Modeling mining in your locale

There are areas of the country, and thus types of layouts, where mining is naturally a dominant industry. Coal in the Appalachians, in Illinois, and other areas, iron ore in the Mesabi Range, copper in the Southwest, precious metals in the Mountain West, and a few other examples, come immediately to mind. But there are lots of other geographical areas where mining is a minor part of the picture. I model the Central Coast of California, and there is certainly no giant category of mining or mineral development in that area. But is it zero? This is a question that can be asked anywhere, not just for my area, and I discuss in this post ways a person might acquire answers.
     The first point of reference is of course the geology of the area you model. Many parts of the U.S. have quite complex geologic histories, and I can think of no more eloquent evocation and elaboration of that history than John McPhee’s Pulitzer Prize-winning volume, Annals of the Former World (Farrar, Strauss & Giroux, New York, 1998, available in paperback). One reviewer called it “geopoetry . . . from a master stylist,” as eloquent as you will find. But this may be deeper than you want to go, and in any case, complex geology alone does not identify the presence of ores. An ore is not just a mineral, it is a mineral that can be economically mined and refined for use. It is an economic definition, not a mineralogical or geological one. So to know about the presence of ores, you need further sources.
     For decades, the U.S. Geological Survey (USGS) and, in many states, State offices of geology or mining, have studied and reported on ore occurrences. These reports are available in hard copy, even now in many cases, both new and used, and many are now on line in digital form. I can illustrate with two such reports which deal with my own layout locale (research will show you whether your own area has the same resources).
     First, part of an older series of USGS guidebooks, USGS Bulletin 614, Guidebook to the Western United States, Part D, The Shasta Route and Coast Line (yes, following the line of the Southern Pacific tracks named), by J.S.Diller and others, from 1915. This contains lots of classic guidebook material, but includes summary geology too. These guidebooks are readily available on line from used book dealers. My copy is a little worn, but intact and useful:

For the region near and south of San Luis Obispo, this 1915 guidebook mentions chromite as the primary mineral being produced, This led me to follow up on the subject of chromite.
     I then found a second and in some ways more valuable source, a California Division of Mines publication, Bulletin 134, Part II, Chapter 2, of July 1953; it is part of a series entitled Geological Investigations of Chromite in California. Part II of Bulletin 134 covers the Coast Ranges of California, and Chapter 2 is about “Chromite Deposits of the Southern Coast Ranges of California.” Authors are George W. Walker and Allan B. Griggs of the USGS. (I’ll say more about what “chromite” is in a moment.) Like the book shown above, it is 6 x 9 inches in size and is paperbound, and includes a number of maps. It covers exactly the area in which my layout is set.

Also of value, though more general, was The Central Coast Geologic Guidebook, Bulletin 61A, Division of Mines, State of California, Sacramento, 1937. One additional article of value for me was one entitled “Chromium,” by Roland D. Parks, published in Mining Engineering, page 469 in the issue for May 1952. Any engineering library near you is likely to have this journal, a source for information about mining of many metals and minerals.
     The great majority of chromite mined in California has been in San Luis Obispo County, and my layout is situated in the southern end of that county. Again, chromite is the major ore mineral found in this area. There were once small pocket mines for silver in this area, but that was in the 19th century, and that kind of mining was long gone by my modeling year of 1953 (coincidentally the year of the report shown above). By 1953, as in 1915, chromite was the mining mineral of significance.
     What is chromite? It’s the primary ore for production of chromium and chromium compounds, all over the world. It is a mixture of ferrous oxide, FeO, and chromium oxide, Cr2O3, nominally with one molecule of each oxide, thus sometimes written as FeCr2O4. But in nature, the ratio of FeO to Cr2O3, is variable, and in addition other elements such as aluminum and magnesium can substitute for the chromium. Accordingly, the quality of an ore can be quantified as the ratio of Cr to Fe, as well as the percentage of Cr2O3 in the ore. Most California ores were good quality, that is, with ratios of Cr : Fe of around 3 : 1.
     In California deposits, the ore was described as either “massive,” meaning large lenses or blocks of the chromite mineral, up to several tons in size, or “disseminated,” meaning individual mineral grains scattered to various degrees in the matrix rock. The latter ore type, of course, required further processing to separate the chromite from the waste rock. The surrounding rock is usually either olivine, a typical intrusive igneous mineral, or if the original igneous rock had been metamorphized, that rock has been transformed to serpentine. Both those rock types are greenish, so that disseminated ores would have such a color. The chromite itself was glossy black or dark brown, and quite dense.
      In one of my posts containing part of a Mac Gaddis interview, he mentioned chromite shipments near San Luis Obispo (here is a link: http://modelingthesp.blogspot.com/2011/01/modeling-freight-traffic-coast-line_19.html). The California report shown above has a photo of the Castro Chrome Co. loader at Goldtree siding. Here’s a scan of that 1943 photo (you can click to enlarge):

Though I can’t include that location on my layout, I show the photo to suggest the kinds of resources that geological reports can contain.
     In the view above, incidentally, are three Enterprise GS gondolas, likely SP. In the interview cited above, Mac did mention the black ore which was so dense that the load in each 50-ton gondola was not very deep in the car. Chromite weighs about 280 pounds per cubic foot, which means that loading a 40-foot gondola one foot deep would give a weight of more than 90,000 pounds.
     Well, that’s the background to the mining in my locale. Yours may be entirely different, but do be aware that there are resources to help you find out about it. I will return in a future post to modeling options for the chromite traffic from the area I model.
Tony Thompson