News and Information: News-November 2006

The Espino district is located in the Chilean Coastal Range to the south of other known IOCG deposits.  The district contains a number of copper and gold veins that have been exploited since colonial times.

The regional setting of Espino is characterized by an Early Cretaceous volcanic arc formed at a continental margin with recurrent marine transgressions.  The district contains a series of intermediate stocks of granodioritic to dioritic composition that intruded the volcanic sequence, and a relatively small marine to transitional sedimentary basin.  Both the intrusive rocks and the volcanic-sedimentary rocks have been hydrothermally altered.

Sodic alteration characterized by albite is extensively developed in the district at low to intermediate structural levels.  Sodic alteration is overprinted by a spatially more restricted sodic-calcic alteration (88.4±0.6 Ma) at intermediate and low levels.  Potassic alteration has been recognized in limited exposures at the structurally lowest portions of the hydrothermal system.  Hydrolytic alteration (87.9±0.6 Ma) occurs at higher levels of the system, and, locally, at intermediate levels.  Hydrolytic alteration is focused along N- to NE-striking veins composed of chlorite-sericite or quartz-sericite accompanied by iron oxide and sulfide with anomalous gold and copper.  Quartz, with minor hematite, is abundant at the highest portion of the system, whereas at intermediate levels hematite is abundant and quartz is minor.  At deeper levels, quartz is rare and magnetite is dominant.

Two weakly deformed Cretaceous pegmatitic tonalite dikes, representing part of the high-pressure magmatic plumbing of the North Western U.S. paleo-volcanic arc system, occur in the North Cascade Mountains, Washington.  These dikes contain quartz, plagioclase, muscovite, epidote and rare garnet, and apatite.  Significantly, the epidote occurs in more than seven texturally and chemically distinct populations, with individual crystals in excess of 13 cm.  Trace element (TE) analyses have been carried out on the epidote populations to test hypotheses of magmatic processes associated with an evolving tonalite magma system (magma recharge, country rock assimilation, differentiation, phase-reaction evolution), as well as to constrain the relative chronology of these populations.

Geochemical interpretations indicate that two of the hypothesized magmatic processes were dominant in the early evolution of this system.  First, magmatic differentiation, resulting from fractionated high temperature/pressure magmatic phases, led to decreasing TE concentrations; second, the breakdown of a heavy rare earth element (HREE) phase significantly adjusted to HREE geochemical patterns of subsequent epidote populations.  It has also been determined that TE variations of these epidotes can be used to construct the relative chronology of the epidote populations.  These results are important because they demonstrate the ability to use distinct populations of trace minerals to acquire insight into early magmatic processes and geochemical evolution.  Early magmatic processes, which often lack constraints due to evolutionary late crystallization processes that dominate the geochemical signature of the final rocks, are important in reconstructions of tectonic regimes, in particular, the deep plumbing systems of continental volcanic arcs.

Denver International Petroleum Society meets the second Friday of each month at the Wynkoop Brewing Co., 18th and Wynkoop Streets. Reception begins at 11:30, luncheon at noon, program at 12:30. Make reservations (required) by leaving message at (303) 623-5396. Reservations accepted after 8 a.m. on Friday until 10:30 a.m. on Wednesday prior to the meeting. Cancellations accepted until 11:00 am Wednesday prior to the meeting. Cost: $15 for lunches; talk only is available for $2 (make checks payable to “D.I.P.S.”). Contact Keith Murray at (303) 986-8554 for information.

Denver Region Exploration Geologists’ Society (DREGS) meets in the Mutual Consolidated Water Building, 12700 West 27th Avenue, Lakewood. Social hour 6:00-7:00 p.m. Technical presentation at 7:00 p.m. Meetings are normally scheduled for the first Monday of each month. For information contact Jim Piper, (303) 932-0137, or the website http://www.dregs.org.

Colorado State University, Dept of Geosciences, Rm 320 Natural Resources Bldg, 4:10 pm. Nov 13, Geovanni Romero, New Mexico State, "Evaluation of the Allochthonous Model for the formation of Waulsortian Mud Mounds in the Sacramento mountains, South Central New Mexico".  Nov.. 27, Dan Centerelli, USFS, "Applying Geomorphic Principles to Restore Fluvial Processes and Aquatic Organism Passage at Road-Stream Crossings."  970-491-5661. http://www.cnr.colostate.edu/geo/seminars