PALOMO MINE: HUANCAVELICA DEPARTMENT, PERU
Jaroslav Hyrsl
Mineralogical Record
Feb 29, 2008 19:00 EST
INTRODUCTION
The Palomo (or El Palomo) mine is situated in a dry valleyat the considerable altitude of almost 5000 meters above sea level, between themuch more famous Julcani and San Genaro mining districts in HuancavelicaDepartment, Peru. A property of the San Genaro Mining Company, the Palomo minebelongs metallogenically to the Huachocolpa ore district (Samame-Boggio, 1985).The last active mining took place between 1995 and 2000, when the Palomo minewas leased by the Buenaventura Company. Now the mine is closed, and only anentrance portal and the remains of a few buildings are in evidence at the site.However, during the last few years the mine has given up mineral specimens to afew independent miners who are mining strictly for specimens. The number ofspecies known from the Palomo mine is not impressive, but some specimens areamong the best to have come from Peru in recent times, and the new realgarspecimens are among the more interesting examples of the species.
GEOLOGY
The Palomo mine area is composed entirely of volcanic rocks:andesitic lavas, pyroclastic rocks, tuffs and breccias. All of the mineralizedveins are found within these rocks. In the depths lies an intrusive dioritestock which is probably genetically related to the mineralization.
Three vein systems are known in the mine. The main system(the Palomo 1, Palomo 2 and Palomo 3 veins) strikes north-south; the second(the La Suerte and 7-A veins) strikes northeast-southwest; the third (thePalomo 6 and 6-A veins) strikes northwest-southeast. The mineralization ischaracterized by banded vein fillings of rhodochrosite with calcite, barite andquartz occurring as common gangue minerals. The main sulfide minerals arepyrite, galena, sphalerite and chalcopyrite, and much less commonlytetrahedrite and silver sulfosalts (pyrargyrite, proustite and poly basite).Realgar, orpiment and stibnite are common in the outer parts of the deposit.The mine was closed mainly because of problems with water flow, and all of theworkings below the main gallery are now flooded. Nevertheless, economicallysignificant ore reserves are known to exist at depth, with the silver contentincreasing downward.
MINERALS
Only minerals represented in recently found specimens aredescribed here; it is probable that no significant specimens of silverminerals, rhodochrosite, calcite or barite have survived from the period ofactive mining.
Chalcopyrite CuFeS^sub 2^
In 2006 a small number of specimens containing chalcopyrite crystals was found. The largest specimen shows a crystal 3 cm across on drusy quartz but, unfortunately, the crystal has an unattractive gray matte surface because small tetrahedrite crystals are epitactically overgrown on it. One broken chalcopyrite crystal shows the typical metallic-golden color inside.
Cinnabar HgS
A red crust of botryoidal cinnabar was identified on several specimens of quartz found in 2003.
Galena PbS
Massive granular galena is quite common in the mine, and a few unusual crystal specimens were found at the end of 2006. These show cuboctahedral galena crystals up to 2 cm, with heavily tarnished surfaces. Because of their idescent colors of these surfaces, Peruvian dealers call galena of this type"pecho paloma" (breast of dove). Some spinel-law-twinned, flattened,untarnished galena crystals to 4 cm with rounded edges have also been found.Some of them are covered by tiny seligmannite crystals, yellow orpiment andpowdery orange pararealgar.
Gypsum CaSO^sub 4^*2H^sub 2^O
Large gypsum crystals, similar to old specimens from Raura,were found in the Palomo mine at the end of 2003, in the vein which producedthe stibnite specimens (see below). Some of these specimens show transparent,colorless, long-prismatic crystals of gypsum to about 25 cm (completelytransparent gypsum crystals occur only rarely in the mine), and one dealer saysthat prisms more than 50 cm long were recently found and broken into smallerpieces for easier transport. Another occurrence in the mine yielded gypsumcrystals to about 10 cm (transparent inside but with corroded, milky surfaces)covering matrix. One 13-cm gypsum crystal is naturally curved into an S-shape.The most beautiful gypsum specimens of the Palomo mine, like no other specimensfound anywhere in Peru, are gypsum cleavages containing clusters of stibnitecrystals. Still rarer, and very attractive, are a few gypsum crystals, found in2004, containing very thin, phantom-like veils colored bright yellow or orange;commonly both colors appear in the same crystal. The inclusions are difficultto identify, but their hues are like those of native sulfur and orpiment. Onecrystal, found in 2006, includes small but sharp crystals of red realgar.
Orpiment As^sub 2^S^sub 3^
Some of the realgar specimens found in 2003 carry yellowcrusts of orpiment. Nice orpiment specimens found in January 2006 showbotryoidal yellow encrustations on quartz "peppered" by tiny orangecrystals of a younger orpiment generation. The best orpiment specimens so farfound in the mine emerged at the end of 2006; diese combine botryoidal yellowaggregates and fan-like aggregates of yellow-orange orpiment cleavages to about5 cm.
Orpiment pseudomorphs after realgar are a specialty of the Palomo mine. The first specimens were found in spring 2006, but the best ones came with the best realgar crystals at the end of 2006. Some of ttiepseudomorphs are complete replacements of realgar by orpiment, while others arepartial, showing relict realgar inside. The complete pseudomorphs are usually hollow and very fragile, in sizes up to 4 cm.
Pararealgar AsS
Orange, crystallographically parallel zones seen in someloose realgar crystals found in 2000 consist of pararealgar.
Pyrite FeS2
Tarnished pyritohedral crystals of pyrite to 1 cm with beautiful iridescence appeared at the end of 2006, together with iridiscent galena crystals (see under galena).
Quartz SiO^sub 2^
Hundreds of specimens showing thin, platy pseudomorphs of quartz after barite were found in 2006. The pseudocrystals measure between 1and 2 cm, exceptionally reaching 10 cm. A banded, agatelike inner structure is evident on some of the larger broken plates. A few of the pseudocrystals arecovered by thin stibnite needles or by orpiment or realgar crystals.
Realgar As^sub 4^S^sub 4^
Loose realgar crystals to 2 cm from the Palomo mine firstappeared in 2000. Some of the crystals are partially altered to orangepararealgar, which forms crystallographically continuous zones. A few specimensshowing lustrous realgar crystals to 1 cm with tetrahedrite, galena andsphalerite were found in 2003.
At the end of 2006, the Palomo mine produced quite a largenumber of specimens (probably a few hundred) showing what are by far the bestPeruvian realgar crystals ever discovered. Most of the crystals are columnar,their sizes ranging to almost 5 cm long and 1 cm thick. Platy crystals, also to5 cm, are very rare. Most of the realgar crystals are slightly rounded, but afew with sharp edges were also found; some crystals are skeletal. Unfortunately,realgar is extremely fragile and virtually all specimens show at least a littledamage.
Seligmannite PbCuAsS^sub 3^
Tiny, brightly lustrous prismatic crystals up to nearly 1 mmseen rarely covering sphalerite or galena surfaces have been identified asseligmannite by Anthony R. Kampf (personal communication, 2007). Some of theseligmannite crystals form small star-like groups, probably because oftwinning. They are associated with younger realgar crystals and orpiment.
Sphalerite ZnS
Complex brownish black sphalerite crystals up to 3 cm werefound with realgar and orpiment. Rarely they are covered by small tetrahedritecrystals. Sphalerite from the Palomo mine also forms lustrous botryoidalchocolate-brown aggregates composed of microcrystals.
Stibnite Sb^sub 2^S^sub 3^
In 2003 the mine produced strange stibnite specimens, whichlocal dealers call "cactus." Columnar stibnite crystals arecompletely covered by tiny, later-generation crystals which growperpendicularly on the earlier crystals. Individual crystals reach 2 cm, andthe druses reach 10 cm wide. Stibnite druses of the same type embedded intransparent gypsum have been found regularly since 2004. The same mine has alsoproduced druses of hemispherical aggregates, each hemisphere formed byradiating acicular crystals of stibnite.
Tetrahedrite Cu^sub 6^Cu^sub 4^(Fe,Zn)^sub 2^(Sb,As)^sub4^S^sub 13^
One specimen found at the end of 2006 shows two sphericalaggregates, each composed of dozens of tetrahedral crystals to 1 cm; the largersphere is 3 cm in diameter. Similar tetrahedrite crystals have been found asovergrowths on chalcopyrite and sphalerite crystals; realgar and orpimentpseudomorphs after realgar are overgrown on the tetrahedrite.
ACKNOWLEDGMENT
Thanks to ing. Julio Meza for information about the geologyand history of the mine.
Source: Mineralogical Record
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