Topographic gradients and lode gold sourcing recorded by placer gold morphology, geochemistry, and mineral inclusions in the east fork San Gabriel River, California, U.S.A

Abstract

Topography plays a major role in the morphology, geochemistry, and mineral inclusion profiles of placer gold from the East Fork of the San Gabriel River. These trends are consistent with a significant change in topographic gradient over a short distance, and coincident with the break in slope along the range-front of the San Gabriel Mountains. Placer gold from above this topographic transition is more rounded and 3-dimensional, with elevated silver in the alloy of the native gold, little evidence of gold enrichment rims, and abundant inclusions of brittle and reactive minerals. Below the topographic transition, placer gold assumes a distinctly flattened form, with lower silver alloy content, thin gold enrichment rims, and rare inclusions of relatively inert minerals. This relationship is observed for placer gold shed from two inferred lode sources within the river catchment, with placer gold from each of these sources being traceable as a unique population over at least 8 km of the river. Significantly, these placer gold properties are distinctive enough to fingerprint gold from specific portions of the placer deposit. These identifying characteristics, with virtually identical morphology and chemical trends, are observed in the large nuggets of the Mel Fisher collection of purported San Gabriel gold. These similarities appear to confirm the provenance of the collection, including identification of the specific locations where each nugget originated.