Sunnyside 29 SGC-042125 Chapter VII Appendices
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A PP E NDIX 7A: GEOLOGY APPENDIX 7B: HISTORY APPENDIX 7C: MINING HISTORY o j tlu* Animus /iu vr' A PP E N D I X 7 A O verview of Geology Anim as River W atershed Above Silverton Prepared for the Animas River Stakeholders Group Use Attainability Analysis Prepared By B.K Stover Colorado Division of Minerals and Geology VOLCANIC HISTORY OF THE SILVERTON CALDERA Water Quality Bedrock Geology Precambrian Metamorphic Roclcs Paleozoic Sedimentary Rocks Upper Paleozoic And Mesozoic Sedimentary Rocks ' Tertiary Sedimentary Rocks Tertiary Volcanic Rocks Surficial Geology Structural Geology nydrothcrmal Alteration CEMENT CREEK SUB-BASTN Stratigraphy Structural Geology nydrothcrmal Alteration Surficial Geology ROSS BASIN WATERSHED Bedrock Geology Structural Geology Itydrothermal Alteration Ore Mineralization Surficial Geology CEMENT CREEK-BONITA PEAK WATERSHED AREA Bedrock Geology Structural Geology Hydrothermal Alteration Ore Mineralization Surficial Geology SOUTH FORK CEMENT CREEK AREA 19 Location 19 Bcdrock Geology 19 Structural Geology 19 Hydrothermal Alteration 20 Ore Mineralization 20 Surficial Geology 20 PROSPECT GULCII-GEORGIA GULCH AREA 21 Location 21 Geologic Setting 21 Bedrock Geology 21 Structural Geology 22 Hydrothermal Alteration 22 Ore Mineralization 22 Surficial Geology 23 LOWER CEMENT CREEK AREA 24 Location 24 Geologic Setting 24 Bedrock Geology 24 Structural Geology 24 Hydrothermal Alteration 24 Ore Mineralization 25 Surficial Geology 25 ANIMAS RIVER HEADWATERS AND BURROWS CREEK 26 Location 26 Geologic Setting 27 CALIFORNIA GULCH 28 Location 28 Geologic Setting Surficial Geology 29 PLACER GULCH 29 Location ^ Geologic Setting Surficial Geology 30 ANIMAS RIVER SITES, ( ANIMAS FORKS TO EUREKA) 30 Location 30 Geologic Setting 3* EUREKA GULCH 31 Location 3* Geologic Setting 31 MINNIE GULCn 32 Geologic Setting 32 Surficial Geology 33 Geologic Hazards 33 MAGGIE GULCH 33 Location 33 Geologic Setting 33 Surficial Geology 33 CUNNINGHAM GULCH 33 Location 33 Geologic Setting 3^ Surficial Geology 35 ARRASTRA BASIN 35 Location 35 Geologic Setting 35 Surficial Geology 37 ANIMAS RIVER SITES 37 Location 37 Geologic Setting 37 Surficial Geology 39 MINERAL CREEK SITES 3 9 LONGFELLOW-KOEHLER 39 Location 39 Geologic Setting 39 Bedrock Geology 39 Structural Geology 40 Surficial Geology 40 BONNER MINE 40 Location 40 Geological Setting 41 Bedrock Geology 41 Structural Geology 41 Surficial Geology 41 Mine Feature 41 RUBY TRUST 42 Location 42 Geologic Setting 42 Bcdrock Geology 42 Structural Geology 42 Surficial Geology 43 PARADISE MINE 43 Location 43 Geologic Setting43 43 Bcdrock Geology 43 Structural Geology 43 Surficial Geology 44 BANDORA MINE 44 Location 44 Geologic Setting 44 Bedrock Geology 44 Surficial Geology 44 NORTH STAR MINE 45 Location 45 Geologic Setting 45 Bedrock Geology 45 Structural Geology 45 CARBON LAKE 46 Location 46 Geologic Setting 46 Bedrock Geology 46 Structural Geology 46 Surficial Geology 47 REFERENCES 48 Overview of Geology Animas River Watershed Above Silverton By B.K Stover Colorado Division of Minerals and Geology Volcanic History of the Silverton Caldera The Animas River headwaters drain the Silverton Caldera, a Tertiary-aged volcanic center on the western margin of the regional San Juan Volcanic Field. In Oligocene through Miocene time, the Silverton Caldera was a focus of repeated volcanic eruptive activity. Hundreds of cubic miles of ash flows and lava were erupted upon a surface of older Paleozoic and Mesozoic sedimentary rock, and Precambrian metamorphic and igneous basement rocks. Through the middle Tertiary, an extensive, thick volcanic complex was formed, encompassing all the present Animas Basin watershed. During periods of volcanic quiescence, retreat of magma from beneath the domed-up center caused widespread subsidence and subsequent collapse of a roughly ten-mile-diameter ring-shaped caldera within the larger volcanic field. Subsequent periods of eruptive activity each caused renewed uplift and doming of the caldera, followed in turn by subsidence along the bounding ring-fauft fractures as volcanic activity waned. These repeatedly active marginal ring-fault fractures, associated breccia pipes, and swarms of antithetic faults tangential and radial to the margin of the caldera, formed preferential pathways for circulating mineralized hydrothermal fluids. Following cessation of volcanic activity, ground and meteoric waters began infiltrating and circulating in the cooling mass of volcanic rock. Heat from the cooling magma below set up broad, regional convection systems, circulating hot hydrothermal fluids through the subsurface for millions of years. Through time, these fluids chemically altered the original rock mass, and became enriched in metals derived from the volcanic and surrounding pre-volcanic country rock. Eventually, mineralizing solutions reached threshold geochemical temperature-pressure conditions, leading to deposition of several types of sulfide ores containing silver, lead, zinc, copper, and gold. The sulfide ores were preferentially deposited within and around the pre-existing fissures, faults, and breccia pipes formed millions of years before. Much of the rock in the Silverton Caldera complex is therefore highly mineralized and hydrothermally altered, particulary along the margins of the caldera, and in the vicinity of major fissure vein systems. Through the late Tertiary and to present day, regional uplift and subsequent erosion of the Colorado Plateau has cut deeply into the volcanic pile. Thousands of feet of overlying rock have been stripped away, revealing the roots of the volcanic center. Canyons around the margin of the caldera, such as the Animas River and Uncompahgre Gorge, have cut deeply into the underlying strata, exposing the underlying Paleozoic and Precambrian rock beneath the volcanic deposits. Water Quality The headwaters of the Animas River begin within and along the margins of the Silverton Caldera. Two major tributaries draining this volcanic terrain, Mineral Creek and Cement Creek, carry excessive metals loading, joining the Animas River at Silverton just before it enters the narrow, steep walled canyon south of town. Water quality sampling in the Animas Basin around Silverton has shown that in-stream water quality can be directly correlated to the specific geologic, mineralogic, and rock alteration attributes present in a stream’s watershed. Rock type, primary mineralization suites, and both pre and post-mineralization hydrothermal alteration systems present in a watershed characterize the observed stream water quality, even in undisturbed watersheds. Analytical data shows that the overall stream water quality of the Animas and its tributary gulches on the eastern side of the Silverton Caldera is generally much better than the water quality of Cement Creek and Mineral Creek Watersheds. The South fork of Mineral Creek is nearly pristine, contrasting sharply with the metals laden waters of the main stem of Mineral Creek. There is a direct association between better in-stream water quality and the geologic differences in mineralization and alteration styles present in the individual gulches in the Animas Basin watershed. Bedrock Geology The Animas watershed drains roughly three-quarters of the total extent of the Silverton Caldera. Extrusive sequences of volcanic ash-flow tuffs and flow breccias, and dacite -to-rhyodacite lava flows and domes underlie essentially the entire watershed. These rocks belong to the Silverton Volcanic series, and underlying San Juan Formation. The Silverton series has been further subdivided into mapable formations in the Silverton Caldera. On the southern and eastern margins of the caldera, Paleozoic and older Precambrian rock are exposed beneath the volcanic flows. Intruded upward into the volcanic flows within the caldera, but particularly along its margins, are younger stocks, plugs, dikes, and sills of a variety of igneous rock. The sequence of rocks underlying and exposed in the Animas watershed area are described here, from oldest to youngest (Ludeke and Burbank, 1987) Precambrian Metamorphic Rocks Irving Formation (Precambrian-X')- Metavolcanic and metasedimentry gneiss, quartzite, amphibolite, and schist “basement” rock underlies the volcanic flows and breccias of the Silverton series on the eastern and southern margins of the caldera. Exposures are limited to the lower valley walls at the head of Cunningham Gulch, in lower Stony Gulch, and just inside the mouth of the Animas Canyon at the Champion Mine. In the Animas Canyon area, an angular unconformable erosion surface forms the contact between the Precambrian Irving section and the overlying upper-Cambrian Ignacio Quartzite. The erosion surface can be traced along the west canyon walls downstream of the Champion Mine. It is characterized by a smooth, undulating surface carved in the metasedimentary Irving Formation, beneath a quartz-cobble and pebble conglomerate at the base of the Ignacio sediments. Locally, the Precambrian contact has been structurally offset several feet across younger vein/ dike structures associated with the granite- porphyry intrusive on the flanks of Sultan Mountain, indicating a dilative emplacement of the intrusive. Compositional layering ofthe underlying Irving metamorphic complex is sharply discordant to the overlying Cambrian sediments and Silverton Volcanic sequence, Paleozoic Sedimentary Rocks Leadville Limestone. Ourv Limestone and Elbert Formation, and the Ignacio Quartzite (Cambrian to Mississippian)- A narrow section of lower Mississippian,