Page 50 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 31

1.11 Influence of pre-Miocene gold distribution on possible recycling
processes of Quaternary placers in northwest Spain (after Perez and Garcia et
al., 2000).

In Spain, the geological history of the gold-bearing surficial deposits of NW
Iberia is interpreted according to a classification (Fig. 1.11), which demonstrates
the influence of the pre-Miocene age of gold distribution on possible recycling
processes of Quaternary placers. The Miocene sediments (mainly alluvial fans)
are derived from quartz veins in the Cantabrian and Leon Mountain uplifts of
Cambrian and Ordovician sandstones and quartzite source rocks. Free gold was
passed to alluvial fan deposits as detrital particles and/or as colloidal solutions
(Perez-Garcia et al., 2000). Ongoing Quaternary erosional/depositional
processes produced deposits comprising principally regolith, fluvial terraces,
moraines and glacial-fluvial fans.
Classification of gold ores in this book is subdivided into primary (Chapter
2), residual and alluvial type deposition (Chapter 3), firstly according to
differences in formation and then in terms of topographic relief, climate and
time. The following constraints are important:
· Limitations to the application of provenance in placer gold exploration are
governed by the overall pattern of drainage or by the existence of a secondary
source of provenance; for example, one tributary of a river may drain
platinum-bearing ultramafics, whilst another might cut through gold-bearing
terrain; ultimately this would bring together unrelated rock-forming minerals
in which quartz would predominate.