New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/19 Geology and ore deposits of the La Plata district, Colorado Eckel, Edwin B. with sections by Williams, J. S. and Galbraith, F. W. Digest prepared by Trauger, F. D., 1968, pp. 41-62 in: San Juan, San Miguel, La Plata Region (New Mexico and Colorado), Shomaker, J. W.; [ed.], New Mexico Geological Society 19th Annual Fall Field Conference Guidebook, 212 p. This is one of many related papers that were included in the 1968 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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Editors Note: This digest of Professional Paper 219 (out of print) has been included in the guidebook so that conferees will have available the important facts concerning an area that will engage our attention during much of the first day of the conference. E. B. Eckel, principal author of Professional Paper 219 was unable, because of a heavy work load, to prepare a summary paper for our guidebook but he graciously authorized preparation of this digest and has given it his ap- proval. Professional Paper 219 is detailed, definitive, and a tightly written work of some 179 pages; it is not amendable to more than slight reduction without serious impairment of its value as an authoritative reference. To achieve the drastic reduction here presented has required the omission of nearly all substantiative data and most of the numerous and detailed discussions that led the authors to the many conclusions made. It is sincerely hoped that no erroneous impression of the thoroughness or accuracy of their work will result from this condensation. If any person has doubts concerning the conclusions as stated in this digest it is suggested that he first obtain and read an unabridged copy of the paper before voicing objections. All footnotes and references have been deleted as have all maps and most other illustrations with the excep- tion of a few photographs. Detailed paleontological, petrographic, and mineralogic descriptions also have been deleted as have all measured sections and much other interesting and informative material. What remains are "bare bones" that, while they reveal the skeletal structure, should not be construed as an accurate picture of the living critter. INTRODUCTION the area is above 9,000 feet and nearly 19 percent is above 11,000. The highest peak is Hesperus Peak, which rises to The La Plata mining district, also known as the Cali- 13,225 feet, but Mount Moss, Babcock Peak, and Spiller fornia district, lies within the La Plata Mountains in Peak are nearly as high, and several other summits exceed southwestern Colorado, 8 to 20 miles northwest of Du- 12,000 feet. rango. The only settlements in the district arc the old min- Except in the outlying parts of the mountain group ing town of La Plata, locally known as La Plata City, most of the slopes are steep, and many of those above which had a population of 33 in 1930, and Mayday which timber line are extremely rugged. is about the same size. Mayday is near the site of Parrott, The mountains have resulted from the dissection by which was the first settlement in La Plata County and was erosion of a domal uplift. Consequently the drainage sys- once the county seat. The principal supply point is Du- tem radiates in general from the central part of the group. rango, which had 5,400 inhabitants in 1930, [10,530 in The master stream, the La Plata River, has cut almost 1960] but Mancos with a population of 646 [839 in 1960] through the north rim of the mountains by headward serves the East Mancos sector. erosion. The La Plata flows south from the mountains and The La Plata Mountains lie about 20 miles southwest empties into the San Juan River in New Mexico. of the main San Juan Mountain front. The intermediate The eastern parts of the mountain group are drained by space is a region of rugged hills carved from sedimentary Hermosa, Lightner, and Junction Creeks. Bear Creek rocks of Paleozoic and early Mesozoic age. The mountains drains the north-central part of the mountains. All the rise abruptly on the west, south, and southeast from the western part is drained by tributaries of the Mancos, of comparatively level Colorado Plateau. which the East and West branches are the largest. All the principal peaks of the La Plata Mountains lie In the rigor of its climate the area here described re- within a 9-mile circle centering in the valley of the La sembles the mining camps of the San Juan Mountains, River near the mouth of Tirbircio Creek. The lesser Plata hut the La Plata Mountains, because of their exposed summits and outlying ridges fall within an oval area from position on the edge of the Colorado Pleateau, probably 12 to 15 miles in diameter. The higher peaks are situated receive even more rain and snow than the main San Juan along the narrow divides on each side of the La Plata River Mountains. or on spurs extending from these divides. The lowest point in the area is 7,250 feet above sea level, but 79 percent of 42 NEW MEXICO GEOLOGICAL SOCIETY—NINETEENTH FIELD CONFERENCE The winters in the district are long, and in the moun- form, but all of them can be grouped in two general types tainous areas they are generally characterized by heavy —porphyritic and nonporphyritic. The most abundant snow. Snowbanks remain in many protected places to the igneous rock in the district is diorite-monzonite porphyry. end of July or August, and a few of them, particularly in The rock occurs as essentially contemporaneous sills, the upper part of Tomahawk Basin, often survive from stocks, and dikes, which were emplaced largely by forcible one year to the next. intrusion. The nonporphyritic rocks, in general younger The first snow usually falls in late September or early than the porphyry, consist of syenite, monzonite, and October. It is often followed by a month or more of clear, diorite and occur as irregular stocks associated with many crisp weather, the boasted Indian summer of the Colorado dikes. The extensive metamorphism of the sedimentary Rockies; not infrequently, however, winter weather sets in rocks in the central part of the district is related to these immediately after the first snows. stocks. The stocks were formed in large part by replace- Precipitation and snowfall at stations near La Plata district ment or assimilation of the older sedimentary and igne- Precipi- ous rocks. Altitude Years of Snowfall tation Place (feet) record (inches) (inches) The most productive of the ore deposits are veins and Durango .............................6,589 38 67.0 19.98 replacement bodies that yield gold and silver tellurides, Terminal Dam .............. 8,300 24 145.4 26.30 but there are also deposits of pyrite and chalcopyrite en- Rico ....................................8,824 29 164.5 26.12 closing native gold, of base-metal sulfides, and of chalcopy- Silverton .............................9,400 24 157.7 26.69 rite enclosing platinum. Many of the deposits are clearly Savage Basin ....................11,522 16 400.2 38.05 related to the hinge fold and associated faults mentioned above. The spring months, especially May and June, are gen- erally the driest of the year. Precipitation during the sum- mer commonly comes in the form of sudden and more or PRE-CAMBRIAN BASEMENT ROCKS less local thundershowers, but in some years there are The precambrian basement rocks beneath the La Plata many days of almost continuous rain. Mountains are buried under 3,000 to 8,000 feet of later A large part of the district is more or less heavily tim- rocks.