The following excerpt is from Report #151, vol. 1, July 1960, Section 4, Geology Dr. H.R. Cooke, Jr., Cooke, Everett and Associates, Reno, NV
Costa Rica is geologically young. Until Mesozoic time, a seaway between the Atlantic and Pacific existed there, until, in late Cretaceous the isthmus was uplifted, with Tertiary sediments depositing on both sides of the rising cordillera. The sediments were faulted and folded as mountain building continued, and volcanoes all along the cordillera poured andesitic lavas and tuffs over them. Many active volcanoes still exist along the backbone of the country. The Meseta Central has been filled has been filled with lava and ash, and the west slope of the cordillera at La Unión is covered by thick andesitic flows, tuffs, volcanic breccias or mudflows and with some volcanic necks exposed in the river valleys. Uplift is continuing there, as indicated by elevated stream terraces at several levels above the Rio Seco and frequent earthquakes. Weyl (1957) also noted uplifted terraces in the swampy Cedral de Miramar.
Farther west, near the Pacific, however, there appears to have been recent submergence. The low mountains of the Nicoya Peninsula seem to represent the peaks of a partially submerged earlier cordillera which continued south into Panama in the Azuero Peninsula and the Pacific coast ranges in Darién National Park, Panama.
Many gold-quartz veins formed during the Tertiary volcanism along west slope of the Central Cordillera Volcánica from magmatic juices which arose along fractures. The Gold Belt trends northwest, parallel to the axis of the Cordillera in a broad zone about 80 km. long.
The veins are steep-dipping and strike variously NE to NW. Their similar mineralogy indicates a , co-magmatic region: strong gouge and breccia bearing faults with banded vein quartz and horses in zones up to 30 m. wide mineralized along vein walls or in intravein fractures by later sulphides and finally gold. Distribution of both sulphides and gold is variable – sometimes even and sometimes pockety. The sulphides are mainly pyrite, arsenopyrite (rare), tetrahedrite, chalcopyrite, sphalerite, galena, and stibnite. Manganese – rhodonite and rhodochrosite – was reported common at Aguacate, as well as a little gold telluride. Some oxidation is reported from the bottom Aguacate workings at 300 m. depth, but is mainly shallower. There appears to be practically no secondary enrichment in gold or other metals: all the ore mined was primary, except for the high-grade manto float in the soil mantle over veins.
The gold-bearing fractures formed during the last major faulting: post-ore cross-faulting is minor. Displacement on the veins is unknown, but horizontal striae were common at Abangares and occasionally see at La Unión de Montes de Oro, indicating that at least the last movement on them was lateral.
Since 1815, recorded or reported gold production of the Gold Belt has been about 1 000 000 oz. However, in view of the very incomplete production information and the probably great amount of gold high-graded (stolen) by the miners and others, the actual production is estimated to be at least twice this.
Central Gold Belt
The major production has come from three districts – Abangares (105 000 oz.) at the north end, La Unión (431 000 oz.) in the center, and Aguacate (536 000 oz.) on the south end. All have much free-milling, very high-grade gold with values measured in dollars per pound, as well as lower-grade gold ore intergrown with sulphides. Aguacate was mined to 300 m. depth and Abangeres to 230 m., but La Unión mining has reached only about 80 m. All the other, smaller districts are also shallow.
La Unión district includes workings on many NW to NE striking, steep-dipping gold-quartz veins – mostly where they are exposed in the steep sides of one of the three main southwest-flowing rivers: the Aranjuez, Seco, and Zamoro. The main production has come from La Unión (Los Angeles and Santa Rita veins) and Bonanza (Lamas, Bonanza, Reina veins), smaller mines along the central Rio Seco. From west to east, the principal veins worked there are the Chispero, Santa Lucia, La Fortuna, Chocolate, Lamas, Bonanza, Reina, Comandela, Esperanza, Los Angeles, and Santa Rita.
The vein outcrops are covered with soil and vegetation except where exposed by stream-cutting – which accounts for the concentration of discovery and mining along the three main rivers. Some ravines follow along the veins, but since the veins usually are not exposed except in ravines, it is difficult to judge whether this is a habit. Soil creep on the slopes tends to cave the portals and distort attitudes of vein outcrops. Where the vein is mostly massive quartz, however, it may outcrop in a jutting ledge, as the Lamas vein below the mine office, and its probable extension south, the Veta Lionzo.
The veins follow nearly vertical, well defined shears generally with free walls. The large, N-trending Lamas and Santa Rita vein zones range up to 30 m. wide, with many wall-rock horses between parallel vein strands up to several meters wide. The Lamas and Santa Rita veins are fairly straight, but most of the other veins are often twisting, especially along strike, and often branching, but cross-cutting EW faults or veins are almost absent. I mapped on one at the north end of the Bonanza 300 L, probably with small displacement, and a SW-striking fault was reported at the S end of the Santa Rita and Los Angeles veins, 3 ½ L (Eliseo Hampton). I looked for E-W faulting along the Rio Seco for 2 km. below the Bonanza mill, where rock exposures are particularly good, but did not see any.
They may be bordered by, or include zones up to several meters wide of crushed and silicified wall-rock, breccia, and gouge, or irregular quartz stringers. They make much water and heavy ground needing much timber. Away from the veins in cross-cuts however, the fresh, hard andesite stands indefinitely without timber. Wall-rock jointing is locally strong parallel and near to veins, and might sometimes be usable as a prospecting guide to them.
Entries to nearly all the extensive older workings of Minas La Unión and Bonanza (as well as workings on most other mines) were caved and inaccessible, and they are known only by fragmentary reported production figures, old mine maps, and reports of the men who worked them. Too little is known of their geology to warrant theorizing about their structural origin. However, the pattern of the M. Bonanza veins, along with occasional horizontal striae, suggests that they may have been formed by dominantly lateral displacement.
A horizontal shear couple may have formed the Lamas fault zone, with right-handed movement (northward on the west side), the Lamitas, Bonanza and Reina veins occupying auxillary tension fractures. Ore shoots would be expected more or less as found near the vein intersections, except in their northern cross-over branches to the Lamas, which may have been closed tight against the ore solutions by compression. Such lateral displacement would explain the localization of ore shoots on strike-bends where bearing surfaces would alternate with the riding surfaces having open spaces for ore deposition.
Oxidation of non-quartz vein material is strong near the surface, forming a rubble down-slope from the vein outcrop of quartz fragments and soft decomposed horse and wall-rock. This surface mantle – manto “veins” – may be very rich in gold-quartz, since it is left behind after much of the less resistant vein material has been weathered and transported. Much early mining was started on these mantle float deposits.
Some oxidation extends along the veins below the water table, as shown on the Bonanza 350 L. There may be residual near-surface gold concentration in veins, but no systematic change of gold values with depth was observed. In any case, the ore observed near and below water-table on the Bonanza and Lamas veins is primary, and may reasonably be expected to continue at least several hundred meters in depth. Similar veins at Aguacate and Abangares were mined to 300 m. depth, most of this below water-table.
In Nevada, epithermal precious metal veins in Tertiary volcanics, as in Goldfield, Tonopah, and Virginia City, were mined up to 1, 000 m. depth. Increasing mining costs with depth often are more responsible for bottoming such mines than significant changes in ore tenor.
The only important gangue is quartz. The gold occurs in quartz veins or stringers, often along one or both vein walls or in intravein shears, alone with argentite in quartz, or with argentite and a little fine grained galena, partly replacing ribbons or inclusions of wall-rock in quartz. Pyrite is only locally abundant in the Bonanza min, but in the La Unión veins, where the ore is of more constant tenor, pyrite is so common with gold that this ore was cyanided; pyrite is also common in the Chocolate vein. Massive chalcopyrite occurs locally, generally without intergrown gold.
The vein structure indicates that gold and argentite deposited last, along fractures in earlier-formed vein quartz and sulphides, though some drusy quartz formed later than gold and silver: a peculiar gold occurrence, cuyol, consists of nodules of rich gold ore, or yema, coated with fine comb quartz on the outside. Some gold is coarse and visible (escarche), as in high-grade ore, but most is in fine intergrowths with argentite, or in very small specks peppered throughout the quartz. This imparts a yellowish-green cast to the quartz, hence is termed yema – albumen. Yema seems to form through replacement by quartz and gold of wall-rock fragments included in the vein; relic outlines of such rounded fragments may be left, and are called sombra – shadow. The cuyoles apparently form similarly, except that late, post-gold quartz deposits around the enriched fragment.
