The 2009 Pozo Seco Molybdenum/Gold Discovery

Transcription

1 Extended Abstract for KEGS Symposium: MAG Silver Corporation (MAG) Cinco de Mayo Project Submitted by Ken Robertson, P.Geo., Delta, B.C. The 2009 Pozo Seco Molybdenum/Gold Discovery MAG Silver Corporation s (MAG) 100% owned Cinco de Mayo property comprises an area of 22,000 hectares located approximately 190 kilometres north of the city of Chihuahua, in northern Chihuahua State, Mexico. During the spring of 2009 Geotech Ltd. completed a 1,920 line kilometre VTEM survey and a 413 line kilometre test of the ZTEM method over the property. The target model was Carbonate Replacement Deposit (CRD) style mineralization. To the best of our knowledge this was the first use of the ZTEM method in the search for buried CRDs and skarn type deposits in Mexico. In the summer of 2009 MAG announced the discovery of a new zone of high grade molybdenum (moly) and gold mineralization in the western part of the project area. The deposit, hosted by silicified micritic limestone and black shale collapse breccia has since been christened Pozo Seco. The discovery hole CM intercepted 75 metres of silicified breccia that averaged 0.307% molybdenum (Mo). Included in this interval were 34.5 metres averaging 0.637% Mo with ppm gold and a 4.65 metre section grading 1.64% Mo with ppm gold. Drilling to date demonstrates grade, width and continuity within the Pozo Seco Prime discovery zone (see following figure) with contiguous holes outlining a flat lying, tabular body measuring roughly 2,500 metres long, 250 to 350 metres wide and at least 55 metres thick. Step out drilling, through thin alluvial cover, indicates that the deposit remains open in at least two directions. In August 2010 MAG announced the first mineral resource estimate for the Pozo Seco Moly-Gold discovery. Results confirm that Pozo Seco is a significant high grade oxide moly /gold deposit Indicated tonnes: million at 0.147% molybdenum and 0.25 grams per tonne gold Inferred tonnes: million at 0.103% molybdenum and 0.17 grams per tonne gold (Note: grade is for Mo, not MoS 2 ) 1

2 North South As with the 2007 Jose Manto Carbonate Replacement Deposit North (CRD) discovery on the east side of the Jose Manto CRD Cinco de Mayo Property (The Leading Edge, Robertson & Megaw, June 2009), the Pozo Seco deposit was discovered by following-up airborne survey anomalies in an area of widespread alluvial Pozo Seco Mo/Au Deposit cover. The project South geologists could easily have missed this unexpected style of mineralization but didn t. The discovery also highlights the importance of collecting multi-element assays. In this case coincident VTEM conductors and a ZTEM conductive zone were observed. There is virtually no outcrop to guide exploration of these blind deposits. Surprisingly, the Pozo Seco Deposit is covered by as little as six meters of alluvium in places and it does not have a surface expression. The target was CRD mineralization but something unexpected was found instead. 2

3 The significance of the Pozo Seco discovery is two-fold: 1) The high-grade moly (approximately three times the average grade of most moly producers) encountered over bulk mineable widths just below surface indicates the likelihood of a standalone moly-gold open pit operation 2) It is perhaps even more significant that the presence of high-grade molybdenum may indicate proximity to an intrusive center of Cinco's silver-lead-zinc mineralization. A much smaller, lower grade proximal moly zone characterizes the San Martin-Sabinas District in Zacatecas, the largest skarn-replacement deposit known in Mexico with upwards of 100 million tonnes of high grade Pb/Zn/Ag. The Pozo Seco Deposit is almost ten times larger than this moly zone. MAG s philosophy and approach to exploring in alluvial covered areas appears to be paying off. We intend to demonstrate how we have applied modern geophysical techniques, alteration mapping, structural geology and multi-element Cinco Range analysis to the search for CRD s and Jose Manto skarn type deposits. Pozo Seco This geological concept model shows a granitic intrusion at depth and the postulated locations for various styles of mineralization. The Jose Manto CRD discovered in 2007, at a vertical depth of 400m, would be distal to the intrusive. The Pozo Seco Mo/Au Deposit would be more proximal. If this concept holds this view would be from north to south along the spine of the Cinco Range. 3

4 Pozo Seco Deposit Jose Manto CRD The Total Magnetic Intensity (TMI) map of the Cinco de Mayo property is dominated by a broad and oval shaped magnetic anomaly trending in the north-western direction, as shown in the following image of Total Magnetic Intensity with VTEM conductors. The magnetic anomaly is interpreted as a deep and moderately magnetic intrusive body. Modeling and 3D magnetic inversions indicate that the top is at a depth of approximately 2,000 metres. The magnetic relief is roughly 250 nt. MAG has enlisted the help of several consulting companies to apply their particular areas of expertise to extract as much information as possible from the VTEM and ZTEM databases. The methods applied to processing the ZTEM data have steadily improved over the past two years. Along with the standard map products Geotech provided ZTEM Resistivity-Depth Sections for selected lines. This flight line (4420) flew directly over the Pozo Seco Deposit. Pozo Seco conductive trend 4

5 Meeting Geophysical Challenges: Difficult Environments and Hard-to-Detect Commodities Mira Geoscience completed some early 3D ZTEM modeling and interpretations, ran 3D magnetic inversions and applied their targeting software to the Cinco de Mayo Project. Condor Consulting has worked with portions of both the VTEM and ZTEM surveys. The following figure illustrates one of their most recent Topography Corrected 2D Occam Inversions for flight line Twenty two VTEM CDI (Conductivity Depth) sections were produced by Geotech with EMflowTM (Encom Technology Pty) on flight lines that crossed anomalous EM trends. The rationale for generating the CDI sections was to provide images of the conductivity distributions with depth, the result of which was to image and map shallow and deep anomalies that could be related to metallic mineralization. The next figure displays a CDI section and related magnetic and time constant profiles for Line The section highlights, in the southwestern portion of the line, a shallow anomalous zone attributed to a highly conductive and thick geologic formation. It also defines a deep and local anomalous zone associated with a steeply dipping bedrock conductor estimated to lie at a depth of 200 to 250 metres. This bedrock VTEM conductor, that appeared to lie above the ZTEM conductive zone, was the intended target of drill hole CM09-130, the Pozo Seco discovery hole. 5

6 TMI VG1 of TMI Tau B-field (red), Tau db/dt (blue) db/dt decays Shallow zone of anomalous conductivity Deep, steeply east dipping bedrock conductor Cinco de Mayo Ridge Working in collaboration with consultants we have continued to refine our in-house handling of the VTEM and ZTEM datasets to extract as much key exploration information as possible. The following figure illustrates a slice through a Karous-Hjelt block model of the 90Hz X In-phase ZTEM data. Superimposed on this block are the Pozo Seco Prime and NW solid bodies defined by Scott, Wilson, Roscoe, Postle (SWRP) and a proposed open pit design. Although faint the VTEM Pozo Seco Open Pit Design with 90Hz XIP ZTEM conductors can be seen tracking the surface expression of the ZTEM. 6

7 We know from subsequent drilling that the ZTEM is mapping an underlying conductive unit that can be used as a marker horizon. We have not adequately explained the cause of the strong Pozo Seco VTEM conductors but continue to work on solving that conundrum. Many weaker VTEM conductors are coincident with mineralized jasperoids and others are apparently related to high angle fugitive veins or mineral leakage along zone of structural weakness. In the Pozo Seco area both the VTEM conductors and the ZTEM conductive zones lie on the east flank of the deep seated elevated magnetic susceptibility body. The VTEM conductors and the ZTEM conductive zones are also more or less coincident with the regional northwest-southeast trending Santa Lucia Fault. Conclusions The VTEM and ZTEM surveys, flown on the Cinco de Mayo Property, have both proven to be effective exploration tools but they only form part of the exploration arsenal. Coupled with multi-element geochemistry, structural geology, geological mapping, detailed drill logging and other geophysical methods they helped to advance the Pozo Seco from a grass roots discovery to an approximately 55 million tonne, high grade molybdenum/gold, deposit in slightly over a year. The exploration potential of this project area is far from exhausted. Many already identified target areas remain to be tested, when manpower and resources permit. MAG continues to pursue a large high grade CRD or skarn type deposit in the chance that the Pozo Seco Deposit and Jose Manto are indicators that there is even more substantial mineralization nearby. Acknowledgements MAG Silver Corp. wishes to express thanks to the following companies for their contributions to the Cinco de Mayo exploration program; Condor Consulting, Geotech Ltd., Mira Geoscience, PhotoSat, Scientific Computing and Applications and SJV Consultants Ltd. References Robertson, K. and Megaw, P., Cinco de Mayo: A New Silver, Lead and Zinc Discovery in Northern Mexico; Leading Edge, June Ward, S.H., AFMAG: -Airborne and ground, Geophysics Vol. XXIV, 1959 Ward, S.H, O Donnell, J.,Rivera, R., Ware, G.H. and Fraser, D.C., AFMAG Applications and Limitations Geophysics, Vol. XXXI, No. 3, June