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of carbon monoxide, hydrogen, hydrogen sulfide, and sulfur dioxide. Aqueous species of these gases are absent from the lake waters as well, and carbon dioxide (CO2) forms 98 to 99 percent of the gas. The Nyos

gas is cool; the lake-bottom temperature of 75 °F is no higher than those in other tropical lakes at similar elevation and latitude.

Stratification in lakes is produced by a layer of less dense water overlying a layer of more dense water. Concentrations of dissolved substances in Lake Nyos waters show a definite increase with depth, implying that the lake was stratified and that the dissolved substances were stored in the bottom waters prior to the event. The high concentrations of dissolved ferrous iron (Fe2+) in the presence of bicarbonate (HCO3) indicate that the lake waters were very acidic prior to the event; otherwise, the iron would have been removed by chemical precipitation of the mineral siderite (FeCO3). A large amount of dissolved CO2 is the only likely candidate for producing the lake's acidity.

Oxygen and hydrogen isotopic compositions of Lake Nyos waters indicate that the solutes within the lake were not acquired through the normal process of evaporative concentration. The solutes, including the CO2, likely entered through springs near the bottom of the lake where confining pressures would be sufficient to keep the gas dissolved. The analytical data clearly indicate that the Nyos gas, although released from magma at great depths below the lake, had cooled and lost its. reactive constituents (such as sulfur and chlorine compounds) as it moved upward through the permeable diatreme. The gas reached the lake as cool, essentially pure CO2.

Calculations show that Lake Nyos can hold more than a quarter of a cubic mile of dissolved CO2 when fully saturated. Laboratory measurements indicate that the lake currently holds 0.07 mi3 of CO2. The lake apparently released up to 0.29 mi3 of CO2, an amount sufficient to account for the deaths and the drop in lake level immediately following the event. The gas cloud most likely was produced by the rapid exsolution of large amounts of CO2. The gas probably made an explosion-like noise and caused surface waves to form as it escaped from the lake. Because CO2 is 1.5 times more dense than air, the gas cloud tended

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Figure 4. Dead cattle littered the hillsides around Lake Nyos after the event.

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metric tonnes, it is assumed that any additional undiscovered deposits will also be small.

The study also indicates that there is a 50 percent probability for the discovery of at least three porphyry copper deposits in Costa Rica. Such deposits typically contain 140 million metric tonnes or more of ore that ranges from 0.15 to 2 percent copper. It is estimated that there is a 90 percent chance of one such deposit, a 50 percent chance of three, and a 10 percent chance of eight porphyry copper deposits being discovered within one domain in the Cordillera de Talamanca in southern Costa Rica. Detailed geologic mapping, rock and soil sampling, ground geophysical surveys, and drilling of defined target areas need to be carried out in order to identify these potential new deposits.

Volcanogenic manganese is known in more than 90 deposits in two domains in the Nicoya Peninsula, and undiscovered deposits like those known at the surface are believed to exist at depth. Bauxite, an ore of aluminum, has been thoroughly explored for in two domains in the area of the Valle General (valley of the Rio General) and southward, almost to the Panama border. There is a distinct possibility of additional undiscovered bauxite in a similar domain in the area of Guapiles in east-central Costa Rica. Three very small massive-sulfide deposits, with major copper and minor zinc content, occur in the Nicoya Peninsula; there is a 50 percent probability that one other such deposit exists in this same domain.

Other deposit types present in Costa Rica for which domains have not been identified include low-grade offshore placer iron (magnetite), polymetallic veins (veins with three or more metals), copper skarn (deposits formed at the contact between intrusive rocks and sedimentary country rock), lead-zinc skarn, and hot-spring sulfur deposits. The mineral-resource assessment of Costa Rica was carried out with funds provided by the Agency for International Development through the Los Alamos National Laboratory. The assessment supports the President's Caribbean Initiative whereby nonmilitary methods are sought to assist Central American countries in improving their economic conditions.

Coal Resource

Assessment in

Pakistan

Exploratory drilling during fiscal year 1987, as a part of the Coal Resource Exploration and Assessment Program (COALREAP), has significantly increased knowledge of the coal resource potential in the Sind Province in southern Pakistan. COALREAP is a technical assistance venture by the U.S. Geological Survey with the Geological Survey of Pakistan under the auspices of the Energy Planning and Development Project of the U.S. Agency for International Development (USAID). Drilling is done by a USAID-contracted Pakistani company, and geophysical logging of holes is done by a U.S. company, also contracted by USAID.

Since April 1986, 32 holes, totaling 25,306 feet (53 percent cored), have been drilled and logged (fig. 6). The holes averaged approximately 800 feet in depth; the deepest hole was 1,247 feet, the shallowest 423 feet. Coal beds that were penetrated by drilling range from a few inches to about 20 feet in thickness; however, in only one hole did a coal bed exceed 7 feet in thickness. Coal beds more than 2 feet thick were drilled in 17 holes; 2 holes contained 4 such beds, 6 holes had 3. Samples of coal were collected from all beds that were a foot or more thick. In all, 141 coal samples were collected from 26 drill holes. The samples are presently being analyzed, but early analysis of the 20-foot-thick bed indicates that it is of good quality. The middle section of the 20-foot bed contains an average of 3.3 percent ash and 0.6 percent sulfur; the parts of the bed above and below contain somewhat higher ash and sulfur percentages.

Preliminary interpretations of recent investigations suggest that:

• The presently known and mined coal fields (Lakhra, Sonda-Thatta, MetingJhimpir) are not isolated occurrences but rather are in a region apparently completely underlain by coal-bearing strata. • Several exploratory drill holes did not penetrate coal, but regularly occurring delimitors for individual coal fields were not found.

• Most coal beds appear to be discontinuous layers, and, although many different

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coal beds within the region exploitable, this discontinuous nature will significantly affect development of largescale mining.

The area east of Sonda and Meting and continuing east of the Indus River to south of Tando Muhammed Khan appears to be the most promising for new coal potential because of the discovery of consistently thick (thicker than 3 feet) coal beds in holes of close proximity. Most potentially minable coal beds in this area, however, are buried deeper than those presently being mined near Lakhra to the north.

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