Guatomo Mine, Thailand - famed for vivid yellow to orange mimetite crystals, lustrous and sharp, from a historic skarn Pb-Sn deposit; prized by collectors.
Key facts
Guatomo is the old “Hat Yai” mimetite locality: an abandoned open-pit lead-tin mine at Tham Thalu in Yala Province, southern Thailand, close to the Malaysian border and beside Wat Tham Thalu. For collectors it matters far beyond its modest footprint because one short collecting interval in the late 1990s put Thailand on the world-class mimetite map. The best specimens are instantly recognizable: vivid yellow to orange mimetite crystals, commonly lustrous, transparent to translucent, sharp, barrel-shaped, hoppered, skeletal, or prismatic, perched on oxidized gossan, altered granite, or quartz. Fine examples have the saturated color and glassy brightness collectors expect from the great mimetite localities, but with a distinctly Guatomo habit—thick hexagonal barrels, stepped hoppers, and brilliant crusts that can cover a specimen face like a field of gold.
Geologically, Guatomo is a skarn-type Pb-Sn deposit developed where Jurassic-Triassic porphyritic biotite-rich granite intruded Permian Ratburi limestone. Its primary ore suite—cassiterite, magnetite, garnet, arsenopyrite, pyrite, and formerly galena—was overprinted by a powerful supergene zone. That oxidized cap is the specimen story: galena was largely converted to cerussite with minor anglesite and goethite, while arsenate-phosphate mineralization filled fractures in altered granite, hydrothermal quartz veins, and porous yellow-brown to black limonitic gossan. The collector minerals belong to a compact but sophisticated assemblage: mimetite and pyromorphite-series compositions, cerussite, anglesite, plattnerite, cesàrolite, carminite, malachite, linarite, and the hidalgoite-philipsbornite-segnitite series.
Regional View
Country View
The locality also has a labeling history that every serious collector should know. When Marco P. Sacchi and Danilo Bernasconi recovered the celebrated mimetites in 1996–1998, the material was introduced to the market as coming from the Hat Yai District of Songkhla Province, a deliberate broadening of the locality intended to protect the find. Specimens still circulate with “Hat Yai,” “Hat Yai mimetite locality,” or even “Songkhla” labels. In modern cataloging, the precise locality is Guatomo Mine, also known historically as Tham Thalu Mine or The Great Mine, at Tham Thalu, Bannang Sata District, Yala Province, Thailand.
Search for specimens: View all specimens from Guatomo Mine, null
Guatomo lies near Tham Thalu, about 10 km west of Bannang Sata town in Yala Province. The mine sits beside the Buddhist temple Wat Tham Thalu and is recorded at approximately 6°15′26″ N, 101°09′53″ E. It is an inactive pit and is not a casual collecting locality in the usual recreational sense; any visit or collecting would need current local permission, and the setting near a temple, old mine workings, gossan blocks, altered granite, and tropical weathering calls for conservative field conduct.
The deposit is a Pb-Sn skarn. Skarn ore bodies formed along the contact between porphyritic biotite-rich granite and Ratburi limestone, and later weathering produced the gossan and oxidized lead minerals that made the site famous to collectors. The primary ore minerals include cassiterite, magnetite, garnet, arsenopyrite, and pyrite. Galena is part of the original lead story, but in the oxidized zone it is mostly represented by cerussite, minor anglesite, and iron oxides. Hydrothermal quartz veins with minor cassiterite cut the altered granite; these veins and the adjacent altered granite host some of the finest mimetite-pyromorphite series crystals.
Mining history reaches back to the nineteenth century. The Guatomo or Tham Thalu deposit was regarded in the 1880s as one of the richest ore deposits of the Thai-Malay Peninsula. Later, before World War II, the mine was opened by Yala Mines Ltd., and it was subsequently acquired by Anglo-Oriental Tin Co. The mine was worked chiefly by open cuts, with some vertical shafts, adits, and exploration workings. A mid-twentieth-century government reconnaissance noted several worked ore bodies but little available information on grade or ore-body form, and also recorded arsenopyrite-rich ore that had to be roasted so that iron minerals could be made magnetic and separated from cassiterite.
In the early 1980s, Guatomo produced low-grade tin concentrate, with reported monthly output in the range of 5 to 30 tons. That concentrate could contain abundant cerussite—reported as high as 37%—and Pb-Sn alloy was produced during smelting. Commercial mining ceased in 1985 after the collapse of world tin prices. The mine’s second life, as a specimen locality, came in 1996–1998, when Sacchi and Bernasconi collected hundreds of high-quality mimetite specimens from the abandoned open pit. Those specimens debuted at the 1998 Bologna show and quickly became the classic Thai mimetites.
The specimen-producing environments are unusually well documented. One productive style consists of yellow-orange to orange prismatic mimetite crystals in fractures of hydrothermal quartz veins cutting strongly altered granite, especially in the central part of the pit. Another classic style occurs in limonitized granite in the area Sacchi called the philipsbornite trench, where gemmy lemon-yellow to yellow-orange long-prismatic crystals, sometimes with pyramidal faces and slightly skeletal terminations, occur with Pb-Mn oxide coatings and microcrystalline hidalgoite-philipsbornite-segnitite material. A third highly desirable style comes from porous limonitic gossan, where short-prismatic to tabular, often strongly skeletal mimetite crystals fill irregular pockets. Orange-brown campylite-style barrel aggregates and massive veins occur in gossan in the upper part of the pit and may prove compositionally to be arsenian pyromorphite rather than true mimetite.
Cerussite is more than a background mineral here. It occurs as white prismatic and acicular clusters, reticulated fracture coatings that can cover broad surfaces, snowflake-like groups, and even massive black to gray pods. Anglesite is rare, appearing as colorless to white prismatic or pyramidal crystals in gossan, commonly altered or replaced by goethite. Plattnerite is a rare late-stage associate, documented as black prismatic microcrystals on mimetite-pyromorphite series aggregates, especially in association with the orange-brown campylite-style material. Malachite and linarite are rare microcrystalline coatings, mostly in gossan fragments from the floor of the open pit.
Guatomo mimetite is the reason the locality is famous: it occurs as lustrous yellow-orange to orange short- and long-prismatic crystals to about 1 cm on quartz veins in altered granite, gemmy lemon-yellow to yellow-orange long prisms to about 1 cm in limonitized granite of the central pit and philipsbornite-trench area, and pale-yellow to yellowish-orange short-prismatic or tabular skeletal crystals in porous gossan, where crystals may reach 2.5 cm; the best specimens show intense color, bright luster, crisp hexagonal form, hoppered or skeletal growth, and attractive matrix contrast with quartz, goethitic gossan, Pb-Mn oxides, cerussite, or greenish to bluish hidalgoite-philipsbornite coatings, while ordinary pieces tend to be duller crusts, crowded damaged pocket-wall fragments, or material whose “mimetite” label needs chemical caution because Guatomo also produced P-rich mimetite, As-rich pyromorphite, and As-enriched pyromorphite in visually similar yellow-orange habits.
At Guatomo, plattnerite is a rare accessory rather than a display species, occurring as black prismatic microcrystals resting on aggregates of the mimetite-pyromorphite series; the best-documented association is with orange to orange-brown barrel-shaped campylite-style aggregates and nearby white to gray, slightly etched reticulated cerussite in gossan fractures from the upper part of the open pit, so collector-grade pieces are judged less by crystal size and more by well-preserved black microcrystals in clean contrast on bright arsenate-phosphate aggregates, especially when accompanied by convincing old Guatomo or “Hat Yai” provenance and, ideally, analytical confirmation.
Other minerals documented from Guatomo include cerussite, anglesite, galena, pyrite, arsenopyrite, magnetite, cassiterite, quartz, goethite, malachite, linarite, carminite, cesàrolite, hidalgoite, philipsbornite, segnitite, pyromorphite, biotite, and garnet-group skarn material. Guatomo is not known as a type locality, but it is important for the chemistry of the mimetite-pyromorphite series and the hidalgoite-philipsbornite series: published microprobe work showed that specimens visually collected and traded as mimetite may range from near end-member mimetite through P-rich mimetite to As-rich or As-enriched pyromorphite, and that green to pale-blue “philipsbornite” coatings and pseudomorphs are commonly dominated by hidalgoite with only minor S-rich philipsbornite zones.
The principal authenticity issue is locality, not artificial enhancement. Many legitimate specimens carry the old protective label “Hat Yai,” “Hat Yai District,” or “Songkhla Province,” but the accepted source is Guatomo Mine at Tham Thalu, Bannang Sata District, Yala Province. A fine old specimen with a Hat Yai label should not be dismissed; instead, it should be reconciled to Guatomo if the habit, matrix, and provenance fit the 1996–1998 material. Conversely, vague Thai “Hat Yai mimetite” labels should be treated carefully, especially if the piece lacks the characteristic gossan, altered granite, quartz, or Pb-Mn oxide matrix.
Species labeling is the second issue. Guatomo is a chemically mixed mimetite-pyromorphite locality. Some yellow to orange crystals are near end-member mimetite, while some similar-looking material is P-enriched mimetite, As-rich pyromorphite, or As-enriched pyromorphite. The “campylite” habit is particularly treacherous: barrel-shaped orange aggregates may have been sold as mimetite var. campylite, but published work demonstrated pyromorphite-series compositions in part of this visual population. For high-end pieces, old collection labels are valuable, but analytical data are better.
Condition matters sharply. The finest crystals stand proud from pocket walls and are vulnerable to bruising at terminations, broken hopper rims, and edge damage where specimens were removed from gossan. Quartz-matrix pieces tend to be sturdier than porous limonitic gossan pieces, but quartz specimens can show missing isolated crystals more plainly. Gossan pieces may shed grains or stain labels and boxes; store them in fitted boxes and avoid repeated handling. Cerussite associations, especially acicular or reticulated forms, are brittle and should be kept away from pressure points.
No well-documented Guatomo-specific faking or treatment tradition is widely recorded, but relabeling and over-optimistic species names are common enough to affect value. Be cautious with pieces marketed simply as “Thailand mimetite” without mine history, with unusually perfect detached crystals on unrelated matrix, or with a locality label that omits the old Hat Yai/Guatomo context. Detached hopper crystals are known, but matrix specimens with convincing attachment, consistent iron oxide or altered granite matrix, and pre-2000 provenance are more desirable.
Handling is straightforward but should be disciplined. Mimetite, pyromorphite, plattnerite, cerussite, anglesite, and several associated phases contain lead, arsenic, or both. Do not cut, abrade, or ultrasonically clean these specimens. Wash hands after handling, keep them away from children and food-preparation areas, and avoid generating dust from friable gossan. Guatomo pieces are not bought for fluorescence; they are bought for color, luster, habit, and provenance.
Market availability is limited but not vanishing. The great supply pulse was the 1996–1998 recovery, and better pieces now appear mainly from old collections, dealer archives, and auctions. Small vivid crystals and thumbnails still surface, while sharp miniatures and cabinet pieces with saturated yellow-orange color, undamaged hopper growth, quartz or gossan matrix, and old Hat Yai provenance are much harder to replace. Plattnerite from Guatomo is a rarity for systematic collectors and is usually of micromount interest unless it occurs as a well-documented association on an attractive mimetite-pyromorphite specimen.
The Guatomo story begins with an old mine whose fame had nothing to do with display specimens. In the nineteenth century the Tham Thalu or Great Mine was already important as an ore deposit, and by the early 1980s it was still producing low-grade tin concentrate. The mine’s ore was metallurgically awkward: arsenopyrite-rich material had to be roasted, and the concentrate could carry so much oxidized lead that Pb-Sn alloy was produced during smelting. Then the tin market collapsed, mining stopped in 1985, and the open pit sat as weathered granite, gossan, quartz veins, and oxidized lead minerals in tropical Yala.
A decade later, Marco P. Sacchi and Danilo Bernasconi walked into what collectors now recognize as one of the great mimetite opportunities of the late twentieth century. Between 1996 and 1998 they recovered hundreds of specimens from the abandoned pit. The find was rich enough to make a sensation but sensitive enough that the true locality was obscured. When the specimens appeared at the 1998 Bologna show, they were introduced as “Hat Yai” mimetites from Songkhla Province rather than as Guatomo material from Yala. That protective disguise worked so well that it became part of the specimens’ identities: decades later, good labels may still read Hat Yai even when the locality is understood to be Guatomo.
The best pieces explain why the misdirection endured. Guatomo produced crystals that looked purpose-built for collectors: yellow-orange prisms on quartz, bright skeletal yellow crystals in gossan pockets, orange barrels and hoppers, and gemmy pieces where a single crystal could dominate a miniature. Later scientific sampling gave names and chemistry to the pocket styles. Sample M1 represented rich groups of lustrous yellow-orange to orange prismatic crystals up to 1 cm on hydrothermal quartz in altered granite. Sample M4 captured the philipsbornite-trench style: gemmy lemon-yellow to yellow-orange hexagonal long prisms, also up to 1 cm, in limonitized granite. Sample M5 was the gossan classic: pale-yellow to orange-yellow short prismatic or tabular skeletal crystals reaching 2.5 cm in irregular pockets.
The locality also played a trick on everyone’s eyes. The yellow and orange crystals were sold and celebrated as mimetite, and many of them are indeed mimetite. But microprobe work later showed that Guatomo’s arsenate-phosphate chemistry is broader than the old labels suggest. Some material crosses into pyromorphite-series compositions even while retaining the visual language of mimetite. For collectors, that does not diminish the locality; it makes it more interesting. Guatomo is both a classic mimetite locality and a case study in why color and habit alone can mislead even experienced eyes.
Brown, G. F., Buravas, S., Charaljavanaphet, J., Jalichandra, P., Johnston, W. D., Sreshtaputra, V., and Taylor, G. C. (1951). Geologic Reconnaissance of the Mineral Deposits of Thailand. U.S. Geological Survey Bulletin 984, 1–197. This reconnaissance records Guatomo’s pre-World War II opening by Yala Mines Ltd., later acquisition by Anglo-Oriental Tin Co., open cuts, shafts, and arsenopyrite-rich cassiterite ore.
Bode, Rainer (1998). “Sicher Weltklasse: Die neuen Mimetesite aus Thailand.” Mineralien-Welt 9(3), 43–44. Early collector-publication coverage of the Thai mimetite find, cited in later locality and chemical studies.
Sacchi, Marco P. (2017). “The Guatomo (‘Hat Yai’) mine near Tham Thalu, Bannang Sata District, Yala Province, Thailand.” The Mineralogical Record 48(6), 731–749. The key modern collector article on the locality, its collecting history, and the mimetite styles.
Števko, Martin; Sejkora, Jiří; and Dolníček, Zdeněk (2020). “Contribution to the chemical composition of minerals of the mimetite-pyromorphite series from the Guatomo mine near Tham Thalu, Bannang Sata District, Yala Province (Thailand).” Bulletin Mineralogie Petrologie 28(2), 237–245. Essential analytical work showing that Guatomo material includes near end-member mimetite, P-enriched mimetite, As-rich pyromorphite, and As-enriched pyromorphite.
Števko, Martin; Dolníček, Zdeněk (2021). “New data on minerals of the hidalgoite-philipsbornite series from the Guatomo mine near Tham Thalu, Yala Province (Thailand).” Bulletin Mineralogie Petrologie 29(2), 270–274. Focused microprobe study of the green to pale-blue hidalgoite-philipsbornite material, including pseudomorphs after mimetite.
Mindat occurrence record for mimetite from Guatomo Mine. Useful for mineral occurrence status, photo count, references, and the world-class rating of Guatomo mimetite.
Mindat occurrence record for philipsbornite from Guatomo Mine. Records philipsbornite, associated photo data, references, and the RRUFF sample link noted for this occurrence.
Mindat locality page: Guatomo Mine (“Hat Yai” mimetite locality) — Best single locality database page for coordinates, hierarchy, alternate names, mineral list, photos, and references.
Bulletin Mineralogie Petrologie paper on the mimetite-pyromorphite series — The most important open analytical source for Guatomo’s yellow-orange collector material.
Bulletin Mineralogie Petrologie paper on the hidalgoite-philipsbornite series — Essential for understanding the green to pale-blue coatings and pseudomorphs associated with Guatomo mimetite.
USGS Bulletin 984: Geologic Reconnaissance of the Mineral Deposits of Thailand — Historical government source for Guatomo’s mining, operators, and ore-treatment observations.
Fabre Minerals reference specimens from Asia, including Guatomo mimetite — Dealer archive with specimen descriptions, sizes, habits, and references to illustrated Mineralogical Record examples.
Mineral Auctions: 1998-find Guatomo mimetite miniature — Useful market archive noting large crystal size, old-find status, and collector valuation.
Mineral Auctions: ex Bedogne Collection Guatomo mimetite — Good example of old-collection provenance and the “one-time 1998–99 finds” market narrative.
Minfind: Guatomo hopper mimetite listing — Contemporary dealer-market example for a detached hopper crystal with price and condition description.
