Saturday, July 12, 2008

California's Motherlode: Where Did All That Gold Come From?



One of the World's Richest Gold Zones

California's Motherlode Region was one of the richest and most extensive gold-bearing zones ever discovered, on this continent or any other. Over 150 miles long and more than 50 miles wide at most points, the Motherlode produced staggering amounts of placer and lode gold. Nearly 30 million troy ounces (approximately $10 BILLION in today's dollars) were recovered from the California goldfields from 1849-1860 alone, not counting the additional millions of ounces that resulted from sporadic production thereafter, right up to today.

Where did all that gold come from? How did it become so concentrated? What was the source of these unbelievably rich placers and lode veins? The answer, my friend, can be found in three words. Sierra Nevada Batholith.

Birth of a Gold Region

Nearly 200 million years ago gigantic "bubbles" of molten rock began forming far beneath the earth's crust where the Sierra Nevada Mountains of California would one day emerge as the pre-imminent high ground overlooking the fertile valleys below. When these bubbles finally broke the surface of the earth's mantle they spewed molten lava in every direction, but in some instances these same bubbles became "trapped" beneath the surface where they, in turn, cooled in place creating what modern-day geologists call the "Sierra Nevada Batholith." What is the Batholith? A huge body of granitic rock underlying nearly 25,000 square miles of California and part of Nevada.

As the Batholith was formed and as it cooled in temperature, many different minerals were either trapped or "squeezed out" of it through chemical precipitation, heat, and intense pressure. Much of this "mineralization" or natural refining process took the form of streaks and veins of natural gold, much of it contained within quartz or quartzitic host rock which, in turn, was "hosted" by the granite of the Batholith itself.

Riches Beyond Belief

Even by modern geologic standards, the gold mineralization of the Batholith was striking and held the possibility of riches beyond compare. Where nature allowed, for the most part, a gold concentration of 5 parts per billion in most areas of the earth, in and along the Batholith gold occurred at concentrations as high as one hundred million parts per billion. Much, much later, this "richness" would provide the foundation for one of the largest human migrations in history, the California Gold Rush.

But for eons the Batholith's gold deposits remained hidden away, stored underground far below the earth's surface. Approximately 4 million years before the arrival of the 49ers in California, the crust containing the Batholith began to shudder, crumple, and then lift upward in dramatic fashion. As the Batholith rose thousands of feet into the air, so did the quartz veins, stringers, and pockets containing all that gold. Then the forces of erosion went to work.

Time and Nature Do Their Work

During decades of intense cold glaciers scoured some of the gold out of its host rock, the gold was "freed" during more temperate times as large volumes of water thundered downslope as melted snow or as heavy rains sluiced the flanks of the Sierra Nevadas. Intense heat expanded rock while freezing temperatures contracted the very same rock. And as it did so, cracks or air pockets were formed, allowing new avenues of escape for the golden metal contained within. And so it went for eons, the gold (because of its weight and density) finding its way eventually downslope into the lowest part of ravines, gulleys, bench gravels, and streambeds, accumulating in pockets or paystreaks of just a few ounces or tens of thousands of ounces.

Then one day in 1849, at a place called Coloma, James Marshall's eye was caught by the glimmer of a few golden bits of metal laying atop the bedrock of a newly constructed millrace. The rest, as they say, is history.....

(c)  Jim Rocha (J.R.)  2008

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