One need not be an experienced placer mining operator to locate bajada placer gold deposits in the Southwest deserts. It is a type that has lain untouched under the noses of uncounted thousands of old-time prospectors throughout the arid Southwest, for countless decades. Only in recent years has the bajada type of gold placer deposit, which is strictly indigenous to the arid regions of the Southwest, come to be recognized by geologists and engineers. And it is still virtually virgin soil to almost all save the geologists.
In a publication of the California State Division of Mines ( Bulletin No. 135, “Placer Mining for Gold in California”) Doctor Olaf P. Jenkins, then Chief Geologist, in the chapter on “Geology of .Placer Deposits” says: “Recent study of the geologic processes at work in the desert has led to a better understanding of the desert placers, which otter a practically virgin field for exploration, holding a potential wealth not yet known.”
However, with placer mining operators who for years have dredged and draglined the gold gravel deposits of California now actively scouting for new fields to work—and more and more of them turning from well-watered California mining regions to the arid and semi-arid desert regions, with increasing success in the latter—the bajada type of auriferous deposit inevitably will come into its own as an added source of gold production.
The word is of Spanish parentage(bah-HAH-dah) and is the Spanish term for slope, locally used in the Southwest to indicate the lower or foot slope of a mountain range—the part consisting of rock debris, standing at a much lower angle than the rock slope of the range proper. The term was first used by C. F. Tolman for confluent alluvial fans along the base of a mountain range, in an article on erosion and deposition in the southern Arizona bolson region, in the Journal of Geology.
In the clear desert air you can spot them for miles. They merely are stretches of eroded detritus from up the mountain—not water-worn gravel and pebbles such as you’ll find in the arroyos, but just plain dirt. Unimpeded by timber or anything more stable than sagebrush roots, the dirt has slid down the steep slopes of ranges for uncounted seasons, perhaps thousands of years, and carried with it the gold eroded from apexes of quartz veins up in the range.
And the gold is utterly unlike any preconceived idea you may have of placer gold, such as is recovered from river or ancient beach gravels, or from the lava-buried Neocene Age river channels of the Sierra Nevada which for decades have been mined by drifting and “breasting.” That placer gold usually has been transported a considerable distance by fast-running water, and hammered and worn smooth by pebble and boulder impact and abrasion. The gold found in bajada deposits along the feet of desert ranges is what in engineering reports and published technical articles have been christened “short-haul gold”—a term that appears to have caught on with professional associates. It is sharp and angular, and bright yellow instead of rusty, or in some cases blackened by iron and manganese solutions as is the “long-haul gold” of California’s present-era or Neocene Age placers.
Another characteristic of some bajada deposit gold is that much of it remains “frozen” to the bits of the quartz matrix from veins whence erosion wore it and washed it to the base of the granite-back-boned range. Gold in quartz matrices ranges in size from mere granules to pieces as big as pencil erasers. That makes it hard to recover gold by ordinary wash-plant sluice-riffles, because the lighter quartz too often carries it along over the riffle-lips and on down to the tail pile; especially where heavy black sand is present in quantity, as it often is. In design of any wash plant for such recovery, a means must be provided for grinding or roll-crushing of such frozen matrix, otherwise too high a percentage of gold will be lost.
So now that you know you don’t have to hunt for and find old-school placer-gravel, characterized by water worn pebbles and “long-haul gold”—and know how to spot and recognize a bajada deposit with its characteristic “lag-line”, perhaps you may be lured to prowl a desert region for such likely-looking spots on your next vacation outing.