Amethyst Amygdules at Temporal Gulch, Arizona

Over the past few months, we’ve spent a fair amount of time tromping around Temporal Gulch, near Patagonia, Arizona. This scenic area lies within open cattle range and upper grassland at an elevation of approximately 4,200 feet. We understand that it is Arizona State Trust land, which makes it publicly accessible for recreational use and light mineral collecting..

Just 15 minutes from Patagonia via a gravel road, the area is easy to reach and makes for a rewarding day of mineral collecting. To get there, drive a couple of miles north out of Patagonia on First Avenue, continuing past the turnoff to the landfill. After the first gate beyond the landfill road, four-wheel-drive vehicles can descend a short distance to a convenient parking area. If you're in a two-wheel-drive vehicle, it’s best to park just inside the gate at the top of the hill and hike down.

From the parking area, follow the path that runs alongside the barbed-wire fence to reach the collecting sites. The area is not only accessible but also offers a pleasant hike and promising mineral finds in a beautiful desert grassland setting.

Fortunately, it’s difficult to get lost in the area: the eastern side of the gulch is bordered by a now-closed landfill site, which often remains visible and serves as a useful landmark.

Basic Geology

The area seems to be underlain by basalt, with a relatively thin layer of overburden and weathered basalt on top. The basalt is vesicular or amygdalodal with the amounts of amygdules varying greatly over short distances. The vesicles or amygdules are generally filled with silica and/or calcite and those are the nodules or geodes that we seek. In places, probably related to certain basalt flows, the size and frequency of occurrence of the amygdules is higher, resulting in a better place to recover specimen grade geodes.

The whole area has undergone several episodes of silicification. All of the amydules have quartz/calcite fillings. In addition., though, there are many areas of quartz veining or breccia infilling that’s often have openings containing quartz crystals. Almost all openings in the veins and breccia in fillings have cores of calcite that formed anfter the quartz and encloses the quartz crystals. At surface, the calcite has been naturally dissolved away. A couple of feet and deeper, most openings are filled with calcite which needs to be dissolved away to expose the quartz crystals.

Mineralogy

Quartz Quartz occurs in many forms, at Temporal Gulch. Most prominent in the silicified areas are large milky-clear, colourless quartz crystals, usually coated with a later generation covering of tiny drusy quartz crystals. The quartz crystals can contain fluid inclusions, sometimes containing moveable bubbles.

Of interest to us are the silica/calcite filled geodes which are mineral-filled amygdules. As mentioned, quartz crystals often have a purple coloration from very light amethystine to darker amethyst. The quartz crystals appear to be of, at least, of several generations of growth, the largest crystals in any given geode being the amethystine ones.

Baryte Sometimes baryte crystals are present as small white, simple, bladed crystals and groups on the quartz crystals. The baryte is most noticeable after calcite is etched away.

Calcite There is lots of calcite in the amygdules and most are lined with quartz with the remaining volume mostly filled with solid calcite. On occasion, geodes are hollow with the final lining calcite crystals which cover everything else

Hematite Often tiny crystals and rosettes of hematite form o the quartz before the calcite formed. The hematite crystals are obvious only after calcite is dissolved away.

Iron oxides Sometimes the calcite or parts of it is impregnated with a high concentration of fine-grained iron mineral like limonite/goethite. This fine-grained material is easily washed away after dissolving the calcite.

Collecting and Preparation

Collecting involves hiking the area and keeping a sharp eye out for quartz crystals and amygdules. When you find one, there’s a good chance more are nearby. In amygdule-rich zones, specimens can often be found on the surface, or you can scratch or dig down to uncover additional geodes.

There’s no reliable way to determine what’s inside a geode without opening it. We typically collect complete nodules larger than 40 mm and bring them home for further examination. Using a splitter, we crack off one end to inspect the interior. If it appears to contain interesting mineralization, it's a keeper.

Most of the geodes are lined with quartz and then filled with solid calcite and iron oxides that conceal any internal crystals. Occasionally, you may get lucky and expose beautiful amethyst right upon cracking—always a pleasant surprise. However, in many cases, the true character of the specimen isn’t revealed until the calcite is dissolved using acid. For this, we use muriatic (hydrochloric) acid to carefully remove the calcite and expose the underlying minerals.

Above: Abby shows off a nice amethyst-in-calcite crystal that she found at one of the areas.

In Conclusion

This locality is especially easy to access and, while not known for rare minerals, it offers an abundance of amethystine quartz crystals. More importantly, it’s a beautiful and enjoyable place to spend the day—whether or not you make a significant find. We’ll definitely be returning. Mat Smith photo.