Polymetallic Nodules Resource Estimation in the Suoi Thau-Sang Than Area, Northeastern Vietnam

1,720 chemical, mineral, and petrography samples in the Suoi Thau-Sang Than area, northeastern Vietnam were collected to investigate polymetallic nodules resource. The similarity-analogy in ore geology, direct calculation method for metallization parameters, and mineral resource estimation based on cutoff grade change methods are employed to estimate polymetallic nodules resource from the Suoi Thau, Sang Than, and Ban Kep areas in northeastern Vietnam. The similarity-analogy in ore geology indicates 1,785,000 tons Cu, 12,005,000 tons Pb+Zn, and 195 kg Au in total. The direct calculation method for metallization parameters shows 2,060,000 tons Cu, 13,648,000 tons Pb+Zn, and 224 kg Au in total. These methods display that the Sang Than area contains mainly polymetallic nodules in the studied area. Following the mineral resource estimation based on cutoff grade changes, estimation results indicate potential Pb-Zn nodules resource in the Ban Kep and Cu nodules resource in the Sang Than, while no potential Au nodules resource in the Suoi Thau and the Sang Than as well as the Ban Kep. The estimation results obtained from mineral resource estimation based on cutoff grade changes are suitable with the results from the traditional geometric block methods. Our study suggests that the Sang Than area can be considered as a potential Cu nodules resource, and the Ban Kep area is potential Pb-Zn nodules resource in northeastern Vietnam for future exploitation, while there is no potential polymetallic nodules resource in the Suoi Thau area. Furthermore, the one resource estimation based on cutoff grade changes method displays an overview of the prospect on polymetallic nodules resources, indicating that this method can serve as a basis for the proposed selection of the prospective areas for effective Cu, Pb-Zn, and Au mineral exploration in the Suoi Thau-Sang Than area, northeastern Vietnam.


Introduction
Polymetallic nodules play an important role in providing valuable metals for industry because of zinc, lead, copper, and gold can be highly found together in the nodules (Rankin, 2011;USGS, 2014;Graedel et al., 2015). Zinc is primarily utilized to make galvanized steel as well as the production of brass, bronze, chemicals, and zinc alloys for the construction and industry. Lead is widely used for the production of lead-acid batteries, cable sheaths, machinery, machinery manufacturing, shipbuilding, light industry, lead oxide, radiation protection, and other industries. Copper contains excellent properties, making it can be used predominantly in electrical power, electronics, energy, petrochemicals, transportation, machinery, metallurgy, light, and some high-tech fields. Gold is considered the most valuable of the pure metals, and it is chiefly utilized for coinage, ornaments, jewellery, and gilding.
Mineral resource estimation is a practical significance in the mineral investigation ( Kužvart & Böhmer, 1986). This provides a scientific and practical basis for establishing geological survey programs as well as evaluating the prospects of minerals and investment. For decades, scientists have focused on researching and proposing many methods of mineral resource estimation in processes of geologic mapping and surveying minerals. Generally, mineral resource estimations are mainly conducted according to the results of geological mapping and mineral prospecting at 1:200,000 -1:50,000 scale. Currently, many geological and math-geological methods have proposed to estimate mineral resources. Among them, the similarity-analogy in ore geology, direct calculation method for metallization parameters, and mineral resource estimation based on cutoff grade change methods have been widely used to investigate mineral resources (Lasky (1950;Byryukov, 1962;Kogan, 1971, Snowden, 2001Hung et al., 2016;Zhang & Huang, 2010;Revuelta, 2018). Different estimation methods have been used for different scenarios depending upon the ore boundaries, geological geometry, grade variability, the amount of time, and money available. To select the suitable methods for the research object, the stage of geological investigation, the completeness of the document, the geological structure characteristics of the object and mineral type should be investigated carefully. The northeast Vietnam is assessed to be high polymetallic nodule resources (Tri & Khuc, 2011). However, the mineral resource estimation methods have been used based on qualitative description and calculation, which used rigid formulas and parameters, suggesting not suitable for the variability of the mineral market. To have a more comprehensive and flexible view of mineral resource estimation, a model for evaluating polymetallic resources based on many different boundary content options should be studied in this area. Therefore, northeast Vietnam is Submission date: 06-03-2020 | Review date: 22-09-2020 an ideal area to apply this asymptotic approach for estimating polymetallic nodules resources.
In this study, similarity -analogy in ore geology, direct calculation method for metallization parameters, and mineral resource estimation based on cutoff grade changes methods are used to investigate polymetallic nodules resource from Suoi Thau, Sang Than, and Ban Kep areas in northeastern Vietnam.

Geological setting Geology
The Suoi Thau-Sang Than area is located in Ha Giang province, northeastern Vietnam (Fig. 1). The area has mainly consisted of Devonian sedimentary rocks (i.e., clay shale, carbonate rocks, and marly sandstone), and minor Triassic gabbroid rocks and Paleozoic granitoid rocks (Minh et al., 1992; Son et al., 2003; Fig. 1A). Structurally, the studied area is Northwest part of a synclinorium complex extending from Northwest to Southeast. The Tung Ba structural block is controlled by the Duong Thuong-Du Gia overthrust/reverse fault in the north, and the Ban Coc-Minh Ngoc reverse dip-slip fault in the south (Dovijkov et al., 1965). These two faults and the faults located between them in the same direction play a key role in controlling the activities of magmatic intrusive rocks and minerals in the region (Son et al., 2003). Besides, other fault systems occurred to are developed to complicate the regional structure.

Morphological and structural characteristics of polymetallic mineralization zones
In the studied area, the Suoi Thau, Sang Than, and Ban Kep areas are investigated to be indicated containing polymetallic and gold mineralization (Bat et al., 1989;Sinh et al., 1985;Minh et al., 1992). Generally, the mineralized zones are developed along the northwest-southeast with a length of 380-3,800 m and extend beyond the study area. The mineralization zones are laid down in the distribution area of the Devonian sedimentary rocks. The concentrations of Cu, Pb, and Zn are unevenly distributed in the mineralized zone, but it is assessed to be industrial ore bodies (Son et al., 2003;Sang, 2011).
In the Suoi Thau area, there are three mineralized zones. Mineralization zone 1 has developed along the north-west-southeast direction of about 720 m, plugged to the northeast with a slope of 10-30°, 90-115 m thick (average 95 m). The zone is located at the boundary between the marble and the quartz-biotite. Analytical sample results showed 0.31-1.06% Cu (average 0.47%), 0.63-1.31% Pb (average 1.01%), 1.32-3.85% Zn (average 2.59%). Calculation results of smelting displayed 0.10-2.10 g/t Au (average 0.53 g/t). The changes related to ore creation are skarnification and quartzization. Mineralization zone 2 extends along the north-south about 380m, width 40-80 m (average 50 m), and it plugs in the northwest with a slope of 35°. The mineralized rock is mainly marbles. This zone contains 0.16-0.57% Cu (average 0.42%), 0.40-2.04% Pb (average 1.19%), and 0.13-3.86% Zn (average 2.82%). Calculation results of smelting indicated 0.10-1.80 g/t Au (average 0.68 g/t). The formation of this mineralization zone can be related to the skarnification and quartzization.
The mineralization zone 3 is suffered highly brittle deformation that impacted by post-ore fault activities. The zone appears mainly marbles, which were significantly broken and fractured. The mineral zone is about 1,800 m length, 230-400 m width (average 320 m), and it dips to northeast 30o. Compositional analyses of sample show 0.20-0.73% Cu (average 0.49%), 0.26-1.56% Pb (average 1.03%), and 1.08-5.56% Zn (average 2.85%). Calculation results of smelting display 0.10-1.30 g/t (average 0.52 g/t). The formation of this mineralization zone can be controlled by the skarnization, dolomitization, and quartzization.
The Sang Than area, the mineralization zone is located in the broken zone by impacting of faults. The mineralized rocks are mainly marbles that were highly crushed and fractured. This zone shows 3,900 m length, 700-900 m widths (average 380 m), and it plugs in the southwest with a slope angle of 40o. Sample analysis results indicate 0.02-3.10% Cu (average 0.59%), 0.19-2.47% Pb (average 0.96%), and 1.01-6.47% Zn (average 2.73). Calculation results of smelting show 0.10-1.50 g/t Au (average 0.63 g/t). The skarnization, dolomitization, quartzization, and chlorination can be considered forming this mineralization zone.
In the Ban Kep area, two mineralized zones have discovered. The mineralization zone 1, the mineralized rock consists mainly of fractured marbles. This zone is 1,300 m length, The formation of this mineralization zone can be strongly related to the skarnization, dolomitization, and quartzization.

Materials and methodologies
This study was carried out based on the results of geological surveying and mapping the Geological and mineral resources map of Vietnam at 1:50,000 scale of the Phuc Ha map sheet group (Son et al., 2003). 518 samples in the Suoi Thau, 503 samples in the Ban Kep, and 699 samples in the Sang Than were collected by the Intergeo Division for analyzing chemical, mineral, and petrography at the Center for Experimental Geological Analysis in Vietnam Institute of Geosciences and Mineral resources and the Intergeo Division. The mineralization zones have been investigated by surface works such as openings and ditches. Based on geological characteristics and level research in the study area, the similarity-analogy in ore geology, direct calculation method for metallization parameters, and mineral resource estimation based on cutoff grade change methods are chose to study the mineralized zone of category 334 resources in the Suoi Thau-Sang Than area.

Similarity-analogy in ore geology:
Similar-analogy in ore geology was developed based on similar geological environments in different areas, which are similar mineralization characterization (Byryukov, 1962). This method is created by using a similar model for other ore deposits, which are similar geological characterization. According to the method, resource estimations are calculated as the formula (Byryukov, 1962).
where Q tn -resource estimation, K-a similar factor among estimated areas regarding the standard area, q c -ore resources in an area unit, volume or length of the standard area, V n '-volume, area, or existed depth of mineralized zone. Metal resources in the studied area are determined by the formula. P tn =Q tn .C (2) where Q tn -ore resources of a mineralized zone (103 ton); C -average contents in the mineralized zone (%). The similarity factor (K ij ) between areas is determined by the comparative method of ore-forming criteria, which are evaluated by using the information on mining geological characteristics of surrounding rocks as well as the composition of each area. The information is digitized and calculated following the formula.
where i, j-comparative objects, a ip , a jp -value of i or j object at p factor (p=1,m ).

Direct calculation method for metallization parameters:
This method is applied to estimate polymetallic nodule resources in the study area. Polymetallic nodule resource area is assessed by ore parameters or ore-bearing coefficients and calculated according to the following formula (Kogan, 1971).
where Q tn -total potentials of a mineralized zone (103 ton), V-a volume of a mineralized zone (103 m3), d-bulk density of mineralized zone (ton/m3), C-average contents in a mineralized zone (%), S sq -area distribution of mineralized zone (m3), M sq -thickness of a mineralized zone (m), K q -mineralization-bearing coefficient.

(4 a)
where ΣM q -the total thickness of ore veins in the mineralized zone.

Ore resource estimation based on cutoff grade changes:
A change in cutoff content (C b ) will lead to a change in ore resource-reserves. In 1950, this method was proposed by Lasky to investigate the change of resources. The method is constructed by determining the relationship between ore resource and cutoff grade, and between ore resource and average grade content. (5) where C b -cutoff grade of metal ore (%), C-average grade of metal ore (%), αβ-factors determined by standard construction measures or calculating resource according to several options on cutoff grade.
However, the law of Lasky (1950) only ensures reliability when metal content is distributed according to the standard log distribution. However, the metal content is distributed according to the gamma function or three logarithmic pa-rameters; the reliability of this method is reduced. Therefore, Margolin (1974) improved the Lasky method and expressed the relationship between ore resource and contents following formula.
The researcher often uses incomplete gamma functions.
The equation for resource and reserve estimation based on cutoff changes is expressed as a function.
where Q(C >C b )-ore resource/reserves, which has an average grade above the cutoff (ton), Q 0 -ore resource/reserves of estimated areas (ton).

Similarity-analogy in ore geology
The combination of information used to evaluate the zones of the Suoi Thau-Sang Than area includes 29 information. The information is classified into four groups and standardized according to the corresponding quantitative principles: Very common-3, common-2, less common-1 and absent-0. Based on the standardized results of research information, a similar coefficient was created in the studied area (Tab. 1). The Kij similar coefficient between subdivisions is very high, indicating a similarly combined characterization Tab. 8. Results of Au resource estimations based on cutoff grade changes Tab. 8. Wyniki szacowania zasobów Au na podstawie zmian zawartości granicznej of indicator elements, mineral composition, geological representation, and variation of surrounding rocks. These suggest that the Suoi Thau, Sang Than, and Ban Kep areas are similar geological conditions and factors controlling mineralization. Therefore, polymetallic nodules resource estimation can be trustworthily guaranteed by using similarity -analogy in ore geology method.
The total estimation results are 1,785,000 tons Cu, 12,005,000 tons Pb+Zn, and 195 kg Au and the Sang Than area highly contains potential polymetallic nodules resource ( Table 2). Cu nodules resource is highly in the Sang Than area

Direct calculation method for metallization parameters
Calculation results show a total of 2,060,000 Cu, 13,648,000 Pb-Zn, 224 kg Au in this studied area, and the Sang Than area consists mainly of polymetallic nodules resource. Cu is mainly in the Sang Than area (1,708,000 tons), minor in the Suoi Thau (177,000 tons), and the Ban Kep (173,000 tons). Pb-Zn is highly in the Sang Than (10,542,000 tons), less in the Ban Kẹp (1,676,000 tons), and the Suoi Thau (1,429 tons). Au shows principally in the Sang Than (182 kg), minor in the Suoi Thau (22 kg), and the Ban Kep (20 kg). According to the similarity-analogy in ore geology and the direct calculation method for metallization parameters methods, estimation results show 1,785,000-2,060,000 tons Cu, 12,005,00-13,648,000 tons Pb-Zn, and 195-224 kg Au.
To evaluate the reliability of these resource estimations above, this study uses one formula.
where P 1 -an estimation result of similarity-analogy in ore geology method, P 2 -an estimation result of the direct calculation method for the metallization parameters method.
The calculation results display 3,2% in the Suoi Thau, 9,6% in the Ban Kep, and 20,2% in the Sang Than, suggesting high reliability of polymetallic resource estimation in the Suoi Thau-Sang Than area.

Ore resource estimation based on cutoff grade changes
Based on polymetallic module resource estimations in the Suoi Thau-Sang Than area, and assessing the reliability of the resource estimations which are mentioned above, we chose results of the direct calculation method for metallization parameters as basis of polymetallic resource potential based on cutoff grade changes.
Based on selected boundary content (C b ), the estimation results show 300 tons in the Suoi Thau, 33,000 tons in the Sang Than, 100 tons in the Ban Kẹp with C b > 1,0%; 6,000 tons in the Suoi Thau, 60,000 tons in the Sang Than, 4,000 tons in the Ban Kep with C b > 0.5%; 14,000 tons in the Suoi Thau, 80,000 tons in the Sang Than, 12,000 tons in the Ban Kep with C b > 0.3%; 18,000 tons in the Suoi Thau, in 113,000 tons in the Sang Than, 17,000 tons in the Ban Kep with C b > 0.1%. This indicates that decreased boundary contents will increase resource estimation results. The total resource estimation will be increased 221.7% with reduced C b from 1% to 0.5%, 154% with reduced C b from 0.5% to 0.3%, 142.2% with decreased C b from 0.3% to 0.1%. Based on Vietnamese industrial contents of Cu (C b =0.5), the Cu resource is mainly concentrated in the Sang Than area.
According to selected boundary content (C b ), the estimation results show 10 tons in the Suoi Thau, 100 tons in the Sang Than, 27,000 tons in the Ban Kẹp with C b > 10%; 8,000 tons in the Suoi Thau, 6,000 tons in the Sang Than, 120,000 tons in the Ban Kep with Cb> 5%; 52,000 tons in the Suoi Thau, 33,000 tons in the Sang Than, 157,000 tons in the Ban Kep with C b > 3%. The total resource estimation results will be increased by 500.9% with reduced C b from 10% to 5% and 182.2% with reduced C b from 5% to 3%. Following the Vietnamese industrial contents of Pb-Zn (C b =3%), the Ban Kep area can be a high potential Pb-Zn resource in the studied area.
Following selected boundary Au content (C b ), the resource estimation results indicate 20.08 kg in the Suoi Thau, 1.00 kg in the Sang Than, 15.50 kg in the Ban Kep with C b > 1.  Fig. 2. Diagrams describe the relationship between ore resources and cutoff grade contents of Cu (A), Pb-Zn (B), and Au (C) in Suoi Thau-Sang Than area, respectively Rys. 2. Diagramy opisujące zależność między zasobami rudy a zawartościami granicznymi Cu (A), Pb-Zn (B) i Au (C) odpowiednio na obszarze Suoi Thau-Sang Than from 1.5 g/T to 1.0 g/T, 131.2% with reduced C b from 1 g/T to 0.5 g/T. According to the Vietnamese industrial contents of Au (Cb=1 g/T), the Suoi Than, Sang Than, and Ban Kep areas can be no potential Au resource in the studied area.
Generally, the combination of mineral resource estimation methods has contributed to clear up polymetallic mineralization investigation in the studied area. However, compared with the direct calculation method for metallization parameters and the similar-analogy in ore geology, the ore resource estimation based on cutoff grade changes method has shown an overview of the prospect on polymetallic nodules resources and gold mineralization in the Suoi Thau-Sang Than area according to cutoff grade changes. These estimation results can be used to serve as a basis for the proposed selection of the prospective areas for effective Cu, Pb-Zn, and Au mineral exploration.

Conclusions
The similarity-analogy in ore geology, direct calculation method for metallization parameters, and mineral resource estimation based on cutoff grade change methods were used to investigate polymetallic nodules from the Suoi Thau, Sang Than, and Ban Kep areas in northeastern Vietnam. Through this study, the conclusions can be drawn as follows.
The similarity-analogy in ore geology method estimates 1,785,000 tons Cu, 12,005,000 tons Pb+Zn, and 195 kg Au in this studied area, and the Sang Than area contains mainly polymetallic nodules 1,421,000 tons Cu, 8,768,000 tons Pb+Zn, and 151 kg Au. The direct calculation meth-od for metallization parameters shows 2,060,000 tons Cu, 13,648,000 tons Pb+Zn, and 224 kg Au, and the Sang Than area is considered as a main potential polymetallic nodule in this studied area, consisting of 1,708,000 tons Cu, 10,542,000 tons Pb-Zn, and 182 kg Au. The cutoff grade changes display potential Pb-Zn nodules resource in the Ban Kep area and Cu nodules resource in the Sang Than area, while no potential Au nodules resource in the Suoi Thau and the Sang Than as well as the Ban Kep areas.
The different estimation results between the similarity-analogy in ore geology and the direct calculation method for metallization parameters methods are from 3.2 to 20.2%, suggesting high reliability of polymetallic resource estimation in the Suoi Thau-Sang Than area. Additionally, the relationship between polymetallic nodules resources and cutoff grade contents show nonlinear relation and inverse correlation. This indicates the polymetallic nodule resources in this studied area can be estimated by using each cutoff grade contents following the variability of the Vietnamese industrial contents.

Acknowledgements:
First of all, we would like to send our deepest thanks to the leaders and staff of HUMG for providing us with good facilities in research and unconditional help to carry out the study. Furthermore, the first author would like to express his sincere gratitude to Prof Jan Golonka, a great supervisor, for giving him the knowledge, indulging in geoscience, and opportunity to carry out geological research.