Pyrolusite dendrites from the Mina Caracoles

--- Atacama desert, northern Chile, South America

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Figure 1. Rock fragments from the Mina Caracoles, displaying fine pyrolusite dendrites. The 100-peso coin is 27 mm in diameter. This is the area of the old Sud-America mine (compania minera Flomax), and there are a number of deep, dangerous shafts and other open mine workings along near-vertical veins, often on approximately north-south structures. The samples are from an area with an old hito (a squat pyramidal survey marker or milestone), a vein trending N.W.-S.E., and at least 3 dangerous shafts. The mine waste is full of a white to greenish-white altered felsic intrusive rock, sparsely porphyritic with 1% 1-3 mm rounded quartz eyes and more abundant white feldspar phenocrysts. This rock is coated with abundant dendrites, as shown here.

"Rock of the Month #182, posted for August 2016" ---

Pyrolusite dendrites

formed on exposed faces of rock fragments from a mine in the arid environment of northern Chile. Pyrolusite is a common manganese oxide, tetragonal β-MnO2, a polymorph of orthorhombic ramsdellite, γ-MnO2. Pyrolusite has a measured specific gravity of 5. Pyrolusite is one of many black Mn oxides. It is found worldwide, across, e.g., Chile, Brazil, Mexico, Cuba, Canada, Sweden, Egypt, Morocco, Ghana, South Africa, India, Russia, China and Australia.


The growth of dendrites has been modelled by analogy to diffusion processes (Chopard et al., 1991). In rock units such as the famed Solnhofen limestone of Europe, the precipitation of Fe and Mn dendrites is often due to a rise in pH associated with CaCO3 in solution (Van Straaten, 1978). Pyrolusite is one of the principal Mn oxides, and was first described by Haidinger in 1827 (see, e.g., Varley, 1849). Mn dendrites are often a part of so-called rock varnish or desert varnish, developed by weathering in arid conditions (McKeown and Post, 2011).

Dendritic form is not restricted to the oxides, but is often reported in native metals, such as gold and copper. In bonanza epithermal Au-Ag deposits, such as Sleeper and National in Nevada, the richest ores commonly contain native Au dendrites 0.05 to circa 5 cm in diameter. SEM imaging suggests that the dendrites formed by aggregation of spheroidal gold particles 1-100 nm in diameter. The dendrites exhibit a fractal structure, and are indicators of turbulence in hydrothermal fluids and random motion of gold colloid particles, consistent with boiling in the ore fluid (Saunders and Schoenly, 1993).


Inland and northeast of the port city of Antofagasta is the Sierra Gorda, and the old Caracoles mining district. The desert hosts the ruins of old salitre mining camps (with oficinas, salitreras, tortas), based on the late 19th century shallow surface excavation of nitrate deposits. Southwest of Calama, the area of the Cerros de Caracoles Pb-Ag mining camp displays extensive vein systems. The Caracoles silver camp has a long history (Enos, 1996, p.67). The camp is located at 23.00-23.10°S, 68.97-69.07°W, E.S.E. of Sierra Gorda.

Extensive beds of the common sulphate gypsum (CaSO4.2H2O), some brecciated, are present in the Caracoles area. Some exposed faces of white gypsum develop distinctive surfaces, smooth but fluted on a centimetre scale. The region is noted for gypsum (Ferraris and Di Diase F, 1978). The gypsum (yeso in Spanish) occurs in Jurassic marine sediments of the Caracoles Group, >800 m of sandstone, lutite, gypsum and limestones (Ramirez R and Gardeweg P, 1982, p.23).

Barite is common in many veins. Galena and barite are often found in banded vein material in the Caracoles area, frequently with white alteration on the rock, the galena itself weathered a dull grey in unbroken surfaces. The city of Calama (1980 population 69,000) is the capital of El Loa province, west of the northern end of the Cordillera de Domeyko, near the giant porphyry Cu deposit of Chuquicamata (Marinovic S and Lahsen A, 1984). Silver deposits in northern Chile include Arqueros, Chanarcillo, Caracoles, Tres Puntas- Chimberos, and Huantajaya- Santa Rosa (Sillitoe, 2007).

Geophysical studies in the region may reveal magnetic anomalies related to fracturing, hydrothermal alteration and subsequent supergene effects. One control on mineralization is the contact between a mid-Tertiary adamellite porphyry intrusion and calcareous sediments of mid-Jurassic age. Intense fracturing, argillic alteration and silicification occur at the contact (Flores V, 1976). Ag deposits in Jurassic sediments are hosted by Dogger to Oxfordian marine limestones intruded by granodioritic to dioritic bodies as recent as the mid-Tertiary: examples are Huantajaya near Iquique and the Caracoles district. Propylitic hydrothermal alteration is noted, and minerals include pyrargyrite, proustite, argentite, native Ag and Ag halides (Oyarzun, 1990).

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Figures 2-4. Old mine workings in the arid region of the Caracoles mining district. Some local views (left to right, Figs. 2-4) oriented approximately to the north, W.N.W. and south. The flanking photographs appear brighter because they capture the early-morning light. All photos are from 12 April 1996.


Two examples out of a multitude.... Pyrolusite may occur with base-metal mineralization elsewhere. An example is the sediment-hosted copper deposits in the Cretaceous Las Vigas Formation in northern Chihuahua, Mexico. Supergene alteration of primary sulphides filling pore spaces between quartz grains has generated malachite, azurite and tenorite, plus minor chalcocite and Fe oxides (Giles et al., 1973). In western Sweden, Mn mineralization occurs in soft Fe ore. Mn oxide dendrites occur in the iron ore, in argillized wallrock (skarn and leptite) and in secondary calcite veins. The `soft iron ore' arose through weathering of primary magnetite ore, wherein the magnetite altered into hematite, and eventually to goethite. The Mn oxides are unusually pure, perhaps because the enclosing argillaceous rocks had a very low permeability (Ljunggren, 1960).


Chopard,B, Herrmann,HJ and Vicsek,T (1991) Structure and growth mechanism of mineral dendrites. Nature 353, 409-412, 03 October.

Enos,GA (editor) (1996) Historia de la Minería en Chile. Comunicación Total Ltda, Santiago de Chile, 2nd edition, 421pp. (in Sp.).

Ferraris B,F and Di Diase F,F (1978) Hoja Antofagasta, Region de Antofagasta. Servicio Nacional de Geologia y Mineria, Carta Geologica de Chile, sheet 30, 1:250,000 scale map and book (48pp.) (in Sp.).

Flores V,R (1976) Control de la mineralización de plata en el distrito minero de Caracoles, II region, Antofagasta - Chile. Thesis (Memoria de Título), Departamento de Geociencias, Universidad del Norte, Antofagasta, Chile, 76pp. (in Sp.)

Giles,DA, Alvarez,A and Beales,FW (1973) Cobre singenetico en la formacion Las Vigas del Cretacico Inferior del estado de Chihuahua, Mexico. Memoria de la X Convencion Nacional de la Asociacion de Ingenieros de Minas, Metalurgistas y Geologos de Mexico, 671pp., 319-326 (in Sp.).

Ljunggren,P (1960) Todorokite and pyrolusite from Vermlands Taberg, Sweden. Amer.Mineral. 45, 235-238.

Marinovic S,N and Lahsen A,A (1984) Hoja Calama, Region de Antofagasta. Servicio Nacional de Geologia y Mineria, Carta Geologica de Chile, sheet 58, 146pp. report plus 1:250,000 scale map (in Sp.).

McKeown,DA and Post,JE (2001) Characterization of manganese oxide mineralogy in rock varnish and dendrites using x-ray absorption spectroscopy. Amer.Mineral. 86, 701-713.

Oyarzun,JM (1990) The metalliferous ore deposits of Chile and Argentina, and their geologic framework. In `Stratabound Ore Deposits in the Andes' (Fontbote,L, Amstutz,GC, Cardozo,M, Cedillo,E and Frutos,J, editors), Springer-Verlag, 815pp., 61-78.

Ramirez R,CF and Gardeweg P,M (1982) Hoja Toconao, region de Antofagasta. Servicio Nacional de Geologia y Mineria, Carta Geologica de Chile, sheet 54, 1:250,000 scale map and book (121pp.) (in Sp.).

Saunders,JA and Schoenly,PA (1993) Boiling-induced chaos and the genesis of gold dendrites in bonanza epithermal Au-Ag deposits. GSA Abs.w.Progs. 25 no.6, 489pp., 413-414, Boston.

Sillitoe,RH (2007) Hypogene reinterpretation of supergene silver enrichment at Chanarcillo, northern Chile. Econ.Geol. 102, 777-781.

Van Straaten,LMJU (1978) Dendrites. Quart.J.Geol.Soc. 135, 137-151.

Varley,D (1849) Rudimentary Treatise on Mineralogy for the use of Beginners. John Weale, London, 164+vipp.

Graham Wilson, 12-15 June 2016

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