scholarly journals Mineralogy, mineral chemistry, and paragenesis of gold, silver, and base-metal ores of the North Amethyst vein system, San Juan Mountains, Mineral County, Colorado

1993 ◽  
Author(s):  
Nora K. Foley ◽  
Stanton W. Caddey ◽  
Craig B. Byington ◽  
David M. Vardiman
Keyword(s):  
Base Metal ◽  
Mineral Chemistry ◽  
San Juan ◽  
Vein System ◽  
San Juan Mountains ◽  
The North ◽  
Gold Silver

Chapter 21: Geology of the Fruta del Norte Epithermal Gold-Silver Deposit, Ecuador

2020 ◽  
pp. 431-450
Author(s):  
Stephen Leary ◽  
Richard H. Sillitoe ◽  
Jorge Lema ◽  
Fernando Téliz ◽  
Diego Mena
Keyword(s):  
Base Metal ◽  
Late Jurassic ◽  
Volcanic Rocks ◽  
Silver Deposit ◽  
Epithermal Gold ◽  
Mountain Ranges ◽  
The North ◽  
Pull Apart Basin ◽  
Gold Silver ◽  
A Minor

Abstract Fruta del Norte is a completely concealed and extremely well-preserved, Late Jurassic epithermal gold-silver deposit of both low- and intermediate-sulfidation type, which is located in the remote Subandean mountain ranges of southeastern Ecuador. Currently defined indicated resources are 23.8 million metric tons (Mt) averaging 9.61 g/t Au and the total endowment is 9.48 Moz Au. The deposit, notable for the widespread occurrence of visible gold and bonanza grades, will be bulk mined underground. Fruta del Norte was discovered in 2006 during greenfield exploration and systematic drill testing of a conceptual geologic model, which predicted that auriferous veins would occur in andesitic volcanic rocks inferred to underlie a zone of arsenic- and antimony-anomalous silicification in fluvial conglomerate. The host andesitic volcanic rocks, crosscutting feldspar porphyry, and associated phreatic breccia are part of a roof pendant in the Zamora batholith. Together, they are products of a continental-margin volcanoplutonic arc of Middle to Late Jurassic age. The deposit lies beneath the northern extremity of the ~16-km-long, Suárez pull-apart basin where it is localized by steep, second-order faults within the regionally extensive Las Peñas strike-slip fault zone. The pull-apart basin was progressively filled by fluvial conglomerate, dacitic ignimbrite, finer grained siliciclastic sedimentary rocks, and, finally, andesite flows. The Fruta del Norte deposit comprises a 1.3-km-long and up to >300-m-wide vein stockwork associated with quartz-illite-pyrite alteration. The deposit comprises two principal vein types, one in the south dominated by quartz, manganoan carbonates, and abundant base metal sulfides and the other in the north dominated by manganese- and base metal-poor quartz, chalcedony, and calcite. Adularia is a minor gangue mineral in both. Both vein types are abruptly transitional upward and westward to a third important ore type characterized by intense silicification and chalcedony veining, with disseminated and veinlet marcasite (± pyrite). An extensive silica sinter horizon directly overlies the andesitic rocks and/or occurs as interbeds in the lowermost 20 m of the conglomerate and, consequently, is in unusual proximity to the underlying gold-silver orebody. Much of the conglomerate lacks silicification except for a narrow, steeply inclined zone exposed above the deposit, which led to its discovery.

scholarly journals Deglaciation of the Colorado Rocky Mountains following the Last Glacial Maximum

2017 ◽  
Vol 43 (2) ◽  
pp. 497 ◽  
Author(s):  
E.M. Leonard ◽  
B.J.B. Laabs ◽  
A.D. Schweinsberg ◽  
C.M. Russell ◽  
J.P. Briner ◽  
...  
Keyword(s):  
Last Glacial Maximum ◽  
Rocky Mountains ◽  
Glacial Maximum ◽  
San Juan ◽  
The South ◽  
Last Glacial ◽  
San Juan Mountains ◽  
The North ◽  
Colorado Rocky Mountains ◽  
Exposure Ages

The availability of almost 180 cosmogenic-radionuclide (CRN) surface-exposure ages from moraine boulders and glacially polished bedrock surfaces makes possible an assessment of the timing and character of the local Last Glacial Maximum (LLGM) and subsequent deglaciation in the Colorado Rocky Mountains. A review of glacial chronologies and numerical modeling results indicates that although glaciers across Colorado responded broadly synchronously, apparent differences in the timing and magnitude of glacier retreat following the LLGM suggest that spatially variable regional forcing, possibly precipitation related, played a role in glacier behavior along with more spatially uniform hemispheric or global forcing. Glaciers in the five ranges examined reached their greatest LLGM extents before ~19.5 ka and abandoned their outermost LLGM moraines between ~23.5 and 19.5 ka. Detailed deglaciation chronologies are available for glaciers in four of the ranges. In the Sawatch Range and Sangre de Cristo Mountains, glaciers were near their LLGM extents at 17-16 ka, before retreating rapidly. In the San Juan Mountains and the Front Range, glaciers may have begun their post-LLGM recession earlier, although early deglaciation is indicated by only a few ages on polished bedrock that potentially contains pre-LLGM CRN inheritance, and thus may be too old. Regardless of the timing of the onset of deglaciation, the equilibrium-line rise associated with deglaciation was earlier and significantly larger in the San Juan Mountains than elsewhere in Colorado. This suggests that the San Juan Mountains, located well to the southwest of the other ranges, may have experienced enhanced precipitation during the LLGM, as did areas farther to the south and west, while LLGM conditions may have been drier in the northern and eastern Colorado ranges. A breakdown in this pattern after the LLGM, with precipitation decreasing in the south and west and increasing in the north and east, may have led to the range-to-range differences evident across Colorado. Deglaciation was nearly complete in all four ranges by 15-13 ka. While some proxy records indicate a later Younger Dryas-age cooling in the Colorado mountains, there is not clear moraine evidence of glacier readvance at that time.

scholarly journals Recognition and significance of Upper Devonian fluvial, estuarine, and mixed siliciclastic-carbonate nearshore marine facies in the San Juan Mountains (southwestern Colorado, USA): Multiple incised valleys backfilled by lowstand and transgressive systems tracts

Geosphere ◽  
2019 ◽  
Vol 15 (5) ◽  
pp. 1479-1507 ◽  
Author(s):  
James E. Evans ◽  
Joshua T. Maurer ◽  
Christopher S. Holm-Denoma
Keyword(s):  
Tidal Flat ◽  
Detrital Zircon ◽  
Source Area ◽  
Alternative Interpretation ◽  
Upper Devonian ◽  
Local Source ◽  
San Juan ◽  
Geologic History ◽  
San Juan Mountains ◽  
The North

Abstract The Upper Devonian Ignacio Formation (as stratigraphically revised) comprises a transgressive, tide-dominated estuarine depositional system in the San Juan Mountains (Colorado, USA). The unit backfills at least three bedrock paleovalleys (10–30 km wide and ≥42 m deep) with a consistent stratigraphy of tidally influenced fluvial, bayhead-delta, central estuarine-basin, mixed tidal-flat, and estuarine-mouth tidal sandbar deposits. Paleovalleys were oriented northwest while longshore transport was to the north. The deposits represent Upper Devonian lowstand and transgressive systems tracts. The overlying Upper Devonian Elbert Formation (upper member) consists of geographically extensive tidal-flat deposits and is interpreted as mixed siliciclastic-carbonate bay-fill facies that represents an early highstand systems tract. Stratigraphic revision of the Ignacio Formation includes reassigning the basal conglomerate to the East Lime Creek Conglomerate, recognizing an unconformity separating these two units, and incorporating strata previously mapped as the McCracken Sandstone Member (Elbert Formation) into the Ignacio Formation. The Ignacio Formation was previously interpreted as Cambrian, but evidence that it is Devonian includes reexamined fossil data and detrital zircon U-Pb geochronology. The Ignacio Formation has a stratigraphic trend of detrital zircon ages shifting from a single ca. 1.7 Ga age peak to bimodal ca. 1.4 Ga and ca. 1.7 Ga age peaks, which represents local source-area unroofing history. Specifically, the upper plate of a Proterozoic thrust system (ca. 1.7 Ga Twilight Gneiss) was eroded prior to exposure of the lower plate (ca. 1.4 Ga Uncompahgre Formation). These results are a significant alternative interpretation of the geologic history of the southern Rocky Mountains.

scholarly journals Colorado Lightning Climatology

2019 ◽  
pp. 45-60 ◽  
Author(s):  
Stephen Hodanish ◽  
Brandon Vogt ◽  
Paul Wolyn
Keyword(s):  
North American ◽  
North American Monsoon ◽  
San Juan ◽  
Lightning Flash ◽  
San Juan Mountains ◽  
The North ◽  
Regional Circulation ◽  
Density Values ◽  
Insight Into ◽  
Cg Lightning

Average cloud-to-ground (CG) lightning flash density values for Colorado are analyzed for the 21-yr period 1996–2016. An annual mean map and monthly mean maps of flash density provide insight into the thunderstorm/lightning climatology over the complex physical landscapes of Colorado. Findings include that 1) the Denver convergence/vorticity zone regional circulation influences the CG lightning distribution across the northeastern Colorado region; 2) moisture associated with the North American monsoon increases CG lightning over the entire state, focusing along the southern exposures of the San Juan Mountains; and 3) the highest concentrations of CG lightning occur where moisture, lift, and instability are maximized.

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