Pecos diamonds–quartz and dolomite crystals from the Seven Rivers Formation outcrops of southeastern New Mexico James L. Albright* and Virgil W. Lueth, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 Abstract Chaves County (Locality 6) and followed Distribution that report with a description of pseudocu- Pecos diamonds, also known as Pecos valley bic quartz crystals from Artesia in Eddy Large, authigenic quartz and dolomite diamonds are colorful, doubly terminated County (Tarr and Lonsdale, 1929). The crystals appear to be confined exclusively quartz crystals that occur in scattered out- term “Pecos Diamonds” appears to have to the Permian Seven Rivers Formation. crops of the Permian Seven Rivers Formation Essentially all occurrences are confined to (Permian) along the Pecos River valley in been first mentioned in print by Tarr and the back reef segment of the Guadalupe southeastern New Mexico. Although authi- Lonsdale (1929), where they note that the genic quartz is relatively common in evapor- crystals were described by that name by reef complex starting in the south at the ite sequences worldwide and throughout local collectors. Other names ascribed to “beginning of the gypsum facies” of Kelley geologic history, Pecos valley diamonds are the crystals include “Indian diamonds” (1971) near Dark Canyon and terminating unique for their large size, variable color, and “Pecos valley diamonds,” the latter near the De Baca and Guadalupe County and crystal morphologies. Single crystals of term favored by collectors today. Practical line in the vicinity of Salado Creek to the dolomite of variable morphologies also uses for these objects are mainly decorative north. In Pecos country, the back reef seg- occur with the Pecos valley diamonds but with present day uses confined to lapidary ment of the Seven Rivers Formation con- are of much more limited distribution. Red sists predominantly of gypsum with sub- pseudohexagonal aragonite crystals are also and jewelry. There is no evidence for their ordinate amounts of dolomitic limestone, present near one locality, but they have not use, decorative or otherwise, by pre-Euro- been found to coexist in the same unit. pean people. red and gray gypsiferous shale, and fine- Although the outcrop area encompasses Initial descriptions of euhedral authi- grained sandstone. East of the study area, an area 100 mi long by as much as 25 mi genic quartz crystals noted the rarity of anhydrite and salt become prevalent. Ward wide along the margins of the river valley, these minerals in evaporite sequences et al. (1986) documented two original the distribution of Pecos valley diamonds is (Tarr, 1929). However, subsequent work depositional modes for the gypsum; these limited to specific depositional environ- has shown that euhedral quartz crystals include subaqueous salina and subareal ments that resemble salinas and/or salt pans are relatively common in ancient shallow sabkha environments. within a larger sabkha setting. Replacement Meinzer et al. (1927) coined the term features within the quartz indicate an authi- marine carbonate and evaporite sequences Seven Rivers gypsiferous member for the genic origin with formation soon after (Folk, 1952; Grimm, 1962; Wilson, 1966; dolomitization of the host rock. However, Zenger, 1976; Ulmer-Scholle et al., 1993). later discredited Chupadera Formation. the large size and suite of inclusions may They are found throughout the Phanero- Tarr (1929) referred to the authigenic indicate a deep burial, late diagenetic origin zoic and exist in rocks as old as Proterozoic quartz occurrence at Acme (Locality 6) as for these crystals. The presence of organic (Grimm, 1962). Other famous occurrences being in the Manzano series of red beds. In matter, formation of dolomite, and the oxi- of authigenic quartz include Herkimer dia- the same year, Tarr and Lonsdale (1929) dation of sulfide appear to be important monds hosted by Cambrian dolomites in continued to use the term Chupadera components to the ultimate formation of New York (Zenger, 1976). Large authigenic Group in describing the pseudocubic crys- Pecos valley diamonds. quartz crystals have been described in tals near Artesia (Localities 1 and 2). These young Pleistocene carbonate-evaporite names were changed to Whitehorse, Chalk Introduction sediments in the Arabian Gulf (Chafetz Bluff, and Bernal by various authors and and Zhang, 1998). One of the most com- petroleum geologists over the years. Sub- Colorful and doubly terminated authi- prehensive studies of worldwide occur- sequently, Tait et al. (1962) proposed the genic quartz crystals with variable crystal- rences of quartz crystals in evaporites was presently used stratigraphic names based lographic forms occur in and weathered published by Grimm (1962). He noted over on the nomenclature applied to the subsur- from scattered outcrops of the Seven 150 localities that displayed similar geo- face rocks of the Artesia Group. Rivers Formation in southeastern New logic and depositional characteristics. Kelley (1971) mapped the surface out- Mexico (Fig. 1). In places, when the sun’s Personal collecting trips by the authors crops of the study area. During his map- rays are at low angles, the desert appears more precisely define the geological set- ping, Kelley (1971, p. 18) reported, “in time paved with diamonds. However, most tings of the localities of Tarr (1929) and Tarr it was noted that their [the Pecos dia- sparkles are only broken or small and and Lonsdale (1929) in addition to docu- monds] distribution is stratigraphically imperfect crystals. Only a small percentage menting 10 other in situ occurrences along related. This is so commonly true that they of crystals are large enough to be of inter- the Pecos River valley. These new localities might be used as a stratigraphic indicator. est to the collector. Crystals in a matrix of highlight areas where authigenic quartz, Most all occurrences are in part of a zone gypsum are only rarely found, but at dolomite, and/or aragonite crystals are rel- perhaps 100 to 200 feet thick from the Locality 1, a 10-cm layer consists of a mass atively abundant, large, or morphological- upper part of the Seven Rivers into the of quartz crystals. ly unique. A discussion of color, crystal lower part of the Yates.” Of the 12 in situ A Spanish miner, Don Antonio de Espe- shape, and inclusion variations is provided occurrences of authigenic quartz crystals jo, first described these crystals in 1583 for each occurrence. Ultimately, we will described in this paper, 10 are within a (Albright and Bauer, 1955). In 1929 Tarr speculate on the origin of these minerals mapped area of Kelley (1971). Localities 12 described the occurrence near Acme in and their significance in sedimentological and 13 also occur in the Seven Rivers For- interpretations. mation based on the mapping of Kelley *Author deceased August 2003, Volume 25, Number 3 NEW MEXICO GEOLOGY 63 FIGURE 1—Location map illustrating the distribution of the Seven Rivers dix. Outcrop segments that contain Pecos valley diamonds are identified and Yates Formations in southeastern New Mexico. Numbers refer to with arrows. Tectonic elements adapted from Kelley (1971). Pecos diamond localities discussed in the text and presented in the appen- 64 NEW MEXICO GEOLOGY August 2003, Volume 25, Number 3 (1972) on the Fort Sumner sheet. We have of matrix pieces can be found on the mar- Features of the authigenic not been able to document any occurrences gins of drainages that dissect the bluffs. quartz crystals of Pecos valley diamonds in the Yates For- Roswell segment mation during our study. The authigenic quartz crystals from each North of the K-M fault at the mouth of the Pecos valley locality are distinctive enough Outcrop segments Rio Felix, the Seven Rivers Formation as to habit, size, color, and inclusions that reappears in the Roswell outcrop segment they can be grouped into suites. Some Both tectonic and geomorphic features east of the Pecos River (Fig. 1). The Roswell suites are traceable for several kilometers, along the Pecos valley control outcrop dis- segment is continuous over a north-south whereas others are restricted to a few tribution of the Seven Rivers Formation distance of 47 mi with an outcrop width square meters. Some localities contain spe- (Fig. 1). Pecos diamonds occur sporadical- 1 1 varying from ⁄6 to 3 ⁄2 mi. The segment dis- cific forms and colors, whereas others may ly within particular segments of an out- appears in the north in the vicinity of the contain a diverse range. crop belt that is almost continuous. These terminus of the Six Mile Buckle of Kelley outcrop segments, and gaps that separate (1971). Pecos diamond occurrences tend to Size them, are caused by specific tectonic and be confined to the top of the bluffs in this Crystals in the New Mexico Bureau of geomorphic controls that were imposed segment except along the Pecos River cut Geology and Mineral Resources Mineral after Pecos valley diamond development. bank at Locality 4. Museum range from microscopic to a max- Descriptions of Pecos diamond localities imum of 6.5 cm (~2.5 inches) along the c Dunnahoo Hills segment within specific outcrop segments are pre- axis. Median length for perfect crystal sented in the appendix. West of the Pecos, between Roswell and forms is approximately 2.5 cm (~1 inch); Seven Rivers segment Acme, a triangular-shaped outcrop is pres- those larger tend to be distorted. The ent in the Dunnahoo Hills (Fig. 1). No largest single crystals found thus far come The Seven Rivers segment starts at the authigenic quartz or dolomite has been from Locality 13, and the largest clusters dolomite-gypsum transition near Dark found in this segment. The Dunnahoo are from Locality 3. Canyon north to the McMillan escarpment. Hills segment is surrounded by Quater- The southernmost documented occurrence nary terrace and alluvium deposits and of Pecos diamonds was mentioned by not defined by tectonic elements, in con- Dake et al.