Gold and other precious metal alluvial nuggets often, if not generally, occur in areas where the relative size of the nuggets and possibly their origin cannot be traced to their original source. In these circumstances, neither transportation by water or eluvial weathering seems to be a logical explanation for these relative size differences.
Many of the largest gold nuggets located have been found in areas where no similar sized gold particles could be found in outcrop. Notable examples are in the Mother Lode country of California and the Piedmont area of southeastern United States. Likewise, alluvial and eluvial nuggets of iridium and platinum have been mined in the Ural Mountain area of Russia where no geologic source has been located other than trace quantities in local ultrabasic and basic rocks. In numerous instances, these nuggets exhibit concentric zonation or rimming patterns which strongly suggest precious metal growth patterns formed externally by some form of chemical deposition. What is the geochemical source and medium of transportation of this external precious metal? Can this information be used to locate possible new areas in which to prospect for precious metal nuggets?
Precious metal nuggets formed in situ should require at least the following conditions: 1) the presence of small precious metal particles to serve as a nucleation “seed" or loci for particle (nugget) growth, and 2) a reasonably consistent source of the subject precious metal either dissolved or suspended in water.
A likely source of precious metal for nugget growth is the widespread presence of precious element-bearing amorphous colloidal silica which is capable of being dissolved or suspended in water. This precious metal is most likely in the form of clusters that are less than critical nuclei size and therefore exhibit few, if any, bulk properties of the subject precious metal; thus making it difficult to identify using standard analytical methods. In this form, precious metal clusters may be converted to bulk metal by adding or accreting with like-element atoms or clusters. After a bulk metal status has been achieved the strong bonding between the cluster and a. c. s. particle is relaxed and they may possibly separate. Thus, precious metal particles may become bulk metal in size by the accretion of two smaller clusters contained on separate a. c. s. particles or by the addition or accretion of smaller like-element clusters on pre-existing like-element bulk metal particles; i. e. , the formation of a nugget. In the latter case, vestiges of a. c. s. may remain accreted to various concentric zone of the nugget.
Without the benefit of preexisting outcrop sources of precious metals, localized nugget fields may form by coagulation and accretion of like-element, less than bulk metal size, precious metal-bearing a. c. s. The presence of pre-existing small precious metal particles increases the odds of nugget growth via the accretion to precious metal clusters from like-element bearing a. c. s. However, these small “seed" particles could also be formed by colloidal coagulation and accretion of like-precious metal bearing a. c. s.
An important remaining factor in nugget formation is that there must be a somewhat persistent source of localized water movement through the forming nugget field; capillary, diffusion, fracture controlled, etc. The purpose of this requirement is to provide a more or less continual supply of like-precious element bearing a. c. s. Caliche zones, certain segments of stream beds, glacial deposits, various sorts of drainage anomalies, springs, etc. are just a few of the many possibilities. Although the pre-existing presence of precious metal “seed" particles increases the odds of nugget formation, it is not necessarily an absolute prerequisite. Good hunting!
Alvin C. Johnson, Jr. , Ph. D.
http://www.acjohnson.us This website considers new and innovative concepts and procedures that pertain to mineral and oil and gas exploration and development.