Gamma ray spectrometric survey of a porphyry copper deposit: The San Antonio copper deposit, Sta. Cruz, Marinduque, Philippines



R.Y. Reyes1,2 and G.P. Yumul, Jr.2


1Nuclear Materials Research Group, Atomic Research Division, Philippine Nuclear Research Institute, Diliman, Quezon City, Philippines

2Rushurgent Working Group, National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines





All rocks and soils contain naturally occurring radioactive elements (radioelements) of which the three major sources are 1) 40K, 2) decay products in the 238U series, and 3) decay products in the 232Th series (IAEA, 1979). These radioelements upon disintegration are accompanied by the emission of alpha and beta particles, and gamma radiation. Gamma radiation, having no mass and charge, has much greater penetrating power than alpha and beta particles. Likewise, each decaying gamma-emitting nuclide produces one or more gamma rays of characteristic energy. These two properties of greater penetrating power and characteristic energy confer upon gamma rays their suitability and utility in the study of the radioelements potassium (K), uranium (U) and thorium (Th) as applied to geological, mineral and environmental studies. The abundance of particular radioactive nuclides can therefore be estimated by measuring the intensity and energies of gamma rays making it possible to conduct direct and in-situ analyses of the naturally-occurring radioactive elements K, U Th in the field. Field measurements can be carried-out by conducting either one or a combination of airborne, carborne, or footborne gamma ray spectrometric survey techniques. The purpose of this study is to present the combined use of carborne and footborne radiometric techniques conducted over the whole Marinduque Island (Figure 1) on a regional scale and the San Antonio porphyry copper deposit in Sta. Cruz, Marinduque (Figure 2) on a detailed scale.


The carborne and footborne gamma ray spectrometers were calibrated to convert their measurements in counts per unit of time into concentrations in parts per million or percent. The calibration of the footborne spectrometer equipped with a cylindrical 3 x 3 inch [NaI(Tl)] crystal detector was achieved from measurements on concrete blocks (calibration pads) with known concentrations of K, U, and Th. The calibration of the carborne spectrometer having a 4 x 4 x 16 inch prismatic detector was achieved by comparing its measured K, U, and Th count rates with the ground concentrations of K, U, and Th as measured with the calibrated footborne spectrometer.


Measurements of K, U, and Th (in concentrations) including the UTM coordinates (eastings and northings) by the carborne gamma ray spectrometric system (Figure 3) were taken every five seconds while the vehicle traveled at about 10 20 kph. A global positioning system (GPS), GW-72, which measures the UTM coordinates is incorporated into the carborne system. The footborne system (Figure 4) was used in large areas uncovered by the carborne system. At the sampling site, five measurements each for K, U and Th were taken, one at the center and four on a crisscross basis at one to five meters from the center depending on the of survey phase. A portable GPS (Magellan NavPro 5000) was used to record locations (easting and northing in UTM coordinates) of each sampling station measured by the footborne system.


Marinduque Island is roughly elliptical whose major axis is parallel to the Philippine Fault Zone. It is located approximately 180 kms. south of Metro Manila and its land area of about 910 sq. kms. is mostly mountainous of low to moderate relief. Its tectonic grain is inherent to the northwest-southeast disposition and elongation of the major lithologic units including the trends of the major shears and fault systems (Figure 5). Undifferentiated and metamorphosed basement rocks unconformably overlain by Paleocene to Oligocene sediments and volcanic rocks characterize the Island. Intrusions of Middle Miocene intermediate rocks subsequently occurred, after which further sedimentation and volcanism took place together with the reworking of the older units (Bureau of Mines and Geosciences, 1982). In the San Antonio porphyry copper deposit study area (Figure 6), the sedimentary member of the Paleocene-Eocene Taluntunan-Tumicob Formation, which is unconformably overlain by the Oligocene San Antonio Formation are both intruded by the middle Miocene Mahinhin Diorite Stock. Mineralization of the San Antonio deposit is attributed to this plutonic event. Northwesterly trending andesitic dikes in turn intrude the above rocks (Marcopper Mining Corp., 1989).


            A combined total of 20,524 and 469 data stations were measured for K, U and Th in the regional and detailed surveys, respectively. Contoured color maps with the scale of 1:250,000 of the whole Marinduque Island and 1:100 of the San Antonio porphyry copper deposit were generated for K(%), U(ppm), Th(ppm), U/K(log U/K), U/Th(log U/Th) and Th/K(log Th/K).


            Results of the regional survey show that the radioelement signatures gave good correlation with the regional geology of Marinduque Island. A remarkable contrast in all radioelement signatures outlined the Malindig Volcanics from the rest of the island. These volcanic rocks gave the highest Th and U values. Broad, continuous and northwesterly trending U, Th, U/K and Th/K radiometric patterns outlined the equally northwest-southeast attitudes of the major lithological units and manifest conformity to the overall NW-SE elongation and disposition of Marinduque Island. The highest K values with relatively higher U and Th values were registered by the hornblende biotite quartz diorite.


The average radioelement content of some major rock types in Marinduque Island were also established. Additionally, section lines were laid out in order to obtain a better display of variability and patterns in the radioelement signatures among the rock types in Marinduque. Based on the radioelement maps, section lines and average radioelement content of rocks from Marinduque Island, ultramafic rocks consistently yielded the lowest values in all three radioelements. Limestone depicted the highest U/K and Th/K ratios. Similarly, the tuff depicted low K with relatively higher U and Th values. In all three radioelements, increasing trends in K, U and Th from ultramafic rocks to mafic rocks (basalt) to intermediate rocks (andesite and diorite) were recognized. These increasing trends are consistent with the generally known fact of increasing K, U and Th content with increasing SiO2 among igneous rocks (Galbraith and Saunders, 1983; Ford and Carson, 1986).


            Results in the detailed survey revealed an inward-increasing pattern by the potassium distribution with the highest K values located at the central part of the deposit. This is consistent with the interpretation that the deposit is centrally characterized by the phyllic alteration. This is comparable with the generally accepted models for alteration zoning of porphyry copper deposits with increasing K towards the core of the deposit (Lowell and Guilbert, 1970; Divis, 1983). Uranium distribution also showed an inward increasing pattern but much of the high U values are within the argillic-altered andesite. No distinct pattern was exhibited by thorium, which seemed to be the least affected, by all geological processes, particularly the hydrothermal alteration at the San Antonio deposit. Additionally, K, U and Th concentrations among altered rocks within the San Antonio pit were compared with their unaltered to weakly altered equivalent outside the deposit manifest. Variable increases of potassium were observed among the highly altered rock units within the pit, with the highest percentage increase at the phyllic altered mineralized host intrusive, as compared to their unaltered to weakly altered counterpart rock units outside the deposit. Variable increases of uranium were likewise obtained, but the highest percentage increase was recorded over the argillic-altered andesite. Thorium gave neither corresponding increase nor depletion. Th strengthens the contention that it was unaffected by alteration processes consequently producing high K/Th and U/Th distributions within the pit vis--vis outside the mine.


            The observed K, K/Th, U, and U/Th highs within the San Antonio deposit can be used as radiometric-based exploration indicators in prospecting for porphyry copper mineralization. Regionally, these indicators were exhibited by the three porphyry copper deposits, hence validating the results of the detailed survey. The presence of a high K in combination with either a low Th/K or high K/Th configuration, including U and U/K if present in an area, warrant its consideration as an exploration target.


            This study represents the first systematic approach to determine and map the abundance and distribution of the naturally occurring radioelement potassium, uranium and thorium through a combined carborne and footborne gamma ray spectrometric survey method in the country. The combined carborne and footborne survey was shown to be a useful additional tool in support of geologic mapping and exploration for porphyry copper deposits. This study likewise highlights the production of the first publicly available radioelement (in concentrations) maps in the country.

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