Table of Contents
Mining has never been a major source of income for Puerto Rico. The Spaniards mined gold placer deposits during 1509-1579, and extracted an estimated 1,200,000 troy ounces of gold. However, economics at the time discouraged further mining, and richer sources of gold and silver had been found elsewhere in the New World. The labor source, which was chiefly the native populace, was also decimated, mainly through disease.
The chief source of the placer gold was in the Rio Mavilla, about 30 km southwest of San Juan. There may be as much gold remaining as was extracted. Periodically efforts have been made to mine more gold from the Rio Mavilla and other placer deposits in Puerto Rico, but these efforts were not profitable. Some rivers draining northeastern Puerto Rico are reported to contain placer gold, and gold placers have also been reported in other areas of the Island.
Regional Geology
The islands along the northern edge of the eastern Caribbean Sea represent an island arc deformed and uplifted from Jurassic through early Tertiary time. The tectonic style is a result of systematic interaction of the Caribbean, North American, and South American Plates. The Mesozoic and Cenozoic strata on Cuba, Haiti, the Dominican Republic, Puerto Rico, and the Virgin Islands were deposited on Paleozoic rocks similar to those in the subsurface of Florida.
Geology of Puerto Rico
The Central Volcanic-Plutonic Province can be further subdivided into three subprovinces. These are the Southwest Igneous Subprovince, the Central Igneous Subprovince, and the Northeast Igneous Subprovince. Puerto Rico also includes two small islands to the east, Vieques and Culebra, and a small island to the west, Mona Island. These islands are not described in this paper due to their lack of reported precious metal deposits.
The Southwest Igneous Subprovince is characterized by the presence of large bodies of serpentinite and extensive thick marine limestones . The serpentine provides the bedrock source for the nickel, iron, and cobalt laterite deposits Plutonic bodies of any size are rare in this Subprovince. The oldest rocks in the area are included in the Bermeja Complex of Mattson which includes the serpentinite as well as amphibolite, chert, and metamorphosed volcanic rocks. The Bermeja Complex is probably Lower Cretaceous. A sequence of lava, agglomerate, marine limestone, mudstone, and tuff, which are believed to be Late Cretaceous, overlie the Bermeja Complex. Paleocene and Eocene lavas, breccias, tuffs, and reworked volcanic rocks in turn overlie this Late Cretaceous sequence.
Although occurrences of precious metals are found in each of the igneous Subprovinces, the majority of the occurrences, and the largest known deposits, are in the Central Igneous Subprovince.
Precious Metal Occurrences
Central La Plata: Mclntyre (1975) described this occurrence of small gold bearing quartz veins that cut the Rio Blanco Formation. The Rio Blanco Formation consists of Tertiary age thick bedded tuff breccia of andesitic-dacitic composition. The formation is also intruded by many aphanitic porphyry dikes. The widest vein observed is about one meter in width, but others seldom exceed a few centimeters. Exposures in the area are poor, and the veins were not observed to extend any great distance. Alteration of the host rocks is confined to the borders of the veins.
Ciales: Two small copper prospects described by Berryhill (1965) were reported by Cox and Briggs (1973) to contain silver as well. No gold was identified by Berryhill (1965) in this area. The copper deposits were in veins along faults and fractures and small irregular masses in the Ciales and Morovis stocks and in the volcanic rocks.
Barrio Pasto: Barrio Pasto is a copper prospect described by Colony and Meyeroff (1928) and reported by Cox and Brings (1973) to contain silver. The deposit is within Upper Cretaceous augite andesite that is considerably fractured along a narrow zone. Mineralization occurs along the fractures and is greatest where two or more fractures intersect. The deposit is small, and Colony and Meyeroff (1928) stated there was no assurance it continues at depth.
Montanas de Corozal: Nelson (1967) mapped this occurrence as a northwest-trending quartz vein in the Upper Cretaceous Los Negros Formation, which is a dark basaltic tuff. The deposit is adjacent to a fairly large intrusion of Tertiary quartz diorite.
Coll Cuchi Sayre: Nelson (1967) mapped these deposits, which are also in the Corozal Mountains, and are within Upper Cretaceous andesite and volcanic breccia of the Avispa Formation. Several prospect pits surround these deposits.
Palos Blancos: Palos Blancos is the main placer district in Puerto Rico. The deposits were worked in the gravels of the Rio Mavilla and its tributaries, and remnants of the pits of Spanish miners can still be observed in the area (Nelson, 1967).
Anones: Nelson (1967) mapped this as a small copper-gold prospect on a hilltop within steeply dipping andesite of the Avispa Formation. The prospect lies south of the inferred trend of the Anones Fault (Nelson, 1967).
El Yunque: Seiders (1971) described a small abandoned prospect in unnamed Cretaceous volcaniclastic rocks in the El Yunque area. Seiders also reported that placer gold was taken from the lower parts of streams that drained this area. However, the El Yunque area is now a National Forest and a popular tourist area, which practically eliminates any further consideration for mining.
Rio Grande de Arecibo: Mattson (1968) described pyrite-quartz veins adjacent to Highway 10 along the Rio Grande de Arecibo. The mineralization occurs in Tertiary quartz monzonite and quartz diorite and granodiorite. Assays from the Puerto Rico Bureau of Mines (1941) indicated values up to 0.5 oz. of Au/ton.
Barranquitas: Briggs and Gelaburt (1962) described the Barranquitas deposit as a blanket of lateritic soil, reportedly gold bearing, that overlies the southeastern part of the Barranquitas stock and a section of pyrite-quartz- sericite rock. The Puerto Rico Bureau of Mines (1941) reported that the area had been drilled, and that “nearly one million cubic yards of workable gravel” (presumably auriferous) had been “developed by a company”, reportedly the Compania Au-Ag de Borinquen with headquarters in San Juan. Trenching and prospect pits were also done, and the principal difficulty in developing the deposit was in disintegrating the clay. Despite encouraging results with pilot processing, the company was undercapitalized, and the project was abandoned. Values reported by the Puerto Rico Bureau of Mines (1941) for a quartz vein that was explored by an adit ranged from <.10 oz. of Au/ton to 0.44 oz. Au/ton. Briggs and Gelaburt (1962) suggested that the area is worth further study for both copper and gold.
Sierra Bermeja: Mattson (1960) described the area of the Sierra Bermeja deposit as within silicified rock of the Bermeja Complex. The Bermeja Complex is the oldest unit in southwestern Puerto Rico, and consists of serpentinized peridotite, spilite, amphibolite, and minor silicified volcanic rock and/or chert. Small barite, barite epidote, and barite carbonate veins were also reported by Mattson in the silicified rocks. Cox and Briggs (1973) classified the occurrence as a vein/shear zone deposit.
Minillas: Minillas is a small, high-grade vein gold deposit that occurs in a suture zone between serpentinite and Cretaceous andesite (Mary Gilzean, written comm., 1992). Mineralization consists of pyrite and chalcopyrite in a steeply-dipping microcrystalline quartz vein. Oxidized portions of the vein contain malachite and native gold. Silicification is prominent near the vein, and propylitic alteration is widespread. Diorite dikes and sills intrude the andesite section. Information from M. Gilzean described the geologic setting as similar to the serpentinite and Cretaceous andesite exposed near Pueblo Viejo in the Dominican Republic, and also stated that the geologic setting is similar to the McLaughlin deposit in California. The deposit reportedly produced 180 tons averaging 0.98 oz. of Au/ton during the period 1916-1918.
Cerro Avispa: Schellekens et al. (1991) described the Cerro Avispa deposit as within an area of hydrothermally altered west-northwest dipping Cretaceous volcanic flows and flow breccias ranging from basaltic andesite to andesite. To the northeast and southwest, the flows are interstratified with volcanic siltstones and sandstones. The volcanic rocks typically contain abundant pyrite, and along a northwest striking deformation zone the pyrite has oxidized and bleached the rocks, leaving pyrite casts and iron hydroxides on the fracture planes (Schellekens, 1987). Mineralization occurs at the top of this section of volcanic rocks at the northwestern area of the hydrothermally altered zone, in intensely altered volcanic breccia. The mineralized area consists of sulfide-bearing quartz veins in an area of generally silicified and pyritized rocks. Bergey (1960) in a reconnaissance sampling of the deposit reported values of 1 to 17 oz. of Au/ton and 0.1 to 14 oz. of Ag/ton. The highest gold and silver values were apparently found in samples with visible chalcopyrite and sphalerite (Bergey, 1960).
Potential for Mining
Precious metals are classified as commercial minerals, according to the Mining Law of Puerto Rico (1979), and as such belong to the Commonwealth. Commercial minerals in Puerto Rico include metallic minerals, hydrocarbons, precious stones, and any other substances that are declared commercial minerals by the Secretary of the Department of Natural Resources (DNR). Exploitation of commercial minerals can only be authorized through leases executed by the Secretary of the DNR and approved by the Governor of Puerto Rico. As noted with the problems resulting from the MOU between CARI and the Puerto Rico government, mining leases can be difficult to acquire.
Mining is also restricted by the population density. Even many of the remote areas are populated, and as the population of Puerto Rico increases, the areas available for mining will decrease. The acquisition of the surface rights in certain areas is also complicated by this density, as multiple, small parcels of land are often involved.
