Paper Number: 2887
Magmatic, mineralogical, and hydrothermal fluid constraints at the Marmato gold deposit, Colombia.
Santacruz, R.L.1, Matteini M.2, Botelho N.F.3, Redwood S.D.4, Molano M.J.C.5, Cecchi A.6
1 MSc Student, Brasilia University, Brasilia, Brazil, lsantacruzr@unal.edu.co
2-3 Professors at Geosciences Department, Brasilia University, Brasilia, Brazil
4 Consulting Geologist, Panama City, Panama
5 Professor at Earth Science Department, National University of Colombia, Bogotá, Colombia
6 Vice President of Exploration, Gran Colombia Gold Corp., Medellin, Colombia
____________________________________________________________________________The Marmato epithermal Au-Ag deposit occurs in one of the premier mineral provinces of Colombia, the Middle Cauca Gold Belt, in the Western Cordillera of the northern Andes, and is associated with late Miocene intrusive and volcanic rocks. Open pit resources are estimated at 15 Moz Au and 90 Moz Ag with grades of 0.94 g/t Au and 5.65 g/t Ag contained in 496 million tonnes, at a cut-off grade of 0.3 g/t Au. Mineralization is hosted by the Marmato porphyry intrusion suite which is formed by five dacite-andesite porphyry intrusions named P1 to P5 from oldest to youngest. The timing of the magmatism is bracketed by LA-ICP-MS 206Pb/238U zircon dates of the P1 dacite stock of 6.576 ± 0.075 Ma, and P5 dacite dikes of 5.75 ± 0.11 Ma. Sericite alteration was previously dated at 5.6 ± 0.6 Ma.
The Marmato intrusions are part of the larger Marmato Stock that intrudes rocks of the oceanic Late Jurassic to Early Cretaceous Romeral terrane, which was accreted along the N-S trending Romeral Fault in the Aptian, and is partly covered by Neogene sediments and volcanic rocks. The Marmato intrusions are characterized by a Mn-ilmenite±Ti-magnetite magmatic association suggesting high ƒO2, 60.8 to 66.1% SiO2, relatively high Sr/Y, EuN/Eu* ratios mostly <1, Mg# <0.5, and (DyN/YbN) <1.5. The geochemical signature shows adakite-like compositions with fractionation and/or restite equilibration effects of amphibole in a hydrous silicic melt [1], with subsequent major plagioclase fractionation. The Sr-Nd isotopic compositions indicate a juvenile, mantle-derived magma with crustal contamination by assimilation as the main source of the magmas.
Mineralization extends over 1,400 m vertically, is open at depth, and is structurally controlled in NW and WNW trends by Riedel shears in a sinistral transpressional shear system related to the oblique collision of the Panama-Chocó arc with the northern Andes. The Marmato gold deposit is divided into the upper (1,600 to 900 m amsl) and the lower (900 to 200 m amsl) zones, characterized by the type of mineral occurrences, gangue and sulfide mineralogy, alteration, and gold composition (electrum in the upper zone and native gold-bismuth associated in the lower). The hydrothermal fluid chemistry is H2O–NaCl, sulfur is magmatic, and fluid inclusion homogenization temperatures and salinities vary from the top to the bottom of the deposit between 305 to 395°C and 5.0 to 11.9 wt% NaCl eq., respectively. The sulfur fugacity was measured between -9.2 to -11 log fS2 from sphalerite in the upper zone, whereas cathodoluminescence shows strongly zoned quartz with some internal zones of amorphous silica indicating rapid variations in the fluid conditions.
The geochemical signature for the porphyry intrusions at Marmato is similar to that of the La Colosa porphyry and Buritica epithermal gold deposits, also located in the Middle Cauca Gold Belt. It is a good regional exploration feature for porphyry to epithermal Au-Ag±Cu±Zn±Mo deposits that formed in the late Miocene arc. Finally, the gold-bismuth association, mineralogy, chemistry, and fluid conditions indicate that the liquid bismuth collector model [2] may have played an important role during mineralization in Marmato, concurrent with tectonic uplift, to generate the vertical zonation.
[1] Richards J and Kerrich R (2007) Econ Geol 102: 537-576.
[2] Douglas N et al. (2000) In: Austral Geolog Convent 15th, 135.