C3^COMMONWEALTH OF AUSTRALIA

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1 C3^COMMONEALTH OF AUSTRALA DEPARTMENT OF NATONAL DEVELOPMENT BUREAU OF MNERAL RESOURCES, GEOLOGY AND GEOPHYSCS RECORD No KNGS PLANS GRAVTY SURVEY NEAR COOKTON, QUEENSLAND 1962 BUREAU OF MNERAL RESOURCES GEOFM..1CAL LBRARY Nei.^. c- cp-t u by HUSSN The information contained in this report has been obtained by the Department of National Development as part of the policy of the Commonwealth Government to assist in the exploration and development of mineral resources. t may not be published in any form or used in a company prospectus or statement without the permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.

2 132 RECORD No KNGS PLANS GRAVTY SURVEY NEAR COOKTON, QUEENSLAND 1962 BUREAU OF MNERAL MOURCES GEOPHYSCAL LBRARY Ref... by J.J. HUSSN - r, The information contained in this report has been obtained by the Department of National Development as part of the policy of the Commonwealth Government to assist in the exploration and development of mineral resources. t may not be published in any form or used in a company prospectus or statement without the permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.

3 CONTENTS Page SULDARY 1. NTRODUCTON 1 2. GEOLOGY 1 3. EQU D^AND METHOD 1 4. REDUCTON OF RESULTS 2 5. NTERPRETATON OF RESULTS 3 6. CONCLUSONS AND RECOMMENDATONS 5 7. REFERENCES 5 LLUSTRATONS Plate 1. Locality and geological map^(drawing No.D55B7-2) No Plate 2. Topography, traverses, and. regional gravity values Plate 3.^Surface and Bouguer gravity profiles; Traverses AA to G Plate 4. Surface and Bouguer gravity H to Q Plate 5. Residual gravity map Plate 6. Drilling cross-section and Traverse A profiles; Traverses gravity profile; (1) 55B7-3 ) ( D 5 5B7-4 ) (D55B7-5) (D55B7-1) (D55B7-8) Plate 7. Drilling cross-section and Traverse N gravity profile; (D55B7-9) SUMMARY A gravity survey to deiermine the course of a possible tin-bearing deep lead in the Kings Plains Prospect, North Queensland was made in Fifteen gravity traverses were surveyed across the prospect and the Bouguer-anomaly profiles when corrected for regional gradient gave information on the bedrock configuration beneath the alluvium. The results suggest a continuous channel which crosses the area in a westerly direction and is considered to be the most. favourable location for a deep lead. The depth to the surface of the weathered bedrock on which tin-bearing wash may lie appears to range roughly between 12 and 18 ft

4 1. NTRODUCTON The Geological Branch of the Bureau of Mineral Resources, Geology and Geophysics requested a gravity survey to determine the course of a possible tin-bearing deep lead in the Kings plains Prospect, North Queensland. This prospect is about 2 miles south-west of Cooktown in a west-trending valley about 16 miles long and up to four miles wide (see Plate 1). A geophysical party of the Bureau of Mineral Resources arrived in Cooktown on 13th May 1962, and completed the survey on 12th July The party consisted of geophysicists J.J. Hussin (party leader) and J.P. illiams, and.one field assistant. ^One surveyor and two chainmen from the Department of the nterior, Brisbane, did the necessary pegging and levelling of the traverses and provided the topographical map of the grid. Fifteen gravity traverses were surveyed at approximately right angles to the supposed course of the lead. These traverses consisted of 967 gravity stations and had a total length of 93, ft (17i miles). 2. GEOLOGY The geology of the Kings Plains Prospect is described by Best (1962). The Kings Plains valley has been eroded in greywacke, shale, and chert of the Hodgkinson Formation (Upper Devonian - Lower Carboniferous); these sediments strike north, dip steeply, and form prominent hills on both sides.. Strike ridges of chert dominate the hills and in places project into the valley, and divide it into several partly closed basins. The floor of the present valley is flat and is swampy in places. From regional geological mapping it app3ared that the Annan River formerly drained westward through Kings Plains into the Normanby River and that the Annan River gorge is an area of stream capture. Alluvial tin has been recovered from the headwaters of the Annan and its tributaries in the Annan River tinfield lying east and south-east of Kings Plains. t was considered that the former westward drainage could have resulted in the deposition of tin in the Kings Plains area. Above the gorge the Annan River valley is alluviated and has been test-bored for alluvial tin. However, the boring did not reveal alluvial tin of economic grade. 3. EQUPMENT AND METHOD this survey. A orld ide gravity meter, serial No. 35, was used in Gravity readings were made by using the leap-frog method. At least one station on every traverse was tied to the base station A13. ith 1-ft station intervals, an average of 21 stations could be obser. in an hour. The normal gravity loop consisted of 18 new stations. loor example, assume that Station on Traverse H was tied to A13 then the stations would have been observed in the following order for the first loop: HO, 1, 2, 3,... 17, 18, 12, 6,.^Stations, 6, and 12 were occupied twice. Then for the second loop the following stations were observed:, 6, 12, 18, 19,... 35, 36, 3, 24, 18, i.e. after Station, Stations 6, 12, and 18 were reoccupied, Stations 19 through to 36 were observed, and Stations 3, 24, and 18 reoccupied. ^The third, fourth, and fifth loops were similarly tied to the second, third,and fourth loop ^'- respectively.

5 2. By employing the leap-frog method it was possible to observe a point on the drift curve every 2 minutes. ith this method of observation, the standard deviation of repeated observations on the same station was +.25 mgal. The relative error of station elevations rarely exceeded one-tenth of a foot. The assumed elevation of Station A was ft and this was taken as datum for the survey. This datum seems to be in error by about +25 ft, according to the Queensland Lands Department topographic maps of the 'Cooktown-Daintree area' (serial number 292, sheets 5 and 6). 4. REDUCTON OF RESULTS The gravity readings were converted to relative observed gravity values and the latitude corrections and elevation corrections were applied. The resulting Bouguer-anomaly profiles are shown in Plates 3, 4, 6, and,7.^the elevation corrections were based on a density of 2.2 gcv ). t was necessary to apply terrain corrections to the gravity data. The terrain corrections were divided roughly into three groups; (a) a regional effect due to the surrounding hills at distances greater than 1 ft from the gravity station, (b) a semi-regional effect of the hill-sides less than 1 ft from the gravity station, and (c) a local effect of the creek galleys less than 2 ft from the gravity station. The regional terrain-correction values were obtained using Hammer's zone chart (Hammer, 1939) from Zone F to Zone J, corresponding to distances of 128 ft to 21,826 ft from the affected gravity station.^beyond 21,826 ft, the terrain-correction values were regarded as negligible. The average elevation values were obtained. using the Cooktown-Daintree area topographic map. The semi-regional terrain corrections were obtained by combining the elevation data from the gravity traverses and the topographic map. The local terrain effects were computed usingagavity integrator and the elevation data from the traverses. The regional terrain effects on the different traverses were roughly as follows: Traverse^ Regional terrain effect kmgai) A, B, F^ The increase of the terrain correction towards Traverses K and L was due mainly to the near ridges of Tableland bluff and Mount Amos which rise about 2 ft above the Kings Plains valley.

6 3. The semi-regional terrain effect could be calculated only very roughly in the Kings Plains area because of insufficient data about the elevations of the hills near the traverses. This effect ranged from.7 mgal at Station ED to.8 mgal at Station LO, and diminished rapidly as the distance from the hills increased. The local terrain effects caused by the creek gulley slopes were rarely as great as.25 mgal and were negligible at 2 ft from the slopes. A regional gravity map was produced on the basis of the Bouguer-anomaly profiles. Most of the gravity traverses were extended to bedrock outcrops at both ends and by using the values near the end of the traverse it was possible to plot a rough regional gradient for each gravity profile. The regional values were then placed on a map and the values smoothed to give a relatively simple regional effect as shown in Plate 2. The regional gravity values were deducted from the Bougueranomaly gravity values and the results plotted in the form of the residual gravity map shown in Plate NTERPRETATON OF RESULTS hen latitude, elevation, and terrain corrections have been made to the readings, anomalies in the profiles may arise from the following causes: (a) large-scale differences in density between major geological formations, (b) small-scale differences in density in the solid bedrock. These could be associated with minor geological features such as fault or shear zones, or igneous dykes or sills, (c) non-uniform thickness of weathered bedrock and alluvial cover. Anomalies arise from this cause because the density of the superficial material is considerably less than that of solid bedrock, (d) variations in density within the layer overlying the solid bedrock. Anomalies due to (c) were those sought in the Kings Plains survey, and it was necessary to remove from the results anomalies due to the other three causes. Anomalies due to (d) are generally of a lower order than the others, except where the layer contains massive rock, such as unweathered basalt. n the Kings Plains area this is not the case; therefore anomalies of this type have not been considered. The separation of regional effects as described in the preceding section is intended to remove anomalies due to (a). Any process such as this involves a considerable arbitrary element, as there can be no direct evidence for the nature of the regional gravity field in the area of interest. The assumption made as to the regional gravity field may ^ often be strengthened considerably if it is possible to extend the survey beyond the area of interest in all directions. n the Kings Plains survey, extension into the very hilly country north and south of the area was practically impossible.

7 4. t is generally impossible to remove anomalies that are due to (b), except for those of such limited extent that they can be smoothed out with safety. f the results show no clear evidence of such changes in the density of the solid bedrock, it must be assumed that they are in fact absent. f it is apparent from the results that anomalies due to this cause make an important contribution to the total gravity field, it is impossible to interpret the results in the detail necessary. n the Kings Plains survey, the results do not suggest the presence of anomalies due to (b) and it is assumed that none are present. hen latitude, elevation, and terrain corrections have been applied, and the regional effect removed, the remaining gravity field may be taken to depend on the depth to unweathered bedrock at the various measuring points. n the present case, some uncertainty remains owing to the incomplete information regarding the regional gravity field. No direct information on the depth to unweathered bedrock is available. However, during the course of the survey, several percussion drill holes were put down on Traverses A and N. The holes were continued until recognisable weathered bedrock was reached, but did not go deep enough to provide information on the depth to solid bedrock. n an attempt to provide some guidance for further drilling, an empirical method of interpretation was used, which involved the further assumption that the relation between the depth to weathered bedrock and the depth to unweathered bedrock was approximately one of simple proportion. At each drill hole, it was assumed that the anomaly present was caused by an infinite horizontal layer of thickness equal to the depth to weathered bedrock. Using the standard formula for the anomaly due to such a layer (Lig T, where Lg is the anomaly in milligals, 67is the density contrast in gicm3, and T the thickness in thousands of feet), the appropriate value of was calculated.^the results are shown below: Traverse A Borehole Depth to bedrock Anomaly Density contrast (T) (ft) Cmgal) --"(irct113) MRK MRK MRK MRK Traverse N Borehole Depth to bedrock Arg2,14Z Density contrast (4) ft) (gal) (gcm3) MRK mrk mrk MEK MRK t must be emphasised that the calculated value of the density contrast has no significance with regard to actual densities. Since the gravity anomaly depends on the depth to unweathered bedrock and the drilling results give the depth to weathered bedrock, the calculated density contrast will be greater than the actual one. On the other hand, the crude assumption of an infinite horizontal slab considerably overestimates the effect of the actual layer causing the anomaly; this will trend to make the calculated density contrast too low. More accurate means of calculation are available, but as no

8 5. information on actual depths to solid bedrock is available, it was considered that there would be no advantage in using them. The value of.44 gicm3 for the density contrast is rather lower than has been calculated in other surveys with better control. The main point of interest is that the density contrasts calculated over the various boreholes are in close agreement. This is, at least, an argument in favour of the accuracy of the various assumptions made.^t is considered tentatively that the residual gravity contour map shown on Plate 5 may be taken as an approximation to a contour map of weathered bedrock, the values shown for the gravity contours being converted to depths according to the relation 1 mgal 18 ft.^t should be noted, however, that on Traverse A and N, for which calculations were possible, the results indicate a rather wide, flat valley. The assumption of an infinite horizontal slat will be more seriously in error if the valley is narrow and steep sided. Depth calculations aremost likely to be reliable on traverses where the valley appears to be wide. Large terrain corrections were required at the ends of Traverses G 2 P, Q, K t and L. These large corrections introduce a further uncertainty. t is considered that the positions of maximum anomaly along these traverses indicate the likely positions of the deepest ground, but no reliable depth calculations are possible. 6. CONCLUSONS AND RECOMMENDATONS The investigation of the Kings Plains Prospect has been based on the assumption that the deepest bedrock would be the most probable location for a tin-bearing deep lead. Subject to several assumptions, which at present there is no means of checking, the gravity results suggest the presence of a continuous channel crossing the survey area in a westerly direction, with two tributaries. The depth to the surface of the weathered bedrock, on which cassiterite-bearing wash may lie, appears to range between roughly 12 and 18 ft. n order to test the channel suggested by the gravity results, bores are recommended at the following positions: B8 1 C15, D11, E3, M45, G33, P16, K26, L REFEPENCES BEST, J.G. HAMMER, S. LUCAS, K.G. 1962^Alluvial tin prospects in North Queensland.^Bur. Min. Resour. Aust, Rec TFT:npU ^Terrain corrections for gravimeter stations.^geophysics, 4 (3), ^Geological sketch map of Kings area.^bur. Mdn, Resour. Aust. (unpub1.3-

9 ^ 7 PLATE OLD a12* REFERENCE TO AUSTRALA STANDARD MAP SERES' COOK TO N ^4 ^1 es,^..., NORMANY^, -\^;1 -,,. ^,,^., \.^,, ,-- ` ^' -,^,., 1 t^'^' ^,^, 1^ \^,^\ ; t^v^ ^1.,^ ^.. ^ ^u;, - ^, 1,,.,'S 4 \ \ ;?-4%.,...,.^, - _ ^\ \ '''^--^_law r oad_ \ X ),C -- :.:*.c,..." ^ 5 4.^*^4 \ c xl '^`)r,... _ \,,.. 47,.. Mjr A M,OS l ^4."`:::-7-^'-'d road^..7.1,.,\, HE LENVALE2-7: t,^1 \ )^ li^.7 ''"'^TABLELAND..\,...,(.).\".^,,,,,^...^...!.\.\. :.... 1^ V.1^--.% ^" 55 D-Ch \,... ( 5,4.4.^4^.1.^a.^4".-', ;::.. ; - ks ANNAN,, 4^4 --'^N. ^,'(.- \ '...-r- 5 \ +' st ;RVER ( +.SHPTONS FLAT 4^4^+ 5 ^ \4 4 'TN -FELD 44' 4 r- 16 ' "o KNGS PLANS, NEAR COOKTON, QLD LOCALTY AND GEOLOGCAL MAP MLES a LEGEND... ^Surveyed area Road Oa^Quaternary alluvium Cz^Cainozoic alluvium D-Ch Hodgkinson formation Finlayson Granite 1-7- :- Palaeozoic granite, undifferentiated Fault Geological boundary Geology after Lucas, (1962) GEOPHYSCAL BRANCH, BUREAU OF MNERAL RESOURCES, GEOLOGY AND GEOPHYSCS' D55B7-2 TO ACCOMPANY RECORD No

10 PLATE 2 LEGEND Ol o ' Traverse with end station numbers Main rood Track Elevation above 6 ft, mainly bedrock outcropping ---3 Regional gravit, values (mgal) Based on Cooktown-Daintree area mops, Sr 292 Sheet 5816 Qld Lands Dept, Topographical Branch, Survey Office, Brisbane X.() KNGS PLANS LAKE X '1; &J.-!... '" 157 fv 48 CJ ) ( " \ \ " ' TOPOGRAPHY, TRAVERSES, AND,. REGONAL GRAVTY VALUES a MLES

11 PLATE 3 C)...J < <.!) 1- :::::i...j :;: -5 < <.!)...J...J J..L_ 1 11 TRAVERSE C o TRAVERSE f ll. o ---.J t- ll "'''''''',,,,,,jll''toltulu'''''''.''''''''.',,, -1- L 1. _....1 o _ L 1 _..1 2 TRAVERSE E TRAVERSE G f.l ll. a l' _..._..L ' o f-._. -.L TRAVERSE F J _ 4 _..L olj-.'- ' t L ---2 J o L L 1 11 Surfoce profile Bouguer gravity profile Latitude and elevation corrections are applied Density for elevation correction 2-2gcm 3 SURFACE AND BOUGUER GRAVTY PROFLES 1 o FEET =4 (Correction 665 mgol ft) TRAVERSES A A, B, C,, E, F, AND G Vertical scales as shown GEOPHYSCAL BRANCH,BUREAU OF MNERAL RESOURCES,GEOLOGY AND GEOPHYSCS TO ACCOMPANY RECORD No

12 1 TRAVERSE H PLATE 4 w LL o -2, w LL TRAVERSE w u.. i ol TRAVERSE K.r? - -1' Vl... <t <!l :::::i Vl... <t S ' L_ TRAVERSE L L L lj... TRAVERSE M ,5 - ll ' Vl... <t (!) '5 Vl...J <t <!l :::::i... -2, -85-2' g o L 1.L J. _ Surface profile Bouguer gravity profile 1 TRAVERSE TRAVERSE P FEET =1 - u _ Vertical scales as shawn - SURFACE AND BOUGUER GRAVTY PROFLES TRAVERSES H,, K, L, M, P AND Q - Latitude and elevation correctia ns are applied Vl... <t S2, ,;; '" "" <. -55 L J _.._L DenSity far elevation correction cm 3 (Correction 665 mgal 1ft 1 GEOPHYSCAL BRANCH,BUREAU OF MNERAL RESOURCES,GEOLOGY AND GEOPHYSCS TO ACCOMPANY RECORD No

13 PLATE 5 c- ') C -1 C LEGEND Area above 6-ft contour (M.S.L) (Mainly bedrock) B M R boreholes V') :;?.. Trend of gravity lows (probable course of deep lead) z «...J a... V) 1.':1 Z '::C. Rood Gravity traverse Gravity contour (mgal) l'2 RESDUAL GRAVTY MAP o MLES Contour interval 2 milligal Yz Elevotian correction for density g cm 3 Regional grollity values are shown on Plate 2 6eophyscal Branch, Blll"eatJ of> Mineral Resources, (je%gyand (jeophljscs TO ACCOMPANY RECORD o 194-l4

14 LL. _ N8 9)8 PLATE 6 LU < V' -^ _o LU z 11- -ft _J - g^ -2 LU cc _o r)11111 tg, _ -2 )18 _ CU _ CU -Q (S1V9MN) AltPONV 83nenosi^ (runi.va Alinutie)..33d N NO1VA313 8 CU DRLLNG CROSS-SECTON AND GRAVTY PROFLE; TRAVERSE A GEOPHYSCAL BRANCH, BUREAU OF MNERAL RESOURSES,GEOLOGY ANB.GEOPHYSCS D TO ACCOMPANY RECORD No

15 PLATE " en...j ct!...j -2,5 >-...J ct -3, ct :: l&j :l (!) :l BOUGUER ANOMALY (based on 2 2gom 3 density) -3' , " - :: :l... ct a >-... :; - ct ::... l&j l&j LL -1 z l&j...j l&j CALCULATED BEDROCK PROFLE -2 o 1, FEET o 1 (Vertical scales as shown) 2 ===t DRLLNG CROSS-SECTON AND GRAVTY PROFLE; TRAVERSE N GEOPHYSCAL BRANCH BUREAU OF MNERAL RESOURCES,GEOLOGY AND GEOPHYSCS 55 B 7-9 TO ACCOMPAN:w' RECORD No