Ioan FILIP¹ & Eugen ORLANDEA^2
1Carmen Bajo Mining Company, P-je Hochschild 113-A, Copiapo, Atacama, Chile, e-mail: ioangfilip@gmail.com
²Carmen Bajo Mining Company, P-je Hochschild 113-A, Copiapo, Atacama, Chile,e-mail: eugenorl@yahoo.com

GOLD POSITION AND DISTRIBUTION IN LOWER CRETACEOUS IOCG DEPOSITS OF THE COPIAPO BASIN, ATACAMA REGION, CHILE

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Detailed geological, petrographic and metallogenic data, including stable isotopes and fluid inclusions have been derived for vein-stockwork mineralization, stratabound to Manto type and replacement-lens shaped to skarn/contact ore bodies of the Lower Cretaceous IOCG Copper Belt near Copiapo, Atacama (Ojancos Viejo-Ojancos Nuevo tectonic alignments). At least three types of gold association can be distinguished in this IOCG environment, as follows:

1. Early iron oxide (magnetite) related gold (M type) is included in early mesothermal magnetite and mushketovite associated with fine grained biotite with/without potassic feldspar. The fluids of the M type gold bearing stage were hypersaline (41-59%wt.NaCl eq.) with temperatures ranging from 350⁰ to >500⁰C. The source of fluids was mainly magmatic (δ³⁴S from -5.3 to 4.7) with possible crustal rock (e.g. evaporite) assimilation during the magma ascent.

2. Later copper sulfides (chalcopyrite) related gold (C type) is genetically linked with, the mostly mesothermal, main copper event and associated with proximal Na-rich (albite) alteration. C type gold was deposited from magmatic or near magmatic fluids (δ³⁴S from -6.9 to 4.1) which, also were highly saline (32-59%wt.NaCleq.) at temperatures varying between 250⁰ and 400⁰C.The evolution of the mineralized system may have included the introduction of an external saline brine and the possible mixing of two different types of fluid.

3. Late pyrite or hematite (specularite) related gold (P/H type). This gold type is recorded either as a subsequent epithermal overprint or a still preserved epithermal suite in the upper part of the structures. The key proximal alteration is a sericite bearing intermediate argillic halo or carbonate – hematite dominant assemblage. The fluid temperatures were between 180⁰-290⁰C, with moderate salinities (15-20%wt. NaCleq.).The mixing of magmatic (δ¹³C from -3 to -9; δ³⁴S between 0-2) and meteoric fluids (δ¹⁸O between 9-11; δ³⁴S extreme values of 7.6 and 8.3), including boiling phenomena characterize this gold bearing stage.