Geology, mineralogy and possible origin of the copper mineralization in marble near Saldán, Córdoba (Argentina)

COLOMBO, FERNANDO; LIRA, RAÚL; RIPLEY, EDWARD M.; GONZÁLEZ DEL TÁNAGO, JOSÉ

Journal of Geosciences

Czech Geological Society

Lugar: Praga; Año: 2011 vol. 55 p. 299 - 316

1802-6222

Copper minerals (chrysocolla>>dioptase>>plancheite>tenorite) plus barite and quartz occur in a small lens of granulite grade calcite-dolomite marble near Saldán, Córdoba province (central Argentina, 31º18´50.7´´S-64º19´50.0´´W). The mineralization is hosted in fractures (striking N30ºE and dipping 78ºW) that were widened by dissolution. Chrysocolla mainly fills fractures. It has variable H2O contents but a Cu:Si ratio close to 1:1. Plancheite forms compact fissure fillings and aggregates of interlocking spherules. Copper is partially replaced by Mg (up to 2.22 wt% MgO), with small amounts of Na, K, Ca, Al and Fe also present. Dioptase occurs as prismatic crystals dominated by {100} and {111} that can reach over 2 cm long, and also as granular fracture fillings. Unit-cell parameters are a 14.5719(6), c 7.7799(3) Å, V 1430.7(1) Å3; refractive indices are e = 1.707(3), w = 1.656(3), with an anomalous small 2V. Magnesium (up to 0.15 wt% MgO) is the only significant substituent. Yellow barite crystals tabular on {001} have traces of Sr. Quartz occurs rarely, as drusy linings. A few cases of botryoidal, opal-like quartz crusts are known. Tenorite forms dark brown masses and stringers included in marble. Very scarce grains of copper sulfides (anilite altering to spionkopite and covellite along rims and cracks) are found scattered in the marble. Copper is partially replaced by Bi (up to 0.15 wt% Bi) and Pb (up to 0.16 wt% Pb). Malachite and brochantite are alteration products of sulfides. Fluid inclusion studies indicate that minerals precipitated from a single-phase fluid initially at a temperature above 300ºC that cooled to ca. 220ºC. Salinity was very low, from 0 to 2 wt% NaCl eq. Isotopic analyses of calcite give d13C values of 0.4-0.9‰ PDB and d18O values of 17.1-15.1‰ VSMOW, coincident with the isotopic signature of regional marbles. Calculated values of d18O for water in isotopic equilibrium with calcite coincide with the Metamorphic Water Box. d34S values of barite  are 3.6-4.6‰ CDT.   The mineralization possibly precipitated from fluids of meteoric origin that suffered an oxygen isotope shift due to a very low water/rock ratio. The sulfur source could be sedimentary or igneous sulfides, or a mixture from two different sources (such as heavy seawater-derived sulfur and light sedimentary sulfides), but not exclusively seawater.  Copper was probably scavenged from accessory sulfides scattered in barren marble or ortho-amphibolite. The heat source remains unknown.