Morphological and compositional analysis of alluvial gold: The Fresnedoso gold placer (Spain)

Abstract

The Fresnedoso creek gold placer (FGP) in the Moraleja Basin (W Iberian Massif) has been investigated in terms of morphological, textural and compositional evolution of gold particles. A mixture of two populations has been identified being coherent with primary sources located at a proximal (2.5–10 km), and distal (20–50 km) range. Shape and compositional evidences suggest recycling of paleoplacers (tertiary) has to be considered to some extend. Primary laminar morphologies point to lode deposits in small-flat veins hosted in the metasediments of the Schist Greywacke Complex. All these features suggest the Fresnedoso creek gold placer is a deposit constituted by with mono- and polycyclic particles. Transport distance – Flattening indexes (CFI, Shilo) models have been tested and result in useful information for exploration of undiscovered deposits, even with a limited dataset. Compositional analysis of gold particle morphotypes has revealed that the Fresnedoso gold is a AuAg bimetallic alloy. Textural and compositional groups include: type 1 (Au1 = Au88-94Ag12-6), type 2 (Au2 = Au99 Ag1) and type 3 (Au3 ~ Au99Ag1). Variations in composition of type 1 (Particle's cores) could reflect differences in primary composition, spatial dispersion of sources or secondary processes, but fit within the compositional range of orogenic gold deposits. Secondary processes have been explored to explain compositional heterogeneity in the particles. Gold type 2 (rim) and 3 (micro-aggregates) represents two different de-alloying stages, from initial Ag-leaching at the rim and/or through grain-boundaries and microfractures (Au2), to complete resetting of primary chemical imprint, pervasive porous texture and Au re-precipitation with Fe-OOH and clays (Au3). A model is proposed which combines: Ag de-alloying enhanced in a Cl-Fe rich environment (lateritic) and the deformation/recrystallization processes related to mechanical cold-work in the river load-bed during transport.