of the day, an antelope seen while heading back to Yerington.

Main Reference:

Dilles, J. H., Proffett, J., and Einaudi, M. T., 2000, Field trip day two: Magmatic and hydrothermal features of the Yerington Batholith with emphasis on the porphyry Cu-(Mo) deposit in the Ann-Mason area, in Thompson, T. B., ed., Society of Economic Geologists Guidebook, 32, p. 67-89.

Chuck has posted another IOCG link. Check it out!