"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.

PYROLUSITE AND DENDRITIC CRYSTAL HABIT

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).

REGIONAL SETTING OF MINA CARACOLES

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).

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.

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