Stop 2 - Rosa Del Castilla and El Fenomeno Mine
Continuing down the long and winding road, past San Faustino which marks the eastern edge of the El Pinal pluton, we passed a fairly prosperous-looking horse ranch. It turns out that said horses are thoroughbreds, and one of our group members passed along the story of an American who raises them, having been swindled out of his apparent property in Mexico, a now-familiar tale. We met Jorge Ledesma and the group from UABC at the townsite of La Rosa De Castilla, the former mining camp for the El Fenomeno mine (I presume). We were greeted just short of the site by a contingent of Mexican army troops, who were quite curious about what we gringos were up to.
Mina El Fenomeno
Mines noted as typifying the Baja California Norte Metallogenic Province are El Arco (copper), El Fenomeno and La Oliva (tungsten), and multiple gold and some iron prospects (Salas 1976, 1980). Placer and quartz vein gold deposits in the region are geographically and perhaps genetically related to the tungsten deposits. Tungsten is a strategically important metal used to harden steel alloys and is known to occur in Mexico in pegmatites and quartz veins in granite, as well as contact metamorphic deposits in limestone. Scheelite (CaWO4) is the primary ore mineral of the tungsten deposits that occur throughout the northern portion of Sierra Juarez, Baja California, Mexico, with the largest known occurrence at El Fenomeno. Several area tungsten-mines, including Los Aliados de America (Allies of America) and Pearl Harbor, produced tungsten during World Wars I and II, Between 1937 and 1943, an estimated 100,000 tons of ore were milled at El Fenomeno, yielding approximately 56,700-units of W03 concentrates averaging 65% W03 by weight.
The El Fenomeno deposit extends along an intrusive contact between quartz diorite and metamorphosed sediments, including a 45-meter-thick bed of white to light blue marble. The main ore body occurs in a zone of tactite, a dark garnet and pyroxene skarn, within the marble and between the marble and quartz diorite contact. Pegmatite dikes containing tourmaline and garnet also occur at El Fenomeno, cross-cutting all the metamorphic rocks near the diorite contact.
Where pegmatites appear to cut tactite, some pegmatite minerals are weakly replaced by tactite minerals including scheelite, suggesting tactite formation had partly occurred later than pegmatite intrusion. The deposition of scheelite likely occurred at the same time, or shortly thereafter, as it often also occurs as well-formed crystals in the tactite. The tungsten ore fluids likely also contained much silica; irregular masses and numerous small crystals of quartz occur in the largest and richest ore bodies.
Scheelite has a high specific gravity for a non-metallic mineral, but can be tough to distinguish in hand sample. It generally occurs as few small masses to distinct crystals scattered in the tactite or marble, so the high specific gravity is no help, and it looks like greasy gray quartz. But it is a bitchin' mineral when it fluoresces bright blue-white under short-wave ultra violet light! Associated minerals in the tactite include garnet, vesuvianite, axinite, diopside, quartz and calcite. Primary iron sulfides (pyrites) are also present in fresh rock, with secondary iron oxides and copper carbonates (azurite and malachite) occurring in the weathered zones.