FLORISSANT SCIENTIFIC SOCIETY
Cripple Creek Mine Tour, 28 September, 2003
Leader: Tim Brown, Anglogold
Attendees: David Atkins, Paul Dunsdon, John Ghist, Ian and Jane Nicoll, Trevor Polley, Beth Simmons, Sheila Steele, Tom Steven, Dan Shawe, Steve Veatch, Andy Weinzapfel
Tim started about 11:15 AM with a detailed geologic presentation of the Cripple Creek District in his office at the Victor hotel, continued with demonstration of 3-D GIS Vulcan/ Arcview model, then lead a mine tour, wrapping up about 5:00 PM.
Anglogold has about 300 non-union employees, and is the largest private employer in Teller County. The mining/leach operation is 24 hr, 365 days/yr (12 hour shifts).
The Cripple Creek district, from 1891-2002, produced 22-23 MM oz Au at depths of 3000 feet or less. This represents nearly ½ of Colorado’s gold production. Surprisingly, the total Au mined can fit in a cube 11 feet on a side (specific gravity of 19.3 helps)! Cripple Creek is the 3rd largest gold producer in the US, after moth-balled Homestake (Lead, S.D.) and Carlin, Nevada. Much of the gold occurs as tellurides: calaverite and sylvanite. These two are not easily differentiated megascopically. Silver content, presumably from sylvanite, is significant. Pour dore from leach pad is 30% Ag. A ternary diagram for Au-Ag-Te system is shown below:
There is no known floor to the gold-rich diatreme. The cumulative production curve reflects the relative ease of mining the high-grade, shallow veins that were exploited in the early years of the district. It also shows the impact of a changing gold price as well as the effect of governmental action such as the War Powers Act signed by FDR that closed all non-essential mines during WWII. Above the mining limit of 3000 feet, there is a significant volume of rock that remains unexplored or under-explored. Reserves have increased with recent (since 1997) successful exploratory drilling and Anglogold is optimistic about the future. Discovery costs average $5.07/oz, while mining costs are $170/oz. Fully loaded costs are in the neighborhood of $240-250/oz., so at current gold prices, significant reserves remain. The present mining permit continues to 2012. At some point, the economic limit of open pit mining will be hit, so the long-term future is really back to underground mining. About 0.6-0.7 oz/ton grade will likely be needed to proceed.
Historically, over 400 individual mining operations have occurred at one time; Anglogold currently controls 90-95% of district. It has been a major task to capture all the relevant historical data and integrate into the GIS world. Past mining activity represents a significant safety hazard. Many hidden drifts intersect open pit operations, but usually are recognized early through blast hole drilling and ground penetrating radar.
Anglo is currently producing about 300,000 oz/yr, or nearly 1000 oz/day. Average ore grade is 0.03 oz/ton, recovery factor 65% using sodium cyanide leachate. Strip ratio is about 2:1. Considering leach pad capacity of 300 MM tons, 600 MM tons overburden would have to be mined to fill it—together nearly a billion tons of rock!
The alkalic (low silica, high Na and K) Cripple Creek diatreme dates as early as 32- 34 ma and covers 7 square miles (18 sq km) across three coalescing “subbasins”, each having their own feeder system. Ar/Ar dates on the phonolites reange from 31.8 to 33.2 MA. Lamprophyre is the youngest unit, crosscutting everything else. Mineralization is post-diatreme. The picture that emerges is one of coalescing volcanic centers, with multiple phases of mineralizing fluid events. There is a granite island and schist island within the overall diatreme structure.
The Cresson pipe, a famous “sweet spot”, has contributed 3.2 MM tons ore averaging .694 Au. Recovery has been 2.22 MM oz. This pipe bifurcates into two legs at depth.
Structure—mostly joint swarms, as faults have little displacement-- is a critical ore control in the district, especially at their intersections. Old mining data are helpful in this regard. Joint trends/lineaments are NW-SE, NE-SW, and N-S. Approximately 25,000 strike-dip measurements have been captured in 3-D space using Vulcan/ArcView software. Borehole image logging is useful for capturing structural information, but this is not done very often due to expense. There is no ring fracture system, thus no organized caldera. Some of the joint trends could be interpreted as radial fractures. The widespread phonolite breccia may partially have been produced by collapse, but there were almost certainly times when overpressure exceeded lithostatic (local top seal present). Meteoric water contacting hot rocks at depth is another mechanism for producing the breccia. Some phonolite plugs have narrow root geometries, flaring out in the shallower section.
Reserves/grade are selective regarding certain host lithologies. The extremely rich Cresson ores occur along the margin of a lamprophyre breccia. Rich, sheeted gold telluride vein systems have yielded 4.3 MM oz from a single mine. Hydrothermal breccia represents 0.55 MM oz. recovered. A large low-grade reserve exists within the widespread phonolite breccia, a major focus of current mining.
The diatreme outline shows up well on a resistivity (900 hz) survey and reduced-to-pole magnetics as a low. There is a gravity low at the Cresson pipe. North of the diatreme, there is a suggestion from gravity of volcanics continuing under granite. Geochemical surveys indicate enrichment of Au, Te, As, Sb, K, and W; depletion of Na and Ce.
Haul trucks have a capacity up to 300 tons. There is a digital readout visible on the truck’s side, indicating weight. Tires on these monsters cost $20,000 each. Blast holes are drilled 40 feet deep, on 20 foot centers. No dynamite is used; ammonium nitrate plus diesel is the explosive. Open pit benches are 35 feet high. The extra five feet of blast hole depth serves to fragment the rock so that bench grade can be easily maintained. Assay results for the exploration work require a 10 day turnaround. Assays for the mine operations group have a 24 hour turnaround. A laser system provides horizontal reference within the pit. The geology of the walls is examined and marked. Ore is flagged so that the driver knows whether his destination is the crusher (ore) or a designated waste area.
About 250,000 feet are drilled annually. Voice recognition software has been developed for geological descriptions of chips. RC drilling comprises 95% of the total footage while core drilling is about 5%. Historic exploration drill holes average 600-700 feet deep while the average depth for recent drilling is closer to 1000 feet.
The Carlton and Roosevelt tunnels drain the diatreme effectively. The former discharges today at 1500 gpm, the latter 15 gpm. Water quality is relatively good. Today the water table is at about elevation 7000 feet, or generally 3000 feet below surface. This means there is no freeze/thaw problem in the open pits. Walls are surprisingly steep.
29 Sept, 2003