Mesa Grande Mine, Gem Hill, Mesa Grande District, San Diego Co., California, USA

At the gambling table, there are no fathers and sons.
-Chinese proverb.

Setting

Location: Near center of the N/2 Sec 20, T11S, R2E, SBM: Mesa Grande District, nearly 2.5 miles northwest of Mesa Grande on the steep east slope of Gem Hill within the boundaries of the DeForest Ranch property, located immediately adjacent to the Angel Ranch property. Livestock raising activities have traditionally dominated the use of surrounding lands, and although conflicts between ranchers and miners occasionally erupt over mineral rights, local ranch hands have played an important part in the development of the gemstone resources in the area.

Geology

The geology of the Mesa Grande mine consists of a local series of three continuous pegmatite dikes averaging 2 to 4 feet in thickness, enclosed within a well-defined gabbroic micropluton. These dikes follow a general north to northwest strike and display an average dip of between 20 and 35 degrees west to southwest. These vein-like bodies are clearly exposed and traceable along the available surface exposure and should be considered prominent southern extensions of the productive Himalaya dike system. The gem-bearing portions of the deposit are located within the central portions of these dikes, characterized as quartz-subhedral perthite pegmatite forming distinctly localized areas of abnormally enlarged crystal structure.

History

The Mesa Grande mine workings were reported to have originally opened sometime between 1903 and 1904, by the Mesa Grande Tourmaline and Gem Company. The officers of the company were: President, Frank A. Seabert, whom spent a large portion of his time at the property; Vice-President, Horace Wilson, a well known business man of Los Angeles; Superintendent, C. O. McCarroll, pioneering California gemologist; Secretary, H. E. Felkenson of Southwestern Securities Company; and Los Angeles Banker, Frank A Liddell. The mine yielded a modest amount of pink and purple tourmaline, beryl (aquamarine and goshenite), and quartz (white and smoky). At least 1100 carats of tourmaline gems were reported to have been cut and sold. The principal development during this period was a cut 60 feet long, 15 to 40 feet wide, and 25 feet deep. From this main cut was driven an adit which extended 80 feet underground into the mountain, cutting through the first two dikes, at which point an appended drift followed the southernmost pegmatite to the northwest for nearly 180 feet. Other cuts and minor adits were driven along the surface outcroppings of the three main dikes, extending down dip no greater than 20 feet. Work on the property by the company had ceased around 1910.

Recent Activities

In November of 1995, several parties involved in consolidation of various gem-bearing properties on Gem Hill began surface reconnaissance and detailed exploration of the Mesa Grande mine. The original mine workings had recently been partly uncovered by the land owner's ranch laborers, so it was decided that work should be focused at these locations. Field work was primarily conducted by Erik Cordova and Scott Ritchie of the San Diego Mining Company.

At this point in time, and apparently since 1957, existing underground workings had become inaccessible due to ongoing caving of decomposed pegmatite and gabbro. Some small pink tourmalines of gem quality were discovered on the brush-covered dumps of the lower, northernmost adit. Additional exploration along the surface exposure of the upper, southernmost pegmatite occurred for a period of 4 weeks.

Within the second week of exploration, Cordova discovered signs of a large gem bearing pocket and began to systematically excavate at the point of discovery. With the weather being exceedingly both cold and wet, work upon the surface was stopped prematurely - although not before several days of digging had unearthed several large smoky quartz crystals nearly a foot in length, as well as several kilos of fine gem-schorl prisms measuring up to 7 inches in length. Additionally, many fine spessartine nodules of exceptional orange color and of top facet quality were recovered. The largest of these crystals was nearly "golf ball" sized in its proportions. Some of these fine garnet crystals were also found on an aesthetic matrix of microcline and cleavelandite accented by cookeite, making for attractive mineral specimens.

Several hundred kilos of quartz crystals, a dozen kilos of slender black tourmaline (schorl) crystals, and three kilos of deep orange garnets (spessartine) were recovered. In all, over forty buckets (5 gallon capacity) of water screened pocket material were taken out. Unfortunately, most of this material was stolen by a self-proclaimed "gemstone mining expert" — a con man from Arizona unwittingly hired by the mine owner to ensure equity of the bonanza. Several months later, a pale pink beryl crystal (var. morganite) measuring approximately 2 inches across was found on the dumps by one of the miners from the nearby Himalaya mine. It was apparent that much gem-material was lost during the ensuing frenzy, having been hastily excavated and pushed over the edge with the backhoe in a mad rush to empty the pocket.

References:
Mesa Grande Tourmaline & Gem Company (1906). Do It Now!! Buy Tourmaline Stock; The Pacific Monthly, Advertisement Section, January 1906. The Pacific Monthly Publishing Co.; Portland, Oregon.

Weber, F. H. (1963a), Geology and mineral resources of San Diego County, California. California Division of Mines and Geology, County Report 3: 309 p., illus., maps.

Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 448 (map 13-1).

Larson, W. F. (1996), Personal communication between Scott L. Ritchie and William F. Larson; underground inspection of recent development works and beryl recovery on the dumps of the Mesa Grande mine by Montes, B.; Spring.

San Diego Mining Company (1996). Gem Hill exploration program; Cordova, E. M., Ritchie, S. L., Rose, C. H., Wilson, R.; Mesa Grande mine field analysis and valuable mineral discovery, Winter 1995-96.