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In the Santander Massif of the Eastern Cordillera of Colombia, intense Pliocene to Holocene sulfide-rich gold mineralization occurs in a six kilometers long section of the La Baja fault zone, where it crosses partially migmatized gneisses and intrusions of Triassic-Cretaceous quartz monzonites and granites. These intrusives are mineralogically and texturally similar to the neosome of the migmatites that they intrude. The mineralization occurs as medium to large veins in the gneisses and as small veins, veinlets and disseminations in the intrusives. The type, volume and metal grades of the veins are dependent on the local structure, and the chemical composition and degree of reactivity of the host rocks. Thicker and higher-grade veins of the Mascota-type occur in chemically favorable magnetite-hornblende gneisses and migmatites, deposited after phases of silicification, and brecciation along the foliation of the host rocks. The Gigante-type shear veins occur in splays of the fault, and are marked by shearing and mylonitization. Lower-grade, flat-banded veins appear in leucocratic quartz-feldspar gneiss and related migmatites. The mineralizing fluids ascended along fault planes, and invaded the gneisses in fractures oblique to them. In all deposits, gold occurs in free form, as electrum and in tellurides, and accompanied by copper and silver.
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