Mineral Resource Assessment In Covered Area

In order to illustrate the general and efficient procedures for mineral resource assessment in covered area,"Tuwu-type" porphyry Cu-Mo deposit in Gobi desert covered landscape of Eastern Tianshan, China, was taken to as an example and well studied. Firstly, on the basis of Geographic Information System (GIS), the mufti-source information spatial database that was consist of geological maps, deposits or occurrences, aeromagnetic, bouguer gravity, geochemical, remote sensing images, digital elevation and exploration and/or mining tables or reports as well as other mineral resources associated data of the studied area was established by means of data inputting works such as coordinate registration, vectorization and attribute table assignment etc.; Secondly, the exploration conceptual model for "Tuwu-type" porphyry Cu-Mo deposit was proposed on the basis of the studies in both geology of region and Tuwu-Yandong deposit, such as the comprehensive analysis of geological, geophysical, geochemical and remote sensing characteristics. Then, based on the conceptual model, various of data processing methods were applied to bouger gravity, aeromagnetic, geochemical and remote sensing data for identifying and extracting mineralization associated anomalies:(a) in the geophysical data processing, the enhancement approaches for boundary information of potential field such as tilt derivation, the total horizontal derivation of tilt derivation and eular deconvolution methods were applied to enhance edge characteristics of bouger gravity and aeromagnetic anomalies and upward continuation was applied to isolate localized and regional anomalies of the bouger gravity and aeromagnetic data. On the basis of the processing results of the bouger gravity and aeromagnetic data, concealed tectonics and intrusions both in the covered and uncovered areas were interpreted by means of the establishment of bouger gravity and aeromagnetic interpretation criteria from outcrops district of the studied area;(b) in the geochemical data processing, firstly, geological units were divided according to the characteristics of lithological construction or combination. Next, elements behavior was analyzed by the way that accumulation coefficient analysis of elements in different geological units so as to investigate possible mobility of the elements for determining a set of proper indicator elements including Cu, Mo, Pb and Zn for porphyry Cu-Mo mineralization and Al2O3, Na2O, Be, and MgO for felsic intrusions, respectively. Then, singularity mapping technique was employed to enhance and recognize weak geochemical anomaly signals of Cu, Mo, Pb and Zn, and Al2O3, Na20, Be, and MgO in the Gobi desert covered district. Finally, the normalized index overly method was applied for integrating single element geochemical anomalies of Cu, Mo, Pb and Zn and principal component analysis method was utilized to Al2O3, Na2O, Be, and MgO derived by singularity mapping in order to delineate integrated geochemical anomaly information associated with porphyry Cu-Mo mineralization and felsic intrusions, respectively; and (c) in the remote sensing data processing, firstly, linear and circler structures both in the covered and uncovered areas were interpreted by means of the establishment of remote sensing interpretation criteria from outcrops district of the studied area. Then, the remote sensing images were preprocessed by means of radiometric calibration, atmospheric correction, and interference (saline-alkali soil, vegetation, shadow, etc.) removed. On the basis of results of the preprocessing, feature-oriented principal component analysis method (Crosta approach) was further used to extract alteration anomalies of iron and hydroxy; In the end, the integrated prediction criteria for "Tuwu-type" porphyry Cu-Mo deposit was drawn on the basis of the exploration conceptual model and the comprehensively processing results of geophysical, geochemical and remote sensing image data. Based on the prediction criteria, weights-of-evidence method was employed to integrate the evidence themes that the favorable geological, geochemical, geophysical and remote sensing anomalies including tectonics, strata, intrusions, bouger gravity anomaly, aeromagnetic anomaly, and the alteration anomalies of iron and hydroxy to calculate posterior probability map for delineating prospects for future’s porphyry Cu-Mo deposits exploration. According to the above study works, the following main conclusions could be drowned:(1) The East Tianshan involves several tectonic units that Tarim, Ili, Kazakhstan-Junggar, and several other micro-plates, which have undergone a series of geological evolutional events such cracking, hyperplasia and collision. Since the original continental crust of East Tianshan in Archean-Proterozoic was formed, it has gone through five important tectonic evolution stages: The evolution stage of Precambrian basement, the formation and early evolution stage of Ancient Tianshan ocean from Sinian to middle Silurian, the stage of Tianshan oceanic evolution and primary collision of ancient continents from late Silurian to early Carboniferous, the stage of post-collision and crust-mantle interaction from late Carboniferous to Triassic and the evolution stage of continent from Jurassic to Quaternary. Among these five stages, the fourth stage that the stage of post-collision and crust-mantle interaction from late Carboniferous to Triassic has undergone many intensive crust-mantle interaction activities, which accompanied with strong magmatism, particularly for the acidic granitoids or igneous that were usually considered to be in favor of migration and enrichment for metal mineralization associated elements, which was generally corresponding with the metallogenic peak periods of various of metals in the Eastern Tianshan such as iron, copper, nickel, molybdenum, gold, silver and other metal mineralizations including "Tuwu-type" porphyry Cu-Mo deposits. However, there are still many disputes for researchers on the issues that the tectonic properties of the East Tianshan and its adjacent region until now. In currently, a new perspective is that taking Dacaotan-Dnanhu fault as the boundary of plates, the East Tianshan can be divided into two main tectonic units that Kazakhstan-Junggar plate in the south and Tarim plate in the north. The tectonic units of the studied area involved in Turban-Hami massif and Ha’erlike-Dananhu Paleozoic island-arc that belong to the southeastern margin of Kazakhstan-Junggar plate, and Jueluotage late Paleozoic rift, central Tianshan block, Ai’erbin late Paleozoic ocean basin, and north Tianshan early Paleozoic rift that belong to the north Tarim Plate. Among these tectonic units, the "Tuwu-type" porphyry Cu-Mo deposits would like to produce in Jueluotage late Paleozoic rift and Ha’erlike-Dananhu Paleozoic island-arc.(2)"Tuwu-type" porphyry Cu-Mo deposits are mainly hosted in the Carboniferous strata that are dominantly composed by clastic or/and volcanic rocks, such as Gandun Formation (C1gd), Qishan Formation (C2qs), Wutongwozi Formation (C2w), etc., and the Carboniferous-Permian acidic porphyry intrusions, such as plagioclase porphyries and diorites. The alterations are well developed and symmetrically around the ore bodies, from the inside to outside, which were the quartz-biotite-sericite-anhydrite, quartz-sericite and propylitic alteration, respectively."Tuwu-type" porphyry Cu-Mo deposits are associated with geochemical anomalies of Cu, Mo, Au, Ag, Bi, Zn, Ni, Pb, W, As, Sb and Cd. Porphyries (mineralized intrusions) usually have high chargeability, medium to low resistivity, high local magnetic anomaly, low bouger gravity anomaly and iron and hydroxy alteration anomalies.(3) The integrated processing and geological interpretation for geophysical data in covered area should follow several important principles:(a) the prior knowledge of geology in the studied area should be one of the most important prerequisites in geophysical data processing and comprehensive geological interpretation. On the one hand, well understanding the geological evolution events and backgrounds such as strata, tectonics, and magmatic rocks, as well as other metallogenic regularities of the studied area could provide a reasonable geological guidance for interpretation of regional structural framework including the concealed faults and intrusions; On the other hand, the physical properties of different rocks or/and minerals such as magnetic susceptibility, density, resistivity should be collected to be assistant for quantitative or qualitative explanations of the proper geological significance for geophysical anomalies;(b) the basic principles and interpretation procedures for geophysical data that bouguer gravity and aeromagnetic anomaly should be strictly followed;(c) application of GIS and geoinformation technologies, which could be employed to analyze the spatial correlation among bouguer gravity and magnetic anomalies with other geoinformations including geological, tectonic, mineralization, remote sensing and geochemical data for improvement of the reliability of the interpreted results by means of spatial analysis approaches such as such as overlay and buffer; and (d) application of both qualitative and quantitative approaches in geophysical data processing and interpretation. Processing bouguer gravity and areomagnetic data with quantitative methods such as the localized and regional anomalies separation, boundary information enhancement, and quantitative or semi-quantitative inversion could provide many important aided information for interpretation of them in covered area, e.g. it could identify and extract the morphological characteristics of some structures or geobodies much more easily from the geophysical data being processed by derivative or/and euler deconvolution methods. However, in practically, the significance for qualitative interpretation could never be ignored because the complicated geological phenomena could not easy to be totally described by several mathematical models or their combination so as the results of statistical analysis for geophysical data usually have no full geological significance and qualitative interpretation guided by the geological regularities have to be done for identifying the possible and more complicated geological information;(4) The geochemical landscape of Eastern Tianshan in which the study area is located is inner continental arid desert. Physical and chemical weathering in these areas is well developed and resulted in decomposition of exposed rocks (bedrock). The wind is so strong that weathered bedrock could easily be blown far away to accumulate in relatively low terrain (basins). Some areas are completely covered by regolith sediments such as windblown sand or soil, alluvial gravel, sandstone and calichehorizons. On the one hand, the geochemical anomaly formed by vertical migration of elements from deep would be weaked due to the Gobi desert coverage; On the other hand, the lateral migration of geochemical elements has commonly existed so as the geochemical composition of different geological units usually have been changed. There is a strong genetic relationship of elements enrichment between Gobi desert covered layer and bedrock:the mobile geochemical elements enriched in bedrock would like to migrate to be accumulated in the covered layer to form pseudo geochemical anomaly in the process of weathering and wind transportation. The pseudo anomaly that is usually considered to be strong noisy signal in geochemical anomaly identification would overprint on the possible mineralization associated geochemical anomaly that the weak anomaly produced by vertical migration of elements from various of concealed geobodies including mineral deposits covered by Gobi desert. Therefore, in Gobi desert covered geochemical landscape, the application that both data processing and geological interpretation of regional geochemical data would be a challenge task in both extraction of the weak and decomposition of the overprinted geochemical anomaly duo to the Gobi desert coverage. In this paper, to reduce the risk for application of regional geochemical data in the Gobi desert covered area as much as possible, the identification and delineation of "Tuwu-type" porphyry Cu-Mo mineralization associated geochemical anomalies has been taken as an example to explore and summarize a general procedure for regional geochemical data processing in the Gobi desert covered landscape of the Eastern Tianshan.(5) The geochemical composition in Gobi desert covered layer probably depends on the composition of bedrock. There is a strong linear relastionship between the elements accoumulation coefficient of Gobi desert covered layer and bedrock outcrops area, which possiblely indicate a genertic relationship in geochemical composition between them. The39geochemical elements (oxides) can be divided into three groups based on their relative enrichment in the bedrock and the Gobi desert coverd layer:(a) the elements that are more enriched in the Gobi desert covered layer, e.g. Au, As, W, Cd, Ag, Hg, Sb, Bi, K, and Ca etc. There are two possible sources for this group of elements. The first one is vertical migration from buried mineralization or bedrock covered by Gobi dersrt covered layer and the second one is lateral migration from bedrock by wind and streams. In the second situation, these elements may have been originally enriched in some lithological units, but their mobility (away from host rocks) was so strong that they readily fractionalized from their host rocks in bedrock because of their unstable geochemical properties, then moved to and accumulated in Gobi desert covered layer. This group of elements were likely to form high concentration geochemical anomalies (secondary enrichment) but without mineralization associated significance in Gobi desert covered layer;(b) the elements that are generally equally enriched in both Gobi desert covered layer and bedrock outcrops area, e.g. Zr, Y, Nb, Ba, B, Si, Sr and Li, etc. These elements can be considered to be stable in geochemical behavior in that they were not limited to be formation by fractionalization from their host rock in bedrock but mainly moved to and accumulated in Gobi desert covered layer with fragments of host rocks by physical processes such as wind or stream transformation; and (c) the elements that are much more enriched in bedrock outcrops area, e.g. Mg, Co, Cr, Ni, Cu, Be, Mn, Fe, Pb and Zn, etc. These elements may have been originally enriched in some lithological environments and their immobility (retention in host rocks) was so strong that, during geochemical fractionation between mobile (e.g. fragments of host rocks that would be moved to and accumulated in Gobi desert covered layer) and immobile (remains of host rocks) status, they would probably remain in the host rocks of bedrock rather than become enriched in fragments of weathered rocks accumulated in Gobi desert covered layer. Thus, the geochemical anomalies formed by this group of elements usually are of possible mineralization associated significance in the study area. There is a source genetic relationship between the geochemical composition of Gobi desert covered layer and bedroak outcrops area due to the strong lateral migriation of the mobile elements from bedrock to be accumulated in Gobi desert covered layer, which coud be generally sumerized to be that the elements such as Au, As, Sb, and Cd could be the most probably come from igneous-volcanic associated sedimentary rocks, the elements such as K, Bi, and Ba could be the most probably come from intermediate-acid intrusions, and Ca could be the most probably come from sedimentary-metamorphic rocks.(6) On the basis of the accumulation coefficient analysis of elements in various of geological units in the studied area, there is a gernal conclusion for the enrichment regular of elements in these geological units could be drowend:(a) the elements such as Cu, Au, As, Cr, Co, Ni, Sb, Cd, Fe, Ti, V, Zn, Mg, Mo, and Ag are more relatively enrichment in igneous-volcanic associated sedimentary rocks, which are usually considered to be generiticaly associated with Fe, Cu, Ni, Au, and Ag polymetallic mineralization in the Eastern Tianshan;(b) the elements such as Al, K, Na, Si, Be, Bi, and Ba are relatively enrichment in intemadiate-acid intrusions, which are generally considered to be closely related with rare elemets metal mineralization including Li and Be; and (c) the elements such as Ca and Mg are more enrichment in sedimentary-metamorphic strata that are mainly composed by carbonate rocks. Though the candidate geochemical indicator elements for "Tuwu-type" porphyry Cu-Mo mineralization could be Cu, Mo, Au, Ag, As, Sb, Pb, Zn, W, Bi and Cd, in which Au, Ag, As, Sb, Bi and W are strongly mobile elements that can be easily moved over large distances, accumulated as secondary enrichment in Gobi desert covered layer to form non-mineralization associated geochemical anomalies, which are usually considered to be pseudo geochemical anomalies providing relatively strong noise signals in mineralization-associated geochemical anomaly identification. Therefore, Au, Ag, As, Sb, Bi and W probably should exclude from the candidate indicator elements and only the remain elements that Cu, Mo, Pb and Zn would be the most appropriate indicator elements for porphyry Cu-Mo mineralization associated geochemical identification in the study area.(7) In Gobi desert covered landscape of the Eastern Tianshan, interpretation of linear and circler structures using remote sensing images in bedrock outcrops area is relatively easy, but it is very difficult for Gobi desert covered area. The most important issue for the extraction of ateration information using remote sensing data is to remove interferences such as saline-alkali soil, vegetation, shadow and clouds. On the basis of the preprocessing of remote sensing data including the references removed, feature-oriented principal component analysis method (Crosta approach) method has proved to be a useful approach for the extraction of iron and hydroxyl alteration anomalies.(8) The integrated prediction criteria for "Tuwu-type" porphyry Cu-Mo deposits could be sumerized as follows:(a) tectonic units:Jueluotage late Paleozoic rift and Ha’erlike-Dananhu Paleozoic island arc;(b) favorable strata:Carboniferous tholeiite-andesite and intermediate-acid volcanic breccia-basalts;(c) intrusions:Carboniferous-Permian diorites, quartz diorites, granodiorites and plagioclase granite porphyries, etc.;(d) bouguer gravity and magnetic anomalies:the reginal anomaly is that the high or/and gradient zone of bouguer gravity and magnetic at the extension of20-30km and the local anomaly is that the medium-high loacalized bouguer gravity and magnetic anomalies;(f) geochemical anomalies:the geochemical anomalies of Cu, Mo, Pb and Zn; and (g) alteration for remote sensing information:iron and hydroxy anomalies;(9) On the basis of postprobality map for "Tuwu-type" porphyry Cu-Mo mineralization in the studied area, there are four high potential prospective areas that A1, A2, A3and A4and two moderate potential prospective areas that B1and B2have been delineated. The A1, A2, A3, and A4prospects are located in the favorable tectonic environments that Jueluotage late Paleozoic rift and Ha’erlike-Dananhu Paleozoic island-arc. They have faverable geological conditions, geophysical, geochemical and remote sensing anomalies. A1, A2, and A3prospects constitute a EW trend porphyry Cu-Mo mineralization zone, in which the Tuwu-Yandong superlarge porphyry Cu-Mo deposit have been found in the middle of the mineralization zone that the prospect A1, Sanchakou, Eastern Sanchakou and other small size porphyry Cu-Mo deposits have been discovered in the eastern of the of the mineralization zone that the prospect A2and there is no discovered in the western of of the mineralization zone that the A3prospect. Therefore, maybe the prospecting potential in the prospect A3would be large. Both the prospects B1and B2are located in north of the porphyry Cu-Mo mineralization zone. There has been found eastern Tudun small size Cu depost and East Gobi large Mo deposit. Thus, it also probably has high potential for prospecting porphyry Cu-Mo deposites.