Mining

AT THE SAN MANUEL COPPER MINE IN SOUTHEASTERN ARIZONA, RECOVERY OF COPPER FROM THE OXIDIZED PORTION OF THIS PORPHYRY MINERAL RESOURCE IS BEING ACHIEVED THROUGH A LARGE IN SITU LEACHING OPERATION USING WEAK SULFURIC ACID SOLUTION. IN THE PAST, THIS ACTIVITY WAS COORDINATED WITH OPEN PIT AND UNDERGROUND MINING, BUT IN TODAY'S ECONOMIC CLIMATE, ONLY THE IN SITU OPERATION CONTINUES. THE ACID SOLUTION (20 GRAMS PER LITER) IS INJECTED IN WELLS UN-PRESSURIZED AT VARYING DEPTHS UP TO SEVERAL HUNDRED METERS, USUALLY AT RATES OF ONLY A FEW TENS-OF-GALLONS PER MINUTE. THE COPPER-BEARING PREGNANT LEACH SOLUTION (PLS) IS RECOVERED EITHER IN NEARBY RECOVERY WELLS OR IN COLLECTION AREAS IN THE UNDERGROUND WORKINGS 350 M TO 500 M (1200 TO 1600 FEET) BELOW THE SURFACE. A THOROUGH DESCRIPTION OF IN SITU MINING IN GENERAL AS WELL AS AT SAN MANUEL SPECIFICALLY CAN BE FOUND IN SWAN AND COYNE (1992). DUE TO THE ECONOMIC EFFICIENCY OF THIS MINING METHOD, THE IN SITU OPERATION AT SAN MANUEL HAS EXPANDED FROM TWO TEST WELLS IN THE MID-1980S TO MORE THAN 900 WELLS COVERING OVER 650,000 SQUARE METERS OF THE OPEN PIT MINE. OVER THE PAST TWELVE YEARS, GEOPHYSICAL SURVEYS HAVE BEEN USEFUL IN BOTH PLANNING AND MONITORING THE EXPANSION OF THE IN SITU FIELD.

A CONDUCTIVE CSAMT FEATURE OBSERVED ON THE SUR DE GUERRERO GRID, NEAR THE TOWN OF TAXCO IN GUERRERO, MEXICO, WAS LATER CONFIRMED AS A DISCOVERY BY THIS YEAR'S IMMSA DRILLING CAMPAIGN. THIS FEATURE IS NOW CALLED THE MANTO ESPERANZA VEIJA VEIN. THIS PAPER COMPARES THE GEOLOGY IDENTIFIED BY IMMSA DRILLING WITH THE CSAMT RESULTS MODELED BY ZONGE ENGINEERING.

 

ELECTRICAL GEOPHYSICAL SURVEYS AT SILVER BELL PROVIDE A COMPREHENSIVE STUDY OF THE NORTH PIT PORPHYRY COPPER ORE ZONE. DATA FOR THESE SURVEYS WERE COLLECTED WITH A ZONGE GDP RECEIVER AND GGT-SERIES TRANSMITTER. THIS CASE HISTORY SHOWS THE HIGH RESOLUTION AND DIAGNOSTIC CAPABILITIES INHERENT IN MODERN GEOPHYSICS. DEMONSTRATED IS THE UNIQUE FLEXIBILITY OF THE ZONGE GEOPHYSICAL SYSTEM IN ADAPTING TO DIFFERENT SURVEY APPLICATIONS.

THE SILVER BELL DISTRICT IS WITHIN THE PORPHYRY-COPPER PROVINCE OF SOUTHWESTERN NORTH AMERICA, LOCATED 35 MILES NORTHWEST OF TUCSON, ARIZONA ON THE SOUTH SIDE OF THE SILVER BELL MOUNTAINS. MINERALIZATION IN THE DISTRICT CONSISTS OF AT LEAST THREE DISTINCT DISSEMINATED PORPHYRY COPPER DEPOSITS AND SEVERAL SKARN REPLACEMENT DEPOSITS. DISSEMINATED PRIMARY AND SUPERGENE-ENRICHED PORPHYRY COPPER MINERALIZATION WERE MINED IN TWO OPEN PITS, EL TIRO AND OXIDE, BY ASARCO, MAINLY DURING THE PERIOD 1954-1977. TOTAL PRODUCTION FOR THAT PERIOD IS REPORTED AT 75.66 MILLION TONNES (MT) AT 0.80% COPPER (GRAYBEAL, 1982). THE NORTH SILVER BELL DEPOSIT IS LOCATED AT THE NORTH END OF THE DISTRICT AND REPRESENTS A LEACHABLE RESOURCE OF IN EXCESS OF 80 MT AT AN AVERAGE GRADE OF 0.40% COPPER CONTAINED MOSTLY WITHIN AN ENRICHMENT BLANKET OF CHALCOCITE. WHEN THE GEOPHYSICAL WORK WAS BEING DONE, IN 1993-1994 AND AGAIN IN 1996, THE DEPOSIT WAS NOT BEING MINED. MINING OF NORTH SILVER BELL BY ASARCO BEGAN IN 1997. IN EARLY 1993, THE AREA WAS SUGGESTED BY J. M. GUILBERT OF THE UNIVERSITY OF ARIZONA AS A GOOD SITE FOR A BASELINE GEOPHYSICAL STUDY OVER A PORPHYRY COPPER DEPOSIT. THE DEPOSIT WAS WELL DEFINED BY DRILLING AND SURFACE MAPPING AND DID NOT HAVE ANY SIGNIFICANT SURFACE DISTURBANCE OR EXCESSIVE CULTURAL CONTAMINATION SUCH AS NUMEROUS POWERLINES AND FENCES. THE APPROACH WAS TO SURVEY THE AREA WITH A VARIETY OF GEOPHYSICAL TECHNIQUES AND DEVELOP A COMPREHENSIVE GEOPHYSICAL SIGNATURE OF THE DEPOSIT THAT WOULD HAVE RELEVANCE TO EXPLORATION FOR PORPHYRY COPPER DEPOSITS ELSEWHERE. ASARCO WAS ESSENTIAL TO THE PROJECT BY ALLOWING ACCESS TO THE DEPOSIT AND MAKING AVAILABLE COMPANY INFORMATION REGARDING THE DEPOSIT. THE STUDY WAS USED AS THE BASIS FOR A MASTER'S THESIS AT THE UNIVERSITY OF ARIZONA BY K. C. FOREMAN (1994). GEOPHYSICAL SURVEYS CONDUCTED OVER THE DEPOSIT BY ZONGE ENGINEERING AND RESEARCH ORGANIZATION (ZERO) PERSONNEL INCLUDED: GROUND MAGNETICS, DIPOLE-DIPOLE COMPLEX RESISTIVITY (CR), RECONNAISSANCE INDUCED POLARIZATION (RIP), CONTROLLED SOURCE AUDIO-FREQUENCY MAGNETOTELLURICS (CSAMT) AND TRANSIENT ELECTROMAGNETICS (TEM AND NANOTEM). ADDITIONAL DATA INCLUDE CR ROCK MEASUREMENTS ON CORE SPECIMENS FROM DRILL HOLES WITHIN THE DEPOSIT AND AIRBORNE MAGNETICS AND EM FLOWN BY WORLD GEOSCIENCE IN 1993. EXCEPT FOR THE AIRBORNE DATA FROM WORLD GEOSCIENCE, ALL DATA WERE PROCESSED AT ZERO'S OFFICE IN TUCSON, ARIZONA. TEM, IP AND CSAMT DATA WERE MODELED WITH PROPRIETARY SMOOTH-MODEL INVERSIONS.

THE LOS OLIVOS PROJECT IS LOCATED 5 KM EAST OF VALLE DE OLIVOS NEAR HIDALGO DE PARRAL IN THE STATE OF CHIHUAHUA. DURING 1996 CSAMT DATA WERE COLLECTED ALONG 5 LINES FORMING THE OLIVOS GRID. THESE DATA WERE ACQUIRED WITH THE HIGH-POWER (30 KW) ZONGE INTERNATIONAL CSAMT SYSTEM. LA PARRENA, S.A. DE C.V. HAS RELEASED SURFACE GEOLOGY AND DRILL HOLE RESULTS, INCLUDING THE LOS OLIVOS CSAMT DATA. CSAMT IS A HIGH-RESOLUTION RESISTIVITY MAPPING TECHNIQUE THAT USES BROADBAND ELECTROMAGNETIC SIGNALS FOR DEPTH SOUNDINGS. FOR THIS CSAMT CASE STUDY, THE ZONGE INTERNATIONAL SCS2D INVERSION CODE HAS BEEN USED TO MODEL THESE CSAMT DATA. THE SCS2D INVERSION IS A TWO-DIMENSIONAL (2-D) FINITE ELEMENT SOLUTION THAT NOT ONLY MODELS FLAT-LYING GEOLOGY AND HIGH-ANGLE CONTACTS, BUT ALSO RESISTIVITY CHANGES ASSOCIATED WITH TOPOGRAPHY. THE MODELED RESULTS FOR EACH MODELED LINE IS PRESENTED AS A SMOOTHLY CONTOURED MODELED RESISTIVITY DEPTH SECTION. THE MODELED RESULTS FOR ALL 5 LINES ARE SHOWN AS STACKED DEPTH SECTIONS VIEWED TO THE NORTHWEST, BUT DISPLAYED IN REVERSE ORDER WITH LINES 4 AND 5 SHOWN IN THE FOREFRONT.

DURING THE PAST TWO DECADES THE COSTS OF MOST ELECTRICAL GEOPHYSICAL SURVEYS HAVE DECREASED SUBSTANTIALLY, DUE TO THE IMPLEMENTATION OF DIGITAL RECEIVERS WITH MULTIPLE INPUT CHANNELS. THE COSTS OF DATA PROCESSING, MODELLING AND THE GENERATION OF COLOR SECTIONS AND MAPS HAVE ALSO DECREASED CONSIDERABLY WITH DRAMATIC IMPROVEMENTS IN LOW COST PERSONAL COMPUTERS ALONG WITH THE LATEST RELATIVELY HIGH-SPEED COLOR PLOTTERS. THE NET RESULT OF IMPROVEMENTS IN DIGITAL DATA COLLECTION AND PROCESSING HAS BEEN A REDUCTION IN THE COST-PER-STATION BY A FACTOR OF 10 OR MORE FOR SOME METHODS IN THE FACE OF INCREASED DAILY COSTS FOR PROVIDING GEOPHYSICAL SERVICES.