Post by
GeneralGogol on Apr 22, 2021 9:18am
North Zone / West Side 3D-IP Analysis (update) - Part 5
Oroco included some additional IP survey cross sections on their website earlier this week under corporate presentation materials as Appendix 2. I spent the past few days reviewing these different cross sections of data. This is informative as to the extent of mineralization in various areas of the Santo Tomas concession and it helps us to better estimate some distances.
In my February 15 post, I made a number of conservative assumptions in estimating the potential additional ore volume contribution from west of the historical 2009 Grade Shell. With the additional cross sections of IP survey data, it becomes apparent that the volume of highly-charged mineralization west of the 2009 Grade Shell is more significant than I had previously estimated by a factor of at least 3.75 times. Based on this, the potential ore contribution estimate from the west side area would go from 121.5 Mt to 455.6 Mt.
Page 95 of the Santo Tomas technical report (“ST report”) shows a plan view of the North Zone historical drilling locations and the Western Fault. There are some seven historical drilling locations west of the fault (STD-06, STD-07, STD-25, STD-26, STD-29, STD-30, and STD-40), most of which are outside of the 2009 Grade Shell. STD-29 extends partially into the Grade Shell. You can now see from the individual IP data cross sections that these drill holes were not advanced deep enough back in the day to make significant contact (or in some cases, any) with the highly-charged subsurface zone.
Compare drill hole locations STD-25 and STD-03 on cross section plates 46 and 48 (look in the legend at the bottom of the image for the plate #) - on PDF pages 2 and 3 respectively. You can see that STD-25 made contact with a grade of 0.30% Cu before terminating, but that chargeability continues to increase with greater depth. The dark orange color representation of chargeability extends another 450 m below the base of where STD-25 terminated. Scale shown in the January Oroco press release represents the dark orange color as > 25.1 mV/V and the purple colors at even greater levels of chargeability.
STD-03 is 153 m from STD-25 and extended to a greater depth into the highly-charged zone and made contact with a modest 207 m of 0.45% Cu grade. Based on the dark orange color representation of chargeability, it could very well have another 400 m of good Cu grade with increasing depth.
Note the termination depths of each historical drill hole at STD-06, STD-07, STD-22, STD-26, STD-29, STD-30 on the other cross sections. Compare those to the levels of increasing chargeability at greater depths. This area west of the 2009 Grade Shell looks very promising.
In my February 15 post, I suggested that as one moves westward and closer to the river, the mineralization may dip in elevation, become thinner or both. I assumed conservatively that mineralization could have a vertical depth thickness of 200 m at the fault and gradually lessen to a vertical depth thickness of 100 m at a distance 400 m west of the fault. Based on those assumptions, I had estimated an additional potential volume contribution, west of the 2009 Historical Grade Shell, of 45 million cubic meters.
After reviewing this additional data and measuring the vertical depth and lateral extent of highly-charged mineralization (≥ 25.1 mV/V) represented on each individual cross section plate, I am revising my volume estimate. Using the same formula from my February 15 post, I am now estimating the following:
500 m mineralization vertical thickness at Fault (A)
400 m mineralization vertical thickness further west (B) - chargeability ≥ 25.1 mV/V (dark orange color)
500 m distance West to East (C)
750 m distance North to South (D)
Volume = [(A+B)/2]*[(C)(D)] = 168.75 million cubic meters
On some individual cross sections, the vertical depths of highly-charged mineralization (values “A” and “B”) appear to be greater than the numbers above, and on others cross sections, the lateral West to East distance (value “C”) might to be closer to 600 m. The North to South extent of mineralization extends greater than the 750 m value “D”, but it appears to narrow laterally on the West to East measurement. So there is likely some additional volume contribution upside not being accounted for in the revised volume estimate above.
Assuming an average bulk ore density conversion factor of 2.7 tonnes / cubic meter would yield 455.6 million tonnes ore. Not all of the volume west of the 2009 Grade Shell can be expected to be economically viable however.
Presented below is a range of average grades, percentage ore viable, and the corresponding Cu resource representing the potential additional North Zone resource to the west of the 2009 Grade Shell boundary. You will note that the table format differs from that in my February 15 post. I now see greater clarity on the vertical depth of highly-charged mineralization as one moves west from the fault. My previous table had presented a range of potential vertical mineralization depths. Since that uncertainty has been reduced, I am using a 168.75 million cubic meter volume estimate and going with the more recent table format consistent with my posts #3 and #4.
North Zone – West of Historical Grade Shell only
Cu resource estimate in millions of pounds
% Cu 20% ore viable 40% ore viable 60% ore viable 80% ore viable
0.55 1,105 2,210 3,315 4,420
0.50 1,004 2,009 3,013 4,018
0.45 904.0 1,808 2,712 3,616
0.40 803.6 1,607 2,411 3,214
0.35 703.1 1,406 2,109 2,813
You can scale the % Cu grade and % ore viability to your own judgement. Keep in mind though that the 2011 historical resource estimate calculated average North Zone grades of 0.461% Cu measured and 0.429% Cu indicated. Neither the historical resource estimate nor the table above take into account additional resource credit for other metals such as Au, Ag, and Mo, which can only provide a positive lift.
I am revising the estimated potential additional ore volume summary from my previous different posts (in units of tonnes) as follows:
South Zone: 400,950,000
North Zone – West of Historical Grade Shell: 455,625,000
North Zone – Below 2009 Historical Grade Shell: 183,600,000
Northeast Zone: 667,575,000
Total: 1,707,750,000 tonnes ore
The 2011 historical resource estimate for the higher grade ore (> 0.35% Cut) in the combined North and South Zones was 332,860,000 measured & indicated tonnes of ore (ST report page 6).
Combined estimates: 2,040,610,000 tonnes ore
I am also revising the summary potential Cu resource estimate (in units of millions pounds Cu). Consistent with the approach in my March 8 post, I am selecting a mid-range value from each table since there is some uncertainty as to the grades and percentage of viable ore that could ultimately be converted into measured and indicated resource above the cut-off grade.
South Zone: 3,857
North Zone – west of Historical Grade Shell: 2,260
North Zone – below 2009 Historical Grade Shell: 1,214
Northeast Zone: 3,974
Total: 11,305 million pounds Cu
The 2011 historical Cu resource estimate for the higher grades in the combined North and South Zones was 3,204 million pounds Cu.
Combined estimates: 14,509 million pounds Cu
* * * * *
One of the projects I worked on a few years ago was managing a geophysical site characterization of a property about one-third the size of the Santo Tomas concession. The project included installation of an extensive groundwater monitoring well network. I worked with several geologists in planning and overseeing the work, and then later reviewed a lot of project data for several years. Prior to making decisions on choice of drilling locations, and then later setting screening intervals on monitoring wells, we reviewed multiple data source types (electrical resistivity, downhole gamma-gamma surveys, heat-pulse flowmeter measurements, etc.).
Seeing mention in the March 3 press release of the other geophysical tools being employed (helicopter magnetics survey with magnetometer and gamma ray spectrometer) was reassuring because those data inputs will be complementary to the 3D-IP surveys and informative prior to drilling. I enjoyed watching the Mexico trip Santo Tomas video series posted on this forum a few months back too because it helped place certain topography features into context with many of the maps and tables of data available to us. Episode 3 was my favorite.
In some ways, analyzing the ST data is easier than the other project because copper porphyry is a static system, and doesn’t move like groundwater. Having the all of the historical drill hole coordinates and depth measurements available in ST report Tables 7 and 8 is very helpful for using as reference points in calculating distances and volume estimates.
I am looking forward to additional data releases as the Santo Tomas resource becomes more fully-defined and the price of copper continues its inexorable climb.