An Article In Meteorite-Times Magazine
by Mark Bostick
The Gold Basin Meteorite
On November 24, 1995 Professor Jim Kriegh was prospecting for gold with a metal detector in Hualapai Wash (Part of a natural drainage system of northern Arizona). He found two small stones. These two stones weighing about 13 grams where the first Gold Basin meteorites officially found. This meteorite was however discovered several times in the past by other prospectors searching for gold in the region. For many years, prospectors have been annoyed by the amount of hot rocks that showed up on their metal detectors. These "Hot Rocks" tossed out of the rock sack at the end of the day or left in the dirt pile turned out to be meteorites!
After these stones where discovered to be meteorites, the strewn field was kept secret for a couple years, Jim Kreigh, Ingrid Monrad and John Blennant working with the University of Arizona made a valiant attempt to map the strewn field and recover as many stones as possible.
Over 4000 specimens have been found so far by this team. Each meteorite is weighed and bagged in the field with its coordinates carefully noted. More then the first thousand where taken to the lab where they were described in detail, dried in a vacuum chamber, re-weighed and photographed.
The Gold Basin meteorite has been
classified by the University of Arizona as an L4 Stone Olivine Hypersthene
Chondrite (Olivine Fa24±1; Pyroxene Fs20 Wo1; Kamacite contains 0.72±0.09 wt% Co;
and a weathering grade W2-3). Studies
have shown the meteorite fell about 14.3 million years ago, on a portion of the
Wisconsin Glaciation. The climate was cooler and wetter when the fall
and most show oxidation that probably happened shortly after its arrival to
Earth. In the meteorite world, many consider this is a fossil meteorite.
While I was at the 2002 Tucson mineral and meteorite show one of these gold prospectors showed me what I believe to be the main mass of the Gold Basin meteorite. This man was in his forties, wearing overalls and sporting a pair of army prescription glasses. Not a person you would picture having an interest in meteorites. After carefully unwrapping a glass dome from a box he present a weathered meteorite over 3000 grams. About 30 meteorite fragments where glued together on a pole that rested in the middle of the glass dome. On the base of the dome a brass plate, "GOLD BASIN METEORITE MAIN MASS". The meteorite appeared potato shaped and was missing a piece about 20 grams. According to the prospector, all the pieces where found together in a small area. Several hours the following day where spent trying to find the missing piece with no luck. The asking price of Gold Basin this year Tucson was from $.75 to $2.00 a gram. His asking price for the Gold Basin main mass was $30,000, about $10.00 a gram. This was hi s ace in the hole and he wasn't letting it go cheap.
The same prospector also gave testimony of searching for gold for years and tossing these "rocks" aside. Several Gold Basins were even recovered at his usual "camp site". Later, this same prospector, after changing to search for meteorites, found a piece of quartz and he didn't understand why it was setting his metal detector off. This gem was almost tossed aside before it occurred to him...it was gold!
Different Meteorites found in the Basin
More then a dozen different meteorites have since been found in the Gold Basin strewn field. The first was the L5 King Tut, A 19.51 g stone found by John Blennert on March 6, 1997. This however would not be the last for Gold Basin or Blennert. In the next couple years, a carbonaceous (Temple Bar) and a mesosiderite (White Hills) were found. John Blennert would make meteorite news a second time by finding White Elephant, an L4 that is indistinguishable from Gold Basin. Only cosmogenic nuclide indicates it is a recent fall. This was found in the center of the strewn field. Cosmogenic nuclides also suggest that it is a fragment of a larger mass.
At least 10 of the meteorites found in this strewn field that have not been identified as Gold Basins where classified L-Chondrites. These non-L4 chondrites whose pairing are still to be determined, are being designated as "Hualapai Wash". The first Haulapai Wash was found November 18th, 2000 by Donald O'Keefe. This was a 206 gram stone that seemed different to Donald from the other Gold Basins he had found that day (Amazingly, his first time in the strewn field). Its matrix is much darker then the average Gold Basin. Classified as an L6, its chondrules are defined poorly. Its crust was very weathered and metal is scattered about. Only the last two observations look anything like Gold Basin.
Hualapai Wash Meteorite Slice
Two other non Gold Basin stones have since been found by O'Keefe, who has made the Basin his currant home. These stones were recognized as being different by the sound they made under the metal detector O'Keefe uses to search. His later find is known as Hualapai Wash 004. "When I swept my coil over it I got a double hit and, when I pumped up and down on it, it really screamed much like a pull-tab would. I believe that the reason for this is that the metal isn't evenly distributed in the matrix, but rather occurs in groups." This meteorite was is an L5. This meteorite has a light matrix like Gold Basin. Like O'Keefe noted the metal in this meteorite is in groups, with several pockets and many armored chondrules. It does not look like any Gold Basin I have seen.
The largest "Haulapai Wash" is Haulapai Wash 10. At 2.35kg it is the largest stone found in the Gold Basin strewn field and at one time was considered the main mass by many. After being classified an L6 with a weathering grade of , it was given its own number. Most of the other Haulapai Wash's are L6's.
Hualapai Wash 4
The distribution of the Gold Basin fall appears pretty evenly, in both mass and stone numbers. This has the experts leaning towards the belief the meteorite exploded in one burst. Comparison with other falls that created many meteorites agrees with this theory. Most individuals weigh under 30 grams and they seem to me, to outnumber specimens over 100 grams, 15 or more to one. The large size of the strewn field has many believing the meteorite was a breccia. This helped it break into smaller pieces and leaves the possibility for the other L chondrites found in the stewn field might be in fact, be Gold Basins as well.
Are they all Gold Basins?
This has been suggested by several people. Olivine percentage in many of the L-Chondrites are very similar:
Gold Basin (L4) : Fa mol% 24±1
Golden Rule (L5): Fa mol% 24.1±0.5
King Tut (L5): Fa mol% 24.7±0.5
Haulapai Wash (L6): Fa mol% 24.6
Haulapai Wash 3 (L5): Fa mol% 24.0 +/1 0.4%
Haulapai Wash 4 (L5): Fa mol% 24.7 +/1 0.3%
From top to
bottom: Gold Basin, Haulapai Wash, Haulapai Wash 4
As shown in the crust comparison photo, most of the L-Chondrites found in the strewn field have a weathered crust. The official weathering stage of Gold Basin is 2-3. It has been noted to vary drastically between W1 to W4 (Wlotzka, 1993). I have talked with many of the hunters that search the strewn field regularly. Most have story of trying to match pieces together at the end of the day.
Basin Meteorite Slice (~ 1 1/2 inches)
Close up of dark region from slice above with metal vein
The Matrix of the Gold Basin varies from a light grey to black. This shows the meteorite is brecciated. However, this does not show up on a thin section. This is confusing to many collectors and sellers. In Gold Basin the olivine in the darker color matrix is very similar to the olivine in the light color matrix. Pyroxene and other values are also very similar. So in other words, it is a breccia made completely from L-chondrite fragments, probably from the same parent body. However, given of the small size of the stones and the large number of uncut stones in collections it is possible we have not found our puzzle key yet. The slice above shows a dark region in the mostly lighter colored matrix. I have observed a tendency for metal veining to appear often in the darker matrix, while metal pockets and armored chondrules appear in the light matrix. Metal veining has appeared in several of the Hualapai Wash meteorites. In Hualapai Wash 4 we have a lighter matrix with metal in pockets and armored chondrules.
The easy answer would be to classify the Gold Basin as a genomict brecciated meteoroid composed of different meteorite types. A person could then say most of the different meteorites found in this strewn field, was in fact the same meteorite, Gold Basin. In the Atacama Desert of Chile, a large number of L-chondrites have also been found. This has led people to speculate that H-chondrites weather into L-chondrites. After all, how could one account for so many different L-chondrites in a small area? It would be hard for this author to imagine all theses meteorites being contributed to one fall. Perhaps the truth is meteorites are more common then we ever thought possible.